Spark plug question

[Veg]

I have changed the head on my Scottish from a 200 to a 250.
the 200 had a ngk bp4hs fitted and the 250 head came with a champion L7 plug is either correct for the 250 head?
thanks Tim

[IanCordes]

NGK B7HS would be the one normally used these days. The 'P' in BP4HS means projected insulator, which may not be a good idea. It would be in the wrong heat range, anyway. The Champion L7 is pretty close, more like a B6HS. Does it work?

[Veg]

Got a champion l82, which is supposed to be the direct equivalent to the ngk b7hs. I’ve fitted the head, etc now won’t even fire. Its also twittering when you kick it over could be the decompresser bypassing?

[John Wakefield]

If you have put a 250 head on a 197 (9E) barrel then it will be very low compression hence why it wont start

[garybam4711]

250 and 197 heads are not interchangeable as bolts are different distances apart

[Veg]

Apparently it’s a 250 barrel with a spacer at the bottom? The 200 head had been machined so the bolts lined up to accept the 250 barrel fitment. The barrel with 200 head is shown below

[IanCordes]

Did it run before you swapped heads? If so, did you dismantle the decrompressor and give it a good clean up before refitting? If not, that could be the issue, or incorrect cable set-up, which needs to have a bit of slack when closed.

If it didn't run before..... A 250 barrel with a spacer under; a couple of options. The early 31A barrel was for the short (5 1/4") rod crank. A 1/4" thick spacer could be fitted under them if a long rod (5 1/2") had been fitted. Which has yours got? If a 1/4" spacer has been fitted and it has a 5 1/4" rod, that doesn't work; too low compression and the porting all wrong.

[John Wakefield]

Looking at the pic it has the 'thick flange' 32A barrel plus the spacer intended for a 31A 'thin flange' barrel when fitted with 5 1/2" rod !

[IanCordes]

I was thinking the same. That combination will never work. So, did it ever run?

[whitehillbillies]

Are you getting a nice blue spark at the Plug? is the plug getting wet after several kicks? it the timing set correctly? The point gap and timing can be a bit tricky if the original Points set up is used.
I have a spacer fitted to my TCS Barrel.
I am not sure of its origins but picked up at a swap meet in the UK many many years ago.
it was on STD bore, and I fitted it to my rebuild.
On turning over, the piston sat out the top by 1/4 inch. I suspect a 32a barrel had been machined to fit to a 9e motor. I had a 1/4 inch alloy spacer made up.
The bike ran well, as it was set up for Daily use on the road with full Rego but I experienced seizures, with the piston nipping up. I discovered the base had not been machined 90 Degrees to the bore. Took it to a machinist mate who squared it up.
I run a B7HS plug here in Aus.
The head on my Greeves has a blanking plug, in place of the Decompressor.
If you zoom in you can see the Alloy spacer.

whitehillbilly

[Veg]

Ran totally fine before I exchanged the head, big spark, timing ok, I suspect it’s the decompressor as it whistles/tweets when you kick it over with very little compression. I’m going to remove the decompressor (which is a pain in itself as it’s solid) tried soaking, heat etc but can’t get a socket or ring spanner on to it because of the actuator at the top so I’ll remove the actuator then socket and a tommy bar. Then I’ll fit a spare plug and see it is better. Then buy a new decompressor from feked classic trials parts

[Veg]

The piston reaches the top of the barrel within a thou or so as well.

[IanCordes]

You shouldn't need to buy a new decompressor. They are simple devices. That one should clean up just fine. Give it a break, it is over 60 years old! I wouldn't have used it without stripping it first.

If it ran ok before, you must have a long rod 32A-type crank set-up, and the barrel must be off a 31A, to work with the spacer.