Damaged Spark plug

[timcosby]

Originally Posted by tomrom

Look like my piston melted. Exhaust valve is gone.

that baby had been running hot for a while.

[blanc12]

Originally Posted by ChrisWNY

If it was so lean that the engine melted down, wouldn't the engine have stalled or bogged down with the choke off? Usually super-lean engines won't run unless they're choked (or they bog badly when the choke is off).

It probably lacked power at top end. He stated he was 30 miles in his commute. He could have been running wide open for a long time. I am sure this is how the bike was ridden for quite a while. When you look up the cause for a melted piston you will see lean fuel mixture is a likely cause. If it was my bike I would up the main jet to a 110 and the pilot to a 25. A little rich is much better than a little lean. It cools the valves.

[ChrisWNY]

What size pilot jet comes stock in a Mikuni VM26 carb? I haven't changed mine out but have the main jet at #120. Is the 25 pilot jet a "sweet spot" when running a #120 main jet?

[blanc12]

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

[tomrom]

Originally Posted by blanc12

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

My ride takes me from 200 ft to 1872 ft. Going home is mostly downhill. Where it failed was on the flats closer to home. Almost wide freaking open but not all the way.

Vermont ain't flat... :-)

[ChrisWNY]

Originally Posted by blanc12

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

Out here in WNY 500-600 ft. elevation is about average...it's mostly flat where I ride. I did order a #25 pilot jet (genuine Mikuni) online just in case I find that I need it. I also modified the air box, installed the performance exhaust, and jetted with a #120 main jet. Seems to run well in all throttle ranges, at least with the current hot weather we're having (mid 80s and humid daily). I rode the Hawk a few miles on the #110 (after the pipe upgrade) and it ran fine but upon inspecting the spark plug it was definitely running a little lean. Doubt it was lean enough to melt the engine down but better to err on the rich side than too lean.

I'll be avoiding those Wingsmoto main jets in the future and will stick to Mikuni brand - the Wingsmoto jets don't thread in so well.

[ben2go]

Originally Posted by blanc12

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

Please see diagram. Main jet (120) is above 3/4 throttle.

[blanc12]

Originally Posted by ben2go

Please see diagram. Main jet (120) is above 3/4 throttle.

Where do you think the fuel comes from when using the throttle valve and needle jet? I think it comes from both the pilot jet and the main jet. Both need to be sized correctly.

[ben2go]

Yes. The needle meters the flow from the main through the carb. A thinner needle or raising the needle equals more fuel from the main sooner. It is the transition from the pilot jet to the main. Yes, they both need to be sized correctly.

[Megadan]

The main jet does not play a direct role in fueling until right around the 3/4 throtte mark. You would be surprised at how big of a difference in main jet you can run and not be aware of the difference below that point. The main jet is the total fuel level that passes through the needle jet (emulsion tube), but the needle restricts flow through the needle jet as low as 1/8th throttle through 5/8 throttle, thus rendering the main jet size itself more or less inert until the passage restriction of the needle has opened up an area at least equal to the area of the main jet at the emulsion holes.

You could put a 110 and a 120 main jet in the same carb, on the same bike, and never know the diffrence until you go beyond 5/8 throttle, and definitely once you reach 3/4.

[ChrisWNY]

I noticed a big difference when going down a notch on the needle inside the Mikuni carb. Definitely made the Hawk run a lot richer, so rich at all throttle positions (combined with a #120 main jet).

[blanc12]

Originally Posted by Megadan

The main jet does not play a direct role in fueling until right around the 3/4 throtte mark. You would be surprised at how big of a difference in main jet you can run and not be aware of the difference below that point. The main jet is the total fuel level that passes through the needle jet (emulsion tube), but the needle restricts flow through the needle jet as low as 1/8th throttle through 5/8 throttle, thus rendering the main jet size itself more or less inert until the passage restriction of the needle has opened up an area at least equal to the area of the main jet at the emulsion holes.

You could put a 110 and a 120 main jet in the same carb, on the same bike, and never know the diffrence until you go beyond 5/8 throttle, and definitely once you reach 3/4.

Thanks for the info Dan.

Although you don't feel the difference, fuel is drawn through the main jet as the needle valve is raised. That is what I was getting at.

[tomrom]

Originally Posted by tomrom

Yes, I sent Don at apscusa.com an email last week. He called me on the phone and promised to cover all the parts. He asked that I open it up first and take some pictures. I emailed him yesterday evening so I hope to hear back on Monday. Don says they have parts at 8 different locations around the country.
He also told be he would work with a professional shop for the parts If I want to bring it someplace for repair.

I did call a shop in my area. http://www.procyclevt.com/. I was politely told that it was not personal to me but they refuse to work on a Chinese, or any 'off brand' bike. He mentioned an bad customer experience but I assume there is some animosity considering he charges thousands more for used bikes.

In actuality, this only took a few minutes to tear down and seeing that I won't need to split the case, it's going to be quick to repair.

7/26/2018
A week has passed since my bike melted down and 4 days since sending pics to APSUSA.com and no parts or engine have have been sent out yet. I have spoken to both Don and Greg and both have told me they waiting to hear back from their supplier and are trying to figure out what caused it before shipping me anything. With the thought that if they simply send me replacement parts, I could end up with the same problem down the road. I have requested a new engine because this one is full of metal flake and chunks of melted piston and an exhaust because I don't know how much metal chunk flew into it and is lodged in it. It's the general consensus here and with Don that it was running lean, but why?
Hopefully this can get resolved by tomorrow and a replacement engine will be on the way.

[tomrom]

Originally Posted by tomrom

7/26/2018
A week has passed since my bike melted down and 4 days since sending pics to APSUSA.com and no parts or engine have have been sent out yet. I have spoken to both Don and Greg and both have told me they waiting to hear back from their supplier and are trying to figure out what caused it before shipping me anything. With the thought that if they simply send me replacement parts, I could end up with the same problem down the road. I have requested a new engine because this one is full of metal flake and chunks of melted piston and an exhaust because I don't know how much metal chunk flew into it and is lodged in it. It's the general consensus here and with Don that it was running lean, but why?
Hopefully this can get resolved by tomorrow and a replacement engine will be on the way.

7/26/2018 update
Received a call from Mr. Don with instructions to pack the engine up in a box and he will issue a pick up to ship it back to the supplier. They will either rebuild or replace the engine with a turn around time including shipping of about a week. This is pretty good news I think. Don has some good southern charm and words of experience that makes me feel like he is taking care of me.

[Megadan]

Originally Posted by blanc12

Thanks for the info Dan.

Although you don't feel the difference, fuel is drawn through the main jet as the needle valve is raised. That is what I was getting at.

It does, but since the needle restricts that flow, the size of the main jet orifice itself is a very small factor. That was what I was getting at.

OP it is nice that they are actually working with you to fix the problem. Hopefully the process goes smoothly.