Topic: Engine runs rough at higher RPMs

[greddm]

Good news is that the bike is done thanks to a ton of help from this forum. It's starts and idles great, but when I hit 3rd gear and give it more throttle it starts running rough. It doesn't bog down seems more like a fuel delivery problem.

I've got pod filters (reference pic) and a pipe from Carpy with a baffle. Carbs are completely rebuilt.

 I believe I have #42 jets. Wondering if I just need to open up the mixture screws a bit?


1977 CB750

[Nic]

Need to post carb setup. Jets, needle pos etc.

[Deltarider]

 

It doesn't bog down seems more like a fuel delivery problem.

Start with the simplest. Fuel cap ventilation? Fuel tubes without kinks or inlinefilters that might hinder fuelflow?
Check spark plug gap.

[Steve_K]

I would also suggest checking plug cap resistance.  Could cause that too.  Brain fade, I can not recall what it should be,
Steve

[greddm]

I've done a bad job describing the problem. What I should have said is that I believe the fuel/air mixture is off at higher rpm.

The bike is a 77 so it has the different style carbs.

Slide needle is at the middle setting

42 main jets (I not 100% sure but as I recall I went a couple steps up from stock)

Mix screw is 1 3/4 turns out.


1977 CB750

[iron_worker]

At what throttle position is the problem the worst?

Sounds like your idle to just off idle is fine indicating your idle jet circuit is likely OK.

If the problem is main around 1/4-3/4 throttle opening then this is a needle jet issue.

If the problem occurs worst from 3/4 - WOT then it's likely a main jet sizing issue.

Note: It could need both a needle jet adjustment and different main size. I would be looking at going to a richer setting because generally the pods will make these bikes run lean. However, the best way to tell is to do plug chops at various throttle positions to determine what your AF ratios actually are.

IW

[greddm]

This is great, I'll do that. One question, what are "plug chops"?


1977 CB750

[iron_worker]

Plug Chop: http://www.mopedarmy.com/wiki/Plug_chop

There are also a couple attachments in this thread from TwoTired that are great info: http://forums.sohc4.net/index.php?topic=141721.0

IW

[iron_worker]

BTW, a #42 jet would be a pilot jet not a main jet. Stock main jet is 105 - 110 for 72-76 750s I think. Your will probably need to be larger due to the pods.

IW