High Speed Mis-fire

darwood112

New member
I would like to solicit some help. This problem is driving me crazy.

Background info:
Red ducar clone
cl2 cam set on a 5 degree offset key
10.8 springs
billet rod
carb has been blueprinted
ARC aluminum flywheel
Big exhaust pipe
87 octane gas

engine starts and idles fine. Engine revs to about 4600 with no problems. Once you pass 4600 it begins to misfire / stutter / act as though its fuel starved / out of time. It will only rev to about 5950 to 6000 max rpm. so says the tach.

However, when you close the choke a little to provide more gas / less air it begins to die, so that tells me its not starved for gas.

I've pulled the flywheel making sure the timing is right, I've changed plugs, changed coils. My next option to tear down the carb and see if there is something causing an issue in there. I was thinking to pull the side cover and double check the cam, to make sure I wasn't off a tooth or something like that. But I figured if the cam was off a tooth the thing wouldn't run right at all.

Does anyone have any other ideas or has anyone else experienced this problem. Thanks for the input.
 

oldasdirt

Member
Cam 1 tooth off probably wouldn't run. Plug gap or type ? I have a mod engine engine that wont run with a 3910X but likes the original Torch plug on alky,go figure. Sounds like it is rich when closing the choke. What size main jet and or idle jet?
 

msquiers

Member
Pull the air filter adapter off and make sure the small hole at the bottom right hand side of the carb isn't partially blocked by the spacer.
 

alvin l nunley

Site Supporter
It could be the spark plug boot is loose. I’ve seen this before. Even seen where the screw on cap on the plug was not tight.
Comments, compliments, criticisms and questions always welcome.
 

darwood112

New member
I run the 3910X plugs in all my engines. Funny thing is, I built the same exact engine that is on my kid's kart and it runs like a top. Same exact parts and pieces. I'm going to tear the carb apart and go through it tonight. If I don't find anything there I am going to change the coil / plug wire / boot assembly. Damn thing has me buffaloed. Starts / idles / revs to 4000 perfectly, then falls apart after that. These engines are so simple I'm sure it's something obvious that I'm missing.
 

darwood112

New member
The main jet is what Dyno cams runs in the carbs that you can get blueprinted when you order the base ducar engine package. So that is something I'll need to check on. Run all size tracks, small indoor, 1/8 mile outdoor, the track at Batesville. So right now the only thing that I haven't changed on the engine is the blower housing. Almost.
 

kart49

Member
iv had them bog down when its to rich. example we run a 39 when its hot out now were running a 38 I live in the same area as you. thy it. if you too rich it will bog on the top end
 

jamiespop

New member
The main jet is what Dyno cams runs in the carbs that you can get blueprinted when you order the base ducar engine package. So that is something I'll need to check on. Run all size tracks, small indoor, 1/8 mile outdoor, the track at Batesville. So right now the only thing that I haven't changed on the engine is the blower housing. Almost.
Yep, that's it....the blower housing. I had the same problem with my red Ducar and I slapped a yellow housing on it, and voila! Whoda thunkit?
 

darwood112

New member
Well, the carb is next on the list. Will definitely look at all possibilities with it, including changing the jet. Thanks for all the replies.
 

W5R

New member
Darwood, try swapping the carb from your kids motor over to yours since that motor runs fine. That would eliminate the carb issue if thats what it is. Also, you may check your fuel pump or check that the pulse line for the fuel pump is not leaking air somewhere. I had the same issue and it ended up being the pulse line leaking
 

Johnnyhustle

New member
I fought a very similar issue for 3 weeks and 3 different coils and it came down to coil gap on mine. I have it set at .050 and runs amazing if I go any less it breaks up at 5900 to 6300 like a rev limiter.
 

Fortunate Son

New member
I've mentioned in other threads about the resistor in the plug end being the culprit, but what Johnny mentioned in the post above does have merit and should be given a try. Good idea that I have never thought of. I have never had this be the problem,but I have always left ample clearance between the coil and flywheel because of the strength of the rare earth magnets in aftermarket flywheels.
 

Yakattack

Member
If all else fails try swapping flywheels from your kids motor.
Had the same issue, runs like a top, then right around 3800-4200 starts to break up and surge and not rev higher.
Ended up being a faulty FW magnet.
 

darwood112

New member
Well, I took the carb apart, as in piece by piece, apart. Main had what looked like a small ( as in tiny) piece of carbon or something in it. Idk. Anyway, reassembled, put it back on and the engine runs smooth all the way to 6890 under no load, so maybe it's "fixed". Had to adjust mixture screw to get a smooth idle and I think I need a new plug, so ill run it and see what happens. Thanks again for all the ideas. They were all seriously considered.
 

Linewalker

New member
If all else fails try swapping flywheels from your kids motor.
Had the same issue, runs like a top, then right around 3800-4200 starts to break up and surge and not rev higher.
Ended up being a faulty FW magnet.

If you get problems under load try the flywheel. I have seen many Fw magnets go. The problem you describe sounds like ignition if the coil has been changed and the problem is still there try a stock wheel or your sons.

Good luck.
 
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