Topic: Cycle X 849 kit spark knock FIXED

[banzaibob]

I know that many of you have put your lives on hold waiting to hear the outcome of this story.  Since May I have been battling this CB750 with the 849cc Cycle X kit.  It has had a nasty spark knock in the lower RPM's under heavy load.  I have had several other projects interfere with getting this engine buttoned up but in the last week I've pretty much solved the death-rattle. 

I drilled out the pilot jets incrementally until I have right now, the equivalent of #45 pilot jets (.45mm).  The engine can be lugged without worry of melting down.  It still can be made to knock very lightly but it is barely perceptible.  I may even go slightly larger (.47mm).  One of the problem I have though is that the air screws right now are turned out 3 1/2 turns on some of the carbs.  I am going to have to shim the springs so they don't rattle out.  So far, the low end performance and idle appears to be smooth as silk.  Anyone care to venture how far those screws can come out before there's a problem? 

I have #125 mains in it now.  When pulling to redline in say 3rd or 4th gear (90mph or so) it feels slightly rough as if leaning it out one main jet size would make it a tad more smooth and aggressive.  I have not done a full throttle high speed plug chop yet but probably will tomorrow. 

All of this good running engine stuff has unfortunately brought to my attention to other problems on the bike such as a wicked fork twist when applying the front brake forcefully.  The forks on there right now are one's I had laying around from a CB750A.  Everything is torqued to specs, no weirdness otherwise.  Coming down hard from 80mph twists the forks ±5º hard to the right.  Applying the rear brake amplifies the problem too.  I haven't had time to check out alignment yet, another project for tomorrow.  Any ideas other than that?     

[74750k4]

I would check carefully the rear wheel to frame alignment, and don't trust the frame marks to be correct. Use a caliper and check bolt lengths in there, and see what you get. I take it the front end is tight, no looseness.

I am going to be building an engine using the CycleX 849 coated pistons. What cam did you use? I am running a Kenny Harmon D grind on stock pistons, and it is totally unacceptable as a street cam. Will not idle. 5000 RPM to over 10K it's very happy. I am thinking with the 849 kit though, compression will be high enough at idle to use that cam without a problem, and no knocking, because the cam has such high overlap.

[iron_worker]

Assuming you're running the stock carbs for your bike ... you have the air screws turned out fully lean but yet you are countering that by going to a larger pilot jet? What gives?

Unless I'm missing something it sounds like you probably could have stuck with the stock pilot jet and just turned the air screws in?

IW