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Carb Adjustment help '72 CB350K4

5864 Views 9 Replies 4 Participants Last post by  Crashoverride
Hello all,

I am having problems getting my carbs adjusted properly and was hoping there was an expert on here that might be able to help me out. I have a 1972 CB350K4 and I had it running pretty well. It had a little bit of a stumble at constant throttle position so I thought I would rebuild the carbs. I bought a very thorough kit from Common motor collective and set about fully disassembling, cleaning and rebuilding the carbs. While I was in there I got the bright idea that I would set the float height as well so that is potentially a second variable along with the rebuild.

The issue now is that you cannot apply more than half throttle without it bogging completely. It happens no matter what speed you are going (even in neutral). I do find that it revs better with the choke on so it seem like it is either being starved for fuel or is somehow getting an over abundance of air.

This was definitely the result of rebuilding the carb so I am focusing my troubleshooting to the carbs for now. So far I have take the carbs back apart several times to see if I could identify any mistake I might have made but I have yet to find any.

One other observation of note; The air mixture screws seem to produce the highest idle at fully closed. I know that they should be around 1 turn out but I don't know what specifically affects this. I also know that it is only in play at idle but I wanted to add it in case it indicates something else wrong that I can look into.

I set the floats at 23mm using the procedure detailed in this video.

I was careful to use all of the correctly sized jets from the kit (there were 2 different sizes included) and I believe that I used the bigger of the 2 for main jets as that is the size that was in the bike.

I synched the carbs carefully and I had just done the points and timing fairly recently (again, was running pretty well before the rebuild)

I really want to start riding the bike but I just can't seem to get to the bottom of the issue. My next troubleshooting step will probably be to put any of the usable original parts back into the carbs and see if I can't get back to where I was before the rebuild but I hate to go that direction.

One other note; I am running inline fuel filters but they were there before I did the rebuild (come to think of it, they might have been the reason for the slight hesitation at constant throttle) but I really don't think that they are the cause of this issue though.

Any help would be greatly appreciated.

Here is a couple pics of the bike.

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Thanks for moving the thread and thanks for the link. I read through the whole thing and I think the only thing that may apply is the felt o-ring part of the discussion. But it sounds like that is only an issue at idle. That may be why I am having problems with the idle mixture screw. I did remove them before dipping the carb bodies and was unsure how they sealed up and kept the air out. I will disassemble them again and see if the felt is still there and try oiling it.

Any thoughts on the bogging at half throttle?

Thanks again
I suspect your FLOAT is adjusted incorrectly and is actually flooding your motor at idle.

(2) reasons:

1 - there are no standard model carbs that were used on the 350's that use a 23mm float spec.
2 - the Air Screw is an AIR BLEED screw so by closeing it completly your are shutting down the amount of air it gets at idle.

What Model Carb do you have?

The model # is molded into the side of the carb.

99% of the K4 models came with model 722a
The float spec for that one is 26mm.

The Float distance has an inverse relationship with the height of the fuel in the float bowl.

The float hangs upside down.
As it "FLOATS" it goes up and shuts off the fuel.
The shorter the float distance the higher the fuel level in the bowl. (the less travel required to shut off the flow.)
Result Over rich at idle.

Note: one VERY Common mistake made on setting the float height on these carbs is not holding it at an approx 45*deg angle but holding it upside down, and letting the weight of the float rest on the needle valve.
The problem with that is the tip of the Needle Valve is SPRING LOADED. (to absorb bumps in the road that could cause the metal on metal valve to damage the seating area)
If that spring loaded tip is compressed while checing the adjustment it will throw off the float measurement by the amount of travel the spring was compressed.

I understand you used the proceedure shown in the video, but it was NOT made by HONDA.
If you reference the HONDA Factory Service Manual, you will see they require the 45 *Deg angle.
Also his reference to the fule level in the float bowl is totally incorrect.
Think it thru for yourself as to what happens to the fuel level as the adjustment changes. - it is NOT rocket science.

I would strongly recommend you go back and re-check your float height.

Then I would check your timing with a strobe light.
That is much more likley to be your original issue.
Thanks for the input. They are 722a's. I will try adjusting them at a 45 and see what that does.

As for checking the timing, I have done it using the manual procedure and, as I said, It was running much better before I rebuilt the carbs. I will recheck it though.

Thanks again
The important thing in working thru performance issues is not losing track of what you know.
Don't just go making random changes without a plan.

You "KNOW" the bike ran better before you rebuilt the carbs.

Therefore something you did in the process was incorrect.
The significant change according to your post was the FLOAT adjustment.
Set the Floats according to the FSM and see if things get better or at least back to where you were before.

Then start to eliminate things one by one.

A FULL Check of TUNE-UP proceedures is likely to be in order.

Start with Valve Adjustments on a COLD motor.
Timing Chain Adjustment. (make sure you follow the FSM here - ALL Valves MUST be CLOSED and LOOSE !)

See how or if that makes any difference.

Then Move onto your Ignition.
If you are running Points check the Gap.
It just has to be in the RANGE of the spec.
Rember Set the LEFT Side, Time the Opening.
Then Set the Timing for the RIGHT Side by adjusting its GAP.
Then Verify that gap is also in RANGE for the spec.

Static Timing is GOOD to get you "CLOSE" so the motor will run.
BUT... If you want full performance the only thing is Dynamic Timing using a Strobe Light. (IT MAKES a DIFFERENCE)
Checking the timing both at IDLE and full ADVANCE (approx 3500 RPM)

Then it is on to the Carb Idle Mixture Adjustments and Sync of the Throttle Plates.

If you have questions about these proceedures check the FSM and ask here.
PS - If you don't have an FSM Download a copy from the Common Motor Collective Site.
They have a PAGE that links to the Manuals in PDF form.
Thanks for the tips. I am going to go back through the entire bike this weekend.

Do you have a procedure for dynamic timing? I think I have a basic idea but a procedure would help.

Thanks again
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