Advice on fitting a non-standard carb

[Glenn RE5]

I have started a new thread on fitting a non-standard carb to a RE5 ..Not least as that is my plan with one of my bikes ....Any advice on this would help me and others am sure ...Not just on the type of carb but if the port valve would still play its part with a single butterfly/slide type .....Always think it would be nice to get rid of all these cables inc fixing the oil pump on 3/4 open and getting rid of that cable too ...not that it is a problem ..As i run mine on premix there is plenty of oil ..maybe just fix at 1/2 open ....most owners maybe now dont use the bikes as Suzuki intended and using a bit more oil is not a problem ....Any thoughts on this

[pmcburney]

Any thoughts?


How about plenty!
I have an alternate carb thread here on the forum somewhere, but I can condense a few points for you...

I have experimented with a 40mm Keihin FCR and a 40mm Mikuni BST40 (from a DR650) mounted on a custom adapter plate on the stock manifold.

I have modified the stock throttle cable by replacing the stock fitting on the end with a larger one to work with the alternate carbs - this means I'll need a stock throttle cable if I want to go back to the stock carb, but they're not that hard to find.

I experimented with the port valve and decided to lock it full open - it really strangles the engine if left in the stock opening configuration as the alternate carbs are (obviously) a single rather than dual throat and meter the air and fuel adequately at low throttle openings.
I have not noticed any contamination of the mixture by exhaust gases returning up the main inlet port by leaving the port valve full open.

I plumbed the stock oil fitting/check valve directly into the fuel line by using a suitable ninety-degree barb-to-barb brass fitting.
The square section of the ninety-degree part of the fitting is large enough to drill and tap a hole to screw the stock oil line bolt into.
Apart from some residual leakage (must use sealant tape), the same amount of oil that would be going into the fuel bowl goes into the fuel feed to the alternate carb - I still see smoke on startup, so it must be working fine!

The BST40 carb (being a CV instead of mechanical slide) seems to be working the best.
I haven't got it jetted completely right just yet, but I've managed to get idle to mid-range quite good and there's no hesitation and the rough vibration section is much reduced.
Fuel economy seems similar if not a little better as the CV carb doesn't have (or need) an accelerator pump.
I intend fitting a Dynojet kit to it in the near future to enable more fuel to be drawn in at lower revs - the stock needle that comes with the BST40 has a very conservative taper and restricts fuel flow - I have raised it as high (and higher) as it goes and I feel it still needs more fuel.

I think it's possible that a 40mm carb is a little large - I could never get the FCR to work properly, although they are a bit finicky anyway.

I'm also considering changing out the pancake-style air cleaner I'm using for something larger and with more surface area - more like the sort of air cleaner that used to be available for SU carbs.

All in all, it's been a cool journey learning about what works and what doesn't.
I reckon there's some untapped potential in these engines with a change in carb too - it's only seat-of-the-pants stuff at the moment, but I am certain my bike pulls harder with the alternate carb than with the stock abomination.

PM me if you need anything more specific.

I have some pics of both carbs mounted on the bike, somewhere - I'll post them up when I find them.

[Glenn RE5]

Thanks for that ....I was thinkiing if the port valve would be needed ...Just a case like you state of finding the right carb one that you can buy a whole range of jets for ....Would you say the DR650 carb would work with more work or better to look else where ....Ive no cables for this bike so for now will just use the one cable to the new carb and fix the port valve and oil pump fully open .....As a RE5 needs a lot of fuel had the mad idea of fitting twin carbs off a 250/350 or twin Amals off a british bike ....As its a retro bike am building the madder the better .....

[pmcburney]

I reckon the DR650 carb is a pretty good compromise and works as well as anything else, just that it might be slightly too large.


SU carb conversions (for example) tend to line up best with the stock carb in a 1.5" rather than a 1.75" size, so I reckon a 40mm carb is a bit of overkill.
I do think that a CV carb is a better prospect than a slide carb as the engine 'dictates' the air flow (and therefore mixture) rather than your right hand...

I need to get my bike on a dyno and do proper stoichiometric measurements to really determine if it's absolutely right or not, but with the small amount of fiddling I've done so far it's better than stock so I reckon it's a winner!

Any day you can give an RE5 a gutful in the lower gears and it doesn't throw you over the handlebars is a good day

[Glenn RE5]

Thanks for that ...Will give one a try when i get the bike to that stage ...All still in bits but have a nice polished proper unit all ready rebuilt to put in ....

[wayne]

Jan 23, 2017 18:24:27 GMT -5 pmcburney said:

Any day you can give an RE5 a gutful in the lower gears and it doesn't throw you over the handlebars is a good day

[goandy]

I put an HIF38 SU carby on mine. Port valve still being used, oil line into a spare drilled and tapped port on the side.

Runs it very well with a much bumpier idle (which I quite like). Downside is that it can be a bit jerky on overrun in lower revs.

My buddy's bike has a 44 which also works well. I think it pulls a bit better in higher revs which makes sense.

[pmcburney]

Jan 25, 2017 18:40:02 GMT -5 goandy said:

I put an HIF38 SU carby on mine. Port valve still being used, oil line into a spare drilled and tapped port on the side.

Runs it very well with a much bumpier idle (which I quite like). Downside is that it can be a bit jerky on overrun in lower revs.

My buddy's bike has a 44 which also works well. I think it pulls a bit better in higher revs which makes sense.

I've noticed that the idle with both the FCR and BST cabs is quite 'lope-y' (like a tuned, peripheral-port Mazda rotary) on my bike compared to the stock carb.
In general I've noticed the richer the idle/primary circuit, the lope-ier the idle...


The stock carb on my bike had a perfectly smooth idle and I can only put this down to the primary venturi being very small and therefore providing a very fast air stream.

The alternate carbs likely don't meter the air/fuel mixture quite as well, nor allow for the same fast, smooth air stream through their idle/primary circuits, however there's a much stronger (more instant) throttle response compared to stock just off idle, i.e. there's no throttle lag at all with the alternate carbs.

There is some jerkiness on the overrun with the alternate carbs as well, although it's much less pronounced with the BST carb.
I'm not too sure exactly what causes it, but I'd say it has something to do with the idle and primary circuit settings - I reckon it can be tuned out.