Matt Harwood

1941 Buick Limited Limousine

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On 12/4/2019 at 5:56 PM, carbking said:

I will disagree on holding the petal to the metal on a flooded engine with today's fuel.

 

Hot starting

 

Jon.

 

What if the engine is cold?  I think that's the situation regarding this particular car.  Floods easily when cold. 

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John - if I misunderstood, my apologies. I may have misread the comments. Flooding an engine when cold, to me, means either (probably) defective spark, or the carburetor is adjusted to rich. If the carb is the issue, then holding one's foot to the floor will activate the unloader circuit, but the choke will only be approximately 1/4 inch open, and the engine would flood worse. Just to prove this to myself, I would start the car, get it to normal temperature, and check the A/F ratios, especially at idle, adjusting if necessary. If the same thing happened again, I would check the firing voltages at the plugs.

 

Jon. 

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Thanks Jon. While it isn't my car thats the subject here I do wonder if this cold start flooding is because Matt is using two primary carbs? 

I have no first hand knowledge of the 1941 dual carb system but I thought I read that the original secondary carb had no choke system. And if I understnd it right the original design was for a progressive linkage to the 2nd carb. But Matt switched to 2 primary carbs with a direct linkage. This causes two chokes to come into play and a prime shot of gas from both carbs. Maybe it would help to adjust the 2nd carbs choke to a lean position so that it provides some air restriction but doesnt actually ever close

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John - I don't think it is the use of two primary carbs that is the issue. I have been building and selling multiple carburetion for 40 years, and except for those building numbers matching showcars, ALWAYS use chokes on ALL of the carbs and solid linkage. Not just Buick, but all makes. My 1968 Ford F100 with a 435 HP 390 has two 625 CFM Carter AFB's running solid linkage, and manual chokes on both. Have done several Buicks over the years, and ALWAYS used two single carbs, as they were less expensive for the customer than fronts. They work well.

 

Jon

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What happens when you floor it to hit the unloader? It seems (without being there to see it), that if one carb doesn't have a choke, then that one carb would be wide open at WOT, and the other carb open whatever amount the choke unloader kicks it open. The choke unloader probably doesn't need to do much if there is another carb wide open. Add a choke to the other carb, and wouldn't you need both of the choke unloaders to be set a lot more aggressive?

 

Jon- What is the purpose of an unloader if not to clear out a cold engine that the owner accidentally flooded? That is the only thing I have ever used them for. It has worked for me getting screwed up cars driven into the shop more times than I can count. (I agree completely that  something was probably set too rich to flood it in the first place).

Edited by Bloo (see edit history)

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Bloo - as you suggested, the purpose of the unloader circuit was to clear a flooded cold engine, BUT one with the choke working. In Matt's case, he is not activating the chokes by not using the footfeed. If he activated the chokes by pressing the footfeed, then the issue could possibly be worse.

 

Much depends on if the carbs ARE too rich, or if the ignition system is weak.

 

Jon.

Edited by carbking (see edit history)
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Thanks for all the additional information, gentlemen. Right now, the car starts instantly and runs beautifully above about 45 degrees. Every time, hot or cold. But lately as I've been driving it daily in the cold, it has been problematic to start. I have found that if I prop open the choke on the rear carb, it starts right up and runs rough for about 20 seconds, then acts like nothing happened. I am not sure that I have the chokes configured properly, and that's something I plan to address. I have them backed way off (springs fairly loose) on the theory that since there are two of them, I don't need a lot of choke. Second, when it was running the factory sequential linkage and a floor starter, the factory starting position would have been front carb on the choke, rear carb wide open, no choke. So maybe I'm over-choking it and backing off the rear carb's choke even more would help. And after reading Jon's excellent information on his website about how the chokes work, including the little piston that is supposed to pull it open once the engine fires, I'm not convinced mine are working correctly. I'm still confused about the hows and whys of the spring mechanism AND that little vacuum piston--why have a gradual spring choke if the vacuum piston just pulls it wide open instantly? Which is more important to be functional? I have always had a slight vacuum hiss from the rear carb's choke area, so I'm not convinced the little piston is working correctly--I'll have to go in after it again.

