Topic: Trouble starting bike/ carb tuning

[nateweisiger02]

Hey guys, finishing the last little tweaks on my 1978 CB750k. The only mod is a chopped exhaust. This past weekend, the bike would start with or without the choke on, and there was not much of a difference whether it was on or off. I will say, it sounded rough when it started up the first of the day, but as it idles for a minute or two and stumbles when I pull the gas, but after it is all warmed up it is good to go. At that time, I had the mixture screws turned an 1/8th turn out from where they were before I chopped the exhaust. I turned them back in to where it was before, hoping I would be able to start it with the choke like normal, and then turn it off when the engine is warming up. Now, it is having a hard time starting even with the choke on. And it bogs down even more for a good two-three minutes until the bike is warmed up, after that it runs great!!
Does anyone have any input on what I should do to correct this, or is it normal without having upgraded jets?
What is the stock tune for the mixture screws on these? I believe I remember reading that it is 2.5 turns out. I am debating just resetting them to that and going from there.

[Floshenbarnical]

Try a slow jet one size larger. The choke restricts air flow, which makes the fuel mix richer. If it's running, albeit poorly, on choke that means the fuel mix is probably too lean. Whenever you make a change to your air system (intake, exhaust) you usually have to compensate by rejetting.

The shop manual I have suggests your stock slow jet is a 40, I would try a 42. Then have fun messing with your mix screws, lol

Waiting for a SOHC vet to tell me why I'm an idiot, but that's my first thought.

[nateweisiger02]

Try a slow jet one size larger. The choke restricts air flow, which makes the fuel mix richer. If it's running, albeit poorly, on choke that means the fuel mix is probably too lean. Whenever you make a change to your air system (intake, exhaust) you usually have to compensate by rejetting.

The shop manual I have suggests your stock slow jet is a 40, I would try a 42. Then have fun messing with your mix screws, lol

Waiting for a SOHC vet to tell me why I'm an idiot, but that's my first thought.

Okay awesome, thank you!! After I rejet, how would you go about messing with the mixture screws? I read an article on cb750 tuning and based on how I perceived it, it said to get the bike up to temp, one carb at a time in until it starts to bog down, then turn out an 1/8th of a turn leaving it for 30 seconds, then repeat until you get to where the bike idles highest, then turn back in an 1/8th of a turn. Repeating down the carbs. Does this sound right? Or does anyone else have a good way of messing with the mixture screws?

[newday777]

Hey guys, finishing the last little tweaks on my 1978 CB750k. The only mod is a chopped exhaust. This past weekend, the bike would start with or without the choke on, and there was not much of a difference whether it was on or off. I will say, it sounded rough when it started up the first of the day, but as it idles for a minute or two and stumbles when I pull the gas, but after it is all warmed up it is good to go. At that time, I had the mixture screws turned an 1/8th turn out from where they were before I chopped the exhaust. I turned them back in to where it was before, hoping I would be able to start it with the choke like normal, and then turn it off when the engine is warming up. Now, it is having a hard time starting even with the choke on. And it bogs down even more for a good two-three minutes until the bike is warmed up, after that it runs great!!
Does anyone have any input on what I should do to correct this, or is it normal without having upgraded jets?
What is the stock tune for the mixture screws on these? I believe I remember reading that it is 2.5 turns out. I am debating just resetting them to that and going from there.

This makes me go back to my standard question of how did you clean your carbs? Your 78 has PD42 carbs which have pressed in slow jets. And it has an intricate accelerator pump circuit. If either were not properly cleaned out of the old hardened gas deposits in the circuits then you will have your results.

Here are some good links for cleaning your PD carbs. Read through them and determine if you cleaned them properly or not. Report back on your results and fixes you need to do.

http://forums.sohc4.net/index.php/topic,48858.0.html

[newday777]

And a video on reassembly after good ultrasonic cleaning and assuring of cleaned jetscand circuits.

[stocky]

And a video on reassembly after good ultrasonic cleaning and assuring of cleaned jetscand circuits.

That video helped a great amount when I was rebuilding my pd41A

[calj737]

If it's running, albeit poorly, on choke that means the fuel mix is probably too lean.

His original explanation stated that with or without choke it ran the same. That would lead me to think his mixture was already “rich”. Turning the air screws. 1/8 should not substantially affect the bike starting with choke unless he’s made the mixture even more rich.

I think he needs to reset his screws, then pull a plug and inspect them. Quite possibly they are fuel fouled. And then follow Stu’s link to accurately and properly clean his carbs.

A chopped exhaust will create more of a “lagging” motor at speed due to the decreased back pressure. I don’t think he’s there yet.

[nateweisiger02]

If it's running, albeit poorly, on choke that means the fuel mix is probably too lean.

His original explanation stated that with or without choke it ran the same. That would lead me to think his mixture was already “rich”. Turning the air screws. 1/8 should not substantially affect the bike starting with choke unless he’s made the mixture even more rich.

I think he needs to reset his screws, then pull a plug and inspect them. Quite possibly they are fuel fouled. And then follow Stu’s link to accurately and properly clean his carbs.


A chopped exhaust will create more of a “lagging” motor at speed due to the decreased back pressure. I don’t think he’s there yet.

When the bike would run with or without the choke, it was 1/8 turn out. I went to turn it in an 1/8th of a turn on each and then the bike started lagging. I am fimiliar with the chopped exhaust lagging because it has a lot less back pressure than the stock exhaust system. I rode with the chopped exhaust  anll last week and it did not do this, until I messed with the mixture screws .  The carbs are also spotless. Can anyone explain how to reset my mixture screws?

[Floshenbarnical]

If it's running, albeit poorly, on choke that means the fuel mix is probably too lean.

His original explanation stated that with or without choke it ran the same. That would lead me to think his mixture was already “rich”. Turning the air screws. 1/8 should not substantially affect the bike starting with choke unless he’s made the mixture even more rich.

I think he needs to reset his screws, then pull a plug and inspect them. Quite possibly they are fuel fouled. And then follow Stu’s link to accurately and properly clean his carbs.

I admit I misread that, I thought he said it was running better with choke.
A chopped exhaust will create more of a “lagging” motor at speed due to the decreased back pressure. I don’t think he’s there yet.

[calj737]

Pull a plug and inspect it. Start there.

[Rosinante]

 Can anyone explain how to reset my mixture screws?

If you are talking about the idle mixture screw, which are in the center of the carbs at the end nearest the motor, then the starting point is 1.5 turns out.  I am fiddling with this on my K8 and finding that 2 turns out works better.  YMMV.

In contrast, my bike needs choke to start and at least partial choke for quite some running time before the choke can be fully disengaged.  I am still troubleshooting.