Topic: Need Help Jetting 836 CB750

[low-side]

     Okay, here's what I have.  I have a CB750K1 (at least that's what the top end is off of) with an 836 overbore, Megacycle 63b cam, Kerker 4 into 1 with an 8 disc Supertrapp spark arrestor, and K&N pod filters.  I'm running stock idle jets (#40s), needle set 1 richer, and a 135 main jet.  1st through 3rd gear, you can run up to redline at full throttle with no issues.  In fourth gear at full throttle, you have to back off and feather it around 6000rpm and get through a rough patch where it seems to misfire so bad as to lose engine and road speed while getting very hot and can then resume full throttle around 7300rpm.  In fifth gear it can't get through the rough patch and you are limited to about 5/8 throttle and 90mph.
     I've tried main jets ranging from 125s through 150s; under 135 it gets too hot, over it runs too rich.  I've tried different needle settings and 1 richer gives the right results.  Drilling the air bleed for the needle/main jet circuit from 100 to 120 helped a great deal, but didn't fix the problem.  My dad has a similiar setup (including the pods) and is doing fine with the same needle and main jet sizes as I'm using.  Has anyone else had the same problem?  Any ideas?  This is driving me nuts and at this point I just don't know what to do with it.

[ev0lution7]

sounds like your advancer unit might have some issues try looking at that! check your points.. or your timming for your electronic ignigtion

[low-side]

Thanks, appreciate the response.  Advancer moves freely, the stopper tabs aren't bent so it's not over-advancing.  The problem isn't as bad 5 degrees advanced, a little worse at normal timing, and horrible with the timing retarded. 

[low-side]

Anyone else have any other ideas?  Anyone else run into this problem?

[ev0lution7]

if you say your points/ advancer is ok how is your charging system? or check your coils... check other outside factors besides jetting sounds like something is holding you back from burning your mixture... when you add MORE power you need more powerful spark to overcome the combustion maybe your dad has the same setup but his parts of the charging system or coils or wires or spark caps are in better shape b4 you jet always check to see WHY you need more jetting also check your float hight you know.... basics...

Spark and fuel and air... are all you motor needs to function check those first! in that order

[HondaMan]

Need more info:
1. Sparkplug type and number.
2. Coils: stock or aftermarket?
3. Ignition: points or electronic (if the latter, what kind)? If points, when were the condensors last changed, what is the condition of the points?
4. What is the battery voltage?

This type of situation can be something other that jetting issues.

[low-side]

Float height is set at 25mm.  Coils are 5 ohm Dynas triggered by freshly cleaned and gapped points.  Charging system works great, no issues with battery.  Normally it's at 12.6V ignition off, system reads 14.2V running at 2000 rpm or so.  Plugs are D8EA.  I might give D7EA a shot and see if it burns the fuel better but I really don't expect it to help.  Thanks for the replies.

[HondaMan]

A couple of things I'd try:
Change the stack on the Supertrap. Go down 2 discs, then drop the needle one notch from where it is, and go to 130 or 135 mainjet.
Drill the emulsifier tubes (in the mainjet holders): the cross-holes (larger ones) should be about .033" (I think the stock hole is .025", .1mm). Going larger than .039" has no effect. Start by drillin gout only the larger ones, if it helps, but not enough, then drill the smaller ones, too.

Once you get this arrangement cleaned up, you can inch your way up, at both ends. You're suffering from the 'on pipe' portion of the tune, where the mix goes from lean to rich, suddenly, because of the cam. One other thing that may help, if this spot never gets smoothed out the way you'd like: advance the cam. About 5 degrees would be a good start: this will broaden the 'start point' of the on-pipe engagement and ease the tuning of the lean-rich transistion. It will also improve the low-end torque.

BTW, the stock airbox, with its built-in velocity stacks, will also smooth out the transition point. Those pods are introducing one lean-rich step (at about 5000 RPM) all by themselves, the pipe another (wherever its band starts), and the cam a (big) third. Together, you're hitting the wall.

[Ben D]

did you ever get this straightened out? i am having a very similar issue..