 

So I guess my question is at this point: should I back off the rear carb's choke a lot or even disable it entirely? I assumed that in cold weather, more choke would be important, but this car seems to be the opposite--on warm days, it fires instantly with both chokes closed, but on cold days it refuses to run and floods instantly. Could the closed chokes be closed too far, creating too much vacuum which is in turn pulling too much fuel into the cylinders (AKA very rich)?

 

It seems counter-intuitive to me.

 

PS: Ignition is strong and all new and I get a big, fat blue spark, although repeated soakings during these flooded starts may have fouled plugs. I should pull them and have another look, too.

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It is distressing to hear one of our forum leaders is waivering.  Many of us rely on Matt and a few others to lift us up when stuff goes south.  I wish we could all meet at your place and collectively sort your car(s) out as payback for what you've given us. Hang in there, man, this too will pass...

 

I wonder if there are any "modern" carbs that bolt up and improve reliability?  It is a further step away from originality, but might be worth it if you drive a lot. 

Peter

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11 hours ago, carbking said:

I would check the firing voltages at the plugs

 

31 minutes ago, Matt Harwood said:

Ignition is strong and all new and I get a big, fat blue spark, although repeated soakings during these flooded starts may have fouled plugs. I should pull them and have another look, too

Check the voltage at the coil while cranking that big old cold engine with a cold 6v battery...

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2 hours ago, Matt Harwood said:

 

"When it was running the factory sequential linkage and a floor starter, the factory starting position would have been front carb on the choke, rear carb wide open, no choke. "

"Could the closed chokes be closed too far, creating too much vacuum which is in turn pulling too much fuel into the cylinders (AKA very rich)?"

"I have them backed way off (springs fairly loose) on the theory that since there are two of them, I don't need a lot of choke."

"I'm still confused about the hows and whys of the spring mechanism AND that little vacuum piston--why have a gradual spring choke if the vacuum piston just pulls it wide open instantly? Which is more important to be functional?"

"I assumed that in cold weather, more choke would be important, but this car seems to be the opposite--on warm days, it fires instantly with both chokes closed, but on cold days it refuses to run and floods instantly."

"So I guess my question is at this point: should I back off the rear carb's choke a lot or even disable it entirely?"

 

 

Matt,  I took the liberty of changing the order of some sentences from your previous post for this response:

 

"When it was running the factory sequential linkage and a floor starter, the factory starting position would have been front carb on the choke, rear carb wide open, no choke. "

This is what made me think disabling the rear carb's choke would be an advantage.  But Jon said he consistently applied two front carbs with their chokes and they worked.  

 

"Could the closed chokes be closed too far, creating too much vacuum which is in turn pulling too much fuel into the cylinders (AKA very rich)?"

For purposes of clarity, when the choke is applied,  the vacuum created within the engine will be forced to pull more fuel through the carb (s).  In your case it used to pull the fuel from one carb.  With the change to two primary carbs, it will now pull fuel from two carbs,  and in my opinion, resulting in too much raw gas in the intake system.  AKA, very rich.

 

"I have them backed way off (springs fairly loose) on the theory that since there are two of them, I don't need a lot of choke."

I believe this application for that theory is flawed.  You may not need more choke than was originally determined (choke on the primary carb only)  but you will need some choke once you have this all dialed in appropriately.  Even in the summer.

 

"I'm still confused about the hows and whys of the spring mechanism AND that little vacuum piston--why have a gradual spring choke if the vacuum piston just pulls it wide open instantly? Which is more important to be functional?"

