O.k. I'm looking for possible solutions if I can't get this problem worked out. I've got the Trail Bikes 88 light bore kit and it knocks at low rpm when under load. I know the jetting is as perfect as it can get with the stock carb. I've tried all the different plugs from NGK6-NGK8. I put two ounces of STP octane booster in the tank full of gas. I'm not too sure the STP is all that great of an octane booster. Today I am going to drain the fuel tank and start fresh using the super 104+ octane booster. I think this is a better product. The other octane booster I've been thinking about trying is made by Klotz. I'll post, if I can get rid of the knock with octane booster. The pump gas I'm using is 92 (so it says on the pump). I've got the stock head,cam,carb,exhaust-slighty opened up. The bike runs awsome--rips like you wouldn't belive. I've been told the TB 88 kit runs 13.1 compression. I would like to find a solution so I don't have to go hunting for race gas. Unfortunatly octane boosters are running $7-$10 for a 16oz. bottle. With what is already avaliable how could I lower the compression slighty (without completely killing the performance) to stop the knock? Would an aftermarket head help? What about running two head gaskets? I can ride it as it is but I know its life will be short lived. I was thinking of a bigger cam but I think that will actually increase compression if anything. I was just wondering if anyone had any good ideas. Like I said today I'm going to experiment with different amounts of octane booster. Thanks!

 

race gas man. get it at an airport. its cheaper there. DO NOT run 2 head gaskets. is ohio the only place in the damn world that has 94 octane at the pump??

 

Fab, whats the name of the gas stations you find 94 octane at?

 

round my parts we get 94 at Sunoco gas station

 

highest we get is 92

 

do not run av gas it is oxygenated for high altitudes
put 11:1 piston in it
its usually lean on the bottom if your using a stock carb
you can drill out the pilot to the right size or use a 70 carb
i dont think octane is the problemo
or you could just put the stk airbox back on it
you can find alot of info about this posted by xidium on his tech post
https://www.stuntlife.com/forums/174-fiddy-tech-help-how/68933-tech.html

 

octane is the problem. its happened to about 5 friends' bikes that i installed these kits on. all have 20mm carbs. 94 octane is found at sunoco here also. i run av gas in all my toys with zero problems. its 100 low lead BP fuel.

 

Highest i've ever seen here is 93

 

You can get 112 leaded from a pump here. One nice thing bout living near a race track. 118 is also avalible but not from pump.

 

nice.. if I head north a little bit towards Moroso ya can get that stuff

 

well today I spent a few hours experimenting with octane booster. I put one ounce at a time with a full tank of gas. I used the super 104+ octane booster (black bottle). Each time I'd shake the bike to mix up the fuel and run it for awhile around the yard. I went up to 8 ounces in a full tank of gas. This was about half the bottle. The bottle says it treats 20 gal. so I figured this was as good as its going to get. Anyway it cured about 80% of the knock. I was amazed that the bike felt more powerful and alot more responsive. My next step is to seek out a higher octane pump gas and run that with an octane booster and see if I can't get rid of the knock completely.

 

Actually they lable it differently because the pump gas isnt exact. They octane can be off badly sometimes. Usually it ranges 5 octane I think. That why in the olden days MX races would run AV fuel, it was cheap and it was more accurate.

 

read the tech info that xidium posted on octane and fuel its a very good read
av gas wont hurt your motor but if you read whats posted it aint recomended either
the more octane you add the richer it gets that is why your knock is going away your to lean on bottom drill the pilot or go up a carb size to a 70
your gonna end up spending 4 bux a gallon for gas when real race gas is only 3 bux a gallon
but its your 4 bux

 

100 octane av gas is $1.99 a gallon at my airport