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TR2/3/3A TR3a stalls on hard acceleration

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J

Jim Lee

Jedi Trainee
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My TR3a stalls on hard acceleration. At operating temperature I can stall out by standing at the engine and putting the accelerator linkage down quickly.
Commonly known as 'punching it' but not even under load. I have been puzzling over this for quite sometime and have checked timing, sprayed
oil on the carb linkage areas that are known to leak and noticed no difference, meaning to me no air leak there. The next thing I am going to
do is check the butterflys to make sure that they are absolutely clean but I am pretty sure I have done this since it started acting this way.
If I slowly accelerate, while in neutral, can get get it up to high rpm. When it stalls, or almost stalls, it feels like it is getting overwhelmed by either air or
fuel. I am guessing air but I am having a hard time thinking of where an air leak could be on such a straight forward setup.

Any advice on what to look at next?

Thanks very much,
Jim Lee
1959 tr3a
 
Jim - first thing I'd check would be fuel flow. Sounds like the wide open throttle lets in the air, but sufficient fuel isn't hitting the cylinders.

Just my two cents.
Tom
 
First thing I would check is the dashpots. You've described exactly what happens when they are empty (or filled with much too light of an oil).

Remove the air filters and lift up on the pistons with a finger or screwdriver. You should feel a very definite resistance to movement upwards, but the piston should rise smoothly under steady pressure all the way to the top. Any binding is a problem to be addressed. Then release the piston, and listen for a definite click as it lands. If no click, then likely the jet is not centered properly.

The function of the dashpots is similar to the accelerator pump on more conventional carbs. When you first punch the throttle, it takes some time for the fuel to respond to the increased air movement and so the mixture would go very lean. But with the dashpot holding the piston down, the velocity through the venturi goes way up and a much stronger vacuum signal is applied to the jet, pulling more fuel through. The mixture actually goes rich (for better acceleration) instead of lean. The effect can be controlled to some extent by using heavier or lighter oil in the dashpots. On my old TR3A, 20 weight motor oil seemed to be about optimum; but 20W50 will also work well enough.
 
Hi Jim,

I agree with Tom & Randall. Sounds like lack of fuel.

What have you found out so far?

Regards, Russ
 
Ya know Randall, even when you're trying, you're helping me! I've been using MMO in my dash pots for several years at the recommendation of either this or the other forum I frequent. I've been fighting through an issue where occasionally my car would start running rough but after timing and checking the mixture countless time, I could never fix the problem.
Compression in in my cylinders, although within about 7 lbs, is low so I assumed I just needed a rebuild.

After driving around 250 miles last weekend, the last 10 miles or so she started running poorly again. I didn't put 2 & 2 together until reading your post - I ran the dash pots out of oil. I filled them up with a heavier weight last night and took her for a quick ride - instantly noticing a difference. I took her out for a 25 mile jaunt tonight and she ran perfect.

Once again, thank you for all you do on this forum!
 
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