Lean is mean?

Daegan

Member
Not sure if I understand this completely. Can you guys elaborate on this for me?
 

69ST

Well-Known Member
Lean can result in highest peak power...sometimes. It's also risky, as it spikes peak combustion temperatures. With these little motors, even the biggest tunes, there's nowhere near enough potential power gain to be worth the risk of "twisting the dragon's tail", unless you're dyno racing down to the last .01hp.
 

Daegan

Member
How would one know? Vapor lock, over heat, taking temp? Seems like i have tons of power and over heat relatively easy.

What is the best way to adjust to fix that. Should be tuned pretty close, no?
 

lukelaw1

Active Member
Let start off a little more basic. What setup do you have stock ct70, big bore kit, stroker??

The best way to tell will be a plug reading.
 

69ST

Well-Known Member
With a bone-stock engine & carb, lean will oftentimes result in bogging...but not always.

The safest way to tune & jet a carb, other than with an O2 meter is to find the rich limits at idle & WOT, then back-off slightly in the lean direction until you just have a clean idle, normal throttle response and the engine will rev-out cleanly at WOT, not go into rich misfire.

But, as lukelaw1 correctly mentioned, you have to have a starting point to work from. Otherwise, it's blind guesswork. FYI, a dipstick oil thermometer is part of that basic starting point. Oil temperature can be a valuable diagnostic tool.
 

69ST

Well-Known Member
You should be fairly close with the stock carb jetting. E10 pump fuel does make a difference but, it's pretty difficult to go too far afield anyway, as long as the carb is clean and in good working order. The stock setup is quite forgiving. Typically you'll have three jet needle height settings that deliver okay throttle response & performance. It's the Goldilocks thing...one will be a bit too rich, one a tad on the lean side and one will truly be best. The changes, between two adjacent clip positions will be quite subtle, to the non-expert tuner. But, with a little practice and some experimentation you'll catch on.

At most, you might want to upsize the main jet, going one step larger. That said, I'd start by checking plug coloration; that should give you a fair idea as to where you are at present. Next step, successive test rides with the jet needle C-clip in the lowest groove (richest setting) and topmost groove (leanest setting) at least one of those settings should deliver a really bad result...either too rich or too lean. If, as you suspect, it'll only go too lean, then it's time to order up the next larger size main jet and retest.

Going by your "tons of power" comment, it seems that you're not running seriously lean. Overly rich mixtures deliver WOT misfire, black exhaust smoke and, if the pilot (idle) circuit is rich, too, overly-easy cold starting, sans choke. Lean mixtures usually mean difficulty with cold starts, having to keep the choke applied for long time until the engine & intake manifold warm up and stumbling/bogging with enthusiastic throttle application.

I'll check to see if I have any of those dipstick thermometers left.
 
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