Author Topic: 1968 750 Interceptor 1a only running on one cylinder.  (Read 283 times)

0 Members and 2 Guests are viewing this topic.

LeonR

  • Neophyte
  • *
  • Posts: 5
  • Karma: 0
Hi all, apologies that my first post is a question but I'm posting this on behalf of my Dad who needs a little help trying to diagnose an issue with his 1968 750 Interceptor (I'm not sure he would know how to sign up and post on a forum  :) )

The situation is that it appears to only be running on one cylinder, he says that the other cylinder does not run/fire at all (apologies if I have got any terminology wrong below).

He has replaced the ignition system with a Pazon system, both spark plugs have a strong spark and both spark at the same time.
The head has been replaced and checked.
It has new carbs , timed and jetted.
The cylinder which is not running is getting fuel as he says the plug is wet/comes out the exhaust on that side.
He carried out a compression test and both were good, the higher compression is actually on the side which does not run.
He's switched coils and carbs which makes no difference.

What is really strange is that he has tried switching the HT leads over (just the spark plug ends) and the problem does not follow the leads, the same cylinder continues to run.

The problem has baffled him and his friends for months now so I thought it may help to ask a wider audience.

Any suggestions appreciated, thanks!






grumbern

  • Grease Monkey
  • ****
  • Posts: 327
  • Karma: 0
Reply #1 on: April 27, 2019, 01:19:11 pm
Are the carbs synchronized? Have the plugs been replaced?


LeonR

  • Neophyte
  • *
  • Posts: 5
  • Karma: 0
Reply #2 on: April 27, 2019, 02:18:51 pm
Are the carbs synchronized? Have the plugs been replaced?

Yep, new plugs and carbs have been synced I'm told.
« Last Edit: April 27, 2019, 02:42:34 pm by LeonR »


Bilgemaster

  • Just some guy
  • Grand Gearhead
  • *****
  • Posts: 749
  • Karma: 0
  • 2005 Bullet 500ES in "Mean Green" Military Trim
Reply #3 on: April 27, 2019, 03:13:29 pm
I don't know the Enfield twins, but does the valve gear on the non- firing cylinder, especially the exhaust valve, have proper tappet clearances and seem to operate properly? A bent, displaced or otherwise goobed-up or "footloose" exhaust pushrod might cause the symptoms you describe on a Norton twin: the inlet valve would still let in the fuel-air mix, but the exhaust wouldn't open up properly, pretty quickly fouling the plug enough so that it wouldn't fire. I imagine a really sploodging oil leak at the valve guides might have the same combustion-quelling effect, but you don't mention an "oil-soaked spark plug". Just some guesses...
« Last Edit: April 27, 2019, 03:27:31 pm by Bilgemaster »
So badass my Enfield's actually illegal  in India.


LeonR

  • Neophyte
  • *
  • Posts: 5
  • Karma: 0
Reply #4 on: April 28, 2019, 04:11:51 am
He says he has double checked the valves/clearances and also had them ground in.

The odd thing is that he has worked on his bikes for years (Triumph, BSA etc) and has plenty of knowledge on how to strip them down and set them up again, we must be missing something really obvious.

Is it possible that the ignition system could only spark on the good cylinder firing stroke but not spark on the non working cylinder firing stroke? (but appear that it's working when turning by hand)


grumbern

  • Grease Monkey
  • ****
  • Posts: 327
  • Karma: 0
Reply #5 on: April 28, 2019, 01:58:31 pm
I'm not sure how the MK1A ignition system is set up, but I'd think there are two points, each controlling a separate coil?


Bilgemaster

  • Just some guy
  • Grand Gearhead
  • *****
  • Posts: 749
  • Karma: 0
  • 2005 Bullet 500ES in "Mean Green" Military Trim
Reply #6 on: April 28, 2019, 02:52:27 pm
I'm not sure how the MK1A ignition system is set up, but I'd think there are two points, each controlling a separate coil?

Actually, he mentioned in his initial post of  having fitted a Pazon (electronic) ignition, which leads me to ask: Did this problem begin just after its installation? If not, what, if any, mechanical work or adjustments were done directly beforehand? And how long had the bike been sitting unridden before the problem appeared?
So badass my Enfield's actually illegal  in India.


grumbern

  • Grease Monkey
  • ****
  • Posts: 327
  • Karma: 0
Reply #7 on: April 29, 2019, 01:47:39 am
Oh, right. Could it be there's only one Signal coming from the "distributor" and the coil is firing only in the exhaust stroke but not the power stroke?


LeonR

  • Neophyte
  • *
  • Posts: 5
  • Karma: 0
Reply #8 on: April 29, 2019, 03:06:53 pm
He said it 'sort of worked' before he switched the ignition system over.
I think it must be ignition related and as suggested, maybe not sparking on the correct cycle for one cylinder as it's odd that the HT leads can be switched over and yet the same cylinder runs ok.

I'll report back on how we get one,I think he wants to try a 'Boyer' system too, thanks for the info so far.


Adrian II

  • Grand Gearhead
  • *****
  • Posts: 1,405
  • Karma: 0
  • Sharing my ignorance with anyone who needs it
Reply #9 on: April 29, 2019, 04:23:52 pm
According to the on-line parts book over at H's the original contact breaker plate looks like the twin CB version of the Lucas 4CA, in which case the Pazon twin cylinder ignition kit ought to bolt straight in, assuming the original advance and retard unit has the same taper.

I do wonder if this is a case of read and follow the installation instructions properly. Boyer systems are quite similar (possibly not as good!), so if you can't set a Pazon up properly you might not do much better with a Boyer...

Some of the following might help.

http://www.pazon.com/news/9/SUREFIRE-FAULT-FINDING.html

http://www.pazon.com/news/12/SmartFire-Technical-Help.html

http://www.pazon.com/files/PDF/PA2.pdf

A.
Grumpy Brit still seeking 500 AVL Bullet perfection! Will let you know if I get anywhere near...


LeonR

  • Neophyte
  • *
  • Posts: 5
  • Karma: 0
Reply #10 on: May 01, 2019, 04:33:35 am
Thanks all, he just let me know that he managed to get it to fire, he had to put some fuel in the cylinder so is now suspecting a carb issue after all (He did say the spark plug was wet with fuel and he tried switching carbs, so I guess he assumed it was working ok  :-\ )

Will update if he manages to fix it.