Number 1 cylinder not firing

Chrisraven

CB750 Enthusiast
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Abbotsford, BC, Canada
I have a 1981 Honda 900F (I know it is not a 750 but this is a problem that will be common to both). For some reason when I am running my bike I get a miss (popping) through the exhaust which I can hear and also feel with my hand. It is starting to drive me a little nuts as I have tried multiple items to source it.

I have done the following: 1) New Plugs, 2) tested all the plug caps, 3) tested the plug wires, 4) tried replacement coils, 5) interchanged the two ignitors to see if the problem moves (nope), 6) Using premium fuel (Chevron 94), 7) Leak down test from 1 to 4...100/97, 100/95, 100/97, 100/99, 8) Sync carbs, 9) Stock aircleaner, 10) Checked the ground path.. clean and no resistance, 11) Valve clearances are all within specs, 12) Battery is good and fully charged. 13) Timing is perfect,

Did a compression test (bike was cold) From 1 to 4 readings are 120 psi across the board, added oil to each cylinder and it jumped to 145/150 across the the board. While I know this is low it is not low enough to give me a problem with the unit running. I managed to track down which cylinder is was through feeling the exhaust, and removing the plug caps. Number one is the bad guy. I actually had the bike running on 2-3-and4 cylinders while I had the plug out of number one. With the number one plug connected the spark was a deep blue and did not seem to miss a spark. I believe the low compression is due to glazing on the cylinder wall (only 50 kms on a complete rebuild), however this will not effect the missing on the one cylinder, since the other 3 were the same compression and were running fine.

The 50 clicks that are on the bike since the rebuild is a ride I took to start the breakin procedure. Interestingly enough I only got the miss when the bike was under load, taking off from a start and going through the gears about 4000 to 4500 rpm it would miss, but cruising down the freeway at 100 clicks at about 4000 rpm's it would run just fine. If I cracked the throttle the miss would come back. Almost sounded like a backfire, hard to describe what it actually sounds like. Don't know how to put a sound bite on here otherwise I would add the audio, it might help someone find a solution. (If you know how let me know)

Looking forward to your assistance with this pesky issue. Thanks Chris
 
Another thing that I just thought of. I have a Tektronic oscilloscope and I am going to try and monitor the waveform/pulse coming out of the spark ignitors (under the seat), maybe I can see where that waveform is breaking down.

On the subject of low compression has anyone tried the "Dutch Cleanser" trick where the powder is sucked through the carb and will do a scoring/hone job on the cylinder to help to get the rings to reseat...versus tearing the engine apart again and using a stone to re-hone the cylinders?
 
On the subject of low compression has anyone tried the "Dutch Cleanser" trick where the powder is sucked through the carb and will do a scoring/hone job on the cylinder to help to get the rings to reseat...versus tearing the engine apart again and using a stone to re-hone the cylinders?
Wow🤓 That sounds incredible! Please, make a video!, I will stand back here and watch!
 
I tried something different today, a Dynamic Compression Test, this is where you have the engine running with the compression gauge installed. I found out that number 1 cylinder is trying to run with 90 psi. So looks like it is time to take the engine apart and install new rings and see it that helps.
 
Hi there: Bore scope said nothing, and I actually tried another carb set that I had from a know good source. So not the carbs, does the same exact thing with either set on. So that kind of eliminates the carbs to being the issue. Also inspected all the intake boots for cracks..etc, all good. (New a year ago but one never knows if there is a problem with the manufacture. Still looking but have ordered new gaskets and rings, when they arrive I will tear the top end apart and rehone the cylinders. This time I will do a bench test for the compression before I take it off the bench.
 
Kinda tough on the motor to drive it that far with one cylinder not firing. Quicker for me just to take it apart and rehone and replace the rings. Only take a week of downtime, could do it quicker but not that big a rush.
 
Hi neighbour :) I'm next door in Langley actually.
Anyways, quick question. What kind of gas (octane rating) are you using? If your compression is lower and you are using high octane gas, it might not fire all the time
 
Hi there I am using rg=eg gas, I have never found high octane to do anything for me. Except in the old days when I built engines that were really high compression. Then I would add av gas, expensive,...lol

Where in Langley are you? I am by King and McCallum. I still have 5 bikes to do a restore on...lol Stop in sometime and say hi
 
Ya, totally agree, only for high compression engines. And also better to have some in the bike when storing over winter.
King & McCallum, so right off Hwy #1? Do you have a shop there where you are doing all these rebuilds?
I'm close to 232nd and Hwy #1.
 
I have a 30 by 40 shop i my back yard where I do all my work. I paint, powdercoat and machine...lol 1533 McCallum Road, you can text me at 604-302-0416 if you would like. Thanks
 
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