Honda Phil UK
CB750 Member
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.
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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