Plug problems.

[brettp]

It feels like the fuel evaporates from the bowls with time.

 

[kirky]

honda Phil i have found my bad stating problem. the battery AGM was showing 12.84 v and would spin the engine nicely but it wouldn't start until it felt like it, this has been going on for over a year. i put a volt meter on it this week because it turned over then died for good i pressed the starter and the battery dropped to 5v . i tested the battery again and yes 12v again so it has lost its punch. i put a new one on today lead acid and she fired up by just looking at her, happy days. carb problems are sometimes elecktrickery problems

 

[Gra]

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.

Hi Phil, I have a CB759K5 and mine didn't start very well so got NGK hotter grade plugs and starts fine now. When I'm at home tomorrow I'll check what they are and give you the reference.

 

[Bo82CB]

I used to let my CB750 drink a bit before I fired it up. I always turned my petcock off and it seemed to start better if I turned the petcock on and took a second to back it out of the garage.

...I do the same thing and bike starts like charm every time.. Also I drain my carb bowls when I am done riding since it may be few weeks when I decide to ride CB again ( have other bikes) and don't want ethanol shit fuel to sit in the carbs.

 

[Honda Phil UK]

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"

did all the valves and compression all good, plugs seemed a bit wet from fuel after have starting problems, but now I've fitted the new plugs all good.
New plugs all look nice now I've given the bike a work out. oil on the plugs was never a problem anyway. thanks

 

[rlangston]

Hey Phil. Just ran across your post. I had replaced my coils and still had issues with stumbling past 1/4 throttle. I found the ignition troubleshooting section in the manual and followed that to test my original ignition coils. Both were barely below spec. I checked the new ones and they were worse that the originals on both primary and secondary coils. Ordered high performance packs and tested. They were on the high side of normal. Also checked plug wire resistance and were good. Installed new coils and it runs perfect. Wet plugs could be from weak, inconsistent spark. It runs better with high octane fuel because it ignites better and compensates for the weak spark.
Just a thought