That spring is thermostatically controlled.  When it is cold it will contract and that's what puts the pressure on the linkage so that when you step on the accelerator the choke should snap closed.  As the spring warms up it expands and releases the pressure on the linkage.  After the spring is sufficiently heated up, operating the accelerator will not close the choke.  The vacuum piston is there to force the choke to open once the cold engine is started.  It does take a few minutes of run time to warm the thermostatic spring enough to take that pressure off the linkage.  And if the vacuum piston was not part of the system, then, when cold,  the thermostatic spring would continually try to close the choke, thus flooding the engine. 

 

"I assumed that in cold weather, more choke would be important, but this car seems to be the opposite--on warm days, it fires instantly with both chokes closed, but on cold days it refuses to run and floods instantly."

I am not sure the concept of "more choke" is applicable.   Choke adjustment is generally described as lean or rich.  But I believe the engine in any car starts with the same amount of fuel to air ratio, and ambient temps are not a major consideration in the adjustment.  In fact, there appears to be an inverse relationship to adjusting the thermostatic spring considering the ambient temps.  Considering that the spring contracts when it is cold, it makes sense to think that it would contract more at 10* than at 45* .   And if the engine needs a certain air to fuel ratio to start and run,  a colder thermostatic spring would be putting tension on the linkage for a longer period as it warms up and expands enough to release the tension on the linkage.  Thus, you might need a leaner choke setting in extreme cold weather than when it is warmer.    When your car starts instantly on warm days it may be because the thermostatic springs are already further expanded and thereby releasing tension on the choke linkage earlier. In addition, if you suspect a problem with the little piston the rear carb it could be that the front choke opens enough to get air into the engine while the rear choke is closed and tending to flood the engine , thus your period of rough engine operation till that rear choke can open enough to stop flooding. 

 

"So I guess my question is at this point: should I back off the rear carb's choke a lot or even disable it entirely?"

That is worth a try to me.  It would be closer to what was originally designed.  Plus it would mean less time trying to coordinate the two chokes.  I would disable the rear choke by loosening the thermostatic spring to the lean side all the way.  Then adjust the front choke per the manual.  Or I would adjust the front thermostatic spring by feel.  With the engine cold, adjust the spring so that accelerator operation results in the choke butterfly closing completely.  And then an extra index mark on the cover.  That little extra is to keep the butterfly closed when the engine is first cranking over, before there is engine vacuum at the little piston to pull the choke off.  Otherwise the choke butterfly may be pulled open during cold cranking thus negating the choke benefits.   But that is just an initial adjustment.  You may have to close that rear choke partially which would be determined by how quickly the engine starts.   You ultimately need the chokes fully open when the engine is warmed up.  And they also must open partially ( which is what the little piston does) even when cold, to let the engine draw air to mix with the fuel.  

 

 

 

 

 

 

 

 

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4 hours ago, Matt Harwood said:

I'm still confused about the hows and whys of the spring mechanism AND that little vacuum piston--why have a gradual spring choke if the vacuum piston just pulls it wide open instantly? Which is more important to be functional?

 

On most cars, the spring sets the start position (and the fast idle), and the piston pulls the choke to the run position, not all the way open.

 

Carter DID make something that pulls the choke all the way open on start, my 36 Pontiac does that. I am not sure how that even works, but it runs good. That setup is NOT normal....

 

I'll bet by 41, the piston works like normal more modern cars, and just pulls the choke to a good run position. That would be closer to open than you would expect! Then the choke opens slowly the rest of the way as the car warms up.

 

If it were me, I would want both chokes doing the same thing on a car with a synchronized linkage like yours. If it floods on start, set the thermostatic coils looser. When it starts, both should snap to the same place, because the little pistons take care of that. There is probably an adjustment for the piston position vs choke position, usually called "choke pull off".

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1 hour ago, JohnD1956 said:

Matt,  I took the liberty of changing the order of some sentences from your previous post for this response:

 

"When it was running the factory sequential linkage and a floor starter, the factory starting position would have been front carb on the choke, rear carb wide open, no choke. "

This is what made me think disabling the rear carb's choke would be an advantage.  But Jon said he consistently applied two front carbs with their chokes and they worked.  

