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Post by dahermit on Jun 13, 2022 22:06:49 GMT -5
VIP Future Champion QMB139 2014.
New plug, new fuel filter, new fuel vacuum valve, new fuel.
I swapped out the new carb which was bogging down when I tried to ride it with the carb that came with it (as a test to see if the bogging down was the carb). The old carb (which I took apart and cleaned the jets) is acting very strange.
When I start the scooter, it goes into high revs (5,000 or more), runs that way for about 15 to 30 seconds, and very gradually slows down and dies when it gets under 1,000 revs or so... like it is running our of gas. If I give it any throttle, it jumps back up to high revs. At first I thought that the idle screw was set way too high, but I can back it out all most all the way and it still will over-rev when I give it a slight goose of throttle (and let up on the throttle completely).
They way it revs and then slowly idles down and dies, I thought maybe the gas tank breather was plugged. However, it does the same thing with the gas cap off, so it is not likely a problem with a vacuum in the gas tank due to plugged venting.
I set the fuel air mixture screw to two turns out to see if that was a problem... no joy, still acts the same.
I cleaned up the intake manifold surface to make sure there was not air leak. Cannot find any other vacuum leak or bad hose.
Tomorrow I will start it and unplug the auto choke when the engine is running as a test to see if that is a problem. I have considered the CDI as a possible source of the problem and will order one of those if I do not stumble upon the cure soon.
Also considered: Float in carb set too high. Valve adjustment.
I am wide open for any suggestions as to what could be causing the high revs and slowly die problem?
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Post by dahermit on Jun 14, 2022 8:44:18 GMT -5
Addendum:
I started the scooter this A.M., let it warm up (it was reving at about 5,000 as per the tach.). I unplugged the auto choke. It made no difference (so that is not the likely cause)... after about 30-45 seconds, it began to loose revs and began die out... just like it has been doing. The Idle screw does not seem to have any control over the revs... when the revs drop way down, I turn the idle screw in to increase the revs...but it seems to change nothing and I have to give a slight goose to the throttle to keep it from dying. When I turn the fuel-air mixture screw in or out, it seems also to have no effect whatsoever. In sum, there is nothing I can do get to a steady 1,500 idle.
Thinking about it, it may also be a pilot jet problem. However, I did crean the carb and ran a wire though that jet (as well as the other orifices) so I do not know how the pilot jet could be the problem. Nevertheless, I think I will take the pilot jet from my new standby carb and replace the one in the carb mounted in the scooter now... just to see what happens.
At this point I am going to order a new CDI and swap it out to see if that makes any difference.
I am still interested in hearing any opinions, etc. concerning my current problem.
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Post by dahermit on Jun 14, 2022 13:35:01 GMT -5
Having done several internet searchers looking for information on QMB134 carb problems, some guy made the statement, "...if you are having an idling problem it is almost always the pilot jet and the fuel-air mixture setting inasmuch as they control the idle in combination." Taking his statement to heart, I pulled the carb, swapped out the pilot jet with the one in my newer (but bogging down), carb. I made damned sure the jet had no obstruction.
I learned a few things about these chinese scooters. They take forever to warm up and any adjustment that is made takes some time to take effect (different to when I had several motorcycles years ago... the changes took effect immediately) and settle down... which I attribute to all the anti-pollution stuff and the auto choke on them. My motorcycles had nothing that was a concession to clean air. I noticed that even revving the scooter it has a noticeable lag before the revs drop all the way to idle where as my motorcycles of yore (1960's) the R.P.M.s would drop instantly when the throttle was released.
I am at the point where I think my problem may be solved. Tommorow I will try to take it for a ride and see.
I will report back here with an update.
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Post by aeroxbud on Jun 14, 2022 15:04:36 GMT -5
A lot of these scooters run real lean from the factory. This probably doesn't help.
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claydoh
Scoot Enthusiast
Posts: 172
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Post by claydoh on Jun 14, 2022 15:54:27 GMT -5
To confuse things further, and one thing that was a head scratcher to me, until I found out, is that these auto-chokes are basically little heaters that melt some sort of material that causes a plunger to to extend, and plug a hole in the carb. it can take a couple of minutes for this to fully extend, so tuning can be a pain if you are impatient, like I am  What jets are you using, at the moment? I found that even with my 50mm BBK, the idle jet didn't need as much a change in size from stock, 38 (I think) as opposed to the 35 it came with. |
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Post by dahermit on Jun 14, 2022 16:36:15 GMT -5
What jets are you using, at the moment? I found that even with my 50mm BBK, the idle jet didn't need as much a change in size from stock, 38 (I think) as opposed to the 35 it came with. I have no idea what size jets they are. They have no numbers or marking on them, on ething of the carbs... not the one that came in the scooter, nor the replacement carb I purchased. |
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Post by dahermit on Jun 14, 2022 19:51:16 GMT -5
To confuse things further, and one thing that was a head scratcher to me, until I found out, is that these auto-chokes are basically little heaters that melt some sort of material that causes a plunger to to extend, and plug a hole in the carb. it can take a couple of minutes for this to fully extend, so tuning can be a pain if you are impatient, like I am  Here is an explanation on how they work: |
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Post by GrumpyUnk on Jun 15, 2022 8:30:15 GMT -5
Runaway revs on most is caused by the low end(idle, pilot) not working, and the throttle plate hard stop being set a bit high. That puts the carb at the low end of the high speed circuit, where it is very unstable and will rev and snort, and die at the drop of a feather. Other than that, it works perfectly... not.
