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Carburettor choices

Hicam

Website User
VOC Member
During a long and tiring search to get my 1950 Rapide to start, over the past few years (20 odd!) I have:-
Had Dave Lindsley rebuild the Lucas mag. Tried - no change.
Bought and fitted a BT-H electronic may. Tried- no change, reverted to Lucas mag.
Replaced with new, carburettor needles, slides and jets to original spec.
Fitted a brand new old stock advance/retard unit.
Checked and re-checked valve and ignition timing.
It occasionally does start after ten or more kicks, going off to recover and having another kick. It may then just fire up and runs perfectly!
It's just that it is totally unreliable when it comes to starting.
I will just mention that I have a Vellocette Venom and very rarely have problems starting that.
Any suggestions please?? Perhaps different carbs?
 

davidd

Well Known and Active Website User
VOC Member
Just curious, have you done the Greg Brillus mod to provide more fuel for low speed running?

David
 

brian gains

Well Known and Active Website User
VOC Member
the first thing I always remember is that if these old machines were a pain to start when in the showroom they never would have sold, the second is that if they don't start by say the third kick then something is not set up right.
I'd look at a fueling issue from the description you have given and also do a compression test.
It's easy to take things for granted but always check. Like the bike I bought that had an increasingly notchy and difficult to engage gearbox on the way home, what could it be, bent selector forks, worn dogs?: the owner had not bothered putting gearbox oil in it!.
All the best with the issue Hicam, slowly, slowly catchy monkey but 20 years really is too long. Sure you'll crack it with suggestions from the knowledgeable folk around here.
 

timetraveller

Well Known and Active Website User
VOC Member
I agree with Brian, 20 years of frustration is not sensible. Get to know some local VOC members. At least some of us are still competent. If the valve and ignition timing is right and there is a decent spark then one needs to look elsewhere. Is the compression strong enough to support your weight for a few seconds? The fact that it runs well once it is going suggests that there is not much wrong but weak compression can cause poor starting and not show up when the engine is running. If that is OK then I suspect your starting technique. Many Vincents are idiosyncratic in that some require a lot of flooding, others cannot stand any. Some require the throttle to be shut, just cracked open or quite wide open. Your Velocette is not an indicator of starting technique for the Vin.. Just that your right leg is strong enough. Seek help and advice locally.
 

vibrac

Well Known and Active Website User
VOC Member
Technique is important. Think of all the variables without getting mechanical. How far to move the kickstart after compression. which cylinder to start on, where to put the choke, to tickle or not a lot or a little.
I guess you must have exhausted all those variations but they are important to try after any major changes that you may try as you follow some of the mechanical solutions that will be proffered here.
After all until I followed a suggestion on here about where to position the engine to then kick my Egli Comet I could not kick start the damm thing I could have spent months changing ignition carbs etc etc and still not have the near first kick starting I get now.
The one thing that helped me and saved all that grief before being told what I was doing wrong was a good mains driven set of rollers I could get and then could tell that the bike was running spot on without getting exhausted starting it.
 

Nulli Secundus

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Perhaps some one local to you, with a Rapide, could visit you and do a temporary carb swap as a test? Also, perhaps they could try starting your bike, which might prove timetraveller's suspicion about starting technique.

I do not follow the rigamarole of finding the correct compression and go from there. I, having freed the clutch first (V Two multiplate) move the kick start to feel compression, then raise the kick start lever back up, pull in the decompressor and go for it, releasing the decompressor once the kick start is about half way down its stroke. My bike has Mk2 Amals and Pazon electronic ignition.

You seem to have ignition covered and the fact it runs well should presumably mean it should start at kick starting speeds.
 

Bill Thomas

Well Known and Active Website User
VOC Member
You don't tell us if it's hot cold or warm, When you have trouble ?.
I have found the old carb chokes are not good, Best left alone and just use the tickler,
Having said that my Brothers Comet choke works perfect !.
The petrol we have now , I have found it needs flooding a bit when warm.
But every Bike is different, I would use a Champion N5 or N9yc Spark Plug.
Good Luck, Bill.
 

davidd

Well Known and Active Website User
VOC Member
Hicam,

I think that I may have sent you in the wrong direction as the Brillus advice works on the 1-1/8"carbs. I do think the fastest route is to have someone else look it over. It is time for a critical eye from the outside to take a shot at scrutinizing the situation.

