Forum

[PeteF]

I've always known that the 24mm carb on the 350 is a bit small. I have a 1 1/16" monobloc available but never bothered changing for what I thought would be a marginal increase.
I just did the maths and I get an increase of 22.47% choke area.
My maths is pretty ropey but if that's right, it might be worth the bother of changing and re-jetting.
The 350 might go up hills then!
I suppose the smaller carb suited the original restricted silencer which is long gone.

[Alan R]

Hi PETE F ------------- Your proposed 1.0625" Amal Monoblock comes out at 26.987 ( 27 corrected)mm.....ie}-- just 1mm diam. less than the 28mm standard for the 500cc bike......When you dumped the restricted silencer did you re-jet accordingly then ?? ......... It might just be that you need to re-jet the existing carb.?? --------- What set-up are other 350 riders using ??

[PeteF]

Yes, I rejetted when I changed the exhaust and filter and mixture is spot on. I just feel the top end of the throttle range doesn't do much. Almost as is the carb can't pass enough air. I realise I would have to do a bit of gas flowing to the inlet to match the larger carb.
Mr H does a Monobloc conversion for 350 - must ask him what size he recommends. Tollgate say 26/28 mm for their gas flowed head.

[simon]

Go for it I say. I've got a 1 1/8" 389 Monoblock on my 350 and it breathes very well. Port the head to match it and it should make a significant change.

[Presto]

The Monobloc kit for the 350 includes a 1 inch carb (approx 26mm). The Concentric Mk 1 for the tuned head is 28mm. With the port opened out to 28mm (approx 1 1/8 inch) the engine is sure to breathe more freely. And with the 350 ‘every little helps’ as they say!

[PeteF]

Simon, Blimey! that is big. If your 350 can swallow that then mine should manage a 1 1/6. Did you port right through or just fair the inlet to match?

[Midge]

Designing a carb application is a complicated subject, its not always a question of bigger is better. The carb has to provide the correct air/fuel mixture from idle to full revs under load conditions from zero to max. If you are constantly holding the throttle fully open and still cannot achieve enough power then maybe upgrading the carb as part of the overall engine design is a good idea. Or maybe buying a 500. Mind you, then you would likely want to upgrade that as well. I like the 350 for what it is personally and if it is running correctly I would just carry on enjoying it.

[simon]

I did quite a bit of work on the head with a die grinder. The important thing is to remove any obstructive metal but to keep the port narrow at one point so as not to reduce the gas velocity. The head I was dealing with was a 92 Indian 350 and it was very crudely finished. The ports both inlet and exhaust were complete un fettled castings apart from a milled carb flange and about 3/4" of the port. There was also a lot of excess metal near the valve guides. I attached the 389 directly to the head with engineer's blue and so left a witness as to the size and position that the port needed to be. I then tapered the hold to the smallest part of the aperture just cleaning any dags of casting out of the way to leave the size basically the same but smoothly finished. There are a couple of quite good pics online of ported RE heads that will give you an idea. The carb is of course only one part of the chain but with slightly higher compression and a bit more cam overlap the performance is considerably improved without I feel compromising the integrity of the machine although I do understand what Midge means and I wouldn't do it to an older Redditch machine.

[simon]

Just worked out the mpg on a decent day ride I did the day before yesterday. I got 69.1 miles per gallon over 146 miles with a reasonably lead welly. Quite pleased with that. S:)

[Alan R]

Hi guys--------------- one other consideration in favour of keeping to the 350cc size is}---------------------------------- it's a cheaper Road Tax band !! ( Now that's GOT to be good maths ??)