Topic: Why won't #1 fire?

[la ca fe]

1977 cb750 k with 78 f carbs (i think?).

I am getting spark to the plug, I checked the valve clearance, cleaned out the carbs...
I am thinking that it is the carbs because i put a different set on it and this is when #1 stopped firing. I changed the carbs because I couldn't get the other set to stop leaking in #3 AND #3 wouldn't fire... Now #1 won't fire.
What would cause the carburetor to not allow the cylinder to fire?

[calj737]

Fuel level too low in that carb. Perform a Clear Tube test.

[flybox1]

+1 ^^

What model carbs DO you have on there now?  Why couldnt you get #3 to stop leaking on the old set of carbs? 

[la ca fe]

# 3 on the old carbs (77k) just wouldn't quit leaking from the overflow; I changed the float level a couple times, , the float, the valve, the bowl, and I still couldn't get the drip to stop nor could I get that cylinder to fire. I swapped them out with a set I had from (i think) a 78f. It is a little different carb set, it has heaver slides,, the needle has no notches for adjustment, and there is an extra air hose coming from #2.

[flybox1]

you need to become familiar with the carb model #.  its stamped on the outside of #4.
that will give us a better idea of what you are working with.
was the overflow pipe cracked.   that would still leak after all those 'fixes' you tried.

Perform a clear tube test as cal posted above.
Be sure to update this with your results.

[Airborne 82nd]

# 3 on the old carbs (77k) just wouldn't quit leaking from the overflow; I changed the float level a couple times, , the float, the valve, the bowl, and I still couldn't get the drip to stop nor could I get that cylinder to fire. I swapped them out with a set I had from (i think) a 78f. It is a little different carb set, it has heaver slides,, the needle has no notches for adjustment, and there is an extra air hose coming from #2.

77&78 do not have slots to move the needle up or down. Look at the numbers and see if they are PD42a or b and let us know what you have.

"What would cause the carburetor to not allow the cylinder to fire?"
Because there is no fuel going threw it. But you have to find out if that is in fact what is happening. If you spray some starter fluid in number 1 and it fires up then you will know it is a fuel issue. Have you cleaned the carbs that are on there now?
Do what the other guys said they know a lot about these carbs.

[la ca fe]

The carb model I am running is PD42a.
I did a clear tube test to see the float level and it was a little above the seam where the bowl meets the carb assembly. I adjusted the float to widen the gap a little and still won't fire. I did notice when I gave it throttle, the carb was spraying a fine mist of gas out of the mouth.

I cleaned the carbs before mounting them.
I'll try some starter fluid and let you know.

[calj737]

I adjusted the float to widen the gap a little and still won't fire. I did notice when I gave it throttle, the carb was spraying a fine mist of gas out of the mouth.

I'm not a fan of starting fluid personally.

If the fuel is misting back out of the carb, it tells me you have your valves adjusted improperly. What's the background on the bike? Is it new to you? Was it running before? Have you just rebuilt the motor? Are the plugs clean? Are they wet and clean, or sooty and dry?

[flybox1]

I adjusted the float to widen the gap a little and still won't fire. I did notice when I gave it throttle, the carb was spraying a fine mist of gas out of the mouth.

I'm not a fan of starting fluid personally.

If the fuel is misting back out of the carb, it tells me you have your valves adjusted improperly. What's the background on the bike? Is it new to you? Was it running before? Have you just rebuilt the motor? Are the plugs clean? Are they wet and clean, or sooty and dry?

^^^  might be a clogged, or turned accel pump nozzle.  Id check there first

Continue to clear tube test the carb bowl till your level is similar....

[la ca fe]

The carbs I took off are PD 1AOF.

The bike is pretty new to me. I bought it from someone who rebuilt the top and bottom end of the motor. P.O. installed a 836 kit with a race cam, he also said he replaced the bottom end bearings. I don't know why he rebuilt the motor other than to make it a hot rod, maybe there was a problem with it ? I'm not sure. His work space/garage was spotless and his tools were quality and he was pretty anal. This led me to believe he did a good job on the rebuild, not to mention he replaced the wheel bearings, swing arm bushings, triple tree bearings, rebuilt and polished master cylinder, caliper, laced the wheels, rebuilt forks, clean wiring... with a ton of new, still in the bag, Honda, nuts bolts screws, gaskets,  He put the bike back together and realized the frame was bent, threw up his hands and sold it to me pretty cheap. .
I straitened the frame put the carbs on and fired it up for the first time since the rebuild, runs great on three cylinders.


I put starting fluid in and no fire on #1 again. I'll do the valves again but wondering if he ever timed it? Dyna electronic ignition plate was not bolted down tight, maybe TDC is not correct according to the plate where the points used to be?

[calj737]

If timing weren't right, the other 3 cylinders wouldn't fire. Sounds like you have a fuel issue with #1.

Pull the plug and post a picture of the insulator after you check the fuel level.

[b52bombardier1]

What do you have for compression on the non firing cylinder?

Rick

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[colvinry]

The fine spray when you crack the throttle is from the accel pump and doesn't tell you anything other than the accel pump is working. I would pull the carb check/clean the jets. I bet you have some debris in your idle jet.


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[la ca fe]

OK, I am going to take the carbs back off and clean out #1 again tomorrow and check the valves.
I'll let you all know.

Thank you.

[colvinry]

OK, I am going to take the carbs back off and clean out #1 again tomorrow and check the valves.
I'll let you all know.

Thank you.

What fixed it?

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