 

"Could the closed chokes be closed too far, creating too much vacuum which is in turn pulling too much fuel into the cylinders (AKA very rich)?"

For purposes of clarity, when the choke is applied,  the vacuum created within the engine will be forced to pull more fuel through the carb (s).  In your case it used to pull the fuel from one carb.  With the change to two primary carbs, it will now pull fuel from two carbs,  and in my opinion, resulting in too much raw gas in the intake system.  AKA, very rich.

 

"I have them backed way off (springs fairly loose) on the theory that since there are two of them, I don't need a lot of choke."

I believe this application for that theory is flawed.  You may not need more choke than was originally determined (choke on the primary carb only)  but you will need some choke once you have this all dialed in appropriately.  Even in the summer.

 

"I'm still confused about the hows and whys of the spring mechanism AND that little vacuum piston--why have a gradual spring choke if the vacuum piston just pulls it wide open instantly? Which is more important to be functional?"

That spring is thermostatically controlled.  When it is cold it will contract and that's what puts the pressure on the linkage so that when you step on the accelerator the choke should snap closed.  As the spring warms up it expands and releases the pressure on the linkage.  After the spring is sufficiently heated up, operating the accelerator will not close the choke.  The vacuum piston is there to force the choke to open once the cold engine is started.  It does take a few minutes of run time to warm the thermostatic spring enough to take that pressure off the linkage.  And if the vacuum piston was not part of the system, then, when cold,  the thermostatic spring would continually try to close the choke, thus flooding the engine. 

 

"I assumed that in cold weather, more choke would be important, but this car seems to be the opposite--on warm days, it fires instantly with both chokes closed, but on cold days it refuses to run and floods instantly."

I am not sure the concept of "more choke" is applicable.   Choke adjustment is generally described as lean or rich.  But I believe the engine in any car starts with the same amount of fuel to air ratio, and ambient temps are not a major consideration in the adjustment.  In fact, there appears to be an inverse relationship to adjusting the thermostatic spring considering the ambient temps.  Considering that the spring contracts when it is cold, it makes sense to think that it would contract more at 10* than at 45* .   And if the engine needs a certain air to fuel ratio to start and run,  a colder thermostatic spring would be putting tension on the linkage for a longer period as it warms up and expands enough to release the tension on the linkage.  Thus, you might need a leaner choke setting in extreme cold weather than when it is warmer.    When your car starts instantly on warm days it may be because the thermostatic springs are already further expanded and thereby releasing tension on the choke linkage earlier. In addition, if you suspect a problem with the little piston the rear carb it could be that the front choke opens enough to get air into the engine while the rear choke is closed and tending to flood the engine , thus your period of rough engine operation till that rear choke can open enough to stop flooding. 

 

"So I guess my question is at this point: should I back off the rear carb's choke a lot or even disable it entirely?"

That is worth a try to me.  It would be closer to what was originally designed.  Plus it would mean less time trying to coordinate the two chokes.  I would disable the rear choke by loosening the thermostatic spring to the lean side all the way.  Then adjust the front choke per the manual.  Or I would adjust the front thermostatic spring by feel.  With the engine cold, adjust the spring so that accelerator operation results in the choke butterfly closing completely.  And then an extra index mark on the cover.  That little extra is to keep the butterfly closed when the engine is first cranking over, before there is engine vacuum at the little piston to pull the choke off.  Otherwise the choke butterfly may be pulled open during cold cranking thus negating the choke benefits.   But that is just an initial adjustment.  You may have to close that rear choke partially which would be determined by how quickly the engine starts.   You ultimately need the chokes fully open when the engine is warmed up.  And they also must open partially ( which is what the little piston does) even when cold, to let the engine draw air to mix with the fuel.  

