150cc GY6 dies after 20 seconds or so of top speed By pazuzu (Read 8,945 times)

[pazuzu]

Hey, everyone. I'm new to this forum. I've notice most scooter forums are mostly dead nowadays, but it looks like you guys are still alive and kicking, so I thought I'd join the fold.

I’m sorry this is so long, but I like to be thorough when asking for help.

I have a 2007 150cc generic Chinese scooter with about 3000 miles on it. It's most similar to the Roketa Capri 150cc. I live in Utah, about 4,500 ft above sea level.

My scooter runs really well in pretty much every way. By that I mean:

• The auto choke works like it should and the scooter starts very easily. I believe the choke is shutting off after the engine warms up as it should.
• It idles just fine.
• It accelerates off the line with no hesitation or sputtering.
• It gets to top speed of about 45mph with no initial problems, which is about the top speed when it was brand new.

The problem happens when I try to run it at full throttle for 10 - 20 seconds. When I do this, the engine suddenly stalls and I coast to a stop. Even if I let off the gas before it has a chance to stall (after 10 or so seconds of full throttle), it sputters and inevitably stalls anyway after another 5 seconds or so. After waiting for about 20 seconds, I'm usually able to start it back up after cranking it a few times (until now. More on that in a bit.)

Before I go on, I should say that the scooter was stored for about 6 years up until this spring. I cleaned the carb, drained/refilled the gas tank and added octane booster, and have ridden it about 100 miles since. This problem started before I stored it (part of the reason I stored it) so I believe it's unrelated to storage issues.

Things I've tried to alleviate the problem:

• Replaced the fuel and vacuum lines with 3/16 and 1/8 (respectively) Tygon lines.
• Replaced the fuel pump with a Mikuni rectangular pump (I have a floor-level tank).
• Replaced the manifold-to-carb clamp (the old one was stripped and loose).
• Replaced the spark plug.
• Drilled a few new holes in the gas cap to eliminate potential vapor lock.
• Checked the air filter, which seems to be clean enough.
• Tested all the electric system and replaced the CDI (as a fix for a failed CDI, which appears to be unrelated) with a performance CDI.

After the above upgrades, the scooter appears to start, idle, and run even better than before. Enough so that I was confident I could go out onto a faster road and try extended full speed again, but found the problem persisting. The difference is that this last time I could not get the scooter to start after it died. I had to walk it home.

Here's a timeline of what I did after I got it back home:

• I watched the clear gas lines as I cranked it and saw the gas making it about half way up to the pump, but no further.
• I sprayed carb cleaner into the air intake and cranked it. Got it to fire up for about 1 second.
• Opened the top of the carb and poured a little gas in. The scooter started and ran for about a minute, probably long enough to burn off the gas in the carb.
• I’m sorry to say that I have yet to actually open the bottom of the carb to see if there is any gas in the float after it dies. I will test that tonight if I take it out again and stall it.
• I gave up and let it sit overnight. The next morning, it start up just fine like nothing happened.
• After it was warm, I spraying carb leaner around manifold and other areas to listen for RPM change to see if there is an air leak. No RPM change.
• I tried to replicate the problem with the scooter on the kickstand but I was afraid I was going to throw a rod, so I stopped.

It sounds like a vacuum problem, but the fact that it runs so well at normal speeds and I replaced all the hoses makes me think that’s not it. I thought maybe I need to re-jet it, but being that I’m so high above sea-level, I’d think I’d be a lot more likely to be running rich than lean, which doesn’t seem to explain my problem.

Any thoughts on what I can try next? I’m so stumped. Thanks in advance!

Edit: I should note that I am normally able to drive it 5 miles to work as long as I keep it below 35ish mph. I haven't tried the drive since my last stall where I had to walk it home.

[jct842]

Number one if you have not already adjusted valves do that. Then from what you say you may be running out of gas....vacuum may play in this also. A vacuum leak can be found by spraying your ether around the base of carb and manifold. It has to be idling when you do it, if you hit any spots where the engine speed changes you have found the leak.

You can check the fuel pump by disconnecting the fuel line from the carb and crank the engine...it should be pissing gas out of the fuel line. a plugged gas tank vent can also cause problems.

[pazuzu]

I believe it probably is a fuel delivery issue, so my first thought was a vacuum leak. After replacing the pump and all the vacuum lines and inspecting the intake manifold and spraying carb cleaner around it to check for leaks, I can't see how it could be the vacuum. Is there another spot in the vacuum I need to be checking?

The other weird thing is that the vacuum is able to easily pump gas up through the fuel line when the bike is cold/warm, but after the long, fast runs, when I crank the engine, the gas doesn't seem to want to make it more than half way up from the tank to the pump. Until I let it cool, then it works just fine.

I will try adjusting the valves, but do you think that could still be a factor if it's running fine most of the time?

