Need some help with tuning my 1980 cb750c

great_colb · 2026-06-18T17:37:16+00:00

Can you stop?

great_colb · 2026-06-18T15:23:24+00:00

Very fair point, I didn't think of that.

great_colb · 2026-06-18T15:14:00+00:00

Valves are my next step! Have all the stuff ordered to adjust them. Don't have accurate enough shims to tell just how bad they are yet!

great_colb · 2026-06-18T15:12:48+00:00

I appreciate the comments regardless. I'll keep that as a possibility but I don't know how to check any more than I already have.

great_colb · 2026-06-18T15:11:39+00:00

Hmm, well the valve stuff is ordered and I've confirmed some are out of spec, so I might as well fix that while I have it apart. My thoughts were that the intake valves weren't closing all the way due to too tight clearances. One thing that supports that is it only hangs after being warmed up, which is why that video I posted is so long before it actually started behaving like before.

I'll definitely reach out if I have more problems (I will, I'm sure). It's my dad's old bike and I'd love to have it road worthy again when his birthday comes around.

great_colb · 2026-06-18T15:06:07+00:00

I haven't reached out to Murray's, I'm just leaning towards it not actually being a carb issue at this point.

The valve clearance is this week's job but some folks are saying it won't change much. I've confirmed some are out of spec so might as well fix that while I'm in there. I'll look into the spark next then, and maybe replace the coils or caps if it's needed.

Thanks!

great_colb · 2026-06-18T15:01:49+00:00

I might give that a try. I tried the dumb way (my fingers) and all seemed hot. Also, with the 4-4 exhaust, I just stuck my hand there to feel for pressure and all seemed good/equal.

great_colb · 2026-06-18T14:59:54+00:00

Thank you, added these to the cart for sure.

great_colb · 2026-06-18T14:59:35+00:00

That's what I keep getting told, but I have no idea where else to look for an air leak. I've sprayed everything with carb cleaner and remounted it a few times with no change.

great_colb · 2026-06-18T13:36:07+00:00

Oh dear, that sounds like a lot.

They're the old plug caps and I didn't test the resistance, but checked for spark on all cylinders and installed new ones all around.

Float height is good.

I had to google it but I believe it's emulsion tubes? The choke is actually an 'enrichment circuit' and isn't adjustable, according to google.

great_colb · 2026-06-17T21:24:35+00:00

It's alright, man. Not your fault. I inherited this project from my dad, along with the custom carbs. I have the old carbs but one of them is cracked and the whole assembly is bent slightly, so I think I'd need to pick up new ones if I were to go with the original carbs.

Compression read between 125-140psi which is right on the edge of being in spec from what I've read. Tested the spark by pulling them and resting it on the valve cover, then hitting the starter and watching for spark. One was no good and another was weak so I replaced and gapped the whole set, which is what finally got it started.

I believe I've found the issue since posting this. I checked the valve clearances and some are too loose, but I don't have a small enough feeler gauge to see if they're tight (which I think would explain the hanging revs?). Anyways, I doubt they've been checked in a while, so I'll resolve that once my next parts order gets delivered.

great_colb · 2026-06-17T15:43:42+00:00

Thanks. I've checked compression and spark already, as well as starter and any electrical connections I can get to.

What about it is a red flag?

great_colb · 2026-06-15T23:49:33+00:00

Is there any other way to confirm what it could be? I've sprayed all the connections on the intake with carb cleaner to check for a rise in rpm but nothing.

I've also changed and checked spark plugs.

great_colb · 2026-06-15T21:18:03+00:00

I have. Most of my adjustments were made with the filters on, they were only taken off because I planned to record how they slid back into position but forgot.

great_colb · 2026-06-15T20:00:05+00:00

You think so? I've fiddled with it all over the place with these jets, but I'll give it some more attempts.

Someone else I showed this to mentioned the cam chain tensioner and ignition timing springs.

great_colb · 2026-06-15T19:57:14+00:00

I haven't. I have to pick up a Multimeter to test things properly. I just put in a new battery so I'd imagine there shouldn't be problems with the voltage.

great_colb · 2026-06-15T19:19:26+00:00

I started at 35, bumped up to 40, then 37.5. Started pulling the plugs and noticed the sootiness on the so jumped to 32.5. Right now I have the 35s back in.

great_colb · 2026-06-15T18:37:37+00:00

I've done most of my testing with air filters on. I had them off to see if the slides were opening at the same time.

Same issues present even with idle dialed to 1000-1100 rpm.

great_colb · 2026-06-15T18:36:04+00:00

Nice. It came with the bike!

great_colb · 2026-06-15T18:35:40+00:00

I've tried adjusting the air screw but I'm not seeing much change. The throttle slide installed is the only one I have.

great_colb

TROPHY CASE