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Engine bucking

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J

JBlubaugh

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Hi All -

Finally got my BT7 back from the Paint and Body shop after a years worth of work. Have been driving it around locally and noticed a problem that I had when I sent it in for paint. Maybe someone has an idea. I did a search but didn't come up with anything so I'll ask here.

The car runs great when it is first started, can run all day with no problems at all. The problem starts when I stop somewhere for anywhere between 10-45 minutes. After stopping the car starts right back up and runs well with the exception that it will stutter occasionally, especially when under load. It will act like this for about 10 mins, then it runs great with no problem.

The only thing I can think of is that the gas in bowls / lines is becoming superheated while the engine has stopped and something in the compression/fire cycle is suffering as a result. Once the hot gas is consumed and replaced with cooler gas from the tank, it runs fine. That doesn't make sense to me though.

Any thoughts?

Some specifics:

Rebuilt engine <5000 miles
Fresh Plugs < 500 miles
Mixture seems good with the plugs a very light grey.
New Coil ( Had the problem with both old coil and new coil)
Fuel Filter in place between tank and pump. ( I'm replacing the tank this weekend so I'll check that while in there)

Thanks for the help,
Jim
 
It sounds fuel related. Id try two investigations.

1) Drive it again, and drive it home. After say 20 minutes remove the float bowl lids and look to see what you have in the way of fuel. The levels should be the same. Check the floats to see if they have fuel in them, sometimes they sink causing running issues, and then check the needle valve seats.

2) Drive it again, and drive it home. After say 20 minutes turn the ignition on, but don't start the car, with the electric pump you should hear a couple of clicks, and then nothing because the float bowls should be full. If it keeps on clicking then the float bowls are 1) empty or 2) the needle valve is not sealing which causes flooding on may be only one carb. If that's the case, the engine will run rough until you have used the extra fuel then settle out.

Try that first.

Johnb
 
Have same issue with my BJ8 on warm days (I know you get them in S. Diego ;). I attribute it to what's sometimes called 'vapor lock,' and possibly ethanol. Heat in the engine bay--esp. from the exhaust manifold--causes fuel in the bowls and or lines to vaporize and form voids. Think best solution might be to ceramic coat the exhaust manifold; but until I get around to doing that I just live with it.
 
have this problem every summer in las vegas. the ethanol boils out of the carb bowls and causes the engine to buck and miss till fresh gas is pumped from tank. I never pull out into traffic and accelerate until I am sure I will not stall out. have removed float bowl covers after engine shutoff and seen the gas bubbling in the bowls as the ethanol boils off (at 167 degrees I believe.) use caution. not sure of fix other than using aviation gas.
 
I have this same problem, and I agree that it is probably because the gas in the bowls or line is vaporizing. I replaced my fan with a 5-bladed fan from Denis Welch and that helped quite a bit. It moves a lot more air through the engine compartment at idle.
 
check your coil...a flaky coil when hot will sometimes cause the symptoms you describe...easy fix if it is the problem.
 
I'd check your fuel pump to see if it's pumping at the right pressure and volume plus the coil.
 
I used the same term "Engine bucking" as you to describe my TR4A's recent problem. I could try and relate what caused and fixed the problem but I am sure I would get it arsh backwards so suffice to say it was a spark plug wire for #4 that while it looked fine on the outside had a break on the inside. This intermittent problem caused the distributor cap to have a problem (electricity takes the path of least resistance I was told). The shop owner that takes care of my cars is wonderful about explaining the problems and fixes to me (just hope I never have a pop quiz) and I try to retain some of the info.
Good luck with the fix.

Cheers,
Irene
 
Thanks all - I'm going with the gas vaporizing as that is closest thing that I can think of. I've checked the coil ( Replaced ), Fuel Filters (Replaced), fuel pump is working well. Just something I will have to deal with on hot days.
 
I have had the same problem with the cause being bad plug wires. Also check the rotor and cap that there is no arching inside. Other possiblity is bad ground on the condenser.
 
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