400EX spark knock problem

[Blown 331]

My cousin bought a 1999 440EX and the owner said it was fine on 93 octane. My cousin put 93 in it and is spark knocked so he started running 110 but it was too late and blew the head gasket and cracked the piston (JE). .

He got it back together with a 11.2 to 1 Powroll kit. At this time he ported the head him self and put on a flat slide carb from a CRF450. It didn't run like it should, he tried swapping carbs back and that didn't really help. He was running 110 in that but put 93 in it when he ran out of gas and blew that head gasket too! He put a stock head on it thinking his port job messed it up but that didn't help and he blew another gasket. Well the bike is sitting apart and we're not sure what to do with it. He wants to run pump gas but it has the worst spark I've ever heard. I'm talking in first gear you give it half throttle and all you hear is spark knock, it's crazy!
He bought another 440 cylinder off ebay for next time around. He also has the biggest cam web cam makes and his valves have been making slight contact with the pistons, both set ups. He thinks that cam is preventing it from making power like it should, too big. But that cam is still no where near the size of the HRC cam in my 450R, so I'm not so sure that's it.

I think it should run on 93 octane with 11.2 to 1 compression. He has tried 2 heads to I don't think he has a milled one. We also check to see if it had an advanced flywheel key, nope. Any ideas why this thing spark knocks so bad? He is thinking about running 2 head gaskets and like 6 base gaskets. lol. Looking for some imput here.

[OldSchoolin86]

I may not have the solution here but I'm going to put my 2 cents in.

1) It's very possible to get pre-detonation in an 11.2:1 air cooled motor on 93 octane. If it was liquid cooled it wouldn't even be a concern but since heat is the cause of pre-detonation and not compression I wouldn't rule it out. Severe nock is a bit confusing but I would expect some especially in a powroll kit. Most 400ex piston kits are based off the XR400 and when installed in a 400ex they produce less then advertised. Take a wiseco 11:1 piston kit. When you install a wiseco 11:1 in a stock cylinder you’re only going to get 9.5:1. Wiseco assumes a deck high of 0 when really it's closer to being .040" to long. Because of this most people that know of this problem will clearance the deck of the cylinder down to get a more accurate compression ratio. Actually it's still a hair less then 11:1. The problem with this is that when someone else goes to install a piston specifically made just for the 400ex it's going to be making way too much compression and most likely hitting things. As common as this is, I wouldn't doubt seeing it on two different cylinders.

2) The 400ex cam should look smaller then a nice 450r cam. A FULL race 400ex cam has about the same intake lift as a STOCK 450R. The valves hitting the piston are a bit troublesome unless there is a deck height problem. Has your friend contacted webcam yet to see if they have had any problems like this? They may know issues with certain pistons.

3) The blow head gaskets make me think of a couple things. One I already addressed in my first idea. The other is movement. Did he have the heavy duty stud kit installed on this engine? A lot of stock 400ex studs are pulling due to extra compression. A slight movement with the studs will make you loose your head gaskets because it's like running the motor without out tightening the head on properly. Installation would seem normal and torque properly but then move less then the eye would notice and allow the head gasket to blow out.

Just some ideas, good luck and keep us posted.