Bottomless fuel pit

iandusud · Post by **iandusud** » Tue Mar 22, 2011 7:09 am

Well done Dave. That's just the sort of info required. Out of interest why did you use a 105 needle jet rather than the std 106? Was just a case of what you had to hand or was it running rich with a 106? I would imagine that 26mm Concentric would be better suited to the 350 than a 28mm. Interestingly I recently bought a Concentric from Burlen Fuel Systems, who make them now, for a Model 20. Their recomendation was a 26mm bore and all settings as per the orignal Monobloc except for -20% on the main jet.

Ian

arsey30 · Post by **arsey30** » Tue Mar 22, 2011 9:12 am

The 105 was either standard or recommended for that carb, can't remember which.
The monoblocs mostly used 106 as far as I remember.

The model 20 cylinders are smaller than the 16ms.

Dave.

iandusud · Post by **iandusud** » Tue Mar 22, 2011 9:22 am

quote:
The model 20 cylinders are smaller than the 16ms.

Fair point, and the Model 20 used 1" Monobloc against the M16's 1 1/16", so a 28mm Concentric would be correct.

Ian

The Sinful Optomist · Post by **The Sinful Optomist** » Tue Mar 22, 2011 9:55 am

"A large carb bore will reduce the venturi air speed and make the engine asthmatic, requiring large throttle openings to get it to pull"

but surely the mix would be the same? (I say surely, but it should be noted that I havent even the slenderest of a clue!)

arsey30 · Post by **arsey30** » Tue Mar 22, 2011 11:58 am

Even if the mixture ratio was set correct, the reduced gas speed results in less power at the lower rev range.

I had a 500 desmo twin with 36mm twin Del Orto's that revved to 10,500rpm and made good power [54bhp at the crank] but was so flat at low revs.

Dave.

The Sinful Optomist · Post by **The Sinful Optomist** » Fri Mar 25, 2011 6:52 pm

So I caved in to peer pressure and bought the correct carb, mpg whooshed up to 67, which is much more acceptable, so thanks for the help everyone.
Next question is, it wont pull 4th gear on any angle of hill. flat is fine, but even a very shallow incline leaves me dropping down, sometimes even to secondd, it just feels completely gutless. If thast something I have to learn to live with, at 67mpg I will, but is there anything I could do to improve the situation?
Carb is new and set up exactly as recommended (although I know some people are saying to go up a jet size because of ethanol in petrol now) and spark plug is a lovely biscuit brown colour.

As an aside, I ordered the carb (monobloc) from surrey cycles, and with delivery cost me £170, I fitted it (no I didnt overtighten it) and the slide stuck hard at any throttle opening. I rang surrey cycles, explained and he knew exactly who I was, said that he'd thought it was a bit tight, and that he didn't know what to say. He eventually offered to send another slide, but didnt have any in so it 'would take a while'. I sent my mate to buy wet and dry paper (g3's my only transport) and spent an hour sanding the slide till it fit. Its not a massive big deal, and I know old bikes will always demand my attention, but at £170, and being brand new I expected more, and the fact that he new it didnt fit properly and sennt it anyway really p*ssed me off. I mainy say this to validate someone elses earlier comment on Surrey cycles.

iandusud · Post by **iandusud** » Fri Mar 25, 2011 7:29 pm

First of all I'm pleased that you've got yourself a new carb and that fuel consumption it back to normal. Personally I think you've every reason to not be happy with the service you've received from Surrey Cycles. As you say he shouldn't have sent the carb if he knew it had a sticky slide. Apart from the fact that it's a case of substandard goods, it's also potentially very dangerous. I can't comment on the performance of your G3 as mine is still a collection of bits!

For information I have had good service from Burlen Fuel Systems who make Amal carbs.

Ian

arsey30 · Post by **arsey30** » Fri Mar 25, 2011 8:05 pm

I found a lot of new pattern parts to be of poor quality and fit.

My 16ms pulled very well, 75mph [flat on tank] romped up hills 50-60mph and acceleration good.
I geared it up to reduce revs and thus vibration and it still pulled well.
Check the ignition timing, it seems modern petrol needs around 34 not 38 or 9 deg, mine went better retarded.
Check gearing, 5.8:1 top.
If you can't get to all the sprockets to count teeth, chalk mark the back wheel, engage top and turn the engine to check ratio.

Dave.