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1980 CB750 Runaway Idle

zhill22

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Crown Point Indiana
Hey guys, I have a 1980 CB750k that I bought for very cheap as a project bike. This is my first bike so everything is new and a learning experience. So let's get to the point of why I am making this post (I've tried researching all over before making this post). Essentially I have got the bike running but it accelerates on its own at idle. The bike starts up very good and idles well for it being cold. The idle drops as it should once it's somewhat warm, but then once the bike warms up for about 5 minutes it starts slowly and progressively idling faster. Then within a split second, something "snaps" and it basically sounds like it's redlining.
Here's a little background on what I have tried so far.
-rebuilt the carbs twice (there are no leaks)
-new carb boots
-the throttle linkage is bottomed out (meaning the throttle linkage is not being pulled and at idle)
-the carbs have been bench synced
-the carbs have pod filters which I know it will probably not run "right" but I can't see that being the reason for the redline at idle
-ive adjusted the pilot screws in all sorts of positions from 3 turns out all the way down to 1/2 turn out with really no difference at idle (no the pilot screw tips are not broken)
(Also would like to mention that the bike doesn't have mufflers and has exhaust wrap) - don't know if that would effect anything that much

I feel like I'm running out of options on what else to try.
Some thing to note is that my valve cover gasket is leaking a decent amount, but I really don't think that could do anything with it (IDK)
I haven't messed with the timing yet. Like I said before I'm in the process of learning about carbs and engines themselves so this is all very new to me, but since it has an electronic ignition system can I static time it? Or would my only choice be to dynamic time it while it's idling really high with a timing light? Or would this not even help it at all? I really don't know and unfortunately, I have no one to turn to that is mechanically inclined enough to help me.

If you have made it this far then I thank you lol. Like I said I'm just a young kid trying to learn about this thing. Any input and advice would be much appreciated. thanks in advance.
 

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Bike looks nice not much advice to offer.

My guess would be carb floats are not adjusted correctly. But then the overflow will be active causing gas to leak out on the floor.
 
Bike looks nice not much advice to offer.

My guess would be carb floats are not adjusted correctly. But then the overflow will be active causing gas to leak out on the floor.
Thanks. Yea don’t think it’s the floats, none of them are stuck. Plus like you said none are leaking from the overflow.
 
i don't know much about these having only owned one i never had to work on and one i bought a couple of years ago that i worked on. have you tried spraying something like WD 40 all around your carb rubbers to see if it is dragging excess air. i know 2 strokes rev higher but don't know if 4 strokes do.
 
i don't know much about these having only owned one i never had to work on and one i bought a couple of years ago that i worked on. have you tried spraying something like WD 40 all around your carb rubbers to see if it is dragging excess air. i know 2 strokes rev higher but don't know if 4 strokes do.
yea i have and I can’t find any leaks. They are brand new boots that i bought from 4into1.
 
I’m having the same issue on a 1980 cb750. Almost the same set up you have. I went far enough to buy Murray’s carbs and try that since I thought it was the pod filters and CV carbs being the issue. That didn’t help. My last option was checking valve clearance and as usual, they were all super tight. So I swapped shims and now I’m ready to try again but the cold weather has kept me from working on the bike recently (NE Ohio).

Let me know if anything you do fixes the issue. I will do the same if my fix works.
 
I'm just wondering if there's a gremlin in the ignition (too advanced, causing rpm rise..which leads to more advance and even higher rpm) - ya'll seem to have all but eliminated carb and intake issues.:umm:
 
Two things you haven't mentioned in your post -- what's the condition of your choke cable and linkage, and did you make any jetting changes when installing those pod filters?

You will never, ever, ever get the bike to run as well with pod filters as it does with the stock intake system unless you swap carbs. You can save yourself a ton of headache (which you are already experiencing) by either swapping to a set of direct-pulls or going back to stock.

You also said this is your first bike. How much experience do you have with carburetors? What was your procedure when rebuilding them?
 
Two things you haven't mentioned in your post -- what's the condition of your choke cable and linkage, and did you make any jetting changes when installing those pod filters?

You will never, ever, ever get the bike to run as well with pod filters as it does with the stock intake system unless you swap carbs. You can save yourself a ton of headache (which you are already experiencing) by either swapping to a set of direct-pulls or going back to stock.

