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Several High RPM symptoms

hje_326

CB750 Enthusiast
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Crawfordsville, IN
Lately my bike has been between 4k and 5k at idle, which is annoying but I think I just need to play with the idle a little more, but what really has me concerned is this.

If I accelerate too hard (or at all sometimes) my tach will jump almost 1000 RPMs with gradual acceleration/ power gain, if it gains at all. There have also been sometimes where it will climb between 6k and 8k (I run around 5k going 65-70) run high for a little bit and then 'catch' and start to accelerate again but no where near where it should in that range.
 
Lately my bike has been between 4k and 5k at idle, which is annoying

What's really annoying is trying to guess what bike we're talking about... not all of us know you well enough to know what you're talking about.

If we are talking about an SOHC CB750 (or most other bikes for that matter), then something is terribly wrong. My 78 CB750K will run 75mph in 5th gear at 5K rpm. Idle setting for the SOHC CB750 (and many other bikes as well) should be 1000 to 1200 rpm.

Sounds like a vacuum leak (or other carb maladies) and a slipping clutch. If you are consistently launching at 4k to 5k (or higher) then you have probably already fragged the friction plates in the clutch. Friction plates are cheap and easy to replace. Carb problems on the other hand can be more difficult to correct.
 
I guess a little elaboration would help. 'My bike' is a '77 CB750K.

Because almost everything on this bike is at least 30 years old I would not be surprised about the friction plates. I have had problems with the carbs before and was trying to get through the simmer so I could send them off to oldskoolcarbs in the winter.
 
Honestly, I have no idea what it would feel like if the clutch were slipping. There always seems to be a distinction between the clutch engaged and disengaged. What are the most common signs?
 
You pretty much described a slipping clutch with your "catch" comment. You should notice it most when the engine is under a load such as when starting from a dead stop, especially with a hard acceleration. If you were to try starting off in second gear, for instance, you'd put more load on the engine and maybe cause more noticeable slipping.

If you can't get the feel for the slippage, the tach and speedometer can give you a visual reference. Again, you actually described it pretty well... running in a higher gear on the highway 65-70 mph at 5k, then the tach showing an increase in rpm but no speed change. Could be you came to a slight hill or even higher wind, and the clutch starting slipping. Hill flattens out or wind drops, and the clutch "catches" as you put it, and things feel right again.

Has the oil been changed recently? Using the right oil? Clutches don't always work well with the wrong oil, and slip. Using synthetic oil by chance?

As for the high idle and possible vacuum leak... are your carb insulators (boots between the carbs and head) hard? They get hard with age, and if you've had the carbs off, might not get sealed when you put things back together. If they're hard but not cracked, you can soften them up by boiling in a mixture of wintergreen oil or even cold in another mixture of stuff that I'm not sure of. Search the net for ways to soften them.

You can check for leaks by spraying around the insulators with carb cleaner, WD40 maybe, an unlit propane torch... a change in rpm would indicate a leak because the stuff is getting sucked into the insulators.
 
I changed the oil about a month ago. I have been using Mobil 1 Racing™ 4T Motorcycle Oil (https://mobiloil.com/en/motor-oils/mobil-1/mobil-1-4t-motorcycle-oil#10W-40) for a couple of years.

My carb boots were changed about the same time as the oil (we ordered replacements from David Silver Spares) as well as the work drive clamps.

How easy is it for a novice, with limited tools and space, to replace the friction plates?
 
Hard kinda depends on attitude and tools available... think it's easy, it probably will be. How well the screwdriver fits the cover screws will be a consideration since the heads can be easily stripped if you don't have a decent fitting screwdriver. It will also probably help if you have a handheld impact driver... the kind you hit with a hammer to loosen and final tighten the screws. Won't cost much at all, this kind, but I'd get a new one with the bits that come with it http://www.ebay.com/itm/5-5-Impact-...171?pt=LH_DefaultDomain_0&hash=item2ee39164cb .

When you take it apart, just pay close attention and lay the parts out in order and in the direction they come apart.

When you get to the clutch, be sure to take the 4 bolts with springs out a little at a time so you don;t break the ears on the clutch pack cover. Same for when they go bak in, and they take very little torque.

And remember, if you have to force it, something is wrong.

You're not all that far away, so if you want to come to Hoopeston I'll help you out. Haven't changed one since the 70's, but was no problem even for the silly kid I was back then.

Do you have a manual?
 
I only have the Clymer manual.

My father, who was the original owner, has been helping me and has more tools including the impact.

Looking at cb750supply.com there are three different plates that fit a '77 K. How do I know which one(s)?

Thank you for the offer, if it gets too out of hand i will definitely consider it.
 
If there blued pitch them, If not place them on a piece of glass to make sure there flat and not distorted I usually sand blast them with fine media if You do not have a sandblaster scuff them up with medium fine sand paper to get the glaze off of them. As far as the disc go I could not tell You I think it,s just a matter of one takes an extra disc and other does not.
 
Replaced all the friction plates, they were almost smooth, and have been running them for a couple days with no more problems. Checked the idle screw and it was tightened down quite a bit, must have been from when the jets were clogged. Thank you for all the help!
 
Replaced all the friction plates, they were almost smooth, and have been running them for a couple days with no more problems. Checked the idle screw and it was tightened down quite a bit, must have been from when the jets were clogged. Thank you for all the help!


Very cool! Thanks for the update. It can help someone else with the same issue when results are posted. Finding and fixing problems can be very rewarding.
 
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