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Detonation

Started by Alain2, June 20, 2020, 10:23:46 AM

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Hawaii-Mike

Quote from: m in sc on June 20, 2020, 04:25:26 PM
Bikes with holed pistons are slow ;)
Well said, we should make a decal with that slogan.

teazer

Our 2R9s used much larger main jets.  240 with LEDAR emulsion tubes which were basically like the OEM #284 which has that tall spray nozzle to richen up high revs.  If you are sure that jetting is OK at mid revs but lean at the top end, try smaller air jets which will tilt the fuel curve.

But your problem may be lack of chamfer on the ports or something loose.

D.e.e.

Just a thought. I had a similar problem on my 400. Pinging at 8k no matter what we did. I  ended up running race gas to confirm it wasn't something else I was hearing. Pinging went away. One day I will get the DG heads reworked and be able to run normal gas but it's running great for now, but expensive. My 2cents

Alain2

Not sure if it's related to my problem, but here the under side of the bottom rings.

Looks like Stuck cylinder material.
1973 RD350, 1977 RD400, 1979 RD400, 1980 RD400, 1985 MJ50, Goped Zenoah 30cc.

sav0r

That looks like aluminum on those rings to me. Are the bores straight?
www.chrislivengood.net - for my projects and musings.

1976RD400C

Something ain't right. You should post more pics of the pistons and rings.
'76 RD400 green  '76 RD400 red   '84 RZ350

teazer

More likely to be material from the piston - being so far in.

I came to the conclusion that our bikes are often lean in the midrange and we are unaware of it because if it makes less power than it should, we just give it a poofteenth more throttle.  I melted an RD400D (IIRC) decades ago running at part throttle.  Can't say I remember hearing it detonate but the hole in the piston and the mess in the crankcase told the story.  That one had stock pipes and inlet system - basically completely stock.

I have seen a slew of other two strokes that the owner said ran fine when running hard and then seized when they were cruising.

Alain2

The taper and side to side on the cylinders are good. There is a bit of wear front to back. I will fit new rings and go from there.
1973 RD350, 1977 RD400, 1979 RD400, 1980 RD400, 1985 MJ50, Goped Zenoah 30cc.

Alain2

I just want to give an update.

I replaced the rings, did a very light hone, re-chamfered and put a 215 main jet.

The bike is running great now. Went for a 200km ride without issue. Running around 1150f at full rpm on the biggest climb I could find.

I suspect the rings were catching in the exhaust port, there were no sign of detonation on the top of the pistons anyway.

That feeling when the bike is finally pulling hard up the revs without suspicious noise!   :celebrate:
1973 RD350, 1977 RD400, 1979 RD400, 1980 RD400, 1985 MJ50, Goped Zenoah 30cc.

quocle603

Quote from: Alain2 on July 27, 2020, 03:53:39 PM
I just want to give an update.

I replaced the rings, did a very light hone, re-chamfered and put a 215 main jet.

The bike is running great now. Went for a 200km ride without issue. Running around 1150f at full rpm on the biggest climb I could find.

I suspect the rings were catching in the exhaust port, there were no sign of detonation on the top of the pistons anyway.

That feeling when the bike is finally pulling hard up the revs without suspicious noise!   :celebrate:

Nice find. I hate it when it something that you do not expect but ultimate fix the solution. Now you know to check that area.
Do not underestimate the power of a two-stroke.

1975 Yamaha RD350 (modified), 1973 Yamaha RD350 (stock), 1971 Suzuki T500, 1981 Yamaha XS650 HS2, 1982 Honda MB5, 1980 Puch Maxi, 1979 Puch Magnum, 1993 Tomos Bullet, 2003 Malaguti Firefox F15 LC