Topic: Head against wall- Midrange problems

[lucky]

If the valves were that bad you would not get those compression numbers.

Forget that.

[lucky]

OK...
Lets weed out the data that really matters.

1973 CB750.
Exhaust? Questionable.
Intake? Don't know.
Main jet 110. (not stock)
Idle jet 40.
Needle position 4th down from the top.(not stock)
Mixture screw open 1 turn.

My suggestions.
Sounds and looks like you have ratty stock exhausts.
Could be clogged with debris. I would get new exhausts if you think they are rust buckets. Probably are . The vacuum gauge will support that.

***You never did tell us what kind of intake system you have.

The needle position is wrong. The clip should NOT be on the 4th groove down ,it should be on the 3rd groove down or middle groove. That 4th groove down is making it too rich in the midrange. When it is popping on acceleration it is too rich.

The stock main jets were #120's.

Idle jet stock was #40 so that is right, WITH stock air box and stock air filter.



Were those idle jets clean? Could you daylight through them?
IS the gas tank full of rust?
IS the fuel filter clean?  Just asking is all.

[josuepdx]

Stock air box, replacement air filter from cb750supply.com. HM341 exhaust, and yes to all of those settings. I've experimented with 7/8ths turns as well.

[lucky]

It sounds like your checking something wrong and timing is over advanced plus lean mixture.
Do you have air screws with holes in? (bleed holes)

He said the timing was correct.

[josuepdx]

On the early 73s, 110 was stock. Honda later changed it to 105s was my understanding and what I've found when researching. 4th notch down from the top, second from the bottom was also the stock setting for the needles.

Regardless, what I'm most concerned about is why cylinder 2 is running extremely lean relative to my other cylinders.

Exhaust is questionable, but other than that crack on #2, they are in great shape.
What do you mean by intake? The carbs are keihin 657A.

The tank had some sediment, but I clean that out long ago, cleaned the petcock filter, and installed inline fuel filters so I could monitor the fuel. They are still clean as a whistle.
And yes, the jets are nice and clean.

[Duke McDukiedook]

Take your carbs off and clean them one more time. Use wire, compressed air and carb cleaner on every orifice you can find.
Second time was a charm on my K3. 
Also, have you checked your HT wires from the ignition coils to make sure those are good and not burnt up?
I went with a K&N filter on recommendation from Mark and I think that helps balance out the 110 I have from my Keyster kit.

[josuepdx]

Well, 4th time wasn't the charm for me. Haha. 

And I have new coils and ht wires. Came with the Ultimate Pamco Kit.

[josuepdx]

Just double checked valve clearance on cylinder 4 in case I was getting valve lash. Everything in spec.

[lucky]

On the early 73s, 110 was stock. Honda later changed it to 105s was my understanding and what I've found when researching. 4th notch down from the top, second from the bottom was also the stock setting for the needles.

Regardless, what I'm most concerned about is why cylinder 2 is running extremely lean relative to my other cylinders.

Exhaust is questionable, but other than that crack on #2, they are in great shape.
What do you mean by intake? The carbs are keihin 657A.

The tank had some sediment, but I clean that out long ago, cleaned the petcock filter, and installed inline fuel filters so I could monitor the fuel. They are still clean as a whistle.
And yes, the jets are nice and clean.

The stingy 105's were on the F models. They had that factory 4 into 1 exhaust also.

[lucky]

On the early 73s, 110 was stock. Honda later changed it to 105s was my understanding and what I've found when researching. 4th notch down from the top, second from the bottom was also the stock setting for the needles.

Regardless, what I'm most concerned about is why cylinder 2 is running extremely lean relative to my other cylinders.

Exhaust is questionable, but other than that crack on #2, they are in great shape.
What do you mean by intake? The carbs are keihin 657A.

The tank had some sediment, but I clean that out long ago, cleaned the petcock filter, and installed inline fuel filters so I could monitor the fuel. They are still clean as a whistle.
And yes, the jets are nice and clean.

Where are you getting this information from?
I do not agree with you on that needle setting.
Ask others on this forum about that.

[josuepdx]

http://archives.sohc4.net/index.php?topic=12403.0

[Duke McDukiedook]

Chuck the inline filters, if your tank is cleaned all they will do is restrict flow.

[josuepdx]

Chuck the inline filters, if your tank is cleaned all they will do is restrict flow.

Yes, I know that. It was after the third time cleaning the carbs (and not finding any sediment or getting any change in preformace) that I installed the fuel filters just to make sure that a)I was getting fuel flow and b) check if I was getting any sediment at all.
Bike hasn't changed at all though.

[lucky]

What if Elgin dePajama was wrong?

You are basing all of your info on a guy named Elgin dePajama?

Why don't you look at this chart instead.
From the CB750 choppers Website.
Garage>Carburator specifications USA.
CLICK on the chart for a larger view.

