Carb Adjustment

[supamandasun]

So
1980 CB750F
Idle with choke engaged sits kinda high at around 3000rpm. Disengage the choke and the bike just cuts out.... so do I lower the fast idle speed or do I simply increase the idle? And in increasing the idle, do I adjust the pilot screws or just the black knob beneath the carbs?

Thanks

 

[brettp]

If it won't idle but revs high with the choke, you likely have a pilot jet issue.

 

[Motogrady]

Brett is correct. The passages that go from the air screws to the throttle body are super small. They get gummed up just a little no gas get thru, no idle.

That being said I’d start with the black knob. Turn it in maybe 1/8 turn see if it doesn’t cut out. If it does repeat and restart till you’ve gone maybe half a turn in, in 1/8 a turn increments.

If no change, go back to where knob was when you started. The 1/2 turn.
Then try turning the knob opposite direction 1/8 increments, restarting each time if it dies.

No change, pull the carbs, guitar string and clean passages that lead from air screw to throttle body. Make sure to count turns in to bottom out air screws so you know how far they go back in when reassembly.

 

[Igor]

On my K2, I have synchronized the carburetors and cleaned them in an ultrasonic cleaner. On a warmed-up engine, I have to idle around 1500-1600. If I lower the rpm, the engine starts to run erratically and eventually stalls. The engine runs best if the air screw is only allowed 1/8 - 1/4 of a turn. If I try to adjust the air screw according to the manual - i.e. 1 + 1/8 of a turn, the engine starts to run erratically (the rpm decreases) until it stalls. Otherwise, the bike runs beautifully when riding, its performance is linear and smooth. What can I do about the idle?

 

[brettp]

Hey, Igor.
First, that screw is a fuel mixture screw not an air screw. So as you open that screw more (too much) is going into the chamber. It sounds like you need more air. So you need to check the air filter, making sure it is snug against the airbox and check the boots between the carb and airbox for leaks.

 

[Igor]

Thanks. Yes, I found that out. I know it's not actually an air screw, but it's called that in the manual. I found that the idle mixture is rich, so I have these screws tightened. But why is the mixture rich at the recommended setting of this screw? I didn't find a leak.

 

[brettp]

That's the million dollar question. But with oem settings or is not rich, so you have a limited air supply going to the carbs. The reason, we are trying to find our.

 

[brettp]

How did you search for an air leak?

 

[Igor]

with cleaning brake spray

 

[brettp]

Is that combustible?

 

[Igor]

probably yes. Someone recommended it here on some forum. If the rpm changes then there is a leak somewhere

 

[brettp]

I've only ever used carb cleaner for that test, not brake cleaner.
Are you using stock carb jets with stock airbox?

 

[Igor]

yes. Everything is original. I'm still wondering if there could be a problem with the choke. For example, it could be left a little open.

 

[brettp]

Yeah, that would richen the mixture

 

[brucewalker]

Your jets are dirty. Ultrasonic cleaner is the best way to get them clean.

 

[Igor]

I checked the choke, the idle jet, the fuel level in the float chamber, and the cleanliness and patency of the air passages and everything is ok

 

[brettp]

Do you have the right plugs? Also, I can't remember what is stock.

 

[Igor]

What plugs do you mean?

 

[brettp]

Spark plugs. If they are too cold of a plug (ie, the incorrect ones) they may not be hot enough to burn the fuel in the combustion chamber. Just a thought that it may not be a fuel delivery issue.

 

[Igor]

I think everything is correct.