Topic: 74 CB750 - Can't figure out high idle

[zerodaydave]

Hello -

I'm having issues with my 74 CB750. The main issue is I cannot drop the idle under 3k. The idle screw is turned all the way out and I just can't drop it below 3k. To me it's running extremely rich. I can smell the fuel sitting at a stop light.

Compression is good.
Fresh gas.
Good spark. Spark plugs are black, gas fouled
Timing is good.
Tappet Clearance is good. .005 I .008 E
Rebuilt the carbs and had them ultrasonically cleaned. (Carbs - 064A)
Floats are measured at 26mm and verified that they float.
Main is 105 Slow is 38
Air screw 1 3/8
Synced (when synced idling at 3k)
Has PODS

Anyone have any ideas?

Dave

[flybox1]

Clear tube test your bowls and adjust floats as needed
Idle plug chop...adjust out the richness with your IMS (idle mixture screws)
vacuum sync your carbs at lowest comfortable idle(1000-1400)....assuming all other maintenance items are complete and accurate

[golfman]

maybe cracked intake boots ?

[TwoTired]

Not super familiar with Cb750 Carbs.

But, on others, the carb sliders can get turned in during carb sync that the slides can't reach the "floor" of travel.  This make the idle adjust knob ineffective.

Since you have pods, take one off a see if backing out the idle knob allows the slides to close completely.

If it doesn't seat, then adjust the syncro adjust for that carb, and lock it down, call it the master.  And then do a vacuum sync to make the other carbs match your master.

Just an idea...

[HondaMan]

1. Look for vacuum leaks on all of the carb boots (mentioned above).
2. Turn the air screws in to 3/4 turn, or no more than 1 turn out.
3. Check the ignition timing with a timing light. See if it drops to the "F" mark for both sides when you pull the throttle closed (opposite normal direction). Timing advances above 1200 RPM, so this might be tricky: if you find the timing is advanced (which seems likely here), then forcibly turn the entire plate (to your right) to slow it down until you can get it to idle: then check for the "F" 1-4 and "F" 2-3 marks at idle speeds.

It is very rich: when the plugs run black, the engine will not idle down until it drowns because leftover fuel that did not burn on the last stroke is still available for the next: then it refuses to adjust to throttle.

[ekpent]

  If you or somebody else had the carbs apart and fiddled with the large adjuster nuts at the top of the carb slider rods then there is a good chance that someone did not leave enough 'headroom' in that adjustment to allow the idle screw to do its job. The nut would be too low making the rods sit to high and never allowing the slides to get low enough.  How many exposed threads are showing on the top of the big and small nuts on the top of the carbs ?

[zerodaydave]

Clear tube test your bowls and adjust floats as needed
Idle plug chop...adjust out the richness with your IMS (idle mixture screws)
vacuum sync your carbs at lowest comfortable idle(1000-1400)....assuming all other maintenance items are complete and accurate

All other maintenance items are complete and accurate. Float Bowels are at 26mm.

[zerodaydave]

maybe cracked intake boots ?

They are brand new but I've checked that anyways and it is good.

[zerodaydave]

1. Look for vacuum leaks on all of the carb boots (mentioned above).
2. Turn the air screws in to 3/4 turn, or no more than 1 turn out.
3. Check the ignition timing with a timing light. See if it drops to the "F" mark for both sides when you pull the throttle closed (opposite normal direction). Timing advances above 1200 RPM, so this might be tricky: if you find the timing is advanced (which seems likely here), then forcibly turn the entire plate (to your right) to slow it down until you can get it to idle: then check for the "F" 1-4 and "F" 2-3 marks at idle speeds.

It is very rich: when the plugs run black, the engine will not idle down until it drowns because leftover fuel that did not burn on the last stroke is still available for the next: then it refuses to adjust to throttle.

Thanks HondaMan. We will check the timing again and adjust the airscrews back to 3/4 -1 turn.

[flybox1]

Clear tube test your bowls and adjust floats as needed
Idle plug chop...adjust out the richness with your IMS (idle mixture screws)
vacuum sync your carbs at lowest comfortable idle(1000-1400)....assuming all other maintenance items are complete and accurate

All other maintenance items are complete and accurate. Float Bowels are at 26mm.

