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.jordan

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cylinder two not firing
« on: August 05, 2009, 07:42:22 AM »
I just got my 1977 cb750f put all back together, I've got pods and a 4 into 1 exhaust, and 115 jets at the moment(i have a jet kit up to 120). The second cylinder wasn't firing before i tore the carbs apart and rebuilt them(bowl gaskets, float needles, main jets, and accelerator pump all replaced), adjusted the valves (twice), cam chain tensioner, and points, and still isn't firing. All the other cylinders pop off like champs at the moment, but two is still pretty cold to the touch. Put new plugs in last night and switched the wires around to see if it solved anything...nothing. I smell gas on the plug, could it be getting flooded? Haven't checked compression, but I'm going to try and rent a tool here soon. What else is there to check?
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Offline Spanner 1

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Re: cylinder two not firing
« Reply #1 on: August 05, 2009, 09:08:19 AM »
Compression, or lack of, would be the next logical thing to check as you have tried plug wire swap and seem to have a wet plug, i.e. getting gas.
One more thing could be a blocked idle/slow air bleed allowing all gas and no air with the throttle slides down ( idle ). Combined with these carbs squirting raw gas into the motor ( accelerator pump ) at even a small twist of the throttle, the plug gets 'drowned' PDQ.
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline TwoTired

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Re: cylinder two not firing
« Reply #2 on: August 05, 2009, 09:36:46 AM »
The 77-78 F model 750 have a history of valve guide wear.  You sure your wet plug isn't oil damp?  It's really best to see if #2 has compression as good as the others.   Can't tune out a bad valve.

You didn't mention pulling out the slow jets for cleaning.  Even though they are pressed in, you gotta yank them out to verify light will pass through them.

Then there is the float height check that differs from earlier carbs.  If you are using Clymer as a reference, that company assumes all purchasers are in a bent over state ready for their stab at pleasure.

My condolences on being forced or coerced into using PODs.  You'll appreciate them as you repeatedly remove the carbs for needed fuel metering adjustments that they inflict on unsuspecting buyers.

Have you measured the plug caps?  Have you looked at the ignition leads in a dark room for arcs where they shouldn't be?  Have you seen the arc happened across the spark plug gap on #2?
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Spanner 1

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Re: cylinder two not firing
« Reply #3 on: August 05, 2009, 10:27:06 AM »
There are 2 small brass tubes which term. at the airbox ( pod! ) side of the carb throat...just pop one of your pods off and you will see them. The right hand one supplies air thru' it's passage to the idle/slow emulsion tube to pre-mix with the idle/slow jet fuel. This air tube makes a 90 deg. turn which makes it v. diff. to clean out with a small wire....best way is with compressed air after a good squirt of carb cleaner into it...lots of posts about this with pics. and detailed description...do a search...!
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline TwoTired

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Re: cylinder two not firing
« Reply #4 on: August 05, 2009, 10:33:29 AM »
The 77's PD carbs have an Idle Mixture Screw (in front of the carb bowl).  This adjustment screw has a very shallow needle tip taper.  If over tightened, it will wedge itself into the carb body and break off, plugging the delivery hole.  But, this doesn't make the spark plugs wet.  It will cause a cold head pipe, though.


You need three things for a cylinder to fire: 
1 - An A/F ratio within the correct range.
2 - Compression of that A/F charge.
3 - Spark that is correctly timed.

Which one is missing?



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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline TwoTired

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Re: cylinder two not firing
« Reply #5 on: August 05, 2009, 02:27:07 PM »
Quote from: .jordan on August 05, 2009, 12:04:13 PM
If it were a timing issue it would be two and three not firing correct?

True.  Have you swapped spark plugs around to prove the spark plug now in #2 will work...somewhere if not in #2 cylinder.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

matthewspradley

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Re: cylinder two not firing
« Reply #6 on: August 26, 2009, 04:26:33 PM »
anybody know what stock compression is on a 1977 cb750f? 
thanks
matthew
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Offline TwoTired

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Re: cylinder two not firing
« Reply #7 on: August 26, 2009, 06:13:50 PM »
Quote from: matthewspradley on August 26, 2009, 04:26:33 PM
anybody know what stock compression is on a 1977 cb750f? 
thanks
matthew

If you are using the same compression gauge as Honda did for the shop manual (Chapter 18) 150 to 170 PSI.  If you are using a gauge for a car or diesel truck, it's subject to whatever volume the gauge adds to the piston chamber.  There is a post about this in the FAQ.

More critical is that all the cylinders are within 10% of each other.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

matthewspradley

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Re: cylinder two not firing
« Reply #8 on: August 27, 2009, 06:19:46 AM »
thanks man.
matthew
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Offline punch455

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Re: cylinder two not firing
« Reply #9 on: August 27, 2009, 07:02:40 AM »
Quote from: TwoTired on August 05, 2009, 09:36:46 AM

My condolences on being forced or coerced into using PODs.  You'll appreciate them as you repeatedly remove the carbs for needed fuel metering adjustments that they inflict on unsuspecting buyers.

LOL. Oscar Wilde couldn't have put that any better.
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