Plug problems.

Honda Phil UK

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hi everyone, I have a Honda CB750 DOHC 1979, I have a question regarding spark plugs.
now a bit of a Backstory on the bike.
Sometimes I can go to it'll start and run beautifully, (very rarely) other times it'll just start on one cylinder, occasionally on 3.
Compression psi around 155 on all barrels.
Had a complete carb rebuild all of them.
I have managed to balanced the carbs when it was running on 4 at 1000 RPM +- 100.
Two new coil packs Leads and caps + plugs gapped correctly.
Both Ignition Unit replace with new ones. I also built two because I wasn't confident in the ones I bought.
Replaced both ignition pulse generator and gapped correctly.
Wiring a checked top to bottom several times.
Also checked the timing with a timing gun. (right in the middle of the Ash marks)
New battery and Voltage Regulator Rectifier.
Good clean fuel delivery.
AS you can see from above I've gone through this with the dose of salts

Still having issues, been off the road now for this for over 7 months now.

I've changed the spark plugs several times because they keep getting wet not so much coked top just wet, so suspecting bad spark under compression.
The bike was delivered to me like this (seller never said a word about) it came with NGK DR8ES plugs, replace them with the recommended ones NGK D8ES.
When you come to buy these plugs everyone seems to recommend NGK D8ES.
Thing is these plugs seem to have an internal resistance of about 5k, given the caps already got a 5K resistance built in it that's a 10k resistance in total, to me this seems wrong.
I've bought a few other plugs to compare with ( Just 1 plug not put them in the bike) like a NGK D8EA identical plug dimensionally but has no internal resistance, I'm thinking this maybe the problem, there's a physical noticeable difference in the visual and crack (sound) of the spark between the two plugs when under test (not fitted in the bike).


Having owned it for 7 months and haven't actually been able to ride it and it's my only transport.
So quick recap sometimes starts on one cylinder, sometimes starts on 3, very rarely starts on 4, mostly one but not always the same one.
When inspecting the cylinders that doesn't fire after trying to start the plugs are wet except for the one that was firing.
When it does run properly it does suffer with some cadence, that goes away the second its throttled up.
Local shops I've tried won't touch it, as they say they're a pain to set up, apparently.

I'm now getting to the point where I'm thinking of breaking it for parts as I need to fix it or replace it.

Has anybody got any advice please, I'm struggling with this one.
 
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I'm not sure what plugs are OEM, but i would run OEM.
If your compression is 150 on all cylinders, your valves are fine.
Have you pulled all 4 plugs and verified spark on them outside of the cylinder head?
 
D8EA is the recommendation in the manual...at least for the USA model..
Isnt the "R" simply for radio interference.
"S" signifies a "std" plug I believe-tho sometimes "shorter"
The coils you changed are they aftermarket or OEM stock?

spark.PNG
 
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phil your problem is very much like mine on my 1979 750FZ it has all new this that and the other and carbs rebuilt and it sometimes fires straight up on all 4 and other times struggles on a couple then 3 then 4 , this after messing with the choke pulling it on and off trial and error. once it's going it ticks over sweet and runs very well , mine is also doing my head in! good luck in finding a cure. i must say that mine doesn't like being stood without firing it up at least once a week and in winter once every day i've run out of ideas now and spent a fortune finding out there was nothing wrong with the electrics i changed.
 
phil your problem is very much like mine on my 1979 750FZ it has all new this that and the other and carbs rebuilt and it sometimes fires straight up on all 4 and other times struggles on a couple then 3 then 4 , this after messing with the choke pulling it on and off trial and error. once it's going it ticks over sweet and runs very well , mine is also doing my head in! good luck in finding a cure. i must say that mine doesn't like being stood without firing it up at least once a week and in winter once every day i've run out of ideas now and spent a fortune finding out there was nothing wrong with the electrics i changed.
I'm coming down to thinking it's the fuel additives in modern fuel.
 
I think it's all coming down to the additives in modern fuel, I have got access to aviation fuel which is extremely clean. so I'll see how that goes.
 
If you had fuel delivery problems, your plugs would be dry. You verified spark on all cylinders, and you have compression. Is the left coil attached to cylinders 1 and 4, and the right coil strange to cylinders 2 and 3? You have an the ingredients for combustion, but maybe your plugs are firing at the incorrect time.
 
I think it's all coming down to the additives in modern fuel, I have got access to aviation fuel which is extremely clean. so I'll see how that goes.
Be careful, it has way, way too much T.E.L. (tetra Ethyl Lead) that will form deposits on the back of the exhaust valves leading to hot-spots and burnt valves. 100LL Avgas actually has about 3 or 4 times more T.E.L. than old 4-star petrol!
 
For what it's worth, my CB750F has the same issue, as does my CB900F that's just had a complete rebuild including rebore, camchains, valves, plus new coils, plugs, leads, carbs rebuilt etc etc etc. If I catch it just right it'll fire up, if I don't catch it right it's a right barsteward to fire up - just like yours.
It seems to be more temperature dependant than how long it's sat for - it really doesn't like anything below 16 deg C.
 
quick up date, banged it on track day ran like a stuffed pig, ran it out of fuel added aviation fuel is I said, totally defiant bike, never missed a beat, ran that out put beck in van and filled it up at the nearest fuel station on the way home, started alright taking to the shed, see what's like in a couple of days.
Be careful, it has way, way too much T.E.L. (tetra Ethyl Lead) that will form deposits on the back of the exhaust valves leading to hot-spots and burnt valves. 100LL Avgas actually has about 3 or 4 times more T.E.L. than old 4-star petrol!
yes i understand that but nothing else i can think of.
 
Are the plugs wet with fuel or oil? Also still do check the valve clearances. Even with the compression where it is if the valves are a little on the tight side or at honda spec when the engine is running it can hold the valves open slightly and cause compression loss, this is why clearances need to open up to .005"
 
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