Topic: How much does syncing affect idle?

[Paintedseat]

I replaced my slow jets to 38 from 35 and now the idle is high and I can't get it to go down. I am going to sync next. Can that be my problem? Carbs are rebuilt, new boots, valves adjusted, timing good and no air leaks. I can figure it out. Plugs a gapped to. 028 NGK. Stock air box with a 4 to 2 exhaust

[dave500]

syncing affects idle and off idle mostly.

[Yamahawk]

I replaced my slow jets to 38 from 35 and now the idle is high and I can't get it to go down. I am going to sync next. Can that be my problem? Carbs are rebuilt, new boots, valves adjusted, timing good and no air leaks. I can figure it out. Plugs a gapped to. 028 NGK. Stock air box with a 4 to 2 exhaust

When you rebuilt your carbs, did you use the idle mixture screws that came with the kit or did you keep the old ones? From what I know, the new ones have a passage drilled through them that needs to be sealed off to get the bike to idle down. Could be your problem. Pull one of the idle mixture screws and check it for the drilling in the tapered part of the screw.
Charlie

[Deltarider]

From what I know, the new ones have a passage drilled through them that needs to be sealed off to get the bike to idle down. Could be your problem. Pull one of the idle mixture screws and check it for the drilling in the tapered part of the screw.

How do you know? I know only one source that stated that.

[madmtnmotors]

From what I know, the new ones have a passage drilled through them that needs to be sealed off to get the bike to idle down. Could be your problem. Pull one of the idle mixture screws and check it for the drilling in the tapered part of the screw.

How do you know? I know only one source that stated that.

And that only applies to some models. What year and model are we talking about here?

[Yamahawk]

From what I know, the new ones have a passage drilled through them that needs to be sealed off to get the bike to idle down. Could be your problem. Pull one of the idle mixture screws and check it for the drilling in the tapered part of the screw.

How do you know? I know only one source that stated that.

Knowledge and wisdom are two things that we attain by experience.... but not necessarily both at once. If he checks that and finds they are drilled, he can replace them with his old originals, and see if that eliminates his problem... applying knowledge, can then lead to his attaining the wisdom of solving his problem. And, really he doesn't even tell us what make model and year bike he's referring to... just something to check.
Charlie

[ekpent]

 Is this the first time the bike has been run since the carb rebuild or did the idle suddenly change from just changing the slow jet ? Does the idle adjustment screw have any effect in lowering it ? If not then the slides may be set too high from the rebuild. Binding throttle cables could be a culprit also.

[madmtnmotors]

Binding throttle cables could be a culprit also.

+1

Seen it more than once.

[Paintedseat]

Slides open too far.


That has to be the issue. Yes first run with rebuild and pilot jets. The accelerator pump was not working because all the passages were blocked. It ran when I bought it but you could tell it was fuel starved. So I guess what I need to do is buy a sync tool and go to work. The cable isn't bindings and the choke is fully open. It is a 77 and as the PD carbs. I did think it was odd that I needed full choke to start in it was still barely running for the first 15 seconds. If the slides were open too much would that not be a good reason so much choke was needed even with the new jets? I will say this, it runs like a completely different bike with the cleaning and working accelerator pump.

[Paintedseat]

Is there a way to adjust all slides at once.?

[Yamahawk]

Is there a way to adjust all slides at once.?

Got four hands...? hehe just kidding. The idle screw will lift all four at once, but to adjust each carb individually is another matter. You do them according to a procedure for syncing your bank of carbs, you have one as a reference, then sync the others to that one.
Charlie

[Paintedseat]

Yeah I was hoping for a magic trick. I am pursuing a tool today

[Yamahawk]

Yeah I was hoping for a magic trick. I am pursuing a tool today

I miss the days when they had mercury in the tubes, being heavy, it gave precise readings. But alas, the EPA says we can't play with mercury anymore... I use a vacuum 4 gauge set, with the restrictor valves. They work well.
Charlie

[Paintedseat]

Can someone point me toward a good Pd41 manual or video on carb syn

[TwoTired]

Can someone point me toward a good Pd41 manual or video on carb syn

Does the Honda CB750 Shop manual and the chapter supplements not give you what you need?

[Paintedseat]

Not really. My problem is when the bike is warm it idles at 2k. The manual says to adjust idle to 950 to 1000 rpms. I can't do that. The idle adjustment screw is not even touching. I'm just trying to figure out why it's idling at 2k. I guess the slides are adjusted to far open. I have on synced carbs once and it was 15 years ago.

[TwoTired]

Do I understand you have not sync'd the carbs and you are complaining about idle issues?

You have 4 motors tied together with a common crank.  You don't think that one cylinder has enough power to make the other 3 motors turn faster?

Have you checked the head pipe temps during "idle".  Do you notice any one head pipe hotter than the others?  Do you think that associated "hot" cylinder might be working harder than the rest?

If you have PD carbs, the #2 cylinder doesn't have a slide height adjuster.  It will be your master reference.  You will adjust each of the other slides in turn to match the vacuum indications on your 4 station manometer.  You may have to interactively readjust your idle knob to lower your idle RPM.

If your slides are out of adjustment far enough.  One cylinder may drive the idle speed beyond the idle knob setting, making it ineffective.

All the MC shops I'm aware of, have made vacuum syncing carb checks a routine part of the 3000 mile tune up procedure, among the other items to be performed.

Cheers,

[Yamahawk]

+1 ^^^ what Lloyd said
   Have you checked the cable adjustment at the linkage and also up at the throttle? If the idle screw isn't touching, then check those parts of the pull throttle cable first.
Charlie

[Paintedseat]

I ordered a tool. I figured it was sync issue. I guess I am just trying to cover all my bases so I can have what I need to actually enjoy it.

[PeWe]

If running points, both must have point gaps set correctly and hopefully equal. If not horrible idle. Set ignition properly before doing carbs, jets or sync.

[Paintedseat]

Dyna S. Timing is on.

[TwoTired]

Have you verified that the mechanical timing advance is functional?  If stuck full advance, it might have an issue returning to idle, as well.   But, the idle adjust knob should allow the engine to drop to zero.

Cheers,

[Paintedseat]

Yes