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Tuning Question - smoke from exhaust   General Discussion

Started Nov-30 by Pharmer Keith (palsieddolt); 337 views.

Hoping to get some insight. I just installed my POD kit from MetricMagic.

- Took off the o-ring on main jet holder as indicated in the KB

- 2 shims on needles

- Tim's PMS screws - currently set at about 1 turn

- 17.5 pilot, 150 mains

- idle set at ~1000 and carbs sync'd

- valves set about 200 miles ago

-New plugs put in a gap set

I've noticed I get a little billow of smoke from the exhaust when revving the engine from idle. It comes out right as the rpms fall back to idle.

Bike seems to run great. Pulls strong and no popping on deceleration. It does seem to pop/backfire (from what sounds like the carb) occasionally during low to mid steady throttle while cruising. This really only seems to happen when it's not quite at temp. I did also notice that last night, after starting from cold, it seemed to bog down and nearly die when I tried to blip the throttle a little to get the rpms up (it was a little cooler than normal but still only around 70 degrees). After warming a little, this issue when away. I turned the PMS screws in a little to their current position this morning and didn't seem to have the same issue this morning but the smoke seemed more significant  after the bike had got to temp.


From: PnutsC


Welcome to the world of engine modification. The black smoke (you didn't say black but I'll assume it is) is a temporary rich condition. Modern carbs (modern carbs??) have sophisticated circuitry and on cars at the end of the pathetic things, even computer controls to try and control mixture under all conditions. The original carb setup on these uses such in a rudimentary way to shut off fuel under decel conditions. You  have altered or defeated the decel shutoff so a little raw fuel makes it past the combustion process in to the exhaust. As long as it's a little no big deal other than a couple bucks worth of gas. The other two symptoms you describe sound like lean conditions which is to be expected with the extra, colder air entering the engine. When we modified engines back in the day for performance we always started like this, do the simpler cheaper things first then work on from there. (bang for buck) Usually the first thing we did was open up the intake and exhaust. The results were usually similar but still varied. Trial and error. These guys have done so much of this they pretty well have it to a science. There is still fine tuning to be done for different machines/riders/climates etc. Another thing is when you modify you give up some of the factory design that is put together as a compromise package to get the vehicle to operate under all conditions as well as possible while keeping big brother happy and the compromise falling mostly on the consumer. When we modify the air filter on a car we not only allow more air in we also defeat the factory design of delivering the air in a heated condition when the engine is cold along with differing timing advance under differing conditions. We were/are willing to live with some cold naturedness to achieve the performance increase we are seeking. We fine tune the best we can and live with the rest. Fuel injection and computer controls have changed all that of course but alas these bikes predate such advancements. I believe in your case I'd start with opening the pilots up some. Try maybe a half round and see if it helps the cold drivability issues and work from there. Some of the more seasoned guys here will have better advice as to what to try for your particular issues. I'm just attempting to lay some  groundwork. 

In reply toRe: msg 2

Awesome. Thank you for your reply. Id still appreciate any other feedback but this is reassuring.

I've spent a bit of time going through this  yesterday. The smoke was concerning to me because... well smoke=bad!... right?! :D

But what you say does make sense. I know the carb will never be perfect but the combination of Lean and Rich symptoms had me confused. To confirm, you mean the Pilot Mix Screw and not the Pilot jet, correct?. I've played with the PMS screw a little more and seem to have removed or at least minimized the lean symptoms but it was a little warmer yesterday. Ill have to see again when I ride again in the cooler air.