Topic: CB750 wont rev properly - with video!

[infinitas]

Hi all!

I´ve recently bought a CB750 to build a cafe racer. The problem is that it wont rev as it should and I hope that you guys can come with some valuable advice on what to do.

As you can see in the video, the engine "blurrs" and wont rev past 3000-4000 rpms when you give it full gas. If I give it gas by slowly twisting the gas roll it manages to increase rpms past 4000, but as soon as I give it full throttle it "blurrs". At idle and between idle and 3000 rpms it runs great.

The carbs have recently been taken in pieces and washed in an ultra sound washer at a shop, so they should be clean. Have not been synced though. Ive checked that it runs on all four cylinders. The spark plugs looks fine.

The air box is not properly mounted, but is that enough to cause this problem? Any tips or advice on what to do?

Video:

http://youtu.be/XYTJuSLeAZk

Thanks in advance!! 

[LesterPiglet]

The carbs have recently been taken in pieces and washed in an ultra sound washer at a shop, so they should be clean. And Ive checked that it runs on all four cylinders. The spark plugs looks fine.

The air box is not properly mounted, but is that enough to cause this problem?

Don't believe everything(anything?) a shop tells you.
Fix the airbox.

[Viktor.J]

Hejja Sverige Välkommen till forumet !

What kind of cb750 is it ? DOHC cb750 ?
There's plenty of guys here with a lot of more expertiece here them me, but have you ruled out that it doesn't run whey rich ? how does the plugs look ? 

[infinitas]

The carbs have recently been taken in pieces and washed in an ultra sound washer at a shop, so they should be clean. And Ive checked that it runs on all four cylinders. The spark plugs looks fine.

The air box is not properly mounted, but is that enough to cause this problem?

Don't believe everything(anything?) a shop tells you.
Fix the airbox.

Yea, I will! Thanks

Hejja Sverige Välkommen till forumet !

What kind of cb750 is it ? DOHC cb750 ?
There's plenty of guys here with a lot of more expertiece here them me, but have you ruled out that it doesn't run whey rich ? how does the plugs look ? 

Haha, trevligt med fler svenskar! Tack!

Oh, I forgot to mention that its a CB750F 1982, DOHC. Have hardly ruled out anything, hehe, but the spark plugs looks quite fine. Not wet nor white. And because the air box is not installed the engine should get too much air = running lean. But it feels strange that it runs this bad only because the air box is not installed...

Thanks for answering!

[lucky]

Think of the air box as a giant choke just as if you were holding your hand over the openings. It richens the mixture. It is part of the equation of 14 parts air and one part gas.

[dave500]

you cant just snap the throttle open on these,theyll gulp too much air and bog or flat spot.