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

[DanDeZinna]

Hey guys!

Just a little update on the project. I was able to get the carbs off and get them broken down and split individually. I immediately jumped to the suspect carb on cylinder 2 finding no immediate evidence of clogged main or pilot jets. Everything is clear. Now I haven’t cleaned the actually carbs yet to see if anything is actually clogged up but that’s coming this week. Here are pictures with my progress.

Another question, there are 4 tiny washers that fell out during disassembly that I didn’t notice. Any ideas on where they came from or where they go!? Thanks in advance!

Also to note, the fuel mixture screws were all different settings. Some 3 turns or even 4 to 4 1/2 turns out. I decided I’m going to set them all at 2 1/2 turns on reassembly.

Dan

 

[Loose Chain]

They go in with the fuel mixture screws. It goes o-ring, washer, spring.

 

[DanDeZinna]

Thank you!!

 

[DanDeZinna]

They go in with the fuel mixture screws. It goes o-ring, washer, spring.

There were no orings on mine! I guess they could be a big issue!

 

[Loose Chain]

Look down the hole, they might still be in there.

 

[DanDeZinna]

Look down the hole, they might still be in there.

I’ll double check them again! Is it possible that cylinder #2 could be getting flooded with fuel causing it not to fire? I did a compression check today as well.

Cylinder #1 - 130psi
Cylinder #2 - 150psi
Cylinder #3 - 140psi
Cylinder #4 - 118psi

 

[brettp]

Compression in 4 is obviously low but rings that have been sitting can come back around after a few miles of use.

Flooding in carbs is not likely because excess fuel will leave the carb via the overflow tube. But if your jets were too big, that would give you too much fuel (but I don't think it would be so much to say flooded).

 

[DanDeZinna]

Hey everyone,

Small update on the carb project. Turns out the o rings were installed in the fuel mixture ports and not missing which is good! I cleaned hell out of the carbs and I literally couldn’t find one orifice that was clogged. They were pretty clean. I’m starting to lean on a it was just a bad carb adjustment. But we will see once they go back together. Also found the intake boots all cracked and torn which could also be why it ran like crap. So new intake boots are on the way from 4in1.com. Thanks for all the advice and help thus far from everyone!

 

[brettp]

Was the cracked boot on the engine side or air cleaner side?
I hope you checked the bowl drain tube while you had them off the bike...

 

[DanDeZinna]

Was the cracked boot on the engine side or air cleaner side?
I hope you checked the bowl drain tube while you had them off the bike...

On the engine side! They’re all cracked after further inspection! Do you mean the bowl drain for each carburetor!? If so I checked and made sure the overflow was clear and the guy before me never put any overflow drain lines on the carbs once he had the carbs reinstalled. I’m going to add drain lines again before installation.

 

[DanDeZinna]

@brettp

Unrelated to the running and carb issues. I noticed the bike had no horn installed. I want to hook up the horns again but I’m not to sure where the connections are. It seems like the wires are in the headlight assembly?

 

[brettp]

The carbs have brass tubes inside the bowls so if the bowls overfill it runs into the drain tube and out of the nipple on the bottom of the carb. Otherwise, the bowl fills up and the fuel goes into the intake boots and into the combustion chamber.
These drain tubes can crack. If you take the bowl off (and empty the bowl of fuel) you can blow into the tube to see if air escapes. If not, the tube is good. If air escapes, fuel will rise in the bowl and then find that broken part, and run out if the carb via the nipple.

 

[DanDeZinna]

The carbs have brass tubes inside the bowls so if the bowls overfill it runs into the drain tube and out of the nipple on the bottom of the carb. Otherwise, the bowl fills up and the fuel goes into the intake boots and into the combustion chamber.
These drain tubes can crack. If you take the bowl off (and empty the bowl of fuel) you can blow into the tube to see if air escapes. If not, the tube is good. If air escapes, fuel will rise in the bowl and then find that broken part, and run out if the carb via the nipple

Recently I cleaned the carbs and I also cleaned those brass tubes on each carb while they were disassembled. I was able to spray carb clean through the bottom up through those tubes without any obstructions. I will also try out your method to be 100% sure!

While I had the bike running before the carb removal, no fuel was running out of the carbs while running or shutoff with the petcock on.

 

[DanDeZinna]

Recently I cleaned the carbs and I also cleaned those brass tubes on each carb while they were disassembled. I was able to spray carb clean through the bottom up through those tubes without any obstructions. I will also try out your method to be 100% sure!

While I had the bike running before the carb removal, no fuel was running out of the carbs while running or shutoff with the petcock on.

Is there a check valve in that brass pipe!?

 

[brettp]

Is there a check valve in that brass pipe!?

No check valve

 

[DanDeZinna]

No check valve

Okay thank you Brett! So based on what I posted last, you think the brass should be in okay condition!?

 

[brettp]

Pushing carb cleaner through it makes sure there is no obstructions, but pushing air through it makes sure fuel can't leak out of the bowl before it reaches the appropriate level.
I wouldn't worry too much about those until you find a particular bowl is overfilling again. Unless, of course, you have the bowl off that carb already.

 

[elcochinato]

¡El mío no tenía juntas tóricas! ¡Supongo que podrían ser un gran problema!

Los mios tampoco tenian y los mandaba al mecanico a xada rato. Que hdp

 

[Davidson750]

Stay away from aftermarket rebuild kits(junk)..and protect all gaskets from over spray from Gumout(carb cleaner)...older gaskets will deform...use a hair dryer or a heat gun on low on your rubber boots(airbox,intake)..it will make things a lot easier..and put all your linkages on..move airbox all the way back,...has to go in first.

 

[DanDeZinna]

Stay away from aftermarket rebuild kits(junk)..and protect all gaskets from over spray from Gumout(carb cleaner)...older gaskets will deform...use a hair dryer or a heat gun on low on your rubber boots(airbox,intake)..it will make things a lot easier..and put all your linkages on..move airbox all the way back,...has to go in first.

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!