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Author Topic: Carb tuning problem - CB750  (Read 622 times)

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Offline thormeyer

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Carb tuning problem - CB750
« on: September 21, 2013, 08:58:26 AM »
Hey everyone.

I have just finished my Honda CB750 cafe project that i started last winter, but I'm having some issues with my carbs.
I bougth the bike as a non runner and the guy I bought it from said it was over 3 years since it last had run. So i have no idea how the carbs and engine ran before i took it apart.  8)

So that was the history of the bike, now the engine has been overhauled and rebuild and I was ready to ride, but no.  :-\

Bike setup is as follows:

I have older type K2 carbs ( I think its the B750A model, I will double check..!)
Main jet - 115 (before new jets)
Slow jet - 40 (before new jets)
4 into 1 exhaust
Pod filter ( the are very old and two of them are in a pretty bad state - should I replace? )
All new Power arc ignition with new coil, cables and iridium spark plugs.

I had it running pretty good but it had some midrange problems, and it stops revving at 6500-7000 rpm. It sounded like it was running lean so i bought new jets from sigma6jetkit.com.
I pulled the carbs of and installed the jets ( they recommended 132.5 in cyl. 2-3 and 135 in 1-4 and 42 in the slow jets )

After I installed the jets the bike has been acting very weird. The nr. 2-3 cyl doesn't fire in idle (exhaust cool) and the plugs get very black.
I thought it had something to do with the ignition or plugs, but there was fine spark in all plugs and I double checked the timing.

After 4 hours of playing with this and getting nowhere I pulled the carbs of and put the old jets in. But surprisingly it ran just as bad.

So now i'm stuck on what to do...

Hope some of you have some good ideas....   ;D

all the best
Thor Meyer

Offline harisuluv

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Re: Carb tuning problem - CB750
« Reply #1 on: September 21, 2013, 09:51:22 AM »
Sounds like you fouled the plugs from running rich, try new ones
Check out my carb rebuild thread! http://forums.sohc4.net/index.php?topic=105640.0

Need carburetor parts?  http://forums.sohc4.net/index.php?topic=115539.0

Cafe hoops regular and upswept version: http://forums.sohc4.net/index.php?topic=120877.0

Offline thormeyer

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Re: Carb tuning problem - CB750
« Reply #2 on: September 21, 2013, 10:31:47 AM »
That was the first thing I thought of but the problem is only in idle. When I give it gas the 2-3 fire. So I think more it's a carb problem then a plug/ignition problem.. :-/


Offline harisuluv

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Re: Carb tuning problem - CB750
« Reply #3 on: September 21, 2013, 11:15:41 AM »
Why would it be a carb problem if you changed it back to the way it was previously? 
Check out my carb rebuild thread! http://forums.sohc4.net/index.php?topic=105640.0

Need carburetor parts?  http://forums.sohc4.net/index.php?topic=115539.0

Cafe hoops regular and upswept version: http://forums.sohc4.net/index.php?topic=120877.0

Offline thormeyer

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Re: Carb tuning problem - CB750
« Reply #4 on: September 23, 2013, 01:44:18 PM »
I found the issue:

The slow jets in the number 2 and 3 carb where clogged, so I took out the jets and cleaned them.
Also the float heights where wrong, so I readjusted the heights. 

now it runs on all four cylinders but i need the to make the final adjustments. ( bigger main jet - 120, bigger slow jet - 42 and new pod filters)

 

Offline nccb

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  • 1974 CB750 Four
Re: Carb tuning problem - CB750
« Reply #5 on: September 23, 2013, 01:51:04 PM »
Do you know what might have been the cause of clogged jets since you just installed them?

Offline lucky

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Re: Carb tuning problem - CB750
« Reply #6 on: September 23, 2013, 07:25:31 PM »
Do you know what might have been the cause of clogged jets since you just installed them?

How about sitting around for 3 years.

Offline nccb

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Re: Carb tuning problem - CB750
« Reply #7 on: September 24, 2013, 04:31:05 AM »
well thats what I was referring too 8)  I just didn't want the OP to think his bike was good to go if he clogged brand new jets right off the bat, could happen again.

Offline harisuluv

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Re: Carb tuning problem - CB750
« Reply #8 on: September 24, 2013, 04:48:14 AM »
I found the issue:

The slow jets in the number 2 and 3 carb where clogged, so I took out the jets and cleaned them.
Also the float heights where wrong, so I readjusted the heights. 

now it runs on all four cylinders but i need the to make the final adjustments. ( bigger main jet - 120, bigger slow jet - 42 and new pod filters)

 

If you are going to put on pods they are something that come with their own baggage as far as tuning. You can't tune with them, you have to tune for them.  Buying new jets to dial in your current setup then adding pods into the mix is asking for headaches.
Check out my carb rebuild thread! http://forums.sohc4.net/index.php?topic=105640.0

Need carburetor parts?  http://forums.sohc4.net/index.php?topic=115539.0

Cafe hoops regular and upswept version: http://forums.sohc4.net/index.php?topic=120877.0

Offline metalmechanix3

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Re: Carb tuning problem - CB750
« Reply #9 on: September 24, 2013, 05:08:16 AM »
I agree pods are quirky and side draft issues. I run velocity stacks now instead of the pods. Mine is later 78 carbs though. I have heard issues with the iriduim plugs I would stick to ngk myself. Sound like you got crap in gas tank clogging jets. clean tank and run inline filter a clear one would let you see any rust etc. If you still have a mid range issue usually needle adjustment is what effects the mid range the most.

Offline lucky

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Re: Carb tuning problem - CB750
« Reply #10 on: September 24, 2013, 07:59:26 AM »
Tuning for pods or for velocity stacks is almost the same. Very close.

Offline thormeyer

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Re: Carb tuning problem - CB750
« Reply #11 on: September 29, 2013, 08:31:49 AM »
Do you know what might have been the cause of clogged jets since you just installed them?

I got crap from the gas tank. So i have put new filter and cleaned the tank

 

;
Honda