Tweaking a Poulan saw

[Muddy]

I posted this in "Chainsaw", but was directed here........


Hi, I have a 42cc (or thereabouts) made by Poualn in 1990.
I am going to replace the crank bearings, seals and put shims on the crank to eliminate the end play.

I am getting over 17,000 RPM out of this motor because of an expansion chamber a mate of mine made.
Am I in the right forum here? Or is there an engine builder's thread this post is better suited to?

My question(s):

1. Will the shims help or hinder the crank?
I would have thought this would be beneficial - no end play, and con rod will stay centred.

2. I am replacing the crank seals. My bearings are rubber sealed, and I think I will be able to leave/keep the shields on the seal side of the crank bearings. The races will be washed out (grease removed) , and the fuel/oil can still enter the bearings from the crank side. As the outer bearing face does not touch the engine seal, and the transfer ports directly above the bearing's cases do not pulse fuel / oil to the seal side of the bearing, will leaving the outer bearing seal installed help to increase my crankcase pressure?

3. I have a Mk IV inlet manifold made, and there are NO air leaks between carb and head. (I moved the carb away from the motor)
I have had to increase the size of the pulse port that operates the carb metering valve. I have also put a kit through the carb (Walbro WT 202) and the diaphragm lifts the metering needle at the slightest whiff of a crank pulse. At high RPM, the motor just fades away. I feel the carb is unable to deliver enough fuel at high demand condition. Ignition, and kill switch have all been eliminated, fuel is top notch, plug is new, etc.

Is there a better carb I might use? I was thinking of a carb from a 80cc saw, or something.
It's not much use plumbing a pressure line from the expansion chamber to the vent on the tank (pressurising the fuel system), as the valve flaps in the carb would just resist the pressure.

The motor starts every time, idles fine, responds to throttle "stabs" but when asked for the goods, fades away still.

Sorry for the long post, but there may be arguments for leaving bearings sealed/ or not. Is there a NO-NO I am comitting here?

Motor is installed in a radio controlled dragster

http://www.youtube.com/watch?v=6LWX9-D8Obc

 

[neutral4x4]

First off I'm guessing the saw was a wild thingy? Correct me if I'm wrong.

1) I don't know where you want to put shims. Are you putting them on the wrist pin or...? I've seen shims on the wrist pin, but those were only put there to keep the needle bearing from "walking" out of the con rod on a big bore kit.

2) Sealed bearings aren't usually rated for that high of rpm loads. If you can find open race bearings, you'll be better off. Better yet, at that rpm a set of ceramic bearings would be ideal. If you insist on running sealed bearings, I would remove both seals so the seal doesn't somehow come loose and grenade your engine.

3) I.m not sure, but at a certain point, reed valves will flutter if you have them. That'll significantly hurt your rpms. On a piston port (which is what I assume you have) you'll have lots of blow back, which also limits fuel delivery. I think you're at that point, so I really can't help you there. The only advice I can tell you is look into a rotary valve intake.

If you want a new carb, look into a dellorto. A pocket bike carb might be better for your application. A diaphragm carb has two speeds. Idle and high rpm. If you want more mid range, look into a fully tunable carb. If you aren't tipping the dragster around, you don't need a diaphragm carb. The only problem that I see with that approach is needing a gravity feed fuel line, unless you can find a mini fuel pump.