Jump to content
  • 0
Bigjohn

As wierd one

Question

After a perfect run to this weekend's event, we had problems getting the our Bedford MW to run properly from the start of the return journey.

Several stops and a lot of head scratching showed that there was no spark on the front two plugs EXCEPT (and this is the nutty bit) when we held the plug cap in close proximity to the terminal of the spark plus.

The spark could be heard arcing from the inside of the plug cap to the nipple on the end of the plug, when this was happening the engine ran fine. However as soon as the cap was pushed onto the plug the engine ran rough as the plugs stopped sparking.

To get home we secured the two plug caps just off the plugs with wire so they could arc to the top of the plug.

 

So why would a spark plug fail to work when properly connected to the HT lead but work well when there is an arc between the plug cap and the plug?

Share this post


Link to post
Share on other sites

21 answers to this question

Recommended Posts

  • 0
After a perfect run to this weekend's event, we had problems getting the our Bedford MW to run properly from the start of the return journey.

Several stops and a lot of head scratching showed that there was no spark on the front two plugs EXCEPT (and this is the nutty bit) when we held the plug cap in close proximity to the terminal of the spark plus.

The spark could be heard arcing from the inside of the plug cap to the nipple on the end of the plug, when this was happening the engine ran fine. However as soon as the cap was pushed onto the plug the engine ran rough as the plugs stopped sparking.

To get home we secured the two plug caps just off the plugs with wire so they could arc to the top of the plug.

 

So why would a spark plug fail to work when properly connected to the HT lead but work well when there is an arc between the plug cap and the plug?

 

Did you remove these two plugs to look at them? They could be fouled, oil possibly. By holding the cap away from the plug it seems to intensify the spark, hence the old trick of a shirt button in the HT lead.. Something else you can do is to swap these plugs to another position and see if the problem moves with them.

Share this post


Link to post
Share on other sites
  • 0

Yeah, we tried all kinds of swapping and the plugs were only wet with fuel as you'd expect.

My only thought is that the coil is on the blink - the plugs, leads, distributor and rotor are all new (less than 200 miles).

Share this post


Link to post
Share on other sites
  • 0

If all the base parts are new, why are only the two front plugs giving trouble? The only place that could occur is at the distributor cap, the only place the spark goes to each seperate plug. I'd change it and see.

 

Though!! I had a similar problem on one of my Dodges, the cables on two plugs had touched and were fusing together, check the lines are seperated, but I'd still change the Dizzy Cap.

Edited by Tony B

Share this post


Link to post
Share on other sites
  • 0

It could be a faulty new distributor but it seems odd that it suddenly went on two plugs. We were getting the occasional very weak spark on the no1 pot.

My hunch is that the coil is very old and tired and that all the sparks are weak and that plugs 1&2 are the worst affected.

It won't break the bank to change the coil and see what happens.

Share this post


Link to post
Share on other sites
  • 0

Like Richard said there is a sort of coil booster that adds another gap in the lead up from coil to the distributor. My thought is that the voltage has to jump two gaps anyway. From rotor arm to cap , then across base of plug. You seem to be missing the first gap on the two front plugs.

Share this post


Link to post
Share on other sites
  • 0

What type of HT leads are you using are they the traditional copper core or modern Carbon fibre ?

Pete

Share this post


Link to post
Share on other sites
  • 0

What spark plugs are you using? I recommend Champion J8C for the Bedford 28hp. If you are using NGK, beware as there are fake ones on the market and sub-standard.

 

regards, Richard

Share this post


Link to post
Share on other sites
  • 0
Hi Pete, they're copper core.

 

Providing the insulation is in good condition, not cracked/soaked in oil and the terminals are good they should be OK.

 

The reason I asked what leads you were using is that I have known the modern Carbon fiber leads give exactly the problem you were describing.

 

Pete

Edited by Pete Ashby

Share this post


Link to post
Share on other sites
  • 0

My MW on initial start up the engine would run like it should ,stop it and restart it ,it would misfire , splutter and sometimes not even start...........NGK plugs swapped for J8C's .............cured it .

 

Now only have to cure the steam problem :undecided:

Share this post


Link to post
Share on other sites
  • 0
Try putting your old plugs back in and see if that cures it.

 

Mike.

The old plugs were binned long ago when I replaced the leads and distributor.

Share this post


Link to post
Share on other sites
  • 0

Hindsight is a wonderful thing but I always kept a working 'old' plug in the toolkit as a spare.

Share this post


Link to post
Share on other sites
  • 0

NEWS FLASH!

 

This problem has gone all the way to the engineers at the BBMF. They say "it's the coil".......apparently they had a similar problem on a Spitfire recently although I suspect that it wasn't discovered mid-journey!

Share this post


Link to post
Share on other sites
  • 0
NEWS FLASH!

 

This problem has gone all the way to the engineers at the BBMF. They say "it's the coil".......apparently they had a similar problem on a Spitfire recently although I suspect that it wasn't discovered mid-journey!

 

I thought a Spitfire had a magneto :undecided:

Share this post


Link to post
Share on other sites
  • 0
I thought a Spitfire had a magneto :undecided:
Damned if I know what they have these days!

Share this post


Link to post
Share on other sites
  • 0

It was a duff coil but I changed the plugs to Champion as well. She purrs now!🙂🙂

Share this post


Link to post
Share on other sites
  • 0

Pleased you've got it sorted and well done for coming back with the result :thanx:,

 

It's always useful to hear what the solution to a problem was.

 

Pete

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...