carb issue 1978 CB750K please help

[subtlySimple]

So for some background. 1978 CB750k. picked it up at the end of last summer it ran until the regulator went out
 I had to replace it. Over the winter I switched to a 4to1 exhaust and replaced all the plugs, fuses and cleaned the carbs and changed the main jets to a 115 . My problem now is that the engine is popping back into the carbs at idle and high rpms. It will idle but will die if I twist the throttle too quick.

I have been in and out of these carbs with a fine tooth comb. Used copper wire on all the circus. Replaced all the needles and seats.

Any help is much appreciated. I am getting far too frustrated.

[TOOLS1]

You have a vacuum leak. Probably the rubber boots between the carbs, and the head. Spray some carb cleaner around them with the bike idling, and if it picks up RPM you have found your leak. Also have you synced the carbs yet. Badly out of sync will cause problems like this also.
TOOLS

[Lucky 1]

4 into 1 exhaust.

What kind of intake?

I would bet you have pods. It will never run right.
Not because you have pods, but because you have to change the jetting and the
slide needle position.

Copper wire for cleaning the idle jets amy not work.
You need a stiff steel wire. I suggest using a #10 steel guitar string.
The hole in the stock jets is .013 thousandths so the .010 thousandths guitar string will do the job.
Then use compressed air and carb cleaner. Protect your eyes.


For pods and 4 into 1 exhaust use #120 main jet.
Drop the clip on the needle one groove, and use #42 idle jets.

The 78 probably has a non adjustable needle so you will have to use shims/washers OR get some 1977  or 1976 needles with the five groves.

[subtlySimple]

I have a stock intake. With a 4 to 1 exhaust

On May 19, 2013 5:28 PM, "Lucky 1 [via Honda CB750'S]" <[hidden email]> wrote:

4 into 1 exhaust.

What kind of intake?

I would bet you have pods. It will never run right.
Not because you have pods but because you have to change the jetting and the
slide needle position.

Copper wire for cleaning the idle jets amy not work.
Tou need a stiff steel wire. I suggest using a #10 steel guitar string.
The hole in the stock jets is .013 thousandths so the .010 thousandths guitar string will do the job.
Then use compressed air and carb cleaner. Protect your eyes.


For pods and 4 into 1 exhaust use #120 main jet.
Drop the clip on the needle one groove, and use #42 idle jets.

The 78 probably has a non adjustable needle so you will have to use shims/washers OR get some 1977  or 1976 needles with the five groves.

Working on a CB750 Digger named Rover

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[shinyribs]

subtlySimple wrote

Problem 1:  My problem now is that the engine is popping back into the carbs at idle and high rpms.

Problem 2:   It will idle but will die if I twist the throttle too quick.

Problem 1: Sounds like either a vacuum leak,like Tools said,or an intake valve set too tight.

When an engine pops,or backfires,thru the carburetor it is caused by either a overly lean fuel condition or an intake valve that is not closed when the spark plug is firing.

The intake valve adjusted too tight is self explanatory: the spark plug fires and the combustion process is ''leaking'' back into the intake tract,and eventually the carburetor.

The overly lean condition causes a back fire because the ultra lean fuel/air charge preignites off of engine compression before the intake valve gets to close completely. Again,leaking back into the intake tract.

Problem 2: Symptom of a vacuum leak or a faulty accelerator pump.<-- Which could also explain your other problem,since a non working accelerator pump obviously will create a lean condition,too.

When it pops at high rpms does it continue to pop the whole time you stay in the throttle,or only when you first crank it open?

If it continues I'd look into a vacuum leak.

If it's only when you initially crack the throttle open,I'd look at the accelerator pump.

[Lucky 1]

With a performance exhaust and completely stock intake you will only need to adjust the mixture screws.

If you have eliminated all vacuum leaks and the accelerator pump system works,
then I would raise the needle by lowering the clip one notch.


To check the accelerator pump system use a bright LED pocket flashlight and look into the intakes
(ENGINE OFF) Turn the throttle a couple times and see if a squirt of gas comes out of the brass post sticking up in the intake of the carb.

 If it does not then you will need to clean the accelerator pump and make sure the two check valves are working.