1981 SR250T Idle problems

Bike: 1981 Yamaha power/engine still stock SR250T

Worst mechanic story ever. I have resorted to throwing parts at this bike. I disgust myself sometimes.

History: Bike ran great. This air-cooled engine was leaking oil at the head gasket. Simple repair. Pull off head, pull of sleeve, clean surfaces, put in new gaskets, line timing chain to original position, check valve clearance, drain old gas from Carb and fill tank with fresh.

Mistake 1: I drained the carb and noticed rust and debris in carb. Checked the tank and realized someone filled tank with my "cleaning" gas from a rusty metal can. Drain gas tank, cleaned fuel petcock. Disassembled carb, jets removed, soaked in Berryman Chem Dip. Sprayed with Carb cleaner, compressed air through all passages. Replaced Air Pilot and set at 2 turns out. Reassembled and started bike.

Primary bike problem: Choke out, runs great. Pushed in choke, ran rough, but ran at least.

Troubleshooting Problem: I have 2 variables now. Rebuilt Carb and replaced Head gasket. Both could cause the symptoms listed below. Note bike was running sort of with choke in until......
.......
Mistake 2: Installed side cover on bike while running, bumped flywheel, bike died. Checked flywheel, not a serious impact, but barely running and caused engine to stumble and died. The bump of the flywheel is likely independent of what is going on, but it is noted due to the chance for weak spark. You will see from the list of parts thrown at the bike.

Current Symptoms: Will only run with choke out or actively and heavily working the throttle.

Parts Thrown at Bike: Coil in case weak spark. Replacement flywheel in case it was damaged. Stator and Pickup in case of weak spark. Carb to engine Air Boot connector in case of vacuum leak. New fuel line and vacuum tubing in case of leak. Air cleaner and tubing in tact and not leaking. Replacement Carb, tested original factory and new jets in new housing in case there was a blockage I cannot find. Carb rebuilt dozen times, carb measurements made carefully, etc..

CONFUSION: If I spray starting fluid on the engine head/case, I have an idle change. I removed the Head, sent into machine shop and had it checked for warping. Said it was warped, which made me very happy. Get engine back together, same issue as well as starting fluid issue when sprayed on block near intake. I can't imagine that a crack developed in the intake during this entire process. I have a new air boot and gaskets as well.

So, what is next? Magnaflex head? Just get a new head? What am I possibly missing. Anyone have a known GOOD Carb for me to give a trial run to completely rule out the carb?

Thanks

If you are still getting fluctuations when you spray starter fluid on the carb manifold....then it's sucking air.


Take off the manifold and check for leaks - is it an O ring type manifold or gasket seal? You might need some 3Bond on it to make sure it's sealing.


Did you remove the butterfly seals before you dipped the carb?

Also have you check the fuel level in the carb?

Thanks.

Sucking air: I agree about the fluctuations. I have a brand new manifold/rubber boot, no help.

Seal: It uses a gasket and both surfaces were polished/cleaned with my 3M buffing pads on the die grinder. Nice and smooth. I have tried multiple gaskets. I do not see any clear lines, etc. where there could be leak. It sometimes backfires through the carb and I was hoping I could see carbon lines or similar. Nothing noticeable. I'll cut a new gasket and try it.

Butterfly seals: No, I did not remove these and I am unsure if they have seals. I'll explore this. I guess the rod/seals are close enough to the starting fluid spray, they could maybe suck a bit of vapor.

Fuel Level: Yes checked with clear tubing with bike on level surface and also watched while running. The correct fuel level is maintained by the float.