82 atc250r engine trouble.

[barnett468]

Unfortunately I can not see your photos in the format you posted them because the owner of the site doesn't like me so he blocked me from viewing photos that are posted in some formats for what he thinks is some type of punishment that is supposed to hurt my feelings or something which certainly is a big help to people like you that need help and post photos to show a problem.

If you can figure out another way to post photos, you can try that and I will tell you if I can see them.

If the head looks heavily pitted, the pits and/or any sharp edges on the pits can contribute to detonation. If it looks unmodified and you use a stock piston, the quench/squish angle between the head and piston will be factory correct, however, this does not mean it will be ideal and it might be able to be improved but the angles in stock form are good enough that they will not cause the problem you are having IF the bike is properly tuned.


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[barnett468]

Your spark plug is likely colder than it needs to be but we need the compression numbers to tell.


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[barnett468]

You could be right with this unfortunately it's all I can get over 100 octane here in nz. I tried 98 octane pump gas 24:1 full synthetic and had same problem after about 5min of riding nothing too hard and no wot for long periods.

You can not buy race gas in NZ?



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[barnett468]

I bought the kart all built the way it is so quite a few things are unknown.

Why don't you call the builder and ask him or her if they know why it keeps seizing? It's not likely that it ran fine for them and then suddenly instantly seizes on you unless all they did was build it and fire it up to make sure it ran then sold it.


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[barnett468]

I couldn't hear any detonation in the video. What time in the video do you think you hear it?

It's hard to say for certain without actually driving it but if anything, it seems to me that it is geared a little too numerically low causing the engine to work harder than it otherwise could and not be in the ideal rpm range as often as it could. Working the engine hard will generate heat.

If you never get very close to top speed in top gear, you might consider increasing the size of the axle sprocket by 2 or 3 teeth but it would help to know what the tooth count is on both drive gears first. My guess from what I can tell is that going 1 tooth smaller on the counter shaft gear would be way too much which is whay I would do a small change with the axle gear.



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[82atc250rkart]

I'm imagining that the previous owner went out to .006 because of this seizing issue. The knock is only coming along after a few laps so as the piston is getting too hot I did wonder if the knock was actually the piston starting to stick as I lose alot of power when it starts to happen going by plug and head and top of piston there is plenty of fuel in there. Is there a reason my head has comE out looking so clean has oil film but no soot or carbon

[82atc250rkart]

I was shot shifting alot that day due to mud so I may have been loading it up myself maybe I'll change it all over to the other piston and cylinder run it in and try not short shift so much I only do it because we it gets on the pipe pretty much all traction is out the window lol. What I think is detonation is about 4:30 in video I pull off briefly when I think it's about to lock then go back on youl here a more profound rattle than earlier in video

[barnett468]

going by plug and head and top of piston there is plenty of fuel in there.

You can't go solely on that . When you are uncertain about jetting, it is safest to start so rich that it burbles and then jet down from there.

The new cylinder combo will need to be jetted.

It would not be wise to run it without doing a pressure test first.

If the compression is around 190 or less, I would not use a plug as cold as 10.

Irregardless of short shifting, it still seems like the gearing may be slightly off.

You did not answer my question regarding race gas.



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[barnett468]

I was shot shifting alot that day due to mud so I may have been loading it up myself maybe I'll change it all over to the other piston and cylinder run it in and try not short shift so much I only do it because we it gets on the pipe pretty much all traction is out the window lol. What I think is detonation is about 4:30 in video I pull off briefly when I think it's about to lock then go back on youl here a more profound rattle than earlier in video

You do not want a "pipey" engine on a muddy track but if the other cylinder is unported or not ported very much, you won't have that big of a problem, however, your compression will likely be fairly low which I would correct.



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[yaegerb]

Help is abound in this thread.....

Unfortunately without you providing baseline numbers we can't magically tell you what's wrong. Don't assume that you should be running race gas, you need a UCCR test to accurately tell you what octane you should be running.

1. Reassemble your top end and as 300R already stated, use your freshly bored jug and wossner piston
2. Post assembly and as Barnett suggested, do a pressure test to confirm your seals aren't leaking and causing a lean condition.
3. Perform a UCCR on the engine to determine exactly what octane you should be running in that engine. I have a spreadsheet with directions I can send you to accurately perform this process and corresponding calculation. Without the UCCR number you are truly guessing.
4. Use an NGK B8ES gapped at .028-.031"
5. Before starting the motor take a baseline compression test

[82atc250rkart]

Ok cool thanks guys I'll reassemble using the other bits and get some figures, like I said I've just bought it the guy that owned it was an engine builder so that's probably a dangerous thing and hence why all this strange stuff has been done to it. Race gas is very hard to come by and very expensive so av gas seemed like the next best thing I'll go back to 98 if it suits the new build. The old owner seems very vague on it didn't want to tell me much and said he freshly built it rode it once. Please do send me instruction to do uccr test thanks.

[yaegerb]

Ok cool thanks guys I'll reassemble using the other bits and get some figures, like I said I've just bought it the guy that owned it was an engine builder so that's probably a dangerous thing and hence why all this strange stuff has been done to it. Race gas is very hard to come by and very expensive so av gas seemed like the next best thing I'll go back to 98 if it suits the new build. The old owner seems very vague on it didn't want to tell me much and said he freshly built it rode it once. Please do send me instruction to do uccr test thanks.

Send me your email via private message

[barnett468]

You could also fabricate some type of shroud to force more air through the fins on the cylinder head if you want . This shouldn't be absolutely "necessary" in your case, and it definitely won't cure your current big problem, however, it will lower engine temps a little which won't be a bad thing, especially when it is hot outside.

It would also add a cool "works" type look which imo is always a good thing.

From vintageklemm.com

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[barnett468]

...the guy that owned it was an engine builder... ...seems very vague on it didn't want to tell me much and said he freshly built it rode it once.

Well without being able to see the photos it's difficult to comment on the "builders" skill level, but imo, it certainly seems questionable at the very least based on the info, especially if he purposely ran .006" clearance on a new piston and bore along with the fact that he was "vague" and not fully forthcoming with info which right there suggests to me he has very little idea about performance engine modification and very little skill at assembling it.

Race gas is very hard to come by and very expensive so av gas seemed like the next best thing I'll go back to 98 if it suits the new build.

Av gas was not the sole contributing factor to your engines demise and you can run 2 strokes on av gas if properly tuned for it, but as was suggested, it is not ideal.

Your 98 octane will be sufficient for a stock or near stock engine, which also means that the head has not been heavily machined to increase the compression by a big margin.

To put it in more simple terms, you can run at least 170 psi cranking compression with your 98 octane if it is properly jetted. Do the compression test with the gas off and throttle wide open and kick it fairly hard. There are other ways to do it but in your case, this should suffice. Use a high dollar gauge. The cheap ones can be off by as much as 15%. A short hose will give you a more accurate reading than a long hose.



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[82atc250rkart]