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Carburetion 101

Stan GalatStan Galat
OK, carb tuners- here goes. I've got some basic questions concerning carburetion jetting regardless of carb type. I recently replaced the exhaust on my car with one that was more free flowing (to the point of being a bit oversized for the present engine)- and two negative things happened (in addition to reducing my oil temperature significantly).

The first is that I developed a flat spot on on acceleration. If I "feather" the gas, the rough spot smooths out, but on wide open accelleration, I have a "stumble"- not really at idle, but just off. I am leaning towards the accelerator pump, since the car runs OK under all other situations (except one, which I'll get to), but what other things might I look for? I realize this is a difficult question, and I'll be in the market for a AF meter, but thought I'd ask...

Second is something I have observed in other cars I've owned. When I move to a more free flowing exhaust with a short (or no) tailpipe, I get some back-fire through the exhaust on downshifts when I am not applying any throttle. For example, when entering a curve, and using a downshift for braking. What exactly is happening? How can this be minimized. I suppose I could add some tail-pipes, but this would only work to a certain extent. Any input would be appreciated.

"BlazeCut®(TM) woulda' saved it!!"

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Stan GalatStan Galat
OK, carb tuners- here goes. I've got some basic questions concerning carburetion jetting regardless of carb type. I recently replaced the exhaust on my car with one that was more free flowing (to the point of being a bit oversized for the present engine)- and two negative things happened (in addition to reducing my oil temperature significantly).

The first is that I developed a flat spot on on acceleration. If I "feather" the gas, the rough spot smooths out, but on wide open accelleration, I have a "stumble"- not really at idle, but just off. I am leaning towards the accelerator pump, since the car runs OK under all other situations (except one, which I'll get to), but what other things might I look for? I realize this is a difficult question, and I'll be in the market for a AF meter, but thought I'd ask...

Second is something I have observed in other cars I've owned. When I move to a more free flowing exhaust with a short (or no) tailpipe, I get some back-fire through the exhaust on downshifts when I am not applying any throttle. For example, when entering a curve, and using a downshift for braking. What exactly is happening? How can this be minimized. I suppose I could add some tail-pipes, but this would only work to a certain extent. Any input would be appreciated.
FM Former Member
Hi Stan:
While i am not an expert on the flat four. i have done alot of mods to the many motor cycle's i have owned. And there is one common theme to any engine. If you increase the air flow into/through/out of the engine you must adjust the fuel air ratio to match it.

sounds like you need to adjust your idle mixture. adjust your accel pump to make sure that any movement will squirt fuel. and maybe change the main jet size. bottom line is that a free flow header and muffler can cause a lean condition, unless the carbs where very rich to start with. you may be close on jetting, as an overly lean condition would rise your oil and head temp. your post said your oil temp went down.

if you have a kadron/solex. go to www.lowbugget.com lots of tuning tips there.
Gordon NicholsGordon NicholsMassachusetts 1993 CMC Supporting Member
Just curious, Stan (I'm working through jetting issues myself and will let others more knowlegeble offer you help), but what are you running for carbs?

The Dellorto tuning book tells me that if you're bogging off-idle, then it's lean in the idle circuit and you can probably adjust the accelerator pump for a little more squirt and that might fix it. If not, or it improves but not enough, then one size larger idle jet should do it.

Also, according to both Jake Raby and Gene Berg, if you run a more free-flowing exhaust, you're getting the gasses out of the heads more quickly and that reduces the overall head temp (and the oil temp along with it).

As for the backfiring on decelleration - I can't help (I get it a bit, too, with a Berg 1 5/8" exhaust).

gn
Stan GalatStan Galat
Its got ICTs on a 1776. The exhaust is 1-5/8"- too big, I know, but I'd like a stroker 90.5 this winter, and I didn't want to buy two exhausts. I'm leaning towards adjusting for more squirt, and was thinking of possibly going up a size on the idle jets, but since the problem is only in transition, I'm going to try adjusting the pump first, unless someone has a better idea.

The back-fire is a bigger issue with me. I can get rid of a lot of it with driving technique, but there are times when nothing works as well as engine braking, and the car sounds like its got a brick of firecrackers in the muffler, just waiting to go off. Should I just add a tail-pipe?

Paging Mr Brown, Mr George Brown...
FM Former Member
stan, your "stumble" is due to you're engine is notgetting enough
fuel during initial throttle up. most better header mfg's
(especially bikes) give suggested jet changes to a stock set up
to correct this situation. i would try the accel. pump first,
your idle would help your trouble but would mean you were idling
rich. george would quote these jet #'s off the tip of tongue.
the a/f indicator is the way to go. good luck.
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