Topic: 'cb750 k8 help with setup

[surveywaters]

1978 CB750k - I have new pistons, new valve guides/valves, cam chain/tensioner, pamco ignition, new wiring harness, Mac 4-2, stock airbox, stock #35 pilots,#120 mains, and the carbs are spotless and synced. The bike fires nicely but after running only a few minutes the headers are already a deep gold and there is the occasional quiet ping/pop at the carbs. Is the mixture screw fuel or air (out is richer right?)? I assume I am lean on the pilot but surely I don't need to go up on it with just an aftermarket exhaust. Any help or tips?

[Stoli]

Assuming you have the PD carbs, out is richer on the screw.

Opinions will vary but changing the exhaust can likely warrant a need to change the pilots (or slow jets as they are often called). The PDs came with 35s, so 38s may be in order.

Before you do that, do you know if the slow jets are clean? They are press in jets and need to be pulled out to access the tiny holes in the recess.

[uksparky]

Out is lean PD46 carbs...try 2.5/3 turns out..i have 100 mains, needle jet 4th ring from top.../pods/stock exhaust but did have Mac 4-1...seems three turns out runs smooth ..spark plugs neutral..

[surveywaters]

Out is lean PD46 carbs...try 2.5/3 turns out..i have 100 mains, needle jet 4th ring from top.../pods/stock exhaust but did have Mac 4-2...seems three turns out runs smooth ..spark plugs nuetral..

You went down on the main when installinf pods and exhaust?
Could the 120 be too rich?
What's up whith what sounds like a bit of a carb backfire?
Did your mac 4-2 color quickly? Could be just the cheap pipes showing there true colors.

[uksparky]

I bought the bike with the Mac..owner also gave me the stock exhaust....One of the POs changed the main jets stock..90 to 100...your 120s could be too rich.....are you getting pop/--pop..at idle/carb...try three turns out like i said....check for vacuum leaks between carbs/ intake

Sorry guys...screwed up me thinking 550k.....but the discussion is about the 750..

[lucky]

Out is lean PD46 carbs...try 2.5/3 turns out..i have 100 mains, needle jet 4th ring from top.../pods/stock exhaust but did have Mac 4-2...seems three turns out runs smooth ..spark plugs neutral..

The 1978 carbs are very different. 2.5 -3 turns would be excessive
 and the screw could fall out.

[surveywaters]

Just finished pulling the carbs. Pulled each pilot, they're still clean and no contaminants is the bowls. The mouth of the carbs was wet, it's possible the carbs are lean. I thought 110 was stock on the K8 and they ran lean from the factory. The 120's were in when I got the bike. Thanks for the catch Lucky I'll try 2 turns.
A side note, it was tough to start today, but fired right up yesterday after the engine was reassembled.
I would like someone with the 4-2 Macs weigh in about the initial color of their pipes.

[lucky]

The main jet has nothing to do with the idle.

The idle is controlled by the pilot jet and the pilot air screw only, up to 1/4 throttle.

All yu would need on that set up is a slight mixture /pilot screw adjustment to be richer.
Make sure you count the turns.
Start at ONE turn open on all of them. Increase by 1/4 turn and then let it have a chance to run.
Make sure they (pilot screws) are all the same.

Since you have the stock air box and filter you do not need to change the pilot (idle) jet.


About the main jet.
Take it out for a ride and when in 3rd or 4th gear at high speed if it just keeps accelerating then the main is ok. If it seems to flatten out and will not accelerate, then it is too lean and in danger of getting hot if you just kept that up.

[lucky]

Did you pull the pilot jets and actually look through them and see daylight?
If not one or more could be clogged or partially clogged.
You will need a .010 thousandths electric guitar string. The pilot(idle jet is about .013 thousandths.  DO NOT use a paper clip, won't work and will cause damage to the soft brass.
Then use carb clean and compressed air to finish.
Then look through them to see daylight.


Make sure all 4 pilot air screws(mixture screws) have a O ring in good condition and a washer and spring. The washer goes between the O ring and spring to keep the O ring from getting damaged by the end of the spring.

Make sure to take a pocket LED flashlight (engine off),(air box off),gas on. and look into
 the intake. Turn the throttle and make sure all 4 brass nozzles in the intakes squirt fuel when you turn the throttle.

I hope you set the floats correctly and MEASURED them the way the book tells you.
Do not use that "clear tube" method until they have been measured.
The Clymers' workshop manual tells you that. And in this case it is correct.


When all of that is done and you get it to idle then SYNC the carbs. DO NOT adjust the mixture screws! Only adjust the slide height by using the 8mm nuts on top of the carbs.
Make sure the big idle screw that pulls open all of the slides  is backed off all the way.

A little tip....
When the floats are set correctly they just happen to line up with the top of the idle jet. like this: 1977-78 models only. Do not press down on the float to make this happen.

[surveywaters]

Yes, I pulled the pilot jets again. I made a slide hammer attachment to replace the bolt in a vice grip, works great. They are spotless. The accelerator pump is functioning properly as well, all four squirt. I adjusted the pilot abit and it fired up much easier and idles better now. I think the color on the header right at the head is just because these pipes are cheap. If the color starts to ride down the pipe further I'll try again. Right now I am going to say this rebuild went off with out a hitch.

Thanks for the help and I would still like someone with the mac 4-2 who installe d them new to chime in about the color.

[lucky]

Yes, I pulled the pilot jets again. I made a slide hammer attachment to replace the bolt in a vice grip, works great. They are spotless. The accelerator pump is functioning properly as well, all four squirt. I adjusted the pilot abit and it fired up much easier and idles better now. I think the color on the header right at the head is just because these pipes are cheap. If the color starts to ride down the pipe further I'll try again. Right now I am going to say this rebuild went off with out a hitch.

Thanks for the help and I would still like someone with the mac 4-2 who installe d them new to chime in about the color.

Do not worry about the color of the exhaust.
Those colors on chrome or on stainless are cause when oxides on the metal when they get hot. Oxides can be from oil that was on the petal,polish,cleanes oil from peoples hands etc.,.
The colors are produced what the metal reaches 975º just like a rainbow colored TIG weld. They are oxides.

You can get a polish for chrome that can turn blue pipes right back to chrome with a buffer. A man showed that to me when I was 16 years old.

[surveywaters]

That's interesting to hear. I don't mind the blueing, it just happened so fast I thought I was lean. Thanks for the help.