78 CB750 Rebuild Update / Questions

[jsmits]

Hello again everyone.

The bike has been street worthy for a few weeks now and I've been doing more work to tune it up. There are just a few more minor things I need to sort out. You guys have been so good in helping me gain insight and knowledge for my new hobby. I have a few questions about the bike's tuning that maybe you guys can help me out with.

Right now I have 120 mains, #42 pilot jets. It's been synced well and the idle screws adjusted carefully with a brand new digital meter.

My main concern is that the bike misses at higher RPMS above 5000. It happens a lot more when the bike is coming down from high rpms when you pull the clutch in and apply brake to start to slow down.  It's predominantly the first cylinder but you'll hear it on 4 as well. I feel and hear it a lot more when engine braking with the clutch when there's more resistance on the engine and it revs up.

My main concern, could the cam chain timing be off? I did the gaskets and valve seals when I rebuilt it, and was pretty sure I got the timing right but I could of been off a link. I'm going to do an accurate adjustment of the ignition advance tonight and see if that resolves it. Anyone whose had a similar problem and has some advice, it would help so much.

My throttle doesn't return all the way on gradual, light throttle but I'm hoping new cables will fix that. If not I might have to look into the throttle linkage as it only started happening after the carb sync. It's not a biggy but I'm sure the neighbors don't appreciate me having to rev it to get it to throttle down at low speeds.

Then there is the noise at low rpm idle. I've read that it could be something with the clutch plates and springs, but I haven't been in that part of the engine yet so I'm not sure. I'm wondering if my cam chain tensioner isn't tightening the chain enough as well. Are these known to gum up and not extend properly? I kind of just put that thing back on with little inspection when I did the rebuild. I'll probably take it off and take a look tonight. It's sort of a metal clanking/pop sound when the idle is set below 1100. It sounds like its at the bottom of the engine.

I pulled 3 and 4 plugs and they were golden brown on the insides, to black on the out. The fuel/air mixture seems to be as close as it'll get with pods on it, but I'm going to pull 1 and 2 tonight after running it a bit.

Thanks again for any help everyone!

[TOOLS1]

If your timing/cam chain was off by one tooth, you would be replacing all the valves, so that is not it. Since you say it is poping on 1-4, I would check the ignition timing. Those two cylinders run on the same set of points, condenser, and coils. It should be the set of points on the left. As for the engine not idling down, that could be the adjustments on the cables too tight. Try giving them some slack. It also could be that the carbs are not in sync, or that you have a vacuum leak.
TOOLS

[jsmits]

Thanks again tools. The carbs are synced up as of Sunday. I'm going to do another idle and ignition timing tune tonight. I'm getting better and better at it every time lol. The throttle sticking is intermittent. Seemed worse yesterday when it was humid out, and didn't stick as much today. I've got fresh cables on the way so hopefully that fixes it.

[Re-run]

With the cables, take off the push cable, the cable that closes the throttle. I have found that this is the cable that sometimes prevents closing all the way. I run mine with the push cable detached. I keep it on the bike in case the pull cable breaks, I can use it for backup.

What gap do you have your points set to? Set it to the minimum gap, I think it was .012. Smaller gap allows the coils to saturate longer and deliver better spark. You must have enough gap to allow the points to open and allow the coil field to collapse. Best dwell is achieved with a dwell meter but setting gap is ok too. A dwell meter is worth it IMO. I got one and once I used it, I saw how much better it is than setting just gap. Has a built in tach for setting idle mix too.

Course I changed to a dyna s and so don't need the dwell part but the tach still comes in handy.