Topic: 72 750k keeps fouling plugs help!!

[CBcafeJunkie]

Im running a 72 CB 750k
Had my carbs rebuilt by Bill Benton
105 mains
40 Slows
air screw 2 turns out
factory air box with K&N filter
de8 plugs

Bike starts with choke on then is run for about 10-15 minutes idling with some good throttle rolls and twists during.

however i go to start it back up having a very hard time  then nothing till spark plugs are replaced.

[Davez134]

2 turns out is a bit much, start with 1. Also bike is rich at idle anyways, so letting it idle for 10-15 then turning off repeatedly would probably foul plugs. You need to ride it around, get rpm's up to "clean" the plugs. Leak around rubber boots can cause these problems too. Have you set points/timing?

[CBcafeJunkie]

2 turns out is a bit much, start with 1. Also bike is rich at idle anyways, so letting it idle for 10-15 then turning off repeatedly would probably foul plugs. You need to ride it around, get rpm's up to "clean" the plugs. Leak around rubber boots can cause these problems too. Have you set points/timing?

Yeah points timing set. guess that means I need to get these brakes finished lol

[harisuluv]

I would check for air leaks first in your boots.  And I mean really do it, it is often times suggested and people just don't do it.  Just check it.

Something like this can happen no matter how good the carb rebuilder is.  Have you contacted Bill?  If I were you I would PM him first as he is probably the one person in the world currently with the most intimate knowledge of your carburetors.

It is probably a matter of settings and he would be best able to point you in the right direction.  Bill is a good guy and he will probably respond promptly.

[CBcafeJunkie]

I have contacted him and yes an awesome man, and always extremely helpful

Still haven't gotten to test for leaks in boots but as soon as I can get a good idle ill do it.

[ekpent]

Not that it gets to the heart of your problem but for me a 105 main would probably be considered a little small on a true 1972. Early bikes up to serial # 220000 ran a 120. From 2200001-2304500 was a #110 and a #105 after that. What kind of exhaust are you running,sounds like you have a stock airbox.
  Are you able to post some pics of the fouled plugs, are they black and wet. Are all 4 exhaust pipes getting hot ?  What else has been done to the engine,did you do a top end or anything and is there smoke coming from the exhaust ? Sorry for all the questions but it is a guessing game at this point.

[Stev-o]

Not that it gets to the heart of your problem but for me a 105 main would probably be considered a little small on a true 1972. Early bikes up to serial # 220000 ran a 120. From 2200001-2304500 was a #110 and a #105 after that. What kind of exhaust are you running,sounds like you have a stock airbox.
  Are you able to post some pics of the fouled plugs, are they black and wet. Are all 4 exhaust pipes getting hot ?  What else has been done to the engine,did you do a top end or anything and is there smoke coming from the exhaust ? Sorry for all the questions but it is a guessing game at this point.

Yes, we need more info to help you.  I think you have another issue, the choke should not be needed for 10-15 min, 3-4 min tops.

[CBcafeJunkie]

Well I think that the problem may have been that i was Idling to much and not taking it to higher RPMs for longer periods of time. I didn't realize that they ran that rich at idle.

Here is what i did
Took a blow torch to my plugs and they turned nice and white again
as soon as I put them back in it started up nice and strong.

and the best part is i took it for its first ride awesome!

As for rest of info I am running 4 into 1 with a tip no muffler ( hope it doesn't get me pulled over
and no engine work as of yet. I have a cb750F bike that I'm running a 72 750k engine in till i get my other engine built.

[CBcafeJunkie]

 well my engine is sort of running I can't get it to fully start now
. I changed the spark plugs. My engine is 1039408 with no modifications , I'm running carbs off of a 1975 CB750F
I know the 750F has 105 mains and 40 slow. Size 20 air jets

[harisuluv]

1039408.  That sounds like a K0 engine, you sure about that number?

[CBcafeJunkie]

Yup

[CBcafeJunkie]

After realizing that the bike has a k0 engine I quickly realized that the jets are obviously the wrong size and need a 120main anyone have extras the can ship?

[CBcafeJunkie]

I would check for air leaks first in your boots.  And I mean really do it, it is often times suggested and people just don't do it.  Just check it.

Something like this can happen no matter how good the carb rebuilder is.  Have you contacted Bill?  If I were you I would PM him first as he is probably the one person in the world currently with the most intimate knowledge of your carburetors.

It is probably a matter of settings and he would be best able to point you in the right direction.  Bill is a good guy and he will probably respond promptly.

Checked for air leaks as well with some starting fluid and a butane torch........obvious not together no leaks

[lucky]

Can you PLEASE show us a photo of the carbs?
If you want solid advice.

[ekpent]

Carbs should have a model number stamped on them also.

[Stev-o]

And what exhaust are you running? This will also determine what jets you should have 

[CBcafeJunkie]

I'm using the carbs off of the 75 750F don't have a picture at the moment. And a 4-1 probably mac with a tip no muffler. From what I understand should be good running stock with that exhaust setup right?I know the carbs that came with this engine were 4wire carbs but traded those to Bill as I am planing to build a 1000cc motor with the F engine

[CBcafeJunkie]

Here's  pics of the carbs, I added the pod filters to see if it was a air problem.. No avail. I couldn't find a number on the carbs

[harisuluv]

They don't look like 75 750F carbs, the backplate is K style and the casting on the side is not "F".  I would guess 657A or very early 657B, educated guess.

The air screw when turned out to about 1-1.5 is usually about flush with the opening, looks like you are turned in a bit past that. 

Here's a question, did you oil your pod filters?

[seanbarney41]

you can see the cast pad where the number is stamped in your last pic...see the brass plug/screw for the vacuum port where you attach your carb sync gauges?  It is right above that.  Stamping is tiny and often hard to read....

[CBcafeJunkie]

They don't look like 75 750F carbs, the backplate is K style and the casting on the side is not "F".  I would guess 657A or very early 657B, educated guess.

The air screw when turned out to about 1-1.5 is usually about flush with the opening, looks like you are turned in a bit past that. 

Here's a question, did you oil your pod filters?

Nope no oil on the pods but did oil my k&n air filter in the box

[CBcafeJunkie]

This is how my carbs are set up now


This is how the K0's are


I'm guessing that getting them closer to that would be a good idea right?

[harisuluv]

Maybe you didn't see, but I am not convinced you have 750F carbs, those settings are pretty irrelevant to your K0 engine anyways.  You need to tell us the stamp on the carbs.  Someone listed where it is located up a few posts.

[seanbarney41]

dude, mainjets are not your problem...you could have anything from 95 to 145 and it will not foul your plugs...why?, because you have to be 3/4 of the way to full throttle under a load before they even do anything(unless your fuel level in the bowls is too high).  First thing I would check?  voltage at the coils should be pretty damn close to 12.  Next, check your fuel level in the bowls with the clear tube method.

[Stev-o]

Maybe you didn't see, but I am not convinced you have 750F carbs, those settings are pretty irrelevant to your K0 engine anyways.  You need to tell us the stamp on the carbs.  Someone listed where it is located up a few posts.

+1......and you don't have stock exhaust!