CARB SIZE

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moppedog

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I have two brand new 38 mm Gardner carbs that I was going to use on the NRE 750 engine that I will (probably) never receive from the current "manufacturer" - Mr Chris Bushell at Nourish Engineering.
I am therefore building a Norton 750 short stroke with a Fullauto 850 head with inlet ports of 34,5 mm diameter.
Now, I am of course wondering if the 38 mm Gardners would work ? As well as if I was to fit a set of 35 mm units.
Any and all experience and advice is welcomed!!
 
Hi Ulf,

I have run 38mm carbs on 750 Triumph twins before and they ran great, I also ran them ona 500cc NRE twin, which also ran great.

If you haven't done so already, you need to pick your way through ALL of this thread: head-flow-testing-t8640.html

If I recall correctly, somewhere in that thread Comnoz explains that a carb bigger than the port is actually a good thing.

My Commando has a RH10 head with 30mm ports and I'm running 35mm carbs which run perfectly. So I would imagine 38mm carbs will be fine on your 34.5mm ports.

However, my concern (again) would be the Gardeners. Basically all of the fuel delivery on those is controlled by the needle if I recall correctly. My worry would be that the lack of inedendant fine tuning ability of not having differnet jet ranges may make it difficult to tune. Basically I would worry that the lower charge velocity through the big carb, at less than WOT, might struggle to draw fuel correctly at lower revs, making it difficult to set up for smooth acceleration / cause flat spots / stuttering / etc.

But I guess you'll not know until you try, so if you already have the carbs, its gotta be worth a try hasn't it?
 
Ulf,

I am still concerned that you are making this bike harder to ride than it needs to be. If you really will be wobbling through the corners, as you have suggested you might, this motor is unlikely to come smoothly on to the throttle on the corner exit. Fast Eddie's comments I think agree with this.

Most wisdom on here would question the large port size and the carb size. I have said that I use 32mm ports and 34mm carbs with a long inlet on my 750 short stroke. This seems well balanced, though I would like to try a pair of 36mm Mikuni I have here. It needs a new manifold to do that since theyare physically large to get close together.

However I have run a 34mm port with 36mm carbs on an 850 running a factory short stroke fully hemispherical head and big valves, they worked very well on the 850 (contrary to the popular wisdom), delivering really good torque and horsepower. I tried this motor on 34mm and wasn't happy with that and went back to 36mm, which on that motor seemed to have a good balance, like my current setup.

I am pleased you got your 38mm Gardeners back. They would be great to experiment with for hours on a dyno, after you have a successfully performing machine on a more conservative set up. And we still don't know what restrictions the race series you would like to compete in might place on carb choice. I use Mikuni for a reason.

Steve
 
SteveA said:
Ulf,

I am still concerned that you are making this bike harder to ride than it needs to be. If you really will be wobbling through the corners, as you have suggested you might, this motor is unlikely to come smoothly on to the throttle on the corner exit. Fast Eddie's comments I think agree with this.

Most wisdom on here would question the large port size and the carb size. I have said that I use 32mm ports and 34mm carbs with a long inlet on my 750 short stroke. This seems well balanced, though I would like to try a pair of 36mm Mikuni I have here. It needs a new manifold to do that since theyare physically large to get close together.

However I have run a 34mm port with 36mm carbs on an 850 running a factory short stroke fully hemispherical head and big valves, they worked very well on the 850 (contrary to the popular wisdom), delivering really good torque and horsepower. I tried this motor on 34mm and wasn't happy with that and went back to 36mm, which on that motor seemed to have a good balance, like my current setup.

I am pleased you got your 38mm Gardeners back. They would be great to experiment with for hours on a dyno, after you have a successfully performing machine on a more conservative set up. And we still don't know what restrictions the race series you would like to compete in might place on carb choice. I use Mikuni for a reason.

Steve
Click to expand...

Yes Steve, I agree with / share your concern. I do believe 38mm carbs will work though, and I'm even prepared to believe that Gardeners might be OK too. But only after serious dialling in time / effort.

Your suggestion to set it up conservatively first, and experiment later, is 100% bang on!

And as Steve say's Ulf, can you please tell us what the carb restrictions are in your intended class?

'Off the shelf' 35mm Keihin FCR flat slides on a purpose designed manifold are available from cNw and if allowed, these would be just perfect for your motor IMHO.
 
Steve/Nigel, with my race motor , we matched the standard manifolds to the ports on the RH10 head that has standard sized In and Ex Vv's , I use 32 mm Premiers with 60mm velocity stacks, open bellmouth , no gauze (screen) of air filter.
I run 300 (sometimes 310) main jets and find this is more than enough for this motor.
It has one of my 2 into 1 exhaust systems. Dyno shows the FA ratio is very good , probably leans out off the throttle a bit, but not drastically.
Admittedly I havent tried a bigger set of carbs. I guess the bigger Vv size you guys are talking about would need the bigger carbs.
Regards Mike
 
moppedog said:
Gents,
Just checked the rules. Flat slide carbs are allowed.
Click to expand...

In that cae Ulf, given the no compromise approach you are clearly taking on this build, its a 'no brainer'...
 
moppedog said:
Gents,
Just checked the rules. Flat slide carbs are allowed.
Click to expand...

Any Flat slides? or period flat slides (which means Gardner for most classics!)
 
Brooking 850 said:
Steve/Nigel, with my race motor , we matched the standard manifolds to the ports on the RH10 head that has standard sized In and Ex Vv's , I use 32 mm Premiers with 60mm velocity stacks, open bellmouth , no gauze (screen) of air filter.
I run 300 (sometimes 310) main jets and find this is more than enough for this motor.
It has one of my 2 into 1 exhaust systems. Dyno shows the FA ratio is very good , probably leans out off the throttle a bit, but not drastically.
Admittedly I havent tried a bigger set of carbs. I guess the bigger Vv size you guys are talking about would need the bigger carbs.
Regards Mike
Click to expand...

I run the 34s with standard valves sizes. You don't seem to be having a problem getting air in through your 32s, so no need to change. Of course with a short stroke I may be revving the motor harder, but from what you have said in the past, not much.

Many of the 32mm Amals in the period were bored to 33mm, including the works ones. 33 or 34mm chokes are unlikely to have any negative effects, and are more likely to have positive effects. You won't know unless you try.

In period on big valves and 34mm ports I ran at different times 36mm MkI Amals and 34mm MkIIs, these were then bored to 36mm, which I judged the best solution. I suspect the 34mm Mikunis are shifting pretty much the same amount of air, aided in velocity by the 32mm port and Fullauto port shape. (all with open velocity stacks/bellmouths)
 
I use an 850 head with 30mm ports except for the first 25mm which tapers up to match the 34mm manifolds which carry two 34mm Mk2 Amals. I run methanol which means I can get away with a lot more than most of you who run petrol. I suggest that once you enlarge the main part of the port, there is no easy way back to regain lost torque. So if your motor performs well, be happy and don't stuff it. There is a thing about using big carbs. If the motor loses vacuum when running on the mains, the motor can lean off. Normally the taper on the needle helps accommodate this, but a very big carb might be more difficult to get right.
 
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