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[ace.cafe]

One thing is mentioned several times is a loss of compression.. A compression test would put that unknown to rest.

Piston slap making the noise?


  Would carburetor problems  in any way effect compression? I'm talking about a loss of compression due to mechanical problems. I know running lean could burn valves and running rich can cause carbon buildup that can stick valves in the long run, but when I think of loss of compression I tend to think of it as a problem with valves, rings, head gaskets.

Well any of those things may be possible.
I'm just looking at the easiest solutions first.
If they don't pan out, we look deeper.

[Andy Cocktails]

HI Guys.

I know of a few bullets around the same area as me, and they dont have any trouble running the stock pilot jet - their owners are as confused as me about the sound....

I would assume that the problem would be consistant (or at least a little more so) if that was the issue, not just when hot and recently started???

As far as piston slab goes - the slug in there at present has less than 500kms on it and it is a nice snug fit, i would also think that piston lap would occur more when cold and just started, rather than hot...

If i had to guess anything, i would be leaning twards the condition of the valves, or maybe a cracked head. But i am only 21 with limited mechanical knowledge.

[chumma7]

did you check for bent pushrods?

[Blltrdr]

You mention you replaced the stock piston with a JP piston from Australia. What were the specs Tim gave you for the piston? Is it a high compression ratio piston and forged aluminum? You may have some clearance issues, piston-bore, ring gap, etc.. Any possibility of the piston interfering with valves? Ace is directing you in the right manner by starting with the easy stuff first but giving him some idea of how you went about installing your new piston may shine some light on your issues. I'm sure it's something you can sort out soon.

[Andy Cocktails]

Hey guys, thanks for you responces.

The piston has the same compression as stock, the only difference really is the construction of the oil ring slits... as alot of us Aussies can't get more than 10000kms out of the OEM slug without the crown separating from the skirt.

The reason i am so quick to rule out the piston though is due to the fact that the noise was present with the OEM as well.

I have had a good look at the pushrods, they were fine, but i replaced them with a coupla spares anyway, just to make sure.

[ace.cafe]

Well, regarding the valves, I was operating under the information that you had new seats and guides installed, so obviously there is a fresh valve job on the engine, and the valves were replaced or deemed in good working order and ground-in to the new seats.

So, I rather doubt that you've burnt a valve in a week on a fresh valve job.
This is why I'm not focusing on valves.
If there is something else we need to know about your valves which leads you to suspect them. please tell us.

Also, if your pals running the stock pilot jet, are not running an unbaffled open silencer and exhaust, they may be able to run a stock pilot jet, whereas your conditions could be quite different, and need richer.
I run the exact same exhaust setup as you have, on my bike, and I'm at nearly 1000 feet elevation above sea level, and I need to have a 27.5 pilot jet to avoid backfiring on deceleration.
And sea level conditions would require richer.
Do you understand my line of thinking there?

Now, your perceived compression loss may not have anything to do with the jetting, but there may be 2 issues that are happening concurrently, instead of just one issue.
See?
And if we can eliminate the backfiring, perhaps it might be easier to then isolate the other issue, if there is one.

That's where I'm coming from.
I'm trying to diagnose a problem by remote control, from 9000 miles away.

[Andy Cocktails]

Firstly, thankyou for your help in this matter guys, this forum is a bit more 'lively' than the aussie one.

I did have new valves seats and guides put in a week ago, but my valve concerns come from the valves themselves, and the springs. At some point there has been a rock or a nut or something hard bangin about in there, as the top of the OEM piston was 'decorated' interestingly, and there wasnt any kind of airfilter on the machine when it came into my possession.... there is a small chip out of the inlet valve accordingly, whilst i have poured gas in the chamber upside-down to see if it was leaking, and it didnt seem to be, i cant be certain.

i have a mate down the road running my exact pipe and jetting setup with no issues. I also ran the bike for a while with the long stock muffler on it, and the noise was still there.

[ace.cafe]

Are you saying that damaged valves were installed onto new seats and guides?


I'll accept your decision that the jetting is okay for your bike.
I'm not sure how, but if it's okay, then we'll move on.

[Andy Cocktails]

yes, i asked the machine shop doing the work to assess the condition of the valves (they have only 5000 mile on 'em) first though.

maybe they didnt look though?

[ace.cafe]

yes, i asked the machine shop doing the work to assess the condition of the valves (they have only 5000 mile on 'em) first though.

maybe they didnt look though?

Can you get hold of a compression tester, and do a compression test?
If so, do that, and tell us what the readings were.
Test with the throttle wide open.

[Andy Cocktails]

Will do ace, i plan on popping down to my local mechanic in the morning and doing a few tests (as well as his opinion on the sound)

will report with results.

[Andy Cocktails]

HAvent had a chance to do anything with a mechanic yet.

I did have one thought... after a turn the key off, it runs (or seems to) for a rev or two whilst backfiring. Could my plug be glowing for some reason causeing these backfires? if so, should i go hotter or colder (i'm not sure whats in it right now)?

[UncleErnie]

Does it do that if you just run it for less than a minute, or just when it's hot?

[ace.cafe]

HAvent had a chance to do anything with a mechanic yet.

I did have one thought... after a turn the key off, it runs (or seems to) for a rev or two whilst backfiring. Could my plug be glowing for some reason causeing these backfires? if so, should i go hotter or colder (i'm not sure whats in it right now)?

Yes, there could be a hot spot in the chamber somewhere, and it might be the plug, or it might be the chipped spot on that valve.

The proper plug is a BR8ES, or a B8ES from NGK.
If you have the proper plug in there, it won't be getting hot enough to cause a problem like that.
But a sharp edge of a chipped valve might.