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

[DanDeZinna]

Okay thank you Brett! I’ll make sure I run a fan over it new time! Since the fuel/air mixture screw only influences idle I may need to explore different main jets or needles. Currently all the jets are stock

 

[Loose Chain]

Just curious, do you have the air box connected with the air filter in it? Your adjustments seem way off.

 

[DanDeZinna]

Just curious, do you have the air box connected with the air filter in it? Your adjustments seem way off.

Yes! Air box is connected with air filter in it

 

[brettp]

If you have all stock air intake parts and your carb has stock jetting and is clean and letting fuel in the engine, and the carb vacuum is set, I'm leaning back to valves or timing maybe. This is odd.

 

[DanDeZinna]

If you have all stock air intake parts and your carb has stock jetting and is clean and letting fuel in the engine, and the carb vacuum is set, I'm leaning back to valves or timing maybe. This is odd.

When I get back from my work trip, the next thing I’m going to check is timing and valve clearances!

 

[DanDeZinna]

@brettp in reference to your one comment about carb vacuum, is there an actual port or connection for vacuum!?

 

[brettp]

I was talking about the vacuum measured by your sync tool.

 

[DanDeZinna]

I was talking about the vacuum measured by your sync tool.

Gotcha!! Just wanted to be sure!

 

[kirky]

in my experience put the idle screws at 1 / 1/2 turns

 

[mrtwowheel]

MacGreggor stated 3 turns out in his manual just so people wouldn't start out running their bike too lean and do damage. He knew full well that three turns out was too rich. 1-1/2 turns out is a better starting point. More than three turns out........something is wrong.

 

[brettp]

I agree with kinky and mrtwowheel. I think your problem is outside your carbs at this point. UNLESS, your piston valve is binding and doesn't raise the needle smoothly.

 

[Davidson750]

Thanks Davidson!! I bought a kit from 4in1.com that specialize in cb750 parts. Quality seems good overall! It was around 80 bucks for a rebuild kit. I ended up replacing every gasket on the carbs so I did an initial cleaning without any rubber parts around! The hairdryer trick works great! I used it to remove the carbs and the old intake boots still ripped! There were hairline cracks in them! Thank you for all of the great advice!! Carbs are currently all rebuilt and put back together! The linkages were quite a pain the first time until I figured it out! Should have them back in on Monday and have an update on how she runs!

Thanks Davidson!! I bought a kit from 4in1.com that specialize in cb750 parts. Quality seems good overall! It was around 80 bucks for a rebuild kit. I ended up replacing every gasket on the carbs so I did an initial cleaning without any rubber parts around! The hairdryer trick works great! I used it to remove the carbs and the old intake boots still ripped! There were hairline cracks in them! Thank you for all of the great advice!! Carbs are currently all rebuilt and put back together! The linkages were quite a pain the first time until I figured it out! Should have them back in on Monday and have an update on how she runs!

Great

 

[Davidson750]

Great

I currently don’t have them on but I’ll put them on and see what they’re showing! I’m going to set my mixture screws to 3 1/4 turns out and hook them up! I’ll report back with what I find!

....1-1/2 turns out from gently seated

 

[DanDeZinna]

Hey everyone, finally back after a long work trip. Got to the bike today and fired it up to see where I last left off. I remembered I had the mixture screws 3 1/4 turns out so I reset them to 1 1/2 turns out. The bike fired right up and with full choke on it screamed to about 4k rpms. After I left it run for a bit I pushed the choke in and got the bike to idle and had zero throttle response. It was popping and definitely seemed super lean. I let it warm up some more and had the same response. If you barely help the throttle open it would really slowly start to rev up, like really slow. I’m starting to lean back towards carb issues again.

 

[brettp]

If the carbs are clean, I would be looking at a valve job.

 

[DanDeZinna]

I may pull the carbs off and go through them again