26mm float height dont mean nuthin
Fuel level sitting at 3-4mm below the top of the bowl does.
Clear tube test your bowl fuel level.  Adjust as needed.

[zerodaydave]

  If you or somebody else had the carbs apart and fiddled with the large adjuster nuts at the top of the carb slider rods then there is a good chance that someone did not leave enough 'headroom' in that adjustment to allow the idle screw to do its job. The nut would be too low making the rods sit to high and never allowing the slides to get low enough.  How many exposed threads are showing on the top of the big and small nuts on the top of the carbs ?

We did fiddle with it and there are 4 threads showing from the top which I'm sure is way to many. We couldn't get the 2nd carb to sync at all until we did this. After HondaMan's post I think we should go back and look at the timing and re-sync.

[zerodaydave]

Clear tube test your bowls and adjust floats as needed
Idle plug chop...adjust out the richness with your IMS (idle mixture screws)
vacuum sync your carbs at lowest comfortable idle(1000-1400)....assuming all other maintenance items are complete and accurate

All other maintenance items are complete and accurate. Float Bowels are at 26mm.

26mm float height dont mean nuthin
Fuel level sitting at 3-4mm below the top of the bowl does.
Clear tube test your bowl fuel level.  Adjust as needed.

Ok cool. I'll check that out and report back. Thanks for the advice.

[ekpent]

 If it is correct what you say and the idle knob is useless to you as there is no more adjustment to lower the revs then slides are probably set too high. Sometimes if you have a goofy carb that will not synch there is a possibility that a rod mount that sits inside the top of the slide may be bent from someone trying to force a carb open and will never be accurately set. One symptom is that the rod flops around more in the carb slide then the others but that has to be checked with the slides out. I have had to fix several of those before that I noticed while I cleaned 750 carbs.

[flybox1]

Even before adjusting the slides, the vac sync screws should be reset to 4 threads, then a check to make sure all slides bottom out at the same time the idle adjuster screw tip comes off its plate...

[BazF]

Inlet 5, exhaust 8 ?
Isn't that supposed to be I 2, E 3 ?

[calj737]

Forgive me, but is it possible you installed the slides backwards when assembling? You would not be the first to do so....

[ekpent]

 The slides have a slot in the sides and a pin in the carb body so they basically just go in one way on these carbs.

[TwoTired]

The slides have a slot in the sides and a pin in the carb body so they basically just go in one way on these carbs.

Even if you swap the left slides for the right slides?

[zerodaydave]

Inlet 5, exhaust 8 ?
Isn't that supposed to be I 2, E 3 ?

I thought it was Inlet 5 Exhaust 8 is that wrong? It's a 74 CB750

[BazF]

Mate, all my books tell me it is I 2 Ex 3 and it is critical. Mine is also '74 CB750.

[ekpent]

 From a service manual.

[BazF]

Well, that explains it!
I'm talking imperial. PO is talking millipedes and centipedes and stuff 

[zerodaydave]

So it looks like my timing was off + I had a vaccum leak around one of my intake boots. I don't know what I was thinking because we did check the timing before the valve adjustment but I'm glad I checked it again because it was off. As soon as I adjusted the timing to where it needed to be we were able to get the bike idling smoothly at 1k. I've ordered new boots and I will do a proper sync when I install them. Now I'm off to try to figure out why the bike is running so rich!

[zerodaydave]

maybe cracked intake boots ?

You were right. Cracked and swollen intake boot.

[zerodaydave]

1. Look for vacuum leaks on all of the carb boots (mentioned above).
2. Turn the air screws in to 3/4 turn, or no more than 1 turn out.
3. Check the ignition timing with a timing light. See if it drops to the "F" mark for both sides when you pull the throttle closed (opposite normal direction). Timing advances above 1200 RPM, so this might be tricky: if you find the timing is advanced (which seems likely here), then forcibly turn the entire plate (to your right) to slow it down until you can get it to idle: then check for the "F" 1-4 and "F" 2-3 marks at idle speeds.

It is very rich: when the plugs run black, the engine will not idle down until it drowns because leftover fuel that did not burn on the last stroke is still available for the next: then it refuses to adjust to throttle.

Thanks Hondaman. I did have a vaccum link. I did adjust the timing and now it's idling at 1k smoothly. The bike is still running rich. My screws are turned out to 1 1/8 per the manual should it be turned into 1?