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Base Gasket Squish


clav3
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New stator in and no difference.

I ran a little test with a timing light though. I now have a TDC mark and 3.5 mm BTDC mark on the case.(the 3.5 mark is where the factory mark on the flywheel lines up with using either stator, at about 1/2 throttle on map 2, which is the more advanced map.)

I held the revs at exactly where the noise just begins and flicked the mapping switch back and forth from map 1 to 2 consecutively.

The timing light showed the noise started at about 3mm BTDC on map 2, then switching to map 1 the timing light showed about 2mm BTDC.

So held at the same revs the timing jumps back and forth but the noise remains stable at just audible.

This seems to prove that the noise is not timing related, but rev related.

I was thinking maybe the CDI was at fault but after doing the above test I think it would be pointless.

It seems to occur at the same revs wheather free reving, under load or riding down hill with the clutch out.

Probably going to try another piston or the old one.

Dunno.

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Seems by most reports by(us) mere mortals, none can tell a performance difference between the two ignition maps. Your info seems to confirm their is an actual change, yet I believe most change would be at really high revs, and logic dictates they prolly taper off the advance a bit earlier to gain revs.

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Today I checked the timing on my mates 2012 300 Cabestany and seemed to show that while the mapping is different on the bikes, my bikes timing seems to be okay.

This also confirmed that sometimes the switch is the opposite way round since the timing light shows my map 2 to be more advanced and map 1 on the Cab to be more advanced.

My mapping shows a 7 to 9 mm difference (measured on the case) at idle and only about 3mm diference at higher revs and nearly no difference when screaming.

(I checked the timing when the bike was only about a year old,and running fine, and this has not changed.)

While my mates Cab shows no change at idle but at higher revs, a big difference and the more you rev the bigger the difference.

This test also shows the at higher revs these bikes can run sweet at a variety of timing settings as long as they advance up to at least 3.5mm BTDC at higher revs.

Oh well, tomorrow I'll pull the cylinder off again to see if I can sought out this knock.

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  • 2 weeks later...

I sent my cylinder to Victoria to Electrosil to be re plated and sized to new piston after trying many different ideas such as different carb (Keihin), new reeds, and original lower compression head. Also another plating company giving the cylinder a light hone to straighten slightly, none of this worked.

It only took 3 days to get my cylinder back from Electrosil. Put it all back together and its all fixed. No noise except the purr of a very tight and quiet sherco engine.

The cylinder only had 0.4 thou variation from bottom to top and even with piston tolerance of 1.1 thou at the tightest point, somehow the variation was enough to cause a very bad piston rattle at mid to high revs.

No one thought that this was really going to fix it, but at even more expense, it was the right move.

I hope this thread can help some other poor bugger, with his head nearly completely done in one day

Thanks all.

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Hi there, Have read your post and I'm guessing the noise is piston slap, If you have a worn bore or the piston you replaced is not up to spec it will happen. Get a proper 2 stroke engine builder to check it or stick the old piston back in to see how it runs, Might cost you a base gasket but will tell for sure if the noise didn't happen before the rebuild..

I've ordered pistons before, size a, b etc as stated on the old one and found the new one to be out and needed the bore matching to the piston.

The problem with bike manufactures is they use different companies, usualy the cheapest at the time like gas gas, They now only supply S3 pistons as they use S3 barrels and pistons now as standard. You can't get a standard piston unless you can source old stock. The S3 pistons in the standard barrel are not always spot on but this is only an example from my experience.

It's probably a piston problem, Ive recieved pistons from S3 and they don't even give proper size circlips for the gugeon pins.

Glad to hear you got it sorted now bud, Have fun.

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Yes, well good to know! Perhaps I was totally off, based upon your initial post which stated you had done all but the conrod. Figured you refreshed the bore. Obviously not.

I can tell these things are critical, fact is on my "12 bike it is a rattler(bought used) at idle. Not much to show on the bore and new piston dod not help. Cyl will come off for replate soon!

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Yes, well good to know! Perhaps I was totally off, based upon your initial post which stated you had done all but the conrod. Figured you refreshed the bore. Obviously not.

I can tell these things are critical, fact is on my "12 bike it is a rattler(bought used) at idle. Not much to show on the bore and new piston dod not help. Cyl will come off for replate soon!

Before going to the expense of replating the cylinder Cope, check the clutch damper rubbers as when they go hard/shrink you end up with a rattle that is most noticeable at idle

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