'78 CB 750 SOHC - Idle and Rev-down issue

jsmits

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I recently purchased a '78 CB 750 from a local. The thing starts right up and runs after some minor adjustments to the choke and idle screw. After warming up, it runs pretty well, but I have been unable to take it for a drive yet. I'm working on it with a friend, and we were able to accelerate through all 5 gears up on the stand with little trouble, but there's an idle down issue that we need to resolve before any attempts at riding it are made...

The main trouble I'm having is with the throttle and, what I believe to be consequently, the idle down. You see, the idle seems to stick, almost like the carbs aren't closing all the way. Except... they are. We've already taken the covers off the tops of them and the little pistons all line up and fall evenly. The problem is that the bike does not rev back down from around 3500-4000 a lot of times. I don't know a lot about carbs, throttle, and idle, except what my dad taught me working on the old 90's Arctic Cat's. If we push down on the throttle pistons while it's idling high, it will idle down a bit, but we usually can't get it to properly idle back down to ~1500 unless it's gunned and then the throttle is let completely loose, allowing it to achieve a full motion and some momentum on the way back down.

The throttle cables seem to have been rerun recently. Some low-slung, cafe-style handle bars were put on. It looks like the the old switches and throttle were then put back onto these aftermarket bars. The throttle is really stiff, you can feel it rub on the bar. The bike has sat for at least two years, so we've been lubing everything as we go, and it's loosened quite a bit, but we're pretty sure it's a combination of the throttle friction, and possible the throttle mechanism that opens up all 4 carbs in unison. We also think the ignition points might have something to do with the problem as well.

We haven't tried much yet, and this is just our gatherings after a few hours in the garage. What do you guys think? I'm eager for some insight!

Thanks ahead of time to anyone who might be able to point us in the right direction. Cheers!
 

dirtdigger

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First do a running carb sync then do idle mixture adjustments. If it still hangs, check to make sure the ignition advance is working properly. Sticky advance and weak springs will cause ignition timing to be higher then spec at low rpm and cause higher engine speeds.
 

Kannen91

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I had the same issue with my 78 cb750. Ended up being the boots on the carbs were not tight and leaking vac. Would idle fine till the revs picked up, then it would take forever to get back down to a idle usually sit around 3k rpm..... try looking for a intake leak, however the carb sync will tell you if it has a leak too! Does the bike run better with the choke on?? I use the K.I.S.S principal, Keep, It, Simple, Stupid. Bikes are simple and more often than not the issues are also very simple. Hope this helps with your issue!
 

Stabler

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I've had the same issue as Kannen91 describes, on both my scoots (CB750 and a XS400). Leaking intake boots - sucking in air, keeping it from getting down to idle after revving it..
An air leak on the intake boots is easy enough to find. Have the bike running and spray each intake boot with starter gas, one at the time and listen for variations in rpm. Variations = air leak. Make sure any spray doesn't get in through the carbs inlet, this might mislead you.

But it could still be any of the stuff DD pointed out.
 

068ball

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do you have the full range of motion of the throttle cable? aftermarket bars, sticky throttle cable, other peoples mods make me suspicious. but intake boots are a good suspect also. but sometimes simple things cause big head aches.
 

ciaochaz

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Yeap, sure enough! I had done some carb work and tightened the boots down while she was cold. When I took her out to ride, the idle hung up around 4000rpm and didn't come back down. A surprising amount of extra cinching down n the straps with a hot engine got her running beautifully. Thanks Boys and Girls!
 

Ricki Brown

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I also had this problem I had put an easier throttle return spring on to alleviate hand cramping.
I hooked up the OEM spring and all is fine, however my 78 has this throttle grip which is way to hard on this
old mans (72 )wrist. Has anyone found a compromise spring to deal with both issues.
 

HondamanUAE

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I also had this problem I had put an easier throttle return spring on to alleviate hand cramping.
I hooked up the OEM spring and all is fine, however my 78 has this throttle grip which is way to hard on this
old mans (72 )wrist. Has anyone found a compromise spring to deal with both issues.
I had the same problem with the OEM spring…it must have lost tension over time…I just cut the hook off one end and twisted the last coil to make a new hook…the throttle is a little bit harder to twist now but the hanging idle problem is fixed.
 

gunr

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Good catch on the leaky intakes.
The 77s and 78s are notorious for stiff throttles. Mine had it. Turns out the cables were rusted.
4into1 has both throttle cables plus the clutch cable for around 30 bucks. Made a world of difference on mine.
 

Jimmy smith

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I’ve had carbs professionally cleaned and rebuilt. 1978 750a First start in many years. So very cold natured. Have to run full choke for 5 or more minutes before it runs with no choke. Idles and accelerates good when finally warmed up. When engine is cold again, same scenario. What’s wrong? Why do I have to choke it for so long?
 

dirtdigger

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Welcome to 40 to 50 year old technology.

Could be a little lean but the choke is normally used until the engine warms up. Thats how carb engines are.
 
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