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Author Topic: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC  (Read 286 times)

Online irk miller

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Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« on: Sep 30, 2018, 21:53:54 »
Maybe you guys can help me diagnose an issue I have had with my CB750 motor for going on 4 years now.  It's why I shelved this motor, before I got the bright idea to put it in a chopper.  I've gone through all the checks.  I've done everything I know to do and nothing seems to work.  It's nothing I've ever dealt with, and it has me baffled. 

It's a 77 motor with PD carbs.  The carbs have been rebuilt and cleaned.  I've synced the carbs and I've adjusted air mixes.  I can turn them all the way in until they start to backfire through the carb and turn them out in 8th increments until they stop, and they still foul the plugs on 1,2,4.  #3 stays somewhat lean.  I've run this motor and these carbs with a stock airbox and currently with a breadbox.  The mouth of the carbs are saturated with fuel on all four carbs.  They have new boots.  Pumper sprays well into all four carbs and appears even.  Maybe too much? 

The motor has a fresh top end.  All new valves and machined seats.  Tappets are set for both intake and exhaust.  Timing has been set and checked and rechecked ad nauseum.  I've run Dyna and points and get the same result.  I've run Dyna ignition with Accel coils and two different plug wires.  When I put the motor in the chopper I went back to points, Dyna coils and new plug wires.  It gets spark on all four.  Brand new, fresh built wiring harness for juice with a well charged LiFePO4 battery. 

Compression is 152 across all four cylinders.  Pretty much on the money.    I've sprayed everywhere looking for leaks and I have found zero.  Fresh head gasket doesn't show any leakage- oil or air.  750f exhaust with my own custom mufflers with fresh gaskets.  No white smoke out of the exhaust, though it definitely is showing signs of unburnt fuel.  I suspect somewhere somehow I'm getting way too much fuel.  The float are set per the manual- 14.5mm. 

I figure it's something to do with these carbs, but I can't figure out what else to look for.  The internet really doesn't show a search for another bike that has had the same thing going on.  I'm comfortable with the condition of the motor and every setting I have, but I'm also open to the possibility that I'm an idiot.  I still think it's these PD carbs.  Any help is appreciated.

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #1 on: Oct 01, 2018, 02:06:43 »
I went back at it, and I think this issue is related to the floats.  During installation and past checks, I measured float height to 14.5" as noted in several places.  I did this several times.  But, tonight when I tested for volume in the bowls with a clear tube, I'm getting a reading that shows fuel too high.  In fact, I had two bowls end up filling into the venturi with the tube hooked up.  I'm not sure if this is a situation where my floats have lost buoyancy or what.  None have filled with liquid.

Offline farmer92

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #2 on: Oct 01, 2018, 07:37:28 »
What makes you thinks itís floats and not just the valves not closing properly?
Unless the floats are filling with fuel thereís no way for them to lose buoyancy. 

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #3 on: Oct 01, 2018, 07:45:58 »
What makes you thinks itís floats and not just the valves not closing properly?
Unless the floats are filling with fuel thereís no way for them to lose buoyancy.
Buoyancy was just me guessing why a mechanical measurement isn't setting the fuel at the right level.  I have set and re-set valves, checked them again and again, with always the same result.  I've pulled the cover and checked the cam and cam chain for alignment.  I know no other reason the valves could be wrong.

Offline farmer92

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #4 on: Oct 01, 2018, 08:17:44 »
Buoyancy was just me guessing why a mechanical measurement isn't setting the fuel at the right level.  I have set and re-set valves, checked them again and again, with always the same result.  I've pulled the cover and checked the cam and cam chain for alignment.  I know no other reason the valves could be wrong.

Sorry i meant the little needle valves in the carb bowl, not the engine valves haha. If the bowls are overfilling then for sure itís a carb related problem. The floats really supply very little pressure on those float valves, so it doesnít take much to have one stick open.

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #5 on: Oct 01, 2018, 08:28:24 »
Haha. Something else I thought about was the possibility that the rebuild kit came with the wrong size needle valves.  If that is even a possibility.  I've never compared lengths between different carbs to know if they vary in size.

Offline farmer92

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #6 on: Oct 01, 2018, 09:31:45 »
Haha. Something else I thought about was the possibility that the rebuild kit came with the wrong size needle valves.  If that is even a possibility.  I've never compared lengths between different carbs to know if they vary in size.

I would think the only variation would be the diameter of the seat, bigger carb, bigger seat but iíve never checked. The needle valve itself is probably standard across a specific brand of carb.

I was trying to stop a carb from overflowing the other day, took it off, took the bowl off and hook a hose to the fuel barb and blew in it. Pressed up gently on the floats it and it would stop the air so i though okay good to go.
Stuck it on and it still overflowed.
Repeated the exercise but this time dipping the carb floats into a container of fuel instead of pushing up on them and it no longer stoped the air. Replaced the valve and seat with a known good one and was good to go.

Tl;dr needle valves are sneaky and finicky.

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #7 on: Oct 01, 2018, 09:32:30 »
Haha. Something else I thought about was the possibility that the rebuild kit came with the wrong size needle valves.  If that is even a possibility.  I've never compared lengths between different carbs to know if they vary in size.

I'd say its possible. When I had the XL350 I had bought an aftermarket rebuild kit and the float needle and would never seal on its own without some taps from a screwdriver. I found out that I had used the OEM seat, and the aftermarket needle and it was a mismatch. I changed it out for the seat that came in the rebuild kit and it fixed the issue.
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Offline teazer

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #8 on: Oct 01, 2018, 09:57:57 »
Set the FUEL levels 3mm +/1mm below the gasket face.  It is common for floats and needle valves to be slightly different in weight or length. I have set float levels and then gone back and checked fuel levels only to find significant differences between them. 

I use a bowl with the overflow cut off or on other carbs I make up an adapter to fit in place of the drain bolt.

Offline pidjones

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Re: Rich on 1,2,4 Lean-ish on 3 CB750 SOHC
« Reply #9 on: Oct 01, 2018, 21:38:48 »
I know that the master GoldWing carb rebuilders specify nothing but Honda float valves. They complain that the aftermarket valves have many issues.
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