Author Topic: Engine runs rough at higher RPMs  (Read 792 times)

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Offline greddm

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Engine runs rough at higher RPMs
« on: September 30, 2014, 10:24:47 PM »
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

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Re: Engine runs rough at higher RPMs
« Reply #1 on: September 30, 2014, 11:03:05 PM »
Need to post carb setup. Jets, needle pos etc.

Offline Deltarider

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Re: Engine runs rough at higher RPMs
« Reply #2 on: September 30, 2014, 11:27:14 PM »
 
Quote
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.
« Last Edit: September 30, 2014, 11:29:01 PM by Deltarider »
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Offline Steve_K

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Re: Engine runs rough at higher RPMs
« Reply #3 on: October 01, 2014, 06:34:21 AM »
I would also suggest checking plug cap resistance.  Could cause that too.  Brain fade, I can not recall what it should be,
Steve
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Offline greddm

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Re: Engine runs rough at higher RPMs
« Reply #4 on: October 01, 2014, 08:28:03 AM »
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

Offline iron_worker

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Re: Engine runs rough at higher RPMs
« Reply #5 on: October 01, 2014, 09:40:54 AM »
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

Offline greddm

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Re: Engine runs rough at higher RPMs
« Reply #6 on: October 01, 2014, 09:45:06 AM »
This is great, I'll do that. One question, what are "plug chops"?


1977 CB750

Offline iron_worker

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Re: Engine runs rough at higher RPMs
« Reply #7 on: October 01, 2014, 09:51:25 AM »
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

Offline iron_worker

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Re: Engine runs rough at higher RPMs
« Reply #8 on: October 01, 2014, 09:52:53 AM »
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