Random Surge when at a stop

[canorthup]

Hello, New to the thread. I have had many CB 550/500s and GS750/550s, but this is my first CB750.

I just picked up the bike, and it is running well, except that sometimes, when I am at a stoplight, it will jump to 3,000 RPM. Sometimes it will return to its original RPM on its own. When I let out the clutch, it drops back down, which makes it fun cause I have to rev the bike when it is already high and let out the clutch. My first thought is that there is an air leak or the needle is sticking. So far, I haven't had time to delve into anything, and I thought I would ask before getting started. The guy said he had the carbs cleaned recently, but I have heard that before.

As far as I can tell, it doesn't happen while moving so far, but I only drove it home since the tabs are expired.

1979 CB750 K

 

[benwa]

Hello, New to the thread. I have had many CB 550/500s and GS750/550s, but this is my first CB750.

I just picked up the bike, and it is running well, except that sometimes, when I am at a stoplight, it will jump to 3,000 RPM. Sometimes it will return to its original RPM on its own. When I let out the clutch, it drops back down, which makes it fun cause I have to rev the bike when it is already high and let out the clutch. My first thought is that there is an air leak or the needle is sticking. So far, I haven't had time to delve into anything, and I thought I would ask before getting started. The guy said he had the carbs cleaned recently, but I have heard that before.

As far as I can tell, it doesn't happen while moving so far, but I only drove it home since the tabs are expired.

1979 CB750 K

check your intake boots for a leak just take some gumout carb cleaner abd spray around boots with it running and see if it revese up if so replace the boots
common ploblem

 

[benwa]

check your intake boots for a leak just take some gumout carb cleaner and spray around boots with it running and see if it revs up if so replace the boots
common problem

 

[Bo82CB]

check your intake boots for a leak just take some gumout carb cleaner abd spray around boots with it running and see if it revese up if so replace the boots
common ploblem

Be careful with that carb cleaner while engine is running, that shit is extremely FLAMABLE!

 

[canorthup]

Thanks, I will give that a try when I get home! I will try not to end up looking like a Pink Floyd album cover

 

[benwa]

Thanks, I will give that a try when I get home! I will try not to end up looking like a Pink Floyd album cover

only do the spray when you first start it up an before engine get to hot to be safe

 

[Old_Crow]

Thanks, I will give that a try when I get home! I will try not to end up looking like a Pink Floyd album cover

You have to be of a certain age to get that. Love it!

 

[canorthup]

I replaced the boots and for some reason it is hard to line up the angle on each one. Anyway I put them on and went for a ride. Now the bike just stays at 3k at idle. I can't adjust out anymore on the idle. So basically I went from random surges to just always at 3 k. I am going to check for leaks tomorrow. The bike runs great otherwise. But I feel like a nut job at the stop light people think I'm rev bombing for attention, so that's fun.

 

[canorthup]

I wrote about this in another thread, because I forgot to add it here. But just an update. I tore them down and cleaned the carbs ultra sonic guitar string and enough carb cleaner to make my garage a hazmat site. So I am fairly positive they are clean. I put them back on with all new rubber front and back of carb. Bike runs perfectly. Then at random the bike will go up to 3k at idle. The only thing I can think of is something that moves is sticking. So that means either the needles or throttle. Or is it that once the bike gets to a certain temp that the idle mix is wrong or something? I don't know I am stumped. The fact that it runs great at first is worse than always being bad.

 

[canorthup]

Also I did check for air leaks and did not find any and successfully did not light my self on fire as well

 

[canorthup]

I'm close to just buying another set of carbs but a pair that I would trust cost about half what I paid for the bike.

 

[canorthup]

Here's a pic just for whatever

 

[Bo82CB]

As I said many times before to other members who had issues with carbs...
After thorough cleaning which you did, I suggested everyone to polish float needle seats and to polish carburtor slide pistons and bores.
ALL POLISHING must be done by hand and with none grit polish paste.
Trust me it does make great improvement in throttle operation and response...

 

[canorthup]

I will definitely give that a try tomorrow thanks I feel like I am running out of things to try

 

[canorthup]

As I said many times before to other members who had issues with carbs...
After thorough cleaning which you did, I suggested everyone to polish float needle seats and to polish carburtor slide pistons and bores.
ALL POLISHING must be done by hand and with none grit polish paste.
Trust me it does make great improvement in throttle operation and response...

More of a general question. I plan to clean everything but the float needle shouldn't effect idle right? I thankfully have not had any issues with flooding or not getting fuel (or at least what I can tell) I cleaned and polished the slides to almost a mirror finish by hand with a gentle polish. I did this with the carbs on the bike. When I got done I took it for a spin and the issue is still there. I was going to do the float needles later, but I just wanted to see if I can fix the issue. But if you think that could be the problem I will take them off.

 

[Bo82CB]

Float needles will not affect idle.
My suggestion was based on people removing carburators and disassembling them, why not to do this one extra step for smoother operation. Also you said it feels like something is sticking / hanging up...now carb slides can cause that...
I am not sure how deep did you go through carb overhaul but I can tell you from my own experience, I replaced every part made out of rubber that could be replaced, bench sync them than sync them again when on the bike.

 

[canorthup]

I did Bench sync them, and I was going to do a full sync soon. I have the Digisync, so hopefully that will take out another variable of me messing it up.