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  #1  
Old 08-24-2008, 03:30 AM
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Default Top End Rattle

So with problem 1 out of the way (the sticking throttle) I figured I was in the clear. Running my bike in for the first 2 lots of 10 minutes went sweet. But today, my third ride with the fresh top end, I take it past half throttle and at the top of 4th and 5th I noticed some top end rattle. The first time I figured since I have a new front tire it was just rocks sticking and getting thrown at the pipe, but it did it again not long after going up a tar road.
Reving it in neutral does nothing, and 1st and 2nd it wont show, but defiantly at the top of 4th and 5th I get this metal on metal rattle. If it was every gear then I'd be happy to say it was something wrong, but it's running awesome up until WOT. I only just recently adjusted my Power valve so it shouldn't be there (02 CR250 is cable) , but what else could it be when it only shows up there?
I also had the barrel ported and milled to boost flow and compression.
Any idea's would be helpful. I have another set of rings and another top end gasket kit, so I think my best option really is to re strip it down and inspect.


Thanks again for anyone that takes the time to read.

Brad
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Old 08-25-2008, 12:12 AM
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Default Re: Top End Rattle

Sounds like classic sound of detonation. When under a heavy load is the most likely time for detonation, ie 4th and 5th gear.
Likely cause is that your running too lean on the main. Other causes could be too low of an octane fuel, engine running hot due to inadequate cooling system, ignition timing running too far advanced,or poor/ineficcient squish space at top dead center.
On a 250 the squish should be around .050" or 1.2 to 1.3 mm.

Start with the easy ones and work toward the harder fixes if the easy ones don't help.

For explaination on squish do a search here or over at thumpertalk. Lots of good info on it.
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Old 08-25-2008, 04:48 AM
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Default Re: Top End Rattle

Thanks AV8R

Under load is definitely the key word, as the first two times I noticed it was pinning up two different hills, and they were about 15 minutes apart. I didn't notice it once in between these hills.
As I mentioned the barrel and head were recently sent here to be ported and milled to boost flow and compression.
This was my first ride above half throttle since having these mods done. Could he may have shaved too much and have not left me enough squish space after decking the head? If so, what can I do about this?
I can check the squish, but even after reading flat out for the last two hours, I'm still not sure what to do with the findings?
The bike was fine before this, with the only other mods being that my JD needles were bent, so I have gone back to the stock needle (39mm Keihin PWK). From memory JD's kit got me to lean the pilot by 2 sizes and take the main to a 172 which I think is one richer?
Bike always gets fresh 98 oct fuel with motol 800 and a splash of octane booster.
Being a fresh build the coolant is new and full.
I'll do the .125 solder trick tomorrow and see what it tells me.

Thanks again


Brad
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Old 08-25-2008, 09:47 AM
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Default Re: Top End Rattle

When shaving the head if the dome is not relieved then compression will increase. If it increases enough then detonation will occur. There are two sure fixes to this. One, as I mentioned, is to relieve the dome, which entails taking metal out of the dome of the head. The other is to run a higher octane fuel.
Running a higher octane fuel these days gets quite expensive over time. And unless you believe that your are maximizing the current potential of the bike then that higher compression and race fuel is getting you little or nothing at an significantly increased cost.

A couple of tricks that you can try are;
Put in an extra head gasket. But it baisically negates the adjustment to the squish band that you just had fixed.

Another is to put copper washer(s) on the spark plug which will recess the plug farther into the head and create more space in the dome dropping your compression slightly. I'm not sure what effect this will have on flame propagation, but worth a try.

You could also try retarding the timing a little at a time until you start to notice a loss in bottom end performance. Once the bottom begins to suffer noticably you've reached the pratical limit of retarding the timing.

You could try one colder heat range spark plug. Put in a 9BREG instead of an 8BREG. or whatever you are using. Each heat range is equal to a change of 100 degrees or so in the combustion chamber as I recall.

Here is some good info on plugs and adnormal combustion occurances.

Tech Info - Spark Plugs Overview
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Old 08-26-2008, 08:01 AM
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Default Re: Top End Rattle

Spoke with the guy that done the mod's to the cylinder today and his instinct was also to go one richer on the main. And low and behold, with it currently having a 172 in it, i believe the next size up is a 175 and I actually happen to have one. So I'll get onto that tomorrow.
He also gave me a small washer to drop under the needle to raise it by a half step. If these two don't work I am going to try the washer under the plug, then the colder plug, and last would be the second head gasket.
As always I will post results as I get things done.
I think the standard beer I would normally owe to AV8R is quickly turning into a carton for the help he has sent my way in the last couple of weeks. The day you head to AUSY land mate I'll pay that debt
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Old 08-26-2008, 09:40 AM
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Default Re: Top End Rattle

I'm surprised that he reccomended putting a washer under the needle as raising it will lean the needle.
One other thing about those JD needles, they can be fixed by you. Just put them in the drill (chuck them on the end where the clip goes) and rotate them slowly observing in which direction they are bent and at what point then straighten them with you fingers just a little at a time until they rotate fairly straight. They don't have to be perfectly straight, pretty close will do straighter is better of course. You'll notice even when installed in the slide the needle has some side to side movement available to it.

Quote:
The day you head to AUSY land mate I'll pay that debt
None required. Glad to help.
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Old 08-31-2008, 07:46 PM
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Default Re: Top End Rattle

The bend that was in the needle was ever so slight as it was. And the consequences that came from it were just far to great for me to run that risk again so that will defiantly be my last option if all else fails.
I'm still not overly confident jetting my bike as I have only been doing it myself for the past 6 months, but from what I have read wouldn't putting a small washer under the needle be the same as going DOWN half a clip, as it would be holding the actual needle up half a step, letting more fuel through and in turn richening the mix?
The main was a 172 and I bumped it up to my 175 on the weekend but the problem was still there again under load at the top of 4th and 5th (occasionally third too I think). I'll get the next two sizes up through the week and see what happens.

Thanks again

Brad

Last edited by B_Rad; 08-31-2008 at 07:54 PM.
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Old 09-01-2008, 05:25 PM
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Default Re: Top End Rattle

Quote:
wouldn't putting a small washer under the needle be the same as going DOWN half a clip, as it would be holding the actual needle up half a step, letting more fuel through and in turn richening the mix?
Yes, Your right , sorry I must have been a little rummy from lack of sleep at the time.
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Old 09-21-2008, 02:21 AM
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Default

Finally got another trail ride in today with the 180 main and it was still there, and I feel like I am actually a little down on top end power now? Not too sure but thats what I thought.
Also I noticed my kickstart lever is REAL close to my pipe and is actually showing some rub marks on it, so before I go any further I am going to put a bit of hose over the top of the lever and go for a stab. I'll be so embarrassed if thats what was causing the noise.
It's really starting to bug me.
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