valk

1941 Roadmaster coupe

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The front carb is the only one with a choke or idle circuit. It should be offline at idle and light throttle openings. Vacuum holds the butterfly under the rear carb closed, it opens at higher throttle openings as the rear carb comes online. If you have it adding fuel at idle, that would explain why the rear cylinders are running rich. It should be offline at idle. When the engine is off, move the throttle linkage through its range of travel and you will see that the front carb is already pretty far open before the rear carb starts to open. It's probably better to think of the rear carb as mechanical secondaries on a traditional 4-barrel carb. I don't think there's anything wrong with your car (other than the missing weight, as Dave mentions), you're just looking at the rear carb the wrong way for tuning purposes. It is usually offline at idle and under normal driving.

 

As far as getting the idle buttery smooth, it probably won't happen. Strombergs are simpler than Carters for tuning purposes, but they don't seem to idle as smoothly. However, I have yet to see a big series '41 Buick with dual carbs idle perfectly smooth. These were high-performance engines and I suspect that they never really idled perfectly. I spent a lot of time tuning and tweaking mine and figured that it should idle imperceptibly, but I was never able to achieve it. I talked to Doug Seybold about it and he said that he can sometimes make it perfect, but it's rare and he believes it has to do with the cam, not the tuning. I have not had actual cams in my hands to do measurements, but there are some indications that the dual carb cams are slightly different and therefore do not idle as smoothly as the single carb cams that came before and after. I think "pretty good" is as good as you'll get, but I also think that's part of the deal with a '41 or '42 large series dual carb car. I've come to accept that it's an acceptable trade-off for the rather extraordinary performance advantage. These are still pretty fast cars!

 

As long as it idles relatively smoothly and pulls cleanly at all speeds without any stumbles or stutters, that's a win. You'll be thrilled with how hard it pulls!

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Thanks Matt. Maybe I am asking too much. I'm still a bit fogged regarding the idle circuits, though, in that the 2 sources I have ('41 shop manual Dave lent me and a booklette titled "Your 1941 Buick Fireball Eight; How to know it better...so it will serve you best!") state the idle systems of both carbs are functioning at idle and partial throttle up to 15MPH or so.  It notes, " There is sufficient clearance around the flies of damper valves to permit idle system of rear carburator to function with damper valve closed".  And I don't know of a way to shut down the rear idle system without turning the idle screws all the way in.  What am I missing??  In any case, something is making the 4 rear cylinders too rich and I have to turn the idle screws in almost all the way to get a decent plug reading. Ugh...

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 I hate to disagree with Matt, as I have great respect for him, but the info I have indicates that the rear carb has an idle circuit. I did not realize that when I first got mine on the road, and had a terrible idle, till I realized that a previous owner had blocked the fuel off to it with a small screw jammed into a passage. I guess to try to save fuel. Then I had a more acceptable idle. This tends to confirm my belief that the rear carb aids in idle. I have had consistent trouble with the butterfly which is under the base of the rear sticking in one position, or the other. When its' closed you don't get the extra performance of the rear carb, but honestly, its' like icing on the cake, the car still performs good without it working. But if its' stuck open, then more air is fed through the rear one, and that will upset the idle quality. I am seriously considering taking it off and machining it for small ball bearings during the winter, in the hope that it will work more reliably.

 I can't agree MORE with Matt on this, it is tough to get that silky smooth idle that I have in other Buicks with the '41's. I have mine set so that it idles decently, and it goes great when I put my foot in it. Driving dymanics is great as well, and when one considers the age of these cars.

 It is possible that there is some internal issue with the rear one that is causing the problem, but hopefully it can be solved with tuning.

 Keith

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Well I understand the confusion. And Matt brings up another pertinent variable, the cam, which on mine was recently reground with what I'm told was a slight "performance" grind, which might contribute to a lumpy idle. Something ain't normal, though, as the vacuum gauge shows. Something I haven't explored thoroughly is a vacuum leak  but not sure how to go about tracking it down if it exists. 

Peter

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 The cam grind certainly could contribute to a lumpy idle, depending on the specs. Though with the some of the plugs showing rich running, it tends to show a fuel issue.

 Got to tell you a bit more about my carbs. A few years ago I had my Strombergs rebuilt by a reputable place, and it ran fabulous for some time, I could go up hills on the highway at 70, with lots to spare, then last spring it gave my troubles on high speed running. Though through all of this the rear plugs tended to run rich, a bit sooty from no5 on back with 7 &8 the worst, no matter how I tried to set it up. Nice idle with those though. So I went back to a set of Carters that I'd had. Late last year I took the Strombergs to a local guy, who came highly recommended, and finally got them back only a few weeks ago. With about 300+ miles of driving on it, I pulled all the plugs late last week, and they were all looking just about right, so it looks like the setup is about right. Hopefully it will keep on running well. The only other issue I have with these, is that I have trouble getting the idle low enough, with matching airflow.

