Misc: Everything Else Starting Problem - Series 'C' BIG Single

[Jez Nemeth]

Jez,
I've been following your thread with interest as I've just finished my Comet special and was playing with carb and ignition issue.
I'm now happy with the bike so far, even though it has only 200 miles under its wheels.
I don't know if this can help you but here are the bike's spec:
90mm bore, 8:1 Omega
1034 concentric: 30, 106, 250, 622/122 needle 2nd groove, 3 1/2 valve.
I'm not sure about the main jet at the moment as all is brand new I don't want to try WOT until it's run in.
Ignition: Ford Fiesta points (by François Grosset), single plug, Lucas condenser.
The bike start first kick cold (don't like to be over flooded though), warm or hot. I mention "warm" because I have a Sunbeam twin which start readily cold or hot but is real pig when just warm.
After 34 years I never solved this problem so I just live with it every day as this bike is my ride to work, I don't own a car or a modern bike.
Cheers.

Hi Eric,
Just jealous of where you live for a start! Used to spend a lot of time avec notre famille française
à Bisacarosse, and windsurfing nr Lacanau and up and down that seaboard, markets etc......ahh memories! Got to ask, and i know this is off track but do hope the fires near you have not been too bad? Bushfires are no fun at all..

Grosset parts - sounds like a beautiful machine indeed, and that info about the 1034 is really interesting -using a setup same as my 500cc engine, same piston ratio -but bigger main at 250...it is 4mm bigger of course, even so the 930 premiere only uses a 220, others are smaller...Carbwise, thinking about a 600 for a starting point, better knowing it will be too rich, and working backwards. Its only at full throttle my engine popped, so in that last 1/3 throttle 320 main, needle middle of 3 groves -its where the fuelling is likely critical, mine was just very lean as we found out...but think the starting is ok, cold and hot -it now just starts...but we did need to set it up with twin plugs to do it -hey ho, c'est la vie!

 

[Jez Nemeth]

There's a drilled out Amal 1034 to 38mm on Ebay at the moment -work likely done by M Bratby, been done well, but spraytube is cut for 2 stroke application, easy fix...tempted for the extra 2mm... Think the 36mm will be fine for now once opened up...interestingly both it and this one use a 3# slide, small stuff but good to know.

 

[Jez Nemeth]

Researching oilpumps -there' a small Mikuni that looks right...no spare halftime pinion, but had one of these spare...surprising how easily a hacksaw will go through it, like tempered cheddar.

Look at those fingernails!...my mum would have a fit

 

[Bill Thomas]

There's a drilled out Amal 1034 to 38mm on Ebay at the moment -work likely done by M Bratby, been done well, but spraytube is cut for 2 stroke application, easy fix...tempted for the extra 2mm... Think the 36mm will be fine for now once opened up...interestingly both it and this one use a 3# slide, small stuff but good to know.

It's your call Jez ,
But I can't agree, If anything I would go smaller Carb',
Can't see what you would get going bigger,
I thought we agreed, Rev's is NOT what we want ?,
Your piston speed would be too high ?.
And the air to petrol ratio , You would need even more petrol with a bigger carb' ?.

Peter is talking about that thing with an aircraft engine, Have you heard it ?.
He only rev's it to a few thousand !, You can count the bangs as he pulls away !.

Heat can be made if the ign's is too high or too low,
There is only one way to find out,
As long as you know the advance is working right , I would retard it 2 degrees.

But we also know it was as WEAK as hell with the carb',
Partly my fault, The way amal britbike gives us settings , For carb's, Not Bikes !,
I thought , They would be in the "Ball Park ",
They was Spot on with my Mk2 Amals, Which I was shocked about !,
So until that is sorted, it's hard to know what to do , "To Me ".

Your call , I am just guessing, I can only say what I would do.


Funny , I have never had pinking with a Vincent, And I have had SO many different Carb's,
On my bikes,
I even tried Strongbergs Off a car, It ran SO smooth slow running, Twin Vin',
But with the rev's It wouldn't keep the slide up !,
They had a rubber Diafram and the slide worked by air pressure,

Often thought of running one carb' between the two cylinders,
Or 2 carb's with a link pipe !.
But never got around to it.

 

[Bill Thomas]

Researching oilpumps -there' a small Mikuni that looks right...no spare halftime pinion, but had one of these spare...surprising how easily a hacksaw will go through it, like tempered cheddar.

Look at those fingernails!...my mum would have a fit

Tell me if you need a Half time pinion and what size, I have a few !.

 

[bmetcalf]

FWIW, Scott Seegers here in Virginia used 32mm CV carbs from a CB450 on a tuned twin.

 

[vibrac]

I am with Bill, the big mistake with a Vincent engine is to go for too big a carb a bit like people who put high compression pistons on Triumph twins nice to brag about poor to ride

 

[Bill Thomas]

I was shocked how well my Ex L/ning went on 28 mm Mk1 Concentrics,
I thought I would be nailing it everywhere, But No , Still 70 to 80 on a 1/4 throttle On the flat.

 

[Monkeypants]

As is normal, I'll go against the grain here.
Jez has a T Prince head on there, not sure which iteration. There were many. Inlets grew as the years went by. Terry improved flow and power outputs as he went along. Steve Hamel started with TP heads then modified them and found a lot more power, albeit at the expense of the low and midrange, from my reading.
That didn't matter, he could take his time feeding the throttle to it as the bike was built for Bonneville.
Terry changed the heads again after seeing Steve's results, but not in a way that killed low or midrange. There seems to be plenty of that!
On carb size, Terry's advice was to use minimum 38mm Dellortos, larger if you can find some.
The largest available size was 41 mm so that's what went on.
It's a sweet running engine. Inlets are 38mm. The little step worked for Roland Pike on the Goldstar and seems to work here. Throttle response is instant. The hard thing is keeping my head forward when accelerating. It's worse for that than the Daytona, which dyno'd 144 rwhp after tuning.

I had fears that the large carbs, high compression, hot heads and big cams would result
in an engine that would be useless for anything other than high rpm.
Didn't get that at all.

Jez, what size is the inlet port on this head?

Glen

 

[Chris Launders]

I'm not that technical but it seems to me that as you have increased the engine size by 66% (833 is 500 +333 or 66%) you need a carb with an 66% bigger cross sectional area to keep the same gas speeds then you will also need jets 66% bigger to supply the fuel (assuming the jet size is based on flow and not diameter, if it is diameter then it's a bit more maths as you want a 66% increase in CSA)

Or is that me being too simplistic.

Chris.