[jct842]

All gy6 engines need adjusting of the valves especially when new and periodically after that. Valves can effect vacuum. Fuel delivery depends on vacuum which as the engine reaches high speed gets less and less normally. If you should have a slight vacuum leak in the manifold it may never show until you open the throttle and keep it there for a period of time. Routing of the fuel line can and will cause problems too. If fuel stops at high speed the engine will still run until the carb bowl runs out of fuel.

[cyborg]

I had a very similar problem with vespa I had,, sustained high speed runs would create a low vacuum situation and the vacuum fuel tap wouldclose andthebike would run out of gas with a full tank,,, went with a manual valve and the problem went away

[pazuzu]

What do you mean by a manual valve? Are you referring to the petcock? I have a pump, so I wouldn't have that issue.

[pazuzu]

Jul 19, 2016 20:52:20 GMT -6 jct842 said:

All gy6 engines need adjusting of the valves especially when new and periodically after that. Valves can effect vacuum. Fuel delivery depends on vacuum which as the engine reaches high speed gets less and less normally. If you should have a slight vacuum leak in the manifold it may never show until you open the throttle and keep it there for a period of time. Routing of the fuel line can and will cause problems too. If fuel stops at high speed the engine will still run until the carb bowl runs out of fuel.

I've ordered a set of gauges, so I'll be adjusting the valves this weekend. I'll also remove the manifold and inspect it and the o-rings. I'm still not sure what caused it to not start until the next morning the last time I killed it. I guess I need to take it out a few more times and try to reproduce the problem. Thanks for your help. I'll update after I get valves adjusted.

[crwford]

Are you sure if you have a vacuum shut off is working it really sounds like it's bleeding back to carburetor floods engine at high speed.

[jct842]

he would not have the vacuum shut off if he has a fuel pump and a floor board tank.

[olivertwisted]

My guess is, the cooled valves, giving wider clearance along with the enrichened fuel mixture making just enough difference for a short while the following morning .

[rks]

You said......I watched the clear gas lines as I cranked it and saw the gas making it about half way up to the pump, but no further.

Sure sign of insufficient vacuum. Could very well be valves are too tight. Could also be those pesky emissions canisters.....are they still attached? If so, it's time to poop can them, along with all of their hoses.

My scoot has the tank under the floor also, it has a one port intake manifold with a hose that splits at the carb.....one goes to the air cut valve, and the other to the fuel pump. That is all the vacuum line you need. Any more, is unnecessary, and a potential problem.

[pazuzu]

Thanks guys. To answer some of your questions:
- No, there are no emissions hoses or canisters. Just the one port out of the manifold.
- Like jct842 said, no vacuum petcock, if that's what you meant. Just the pump.

I'm about to adjust the valves now. I've never done this, so I need to watch a couple videos first. Hopefully I can get it all done tonight and give it a test drive.

[pazuzu]

An update for you guys:

I adjusted the valves. I must say, that was a much bigger pain that I'd hoped. On my scooter, there are two things preventing easy access to the valve cover. First, the floor tank is about 1.5 inches from the bottom two bolts. Second, there is a part of the frame that goes over the cover to bolt the seat onto. Regardless, I eventually got it off and got the valves adjusted.

I got it started up and it sounds like it's running great, but it was running great at idle before. But then, out of nowhere, it just died and wouldn't start. Once again, the fuel wasn't making it up the fuel line to the pump. After trying several fixes, I ultimately bypassed the fuel filter and plugged the hose straight into the pump from the tank and suddenly I had no problem getting fuel up the hose. So, bad/leaky fuel filter. I ordered a new one that should be here today. I think that's a separate issue from the WOT issue, but may be the reason I could not get it started on the street a couple weeks ago. I'm out of town for a week, but I'll give it another test drive when I get back.

Thanks for your help so far.

[olivertwisted]

GREAT! I dunno pazuzu , a clogged fuel filter definitely could have been a problem at WOT. Letting the bike sit or "cool" actually could give the blockage time to fall away from the "In" side of the filter, long enough to start up again at idle or low speed. I hope that's the case.
I empathize with you on adjusting the valves. I triple checked mine, bent over , upside down , through the small access panel under my seat compartment. I'm sure ya could use an epsom salt soak after that, at ANY age !
Keep us posted

[pazuzu]

Hooray! I just took my scooter out for a test drive and I was able to make it up and down main street quite a ways with no problems! Sounds like the initial problem was caused by the tight valves. The secondary problem of the scooter not starting at all was likely a leaky/faulty fuel filter.

I did notice a bit of a "clackity" sound from the engine, so I'm guessing the valves are now a little too loose. I'm dreading opening it back up from the placement of the gas tank. I may just have to drop the tank and go buy a wrench in the right size (I as using sockets before). How long is it OK to run the scooter with loose valves?

Thanks everyone for your suggestions!