The idle adjustment needle likely has some decomposed fuel/stuff, as does the pilot jet. The pilot MUST see light through it or it is plugged and will not work. I use a single strand of a wire brush(scrub brush style) bent away from the handle to poke through pilot jets that are clogged. I have tried chemical(acetone, carb & choke cleaner, and about everything else) to get them to clear with not much success.
The idle mixture screw generally runs about 1.5 turns from bottomed(gentle!!). Adjustment instrux are posted on site in a video by the host.
A good idle setting will not turn the rear wheel when on the center stand. It will idle without droop or variance in rpm when warmed up. It will allow full WOT without bogging. If you get bogging, check the accelerator pump as the little tube can get clogged. Easily, it seems. Bogging can also be caused by a too-lean idle mixture or a lean pilot jet.
tom
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Post by dahermit on Jun 15, 2022 20:14:34 GMT -5
The idle adjustment needle likely has some decomposed fuel/stuff, as does the pilot jet. The pilot MUST see light through it or it is plugged and will not work. I use a single strand of a wire brush(scrub brush style) bent away from the handle to poke through pilot jets that are clogged. I cleaned the pilot jet and I can definitely see daylight through it, so that is not the problem. |
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Post by wiscootsin on Jun 15, 2022 22:50:28 GMT -5
What is your airbox situation? What kind of filter is in it and what condition is it in?
Are your vacuum lines still stock and setup with the basic emissions components?
Is there any chance your carb needle valve that's attached to the float doesn't seal the gas intake and it results in overflowing the carb?
Have you tried running gas straight into the carb and not through the tank/vacuum petcock? Going gravity feed can help isolate some problems.
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Post by dahermit on Jun 16, 2022 11:10:06 GMT -5
What is your airbox situation? What kind of filter is in it and what condition is it in? Are your vacuum lines still stock and setup with the basic emissions components? Is there any chance your carb needle valve that's attached to the float doesn't seal the gas intake and it results in overflowing the carb? Have you tried running gas straight into the carb and not through the tank/vacuum petcock? Going gravity feed can help isolate some problems. New airbox, new foam air filter. The vacuum lines appear to be stock with all the basic emissions components... however I bought this scooter non-running and I do not necessarily know anything for sure. I took the carb needle out last night and made sure it was clean as well as the jet again. However, as I posted, the RPMs drop off and the engine gradually dies... sounds to me like running out of gas. If I catch it before it dies, and give it a little throttle, it is alright for about 15 seconds... idles at about 2,000, then slowly dies. Although I have a new vacuum petcock installed and it appears to be working, running the gas line from the tank (and new filter), directly into the carb is definitely something on my list to try. Also on the list, beside swapping out the CDI box, is to set the valve backlash (A.K.A., clearance). |
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Post by wiscootsin on Jun 16, 2022 11:49:22 GMT -5
Checking valve clearance only takes a few minutes, maybe 30 minutes your first try. It's a good idea to check on these engines just in case.
Are you able to start it and drive it? Like, could you start it and drive off for five minutes and then see where the idle is at? Your info here is mostly about starting it and letting it idle. Just wanted to clarify.
For the direct gas idea i typically use a 30ml syringe because i can easily run a fuel line from it to the carb, that way i dont have to mess up the gas tank/petcock at all. Just for testing - zip tie it to stay vertical and you can actually drive around a bit.
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Post by dahermit on Jun 16, 2022 19:46:20 GMT -5
Checking valve clearance only takes a few minutes, maybe 30 minutes your first try. It's a good idea to check on these engines just in case. Are you able to start it and drive it? Like, could you start it and drive off for five minutes and then see where the idle is at? Your info here is mostly about starting it and letting it idle. Just wanted to clarify. For the direct gas idea i typically use a 30ml syringe because i can easily run a fuel line from it to the carb, that way i dont have to mess up the gas tank/petcock at all. Just for testing - zip tie it to stay vertical and you can actually drive around a bit. I opened the valve cover and checked the valve clearance. The intake would not accept a .0015 (inch) feeler gauge, so I opened it to .003. The exhaust valve accepted a .003 with some pull, so I left it where it was. I will try this setting and see how it runs/idles. I have not attempted to drive it since swapping the new carb (which was bogging down, not moving unless I pushed it first with my feet to get it moving), for the original which I have been cleaning. I have been running it on the center stand only. I have installed a Briggs & Stratton shut-off after the tank to enable experimentation. I don't think I have any 1/8" gas line lying around so I have not tried bypassing the vacuum petcock yet... that will happen as soon as I can dig up some 1/8 gas line that is long enough. |
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Post by wiscootsin on Jun 16, 2022 20:53:37 GMT -5
I typically do .004 for both.
Is there any chance your exhaust got restricted or anything like that?
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Post by dahermit on Jun 16, 2022 21:19:59 GMT -5
I typically do .004 for both. Is there any chance your exhaust got restricted or anything like that? I don't think the exhaust is restricted... but I will shine a light up there and check for rat's nests and such (it had been in storage for awhile after all). However, I suspect that such debris would catch fire and/or produce non-typical smelling smoke, but will check nonetheless. |
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