All the best,

Davi
 

Sakura

Well Known and Active Website User
VOC Member
the first thing I always remember is that if these old machines were a pain to start when in the showroom they never would have sold, the second is that if they don't start by say the third kick then something is not set up right.
I'd look at a fueling issue from the description you have given and also do a compression test.
It's easy to take things for granted but always check. Like the bike I bought that had an increasingly notchy and difficult to engage gearbox on the way home, what could it be, bent selector forks, worn dogs?: the owner had not bothered putting gearbox oil in it!.
All the best with the issue Hicam, slowly, slowly catchy monkey but 20 years really is too long. Sure you'll crack it with suggestions from the knowledgeable folk around here.
Careful reading of original road tests indicate that twins were often not easy to start. Bearing in mind they were carried out by experienced motorcyclists with a reluctance to denigrate bikes I suspect many were difficult to start even when relatively new.
 

Bill Thomas

Well Known and Active Website User
VOC Member
It was always a joke that they won't start when people are watching :D .
I think as we get older, That doesn't help, Maybe a bit of stress, Or is that just me ?,
But if it doesn't start quick, It's easy to get it wrong, And this petrol is Pee !,
On my Bikes it took me a long time to work out when warm, It needs flooding or choke,
Which is totally wrong for an old mechanic,
But my Comet is fine most of the time.
 

Hicam

Website User
VOC Member
Thank you for all your replies.
It's cold starting I am struggling with now. I have decided to take the carbs and fuel lines apart and check for foreign matter.
I will do a compression test but when I first rode it in the late last century it climbed Hardknott and Rynose passes in the Lake District which are hairpin 1:4 hills without a problem even with its Norton clutch conversion!
I have tried feeling for compression and the variations of front and rear cylinders!
A local member is going to take a look at it for me next week, so I will keep you posted.
Regards Chris
 

Bill Thomas

Well Known and Active Website User
VOC Member
From cold, Forget the chokes and give it a good ! flooding, With new plugs It has to start ?.
As I said on another thread, Get someone to give you a push start in first gear !!, Some say use second gear,
But work out how FAST you would have to push !!!,
For me, First is the one to use, Just to prove it's a runner.
 
Last edited:

vibrac

Well Known and Active Website User
VOC Member
As the title of this thread is Carb choices I would put down my thoughts

Original 276 Family Great Carbs not a lot wrong with originals except wear after all this time and I dont know about their new expensive copies.
Monoblocks again not a lot wrong wear a bit more prominent as the bean counters moved in on quality
Concentrics I have only met these 2 variations on other peoples machines who had a lot of problems with tickovers and having to drill etc I believe the latest versions have improved I would not take them off if I bought a bike with them on, but I would not buy one to put it on a bike I had.
Mikuni Cannot fault these once Allens have set them for a machine and the rubber fixing has been sorted I miss the tickler and also have to think hard about the choke but thats just habit
TT Had a lot of time with these wouldn't hesitate to use them on the road but alloy body can wear fast
GP Ditto TT but not cheap just got a big pair for the racer (ouch!)
Bing,Del Auto etc etc no idea I have two Bing to sort on a BMW ...
 

Chris Launders

Well Known and Active Website User
VOC Member
I am with Bill, flood it and kick, I have concentric's on mine and flooded it is normally one kick, I don't use the decompressor but have the advantage (for this anyway) of weighing 20 stone, I don't have chokes.
 

brian gains

Well Known and Active Website User
VOC Member
10 kicks + and I'd be thinking the motor was flooded so do the usual of crack it wide open and full choke, although with a valve lifter shouldn't you be able to clear flooded pots?.

on the twin i can't fault the Mikunis although early days so far. Trouble with rear carb' over flowing seems to have cured and probably was due to standing idle too long. Other than that i have just leaned out rear by using smaller needle jet and using 3rd groove as opposed to 1st groove with P2 jet, slightly browner appearance.

i don't want to jinx it but the twin starts so easily (when all is right) that I don't use the decompression lever, I always thought deco' levers were relevant to 40's - 50's man who maybe was less in stature and weight than his later counter part.
 

Bill Thomas

Well Known and Active 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.
 

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