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Very high involuntary reving when idle!

lukenukem

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Hi there,

I am very new to the bike game and would greatly appreciate your help.

I Just purchased an 82' cb750 from a person with good credibility and bike knowledge and was told that the bike would "run like a dream" and honestly needed nothing.

The bike was shipped from Northern Ireland to the London and took a day over see in the cold. On arrival it seemed someone left the ignition on and killed the battery and there was no working brake light from front brake.

After a day of charged the battery, with the choke open fully the bike started, then died and again and again. After it was warmer I got to give it a ride for about 20 mins. What I found was that when I was going at a low speed the engine would nearly cut out unless I revved it again, this isnt what really bothers me. The main problem I am having is when I am travelling at a low speed, in neutral stationary or when I am slowing down and pulling the clutch in to stop, the engine makes a horrific sound like an extremely high revving although I am not touching the throttle. There is also smoke coming out of the engine somewhere and LOTS of backfire. Another strange thing it seems is that the bike seems to travel on its own as in wont stop accelerating so I could literally cruise at 10-20 mph for a good while without touching the throttle, I then slow down and clutch and then the horrific sound and bike firing.
My good friend had a look at the bike as he has better then average knowledge and he found that the the carbs were extremely loose and the rubbers wouldn't tighten around it. He suggested a vacuum leak and after reading up about it, it sounds like I need to replace the rubber inlets and hose clips.

Would be great if you guys could shed any info on the possible problem/problems that it could be...

Pretty pissed off to be honest as I feel ive been messed about with, although the guy sounds very genuine, has good credibility and is a good lever racer on the N.I circuit.

Cheers,

Luke
 
Your pal is on the money on this one from what I can read in your post. Classic extreme lean condition from sucking in air through the intake rubber manifolds. I would pull the carbs and the intakes and examine them for cracks before buying new ones. Could be they are in OK working order, just needs to be clamped down properly (mind you PROPERLY, not ridiculously tight).
If not, your intake rubbers are plentiful on eGay and not to expensive either.

Let us know how it goes.
 
Hey

Cheers Stabler, much appreciated!

Tried to tighten but not much give in the rubbers they are pretty damn hard, ordered new ones for an 81 dohc hopefully they are the right ones.
They arrive tomorrow so will keep you posted!

Could the firework display that People were witnessing whilst I was riding damage the engine, the smoke and noise was my concern !?

Thanks
 
Sounds like a vacuum to me.. That would totally explain the behavior. New or referbed rubber inlets would help seal them up. I don't know the 82, but often regular hose clamps from any auto parts store will work. The trick is not to over tighten them, they will either jump a groove on the strap or you might buckle the rubber inlet.
Here's how I referbed my intakes on my 75 http://www.cb750.com/threads/595-CB750-K5-Restoration/page2?p=8156&viewfull=1#post8156 They were hard as a rock, but came out pliable.
 
I don't know enough about those DOHC motors, but I wouldn't buy anything to my SOHC without checking the OEM parts number first. Ask me why.. It's expensive to buy the wrong parts. You might be lucky though.

I don't think the backfiring and revving have hurt your motor, depending on how long you had it running like that.
 
Cool

Well as long as I haven't properly screwed anything, that's all good.
Appreciate all your help, massively, it's freezing over here wanna get riding as much as I can before the ice and snow kick in!
 
The boots should fit. need to check the air cut diphrams under the round side covers on the carbs as they are more than likely crack and leaking also.
 
Received the rubbers pretty sure they are the correct ones!

Does anyone know how these should be correctly aligned as I'm running out of patience?
They are all secure to the engine but getting them on the carbs is a fucker !!!
 
UPDATE!

So, the original rubbers were cracked all over and in pretty bad condition. So I spent nearly a whole day replacing them and fitting the carburettor back on. Once on I started her up from cold (full choke), she was revving like crazy but after adjusting the Idle screw she seemed to calm down, although still not perfect.

The problem I am trying to solve now is the battery or charging system which if you guys can help again, would be much appreciated!
My friend again suggested that the charging system my not be working 100%, I just bought a new yuasa 12v 6a battery (as I thought the old one was probably knackered) and after a few hours use the battery is now flat and bike cant start! (KILL ME)

I did start the bike about 10-15 times as I stalled (cut out) it a few times, the bike ran out of petrol and I parked up a few times. So would all that activity kill a new battery in a couple hours? If so I will just charge it up over night. It was showing 12.6v when it arrived...
If its not the battery, then I have to start digging in to the bloody charing system...

Look forward to hearing from you guys.

Thanks

Luke
 
If it is truly a 6amp. battery it is to small should be 20amp. Early models were 15 amp. should show around 13.5 volts across battery at around 2,500 to 3,000 R.P.M. s
 
My manuals only go up to 1978, After a little looking it seems like they downsized to a 14 amp. hour batt. I stand to be corrected on this one, 6 amp. is to small of a reserve.
 
Again, another problem.

The charging system isn't working.
I stuck a meter on the battery and revved the bike, no change to the battery at all. Very annoying.

Can someone suggest the first step to look for, is it worth going straight in for the alternator ?

Cheers
 
Check the obvious first loose or corroded wires, As I said earlier I do not have a manual for Your model so the color codes might have changed if all 3 stator wires are still colored yellow You can check with an ohm meter across any two wire combinations of the three, Reading should be approx. .2 ohms on each pair. If the color of the field coil wires stayed the same a white and green wire the resistance should be around 7.2 ohms between the two if all these are within tolerances this part of stator charging system should be good. Leaving the regulator/ rectifier as possibly bad. You still need a good manual to conduct other tests. But going back check all wire connectors first.
 
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