B31 1949

[Pete Gill]

Hi all,
I took out my B31 rigid yesterday for a run out, about 50 miles all together. Im pleased generally with the bike, great fun with the sprung seat etc. I rebuilt the engine about 2 years ago and it starts and runs easily. Magneto had been refurbished at the same time as rebuild of engine.
It accelerates well I think for an old 350 with my 95kg sat on it, but one thing i have noticed is at top speed (60 mph or so is a fast as i dare on this) the bike engine drive becomes irregular , it doesn't misfire but sort of 'pushes and pulls' instead of having an even balanced throttle.
Any ideas?

I wondered if it might need a bigger main jet?

[Jean-Luc]

Changing the needle position by one notch solved the same problem on my B33 .
I don't remember if it was higher or lower.

[idie]

I the throttle was open at the time you may need next size up on the main jet.

[Jim S]

Hi Pete

See the attached document on tuning amal carburetors. On page 2, last 2 paragraphs, it explains how to determine if the main jet is too large or too small by riding flat out and closing the choke by 1/3.

Be careful, grit your teeth and let us know what you find.

Jim

[Pete Gill]

Hi Pete

See the attached document on tuning amal carburetors. On page 2, last 2 paragraphs, it explains how to determine if the main jet is too large or too small by riding flat out and closing the choke by 1/3.

Be careful, grit your teeth and let us know what you find.

Jim

Thanks Jim, thats a good idea. By the way I have a Monobloc carb fitted, but same applies i expect.

[Pete Gill]

Changing the needle position by one notch solved the same problem on my B33 .
I don't remember if it was higher or lower.

Thanks Jean Luc I will try

[Pete Gill]

I the throttle was open at the time you may need next size up on the main jet.

Thank Idie , that was my thought too but might be worth the choke trick as has been suggested below. If i can find a bigger main jet in my spares i might just replace it and see. It's very quick to change out.

[idie]

Standard size jet is 260 so you will need to try a 270

[Steve.S]

Sounds a bit like the engine is "hunting", but this normally occurs at a smaller throttle opening and indicates a rich mixture. If you have no success with the carburettor, I wonder if the cush drive might need tightening a little?

[Pete Gill]

Sounds a bit like the engine is "hunting", but this normally occurs at a smaller throttle opening and indicates a rich mixture. If you have no success with the carburettor, I wonder if the cush drive might need tightening a little?

Thanks Steve I will look into I this.

[Pete Gill]

Just an update.
I decided to go with changing the main from a 250 to a 270 (mainly as i had a spare one in the spares box) did another long run a couple of days ago. No difference . I tried the choke trick but I'm not really sure it helped. Backing off the throttle a little made it run smoother. Im now thinking I've got fuel starvation on wide open throttle for some other reason. I will recheck my float height and report back.

[neil1964]

Generally you need to make sure everything else is correct before fine tuning the carburettor, I am sure that you have done this already.
Fuel starvation usually causes a cough and splutter as you empty the float chamber and you need to back off for a few seconds for it to refill.  As long as your fuel tank is clean and the filters (tank and carb banjo) are clear a supply issue would be odd (does your tank have a breather or the tank cap have a clear breather hole?) so fuel level in the float chamber is a good thing to check.
Is fuel fresh - ie modern fuel sitting for more than a few weeks 'goes off'.
Is compression good?
Brakes not binding?
Are the tappets set correctly (assuming that the valve timing is correct)?
Is the points cap 12-14 thou'?   Points surface clean (not pitted or oxidised).  Valve timing correct (7/16" BTDC = 38 degrees at the crank)
When the mag was rebuilt was a new capacitor used?
'High speed' misfires can be traced to a failing capacitor (in the same way that a weak armature and cap will cause poor hot starting).
At 60mph on standard gearing your engine will be doing 3750rpm or thereabouts.
What gearing are you using?  I think final drive is 19/42 and primary 17/43?
Overall ratio is 5.591 with a 3.50x19 Speedmaster II tyre (26.5" diameter)
The B31 in good order makes 17bhp @ 5500 rpm
So at 5500 rpm a healthy B31 will be doing just under 80mph on the flat.
You know everything is maxed out but running well when you simply run out of steam and curiously I find everything seems to smooth out but you just go no faster.  Sounds like you have a few things to double check?

[Pete Gill]

That's  good food for thought, thank you Neil. Although i must say that i would run out of courage a long time before 80 mph considering my steering and suspension unless it was a pristine quiet straight dual carriageway!

[Pete Gill]

 I made a crude Perspex replica of the monobloc float chamber cover. Ensuring the dimple is marked in the precise location below the name. I'm informed the fuel level should coincide with the dimple. So is this a bit low? I'm considering shims.
Pete

[neil1964]

I made a crude Perspex replica of the monobloc float chamber cover. Ensuring the dimple is marked in the precise location below the name. I'm informed the fuel level should coincide with the dimple. So is this a bit low? I'm considering shims.
Pete

Looks like a nice way to do this.
Assuming your float moves nicely can't you bend the little brass tab on the float, or even trim the length of the float needle (on the side that the float tab touches!!)
Does not look desperately off.
Is the carb actually on the manifold during this measurement?