139qmb 2006 key west 50cc moped (Help Needed!) Witts End.

[madisonmarineman]

having trouble getingthe valves dialed in, had the scooter running once and didn't restart cold. The valve adjustment on this site says .002-.003. Is this accurate, the bike has 3000 miles on it. I was told the top end is bigger but cant verify the old owner isnt reachable. (For what its worth i am a certified mechanic, i don't work on these 50cc Chinese engines very often) 

Coil, plug, carb, intake, fuel pump, fuel sender and vacuum lines replaced. I cant not get this thing to start easy and stay running unless its hot, it sat about 3 years, the variator shaft has a oil leak i believe this is resolved but will not know until it gets hot. 


Did a compression check and i got 60psi only with a long hose automotive tester, am i getting lower reading with this tester...?

i have been trying all different valve combos and decided I'm giving it 2 hours and i am junking the thing. My son wants to take it to college and i have 20 hours in.


any help with valve clearance is helpful before tomorrow. I have spark and fuel, it seems extremely touchy with the valves the one time I had it running I think I had .004 for both and it wouldn't restart again when cold. 

Valve adjustment if the top end is a larger add on?

Valve adjustment if the top end is stock?

How critical is the valve adjustment for running, should it be running, i have tried every variation except going super tight. maybe this where i need to direct the repair. 

Thanks for any help..

[FrankenMech]

.004" will work on both valves. There is nothing super critical about the valves but if you get them too tight they will leak. The long hose tester will take a while to build pressure because the engine displacement is very small. Just three to four strokes will not work like it does on most automotive applications. Make sure the compression tester is in good shape and does not leak down by itself.

There is a automatic enrichment circuit on the carb that operates with electricity to shut the fuel off just the opposite of a thermostat. The expanding wax pellet closes off the enrichment passage.

The carb is very strange. It has to be tuned to the intake passage and filter. Without the air filter box it will not deliver enough fuel. Matching the carb with the inlet filter box is essential. There are lots of things that can go wrong with an old scoot that has been sitting for three years, both mechanical and electrical.

Personally I would not send the scoot to college with my son unless he is thoroughly familiar with upkeep, tuning, and maintenance on it. It will be a major distraction from his studies and a long distance headache for you.

Get rid of it.

[madisonmarineman]

Franken mech. can you explain more about matching the carb , i bought a carb from amazon. was i wrong in maybe i should have tryed another carb. everything matched identically to the one i took off. i did not do a comparison of the jets but i dont think that is critical. with it running on and off could it still be carb related...?

thanks for any additional information

[FrankenMech]

The stock replacement carbs found on Amazon and eBay are usually the leftovers from a production run and only roughly match the OEM carb. With amazon and eBay they also may be a defective batch of carbs or factory rejects that did not adhere to the OEM's specs. YMMV

[classacted]

when I am checking the valves, I leave the compression tester hooked up and make numerous adjustments if necessary, carefully documenting which valve and which way I am making the adjustment with the purpose of getting the highest compression possible. don't forget to have the carb open (I manually prop the cable full throttle) when testing compression and the exhaust can't be plugged up either. if you don't, you are compressing vacuum. the first time I did a compression test without opening the carb, I got about 60psi and spent over an hour worrying and searching google, then realized I was simply doing the compression test wrong.

[madisonmarineman]

set intake to .10mm exhaust .12mm. Starts one kick got it running. thanks for the help.

Note: i was having more difficulty then normal because it soaked theair filter in fuel trying to set valves for 6 hours...


the stock airbox is in the garbage, i removed the whole assembly. cool air intake right on carb runs better then the box. 


Heading to aimes iowa for fall semester. 

[classacted]

does it have lots of power, and what does it have for compression now?

[FrankenMech]

If you used a 'stock replacement' type carb it will have to be tuned to having no air filter box unless it is a 'performance' carb (that usually comes with a filter) already set to run with a pod type air filter. The problem with running with an open or pod type air filter is water getting in the intake from rain, spray, or puddles.

Ames is roughly 1000 feet altitude. Carb adjustments may be required.

[GrumpyUnk]

10mm and 12mm clearances must make for some noisy valves. Both valves are opening late and closing early. In theory you have reduced the possible airflow and thus developed power. But if it runs and that's the only way you can get it to run, then it is what it is.
If a carb is tuned(idle mix and setting correct), and the 'byvalve' (choke/enrichment) is working, these engines will start withing one or two engine revolutions. If there are clogged jets, idle passages, or fuel bleeds to the accelerator pump and byvalve, it will not start well nor have a good cold idle. If the idle is set too high, it will be difficult to start when cold, and may want to increase rpm way beyond normal with fractional movements of the throttle. It will be super sensitive...
Other than that, hope it works. I am not too sure that the factory airbox & filter are much restriction, but people have their own opinion as to their usefulness.
tom

[madisonmarineman]

Big Update, Recommendation needed. 



Correction valves set to .010 and .012. I plan to bring it back to .06 and .08 per engine service manual.

When hot the motor is not running very good, popping hesitation. only hiting low 20mph range. It got worse with the airbox vs. cold air intake.

The next 48 hour plan (almost out of time)

Puting on a different carb and jetting as required
new air box assembly installed
Setting valves to factory settings .006 & .008 ...Thoughts on this>
Replacing spark plug
Possible switching gas tank to petcock vs. the vacuum pump style, i plan on caping off vaccum on intake if i switch to petcock.

Question: for the valves what do you recommend for valves and what jet size for wisconsin/iowa elevation and typically cooler weather. 40-50f . THANKS!

[FrankenMech]

Remember to specify valve lash settings in MM or INCHES. I set mine to .004 INCHES for both. I used the same specs for my old '68 VW.

Tuning a carb is a long process that is detailed in many locations here. Follow the directions and methods that the site owner specifies. It is a localized process that depends on the location. There is no way I could detail jets for a location although some here may be able to give you a starting guess.

If your compression is only 60psi carb adjustments are a moot point. Fix the engine first.

[madisonmarineman]

Thank you

[madisonmarineman]

without thinking i did the compression test wrong...i will report back the accurate reading when im done with everything. Franken on valves- i am going to start with setting to  .004 INCHES for both.

Thanks for all input so far, first time with 139qmb engine, was in rough shape when i recieved it.

[madisonmarineman]

I set valves to .004inch and was exactly the same. When it get to 15mph it starts popping and not running right. Put on improved carb no change....I'm wondering if the fuel pump is not opening all the way. I took line off and turned it over and it's very light and sporadic...I think it's a lean issue but I feel like it might be fuel. I replace the pump as they call it. I have a petcock coming tomorrow to keep it on all the time. Is this a common issue?

Airbox came today. Installed that before carb swap. No better.

[GrumpyUnk]

When using a compression test gauge, be aware that the displacement of the engine is so small that many gauge mechanisms are unable to provide accurate pressure readings. They seem to need enough pressure buildup to fill the hose and work the mechanism that the pressure fails to register. In theory, cranking for a longer period of time should fill the assembly and finally register true PSI, but my take is that it won't happen unless the gauge is a good one. An imported set, bought to get the adapter that fit the spark plug threads/diameter, would not read properly and after much fiddling, I used my 1969 version JC Whitney compression tester. It had an actual pressure gauge with the makers name on the dial. It worked. The Far East product would not.
Just be aware if you want actual compression readings...
What happens when you reach terminal velocity of ~15mph? Does manipulating the throttle have any effect? Does opening wide do anything?(if not already at WOT) Does backing off for a few seconds improve things? That may indicate a fuel delivery problem, which could be the petcock limiting flow or similar.
tom