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Carburettor choices

Bill Thomas

Well Known and Active Website User
VOC Member
The last, Can't mess with old plugs, Will only give hard to find misfire later, Or non start.
We have got a few old bikes going over the years, And it's a joke where I buy my Plugs !,
Not another Box Bill ??, What do you do with them ?.
I met a West London Man on the road years ago, No spares !, Gave Him a couple of plugs and away He went.
 

Hicam

Website User
VOC Member
And if you wet the plugs, Put them in the BIN, Spin the engine over with the plugs out to clear it,
And with new plugs start again.
Thanks for all your feedback.
My wife has suggested I leave the plugs where they are and put the bike in the bin!
Stripped the fuel system this morning. Pipes, taps, carbs and float chambers. Thought I'd replaced the slides but hadn't. They are worn so will buy new ones. Filler cap breather a bit gummed up but not enough to stop fuel flow. Everything else is ok. Contents of fuel tank not being reused, will use fresh. Some debris found in tank when left hand tap removed but right hand tap clear.
It is slightly down on compression on one cylinder when I checked. May have to revue that when I get it running.

Will report back when new carb slides (6/4) fitted. I would like to keep it original if possible.
Regards Chris
 

MarBl

Website User
VOC Member
Did you check spark plug gaps recently? With a magneto they should be 0,5mm maximum (also with the new BTH). A weak Lucas may even need 0,4mm. Out of the box ngk have 0,7mm.
 

Roslyn

Well Known and Active Website User
VOC Member
I am not sure put me right, if I am wrong please let me know, did not Stevens of Know Thy Beast recommending to use the decompressor to start the bike as the excess strain on not using it can cause problems with the kick start mechanism, sorry if I got it wrong everyone.
 
Last edited by a moderator:

Bobv07662

Well Known and Active Website User
VOC Member
Not just Eddie Stevens. I wince every time I see someone trying to start a bike without the valve lifter. The strain it puts on the mechanism is asking for trouble.
My Rapide isn't the easiest to start but 3 kicks when cold is about normal...I have only been successful once when using the decompressor and I envy all who have mastered that technique. Any tips?
PS My bike loves it's chokes to start and run for about 2-3 minutes when cold. When hot it likes the chokes just to start, not needed to run
 
Last edited:

Robert Watson

Well Known and Active Website User
VOC Member
I see people using the chokes on the original carbs and often wonder why they bother.
Unlike perhaps a choke on say an AFB four barrel (American car stuff) which is like a big butterfly across the whole intake, the chokes on our original carbs are a slide which goes down in the middle of the throttle slide. So I see people just say, Oh it needs just a bout 1/4 choke and a closed or slightly open throttle. Unless you open the throttle slide at least 3/4 of the way the choke is just hidden up in the throttle slide. To have any effect at a small throttle opening the choke must be dropped down to the bottom to have the desired effect of choking off some of the air flow, thereby as the intake stroke still pulls the same amount of air, the velocity of the air must increase thereby sucking more fuel from the pilot circuit.

Mikunis on the other hand have a small fuel reservoir with a jet in the bottom. When "choking" these you actually lift the plunger allowing the fuel to flow directly into the intake path thereby enrichening the mixture. In fact these are called enricheners as opposed to chokes.
 

Glenliman

Well Known and Active Website User
VOC Member
There are a couple of ways to use that decompresser. Not using it all makes life unecessarily difficult.
One way is just to give it a boot with the lever held up at start then drop the lever as the boot goes down. That seems to be about the best method, or at least the least bothersome method.
The other way is to just use the decompresser to slowly squeeze through most of the compression stroke then release the lever and give it a good kick. This works best if you find the long space between compression strokes, that way the heavy flywheels are really spinning when the next compression stroke is encountered.
Here is the first method. This bike is pretty easy to start. A bit more effort ( speed) might be required for some machines.

 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
My Rapide isn't the easiest to start but 3 kicks when cold is about normal...I have only been successful once when using the decompressor and I envy all who have mastered that technique. Any tips?
PS My bike loves it's chokes to start and run for about 2-3 minutes when cold. When hot it likes the chokes just to start, not needed to run

Watch the video in post #30 by Glenliman. If you click the settings "cog" at the bottom of the video window you can set the playback speed to 0.25. You can see how Glen co-ordinates the kick and decompressor lever release near the bottom of the kickstart swing.
Re chokes needed: Sounds as if you might have a too weak setting somewhere. (Pilot setting?) Or maybe you live in a very cold place.;)
 

Chris Launders

Well Known and Active Website User
VOC Member
I'm sorry but I've never been able to master the use of the valve lifter, it also seems a very awkward position, I have trouble reaching it and trying to use the kickstart at the same time unless I'm astride the bike, which if on the centre stand I can't reach the ground (I only have a 28" inside leg). In 15 years I've just had to replace the bush in the ratchet mechanism once and the engine is quite worn I suspect as it turns over easily.
 

Roslyn

Well Known and Active Website User
VOC Member
I'm sorry but I've never been able to master the use of the valve lifter, it also seems a very awkward position, I have trouble reaching it and trying to use the kickstart at the same time unless I'm astride the bike, which if on the centre stand I can't reach the ground (I only have a 28" inside leg). In 15 years I've just had to replace the bush in the ratchet mechanism once and the engine is quite worn I suspect as it turns over easily.
I also have a pre war triumph that has a valve lifter to start it and stop the engine no problems
 

timetraveller

Well Known and Active Website User
VOC Member
I too am somewhat vertically challenged but my technique, with a Dave Hill centre stand is to stand on the clutch side footrest, astride the bike. This gives me plenty of height to push down on the kick starter lever. I had the first, or one of the first, stainless steel versions of the centre stand and Dave asked me to start the bike in the above way to test out the strength of the stand. It has never been a problem. The alternative is a high kerb to stand on but one cannot guarantee the availability.
 

Bill Thomas

Well Known and Active Website User
VOC Member
My Rapide isn't the easiest to start but 3 kicks when cold is about normal...I have only been successful once when using the decompressor and I envy all who have mastered that technique. Any tips?
PS My bike loves it's chokes to start and run for about 2-3 minutes when cold. When hot it likes the chokes just to start, not needed to run
Bob, You must be hanging on to the lever a nats too long ?.
 

vibrac

Well Known and Active Website User
VOC Member
I think on a single the valve lifter is just a way of getting past compression I just have a smaill trigger lever to do that on both comets . on my 35 Cammy Velo you could not kick it without getting over the hump. I think the gear change lever is longer than the kick start
 

Chris Launders

Well Known and Active Website User
VOC Member
I too am somewhat vertically challenged but my technique, with a Dave Hill centre stand is to stand on the clutch side footrest, astride the bike. This gives me plenty of height to push down on the kick starter lever. I had the first, or one of the first, stainless steel versions of the centre stand and Dave asked me to start the bike in the above way to test out the strength of the stand. It has never been a problem. The alternative is a high kerb to stand on but one cannot guarantee the availability.
I have longer than standard hangers that are angled forward 45 degrees to move my feet forward and I am reluctant to stand on them, I shall just have to practice. There again I have installed a pair of the chainsaw decompressors so I should use them.
 

Bill Thomas

Well Known and Active Website User
VOC Member
I had to when racing in the 70s and 80s, I was not the best and could never do it side saddle,
I don't have good balance !.
 

Roslyn

Well Known and Active Website User
VOC Member
it is one of those thigs best left to younger fitter riders but I still can do it on rigid triumph as it much lower down and a bit lighter then a vincent twin.
 

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