VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Surging

Is it normal for my 29 to surge some especially at lower RPMs? There's not much to adjust on the carburetor, however, I did lean out the idle screw and that seemed to help some. I thought maybe my foot was not steady on the accelerator pedal, but it runs the same with the throttle knob. I can try to post a video if that would help.

Hello Bare Feet,
If leaning has an effect, suspect that the needle and seat may not be completely sealing the float bowl where an over flooding may cause a surge. After the overage of fuel is consumed and the engine returns to normal, the cycle repeats itself. If that is not the case, then I'm OVERFULL of it. Let us know what you discover.

generally surging is cause by fuel/air mixture lean = not enough fuel

does it surge under acceleration/throttle ?

friends 31 had this issue

pull carb, clean out bowl, check float valve/seat and then i would check all jets make sure they are clean and clear. blast each port with some brake clean and tube (just be cautious as to where it will exit )
can also run small gauge wire in some of hte jets to clean them, but the brake clean worked for us.
install it and see how it does.

My 24 and 31 will do that when cold but smooth out when up to operating temperature, Does yours happen hot or cold?

Dave

Dave, I think so, but I only have 110 miles on it since rebuild 6 years ago. I need to do some more test runs. The carb was rebuilt but sat with fuel in it for about three years off and on. It seems that it did surge when I started testing the engine three years ago. I will get back on it in a week or so.

Hello Bare Feet and Others,
From what I have been reading about engine surge issues, it appears that an imbalance of either air or fuel can cause an engine to surge at idle with no input of the throttle. As Bears post suggests that it's usually too much air causing a lean ratio, I guess that a hotter burn will produce a surge/RPM increase. I'm visualizing a billows blowing air onto a fire and creating a flame increase. Inside a cylinder would increase pressure and a surge. OK, with air being less adjustable within a carburetor than fuel is, a restriction in a carburetors fuel passages and/or calibrated openings appears to be most likely. Less fuel more air.

Remember that the original carburetor on the 1929 has an inverse idle circuit. The "mixture" screw meters AIR, not fuel. In to richen, out to lean.

Lightly seated would be as rich as possible.

Jon

When I first started trial runs after the rebuild, I noticed the car spitting carbon/water when cold so I mistakenly turned the idle screw the wrong way, even closing it completely and found that it was blowing out carbon even when hot so I called the person who rebuilt the carb and he explained that I was going the wrong way. I have since been adjusting the screw to add more air and there is much less carbon coming from the exhaust, but still some. I am wondering if surging is only caused by fuel or are there other causes (distributor automatic spark advance or timing)? Do I need to check the fuel valve on the carb bowl? I have an inline fuel filter, but dirt is a possibility.

you could do a reboot...

check all valve settings, then timing. pull carb, clean out bowl, check float valve/seat and then i would check all jets make sure they are clean and clear. blast each port with some brake clean and tube (just be cautious as to where it will exit wink ) can also run small gauge wire in some of the jets to clean them, but the brake clean worked for us.
install it and see how it does.

could swap out carbs, but like most we only have the one on the vehicle

like chasing your tail around in a circle

that way you KNOW everything is where & how it is supposed to be. the fact that it runs means things are in order, surging is mainly a fuel/air mixture issue.

Thanks, BearsFan3
I think the carb cleaning is the next step. I wish I had an extra to swap over, however, they are expensive.

If the mixture is too rich at idle, carbon tends to build up in the muffler/tailpipe, even when the engine is hot.

When the engine is run at a higher RPM, additional exhaust velocity will cause some of the built-up carbon to blow out the tailpipe, even if the engine is now running at a lean condition.

Generally, surging is caused by a lean condition, rather than a rich condition.

Are you getting black smoke from the tailpipe, or just carbon particles?

Perhaps you have gone too far leaning the mixture because of the presence of the carbon particles.

Jon.