My bike is 4 stroking like crazy. I have tried almost everything to fix it. Changed plug gap, changed oil mixture, changed the jet and needle to the leanest possible (60 jet is the smallest that I have), while it worked for a while it started doing it again (probably because it was colder that night than now), I changed the timing with an offset woodruff key, I switched CDIs, but nothing. One thing that stands out to me is that I got it working pretty well and then it started 4 stroking a lot when I tightened down the air filter tighter than before I might loosen it a little or find a less restrictive one but I ran without a filter and it still 4 stroked. Here is the spark plug after a few WOT pulls with 4 stroking. It looks rich to me and the engine stays really cold even during hard pulls. It usually stays between 120-130 degrees celsius and after hard pulls 140-150. Highest I've ever seen was 160 but it took a lot of WOT pulls. I think either the head gasket or the exhaust gasket is leaking as well. I wonder if that could reduce effective exhaust scavenging or something. I did some pulls with WOT and the spark plug was very dark at the ceramic and the band was kinda grayish. There is also one very noticeable (but hard to see with the angle of the picture) very dark spot on the ceramic portion next to where the spark occurs. Any help would be greatly appreciated. Thanks.
4 stroking after many attmepts to fix it.
- Thread starter fdnjj6
- Start date
Well it looks to me like my cylinder head is warped. It is terrible with 4 stroking now. I can't get it to go past 20. I guess I'll sand it down and see what happens. I was able to feel the air slip through the warped part and on my homemade copper gasket I could see that the area that's leaking isn't being clamped down enough. Earlier I was even able to see oil bubbling through the little slit that was formed. I'll update if it works in case anyone cares and has the same problem as me.
I sand every head I buy, I have found several that were not true. Be sure to torque the head in a x pattern to 12 ft lbs, don't over tighten it or it will warp again.
Can over tightening warp a head? I'm 99% sure I have over tightened it. Thank you for your advice.
This is a common problem, try to sand the head on a known flat surface. Sandpaper taped to a thick piece of glass will do it.
You can produce 40 to 50 foot pounds of torque with a standard 3/8 drive ratchet, yes overtightening will warp the head.
Yep I believe it. I sanded it down and the gasket doesn't leak anymore. It still 4 stroked like crazy. I have an aftermarket exhaust that hit the frame and didn't make a good seal at the point where the exhaudt met the cylinder. I got the bike to work and hit a new time top speed after downjetting with the seal working just fine. Then the seal must have gone downhill from there and the 4 stroking got worse than ever. I put the stock exhaust with the baffle cap off and it made a good seal and went faster than it would before. My plan is to make an extension pipe and put the expansion chamber back on. That should work the best. I'll keep updating and hopefully it'll work.
Ok just another update. I found out my exhaust had a small hoke and since it was still hitting the frame it probably didn't make a good seal. Today I fabricated a mounting plate and jb welded (I sadly don't have a welder) it to a pipe. I also cleaned up the very misaligned mounting plate on the exhaust in order to almost port match it. I plugged the hole with jb weld. Everything is still drying and I will jb weld the exhaust to the extension piece I made together tomorrow. I also discovered that the piston still has about 1-3 mm of it still sticking out from the exhaust port. It'll make more sense from the picture. I'm pretty sure that that's not normal. I'll do some research and once I find out that it is wrong I'll raise the ports with spacers. I'll lose some compression but I can always sand down/get a new cylinder head for more compression. If anyone knows if what you can see in the picture isn't supposed to happen, please let me know. In the picture the piston is at bottom dead center.
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If it were mine I wouldn't worry about the port timing right now, if you can solve the four stroking it may not make a difference.
Is this engine broken in? Have you run at least two full tanks of fuel through her? You changed fuel mix but what ratio is it now running? Are you using premium fuel? Are you living at low altitude or high altitude? You've tried every thing but not jetting under 60? Buy a 50 & 55 jet. Have you changed float clip height? Does the bike four stroke or 4 stroke less with air cleaner removed? Have you tried different, high quality NGK plug & heat range? Stock CDI's (kit and replacements), including wiring are real iffy components new and fresh out of the box. Much you don't seem to have checked to this point or haven't shared the details with us. Like Greg said not to worry with port timing though it can be changed with base gaskets if you like, but remember to change back to stock if the 4 stroke problem isn't corrected, cause power will likely suffer.
Piston crown stickup at bottom of stroke above the exhaust port is normal with stock port timing. Some of us have "ramped" the edge of the piston to flow smoother into the port, but overdone can cause top ring problem...plenty of piston material needs to be above the ring for support. Ramping is not something I'd suggest doing to your engine without supporting power modifications.
I actually prefer a bit of 4 stroking at low & narrow rpm band as it indicates adequate oil in the mix in an otherwise good running engine & disappears as rpm builds, but with good throttle response on demand.
Rick C.
