midrange stumbling on 157qmj By blueboy5000 (Read 1,178 times)

[blueboy5000]

So on my 2011 Baccio vx150 I have had a stumble (misfire?) Between 3000 and 5500 rpms.

So far I have done the following to no avail:

Changed carb to Keinhin 24mm w 105 main

Changed petcock to high flow Miniki vacuum pump

Changed CDI to Hoca unlimited

Changed coil to Hoca

Removed PAIR and blocked off exhaust port (and confirmed no leaks)

Changed exhaust to scrappydogscooters one piece can.

Changed plug to c7hsa NGK R

Adjusted valves to .004 intake .005 exhaust.

For one day it ran excellent, no stumble. But then after a 20 mile ride the stumbling returned. And then has become worse progressively to the point where I am only getting 30mph on flats, all the while bogging /stumbling.

This problem has been present since uncrating. It's a real head scratched as none of my other bikes have done this. Everything seems properly adjusted.

My PAIR did fail, causing lots and lots of backfiring around 400 to 450 miles. I am wondering if this may be a damaged exhaust valve?

Compression is spot on, no smoke or excessive oil venting.

What could this be? Why would this problem still be present after changing the entire fuel system and ignition system? Is this indication of inner engine valve damage?

[tvnacman]

I would recheck the valves. And gas filter. A vacuume gauge might be helpfull in this case. Check the intake for cracks. While at idle flex the carb, see if there is any change.
john

[blueboy5000]

Just checked the valves they're spot on. Checked vacuum with propane and carb cleaner, no leaks.

Just checked plug and it is sooty, so I'm thinking rich. But this is set to 2.75 turns counter clockwise, which should be pretty right on.

Which way do I move the needle to lean the mixture? It's currently at the second lowest of five notches.

[JR]

You raise the clip to lean and lower the clip to richen. Just for curiousity sake do you have the OEM CDI?

Reason I ask is there have been occasions that people with an unlimited CDI have had these issues? It has something to do with the firing getting to a certain what they say threshold and the engine and CDI aren't working together? Does that make any sense?

I rem,ember some time back this very problem happened to a fellow, he cahnged back to the OEM CDI and it ran like a scalded ape!

I didn't see where you changed the the OEM filter? Open exhaust, more fuel, = needs more air.

JR

[alleyoop]

Jul 19, 2012 9:10:12 GMT -6 blueboy5000 said:

Just checked the valves they're spot on. Checked vacuum with propane and carb cleaner, no leaks.

Just checked plug and it is sooty, so I'm thinking rich. But this is set to 2.75 turns counter clockwise, which should be pretty right on.

Which way do I move the needle to lean the mixture? It's currently at the second lowest of five notches.

You answered your own question and problem, "THE PLUG IS SOOTY"

The Fuel Ratio being out 2.75 out doesn't mean a thing, when someone has problems and they messed around we tell them to turn it out 2 1/2 turns out because that should be enough for the motor to start and stay running(BUT THAT IS JUST A STARTING POINT).

In your case your running RICH so you have to LEAN IT OUT no 2 ways about it. So try ONLY 2-2 1/4 TURNS out and see what that gets you.

You have to take in consideration the HOT Temps we have been having the hotter it is the LESS GAS the motors need. IT is the OPPOSITE when it gets around 60 and below in that case the motors need more fuel so you have to AGAIN adjust the Carb to feed it more fuel.

So give it a shot 2 - 2 1/4 turns out that should also give you a little more power as well.
Alleyoop

[blueboy5000]

The carb has been adjusted as follows: mixture screw was seated, then turned counterclockwise until the highest rpms,then set 1/8 clockwise. I don't think the carb is the problem.

I thought it may be the CDI, but after swapping my Hoca with a known good one, still had the same problem. Did the same with the coil.

Running out of ideas, I took off the CVT (lots of belt dust and chunks of belt in there) and kickstart and then left the kickstart off and put the CVT back together and started the bike. The belt flopped like crazy at 5000 rpm (where my stumble occurs). I am thinking the belt might just be the problem. I ordered a new Gates Powerlink belt from Napa that will be here today.

I also removed my carb and freshly cleaned and rebuilt my spare carb and put that on. The carb made no difference.

Other than having a shredded belt, the rest of the CVT looked good after I cleaned it. No flat spots on the rollers or heat checking or glazing on the clutch bell or shoes, no broken springs.

The belt was a Zhianping? Could a screwed belt cause stumbling if it's flopping around? Isn't 700 miles a bit soon for a belt to fail?

And lastly, my brother suggested running the bike sans belt to see if it was truly the CVT and not an engine problem. I told him that would cause my rollers to go flying out of the variator. Is it okay to run the engine without a variator? I would guess not as it's probably counterbalanced by the CVT.

[blueboy5000]

Jul 19, 2012 9:50:34 GMT -6 JR said:

You raise the clip to lean and lower the clip to richen. Just for curiousity sake do you have the OEM CDI?

Reason I ask is there have been occasions that people with an unlimited CDI have had these issues? It has something to do with the firing getting to a certain what they say threshold and the engine and CDI aren't working together? Does that make any sense?

I rem,ember some time back this very problem happened to a fellow, he cahnged back to the OEM CDI and it ran like a scalded ape!

I didn't see where you changed the the OEM filter? Open exhaust, more fuel, = needs more air.

JR

I changed to a stock style muffler, not a performance type, so I still run about the same back pressure. I'll post a pic later when it stops raining.

[blueboy5000]

Also Alleyoop you were correct, new carb (er well not exactly new) is set considerably leaner, after 45 mins of running on the new carb after setting mixture plug is nice light coffee color with no soot.

Previous carb was set with needle two notches lower than the current one (not by me).

The dirtbiker neighbors seem to think my plug looks good "Pretty damn close" says the Old Man.

Now I have no bogging or gas stink from exhaust, but I still have the stumble

[blueboy5000]

Lol I hope it's this belt. I'm running out of things to check

[blueboy5000]

So I found nice big cracks in the drive face, so I replaced the entire variator with a stock one. And replaced the belt with an 842-20-30 Gates powerlink.

And the bike won't break 30mph. After watching it run on the centerstand with the belt cover off, the belt flops at 5000rpm.

I am now thinking the clutch? It's especially bad when I try to climb any hill or incline, dropping to no more than 15mph.

Is there a way to run the engine without the belt and variator? I'd like to find a way to do this so I could determine if its the engine or CVT causing this.

Of my four gy6 150 bikes, only this one is doing this crap. The bike has a top speed of 15 on hills and 30 on flats.

If this was a car I would swear it was a timing problem. But there is no timing adjustment that I know of.

So, I've replaced the carb. The plug is nice light brown. I've swapped my coil and CDI with known good ones, changed the variator and belt.

What is left to check out besides clutch?

[jct842]

Is it possible the cam jumped timing by one link. They say they will run,,,,, but not well. easy to check....top dead center on piston, screw the timing mark and see the two side holes on cam are lined up with the side of the head and the big hole in gear at top. A compression check would show it too/ I do know some engines will jump timing if they are turned backwards. john

[JR]

A good suggestion John, actually had that happen to me.

JR