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[Zelda]

Hi all,

Im looking at upgrading the standard carb to the Amal Mk1 on my Bullet-65. I understand that there are performance benefits to be gained from this, as well as improved cold starting and a better sound (?). What else can I expect, and is the difference noticeable?

Would you reccomend any other upgrades for that model? Perhaps an exhaust which would benefit the carb?

Cheers guys

[simon]

An old Glaswegian engineer told me aeons ago "If it works leave it alone!" That said if you are after better breathing then a larger choke carb does make a difference. I also personally prefer the look of an Amal but would go for the 376 or 389 rather than the concentric as they are more in keeping with the original era of the design of the classic iron barrelled version. Remember though that to take real advantage of a larger carb you will need to address the exhaust flow with a free flowing pipe and muffler and a carefully ported head also the valve timing via improved cams. Then of course there is increased compression with a higher comp piston and the issue of dealing with the increased horse power. Oh the humanity!

[Zelda]

Thanks for that simon. So really what you are saying is if im going to do anything, i may as well do it all? - carb, cams, exhaust... whats porting the head?
I suppose just the carb and silencer wont make it worth it? May have to re-think this one.

[Mark M]

Zelda, you don't need to do it all at once, you can progress through the stages, probably a good idea if outright performance isn't your aim as you can judge the effect as you go. Others may have tweaks on this list but I'd say 1: exhaust, 2:carb, 3: ported head and valve job, 4: adjustable timing pinion, 5: high comp piston, 6: performance cams 7: performance crank. Obviously there are additional parts to add at the various stages according to taste!
REgards, Mark

[Anders F. R.]

Simon wrote: "That said if you are after better breathing then a larger choke carb does make a difference." -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

There seems to be a common belief that a bigger carburettor automatically gives better performance. It MAY be true in some cases, but this is NOT a simple truth without any modofications or exceptions. What you may achieve is MAYBE some more power at the top end (high revs), IF the rest of the engine can go that fast. The price for this will however most likely be loss of low end torque. This may be OK for a racing machine where maximum power is important, but it is not a good thing for a road bike. For a road bike a broad power band is more important than maximum peak power. -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

Also, there are better ways to improve breathing than just fitting a larger carburettor. I have earlier this summer experienced that removing the K&N-type filter from the Amal Concentric Mk.1 626-300 and replacing it with a long bellmouth really made a difference, increasing both low rev torque and peak power. Why? Because even if the filter itself has low flow resistance, the shape of the inlet tract is FAR from good, with square corners etc. reducing the effective inlet area by 30 % or so (according to a friend of mine who is an hvac engineer and knows more than a fair share about fluid dynamics). -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

So before "messing" with the carburettor size, I would make shure that the one you already have, including air filter, manifold, cylinder head and exhaust in fact are as good as possible. Maybe not as easy as changing the carb, but a better way if you want to get the best out of your engine and maybe learn something too! -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

The book "4 stroke performance tuning" by A. Graham Bell (not the man with the telephone) is recommended reading for those who want to understand their engines and not "tune" them without knowing what they really are doing. It can be downloaded as a pdf file. Google it like I did. -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

Regards, Anders F. R.

[simon]

Porting the head is a process where you contour the inlet and exhaust ports on the cylinder head to creat the least interference and the best gas flow. I don't know if all the Indian bullets are similar but mine had an incredibly under machined cylinder head in that the casting had been given the bare minimum of fettling and machining to get it running. This meant that there was a 1" or maybe 15/16" hole drilled about half way into the port after which it was rough casting. The secret (I'm told and this might start another heated debate) is not to simply open out the inlet port to e maximum possible size but to match the opening of the port to the carb you intend to fit. Clamping it together with a wipe of engineer's blue will give you the shape to grind it to and then to make a smooth un impeded passage to the valve but in an hourglass shape so the the gas that leaves the carb is accelerated by the constriction. The exhaust needs to be a clear path with minimal interference. Anders is certainly right when he suggests that lots can be done to improve the breathing without changing the carb but I do think that if you have the 23mm mikuni on a 500 or even a 350 then an upgrade could give you a bit more legs. If its a 500 then I'd up it to a 28mm which is much the same as the 1 1/8" 389 I've put on my 350. It pulls like a schoolboy and is capable of higher speeds than I think is good for it although I have raised the compression and I did make a very free flow exhaust system. It is fun getting these machines to go better and you will notice a difference so go for it I say.

[Durham Tim]

Last year I removed the standard long "torpedo" silencer and air box from my Bullet 350 and replaced them with a freer-flowing exhaust and down-pipe and fitted the conical filter to the standard Indian mikcarb.

A few weeks ago I went a step further and replaced the mikcarb with our hosts Amal concentric carb. I must admit, the biggest performance gain by far, came from the freer-flowing exhaust and conical filter. The further improvement in performance from the Amal carb is at best, negligable. (In my humble opinion!).

To be honest, if anything, I've found the bike HARDER to start with the Amal, even after spending a few seconds "tickling" the carb to ritchen it up. The Amal carb does LOOK better, but to be honest I'm struggling to recommend the additional cost....

As many others on this site often say. "If it aint bust....."