1980 Honda CB750F Runs with choke then dies with choke off.

D

DanDeZinna

CB750 Enthusiast
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Hey Everyone!

New to the forum here! Looks like a great forum to be apart of! I recently just purchased a 1980 Honda CB750F for a good price I thought and the bike runs and drives. The previous owner said he had the carbs overhauled at a bike shop and found some issues with them. (I have no idea what the found wrong). Anyway the bike will start with choke then rev up really high then if you push the choke in, the bike just dies. I was thinking about doing a complete carb re tune and synchronizing them, but I am not sure where to start. I have a video I can post showing what it’s doing! Any help would be greatly appreciated!

Thanks in advance!
IMG_1901.png

 
Don't assume because a Bike Shop worked on the carbs that they are good. You could start with a Sync first so you have that confirmed. If that doesn't help then you may have to pull the carbs. I worked on a bike awhile back that had just come out of a local bike shop and it was a mess. I had to do everything they had done over. A lot of bike shops today won't even touch these kind of bikes and that's probably a good thing. Did you buy this bike knowing it had issues?
 
Loose Chain said:
Don't assume because a Bike Shop worked on the carbs that they are good. You could start with a Sync first so you have that confirmed. If that doesn't help then you may have to pull the carbs. I worked on a bike awhile back that had just come out of a local bike shop and it was a mess. I had to do everything they had done over. A lot of bike shops today won't even touch these kind of bikes and that's probably a good thing. Did you buy this bike knowing it had issues?
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Thank you for the advice! I definitely knew there were issues, which reflected in the price! He also installed a dekelvic exhaust from a stock exhaust which probably changed things.
 
I have an 81 with a dekelvic exhaust. Bike runs well with stock air box and carbs.
Is your idle set too low? That is something to play with along with the sync before pulling the carbs to look at the pilot jets.
 
brettp said:
I have an 81 with a dekelvic exhaust. Bike runs well with stock air box and carbs.
Is your idle set too low? That is something to play with along with the sync before pulling the carbs to look at the pilot jets.
Click to expand...
Okay that’s good to hear!! I will check idle on the carbs along with doing a sync on them! Thank you for the advice!
 
Hello everyone,

I was able to get the sync gauge hooked up to the bike. From what I can tell, everything is pretty close to the second carb. Could use some fine adjustments to get it closer. I was having trouble keeping the bike running without choke though. As soon as you pushed in the choke the bike wanted to die. I guess the next route would be to adjust idle? I have another video to see for reference. Any help or advice is always appreciated!CB750 Sync gauge
 
brettp said:
Don't mess with the choke unless you know the jets in the carb are clean and the spark plugs are good.
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Okay thank you Brett! I will pull the carbs and go through them! I was reading the clymer maintenance manual and was reading how to adjust idle settings, and it said to turn the black knob in the middle of the carbs until you achieve 1000 rpm +/- 100 rpm. I was also going to try this tomorrow before pulling the carbs.
 
Update for anyone interested..

I started the bike today and checked cylinder temps with an infrared laser thermometer and cylinder #2 was cold. Checked spark and compression and both were good! Pulling the carbs this weekend to go through them!
 
brettp said:
Yup, sounds like a clogged pilot. Did you try revving it to see what would happen?
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I think you’re 100% correct! I tried revving it and nothing made a difference. Seemed like it wasn’t getting any fuel at all. The spark plug was a tiny tiny bit wet but not enough to fire. Also noticed cylinder 3 was running alittle lean based on the temperature on the header. So definitely need a good going through and cleaning
 
brettp said:
I have to ask; the spark on cylinder 2 jumped between the electrode to the core? I recently had a spark plugs arc down the porcelein. It had spark, but in the wrong spot.
Click to expand...

Oh wow! That’s a good find for sure!! Plugs seemed new when I looked at them ( but that doesn’t mean they’re always good lol). I checked spark and it had really nice blue spark from the center electrode to the ground electrode! Didn’t notice it going to the core at all! I’ll have to double check it this weekend again and report back!
 
With my ‘79 SS, I had the same problem, it ended up being the passageways that went from the idle screws to the throttle bodies.
The passageways are very small.
After 3 times cleaning it was a guitar string that picked the hardened fuel that had turned to varnish most of the way thru on all 4 of them.

Turn the idle adjust screw all the way in.
Write down how many turns it is.
1 1/4, 2 full turns, whatever.
Take the screw completely out.
Make up or buy something that you can squirt into the idle adjust hole.
See if whatever you use comes out of the little hole in the throttle body.
If nothing comes out the passage is clogged.

Until those passageways are cleared, and fuel dumps into the airflow going to the cylinder, you can clean them till you’re blue in the face, and it will not idle.

Hope this helps.
 
Correction to the above: what is being called the 'idle adjust screw' is the fuel mixture screw, found at the front of the carb by the bowl. It adjust the amount of fuel provided to the cylinder at idle. The idle adjustment screw is the black knob that opens the butterflies of the carb and increases (or decreases) the idle RPMs.
 
Motogrady said:
With my ‘79 SS, I had the same problem, it ended up being the passageways that went from the idle screws to the throttle bodies.
The passageways are very small.
After 3 times cleaning it was a guitar string that picked the hardened fuel that had turned to varnish most of the way thru on all 4 of them.

Turn the idle adjust screw all the way in.
Write down how many turns it is.
1 1/4, 2 full turns, whatever.
Take the screw completely out.
Make up or buy something that you can squirt into the idle adjust hole.
See if whatever you use comes out of the little hole in the throttle body.
If nothing comes out the passage is clogged.

Until those passageways are cleared, and fuel dumps into the airflow going to the cylinder, you can clean them till you’re blue in the face, and it will not idle.

Hope this helps.
Click to expand...

Thank you for the advice! Definitely going to be tearing the carbs down at my job! We have an ultrasonic cleaner there which should work awesome for getting them clean!
 
brettp said:
Correction to the above: what is being called the 'idle adjust screw' is the fuel mixture screw, found at the front of the carb by the bowl. It adjust the amount of fuel provided to the cylinder at idle. The idle adjustment screw is the black knob that opens the butterflies of the carb and increases (or decreases) the idle RPMs.
Click to expand...


Thank you for clearing that up Brett!
 

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