 I bought an airflow meter off of eb.y a few years ago, and I find that it really helps with trying to balance air draw from the front carb to the rear.

 Vacuum leaks can be tough to find, one way that was recommended to me, which I've used, is too use acetylene or a propane torch to feed raw gas to various suspect areas when running, and see if the idle changes. It could be a simple as a bad gasket somewhere.

 Don't know if I missed something in a past post about the vacuum gauge reading you got. I'll can read back through your thread, and see I can find the info.

 Have you had the car out on the road?

 Keith

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I have an air flow meter that I use to balance the SU's on my TR3. I can make up an adapter for the larger carb. Good suggestion.

 

After some mixture tuning, his vac gauge at idle shows readings close to 20 but periodically dropping 2 or 3 inches. Definitely missing or very weak combustion. Cylinder balance test showed very little RPM drop on rear 4 cylinders. This matches with what his plugs are showing. He is going to see what the damper position is. It looks right now like the rear carb is dumping in way too much fuel. Carbs are Carter WCDs by the way.

 

Cheers, Dave

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14 hours ago, valk said:

Well I understand the confusion. And Matt brings up another pertinent variable, the cam, which on mine was recently reground with what I'm told was a slight "performance" grind, which might contribute to a lumpy idle. Something ain't normal, though, as the vacuum gauge shows. Something I haven't explored thoroughly is a vacuum leak  but not sure how to go about tracking it down if it exists. 

Peter

Usually you can hear a vac leak but not all the time. I use wd-40 to locate a vac leak. with engine idling, spray wd, one place at a time, intake to head, base of carbs. Engine will speed up when detected. Your rear carb should not be putting any fuel in the system until kicked in progressively. Makes me think flooding over(not stopping the flo of gas) trash in needle/seat or too high float level.

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Yea, I think we are all zeroing in on the rear carb. Thanks for all the vacuum leak detection suggestions. A rebuild kit is on the way courtesy of the guy I bought the car from (is that cool or what?) and I'll check the float and needle valve in the meantime.  But here's a mystery;  both "heat control valves" -  which are located between the carbs and exhaust manifolds and have a wound spring on one end and a counter weight on the other -  are stuck solid. I would think I should be able to at least budge these with my hands and makes me think they were set up this way since both are the same. 

 

And thanks all for hanging in there for me even though this is getting somewhat tedious. 

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Posted (edited)

 Number one, we have seen far more tedious than this,  so don't feel bad.   Number two, I would wager that the heat riser valves on most cars from this era are stuck closed because people don't need them since they only drive in warm weather, and they are a big hassle to keep working correctly. They are stuck closed on my '41.

Edited by neil morse (see edit history)

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Congratulations Peter  !!!!

So glad she's found a good home.

It's just too bad Colorado has lost another beautiful Buick.

We don't have that many to spare.

 

Mike in Colorado

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Man, not much to these rear carbs, is there?  No real obvious problem detected other than it's got a gritty residue all in it and it stinks to high heaven of stale gas. The crud in this one is telling me to clean the main one too but it won't be this simple. 

carb.JPG

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Peter, the heat control,  or heat riser valves were stuck solid on mine. I sandblasted them clean, soaked them all kinds of penetrating solvents, and as much heat from the torch and hammering as I dared. This went on for about two weeks, but all to no avail. They were stuck in a mid open position, and bent them to fully open. My bet is that they hadn't been lubed since they left the factory.

Hope the cleaning and rebuild goes well.

Keith

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Yes Keith:

 Been there done that on my 1937. Heat-Tap-Spray-Repeat.  I did finally get the damper to be loose and work with a new spring from Bob's.

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I think Peter is OK for now on the exhaust bypass. Both weights were in the correct 2 o'clock position.

 

The 2 longer tubes in your pic are the low speed jets. They are spec'd to a 65 drill. This is at the lower pinched part. You also have the bypass and economizer ports to check in the carb body, the spec should be on the sheets I sent you.

 

So what position is the damper valve in, does it move freely?

 

Cheers, Dave

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13 hours ago, valk said:

Yea, I think we are all zeroing in on the rear carb. Thanks for all the vacuum leak detection suggestions. A rebuild kit is on the way courtesy of the guy I bought the car from (is that cool or what?) and I'll check the float and needle valve in the meantime.  But here's a mystery;  both "heat control valves" -  which are located between the carbs and exhaust manifolds and have a wound spring on one end and a counter weight on the other -  are stuck solid. I would think I should be able to at least budge these with my hands and makes me think they were set up this way since both are the same. 

 

And thanks all for hanging in there for me even though this is getting somewhat tedious. 

Both flappers were froze up on my `41 248 dual carb engine ex manifolds, I also tried the soaking and heat, wouldn`t budge. So taking a closer look at the flappers both were a little offset(larger gap on one side), I then set it up in my press and applied pressure to the shaft(oppisite end of the gap)to move the shaft sideways, both came right loose, little stiff at first, worked them manually with some spray lube and both were free moving as they should.

