HELP! RPMs up and down...

[lilredscoot]

When I initialy start my LBT150 the idle shoot to about 2000 rpms on the tach, after warm up it settles to a 1000 rpm...
Then it will shoot back up again and I can feel it engaging the drive.
When I try to adjust the idle it will settle down but then go up...
I'm sure the problem is the automatic choke. Tried to adjust mixture to no effect, adjusted valves to spec.
Can I just unhook the wires to the autochoke and this solve the problem???

[harrywr2]

You can unplug the choke/enricher.

My money would be on cracked intake manifold.

[Rebel]

It is my understanding that if you unplug the choke/enricher, it makes it stay closed. That might compensate for a cracked intake manifold though, but not forever.

[lilredscoot]

the intake manifold would be the rubber elbow from the carb to the engine?
Would the crack be noticible or hairline and hard to discern?
Is there some sort of testing to rule one way or another?
Thank you Gentleman!

[Rebel]

Yes, it is a rubber tube on most scooters. Could be a small hairline crack and most probably is. You can wrap it with tape and see if it helps.

[jct842]

long time ago a buddy showed me how to find a vacuum leak.....take carb cleaner and spray around where you suspect it. if you hit a leak the engine speed will change. usually speed up, but not always. john

[lilredscoot]

Interesting Idea on the carb cleaner...
Went for a long ride this morning,150 miles by the Chinometer...
No issues...how would this problem manifest itself on a ride like that?
The only thing I think is off is the fuel consumption. Uses more gas than I would think it would.
and of course the mileage is a lot less if the speedometer is off.

[Rebel]

If you have a vacuum leak like that eventually you will burn a valve, best to eliminate that as a issue before you do further damage.

[harrywr2]

When you are used the carb cleaner method of finding a vacuum leak...it helps to apply a little random pressure to the top of the carb...just to simulate the boucing around on the road.

[Rebel]

I think I'd just remove it and see if i could find any cracks, never know that the carb cleaner might do to the rubber. If you find cracks you can wrap it up with some electrical tape until you get a new one in to replace it with.

[oldchopperguy]

It certainly SOUNDS like a cracked intake, and you might not be able to see the crack. Any place that lets unwanted air in could cause that problem though. Please check ALL potential air leaks, like vacuum hoses, fuel lines, even the carb may not be completely seated in its "groove" in the inake manifold.

The clamp on my carb was loose, letting in air. Also, check the nuts holding the intake manifold to the head. Mine were loose from the factory.

Could even be a DEFECTIVE intake, with the rubber "O" ring not seated right. Don't be surprised to find absolutely anything "not right" from the factory. These Chinese scooters ARE a bargain, but sometimes have little glitches from the assembly line. (ALL my muffler/exhaust mount nuts, AND center stand nuts were totally stripped from the factory, with the bolts cross-threaded...)

Make sure your spark plug is tight also, and not cross-threaded (don't over-tighten though).

The culprit IS most likely air entering the engine from SOME unwanted place, most likely the intake manifold... BUT, I've seen loose (and cracked) vacuum hoses that don't seal, and loose fuel lines that let air in where they plug onto the carb, tank, or petcock. Check ALL hoses connected with the carb for cracks, or loose fitting onto the barbs. This would be a GOOD time to just replace any suspicious pieces with inexpensive upgrades.

Best wishes on the fix, you'll get it done!

Leo in Texas

[jct842]

just about any airesol that burns should be fine for subbing carb cleaner if your afraid of using it,

[jpn]

Aug 3, 2009 0:45:44 GMT -5 @jct842 said:

just about any airesol that burns should be fine for subbing carb cleaner if your afraid of using it,

WD40 should work.

An intake leak would have a more noticeable effect on the engine at idle, than at full throttle. At idle, the throttle is closed off and letting the engine pull more vaccuum in the manifold (pulls more air through the leak). At full throttle, the only restriction to airflow is the carb's venturi and the air filter, so the engine pulls very little vaccuum in the manifold (less air pulled through the leak).

The leak causes the mixture to go lean, which can increase rpm (to a point). If the leak opens and closes (the rubber tube flexing at the point of the leak), that would most likely cause surging rpms.