Set up Carb Lean or Rich?

R

Rdjefferies

New member
At a recent event had issues with Motor feeling 'dead' after a couple of laps. It happened repeatedly during each session during the weekend. Came off grid feeling good and would fall off within a lap or two. Float was set to 890, pin 2, 1 1/4+ turns air screw but when tuning carb from idle, rpms would increase when leaning out further which I tried running with same results. The very last session we ran the carb a little richer which turned out to be better but was still off the pace by .5 secs.

For Al;) Barometric pressure 1014.9, Rel Hum 70.3, Air density 1.134

My question is, does the Carb lean out or become rich as it heats up? Should it be set up Rich or lean and if so how much, 1/4 turn, 1/2 turn from setup? what carb setup process are other guys using?
 
First thing I did, should have mentioned that, leak down was good, 4%.
Spark plug showed as very lean, white tip, even though when setting up the carb before each session when opening the mixture screw would increase the RPM (as per the Briggs Carb setup guide). Big event this weekend and I don't want a repeat or to leave anything to chance. Please PM me if there is a trick I am missing...
 
Rdjefferies said:
First thing I did, should have mentioned that, leak down was good, 4%.
Spark plug showed as very lean, white tip, even though when setting up the carb before each session when opening the mixture screw would increase the RPM (as per the Briggs Carb setup guide). Big event this weekend and I don't want a repeat or to leave anything to chance. Please PM me if there is a trick I am missing...
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the mixture screw should be an idle adjustment with very little effect on mid range or full throttle.
a call too a lo206 builder may be in order.
seems like fuel delivery or clogged wrong jetting.
 
Have you checked for air leaks around the carb? Could be drawing in air and causing the lean condition
 
Agree with Flattop1, no way the carb adjustments will cure or cause this particular problem. Look at valve lash, fuel restriction, chassis set-up.
 
Rdjefferies said:
For Al;) Barometric pressure 1014.9, Rel Hum 70.3, Air density 1.134
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Thank you, but I've never seen barometric pressure and air density expressed in those numbers. SAE uses 29.250, at 77°, as a sea level standard barometric pressure and temperature. I don't know for sure but I believe that the Longacre air density gauge uses the number 100 to represent that barometric pressure and temperature. Relative humidity has little to do with performance. The torque correction factor, on a dyno, for relative humidity, is a very very small number.

You can't tune an engine in the pits. Not recommended. Engines will lean out when they get hotter and the load increases. They must be tuned for racing conditions. This idea that an engine has to idle in the pits, and that's a good setting for racing, is all wrong. You're going to find, once you get the carburetor set for racing conditions, the engine is not going to idle in the pits very well, if at all. It just has to do with load and temperature levels. I always smile when I see people revving their engines in the pits. The engines not up to racing temperatures and there's no load. Completely different from the conditions on the track.

If you don't have a gauge, I highly recommend getting one.
 
Al, I know we've had this discussion before but, relative humidity is very important in tuning. In the pro ranks it is usually expressed in grains of water and you wont find many professional race teams that don't watch it like a hawk on race day.Fire away.
 
Al, I use an app called 'AutoDens' which pulls info from the closest airport. I usually check it first thing in the morning and before the prefinal to see if there has been a major change which might indicate a need to change the carb tuning.

If the carb does lean out as it heats up, how much richer should it be set up from Idle? 1/4 turn, 1/2 turn, or more?

Valve lash was checked and was Ok, depending on who you talk to, between zero and .004.
Track was grippy and chassis setup was probably part of the problem but it felt more like lack of power was causing the issue and not visa versa which is usually the case, couldn't even stick with a mid pack in the draft when normally I pull up on the draft when running top 5.

Thanks for all the input everyone:)
 
Rdjefferies said:
Track was grippy and chassis setup was probably part of the problem but it felt more like lack of power was causing the issue and not visa versa which is usually the case, couldn't even stick with a mid pack in the draft when normally I pull up on the draft when running top 5.

Thanks for all the input everyone:)
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The carb doesn't have enough adjustment in it to get that far off. I think you have more than a "carb tuning" problem. I am not an expert, but if leakdown was good and valve lash is good, it might not be engine. Does a buddy have a 206 that you can swap and see if your time immediately picks back up? If none of this helps, it sounds like the kart got super tight. Was there anything different about this particular race than others? Hotter day? Different rubber in the track? More karts? Try something different in set-up?
 
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