Stock maxi high end issue

Jeff Woodin /

Bike is stock except for Dellorto SHA 14.12 carb, currently with a 58 jet. Bike runs well overall except has started having this issue lately - the bike will get up to 25 just fine but then kinda struggles around 28-30 and there an intermittent buzzing/grinding noise (hard to explain) seemingly coming from the top end of the engine.

Just tried fully cleaning the carb and not much difference. Exhaust is reasonably clean I think. The bike used to get over 30 pretty easily and wasn't making this noise. Like I said it only does it as it approaches max speed, everything else is pretty normal. Any ideas?

Re: Stock maxi high end issue

aaaaaaah, take the top end off?

Re: Stock maxi high end issue

Cleats Onionpockets /

Agreed, take the top end off and look for signs that metal is hitting metal. Could be that you have a spark plug that is a tad too long, and the piston crown is whacking it at high speed (rough theory). Could be you have a piston ring that's cracked and acting funny at high speed, could be you have a scuff on the piston wall.

Re: Stock maxi high end issue

Jeff Woodin /

Does the gasket have to be replaced if the head is removed? I'd prefer to avoid removing the head for now because I ride the bike to work everyday and want to keep it up and running.

This morning I noticed that the bike will accelerate better and is more likely to get up to it's top range (30+) if I only hold the throttle slightly open - like 1/4ish. When I open the throttle all the way it's more likely to struggle and hesitate and make the noise. Could this be from the exhaust not being open enough and creating back pressure when the throttle is full opened?

Re: Stock maxi high end issue

if you back off the throttle at wot and you get a slight increase in speed you're running a little lean...go fatter on your main jet...lean on a 2 stroke is asking for a sieze

Re: Stock maxi high end issue

Cleats Onionpockets /

You won't have to replace the head gasket, but the base gasket would need to be changed out if you take the whole jug off. Just taking the head off won't hurt it. But yeah, I'm inclined to agree with Matt...it sounds like you've been running lean and on the edge of a seize with that sound. If it started doing that completely on its own, then it's possible that you have an intake gasket that's leaking. Or, that the filter element in your airbox has come undone and isn't restricting the flow enough.

To be safe, I'd bump it up to a 60 or 62 jet if you can't nail down where the extra air is getting into the system.

Re: Stock maxi high end issue

Jeff Woodin /

I did try bumping it back up to a 60 jet, where I had it originally. The top end improved somewhat, back up to 30+. The noise still occurs when it gets up to the top end, which is now higher. I guess ideally I should try a 62 jet and see what happens with that. I don't have one yet, though.

Regarding the air filter - it's a Dellorto SHA and all it currently has is a double mesh screen thing. Doesn't seem like that would restrict the air that much. I should probably put on a proper filter, huh?

Re: Stock maxi high end issue

try all the screens in it and see if it runs better...if it does then you're going in the right direction...meaning you need to get more fuel flowin'....put in the bigger jet when you find one, then you can remove a couple screens from the filter

Re: Stock maxi high end issue

Why switch from a 12mm Bing to a 12mm Dellorto? There are more options for tuning on a bing and they are the same damn size.

Re: Stock maxi high end issue

Jeff Woodin /

The Dellorto was on the bike when I got it. It's strange, like I said I put the 60 back in and the bike was running much better. Then the next day it was running crappy again, all kinds of acceleration issues and even a couple misfires I think. I can't understand why it would be reasonably fine one day and then crappy the next.

It seems like there must be some unknown variable at this point that is causing intermittent issues - maybe something electrical? I did take the head off and the cylinder looked smooth and unmarred. Is there anyone in the Boston area that might want to help a noob out and take a look at it with me?

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account