New MC-70-150 Dies at High Speed By kountryguy (Read 2,937 times)

[kountryguy]

I just got a MS-70-150 about a week ago. I have been following the break in suggestions.for the most part. Today, with about 150 km on it, I stepped it up to about 50 mph and after about 20-30 seconds it died. It coasted to a stop. I loosened the fuel cap, thinking it may be air locked. I let it sit for a minute or two and it started right up and got me home at lower speeds. Checked things to make sure nothing obviously wrong. Tried the same test one more time, same results. Am I hurrying the break in or does someone know what I should check? Thanks for any help.

[sprocket]

Loosen the tabs on the gas cap a slight amount to let a little air in the tank. Failing that, drill a 1/16" or smaller whole right through gas cap...

air lock are common.. if you have a charcoal filter rebreather on the system, check the connection to the tank, they are sometimes not open...or painted shut...

Be certain you don't over fill the tank...

[kountryguy]

That was my first thought. I took the cap off and removed the gasket. Then I left the cap loose with the same results. Then I disconnected the fuel line from the vacuum valve. The flow was great. Not sure how to check the operation of the valve. That's where I'm at. Thanks

[Bashan]

Yes, I'd check the flow to the carb and get that off of the table. The float bowl may be running dry causing it to stall.

Scooter Doc gas flow test

[kountryguy]

I did the gas flow test, and it seems fine. I wonder what I should check next? Should I just continue the breakin and hope it fixes itself? I have about 120Km on it now. Thanks

[ce]

Check the connections to the coil, and the wire to the plug. These usually loosen up with the first vibrations of break in and cause erratic ignition. Squeeze the connectors tight.

[kountryguy]

I will do that, but this is not erratic. It happens every time I hold the throttle fairly wide open for more than about 20-30 seconds. The rpm have to be above 5000 for this to happen. Slower it runs flawlessly.

[ce]

If your tank is higher than your carb, you have a petcock, and they're reliable unless there's a vacuum leak, so fuel delivery should be fine. Inside the carb the float valve is open as long as the bowl is draining, ie. WOT delivery, and closes at idle to keep from overflowing.

Even with a too small main jet, you shouldn't get a flameout, it would kinda sputter and recover, but if you're sure it's a fuel problem, open the carb up and see what main jet you have, probably a 105, and put in a 110, and also examine the float level setting.

[Alleyoop]

Also check your Fuel Filter, since it is New pieces of crud could have been in the lines and or in the tank and now your Fuel Filter is all Clogged up. If you see it is dark sort of black particles put on a new fuel filter.
Alleyoop

[kountryguy]

I replaced the fuel filter even though the existing one looked ok. As to the suggestion of opening up the carb and changing the main jet, I am not a mechanic. How would I know what size is currently in it? How involved is this project? I guess I am going to have to learn some of this stuff! I really appreciate all the suggestions and help.

[sprocket]

>>>>I replaced the fuel filter

What kind.. one designed for a gravity feed system? Automotive fuel filter can cause flow problems...

These work well...

www.bikebandit.com/brand/visu-filter

You don't change jets like underwear! There has to be a reason...usually putting on a new free flow exhaust and sometimes a UNI filter, but only if your original filter is paper...

I suspect your problem is the CDI unit...it may have a poor capacitor...they usually crap out at high RPMs

[kountryguy]

OK, what is a CDI and how can it be checked? Like I said, I am not a mechanic. Thanks

[kountryguy]

Sprocket, the filter I put on looks like the Visu filter. After I installed it, I allowed some fuel to run through and the flow seemed great. Thanks, I never knew there was a difference in fuel filters. This is going to be a real learning experience!

[sprocket]

That is good...no problem with the filter then... what is it doing now? Does it run and then stop?

There is a situation that can occur, before getting to the CDI, where the petcock stops functioning at high RPMs, because there is not enough vacuum to keep it open. This is usually caused by a vacuum leak somewhere. Perhaps a vacuum hose has a loose spring clamp...

The petcock could be mounted in the fuel line or on the exit of the gas tank. The hose in the middle is the vacuum line...

When the engine starts the vacuum is high and the petcock opens, fuel flows. It is like a tap to turn the fuel flow on and off.

At high RPM the vacuum is less and the petcock could be closing and stopping the fuel flow and it stalls out...

Check those vacuum lines. I like to replace the spring clips with 5" nylon zip ties and pull them really tight with pliers...

I have never had a leak in anything...

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[kountryguy]

It starts and runs great until I ask it to exert itself such as going above 40 or trying to maintain speed going up a hill, then it just dies. Sometimes it will restart but usually coasts to a stop. I let it sit for a minute or two andthem it starts and runs fine at low speed.
I did replace all the vacuum lines, I hope I put them all back in the correct place. I think I did because I did them one at a time. I will go back and check the clips. I like your idea of the zip ties. Will keep you advised. Thanks