Topic: Purists BEWARE! 1978 CB750K Totally Stumped!

[Hayley]

I get it! A Chopper! Dreaded awful not stock Chopper!
I also get that without stock Air box and exhaust this is a crap shoot.
I'm going to try anyways..

This Bike has me so stumped!
First and foremost the bike is a 1978 cb750 with custom 4 into 2 exhaust, no baffles and a breadbox air filter (so gross right? just awful!)
There is absolutely 0 middle ground on this bike right now. I've rejetted, went back to stock, taped up air filters, untaped air filters, put baffles back in and took baffles back out.
The bike runs on points (also gross right? I should probably get an electronic ignition.. I agree but right now points).

The bike is either too lean all the time (popping nonstop on acceleration and hesitating), or it's too rich and completely unable to accelerate with no popping (with only change to taping up the breadbox even mm).

Whats new on the bike..
I rebuilt the entire top end of the bike, it's mint. All clearances are proper and it's been done with proper tooling.   
New pistons, valves, gaskets the whole works
New points and condensers
New wiring
Carbs are spotless and cleaned.
Accelerator pump works
No boot leaks
DR8EA/D8EA Firing properly
Reg/Rec checks out good
Coils good
Tank cap well vented
Fuel filter clean and has lots of flow

No matter what combo of bigger jets it runs rich, take it back to stock it's way too lean (runs like a night after drinking Coors Light lean)
If I run stock jets and tape the air filter off 3/4 it runs better than any other setup, (meaning it only sounds like a night after Heineken and not Coors) but it's still crappy.

What's weird is that I had it out last week and for 20 min it ran so beautifully, after I turned left from a stop.. it started popping like crazy.

I'm stumped.

[bryanj]

PD carbs?

[Hayley]

Pretty sure they're PD42A

[Bankerdanny]

One of the nice things about the PD carbs is that using the 'clear tube method' to double check your float settings is dead simple because they drain out from the overflow tube instead of the drain screw like all other SOHC4 carbs.

Attach a clear piece of fuel line to the drain tube that is long enough to hold the open end higher than the float bowl edge. Open the drain screw and observe where the fuel level is in relation to the seam where the float bowl meets the carb body. The fuel level should be a couple mm lower than the seam. Incorrect fuel levels in the bowl can impact the mixture.

[rotortiller]

What side is the petcock on, maybe something slid over the thing. Being a chopper and all it may be seeking its revenge upon you

I had it out last week and for 20 min it ran so beautifully, after I turned left from a stop.. it started popping like crazy.

[Hayley]

One of the nice things about the PD carbs is that using the 'clear tube method' to double check your float settings is dead simple because they drain out from the overflow tube instead of the drain screw like all other SOHC4 carbs.

Attach a clear piece of fuel line to the drain tube that is long enough to hold the open end higher than the float bowl edge. Open the drain screw and observe where the fuel level is in relation to the seam where the float bowl meets the carb body. The fuel level should be a couple mm lower than the seam. Incorrect fuel levels in the bowl can impact the mixture.

I'll try that!! Thank you! I set the floats but because the bike is 'up and out' it has crossed my mind that it could have some kind of issue.

[Hayley]

What side is the petcock on, maybe something slid over the thing. Being a chopper and all it may be seeking its revenge upon you

I had it out last week and for 20 min it ran so beautifully, after I turned left from a stop.. it started popping like crazy.

Haha They do have a mind of their own, I know this CB is mad at me.. she just wants to go fast but is so disgusted with itself it cant lol

[Doobie]

Haha They do have a mind of their own, I know this CB is mad at me.. she just wants to go fast but is so disgusted with itself it cant lol

It ain't the bike.

[TwoTired]

Have you tried adjusting the slide needle?   This adjusts mixtures from 1/8 to 3/4 throttle position.  It likely needs to be raised to enrich.  If it doesn't have adjustment grooves, you'll have to shim them, add grooves, or get needles from and F model that do have adjustment groves.

Cheers,

[Hayley]

Haha They do have a mind of their own, I know this CB is mad at me.. she just wants to go fast but is so disgusted with itself it cant lol

It ain't the bike.

 

[Hayley]

Have you tried adjusting the slide needle?   This adjusts mixtures from 1/8 to 3/4 throttle position.  It likely needs to be raised to enrich.  If it doesn't have adjustment grooves, you'll have to shim them, add grooves, or get needles from and F model that do have adjustment groves.

Cheers,

I will try this!

[754]

 If you want to sell the breadbox pm me.
 Something changed, you said when you turned.. I would check fuel lines first. And look for dirt in filters or carbs   I think outside bowls are easy to get off and will show if it's getting dirt. Not uncommon for a tank that sat dirty to have stuff working loose.

[bryanj]

Pd's clog the pilot jets for a passtime and as they are pressed in they get neglected

[Hayley]

If you want to sell the breadbox pm me.
 Something changed, you said when you turned.. I would check fuel lines first. And look for dirt in filters or carbs   I think outside bowls are easy to get off and will show if it's getting dirt. Not uncommon for a tank that sat dirty to have stuff working loose.

Hi again!
Hope your summer is going well!!
Gas Tank is brand new, but that's what made me think there was an issue with ventilation within the gas cap.  Added bigger vent holes in the cap, and even ran the bike around without a cap on at all. No change.
Carbs, filter and line are all clean. I put clear line in until I can figure out whats going on with the bike so I can watch it.

[754]

 Are the emulsion tube holes clear ?

[Hayley]

Are the emulsion tube holes clear ?

Totally clear..

[Keith]

Don’t trust NGK plugs!

[Hayley]

Whys that? I mean I'm on my 16th plug in the last lol...2 months

[BenelliSEI]

Read it all and here is what tormented me for months on my totally stock 1978 K8. Same symptoms as you. Run, then pop bang, fart and run again. Then worse, then better. I did everything, ignition, coils, points, valve clearances, float levels, jets, etc......

The FUSE BOX.

It had repeatedly, for years, over heated the main fuse. Melted the fuse box plastic base and the clips were loose. Clean, good fuse, but as it warmed up, intermittent connection. One day, instead of just running intermittently crappy, it totally stopped. No power at all. While poking around with a test light,  I spotted power across the fuse, but the load side clips were dead! Replaced it in total, and the bike has been running great since........ probably not, but have a look!

[PeWe]

Power glitches to coils tricky thing that might make the owner to think carbs and start to change things that will do things worse and permanent the problems when the initial issue is sorted.

I have wasted a lot of time and money changing jets when the problem was a H-T lead with intermittent disconnection 5 years ago.

The 12V all the way from battery to coils important maintenance point.
I'll check my K6 that has got some strange misfires 2 months ago, mostly at lower rpm/ throttle lifts.
I even tested my Dyna-S as active faulttracing.
I have still nightmares about it
 Back to points since problem remained or even worse with Dyna-S
Replaced all HT leads with caps, now even other coils and recrimped all connections.
Last thing is to take carbs apart, winter job.

Important to write a log about initial status and parts used, carb setting included. Write down what changed and test result.
If electrical issue, change carbs back to the initial setup.

[Hayley]

Power glitches to coils tricky thing that might make the owner to think carbs and start to change things that will do things worse and permanent the problems when the initial issue is sorted.

Last night the coils tested good, then tested bad, then tested good.. I think you're onto something!!
Also think there's a slew of other things going on, but after work i'll take it for spin.
Thank you