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14 hours ago, valk said:

Man, not much to these rear carbs, is there?  No real obvious problem detected other than it's got a gritty residue all in it and it stinks to high heaven of stale gas. The crud in this one is telling me to clean the main one too but it won't be this simple. 

carb.JPG

You might consider cleaning gas tank also..

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When I pulled the manifolds on my '40 LTD (single carb) the damper was so stuck open, there was no hope of freeing it up, so I fabricated a plate from 14 gage sheet metal and brazed it to what was left of the shaft in the closed position and never looked back.

Pont35cpe is right in that pulling the gas tank is a must.

Then put a BIG clear fuel filter back at the tank, so you can check for trash.

 

Mike in Colorado

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I regret to report no progress on my '41 Buick woes. Rear 4 plugs still wet with fuel and now it smokes worse than ever. I'm afraid the fuel will wash the oil off the  cylinder walls and ruin the rings so I'm chicken to drive it.  Best looking car with a shitty engine I've ever seen. Anybody want to buy a Roadmaster coupe? 

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Sorry boys.  After a belt or 2 of Woodford Reserve, I am braced to face whatever lies ahead. Damn the torpedos....I will plow ahead and get this thing back on the road. 

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I feel your pain, Peter. My 1935 Lincoln has been moments away from a sledgehammer more than once.

 

Fortunately, this is just a case of carburetor adjustments and that's a very curable condition. I think my next step might be disconnecting and blocking the fuel line at the rear carburetor and seeing how it runs.

 

After that, I'd check the linkage. The rear carb really shouldn't be doing much until you have the throttle pretty far open. Is your linkage working properly? There should be two separate rods from the accelerator pedal assembly and the one to the rear carb should not open it until the pedal is about 3/4 of the way through its travel. And I still don't believe there is an idle circuit in the rear carb. Yes, some fuel might get pulled in just as a function of vacuum, but it is not designed to feed fuel at idle. The disassembled carburetor verifies this. 

 

The flapper, meaning the little one under the carb, might be stuck. I believe it is expressly designed for this very situation--it should be closed at high vacuum levels and open at low vacuum levels (that's what the weight is for). If it's not working correctly or is open at idle, of course the rear carb is going to dump fuel. 

 

I have a fairly comprehensive training book that I've been meaning to scan. It has very detailed carburetor adjustment procedures. I'll try to get on that job in the next few days and post it so we can all use it. But in the meantime, try running it without any fuel going to the rear carb. Yes, it might lean out a bit at big throttle openings, but I think that'll be OK while you test. That will at least narrow down the culprit to the rear carb.

 

Keep us posted.

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Try pouring a little of that Woodford in the rear carb -- should fix the problem immediately!  ☺️

 

Although I'm the lucky owner of a dual carb Buick that runs flawlessly (not because of anything I did -- it came to me that way), I'm afraid it's above my pay grade to give you much help.  I'm still stumped on why you are focusing only on the rear carb when the shop manual shows that both carbs feed all eight cylinders.  You say that you "plotted out" the set up, and confirmed that the rear carb feeds the rear four cylinders.  I'm not following that.  I really like Matt's suggestion of cutting off fuel to the rear carb and seeing what happens.

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Thank you Matt and Neil very much for your suggestions and response.  I'm going to take Matt's advise and plug the rear fuel line. Blocking off the rear carb entirely is also an idea supported by a couple folks I've talked to.  And I have a line on a stellar rear carb damper from Greg Johnson thanks to Dave Stovall.  Even though Dave and I already tested a few, I'm going to hook up a compression gauge to all cylinders tomorrow and start anew. The only thing I think I can rule out at the moment is ignition - have strong spark going to at all 8 and timing is correct...and got new plugs. 

Thanks boys, more later.

Peter

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Posted (edited)

While Peter is imbibing, perhaps I can shed a bit more light.  Matt - the rear carb definitely has an idle circuit, including 2 needle valves and 2 low speed jets. His flapper assembly has no flapper. It is missing. I was afraid of this when I saw the counterweight was missing. He is working to get one from Greg Johnson. But your suggestion to plug the rear fuel line is good, and maybe an even better idea is to remove the rear carb completely and plug the manifold opening, like they did in WW2 to try to save on gas.

 

Neil, we know they are in parallel, but are assuming that one of the carbs dumping fuel would likely affect the closer cylinders more. In theory anyhow lol!

 

Cheers, Dave

Edited by Daves1940Buick56S (see edit history)

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Posted (edited)

Thanks for addressing my question, Dave.  That makes a little more sense to me than what Peter said, but I'm not knowledgeable enough to know whether your "theory" is correct.  At least we all agree that blocking fuel to the rear carb is a good diagnostic move.

Edited by neil morse (see edit history)

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