200x spark knock with 10.25 piston

[big specht]

I rebuilt a 200x and put in a 10.25 piston, that I thought would be ok on pump gas this thing spark knocks bad when under a load ! I tryed playing with the timing but it didn't help it just made it run worse. I haven't tryed racing fuel yet! But is it common for these 200x's to be this way
? Every thing else we have done with a 10.25 piston has been fine?

[leevarnado]

sure its not a 12 to 1 piston

[yaegerb]

Shouldn't knock at 10.25. 11 and 12 it will. Try a little race fuel and see what happens

[dmfaulks85]

Are you sure your head gasket isn't leaking? That can make a slight knocking noise.

[big specht]

I should of double checked the numbers on the piston, but I ordered a 10.25 piston. I havnt tried any racing fuel yet

[oscarmayer]

for me when i get that it means a few things. here is my list of things to start with.
1- confirm everything has fresh fuel (super unleaded only. if running 12:1 as long as you properly "prepped" it, it should be fine.)
2- check valve lash
3- check to ensure carb is clean and no plugged jets. if you have not done so, always rebuild carb with the engine no mater what and replace everything you can (especially jets)
4- check to ensure your using a good spark plug
5- did you properly let the engine seat the rings? new engines require specific break in procedures. you need to ensure your following those properly for your piston or you will cause a problem.
6- carb/jetting ne3eds to be checked and validated. start with idle and startup, then move to mid and finally fine tune main jets for top end.

this is my basic list of new engines and spark knock issues. hope it helps.
#1 is VERY important and the prepping of the piston can make or break an engine.
if your not sure how to prep a piston,then do a Google search and you will find the answers. but pistons are never bolt and go unless the vendor sells them pre-prepped. (which wiseco does not)

good luck!

[big specht]

I have been thru the carb twice and rechecked valve clearance and double checked cam timing and advancer timing to. It did it with the old plug in too it has a new ngk in it now. I will have to try and stop and get some racing fuel to try in it. But it's going to in the low20's most of this week. That might make a differance in the detonation

[Dave8338]

What was the reason for the piston change? If it was to freshen the top end, often times the old valves will have carbon deposits either on them or is and around the seats.
Did you rebuild the head? Thickness of the head gasket get thinner on the new top end? Several reasons for pre-detonation.

One other thought... if the timing chain has stretched over time (they all do) you may be off a little on the valve timing, even though everything is "set correctly".

[oscarmayer]

^ good point, if he did not do a valve job during the rebuild a valve could be not properly sealing.

[NeverLift]

How about a compression test for starters. I believe when you get over 190 PSI you need to start bumping up the octane(Correct me if I'm wrong).

[Dave8338]

How about a compression test for starters. I believe when you get over 190 PSI you need to start bumping up the octane(Correct me if I'm wrong).

There is no "specific" chart referring to CR vs. Octane, that I know of. There are just too many factors that can lead to the problem. Could be something in a weak CDI department, sharp edges on the cylinder head crown, from a blown piston, valve hitting a piston that just "sounds" like a ping, Just don't know.

[big specht]

What do you guys think of aviation gas? It higher octane. As for the head I went thru it to its all good and the spark knock only happens when under a load, pulling a hill in a higher gear. As for possible cdi probley I can see that I will have to try and switch it. Does anybody have a a/c voltage for the ingnition coil I know they can ohm good but brake down under rpm?

[oscarmayer]

That sounds like jetting then. try going up 2 jet sizes on the main and see what happens. also se the needle on the center e-clip and start from there.
the mixture screw should be 1 1/4 - 1 1/2 turns out only. any less can cause problems, anymore can cause problems. once in a very rare moon I see a machine that needs 2 turns, but that is rare.

[yamasaki760]

aviation gas should be 100LL

Thats a lot better than pump 90/91/92

[pcs]

what is pump gas in your area? 91/93 octane? also pump gas recomendations for people that live in other areas might not be valid bc pump gas is not the same everywhere. most(not all) pump gas in the USA is actually blended with ethanol or is e10 fuel. if you read the pump label it might say this fuel is oxygenated and may contain up to 10% ethanol, the exact amount is not the same and even in a given location it varies based on seasons. i would say dont be cheap and try race fuel, no sense in spending money on a rebuild only to risk damaging your engine bc you wanted to be cheap on fuel. try a 50:50 blend of 100 octane with 91/93 and replicate your riding conditions.
other things to consider is the load you are putting on the engine, the inline of the hill, your weight, gear ratio, tire diameter, and grip such as paddle tires.

[big specht]

Ok I'm going to double check the jetting and probley try the av gas anyways it's 5.75 at the air port. It's going to be in the low teens to high 20 around here this week so I will see what it does

[big specht]

Well I got some aviation gas and tryed it. It is 18deg out tonight but I heard no spark knock, all I did was dump the pump gas out and add the aviation gas, I'm going to retry it with reg gas again this week some time to see if it was the cold or gas that made it stop spark nocking