Topic: 1978 CB750K performance question.

[Billy Bones]

1978 CB750. Runs on all four, but only has any real power at the low end of the throttle. Ie, I pull away from a light and it don't go fast and when I give it more throttle, I hear the engine rev more without it delivering more power.  Spark advance seized? Plugged main jets? Too low compression? Timing is right. Valves are right. Hard to start when cold and the compression is low. The two jugs I checked had 130 and 140 while the book says it wants 150-170. Haven't checked the inside jugs cause they are hard to get at.

[brewsky]

Clutch slipping?

[Terry in Australia]

Clutch slipping?

Bingo. As for your compression numbers, they're not too bad, and you didn't say if your compression test was done hot or cold? (should be done at operating temperature) You didn't mention any mods, but if you've removed the OEM airbox and replaced it with pods etc, it'll run very lean, which will make it very hard to start.

The PD carbs have more issues with blocked pilot jets than mains, does it idle well on all 4 cylinders? If not, it may be that you've got one or more blocked pilot jets. Come back to us and let us know what mods have been done, how it idles, etc etc so we can help. Cheers, Terry.

[Billy Bones]

Stock airbox. Stock pipes, but one has a big hike I need to patch. I don't feel any jerky ness that I would expect from a slippy clutch. Just no power delivery. Compression test done hot. Possible fuel delivery issue? I was having a hard time starting, and it was only starting after a million kicks with the throttle just barely cracked open. Today it started fine after I had fully opened the throttle a couple times and held it wide open to start.

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[flybox1]

assuming you have PD carbs...they need to be pulled to verify your accel pump and fast idle cam are operational.
check and clean jets/emulsion tubes.

[iron_worker]

I hear the engine rev more without it delivering more power.

This is why they are saying it's a slipping clutch. If a clutch slips it means you can watch the RPM climb but the speed does not increase. Is this the case? Or are you describing something else?

IW

[flybox1]

I hear the engine rev more without it delivering more power.

This is why they are saying it's a slipping clutch. If a clutch slips it means you can watch the RPM climb but the speed does not increase. Is this the case? Or are you describing something else?

IW

serves me right for skimming the first post.
IW is correct if your description is true. 
clutch.

[Billy Bones]

Here's hoping it's clutch. I'll hook up a cable and check. I an describing a motor that feels like it's revving higher, but not delivering power. I have just felt clutches slip before in cats and this seems too smooth. It does stand to reason though. I have another 750 earlier motor tho, I think 71. Can I just switch the plates over to check, or its it possible I just need to tidy up the current plates?

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[Billy Bones]

Riding home from work and it really does seem like the clutch is not fully disengaging. Gonna check it after supper.

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[Billy Bones]

Totally the clutch. Took off the cover and found the adjuster and she bangs!

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[Billy Bones]

Thanks everyone, I really appreciate the help. This bike was real hot by the time I got home. Hoping it was because it was just revving so high. Any other reason it would be hot? Has oil.

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