Bottomless fuel pit

The Sinful Optomist · Post by **The Sinful Optomist** » Mon Mar 21, 2011 5:08 pm

I called surrey cycles, in mind to just bite the bullet and buy a new carb, but the chap who worked there said it would be pointless, that even though the carb on it was the wrong size, it was jetted to suit already, and if the bike ran well (which it seems to) that theres nothing wrong with the carb and the fault lies elsewhere.

I ran a tank through it today and got 44.6 mpg. And ive found a period roadtest that says this bike should do 93mpg at 30mph and 67mpg at 60mph. Now I know that petrol has changed since then, and different people use different throttle openings, but even allowing for that error, Im still miles away from acceptable fuel consumption. Im bothered about this because I do 300+ miles week. And yes, I know I should be riding a jap bike, but I dont want to, so dont say it!

Any advice appreciated

iandusud · Post by **iandusud** » Mon Mar 21, 2011 6:46 pm

Despite what the chap at Surrey Cycles said i think you'll find that the main jet is far too big. It should 220 on a Monobloc but Concentrics should use a main jet 20% smaller according to Burlen Fuel Systems who make them.

Ian

The Sinful Optomist · Post by **The Sinful Optomist** » Mon Mar 21, 2011 7:01 pm

Ah, I forgot to mention that todays run was with the new jet, down to 210, recommended by the chap at feked.com

My maths is fairly appalling, but 230 down by 20% is what, 184? So you think I should be aiming at a 180 jet? All this re jetting stuff is new to me, the difference in hole sizes seem inconsequentially small to account for a 30mpg loss. Am I wrong?

bjork · Post by **bjork** » Mon Mar 21, 2011 7:08 pm

Until you fix this you definitely should be riding a Japanese/Indian/Chinese/Italian bike, sorry!

If I remember rightly, a 932 concentric means it has a 32mm bore. Again from memory, a 350 single should have a carb of around 26/27mm? So, if you have a 32mm carb and a 26/27mm inlet port you also have a great big step for the mixture to climb up before it dribbles down to the valve. Also the idle/pilot circuit will be badly compromised by the mismatch, surely? my guess is that to get the bike to pull reasonably well you are probably having to open the throttle a lot more than normal too. All this would lead to bad fuel consumption, and if your choke upsets things so much then it's all very wrong, innit?

I'd say a proper size carb will be the way forward. That could be a Monobloc/concentric/Enfield/Mikuni/Dellorto/Keihin or others too.
The easiest and least painful would be the correct size monobloc because the settings are all ready worked out for you in the manual.
Btw, Surrey cycles changed hands a little while ago, and my personal view is that the new owner hasn't got the same in depth knowledge, even if he means well enough and I will still buy stuff from him. (but I'll use my manuals to decide which products-there is also JSL to consider, of course)

I use my G80 for everyday transport and it's doing 90+mpg at the moment-while I run my new piston/bore in. When i fix the leaky fuel taps it'll be even better. We plan to do the International two up on this bike and I hope to do it on about 4 tankfuls each way.

You need to fix this if only to stop paying so much tax to our glorious bleeders.

iandusud · Post by **iandusud** » Mon Mar 21, 2011 7:08 pm

Hi S.O. I would think 180 or 190 would be right.

Is anyone here running a Concentric on a 350 H/W? What main jet do you use?

Ian

bjork · Post by **bjork** » Mon Mar 21, 2011 7:28 pm

While I agree with all that Ian has said above, I still think you need to address the mismatch of carb bore and inlet port diameter. have you looked at them? That step (if it's there) means you get a lot of mixture hanging about there, going back to liquid from vapour and making a mess of the whole process. It probably answers the question of the choke killing the engine too. With the (choke) air slide at the bottom it might even be lower than the level of the port. And you are using the air slide/lever the right way round, aren't you?
Eggsucking info on www..........

The Sinful Optomist · Post by **The Sinful Optomist** » Mon Mar 21, 2011 8:58 pm

Ian, I wrote that last msg, then reread your earlier replies, realized you'd already told me 180, felt quite stupid for not paying more attention, then the internet connection timed out, my point being, oops, and sorry!

last thing you need when your trying to help is to have to say it twice, cheers for your patience.

