Motorcycle Repair: Desperate need for advice on an air cooled 2-stroke engine, mikuni carburettor, attex atv

Question
With the bench test, I only have this info second hand. As far as I know, the mechanic ran the engine out of the vehicle on a test bench...

At the moment, I cannot get an accurate compression test using a gauge. The battery in it is dead, so we need to start using the re-coil starter. Going by the re-coil, the engine still has good compression!

Do you have any possibilities on what the problem could be?

Thanks!

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Hi Again Colin,

Thanks for getting back to me, I appreciate your time and advice.

Unfortunately, I will not be able to do a compression test for a few weeks as the vehicle is stored at my folks holiday house.

The engine is air cooled, 2 cylinders, single (mikuni) carburettor. I am guessing the engine was manufactured in 1982-1984? Engine does not have all that many hours on it. New points were installed, engine was timed by a mechanic. New coils were fitted. new condensers were fitted and new plug leads were fitted.

I would LOVE to fit CDI, but so far have not been able to find a conversion kit to do so.

Do you know what could be wrong?

Thanks,
Steve



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Hi Colin,

We have an Attex ATV with the G50B Chapparral engine (484cc - Fuji Xenoah Corporation. The engine is a snowmobile type. For some time now, it has been rather un-reliable. The engine has been bench tested, new points, serviced and timed by a 2-stroke mechanic. Just the other day I had an issue with it flooding, not wanting to start. Took carby off and checked it over and put it back (didn't really do anything, so maybe the floats or needle was stuck?). It then started, but idle was really low and just wanted to die if I took my hand off the throttle. I ended up getting it to idle a bit better and took it for a run down the road. Poping was coming out of the carb and it was missing as well as laboring under load. I gave up and put it away... I had someone think it is a connection near the coil... I guess this makes sense when spark on one of the cylinders drops out at times... We plan to check connections and solder up any loose connections.

To give a bit more history to the reliability problems... In the past, we have had it act up after about 1/2 an hour of driving. When I say act up, it starts to "pop" (through the carburetor I believe) and the engine starts to lack power and when you give it more throttle, it can "pop" even more. I am pretty sure it is a "pop" through the carb as I also had it backfire through the exhaust only once and that was like a gun shot with a "bang". Once, the engine quit on me when I was out driving and I was not able to get it re-started for the next 15-20 minutes. It nearly seemed like it needed to cool down? We thought the electrics were getting too hot, so we ended up relocating the coils and condensers from the original place and placed them away from the engine. Our reason for this was we believed these items were getting way too hot in that little compartment. I haven't had the chance to really take it for a good long run to see if this has worked or not. We took it along a beach for a few miles where the sand was really soft and the engine really worked hard. On that occasion, it all worked fine.

I am desperate to get the vehicle running in a reliable state and thought I would drop you a line to get your opinion. We cover long distances and reliability is very important as you can imagine (ha, ha - nervous laugh)

Thanks,
Steve
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Ok, first of all let's do a compression test.  How many cylinders and is it just a single carb?  Let me know these answers as I believe u may have a couple issues on your hands and a compression test will eliminate a head problem.  Secondly, please tell me what u mean by "bench tested".
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Without a compression test all I can really say is that the mechanic who timed it did it wrong. U never told me about the bench test.

Answer
Ok, without a guage for compression there's no way of knowing if it's good or not.  The only things that can cause your problem is bad valves, secondary ignition (points, coils, etc), or that the timing is off by a lot.  It may have even jumped a tooth on the cam.  Get the timing checked by SOMEONE else.  I'm sorry I can't b of more help, but without the bike I'm stuck.  If it's getting gas and has compression then it must be igniton/igniton timing.