Topic: CB750K8 Issues with Carbs

[MrJey]

Hi everyone,
I need some advise on my carbs on my CB750K8.
Bike starts great however, as soon as I pull on the throttle it basically wants to die.

Carbs are cleaned and rebuilt.
Floats are adjusted (please see picture below)
Valve clearance is adjusted.
Chain tensioner is adjusted as well.
Using a new Dyna ignition.
Plugs are new
Wiring is new
Ignition coils are new as well.
Carbs are PD42B with 115 main jets
4into1 exhaust
Open pods

Haven't received my vacuum gauge yet, so carbs are only bench synched at this point.

One thing to notice, exhaust 2 and 4 are heating up much quicker. #3 heats up the slowest.
It also backfires quite happily..

I'm a bit clueless at the moment and was hoping someone here could give me some advise on how to proceed/what else to check.
I also uploaded a video. Unfortunately the sound isn't that great.

https://youtube.com/shorts/_mYVh5d4AcY?feature=share

thanks in advance,
Tom

[newday777]

The accelerator pump circuit is plugged. More cleaning of the carbs is needed.

[bryanj]

PD carbs again! The pilot jets need to come out and the carbs ultrasonicly cleaned, cleaner spray and air is not good enough

[denward17]

Is the choke working properly?

I just went through this with mine, these beasts are cold natured and I think I had the timing off a little.

[madmtnmotors]

Good info on PD carbs:

http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204

[TwoTired]

The pilot jets are pressed in.  Must yank them out to clean and then tap back in. The very smallest fuel delivery orifice in the carbs.

The accelerator pump system is unlikely to be working.  It has fuel circuits with balance restrictor tubes between carbs, don’t lose them.   The check valve in the pump has small balls that need cleaning and proving they can move and seal in their one way flow direction.  The pump diaphragm must not be torn or brittle.  And the “mouse ear” holes must be open to the same diameter as the holes they surround.
The throttle link actuator rod must have proper engagement clearance.  Some replacement rods are too short.
 At the entrance of each carb is a post with the squirter jets.  When the throttle is twisted, a stream of fuel should squirt all the way to front of carb in a very strong stream.

The pod filters are leaning the carb’s mixtures, so, that will require new settings for mains, midrange, and pilot circuits in each carb. 
The pilot mixture screws have very small narrow tips that can jam and break off in the carb throat exit holes if overly forced when seated. Check these holes for blockage.  But, even when clear and properly shaped, the stock setting won’t be correct without the stock air box and filter installed.

The 4 into 1 exhaust likely has less restriction than the stock 4to4. Less back pressure means stock carb settings will deliver too lean in all throttle positions.  Expect to enrich all three fuel paths in each carb to compensate for that change, too.

[newday777]

Hi everyone,
I need some advise on my carbs on my CB750K8.
Bike starts great however, as soon as I pull on the throttle it basically wants to die.

Carbs are cleaned and rebuilt.
Floats are adjusted (please see picture below)
Valve clearance is adjusted.
Chain tensioner is adjusted as well.
Using a new Dyna ignition.
Plugs are new
Wiring is new
Ignition coils are new as well.
Carbs are PD42B with 115 main jets
4into1 exhaust
Open pods

Haven't received my vacuum gauge yet, so carbs are only bench synched at this point.

One thing to notice, exhaust 2 and 4 are heating up much quicker. #3 heats up the slowest.
It also backfires quite happily..

I'm a bit clueless at the moment and was hoping someone here could give me some advise on how to proceed/what else to check.
I also uploaded a video. Unfortunately the sound isn't that great.

https://youtube.com/shorts/_mYVh5d4AcY?feature=share

thanks in advance,
Tom

Tom
Welcome to the forum
I see you haven't replied back on this thread of yours, it's important not to leave your threads hanging. Did you get it fixed and running properly? What was the outcome?