Two stroke knocking problems
Two stroke knocking problems
(OP)
I have a Velocette 250cc flat twin two stroke.
As nothing was available for the engine, we made the following modifications:
Wiseco Pistons
Nikasil steel cylinders
Modernized combustion chambers.
We just finished rebuilding it and started jetting.
The bike was very over-jetted and we brought it into line.
We are having a knocking problem. We have to severely over-jet the bike to get the knocking to go away. Like smoke screen over-jetting.
The original compression ratio was 7.5:1. Our current compression is 11:1
The knocking is heat related, but we don't see evidence of detonation on the plugs. Plug color is good, but rich.
We are running AMSOIL AT 32:1, which is less than the manufacturer's 20:1.
The bike does have smallish cooling fins:
<<http://pic asaweb.goo gle.com/od dssodds/Vi ceroyScott #543865613 4821149778>>
Here is the engine design notes:
<<http://picasaweb.google.com/oddssodds/JerryNotes#>>
Any ideas?
Thanks,
Paul
As nothing was available for the engine, we made the following modifications:
Wiseco Pistons
Nikasil steel cylinders
Modernized combustion chambers.
We just finished rebuilding it and started jetting.
The bike was very over-jetted and we brought it into line.
We are having a knocking problem. We have to severely over-jet the bike to get the knocking to go away. Like smoke screen over-jetting.
The original compression ratio was 7.5:1. Our current compression is 11:1
The knocking is heat related, but we don't see evidence of detonation on the plugs. Plug color is good, but rich.
We are running AMSOIL AT 32:1, which is less than the manufacturer's 20:1.
The bike does have smallish cooling fins:
<<http://pic
Here is the engine design notes:
<<http://picasaweb.google.com/oddssodds/JerryNotes#>>
Any ideas?
Thanks,
Paul





RE: Two stroke knocking problems
To low octane.
Inadequate cooling.
Hot spot in chamber.
To much piston to bore clearance.
To much crank end float.
Insufficient piston to head clearance.
To much dudgeon pin clearance.
Regards
Pat
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RE: Two stroke knocking problems
RE: Two stroke knocking problems
RE: Two stroke knocking problems
@patprimer
To much ignition advance.
To low octane.
Inadequate cooling. DON'T THINK SO, TEMP ON CYLINDERS IS 252F
Hot spot in chamber. DON'T KNOW
To much piston to bore clearance. NO
To much crank end float. NO
Insufficient piston to head clearance. DON'T THINK SO, LOOK AT THE DESIGN SPECS I POSTED
To much dudgeon pin clearance. NO
RE: Two stroke knocking problems
To much ignition advance. NO, ALMOST NONE
To low octane. NO
RE: Two stroke knocking problems
What kind of seal is used in the crankcase to separate/isolate the cylnders?
Under what operating conditions and throttle position does the knocking occur?
What were you looking for on the plugs as evidence of detonation?
Have the heads been off lately?
What do the undersides of the piston crowns look like? Dark brown varnish, or flaking char, or ??
That's a high compression pressure on a modern (?) air cooled 2-stroke, although the exhaust timing is pretty short. I think it would need an effective squish or quench type combustion chamber to help with detonation. Around 0.030 inch piston to head clearance. What is the current piston-to-head clearance? Also ventilation under the piston via holes would keep things cooler and reduce stagnation.
Head temp with under spark - plug washer sensor is a useful measurement. Perhaps more important than any cylinder measurement. The cylinder by the ex port is a lot hotter than over the transfer ports, and the mixture gets compressed prior to and during firing in a doorway in a neighborhood across town anyhow.
If the exhaust system is restrictive it will cause overheating and detonation. Can happen By design, or if a screen type spark arrestor plugs up with carbon, or .... ..............
Does it use fixed ignition timing?
What was the stock ignition timing?
What is the ignition timing now? ( close to 20 BTDC? )
I'd look at the spark plugs to evaluate timing based on this info - http://www.strappe.com/plugs.html
I think some folks are concerned that the knocking is possibly mechanical rather than combustion related.
RE: Two stroke knocking problems
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Two stroke knocking problems
What model is that?
RE: Two stroke knocking problems
sometimes if a copper or alluminium gasket is reused several times then it thins out and protrudes causing a hot edge . just my 2 cents
cheers malbeare
RE: Two stroke knocking problems
Please keep us informed on your progress.
RE: Two stroke knocking problems
http://en.wikipedia.org/wiki/Velocette
Benta.
RE: Two stroke knocking problems
RE: Two stroke knocking problems
Sorry to say, I think you should revisit a lower compression ratio, whilst preferably maintaining a squish area. Maybe also try a twin spark plug setup.
RE: Two stroke knocking problems
RE: Two stroke knocking problems
RE: Two stroke knocking problems
That gave me knocking in that cylinder. We are still getting some light knocking, so we are going to drop the compression a little more.
P.
RE: Two stroke knocking problems
Was the knocking detonation, or mechanical contact?
Depending on the other equipment especially complexity of the exhaust system on the engine "raising" the exhaust port would lower the effective compression ratio. For the last few decades 27 degrees of "blowdown" between exhaust and transfer opening has been a pretty desireable number, even on relatively low revving 400 cc CZ MX engine (whose exhasit timing was pretty close to 90 degrees B/ATDC. The first page of notes suggests you have less than 10 degrees. A later page (4) suggests the cylinder was shortened 0.092", yet the transfer timing remained the same ( different piston?) and the ex port was raised about 5 degrees.
RE: Two stroke knocking problems
Sounds like a fun project, best of luck!