Bjork, interesting what you say about surrey cycles changeover, the chap did seem friendly enough, and both he and the guy at feked.com both said 210 main was fine, so with absolutely no trace of disrespect to Ian (and freely admitting my own ignorance) the conflicting advice is confusing me! It does seem easiest mentally to get the correct carb, its just a hell of an outlay for only a 'possible' fix, this bikes already sucked me dry in the last week with tyres and leaky taps, Im loathe to throw good money after bad, starting to think I bought a lemon.

Chap at surrey said it could only be timing or valve clearance, setting timing is something I need to practice, but the bike starts first kick, idles fine, doesn't pink or backfire and pulls ok, and pushrods spin at compression with no noticeable up and down play, so I presume thats all fine?

Anyway, thanks for the input (although not the get a different bike dig, finances simply dont allow that!) The G80s a 500cc isn't it?Is it supposed to get 90mpg or have you fiddled with it? And are you matching A+B road speeds or taking your time at 40mph? whats your non running in mpg? Interesting about the possible step, I'll have a look when the sun comes up. Though would 4mm spread over a diameter make a big difference? and if you'll excuse my ignorance of jets, the hole sizes seem inconsequentially changed between a 230 and 210, would going down to 180 possibly account for 30+ lost mpg?

Again, thanks for your help.

bjork · Post by **bjork** » Mon Mar 21, 2011 9:31 pm

As Ian says, a 180 main would be a good size (I think) but that's only the main jet and it doesn't come in to play till around 1/2 or 3/4 throttle. You shouldn't be spending much time up there.
The idle circuit, slide, needle/needle jet and float level will be the most important settings.
The more I think about it, the more I think it's a 1" carb/port as standard, so that's 25.4mm dia. Even more of a step to climb. And yes it is important. Try googling 'inlet port obstruction' there is a lot about it on some Norton sites and it's well explained there.
Your timing and valves etc do need to be right and should be checked first or you won't know where you are.
My bike is a 500 and completely standard state of tune. It's still running in, so yes I'm going very gently, but 90+mpg is genuine. Once run in and normally ridden I expect 60/70mpg without trick riding. Your 350 should do a couple more treated the same.
I doubt your bike is really a lemon, just been fettled by wannabes from Kwik-Fit. They are simple enough and that encourages all and sundry to have a go at 'improving' them. It may take time to work through everything, but it's do-able. Just don't expect to outsmart the factory!
Sorry if the other bit sounded like a dig, but you really don't want to be relying on this bike for going to work etc till you know it's right.
Course, if to does turn out to be all a bit too much, you could always swap it for a Y----a I've got spare, electric start, lights everything-and it's only supposed to do 45 mpg.......off to bed now, up early for work (via G80 of course)

iandusud · Post by **iandusud** » Mon Mar 21, 2011 10:06 pm

quote:
Ian, I wrote that last msg, then reread your earlier replies, realized you'd already told me 180, felt quite stupid for not paying more attention, then the internet connection timed out, my point being, oops, and sorry!

last thing you need when your trying to help is to have to say it twice, cheers for your patience.

There's no problem and I understand that you're getting conflicting information, particularly from Surrey Cycles who should know. I do agree with Bjork about the importance of haveing the right size of carb.

It would be good if someone running a Concentric on a 350 H/W could chip in.

Ian

arsey30 · Post by **arsey30** » Mon Mar 21, 2011 11:20 pm

I no longer have my 1961 16ms, so go from memory.
Surrey cycles said jet down 10% from monobloc to mk 1 concentric.

A large carb bore will reduce the venturi air speed and make the engine asthmatic, requiring large throttle openings to get it to pull.
It will go well enough at high revs.

Should have 376/5 1 1/16" bore monobloc. [26.9mm]

I ran 928 [28mm bore] with 180 main jet, no air filter.
105 needle jet needle, clip 2 from top notch.
Fixed pilot jet removed and normal brass screw threaded type fitted,
size 30, pilot screw 1 and 3/4 turns out.
Bike ran well, around 60mpg, at speeds up to 55mph.
Any more was limited by vibration.

Dave.