[josuepdx]

This is what I was looking for. This is much more detailed than the choppers info sheet, and from a much more reputable site
http://www.sohc4.net/cb750k3/

I can't remember where I found the info about the needle placement, but I could have sworn I read it somewhere.
Anyways, I know I'm running a bit rich right now, but that doesn't solve problem. My problem is that one cylinder is running LEAN. Once I can get all of my cylinders to be doing the same thing, I'll be concerned with dialing the mixture in.

Last night I took off pipes 1 and 2, and blew them out with an air compressor. Seemed like it had nice flow. I did a temporary patch on the cracks until I can afford getting a new replacement pipe. They are ridiculously expensive! I might just have a shop patch it for me later. I also took the hard rubber from the air box to the carb off and soaked it in paint thinner to soften it up to see if maybe I'm getting a leak there. Doesn't have any cracks though.
Tonight I'll throw everything back together with new exhaust gaskets and see if I get any different results.

[lucky]

This is what I was looking for. This is much more detailed than the choppers info sheet, and from a much more reputable site
http://www.sohc4.net/cb750k3/

I can't remember where I found the info about the needle placement, but I could have sworn I read it somewhere.
Anyways, I know I'm running a bit rich right now, but that doesn't solve problem. My problem is that one cylinder is running LEAN. Once I can get all of my cylinders to be doing the same thing, I'll be concerned with dialing the mixture in.

Last night I took off pipes 1 and 2, and blew them out with an air compressor. Seemed like it had nice flow. I did a temporary patch on the cracks until I can afford getting a new replacement pipe. They are ridiculously expensive! I might just have a shop patch it for me later. I also took the hard rubber from the air box to the carb off and soaked it in paint thinner to soften it up to see if maybe I'm getting a leak there. Doesn't have any cracks though.
Tonight I'll throw everything back together with new exhaust gaskets and see if I get any different results.

"Much more reputable website" wow, The CB750 Choppers website is older than almost everyone on this forum>

[josuepdx]

Oh, have a sense of humor!
I'm trying to flatter you 

[lucky]

If 3 out of 4 cylinders are Rich what does that tell you?

I would think if one plug is different than all of the others then that one that is different may have a problem ,not the other way around.

[lucky]

Oh, have a sense of humor!
I'm trying to flatter you 

OK..I understand now.

[josuepdx]

Alright! I moved all of those pesky clips to the center position. Plugs look much healthier, except for cylinder 2...

Also, in the process of resyncing the carbs, I stripped the vacuum plug on carb 2 (my problem cylinder/carb). Just I was going to just tap the treads but instead I just got a replacement carb body off of ebay (in case there was damage to the carb body that I was just missing). I bought a carb rebuild kit and put it all together and thew it on. I also replaced both the rubber boots that go on each side of the carbs so that I could eliminate those as possibilities.

I STILL HAVE POPPING IN MY MIDRANGE AND A FUEL DELIVERY PROBLEM IN JUST CYLINDER 2.
Is it possible that one of my valves are sticking? Maybe a worn cam? I had my carb torn all the way down to literally nothing, so I am positive that my carbs are not the issue. The bowl is getting sufficient flow, height is correct, nothing is clogged in the carb. I'm getting spark. I have good compression. My head is about to explode. I'm pretty sure I'm going to be taking my top end apart soon to investigate what's going on in there. Does that sound like my next plan of attack here?

[Duke McDukiedook]

You never mentioned that you did a compression test from what I read- dry and wet.
Is this new carb body fully cleaned too? Every orifice with carb cleaner/compressed air/wire?
Do you have the old throttle needles? Try putting those back on if they are not worn or bent, HM says the Keyster needles have different profiles than stock.
 

[josuepdx]

Alright, ran my compression tests. I did the test COLD, with wide open throttle, and no air filter. I have heard some people say cold tests are fine, not sure what your opinions are about the matter but this still seems telling.
Cylinder 1: 140
Cylinder 2: 120
Cylinder 3: 130
Cylinder 4: 135

I fully cleaned the carb body with carb cleaner and a guitar string. I literally went through every possible orifice that I could find and sprayed the s*** out of it to make sure it was getting good flow.
I used the old throttle needle since it looked fine. Just in case though, I swapped the needle with a different carb to see if maybe my problem would move. If it did, my logic was that it mean that the needle was the problem, but it didn't of course.

[Vinhead1957]

Did you remove the tubes the needles mate with?   Those are the ones you miss.   Other than that I would see how it syncs up.

[josuepdx]

Did you remove the tubes the needles mate with?   Those are the ones you miss.   Other than that I would see how it syncs up.

Are you talking about the one that you have to push out from the top of the carb? If so, I replaced it already with a new one. The old one looked fine though.

[Vinhead1957]

Yes those that push out.    Mmmmm I still see what kind of vacuum it was pulling in the sync test/ adjustment