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1941 Special engine dies when warmed up


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Hello all,

I'm new to the forum, so here goes.  I have a 1941 Buick Special with the stock Stromberg compound carburetion setup on it, the 7-42 B on the front and the 7-43 for the rear carb.  I rebuilt them both with full kits very carefully.  The car runs beautifully overall.  It idles extremely smoothly, no puffing or missing at all.  When you start out it runs really well, pulls in all gears smoothly, lots of power, the second carb kicks in and pulls strong when you floor it up to 60mph, etc.  The problem is, once it warms up to 180-190 full operating temperature, the engine starts to die.  It loses power and starts to shut down entirely, and you have to quick push in the clutch and bobble on the gas pedal to keep it running, and it will idle that way fine, and you can rev it up ok, but the minute you put it back in gear and try to drive the car, it dies.  When it does die this way I can restart it fine, rev it up, etc.  But if you put any load on it at all it dies.  I have to nurse it home in first gear with high revs and periodically pushing in the clutch in and bobbling the gas pedal, and so forth.  Scary and frustrating.  Next day, starts right up, runs super fine in every way, warms up and starts to die. 

That's about it in a nutshell.  Any idea what would cause this?

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Welcome to the AACA Discussion Forum. What have you done so far to try to fix it?

 

Under the wild guesses with limited information category, I wonder if the coil is failing when hot. The most important thing to remember is to only try one "fix" at a time if you want to know what solved the problem.  

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Ah no, no tiny air filters, just the stock one.

And my attempts so far at trying to fix it have been to replace the points and condenser, set the gap/dwell, and time the engine.  I also have opened the observation holes in the float bowls at idle to check the level of gas in the bowls, and I think that the surface of the gas is right at the bottom of each hole.  Nothing dribbles out.  But, maybe it's too low and I just wasn't able to see that.  In any case, it does run really well until hot.  I think it's the original coil.  Can coils work well until hot?  Maybe I should consider a new coil?....

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Also if the exhaust valve body flapper valve is stuck in the "warm" position, it could cause some weird problems due to trying to heat up the intake air unnecessarily.

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Thank you very much for the responses. 

- I'll look into getting a new coil, it's worth a try.

- I think my manifolds are all tight everywhere with no leaks.  I never detached the valve bodies so that joint is tight and original.  And I replaced the manifold

   gaskets and the studs and nuts and so the attachment to the head is really solid and tight.

- The valve body flappers were really tight so I wired them to be full open all the time, no carburetor preheating.  I only drive it in summer so it's not an issue.

- Likewise my choke is wide open when it's warmed up, no problem there.  And I just replaced the fuel filter element so that's flowing fine.

 

The carbs on these cars, and all cars I suppose, get pretty darned hot when the engine's warmed up.  Something about my front carb seems to change when the engine is hot.  Or for all I know it's not carburetion at all, I'm stumped. 

It wouldn't be vacuum advance, would it?  I mean if that was the problem it wouldn't run great before warm up, right?

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The gasket between the carb and the manifold is like 3/4 inches thick on my '40. The previous owner had stacked around 8 normal-thickness gaskets to make a total thickness of also around 3/4. I'm just guessing, maybe more like 5/8. I'm sure it's to slow the conduction of heat from the manifold to the carb. How thick is your gasket?

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Right, good question.  Mine are also the big thick gaskets, at least 3/4 inch.

Strange that the key factor is load on the engine.  It runs fine hot when not in gear.  It revs, idles, no problem.  But put any load on the engine and it starts to die.

To quote Bob and Doug Mackenzie-   at first I thought it was kind of funny but now it's really starting to bug me....

 

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33 minutes ago, Tinindian said:

Maybe inches of vacuum??  Well my apologies on further thinking I suppose 21 inches could be written as 1' 9".

 

 

Or maybe an inch of mercury is a foot of water since mercury is 12 times denser than water. That's why a vacuum pump can only draw 30 feet of water. Or 30 inches of mercury, but who the heck draws mercury?

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Tried the wiper test last night, and they work as well hot as they do when the engine's cold, no difference.

Regarding the photo, I've included one here.  Probably not too useful, it's just the usual '41 setup.

I looked into a coil at CARS and they have a replacement but not one with the bottom cap connection like the original.  So it would work and enable a test of that theory at least.  I'd just have to jerry rig the wiring a bit to hook it up.

I suspect it's the front carb that I rebuilt.  Not sure of course, but the carbs are the main thing that's changed.  Anyone got a rebuildable AAV 16 Stromberg they're willing to sell?....

Engine2.JPG

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Is there a chance that the heat from the manifold is vaporizing the fuel in both the glass bowl and the fuel lines when she warms up?  At this angle, the glass bowl really looks close to the heat source.

