Topic: Diagnostic Trivia **SOLVED**: No Combustion 4th Cylinder in 5th Gear

[YoungBlood]

Here's what I have done (and the problem existed before tune up, and I double checked all tune up specs--problem still exists):
Cam chain adjustment
Valve adjustment
Contact Breaker adjustment
Static Timing
Carb Synch (Bike will idle at 1000, 1100rpm)
Replaced 1/4 and 2/3 Coils
New plug resistors: NGK VD05F and SD05F
New plugs: all 4 ~.026 gap, NGK D8EA
New rectifier
New condensers
Oil change

My problem: Excessive backfire on deceleration, I suspect it is the unburnt fuel because I am running rich. On highway, fourth cylinder cuts out, loss of power in 5th gear. Once getting off highway, engine is running on cylinders 1,2,3. If the bike sits for a few minutes (still running, or not), the fourth cylinder will slowly start to get hot. Cylinders 1,2,3 get hot hot, #4 hot, but not as hot as 1,2,3. Exhaust smells like gas.
My suspicion is I am running rich, and for some reason the bike is being suffocated with gas in cylinder four when in 5th gear. Thus, the bike cannot handle sparking the gas, gets flooded or something?
I need to know where to go hunting next. Carburetor? Float level? Main jet sizes? Emulsion tube (whatever that is?) Needle position?

[Alan F.]

Test ride with points cover off, no change swap condensor positions. I believe this might be similar to what another member is/was going through.

Trying to approach from a least cost/least effort perspective.

You could also swap plugs 1&2 and 3&4 to see if any change is made.

Move the easy stuff and watch for the problem to follow, no need to switch back if no improvement or difference is noted.

[Bodi]

What model bike?
Seems like carbs are all that remains. A thorough cleaning is suggested.
The main jet emulsion tubes are under the main jet, sticking up into the carb throat so the needle pokes into its end there - on most Keihin carbs it is also the needle jet. Fuel is sucked up through the main jet but there they have a bunch of little cross drilled holes where air also gets sucked in, making a gasoline foam passing through the tube into the carb throat.
These cross holes can be gummed up and blocked. Result: just gas, no air, not foam. Naturally that means richer mixture into the engine.
Fairly common cause of too-rich condition at higher throttle on these old bikes.
The pilot jet also has the emulsion tube and holes but it's all one piece. Some carbs adjust idle mixture with a fuel metering screw, in=leaner... others the screw meters the air that foams the gas, in=richer.

Possibly a PO rebuilt the carbs with aftermarket brass parts. Usually a bad idea.

Pedants: yes I know there is no sucking, just pushing.

[YoungBlood]

Thanks guys... Alan, when you say "no change swap condenser positions", do you mean to move one of them up, and the other down? So the condenser will still be linked with 1/4, or 2/3 as before, but they will be flipped 180 deg. from where they are orginally positioned?

Bodi, the bike is a 1975 Honda Cb750F. Thanks for everything you said, this was informative and helpful. I do have a rebuild kit, I scored it off of 4into1.com, this link:  https://4into1.com/set-of-4-carburetor-rebuild-kits-honda-cb750f-super-sport-1975-1976/ I know the guy before me "cleaned the carbs", but he didn't say he rebuilt them. Are these aftermarket brass?Sounds like the emulsion tubes would be the culprit, does ethanol gas have anything to do with it if not brass, but copper?... I'm going to try out Alan's idea first, but if it's a no go I'm pulling the carbs and cleaning. Would you recommend against a rebuild, just clean?

[Alan F.]

Thanks guys... Alan, when you say "no change swap condenser positions", do you mean to move one of them up, and the other down?
So the condenser will still be linked with 1/4, or 2/3 as before, but they will be flipped 180 deg. from where they are orginally positioned?

Yes, exactly that, sorry for being unclear.

The points cover can at times touch something underneath and short out causing an issue, running with the cover off for a short time is a very easy test.

Yes, swap positions on the 2 condensers and look for the problem to move to a different cylinder.
If you have the old ones swap them in too. Some new ones can be bad.

Making sure of the easy stuff that can cause problems is easy so do all of the easy stuff first.

[YoungBlood]

Problem solved: #4 carb float was way too lean. I push the tab out with a flat head, richening it, and did it eye-ball with where the #1 carb float was set. It was closing the valve early, not allowing gas in, starving the bike in 5th.

I pulled the main jet, 105, and the emulsion tube. Cleaned both up, they were not really plugged.

New ignition system runs like a top. 5th gear is a hound--100mph--hell is easily behind me.