 

 

Wow, thank you, John! That is EXTREMELY helpful. I may go fiddle with it tomorrow (Sunday) and see what happens. I have to admit that even after all these years, the science of carburetors is still a bit of a mystery to me. Fuel injection? No problemo. But this is like black magic sometimes and I get things right more by accident and trial and error than by procedure. This is an extremely helpful guide. Thank you!

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As I have been running a matched carb parallel linkage for three years now using both 60 series Strombergs and smaller series 528s Carters I have experienced none of the issues Matt has encountered with cold starting. I did not find that flooding was ever a problem even with the larger Strombergs. I did shorten the accelerator pump adjustable linkage to reduce the volume of fuel . When I first switched over I did not hook up the rear choke thinking I would have enough just off the front. The result was lean popping and flame belch in the rear carb. Sync hing  up the chokes was really easy and only required a couple of adjustments. Like Jon said multiple matched carbs is not a problem any more then a single carb is when properly adjusted. This discussion has been incredibly informative so thanks for some enlightenment by expert members. Good luck Matt!

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Disconnecting the choke on the rear carburetor will violate the "Law of Unintended Consequences" ;)

 

WHY?

 

When running with the original progressive linkage, the throttle on the rear carburetor is closed at start-up, forcing all A/F mixture to be supplied by  the front carburetor.

 

When running with solid linkage, the throttle on the rear carburetor is open at start-up, thereby causing a severe imbalance of the A/F mixture density to the cylinders fed by the rear carb, thus creating the condition experienced by Lawrence in the post above this one.

 

Matt - how far out do you have the idle mixture control screws on both carbs? My GUESS would be somewhere around 1/2 turn would be about right.

 

At this point in time, why not start the adjustment procedure over. Pick a day when the ambient (or the temperature in your garage) is in the window of 65~70 degrees F., disconnect the linkage connecting the two carbs, one by one adjust the chokes by first working the throttle on each carb to wide open and then release. Open the throttle approximately 1/2 way and HOLD. Now loosen the screws holding the retaining clamps, and rotate the choke cap gently in each direction, looking for the direction that allows the choke butterfly to fall open. Now, gently rotate the choke cap in the opposite direction until the choke butterfly just touches closed with zero tension. Once both chokes have been adjusted, reconnect the two carburetors. Start the engine. Run the engine at a fast idle until the engine is totally at operating temperature. Turn one of the throttle positioner screws clockwise to give an idle of maybe 1000 RPM. Rotate each mixture control screw clockwise until lightly seated, then counterclockwise 1/2 turn. Which one is done in sequence is unimportant. Once all four have been adjusted to 1/2 turn, rotate the throttle positioner screw you used to set the fast idle counterclockwise SLOWLY, blipping the throttle each 1/8 turn to try to get your target idle RPM. Some further adjustment of the mixture control screws may be necessary, but shouldn't be much.

 

Now, the next time you go to start the engine cold: remove the air cleaner, and verify both chokes should be wide open. If not, we have an adjustment issue somewhere. If they are, GENTLY push the throttle wide open and release (take about 4 seconds to push the throttle open). Verify both chokes should completely close. Now, with the transmission in neutral and one tire blocked, open the drivers door window, stand outside the car, and start the engine! (no foot on the footfeed). It should start, and immediately go to a fast idle. You may need to adjust the fast idle to your target RPM.

 

The above procedure is assuming that you have previously synchronized the throttle operation of the two carburetors. If not, then that should be done as well.

 

Jon.

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On 12/8/2019 at 10:06 AM, carbking said:

Disconnecting the choke on the rear carburetor will violate the "Law of Unintended Consequences" ;)

 

WHY?

 

When running with the original progressive linkage, the throttle on the rear carburetor is closed at start-up, forcing all A/F mixture to be supplied by  the front carburetor.

 

When running with solid linkage, the throttle on the rear carburetor is open at start-up, thereby causing a severe imbalance of the A/F mixture density to the cylinders fed by the rear carb, thus creating the condition experienced by Lawrence in the post above this one.

