O/T two stroke motor help

Anonymous-0 · Jul 11, 2008

To save some gas I am putting my 68 Yamaha 250cc back on the road. It seams to have this problem. When I give it gas I can see the fuel mixture blow back through the carb. It runs crappy at low speeds, but cleans out at high speeds, and pulls good at the top end. Any thoughts? Stan

Ryan - WI · Jul 11, 2008

(quoted from post at 21:28:09 07/11/08) To save some gas I am putting my 68 Yamaha 250cc back on the road. It seams to have this problem. When I give it gas I can see the fuel mixture blow back through the carb. It runs crappy at low speeds, but cleans out at high speeds, and pulls good at the top end. Any thoughts? Stan

In a 2 stroke engine there is a valve between the carburetor intake boot and the crankcase. This valve is either a reed type of rotary type valve and is usually located inside the crankcase. How it operates is that as the piston goes up there is a partial vacuum created in the crankcase. This vacuum pulls the fuel mix into the crankcase. As the piston comes back down the fuel is supposed to be pressurized and then be pushed up through a transfer port into the cylinder. In your case the valve is not sealing correctly so some of the fuel is being pushed back out through the carburetor.

Clear as mud?

This most often happens with a reed valve type of engine. The reed (which is basically a thin piece of plastic like and overhead transparency) gets brittle over time and does not bend smoothly like it is supposed to. However, it can also happen with a rotary type valve it is just not nearly as common.

Mathias NY · Jul 11, 2008

Did you clean the carburator before you started the project? If so, it is possible something is out of adjustment on the reassemble.

Also was the engine run dry when it was stored? If not there could be some gunk left into it.

A little bit of spit back I think is normal and poor performance at low RPMs may also be normal. 2-strokes are usually designed to run wide open. I know my 2-stroke sleds of that era also spit some fuel at lower RPMs and tend to bog at low speeds.

Wayne in MN · Jul 11, 2008

If the engine is piston ported (the piston skirt is the "valve" that controls air/fuel flow) the piston skirt could be worn or damaged.
If the engine has a reed valve, the reed may need replacing or re-positioning.

JBMac · Jul 11, 2008

Left a reply earlier, but it disappeared! This engine is almost certainly pre-reed valve. Like others have said, performance will be iffy at low rpms. Carb needs a good cleaning-soaking. Maybe new jets. Make sure air filter is clean and good spark plug gapped correctly and the right heat range. Good luck!

Dave in Tx · Jul 11, 2008

On a 2 stroke high rpm engine the timing of the ports or rotary valve is far enough advanced that they will spit fuel at low rpm. as the r's increase they will start to clean up and at higher r's they will run really well. sounds like yours is running as it was designed to do. you can feel real easy when you get to the power band. a maverick ignition with timing in the mid to high 30's degrees of advance will also help a bunch. at 20,000 rpm's a spark plug firing at the instant the rings get above the exhaust port is pretty close.

jose bagge · Jul 12, 2008

As most have said, this is a characteristic of high RPM two strokes...if it is really loading up at low speeds, lower the needle in the carburator- should be a set screw in the throttle slide that will allow this. do it in small small small increments until lower RPM performance cleans up a bit.

O/T two stroke motor help

Anonymous-0

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Ryan - WI

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Mathias NY

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Wayne in MN

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JBMac

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Dave in Tx

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jose bagge

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