Mid range throttle issue

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BIKE MIKE

Active Member
I have an 81 cb750 custom. It starts and runs well but... the throttle is smooth if I'm gentle on it, as soon as I give it a good pull the engine will rev from 2000 ish rpm to 6000-7000 rpm before it pulls. This happens in any gear it just hesitates for a second. What could be causing this? I should say I've changed the air filter plugs and wires.

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Usually hesitation or if a bike continues to rev after you back off the throttle is attributed to an air/vacuum leak on the carb side. Possibly when you changed the air filter something was left not sealed completely or at the manifold u've got a leak. Could also be attributed to a carb issue (too rich, floats stuck, valve slide sticking). Or could simply be the way your throttle n cable are adjusted (check for slack in line at carb mount adjuster and that it's moving freely). Do the easy stuff first cable adjust and vacuum leak first before moving on to carbs. You can find vids for vacuum leak easy on web, cable is simple just follow it along and see how it moves from end to end. Hope that helps


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I appreciate your help. I may not have been completely clear with my issue. It was happening prior to the filter change. I'm not backing off the throttle, I twist the throttle, the engine will rev momentarily from 2k to 6k rpm, after its revved up then the bike will begin to accelerate. I will check for a vacuum leak. I was thinking this may be a carb issue.

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Change the clutch springs, they were marginal when new and designed to slip a little before gripping. The clutch cable is lubricated and routed properly? Clutch is adjusted correctly? The inspection cover on 650's allows all sorts of nonsense to be carried out
 
I did a vacuum check and cant find any leaks. I hadn't looked into the clutch much but yes it is routed properly and lubricated. It may need some adjustment, I'll look there next. It engages when the lever is almost all the way extended, I haven't messed with it since it shifts with no problems. I would prefer it to engage a little earlier so I'll adjust it and swap the springs. Then see what happens. Thanks a ton!

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Should I only replace the springs or is a complete kit the way I should go? There is a big price difference between the two, I'd hate to just replace the springs and have some other issues come up. Forgive me, this is my first older bike and I'm somewhat a noob to the scene.

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Just change the springs.In my experience the stock clutch will take tremendous abuse before plates need changing but the springs weaken very quickly (on all Honda's) It also sounds like clutch adjustment isn't right, I'll have to check proceedure as I haven't done 750/900 since late 80's.
 
When is the last time your carbs were cleaned? I'd double check the needle and slides. Also, how did you check for a vacuum leak? Did you spray something around the boots while it was running? You may not have your airbox seated properly. I'd triple check everything.
 
I've only owned the bike for a couple of months so not sure when the carbs were cleaned last. I plan on pulling them over the winter and cleaning them. Just trying to eliminate possibilities before then. I used wd-40 while idling and sprayed all over the boots one by one. Did that twice with absolutely no change to the idle.

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I did put seafoam in the tank when I first brought it home and also when I filled the tank up again. Is this redline better or is the seafoam sufficient to do the same?
 
I’m not a huge fan of the sea foam. I prefer something a little stronger like Berrymans. I don’t know anything about the redline. These really are just temporary fixes. The best thing will be when you pull the carbs and clean them.
 
I appreciate the help from you all, the weather is starting to cool down here in new england so once its too cold for me to ride Ill be pulling the carbs.
 
esmoojee said:
I’m not a huge fan of the sea foam. I prefer something a little stronger like Berrymans. I don’t know anything about the redline. These really are just temporary fixes. The best thing will be when you pull the carbs and clean them.
Click to expand...
Oh no. Not at all. Some of the cleaners are so strong the jets come out looking polished! just gotta find the real deal.
 
Problem with Berryman's carb cleaner is it destroys any rubber parts so with a full bank of carbs you need to completely dismantle them off the rack.
If you don't the 'O' rings between carbs will deteriorate and you'll have fuel and air leaks. When you have carbs stripped that far down you may as well just clean them by normal means.
'81 CB750 will have air cut valve (or valves, I forget)
They are often corroded in place or if free the diaphragms are damaged and need replacing.
CB750/900's are also pretty impossible to get working without stock air-box.
Even with all stock parts the bike runs close to too lean and could use a slightly larger pilot jet and primary main jet.
I spent about 3~4 years in mid 80's messing with 750/900, (did an awful lot of cam chain changes ::) )
 
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