Engine running rich, tried everything twice and no luck!

[rudyauction509]

Got a 66cc engine kit used on craigslist about a week ago. Engine had less than 1hr use on it. Mounted everything to mountain bike frame and have out about 600 miles on it. However, the whole time it runs really rich. Spark plug black and wet every time and starts 4-stroking at about 25mph. Needle is lowered all the way, tried 2 carbs, lowered gas level in float bowl, tried new throttle cable, new intake, new spark plug and cable, new fuel line, muffler end cap on or off, checked and re-checked jets, float, etc. and it always runs rich. With muffler cap on and/or needle not in lowest position it barely gets to 15mph before 4-stroking.

I've built dozens of these kits and my friend has been a small engine mechanic for 30 years and we still can't figure this out. I found an old intake gasket and cut a 1/4" section of it out and installed it, other than a high idle the engine ran great.

Anyone have any ideas? I'd prefer not to need an air leak just to get to top speed.

Also I've switched carbs, spark plug, etc with my friends motor and they all work fine on his. The only difference in our engines is he has a slant head and I have a flat head.

 

[crassius]

clogged muffler, gasket leaking at head or case or seals maybe

 

[rudyauction509]

Mufflers clear (also just tried another muffler from my friends engine), head gasket fine and wouldn't it run lean if there was a case gasket leak? Also it ran better when I made the intake gasket leak. I'll spray starter fluid on the case while idling just to make sure.

Lowered the fuel level in the bowl a bit, seemed to run well for a minute but started 4-stroking again about a block later. It does seem to get a bit faster though.

 

[rudyauction509]

No leaks, double checked cylinder gasket.

Trying a tiny hole in the intake, hopefully it'll work. If not I can plug it with a bit of glue.

 

[rudyauction509]

The tiny hole got me up a couple mph, I'll try a second tomorrow. This whole thing is just odd. Normally I run lean because of vacuum leaks and stuff, never seen this before.

Also, I removed the air filter completely, no difference.

 

[2door]

Sounds about normal when using factory jet size. There are aftermarket jets available but many of us prefer to solder and re-drill our jets. There is no reason you'd have to introduce more air through the intake (drilling holes). That isn't the way to lean out the carburetor.


You didn't say what fuel/oil ratio you're using. The kit instructions will tell you 16:1 which most of us agree is too much oil. It can cause symptoms that can be mistaken for 'too rich'.

Tom

 

[crassius]

you're not running rich because of too much fuel
- carb is OK on other bike
- muffler is OK on other bike

that means you;re getting right amount of fuel in & getting exhaust out properly

fuel could maybe blow past rings and come back from cases next stroke making it seem rich

head, cases, seals could leak & just cause a misfire that will bog above about 1/2 revs

the way to check for head/case leaks is to clean it very well - then run it quite a while - then wipe with paper towel to find residue around head or case seams

checking seal leaks can be hard - first, pry up on crank end with a screwdriver to see if mains are loose - next, pull bevel gear and rotor, clean well around that area, replace rotor & gear & run a while, then check with paper towel again

 

[fatdaddy]

I know I've said this AT LEAST a hundred times, LISTEN to 2DOOR. The stock jets are almost always too big. You tried everything BUT a smaller jet size. So what'cha got to lose. Jets are only a few bucks each. OR solder and drill yer own.
fatdaddy.

 

[maniac57]

Jetting on stock china motors is ALWAYS blubbery rich since the factories don't care HOW the engine runs, as long as it runs longer than the warranty period. This is also why they say "16:1" on the oil ratio.
drop the main jet size and it will wake up.

 

[rudyauction509]

I'll pick up a micro drill bit set today and drill the jet. Thanks.

 

[pulsar2121]

I hate to be captain obvious here, but have you tried turning the set screw out or in on the carburetor?

 

[Chainreaction]

Maybe plug is black from 4 stroking, erratic spark?

 

[maniac57]

I'll pick up a micro drill bit set today and drill the jet. Thanks.

Drilling the jet will make it WORSE!
Rich condition requires a SMALLER main jet.
You have to solder the jet closed BEFORE you drill it out to a SMALLER size.
Please do some research before you mess things up.

 

[2door]

Ditto:
You'll need to find the drill bit size that just fits your jet orifice then solder the jet closed and re-drill it to a size or two smaller than the original. It might take some experimentation to find where you need to be. The jet needs to be smaller, not larger.
A larger jet will increase the amount of fuel to air and only make your situation worse.

Tom

 

[crassius]

still concerned that 4-stroking due to richness isn't the problem - why would a carb that runs fine on one engine run so rich it limits to 1/2 revs on another engine?

 

[fatdaddy]

I'll pick up a micro drill bit set today and drill the jet. Thanks.

First, ya gotta solder it closed, Start small, then drill larger if needed. easier than resoldering every time ya wanna check out a jet size.
fatdaddy.

 

[rudyauction509]

Sorry I forgot the word solder. Soldered the jet, drilled it out one size at a time, now its almost half its original size, still 4stroking. Will swap CDI and magneto later tonight.

 

[rudyauction509]

Also plug still black after run at 1/2 throttle, no 4stroking at that speed.

 

[fatdaddy]

I hate to be captain obvious here, but have you tried turning the set screw out or in on the carburetor?

The set screw on these carbs is just the idle screw. it has nothing to do with mixture. All it does is raise or lower the slide to set the idle.
fatdaddy.

 

[rudyauction509]

I did mention that I've been working with these engines for about 8 years now right? If this was caused by something obvious I'd have figured it out.

I can barely get it to run at all with muffler cap on. 4-stroking at 10mph. so something other than oversized jets is going on. Piston and cylinder have no scratches and no sigh of blow by, compression can lock up rear wheel, remove spark plug and engine spins freely, no change in RPMs when starter fluid is sprayed on bearings, crank case and lower cylinder gasket, head gasket shows no signs of leak, carb now runs extremely lean on my friends engine and still rich on mine.

Tried about 6 different spark plug gaps from .018 to .040 no difference, tried new magneto and CDI, no difference, tried about 4 different spark plugs, no difference, swapped cylinder head with my friends engine, no difference, flushed crank case with carb cleaner and drained completely, no difference, removed choke and air filter and swapped intakes again, no difference.

What am I missing!?! Could port timing do this?

If I turn fuel off, it runs great for about 2 seconds before running out of fuel.