Which side for the sparkplug?

[Tom Gaynor]

My first Manx Norton head, which had come from Joe Potts' "lab" at Bellshill, had had twin plugs. The extra plug 'ole had later been welded up. Draw your own conclusions. There might be an advantage if the head is ill-designed, and leaves a pocket of unburned gas, but by the fifties the advantages of "swirl" were well accepted, and gas was "injected" into the periphery of the combustion chamber "at a tangent" past the plug, so that it would sweep right round a full circle, then exit via the exhaust port.
My single plug 86 bore Manx clocked 146 mph at Chimay. Since you ask, it was positively orgasmic.
Amazing how many 60 bhp (at the back wheel...) Manx Norton classic racing 90 or 92 bore rocket-ships, faster than mine (a measly 52 bhp atbw), DO NOT have twin plugs. Maybe the builders know something.

I do not think it will make a difference. I have had engines that produced more power on single plugs than twins for reasons unkown. A fellow member runs a Manx with a number one plate that did 150mph at Daytona and he runs a single plug. There are certainly benefits to twin plugs. The timing in the bike should be set acording to the speed of the burn. If the charge burns quickly you can time it close to TDC where the effective compression ratio is as high as possible. A quick burn should keep the heat exposure low which will increase the density of the charge. I am not an engineer so I will stop, but in a street engine I would not worry about it.

David

 

[Howard]

I think the idea was to put the plugs in line with the flow of gas in. I would suggest you put the front plug in the left hole, and the rear in the right. That`s provided you have standard heads, i.e. front head and rear head. Not two front heads.
Try it and report back, I think you might find a difference.

I've put the plugs on opposite sides, and it seems to run more evenly. Only a feeling, nothing I can put numbers to.

H

 

[Tom Gaynor]

According to Eddie Stevens, some bikes have twin plug heads because some numpty drilled a batch with the plug holes on the wrong side: on the right for the rear, left for the front. Rather than scrap the heads, the factory drilled another plug hole, this time on the correct sides, and plugged the wrong-'un.

I've just read Roy's comment in the Vietnamese Vincent thread, about the position of plugs.

I've got twin plug heads. Don't know why, they were like that when I bought the bike, and it was using the standard valve lifter then - I use decompressors now.

The thing is, I've always put both plugs in on the right of the engine (easier to get at). Is there any advantage putting the rear plug (it is a rear head) in the left hand side plug hole? If not, why did Vincent put the plugs on opposite sides?

H

 

[Howard]

According to Eddie Stevens, some bikes have twin plug heads because some numpty drilled a batch with the plug holes on the wrong side: on the right for the rear, left for the front. Rather than scrap the heads, the factory drilled another plug hole, this time on the correct sides, and plugged the wrong-'un.

Now you're making me feel sorry for my engine - started life with numptys working on it, then Slater's now me!

That makes sense actually - the heads were fitted with proper blanking plugs, not welded up spark plugs or something similar.

H

 

[john998]

Hello,
As an aside, Enfield de-compressors, I fitted them too many years ago. What you get is two oil leaks instead of one.
On the Enfield the valve is in a recess ported to the exhausts, on the Vincent all the rubbish is dumped on the heads.
This is made worse if the engine is burning oil.
Regards John.

 

[vibrac]

Hello,
As an aside, Enfield de-compressors, I fitted them too many years ago. What you get is two oil leaks instead of one.
On the Enfield the valve is in a recess ported to the exhausts, on the Vincent all the rubbish is dumped on the heads.
This is made worse if the engine is burning oil.
Regards John.

You used them when the engine was running?
I only use the decompressors on starting full pull works both heads to turn the engine, half hearted pull closes rear
so I when I get compression I know I am on the right cylinder to kick. ease it over the top, then leave the lever alone
stoping? thats what your cut out button is for

 

[Howard]

"I only use the decompressors on starting full pull works both heads to turn the engine, half hearted pull closes rear
so I when I get compression I know I am on the right cylinder to kick. ease it over the top, then leave the lever alone" ......... Vibrac

All these years I've been listening to see which decompressor is letting air out - I knew there'd be a simple answer!! I'll be adjusting the cables tonight.

H

 

[timetraveller]

Isn't it strange that after well over 60 years of Vincent twins there is still no agreement as to how to start them. I never did go down the route of trying to find the correct cylinder on the correct stroke and then abusing the kick starter. Vibrac is a very experienced Vincent man and yet he is doing something that looks like an abomination to me. I just turn the engine over a few times with the valve lifter operating to ensure that the bike is out of gear, that there are no unusual noises and that the oil is starting to go round then I just kick down on the starter and let go of the valve lifter half way down the stroke. It works for me and if you incorporate the mod I mentioned here a few weeks ago into the original valve lifter (cost near to zero ) then there is no leak there either.

 

[BigEd]

Wearing ear-plugs and can't hear well anyway. Just pull in valve lifter and just give swinging stroke to the kick-start lever releasing the valve lifter as you near the bottom of the swing. Takes the strain off your knee joint as the flywheel inertia takes the pistons over compression. A little practice soon gets your swing/release timing coordinated. Easy.

"All these years I've been listening to see which decompressor is letting air out - I knew there'd be a simple answer!! I'll be adjusting the cables tonight.

H

 

[Howard]

It ain't that easy with decompressors. I think it's due to the opening being much smaller than a lifted exhaust. There's still too much resistance for my short legs, and like Vibrac I have to resort to the Velo method. It works fine once you get the technique sorted. I have to say valve lifter sounds easier, but my valve lifter remains were thrown in a drawer following an expensive failure.

H