You also said this is your first bike. How much experience do you have with carburetors? What was your procedure when rebuilding them?
so my choke cable and linkage is in working order. I have not made any jetting changes to the carbs. The reasons i have pod filters is mainly because i bought the bike not even running and the previous owner didn’t have the original air filters. I’ve read all over seeing how it wont run right without it but i have a hard time belive that’s what’s causing the bike to run away like this. As far as rebuilding the carbs i did it with my grandpas who has tons of experience with carberators. We failed a very specific pdf document explaining on how to clean everything and break the carbs apart and put everything is back together. I’m almost 100% certain everything in the carb is where it’s supposed to be. The previous owner actually broke the tip off two of the pilot jets that took hours to get out. ( no we did not drill them out we were able to tap them out). What i did find yesterday is that my timing is wayyyyy off. i’ll see if i can send a picture to the group, but it’s so far off that the adjustment plate can’t be turned anymore. I’m talking like a good two inches. Could this be the cause of it?

thanks again i’m advance. I know this is a lot but i seriously don’t know what my next step is supposed to be😂.
 
598D7AB3-75BF-4AA9-AE6E-A539919D108A.jpeg

ok sorry it’s not the best pic, but as you can see the black arrow i drew is where the timing is supposed to be set and right now it’s way to the left, but i’m out of room to adjust it with the plate.
 
How are you checking timing? There is an advance unit.......does it advance and return smoothly? If it is that far I would be inclined to remove the advancer from the end of the crankshaft and see if the locating dowl pin is still there or missing or broke off.
 
How are you checking timing? There is an advance unit.......does it advance and return smoothly? If it is that far I would be inclined to remove the advancer from the end of the crankshaft and see if the locating dowl pin is still there or missing or broke off.
I was using a timing light, and yes i’m aware of the dowl pin. I’ve take the mechanism of and made sure the dowl was set in its correct place. This picture was taken after i had done that and as you can see the timing is still way off, and out of adjustment.
 
So this picture shows what exactly? Is this verified top dead center of #1 piston? Does the advancer work? Is it stuck advanced? maybe previously taken apart and reassembled incorrect?
 
So this picture shows what exactly? Is this verified top dead center of #1 piston? Does the advancer work? Is it stuck advanced? maybe previously taken apart and reassembled incorrect?
so the picture i took was from when i took a video of the bike idling with a timing light. the picture shown is when the light on the timing light goes off. I haven’t check the advancer yet (because i don’t even know what it is. remind you this is my first time working on anything like this)
 
The advancer is the part you said you took off the crankshaft to check the pin
 
gotcha, okay thanks so much for the advice. i’m going to take off the advancer tomorrow and try to find some youtube videos on cleaning it and make sure it works properly. I’ll let you know what i find out. Thanks again!
 
so my choke cable and linkage is in working order. I have not made any jetting changes to the carbs.
I am astonished that nobody else has commented about this yet.

The advancer may be contributing, but what I italicized up there is your biggest issue.

With pods and no jetting changes, I'm surprised the bike is running at all at idle with no choke and no throttle input. You are letting in a ton more air than these carbs were meant to take in at idle without adding the extra fuel necessary, and that means it is running extremely lean. It's running like a stock bike with a giant hole in the airbox.

One of the common symptoms of lean running is excessive idle RPM, or a "hanging" idle when the throttle is closed.
 
I am astonished that nobody else has commented about this yet.

The advancer may be contributing, but what I italicized up there is your biggest issue.

With pods and no jetting changes, I'm surprised the bike is running at all at idle with no choke and no throttle input. You are letting in a ton more air than these carbs were meant to take in at idle without adding the extra fuel necessary, and that means it is running extremely lean. It's running like a stock bike with a giant hole in the airbox.

One of the common symptoms of lean running is excessive idle RPM, or a "hanging" idle when the throttle is closed.
thanks for the info. To probably make matters worse there just cheap air filter pods off of amazon lol. Not even the nice k&n ones people talk about using lol. I’m gonna try to find a stock one on ebay since the bike didn’t come with one when i bought it. Thanks!
 
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