 

Matt - how far out do you have the idle mixture control screws on both carbs? My GUESS would be somewhere around 1/2 turn would be about right.

 

At this point in time, why not start the adjustment procedure over. Pick a day when the ambient (or the temperature in your garage) is in the window of 65~70 degrees F., disconnect the linkage connecting the two carbs, one by one adjust the chokes by first working the throttle on each carb to wide open and then release. Open the throttle approximately 1/2 way and HOLD. Now loosen the screws holding the retaining clamps, and rotate the choke cap gently in each direction, looking for the direction that allows the choke butterfly to fall open. Now, gently rotate the choke cap in the opposite direction until the choke butterfly just touches closed with zero tension. Once both chokes have been adjusted, reconnect the two carburetors. Start the engine. Run the engine at a fast idle until the engine is totally at operating temperature. Turn one of the throttle positioner screws clockwise to give an idle of maybe 1000 RPM. Rotate each mixture control screw clockwise until lightly seated, then counterclockwise 1/2 turn. Which one is done in sequence is unimportant. Once all four have been adjusted to 1/2 turn, rotate the throttle positioner screw you used to set the fast idle counterclockwise SLOWLY, blipping the throttle each 1/8 turn to try to get your target idle RPM. Some further adjustment of the mixture control screws may be necessary, but shouldn't be much.

 

Now, the next time you go to start the engine cold: remove the air cleaner, and verify both chokes should be wide open. If not, we have an adjustment issue somewhere. If they are, GENTLY push the throttle wide open and release (take about 4 seconds to push the throttle open). Verify both chokes should completely close. Now, with the transmission in neutral and one tire blocked, open the drivers door window, stand outside the car, and start the engine! (no foot on the footfeed). It should start, and immediately go to a fast idle. You may need to adjust the fast idle to your target RPM.

 

The above procedure is assuming that you have previously synchronized the throttle operation of the two carburetors. If not, then that should be done as well.

 

Jon.

 

Thank you, Jon. I'm going to spend some time on it one night this week. It's still pretty cool so hopefully I can get the shop up to a reasonable temperature for tuning. I also have a training manual here somewhere with pretty detailed instructions for tuning the carburetors and I seem to recall it said something about setting the choke so a 3/32" drill bit could fit between the blade and the carburetor throat--I'll see if I can find it before I tear into anything.

 

My hunch is that even though I still set up the chokes to put very little spring pressure on the choke blade, I may have to back it off even more given how cold it is. That shouldn't affect warm weather operation, either other than the chokes might open a little sooner.

 

Thanks for all the advice. I'll report back once I get some time to really play with it. You guys rock!

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46 minutes ago, Matt Harwood said:

 

Thank you, Jon. I'm going to spend some time on it one night this week. It's still pretty cool so hopefully I can get the shop up to a reasonable temperature for tuning. I also have a training manual here somewhere with pretty detailed instructions for tuning the carburetors and I seem to recall it said something about setting the choke so a 3/32" drill bit could fit between the blade and the carburetor throat--I'll see if I can find it before I tear into anything.

 

My hunch is that even though I still set up the chokes to put very little spring pressure on the choke blade, I may have to back it off even more given how cold it is. That shouldn't affect warm weather operation, either other than the chokes might open a little sooner.

 

Thanks for all the advice. I'll report back once I get some time to really play with it. You guys rock!

 Hi Matt, I will tell you that I tried the 3/32 drill bit setting and could not make it work at all as it requires a very light and ultimately inaccurate setup as it never could maintain this position. Also the NOS carbs in the box with from the factory did not employ this setting.  Just saying I found it to be totally useless. The factory  set chokes were lightly closed all the way and from there I left the front as is and just tweeked the rear to match. Mostly an adjustment to compensate for the slight lag from the slower heating of the coil due to jury rigged heat tube which was just a few moments behind in actuation.  

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