87 125m won't run right

[3>4]

I recently bought an 87 125m. It will start and idle fine but when you rev it up it cuts out and starts smoking. The plug fouls. it does not smoke when you start it. What could it be?

[atc007]

Sell it to me and your worries are over :)...Sounds like a carb problem. Carefully tear her apart and blow out with compressed air. paying close attention to all the air/drain/breather ports.

[3>4]

I cleaned the carb.

[Dave8338]

When you say that you cleaned it, what all did you do?

A can of carb spray and remove the bowl?

A complete tear-down, soak, and reassemble?

Somewhere in the middle?

[3>4]

I tore it apart cleaned everything and blew air through all of the holes.

[Dave8338]

Sounds like it is flooding out at anything above an idle?
Where have you got your adjustment screws set?

[3>4]

It runs good past idle. Then it hits this spot and won't go anymore.

[atc007]

I would reclean it,checking jets are both correct sizes. Spray carb cleaner through EVERY single hole there is to verify it is coming out somewhere,proving that passage IS open. After you have cleaned it again with preferrably 170+ PSI of air. If all jets ,needle,and parts are correct and in their correct places. You're looking at ignition. CDI or weak spark. I think that is Not your case though/

[Dave8338]

Oh... got it. That makes a bit more sense to this old brain. You mention that it fouls the plug and smokes. Is the smoke white or blueish black?

When it "hits the spot", does it just fall flat on its face, as in wants to die? Pop, sputter, etc.? What color is the plug when you pull it? Black or White?

When it gets to the point it wont "go anymore", what happens if you apply a little choke, while NOT falling off the bike?

[3>4]

The smoke is blackish. It wants to die. The plug is black. I don't ride it like that.

[atc007]

Dry black= gas,wet black = oil

[3>4]

It's wet. How could oil be getting in there?

[sweetip2000]

look inside gas tank and check for rust, remove fuel lines and check for clogs, try to rev it with gas cap off tank make sure cap breathes, make sure air filter is clean, make sure petcock (fuel valve) flows....if still a problem then you have to take the jets out of the carb, check to see if you can see through the pin hole in the jet, see if you can blow through it in your mouth,
sounds to me like you got that alcohol varnish gummed up, use fresh gas and add this stuff called startron in your fuel and make sure you run the motor every week, alcohol in gasoline screws up a carb big time,,,,

[3>4]

I don't think it's a fuel issue.

[Dave8338]

Did the bike run correctly when you bought it? If not and you are trying to sort out a few issues, I would start with a compression test to verify the top end, to some extent.

If the plug is wet, black, and fouled when you ride at mid throttle or above, either you have an issue of too large a main jet, the float isn't doing its' job, OR the engine is worn beyond limits. Weak coil as mentioned, can cause popping and stall issues under a good load when warmed up. A highly worn cam chain or malfunctioning cam chain tensioner can also cause valve timing issues at higher speeds.

It is likely something simple so I always start at a spotlessly clean, well assembled/set carb and new plug. From there at least two of the potential issues are completely ruled out. I would re look the carb, completely. Do you have a manual for that bike/carb?

[3>4]

I don't have a manual

[Dave8338]

Here is a link to your Honda Service manual. Print out the parts you need.
http://campaigns.hondampe.com.au/Motorcycles/manuals/atv/own85atc125m.pdf

Start at page 41 in the manual.

Here is an exploded diagram of your carb and below the picture, is a complete list of the internal parts.
http://www.bikebandit.com/oem-parts/1987-honda-atc125m/o/m1877#sch6727

[3>4]

So if it needs the top end done that would cause it to do that?

[Dave8338]

It "could". There are several factors at work, here. The first thing you need to do is establish a check list of potential problems.

I've seen some pretty loose engines that still run amazingly well BUT, everything else was in check.

You have Three sections that you need to cover, thoroughly:
1) fuel / air
2) spark
3) compression

If all of those check out correctly, it will run.

The 185 engine in my little trike is SHOT, SHOT, SHOT. As soon as it warms just the slightest, I can leave a trail of blue smoke for 1/2 mile. Louder than a bag of hammers in a paint shaker, and leaks from EVERY seal. Just add oil. Carb is spot on and ignition components are new. Still runs strong...

[3>4]

I think it needs rebuilt. When it warms up it starts smoking.

[3>4]

I tore the top end down. The piston and rings look fine. When I cranked it over I could hear air getting out. I think it might be a bad gasket or something.

[Dave8338]

Are there any signs of carbon build up or "burnt" spot on the head gasket? It is nearly impossible to visually inspect a piston and rings, unless it hatched in the jug. You need to measure your piston to wall clearance, ring end gap, and the ring groove, for wear.