`78 CB750 Trouble After Rebuild

[jsmits]

Hello everyone!

I've been working on this CB over the winter. It's my first project so it's been a tremendous learning experience. I did all the seals and gaskets in the engine top end, lapped the valves, struggled with figuring out the timing on the shaft but got it there after some study and trial and error. Struggled a lot with getting the bike wired back up correctly but again, got it there eventually. I've installed one of the Pamco replacement ignition with brand new coils as well.

Now, I've been having a hell of a time with the carbs and getting the thing to idle. I've tested spark numerous times, and can even get the thing to run for a bit by spraying a little carb cleaner into the carbs directly, but it doesn't seem to ever want to stay idling. I've tried numerous positions on the idle screws, with a sweet spot around 2 turns out. The bike fired up and ran for a bit, but I think it was because fuel had gotten up into the chamber during mounting/un-mounting. I put 120 mains on them and #43 pilot jets as the previous owner seemed to have way over-done it with 135's. I also made sure all the fuel passage ways were clear. The one thing I still haven't really looked into yet are the little tiny accelerator jets to make sure those are spritzing gas on throttle turns.

The plugs are dry but not white when I pull them after trying to start it, so my best guess is that it's just not getting enough fuel on idle, or the slides are not sitting right and it's getting too much air at idle. There are pods on the carbs and I have heard how troublesome that mod can be. Unfortunately I do not have the stock airbox and would have to find one on Ebay or something.

So in short, I can get it to start and make a few turns, but never idle. Do you guys have any ideas?

[TOOLS1]

Sounds like the pilot jets are plugged, or you have a vacuum leak. These carbs are well known for both problems.
TOOLS

[jsmits]

Pilot jets are brand new so that's probably ruled out. Vacuum leak sounds much more plausible. Do you have any advice how to trouble shoot or fix this problem?

[jsmits]

I've found some of the other posts on this site for these carbs. I'm betting the hoses are leaking. When I got this bike they were stiff as a rock. Early on into this project I boiled them in winter green. They are looser but cracked all along the brim and I've been trying to start it without securing the clamps.

To anyone landing here, these threads are helpful

http://honda-cb750-s.456789.n3.nabble.com/carb-issue-1978-CB750K-please-help-td4040678.html

http://honda-cb750-s.456789.n3.nabble.com/How-to-1978-CB750-slide-needle-shims-td3688760.html

[TOOLS1]

jsmits wrote

I've found some of the other posts on this site for these carbs. I'm betting the hoses are leaking. When I got this bike they were stiff as a rock. Early on into this project I boiled them in winter green. They are looser but cracked all along the brim and I've been trying to start it without securing the clamps.

To anyone landing here, these threads are helpful

http://honda-cb750-s.456789.n3.nabble.com/carb-issue-1978-CB750K-please-help-td4040678.html

http://honda-cb750-s.456789.n3.nabble.com/How-to-1978-CB750-slide-needle-shims-td3688760.html

Oh, for crying out loud!
TOOLS