JMStar 250 won't go over 45mph By acvw74 (Read 1,600 times)

[acvw74]

My newest acquisition a 07 JMStar I was getting ready to switch over to waterless coolant...Had issues the day I bought it...it burned the CDI up on the way home, got a new one off EBay and fired right up.

Idles great and if I go easy on the throttle it will run smooth up to 45...then it feels like someone is pulling hard on the back of the bike. Does the same thing at about 25 if I start at wide open throttle.

Things done so far...Replaced all the fuel lines and the fuel filter.  Cleaned the carb (looked really good for a 10yr old bike)...I think it has been replaced because it has a Keihin carb on it....Traced all vacuum lines - cut the ends off so the fit tight. I also took the screen out just in front of the carb.

What is the next step?  Do I have a bad or restricted CDI? It sounds quiet...but, would valve cause this issue?

[cyborg]

I'd be giving it a real long look,,,a "pdi",,,check air filter ,,plug ,,gap it " also check fuel tank cap for venting,,,also I'd be looking at the variator it may have "bad" rollers

[acvw74]

Thanks,
Air filter is new, new NGK iridium plug...the fuel tank is double vented...it has one going to the carbon filter and one from the previous owner(drilled a hole in the cap).
The engine does the same sputtering or cutout while on the stand...idea?

[cyborg]

Check the intake manifold for snugness to carb and head also check for cracks,,,very common to be cracked ,,,also give all the vacuum hoses the long look for cracks,,,it sounds like a vacuum leak to me

[acvw74]

Thanks Cyborg,
I have looked at all the vacuum lines and the intake manifold - I didn't see any issues. The bike runs smooth up to 40-45 then starts cutting out/surging/losing power.

I have a new intake manifold on order, a set of bigger carb jets (it feels like it is running out of fuel) and will replace all the vacuum lines.

Any other ideas? FYI I think the previous owner had the same issue there is a lot of evidence of replacement parts.

[blu]

Food for thought. I had (have) something in my tank that blocked off the fuel from leaving the tank. In my case, it shut it right down. I cleared it by blowing into the fuel line going to the tank. I can see a partial blockage doing what you describe. Allowing fuel flow to a certain point.  the thing is still in there, when I get to about half a tank, it will plug things up.  If I just stop and rock the bike it will fall out, and I'm good.  Some day before I die I need to pull the tank and clean it out.

[elcamino121]

Can you grab it with one of those 3 or 4 fingers reachers?

If its soft try a siphon.

[acvw74]

I don't think it is something in the tank...it only has issues above half throttle. Anything below half runs smooth...over half and it wants to throw me over the handlebars.

Read somewhere about the carb return spring being too stiff...cutting a few loops?

[cyborg]

Check the carb diaphragm for holes or tears

[acvw74]

Diaphragm is good  I even adjusted the needled (up? - richer)...no effect

[cyborg]

I still think you have an airleak

[acvw74]

I haven't totally ruled that out yet...Just finished replacing ALL the vacuum lines. All were without leaks...one was really soft (going to vapor canister from the fuel tank) another going to the vacuum petcock was partially (almost completely) block from being zip tied to the frame.

Cooling off a bit...going to give it a ride before changing anything else...only one change at a time.

[cyborg]

There's your problem,,, high rpm low vacuum situation ,, auto petcock closeing partially from low vac ,, that's it you found it!!!!!,, had a very similar situation on a Vespa et4,,, it ran out of gas at high throttle,,, i put anMikuni vm series carb and I bored out all inlets and outlets,,, problem solved,,, never had another prob with it again

[acvw74]

I thought that was it!

The problem persist...Thinking the petcock was the issue...I put in a brass hose connector...at 45 stops/sputters and won't accelerate.

[rks]

Classic main jet issue......runs great at low throttle settings.....bogs, wants to die, etc, at or above half throttle.