Topic: CB750 Warm Deceleration Idle Stall

[Simpson]

What could be a reasons for a cb750 to stall coming to a stop off a hard deceleration?
But, when just running at an idle its fine.
Assume that the carb is sync'd, and the timing is correct.

Thanks

[Inigo Montoya]

Either too much gas or not enough I suppose. Maybe pulling the plugs and checking them next time it dies would tell which it is. It might depend on if it is a sudden kill or if it gurgles and dies.
If it is sudden, it may be too lean. If a gurgle, then possibly too rich.

[HondaMan]

Check your air screw settings. They should be at 1 turn. See if the plugs are black: that will do it, too.

You didn't mention which year it is: starting with the late K3 there are tiny holes drilled in the ends of the air screws and the internal carb passages were different. Make sure those holes, if yours has them, are VERY clean. Use a tiny drill bit to clear them out, a wire won't do it. 

[Simpson]

Check your air screw settings. They should be at 1 turn. See if the plugs are black: that will do it, too.

You didn't mention which year it is: starting with the late K3 there are tiny holes drilled in the ends of the air screws and the internal carb passages were different. Make sure those holes, if yours has them, are VERY clean. Use a tiny drill bit to clear them out, a wire won't do it. 

Glad you all chimed in!

Here's my #029054 CB750 K0 "issue"...
Last year trying to dial in the carb (first year after sitting 20 years) I had to changed the slow idle jets to #42-44. Additionally I raised the needles up one step for balance I guess. This help over come a nasty take off stutter and poor idle especially when cold. The idle was okay and acceleration was good after re-jetting. The engine had a tad too much vibration seeming to run too rich. But it was okay for the season.

This year, (keep in mind I took the bike apart this winter replaced intake clamps etc etc...) about 1-2 weeks ago it suddenly acts if I'm running rich. I come hot into a stop with high rpm and the engine dies out. Therefore yesterday I move back to #40 slow jets and the engine runs smoother but still suffer from a stall.

Which actually great news. It should be because I adjusted it way rich to compensate for a air leak or a factor I couldn't understand last season. Believe me, I looked everywhere for an air leak!!!!!

Today I adjusted the needles down a step to the middle and it accelerates smoother but still has the grumpy hard decel idle issue. Its better but can be very flighty with a possible stall. I've been fiddling with the idle screws to compensate.

Setup:
#40 slow jet
Air screw at around 1 turn
needles in center
sync carb
timing correct per timing gun

Running Characteristics: Everything outside of the hard decel stumble runs like an absolute dream. Highway speeds and accelerations are very smooth strong.

[sellitsoon]

Im having carb problems as well so I can't help.....but lets ride.  My bike is done!

[Simpson]

Im having carb problems as well so I can't help.....but lets ride.  My bike is done!

I'm ready, a stall here and there will not stop me. Of course I would probably take the 360. That bike is a hoot. Went to the post office this morning to pick up some bike parts with my 750. I didn't realize one of the bike packages was a rear rack. The box was huge as you might imagine. Sat and thought about it in the parking lot and ended up setting a corner of the box in one side saddle and wrapping the bike cover I had in storage around the entire rear end of the bike to strap it down. Made it home no problems. 

[HondaMan]

Check your air screw settings. They should be at 1 turn. See if the plugs are black: that will do it, too.

You didn't mention which year it is: starting with the late K3 there are tiny holes drilled in the ends of the air screws and the internal carb passages were different. Make sure those holes, if yours has them, are VERY clean. Use a tiny drill bit to clear them out, a wire won't do it. 

Glad you all chimed in!

Here's my #029054 CB750 K0 "issue"...
Last year trying to dial in the carb (first year after sitting 20 years) ...

After a sojourn like that, 2 things come to mind:
1. Have you changed the condensors? They will be very leaky if they are more than 5 years old with no operation: without electrical activity inside, the insulation thins out and even cracks. This will make them sap away low-RPM spark.
2. The emulsifier air bleed tubes may be partially blocked. The main symptom of this will be chronically dark plugs, fouling out in less than 100 miles. Search for "emulsifier bleed" and my pictures on how to clean that should show up (maybe in the Thoughts of Hondaman collection).

[Inigo Montoya]

I could see the condensers being possible. What air filter is being used on this bike?
From reading, the emgo filters are too restrictive and so if you have a cheap 10-12 dollar filter, it is probably an emgo.
Might be something to check out.

[Simpson]

Alright, I hoping people are still tuning into this thread, as it's still acting odd.

To answer some questions:

After a sojourn like that, 2 things come to mind:
1. Have you changed the condensors? They will be very leaky if they are more than 5 years old with no operation: without electrical activity inside, the insulation thins out and even cracks. This will make them sap away low-RPM spark.
2. The emulsifier air bleed tubes may be partially blocked. The main symptom of this will be chronically dark plugs, fouling out in less than 100 miles. Search for "emulsifier bleed" and my pictures on how to clean that should show up (maybe in the Thoughts of Hondaman collection).

1.) New condenser bought last year.
2.) My plugs are mostly tan or tan-ish across the board.
3.) New stock air filter with box.

Here's a better description of what is happening...

When riding in cold weather tonight (39) the engine would stall easily coming to stops and idle poorly. During the day today the temp was around 65 and the idling climbed up high to 1200-1300rpm and I had to adjust it down. Seems like a lean issue as I type it. Perhaps my slow jets holes have deposits in them rather than the emulsifier holes?

All other parts of riding is absolutely wonderful. Fast, smooth and responsive.

I'm glad to be back to running to manufacture spec. I have no idea what caused this suddenly.
#40 idle jet
#120 main
needles at center
air screws around 1 turn

[w1sa]

If it has good colour on plugs after normal running and deceleration and also after some warm idling and yet is cold blooded/cranky until warmed right up, I would (after checking tubes, screws, jets) also consider the possibility that the fuel level in the bowls is being maintained at a slightly lower level than desirable.

The warmed engine will assist atomisation and normal running revs may enable the fuel to be drawn successfully from the bowls. But a sudden closing of the throttle from higher revs may momentarily catch the fuel at too low a level in the bowls/jets to be drawn efficiently for idle.

[Simpson]

If it has good colour on plugs after normal running and deceleration and also after some warm idling and yet is cold blooded/cranky until warmed right up, I would (after checking tubes, screws, jets) also consider the possibility that the fuel level in the bowls is being maintained at a slightly lower level than desirable.

The warmed engine will assist atomisation and normal running revs may enable the fuel to be drawn successfully from the bowls. But a sudden closing of the throttle from higher revs may momentarily catch the fuel at too low a level in the bowls/jets to be drawn efficiently for idle.

Some good advice in there. I guess looking back over my last post, it doesn't seem likely it would be a slow jet issue, seeing how I just exchanged the #42 to the #40 and I'm still having the issue.
It is running nice outside this problem. Good news is summer is coming! 

Its definitely temp related behavior. warm temps give high idles, low temps create stalls. Rolling to stops last night really showed that coming off the highway in 39 degree air made it stall right out. Couple lights later allowing it to warm up in low speed operation (less cooling), it begins to idle better.
hmmm 

[Inigo Montoya]

What brand of air filter are you running? It might be a paper but is it honda?

Also, maybe clean out the vent hole in the gas cap. this will help the fuel flow better to the bowls.

[Simpson]

okay, i adjusted the air and idle screws more this morning with good results so far.
I feel like a jackass!
Should have explored that more before posting up.
Had some paradigms stuck in my head from adjusting with last year jetting...

To answer the previous question, I have a new stock Honda air filter.
My bike is now almost totally stock setup (except a very slightly taller sprocket gearing).