SOLVED - Yet another stalling mystery

[MN_Scoot]

I solved my last stalling problem when I realized the carb had loosened from the intake. Tightened it up again and it ran great for about a week.

Now, I have the same symptoms, even though the clamp is now tight. Everything runs great for about the first 3-4 miles. Then, I can feel it start to lose power and if I approach a stop light, it dies as I'm slowing down. I can always get it started again by holding the throttle open all the way like you would for a flooded engine. Could the engine be flooding itself as I drive?

It often seems to happen on straight-aways with full throttle (after the first 3-4 miles). Not sure if this is a red herring, but it also doesn't seem to happen at night when it's slightly cooler.

I somewhat suspect the choke needs to be replaced since the problem is somewhat sporadic. -it also won't start in the morning without me holding the throttle open a bit for the first 2 minutes before it will idle on its own.

This problem developed after the following maintenance completed a month ago:
cleaned carb/jets (added new gasket and o rings)
adjusted valves
new fuel filter
new air filter
new spark plug
adjusted idle to rpm spec in manual

Appreciate the help!

[zombie]

It does sound like the choke is sticking part way open. Not enough for a cold start but enough to make running difficult. Pull it out and check it. It MUST move at least 4mm.

OR it could be cracks in the rubber boot that attaches the carb to the intake manifold. That would create an air leak, and lean running. The throttle being held wide open will allow more fuel thru the main jet allowing it to start.
An easy way to tell is pull the spark plug, and see if it is black or white.
Black for a bad choke, and white for a leaking boot.

Could be a few more things like a bad float level but I suspect these first.

[MN_Scoot]

ok, yeah, I read somewhere else about checking the choke, but I didn't know what "checking" entails. I've seen mentions of checking distance and electrical resistance, but didn't exactly understand how to go about doing it.

[zombie]

You pull it out of the carb. (two tiny phillips screws, and a C shaped retainer).
Leave it plugged into the harness, and turn on the key. It takes about 90 seconds to 2 minutes to fully extend (4mm minimum). If it is not moving verify there is 12 volts across the yellow, and green wires.

[MN_Scoot]

Perfect, thanks Zombie. …I almost felt inappropriate watching that clip at work. 

[zombie]

Yeah... It got me too. Ive never seen anything grow that fast. (Zombie curse)

[JJJoseph]

I solved my last stalling problem when I realized the carb had loosened from the intake. Tightened it up again and it ran great for about a week.

Now, I have the same symptoms, even though the clamp is now tight.

The answer is so simple, you'll fall off your chair.  All qmb139 engines have the same problem.  The intake manifold has an O-ring between the manifold and the head.  The stock O-rings are cheap neoprene and they go flat from the temperature.  The result is a tiny air-leak, and your scoot won't start or idle properly.  Replace the O-ring.  If you live in a big city, look for Viton (heat resistant) O-rings at an industrial supply shop.  Cost: maybe $1  You'll have to replace the O-rings every 5000 miles or so forever, so buy several at a time.

[MN_Scoot]

The answer is so simple, you'll fall off your chair.  All qmb139 engines have the same problem.  The intake manifold has an O-ring between the manifold and the head.  The stock O-rings are cheap neoprene and they go flat from the temperature.  The result is a tiny air-leak, and your scoot won't start or idle properly.  Replace the O-ring.  If you live in a big city, look for Viton (heat resistant) O-rings at an industrial supply shop.  Cost: maybe $1  You'll have to replace the O-rings every 5000 miles or so forever, so buy several at a time.

Hmmm, I'm about at 8,000, so I will give that a try as well. Thanks.

[zombie]

Check that "bako-lite" spacer for cracks, and warping. Also check the rubber manifold when you have it off. They all eventually crack/leak.

I have a link to an aluminium spacer, and a cure for the sh** manifold if you find they are an issue.

[MN_Scoot]

Check that "bako-lite" spacer for cracks, and warping. Also check the rubber manifold when you have it off. They all eventually crack/leak.

I have a link to an aluminium spacer, and a cure for the sh** manifold if you find they are an issue.

Yeah, would definitely like to see links for future reference. Thanks.

[MN_Scoot]

On this page, there's two different gaskets, an O-ring and the entire intake all claiming to be for QMB139 engines. I didn't think we needed that grey or black gasket...

http://www.partsforscooters.com/151-7-Carb-Gasket

Are there different versions of QMB139 engines?

[zombie]

The one you linked I THINK is the OEM plastic type. Its hard to tell from the Pict.

This is the one I use on all the conversion engines I build...

http://www.racingplanetusa.com/intake-manifold-spacer-naraku-flow-aluminum-milled-139qmbqma-p-5390-1.html?cPath=7534&sessID=2642caa5d879b95abc6dce8485fff653#.U5-1XtTbzl8

Sucks it has to be from RP of all places...

Start a dealer account under whatever company name you want. You don't need the TAX ID number because you will be paying sales tax on every purchase. It makes the parts like 40% off for most items. Electronics have less of a discount.

[MN_Scoot]

Check that "bako-lite" spacer for cracks, and warping. Also check the rubber manifold when you have it off. They all eventually crack/leak.

I have a link to an aluminium spacer, and a cure for the sh** manifold if you find they are an issue.

Thanks for all the info Zombie. Going to open it up again tonight. You think intake cracks will be immediately obvious or can they be hidden?

[zombie]

You'll see it. That spacer usually comes out in pieces. If it is whole, and nothing obviously wrong I would take a piece of 100 grit sandpaper, and use a flat surface to block sand it. That will show/correct if it is warped.

Two new O rings, ans a little Indian Shellac will seal it up. Don't use that RTV crap.

Also check real close on the rubber intake for cracks. That is the next weak link.

[MN_Scoot]

Two new O rings, ans a little Indian Shellac will seal it up. Don't use that RTV crap.

Also check real close on the rubber intake for cracks. That is the next weak link.

I didn't even think of a sealant…I looked that stuff up and it said it's for low temp applications (350 max). That area won't exceed that?

Also, do you put the sealant on both sides of the spacer? Do you put it over the seated o-ring as well or just a little around it? 

Appreciate all the help. Hopefully someday I'll send it the other direction, but I clearly have a lot of Jedi training to go!