srinath wrote on 08/22/09 at 13:25:00:DrunkenDwarf wrote on 08/21/09 at 06:30:52:LANCER wrote on 08/20/09 at 17:16:10:srinath wrote on 08/20/09 at 07:43:40:Surging is not rich, surging is lean. Most rich issues relate to lack of power and soppy sound and soppy feel as well as black or white smoke (if you can see that from the exhaust) and smelling of gas when you try to accelerate.
Surging, misfiring etc on cruising and steady throttle is lean.
Trying to accelerate and you get a weak and lousy response that is rich.
Cool.
Srinath.
Your right dude, my brain must have been running in reverse.
So the pilot circuit is both lean and rich. Whoohoo!
That's why I haven't already moved down a pilot size.
The weather here hasn't been cooperating either.
-D. Dwarf
No we didn't say it was rich and lean ... I think Lancer agrees, but we are saying its lean.
However you really need to modify your method of test running it.
Precise throttle position and correct symptoms will get you far better diagnosis.
BTW this carb is so simple I will jet it in my sleep to run like a swiss watch. That would also be lancer's thought I am sure.
Cool.
Srinath.
If it's lean, why would I want to go down a jet size?
Maybe I should start over.
I bought the S40 with 55 pilot (brass cap still over screw), full spacer, 155 main, Dyna muffler that leaked, and noticeable surging between 1/8 and 1/4 throttle.
I've adjusted the idle mixture screw by following "Lancer's Straight & Simple Carb Tuning". Although I didn't turn up the idle speed when adjusting the screw. Perhaps that was a mistake.
I wanted to use a tach for this but the pickup on the Harbor Freight special won't fit under the tank with stock petcock and fuel/vacuum lines.
To fix the surging I removed the white spacer. I've since put 3 washers in (1.5mm vs 2.8mm) because of extremely bad gas mileage.
I've toyed with 145, 150, 152.5 and 155 main. I know 145 is too small, but I've been unsuccessful in testing the other three. I'll keep trying.
I have not checked the float level.
I have not replaced any of the air jets or the needle.
I have read Haynes Motorcycle Fuel Systems Techbook.
I do not have much experience with engines or carburetors.
What started this thread is the requirement of a cold engine for a richer mixture. A cold engine doesn't allow you to determine the correct jetting for a warm engine, but it will respond badly to circuits that tend toward lean.
The fact that the cold engine idled better without enrichment, but ran (1/8-1/4 throttle) better with enrichment says that the mixture delivered to the engine is richer at closed throttle than slightly open throttle. Whether either mixture is too lean or too rich for a warm engine is unknown. It may be perfectly normal.
I'm doing this because I'm genuinely interested in not only riding my motorcycle, but understanding how it works and what I can do to make it run as well as it can.
I'm looking at the Techbook and it shows one circuit for idle:
- pilot jet + pilot air jet
It shows three overlapping circuits between 1/8 and 1/4:
- pilot jet + pilot air jet
- air slide cutaway
- top part of needle + needle jet + air jet
We don't have an air slide cutaway. I've never seen a suggestion to replace the pilot air jet, needle, needle jet, or air jet. That leaves pilot jet, mixture screw, and needle height as the available adjustments for these throttle ranges.
My understanding is that the pilot jet feeds multiple outlet holes. One of which is metered by the adjustment screw. The rest of which are brought into play by opening the throttle slightly. Opening the throttle slightly also brings the needle and needle jet into play, but only slightly because the thickest part of the needle is still in the jet.
Theoretically:
If I wanted to make idle leaner and 1/8 to 1/4 throttle richer at the same time, how would I accomplish that?
And now for something completely different...
The Techbook indicates that the lift of the piston valve (slide) at WOT is proportional to the engine RPM.
This means that at WOT and low to mid RPMs that the needle and needle jet are the limiting factor.
Which brings me to back to question the Roll-Off testing procedure.
Is the intention of the Roll-Off test to experience the transition from the piston valve (slide) at it's fully lifted position to a slightly lower position?
I apologize for how long and random this has become. The theory posted by Diamond Jim, his "Progressive Jetting Guide", and reading the Techbook have got me thinking about how the engine works and what the difference is between running and running well.
diamond jim wrote on 06/04/09 at 10:39:39:Hypothesis- I think the common spacer mod- thinning the spacer or replacing the spacer with a couple of washers- still leaves a too lean midrange that we often compensate for by over-jetting the main and sometimes the pilot.
-D. Dwarf
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