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Which side for the sparkplug?

Howard

Well Known and Active Forum Website User
VOC Member
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? :confused:

H
 

ET43

Guest
I reckon that it is all to do with gas swirl, as when I ran twin plugs, the second plug in each head was burning significantly weaker leading me to believe that most of the mixture was being burnt by the first, or original position plug. Cheers, ET43
 

davidd

Well Known and Active Forum Website User
VOC Member
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
 

clevtrev

Well Known and Active Forum Website User
VOC Member
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? :confused:

H

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.
 

vibrac

Well Known and Active Forum Website User
VOC Member
And when you have found the best side stick some RE decompressors in the other side and throw out that cats nest of springs and levers waiting to entangle the cams-and stop a regular oil leak point as well
 

Black Flash

Well Known and Active Forum Website User
VOC Member
And when you have found the best side stick some RE decompressors in the other side and throw out that cats nest of springs and levers waiting to entangle the cams-and stop a regular oil leak point as well

this is almost blasphemia regarding the merits of the two Phils ! ;-)

Bernd
 

ET43

Guest
I have elected to use a 3/4 inch reach Venhill decompressor on the trike engine even though it might sound like a wet fart, but it's got to be better than that heap of mechanical rubbish found in the timing chest. Plug holes in standard heads are directly in line with the slug of gas from the carb, but poor porting will alter the way the charge enters the head. I have been told that the 636cc engine should be timed at 24 degrees btdc given that it it not overly tuned, but there again, what do I know. I do not really care as today, I got the engine into the chassis and I had a cream cake to celebrate. Now if only the Hamons' would extract their digits, I could start the damn thing. Phil Primmer.
 

Howard

Well Known and Active Forum Website User
VOC Member
Trev - I think that's oposite to standard (yes, Iv'e got B heads).

Vibrac - I've had decompressors since the valve lifter did as you describe in the 70s.

Bernd - everyone has an off day.

Phil - Decompressors are good, but they don't make the engine as easy to turn over as valve lifters. 24 degrees seems a bit late, I started at 34 on 9:1 compression (since rebuilding for new fuel) on overwhelming advice. I'm up to 39 now and I must have 10% more power - I'm going to try a couple more degrees at the weekend.
Less of the creamcakes, that's how I started, and I need new leathers for Mallory now. :)

Thanks all.

H
 

ET43

Guest
I know that I'm a tub of lard, but at the moment, Grub is my only pleasure. Because of the lack of a starting device, I have no motorcycle insurance, the first time since 1958 having now been off the road since September 2009 building the trike. The ignition timing was advocated by Peter Barker who got it from Bob Dunn. This is for a 636cc engine which has a squish band and a c/r of just over 8-1 so that is where I will start because that is what I have got. Cheers, ET43
 

Howard

Well Known and Active Forum Website User
VOC Member
I know that I'm a tub of lard, but at the moment, Grub is my only pleasure. Because of the lack of a starting device, I have no motorcycle insurance, the first time since 1958 having now been off the road since September 2009 building the trike. The ignition timing was advocated by Peter Barker who got it from Bob Dunn. This is for a 636cc engine which has a squish band and a c/r of just over 8-1 so that is where I will start because that is what I have got. Cheers, ET43

It'll be interesting to see where your timing ends up. I'm seeing anything from 19 to 42 advocated for the wide variation of motors we've got now.

Hope you sort your starter supply problems soon.

H
 
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