I realize this is a place for CBR owners, but since we are all grown-up (see the philosophy thread in the Saloon), it seems possible that someone out there knows a thing or two about two stroke engines. I, on the other hand, don't. Never owned one, never worked on one.

My grandson bought a 1976 Honda CR250 Elsinore. My son, grandson and I are trying to make it run. It starts, idles, runs fine at part throttle, then gains rpm and runs away at approximately half throttle. Closing the throttle has no effect, although twisting the throttle does change rpm somewhat. The only way to shut it down is to use the kill switch or choke it.

The PO claims to have cleaned the carb, and it did appear quite clean on opening it up. There are no scuff marks on the needle or throttle slide, no binding apparent in the throttle cable. As best we can tell the throttle opens and closes properly. The needle valve holds pressure when blown through, the floats do not appear to be soaked. We can find no vacuum leaks around the carb.

Is there anywhere a two-stroke can pull fuel other than through the carb? (Remember now, there are NO stupid questions) Is it possible the engine is pulling fuel through the enriching circuit? The idle circuit? We're stumped.

Thanks everyone for any ideas.


DRam

 

Hey:

No not unless an aux circuit is ran to it

Sounds like dirt in the main jet or other air jet clean both out ( remove and view thru them they must be clear) and make sure the fuel to oil ratio is correct for it. A wrong mixture (to much oil will run poorly) (to lean will seize up) is not good. A new or clean spark plug is a common fix for when they are blubbering. Does this bike have the stock Kiehin carb on it or something else (let us know), Also they need a fresh fuel oil mix, old fuel won't cut it.
A CR250 is not a trail bike and is meant to be ran in its powerband at the top of its range 6000+ RPM or if is was a ported race bike it will load up fairly easy so keep this in mind (run poorly do to a fouling plug). Lastly make sure the air filter is not real dirty as that is a big problem on a dirt bike.

Once all thats done get back with us as it may be the CDI unit.

Dub

 

Thanks for the information. The carb is a Kehin. We'll check the items you mention and see what happens.

 

2 stroke or 4 stroke, it makes no difference when it comes to the basics. Is there a strong spark (at the right time), adequate fuel getting through and compression?

 

Great choice, when you get it running right that thing will kick your ***.

Those old girls do have a tendency to 4 stroke, so nice fresh clean fuel, good new spark plug, and just rev that baby until it recovers.

The other guys are probably right in that it may have some carb issues, give it a most thorough clean.

CDI? I seem to remember those old things still had points?

Quite happy to be corrected.

 

DRam,
A 2-stroke draws air in the carb and the air/fuel mixture is drawn into the lower crankcase as the piston is moving up (sucking the mixture). When the piston is traveling down it forces the mixture up through the ports and into the cylinder where it is ignited. Push/pull, push/pull, push/pull hence a 2-stroke. There is no oil in the journals to be thrown on the cylinder walls so oil is either per-mixed with the gas or it is injected into the lower crank case where it is blended with the gas like a blender. There are no valves needed in this configuration. There is a reed (like a reed in a wind instrument) that does not let the mixture go back into the carb.

The fuel is coming from the carb. If you say the slide is returning, it may be when not running but there should be a spring about the dia. of the carb, pushing the slide down. Un-screw the top of the carb and the spring should be pushing the carb top up.

If that's not the problem the enrichner may be messed up.

Maybe replace the carb?

LINK to carb on e-bay

 

Hey:

Yes on having a CDI with magneto

1976 Honda CR250 C.D. IGNITION UNIT Genuine OEM Dirt Bike Parts | MotoSport

Also make sure the air jet on the carb is not all the way out, (right side slot screw on carb) or in to far as this will cause problems as well.

Dub

 

Thanks everyone. I'm relaying this info to the son and grandson - they're some 170 miles from here - we'll see what they can do with it. I'll report back.

 

What's happening with the old CR?

Great bike, is it kicking some *** yet?

 

if you are POSITIVE the carb is fully closing the throttle and KEEPing it closed then the crank seals are probably leaking.

Spray round the ends of the crank see if that drops the rpm's the pick up side will be the easiest to access.