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Yeah, it looks crazy close in that photo doesn't it.  It's not as close as that.  I've attached another photo.  But it looks closer than it really is in this photo too.  It's actually around four inches away from the exhaust manifold and around three inches away from the intake.  In general the fuel lines and filter are pretty much where you see them on a lot of these cars.  But, you make a good point, that could be too close.  I'll have to do some testing with shielding the filter and lines in that area from the heat of the manifold.  Thanks a lot for the suggestion!

 

 - Larry

Engine3.JPG

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Yes, nothing would surprise me, maybe it's that.  I've always tried for E0 but there might be some E10 in there, a lot of times I can't find E0.  I know what vapor lock is like when trying to start a hot engine, but I don't know what the experience is like when you're driving the car.  Is it like this, where the engine seems to die off ?

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I am leaning towards coil. It matches the symptoms and since these coils are kind of unique with their armored ignition cables, people tend not to replace them even when they need to be replaced. A car will run fine when it's cool but as soon as it warms up, the coil can't fire the plugs anymore. Very common in old cars. I would start there.

 

I might also replace the condenser, which are known to go bad and even new ones aren't always good. But do one thing at a time before changing a bunch of parts. That way you know what solves the problem.

 

I don't think that it's fuel, although it could be the fuel pump. Ethanol brings headaches of its own and modern fuels evaporate too easily, but they usually cause problems in these cars only after prolonged sitting, like in traffic on a hot day. You might want to check the butterfly valves under the carburetors to make sure the exhaust heat is being diverted out the pipes rather than up to the carburetor heat boxes. If the carbs are getting too hot, that might be an issue, although an unlikely one. Adding an electric fuel pump isn't a bad idea, either. It will help with priming and starting after the car has been sitting and it will push fuel into the carburetor when it's hot enough to boil in the lines and the mechanical pump can't move vapor. If the mechanical pump does not have an ethanol-safe diaphragm or check valve, it might not be moving as much fuel as it should with each cycle.

 

Try one thing and evaluate. I would start with the coil. My Limited runs on a standard coil and it has been rewired so it doesn't have the armored cable connector. If you take it apart, you'll see it's the standard setup, just enclosed in metal. No big deal to change it and that gives you access to lots of new coils rather than NOS ones that might also have shorts. 

 

After that experiment, try the next thing on the list. This is a common problem that's usually one of these simple things. Shooting in the dark and throwing increasingly esoteric guesses at the problem does nothing until you're actually in there testing and changing parts to see if you get results. So let's start that process.

 

Keep us posted.

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Wow, I'm grateful for the responses, I have some possible solutions to try now.  Per Matt and MCHinson, I'll order a coil from Bob's or CARS.  While I'm waiting for that I'll experiment with some temporary Kevlar shielding on the fuel lines and filter, just to eliminate that.  I'll then try the new coil when it arrives.  If that doesn't solve it I'll try a new condenser.   

I see what you mean about coils-  you can't get a new one with the bottom connection like the originals, they now all look like the usual kind with both connections at the top.  So you must have to add a section of wire to extend the armored wire to reach the second lug at the top of the coil.  I can do that easily enough. 

Thanks tons again.  Hopefully I'll have something positive to report soon. 

 

       - Larry

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I meant to say-  also one more suggestion that I can try is the electric fuel pump.  The car has one but I've only used it prior to starting a hot motor, I've never tried running it all the time, while driving the car.  Good idea though, definitely something to try.  I'll add that to the list.  And as you say, try each suggestion one at a time.  Thanks again.

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If you have an electric fuel pump on the car, when the engine starts stumbling, hit the switch to turn the electric fuel pump on. If the car starts running better in a few seconds, that would indicate that a vapor lock condition is your problem. I still think that your coil is a prime suspect. I would try another coil as soon as possible. 

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If the coil overheats, why would it only fail when the engine labors and not when it idles or "revs"? The coil only makes a spark, it has no way of knowing when the engine is laboring or when it is revving at high RPM's but no labor. The coil does the same thing at 4000 RPM un-labored revs as it does at 4000 RPM under labor. But the car stalls at low RPM under labor. Failing under labor means fuel starvation or air starvation.

 

Not the coil.

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Actually, there is considerably more resistance inside the cylinder under load than at idle or in neutral. More air and fuel being compressed, a denser charge to light, more heat (as heat goes up, so does resistance). Idle and load are not the same at all when it comes to making a spark. It's more than just the coil itself heating up, it's the ability of the coil to generate a spark powerful enough to light under heavy load conditions. As the coil gets hot it becomes more difficult to make a hot spark (resistance again). And it's more than just ambient heat making it hot--the current running through it also heats up the windings. That explains why it'll work just fine when you first start out but start to sputter as it warms up, and why it will idle and rev just fine but seems to struggle under load. I'm still betting on coil in this situation.