Piston crown stickup at bottom of stroke above the exhaust port is normal with stock port timing. Some of us have "ramped" the edge of the piston to flow smoother into the port, but overdone can cause top ring problem...plenty of piston material needs to be above the ring for support. Ramping is not something I'd suggest doing to your engine without supporting power modifications.
I actually prefer a bit of 4 stroking at low & narrow rpm band as it indicates adequate oil in the mix in an otherwise good running engine & disappears as rpm builds, but with good throttle response on demand.
Rick C.
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I got the bike to stop 4 stroking. Ran a 60 jet and drilled a large hole in the nt carb air filter housing. Also the engine has to have been running for a few minutes with some medium pulls in order to get warm enough to stop the 4 stroking.
Ok so here is the info:
Engine has 630 miles
Used to run 4oz per gallon
Now run 6 oz to gallon due to much leaner jet and better air filter keaning it out further.
91 octane (I know it could be lower but 91 is what we have at my house. Also doesn't higher octane burn slightly slower than lower octane? Could that be contributing to the 4 stroking?)
Altitude is 3,202 ft
Bike did 4 stroke with air filter removed initially with bigger jet.
Now the bike has stopped 4 stroking with a 60 jet and a large hole in the air filter housing.
Taking off the baffle cap of the stock exhaust helped with power. The other expansion chamber pipe extension failed because I do not have a welder and my attempt with jb weld and bolts didn't work. Got too hot and also cracked. The pipe itself won't fit.
Spark plug: NGK b6hs (7534)
Average temperature under normal casual riding: 125 C
Temperature after hard acclerating and long top speed runs: 150- 160 C (I don't remember seeing 160 before but I'm sure that by the time I pull out my thermometer and slow down the temperature already dropped.)
Very rough exhaust port timing( porting to better dimensions should help with 4 stroking right? I don't think I'll try it yet unless I want more power later on)
Port opens ~98 degrees from TDC
Port duration ~158 degrees
Port closes ~256 degrees TDC
Port rough diameter ~20mm already relatively smooth
Haven't measured transfer port timing yet so I don't know blowdown.
Intake also hasn't been measured yet.
Crankcase is stuffed from factory.
Running NT carburetor.
80cc chinese spec.
Bore 47mm I believe stroke is 38mm.
That's basically all the info I got.
I was thinking about using the base spacers to have the piston clear the exhaust port completely but I would probably change port timing and compression too much for it to fit my needs. I like fast acceleration and a decent top speed which is how it's running right now. Top speed is about 35 mph without 4 stroking and it gets from 10-30 mph pretty quickly.
Thanks for everyone's help and great advice. I think the bike is running pretty good right now.
Ok so here is the info:
Engine has 630 miles
Used to run 4oz per gallon
Now run 6 oz to gallon due to much leaner jet and better air filter keaning it out further.
91 octane (I know it could be lower but 91 is what we have at my house. Also doesn't higher octane burn slightly slower than lower octane? Could that be contributing to the 4 stroking?)
Altitude is 3,202 ft
Bike did 4 stroke with air filter removed initially with bigger jet.
Now the bike has stopped 4 stroking with a 60 jet and a large hole in the air filter housing.
Taking off the baffle cap of the stock exhaust helped with power. The other expansion chamber pipe extension failed because I do not have a welder and my attempt with jb weld and bolts didn't work. Got too hot and also cracked. The pipe itself won't fit.
Spark plug: NGK b6hs (7534)
Average temperature under normal casual riding: 125 C
Temperature after hard acclerating and long top speed runs: 150- 160 C (I don't remember seeing 160 before but I'm sure that by the time I pull out my thermometer and slow down the temperature already dropped.)
Very rough exhaust port timing( porting to better dimensions should help with 4 stroking right? I don't think I'll try it yet unless I want more power later on)
Port opens ~98 degrees from TDC
Port duration ~158 degrees
Port closes ~256 degrees TDC
Port rough diameter ~20mm already relatively smooth
Haven't measured transfer port timing yet so I don't know blowdown.
Intake also hasn't been measured yet.
Crankcase is stuffed from factory.
Running NT carburetor.
80cc chinese spec.
Bore 47mm I believe stroke is 38mm.
That's basically all the info I got.
I was thinking about using the base spacers to have the piston clear the exhaust port completely but I would probably change port timing and compression too much for it to fit my needs. I like fast acceleration and a decent top speed which is how it's running right now. Top speed is about 35 mph without 4 stroking and it gets from 10-30 mph pretty quickly.
Thanks for everyone's help and great advice. I think the bike is running pretty good right now.
If it's fairly close I'd keep riding, my main bike four strokes until it gets to full temperature. It does help here in Georgia to keep the engine cooler, leaner is faster at a price.
Pulling good and running 35mph is quite good for a kit stocker running 44 tooth rear which is pretty standard gearing with the kits. I should think this would please most.
Rick C.
Rick C.
