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Discussion Starter #1
Hey guys new here, wanted to bounce this around. Just picked up this 79 Cb750F. Only idled with full choke and was backfiring through exhaust. I replaced plugs, drained fuel tank replaced with fresh fuel and seafoam, replaced fuel lines and inline filter. This took care of backfiring completely but still wont idle without full choke. Plug 1 = white with black sooty deposit. Plug 2 = Lean. Plug 3 = Lean. Plug 4 = Rich. Waiting on manual to show up. Plan on soaking carbs, adjusting and then if I need to take em and have them synced. Any thoughts greatly appreciated.
 

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Ghost in the machine
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My guess is that you are on the right path. Once you have properly cleaned the carbs and synced them we can go on from there.
 

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Discussion Starter #3
Rebuilt carbs, installed, working on adjustments. I havent had them synced with vaccum gauges yet. Starts great, idles great, poor performance in low rpms. mid and high are fine.
 

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Discussion Starter #4
Plug #1 #2 and #4 show same signs of mixture, which has been lean until now as I have been adjusting them. #3 tho and this is how its been before rebuild, after rebuild and bench sync, #3 is black dry soot.
 

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Discussion Starter #5
#3 carb was the one oddball also air/mixture screw doesnt sit like the rest. the rest of the carb mixture screws when seated are below the bowl, this one seats above. So as you open them back up all 1 2 and 4 portrude the same distance, where as three's portrudes more....possible someone damaged threads in that float bowl.....?
 

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Ghost in the machine
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Just because it is quick, simple, and cheap, I would do a compression test on all 4 cylinders just to eliminate the possibility of a bad cylinder.
 

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Discussion Starter #7
Compression test results.


Cylinder #1 - 120 Oil added - 137
Cylinder #2 - 110 Oil added - 109
Cylinder #3 - 98 Oil added - 149
Cylinder #4 - 107 Oil added - 125
 

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Retired twice: Navy and as a govt contractor
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Throttle wide open with those readings?
 

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Retired twice: Navy and as a govt contractor
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To get a true reading the throttle has to be wide open.
 

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Ghost in the machine
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Compression test results.


Cylinder #1 - 120 Oil added - 137
Cylinder #2 - 110 Oil added - 109
Cylinder #3 - 98 Oil added - 149
Cylinder #4 - 107 Oil added - 125
I would start to worry about any reading under 120 psi.
 

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Ghost in the machine
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Those numbers are starting to get a little low, but the bike should still run ok. There doesn't seem to be a dead cylinder.
Next thing I would do is remove the carbs (again) and find out what is causing the the position of the mix screw on #3. Is the milling/tapping depth different? Is the needle different? Is there an obstruction in there stopping it from seating fully?
Maybe it was meant to be different, but I would want to know just for my own peace of mind.
 

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Discussion Starter #16
When I was re-assembling the carbs I tried the old mixture screw, a couple of the new mixture screws. Visually look inside for obstruction or damaged threads. Couldnt find anything, asked local honda shop they said possibly different. I proceeded. After carbs where installed I tried bringing the screw in to match what was visual portruding on the other three and it ran worse. I then brought it to where it feels seated again, backed it back out 3 turns, like the rest, it runs fine. Until you put load, in gear, riding it. Then it struggles in low rpm.
I'm wondering if the final carb sync will fix that. And if I want to replace the rings anyway. I dont want it to run ok, I want to be able to travel on it.
 

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Ghost in the machine
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The sync will surely help. 3 turns out seems a bit much, I usually start at 1-1/2 turns and end up between 3/4 and 2 turns out.
 

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Discussion Starter #18
I started at 1 and 1/2. Judging by performance and spark plugs, 3 is what the bike wanted. See what a carb sync does.
 

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Discussion Starter #19
Ring gaps are in spec, they seem new. Cylinder diameters and shapes are fine. Pistons themselves are below wear diameter, spec is 61.90mm minimum diameter, theyre all about 61.45mm
 
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