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My car ran just fine as long as it was revving in the driveway. But while  driving it began to miss when I put it in gear.

It was most evident in third. I found that one of the spark plug cables was bad. The distributor end was not

seating tightly .. the terminal appeared to be of the wrong type.

I made a new cable with the proper terminals and the problem is now gone.

So, are all the cables good? How about the distributor cap? So many choices. I'm sure you will figure it out.

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 I had nearly the exactly the same thing with my '41 Roadmaster. I verified it when I soaked a cloth in cool water and placed it over the coil, then it started and ran great after about 5 minutes of that treatment. This was on a hot day and in traffic, but the same thing can happen at more normal temps.

 I have an original type coil that seems to work fine, but I carry a modern one, with a by pass wire as a spare. This is the set up I used till I located a correct, and working coil.

 Keith

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Thanks again for the responses.  I was gone all weekend and unable to do anything but I hope to try the things I outlined above this week, barring bad weather.  Boy, will it ever be great if the problem actually is one of the things we've been talking about....

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  • 3 weeks later...

Sort of an update but I'm not out of the woods yet.  Holidays and kid's graduations and all kinds of other stuff caused delays, but I finally replaced the coil.  That changed things, it doesn't die off so hard now, I can keep it running in gear when warm.  But, it still acts up when hot.  There's a sort of "chush" sound from the carbs and a loss of power and I have to get the rpm's down to keep it running.  So for the heck of it I switched on my electric fuel pump and just left it on while driving around (the switch is under the hood for some reason).  I think that might have stopped the problem.  It's as if when the engine is hot the fuel is too hot and the stock pump can't supply enough of a flow to overcome the heat in the lines, in the filter bowl and/or in the carb itself (it gets so hot under my hood that I can't touch the fuel lines or filter bowl).  At any rate, to be sure I need to make some more, longer test drives with the electric pump running all the time.  So far then, it seems like a combination of weak coil and insufficient fuel flow.  I'll report back again in a day or two.

By the way, is it ok to run the electric pump continuously?  If it's a 6 psi pump (I think), will that not overpower the float valves and flood the carb?

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Also-  If I'm right about this problem, does that indicate a faulty stock fuel pump, and I should rebuild it?  I would think so.  Or, my engine simply runs way hotter than it should.  The stock temp gauge works and it stays below 200, so I thought that was acceptable.  It doesn't climb into the hot (220+) zone until I switch the engine off.  But it seems to me that the fuel lines and the carbs shouldn't get so hot you can't touch them, right?  Or is that normal?

 

   Larry 

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What is the mechanical fuel pump on the car? It should have a number cast into the top and there will probably be a number stamped into the mounting flange, either on a boss just above the flange or on an edge of the flange. In other words, is it the correct fuel pump?

 

A 6 psi electric fuel pump may very well overpower the float and needle in the carb. and cause flooding.

 

Your engine should probably have either a 160 or 170 oF thermostat?

Edited by Spinneyhill (see edit history)
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Your current problem sounds like vapor lock due to engine heat heating up the fuel lines. Are the fuel lines in their original position? If someone moved them closer to the engine block, that can cause this problem. Today's fuels, especially those with ethanol also boil more easily than the fuels back in the day. For solving this problem in my 1937 Century, I avoid ethanol fuel when possible. I also have made sure that my fuel pump is rebuilt and operating properly. I have an auxiliary electric fuel pump located near the fuel tank which is activated by a switch that can be reached by the driver. I almost never have to use the electric pump when using non-ethanol fuel. When I am touring and only have access to fuel with ethanol in hot weather I typically have to run the electric pump quite a bit.

 

I would rebuld the mechanical fuel pump. I would try to avoid fuel with ethanol. I would make sure that the lines are no closer to the engine block than they were originally. I would put the electric fuel pump switch where you can reach it when driving.  That should solve this problem.

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I don't think the engine should get that hot. Something is wrong if the engine gets so hot you can't touch the fuel lines. Run a can of evaporust Thermo-whatever it's called through the system to get the rust out. I did that and my car runs cooler now.

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Thanks for the replies.  And yes to all of them.  It does seem that the remaining problem is the fuel getting too hot, either because my lines are too close to the engine and/or the pump is too weak to move the fuel very well.  I'll rebuild the pump-  if it was ever rebuilt it was in the late sixties when this car was first restored, so it could use another rebuild.  And I'll move my electric pump switch into the car where I can get at it while driving.  And thanks for the Thermocure tip, I'm going to do that-  I also wonder if my engine is running too hot, I think it might be.  Sure hope the Thermocure helps.  Anyway, I will:  possibly move some lines, definitely rebuild the pump, clean out the cooling system, verify what temp my thermostat is.  I'll move lines only after trying all the other things first.  Thanks tons again for all the help.

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