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[Revband]

Yes OK

[Dennis C]

I have a BSA B31 overbored to take a Triumph T140 piston also fitted with Gold Star clubmans cams and an original Gold star silencer, the carb is still the original monobloc on original jetting and settings.

The chart Mauri put on from Hitchcocks Bullet catalogue shows that all the carb settings for the 500cc with a micarb are the same for both the standard silencer and the free flow silencer only needing to be changed if the air filter is changed.

[coley]

Hi, just checked the chart in the catalogue and our hosts do recommend changing the jets for a free flow silencer even with the standard air filter, all richer apart from the slide. Basically the same setting for free flow pipe and standard air filter or standard pipe and S&B air filter. If you change both then you go even richer, plus a richer slide.
From the catalogue - 500cc VM-28
Standard setting is 110 main, O8 needle jet, 25 pilot and no.3 slide
S&B air filter with standard silencer is 117.5 main, P0 needle jet, 27.5 pilot and no.3 slide
Standard Air filter with open silencer is 117.5 main, P0 needle jet, 27.5 pilot and no.3 slide
S&B air filter with open silncer is 125 main, P2 needle jet, 30 pilot and 2.5 slide

These are only a guide, I have just changed to an S&B air filter with the standard pipe and fitted the above recommendations and she is too weak low down, needs some work still.
ta
Pete

[coley]

Hi, just checked the chart in the catalogue and our hosts do recommend changing the jets for a free flow silencer even with the standard air filter, all richer apart from the slide. Basically the same setting for free flow pipe and standard air filter or standard pipe and S&B air filter. If you change both then you go even richer, plus a richer slide.

From the catalogue -

500cc VM-28 Standard setting is 110 main, O8 needle jet, 25 pilot and no.3 slide

S&B air filter with standard silencer is 117.5 main, P0 needle jet, 27.5 pilot and no.3 slide

Standard Air filter with open silencer is 117.5 main, P0 needle jet, 27.5 pilot and no.3 slide

S&B air filter with open silncer is 125 main, P2 needle jet, 30 pilot and 2.5 slide

These are only a guide, I have just changed to an S&B air filter with the standard pipe and fitted the above recommendations and she is too weak low down, needs some work still. ta Pete

Edited to make more sense with the line brakes!!!Hope it works!

[Dennis C]

Hi coley I see that you checked it twice, so I thought I should do the same. The picture mauri posted has a bit chopped of and I can now see that I misread it.

As I have said before I do not own and never have owned an Indian Bullet, however I have owned and maintained many British bikes over the years, apart from the B31 I said about earlier I also have a 1959 Super Meteor which I restored from a wreck a few years ago, this bike has a two into one exhaust, constellation cams and higher compression pistons, the Amal monobloc is standard with standard jets and runs perfectly.

I have altered and adjuster many of my bikes over the years but like Revband have never found the need to change the carburetor setting when changing exhausts ETC. Could it be something about the Micarb that is different or possibly that these bike are set up to be as fuel efficient as possible for the Indian market which makes even small alterations cause them to run too lean?.

[coley]

Ha! No i reposted with line breaks as it looked illegible all as one paragraph!



I think it is actually the other way round, they are very lean to meet Emissions in the west I think!




Pete

[Tomo]

Ok, the bike is now running properly, happy plodding along at a steady 60mph today, occasionally up to 65 with no issues. After my initial post I was getting a bit confused, so spoke to Richard at Hitchcocks, explained the situation. He confirmed what I suspected, that the carb may well need upjetting after fitting the new silencer, but suggested first raising the needle. He thought that might be enough, so I moved the clip down to the first notch. Result, no rough running. So while I don`t claim to understand why fitting a different exhaust can weaken the mixture, it obviously does. Maybe it is just a Mikarb thing, but then again I`ve had to upjet other bikes before where there are no needle clips to adjust.

[papasmurf]

Tomo it is a case of you have fitted a silencer that creates less back pressure, the means the exhaust gases flow out quicker, which alters the air flow through the carburettor when the overlap on the inlet and exhaust valves (basically when they are both open) which allow a faster air flow through the carburettor and weakens the mixture.
The free flowing exhaust in theory give you an extra 3bhp.
(It also why you need to be very careful fitting "bespoke" exhaust systems.)
It is a technical subject and far too technical for me. (On a two stroke "circumcising" two inches off the exhaust pipe can raise the revs by up to 20% but will make the bottom end power a lot less whilst increasing the top end power.)

[Tim NZ]

Air is compressable, petrol is not, thus any changes to flow of one is not linear to the other.
Any thing that is done that effects air-flow both before and after the venturi, effects fuel supply requirements.