Carburettor choices

Here's Cash's fuel tap upgrade photos:

I use similar taps, with an aluminium tube pressed into one, to give a reserve. Very good flow and no leaks, even after many years.

I don’t think gauze strainers on the taps are a good idea. I use inline filters.

Been using them for about 30 years on all my race bikes

jseng1 said:

Carb width is the problem. Dell Ortos must be splayed outward at an angle which hurts the flow. Its hard to get them to fit. Same with Mikunis. The only available carbs that fit side by side are Amals. Keihins and PWKs.

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Hi Jim, how does you CRS carb kit mate with an RH10 head? Widening the inlet port is not desirable. Is there another manifold available? The original manifolds for Mk2/3 provided a taper from 32 to 30mm diameter. Your port sleeves mimic the RH10 head's inlet also.

- Knut

jseng1 said:

Carb width is the problem. Dell Ortos must be splayed outward at an angle which hurts the flow. Its hard to get them to fit. Same with Mikunis. The only available carbs that fit side by side are Amals. Keihins and PWKs.

I modified the frame in the racebike below to accept Dell Ortos by removing the gusset and changing the oil tank etc so I could extend the manifolds. The extra length allowed them to be splayed outward from each other. A tight fit.

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That is a very nice bike, however the rear sprocket looks too large, and I usually ride on tight circuits. It is difficult to know when you are geared too low with a Norton motor. It can feel and sound as though it is pulling as hard as it can, then when you raise the gearing the bike sometimes goes faster. I use very high gearing with close ratios. With the Manx cluster, first gear was always a problem during starts. I should have fitted the Commando first gear and lived with the gap.

I use 34mm MK2 Amals with the ports tapered from 34mm to 30mm in the first 12mm. I do not use Amal needles - only Mikuni. The needle jets and needles are critical. The slightest bit too rich means slower. If the motor misses, I stop and raise the needles, one notch.

acotrel said:

It would make sense that the pump rate would be adustable, and if is done by turning a screw, that is much better than opening a carb and changing needles and needle jets, The trouble might be in getting two carbs to be identical in adjustment.
If you are seeking performance with petrol, the margin for error is very small.

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Al, you do know the term "pumper" carb, merely denotes that it is designed with an accelerator pump, (like every simple car carb for 75 years).
An accelerator pump only covers the lean bog experienced when opening the throttle quickly.
It does not magically remove the need for needles & needle jets for tuning.


There must be a youtube video that would explain it, rather than reason it through.

I know how a pumper carb works. The problem with many carburetors is the amount of control they offer. Most old cars used to have a pump in the carburetor - that does not mean they were good. My old Ford Mercury used to get 15 miles to a British gallon. Most of the improved performance of modern motorcycles probably comes from computerised engine management, and not 4 valves per cylinder.
When I run my 850 on methanol, changes to jetting have half of the effect compared with using petrol. And very small errors in jetting still have a large effect on performance. What hope do you think you have when you use petrol in a normal motorcycle carburetor ?
I have used Amal petrol needles in my 34mm MK2 Amal carbs with methanol. I replaced them with 6D Mikuni needles - the leanest - and gained a significant improvement. When racing, some guys whack the throttle open - I never do that. If the mixture richens too quickly, you go slower. I feed throttle on in a controlled manner. Most Amal methanol needles have absurd tapers. I would not trust their petrol needles.

"When racing, some guys whack the throttle open - I never do that. If the mixture richens too quickly, you go slower. I feed throttle on in a controlled manner."

The mixture goes weak/lean when the throttle is snapped open, vacuum suddenly goes low, fuel isn't sucked up through the jets.
Please do try to follow along.
Surely there is a YouTube video

mdt-son said:

Hi Jim, how does you CRS carb kit mate with an RH10 head? Widening the inlet port is not desirable. Is there another manifold available? The original manifolds for Mk2/3 provided a taper from 32 to 30mm diameter. Your port sleeves mimic the RH10 head's inlet also.

- Knut

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I can provide manifolds with a short tapered sleeve installed to reduce the ID to 30mm to match the head. I just added that option to my shopping cart.

"choose motor and head port size"

"750 head 30mm | 850 or larger 30mm head | 750 32mm head | 850 or larger 32mm head"

I prefer the taper from 34mm to 30mm to be in the beginning of the port, rather than in the manifold. And I do not know the reason. The shape of the inside of the carburetor might matter ?

Schwany said:

Are pumpers still illegal, or do they put an old Norton in another classification if used?

I had a heck of a time making a big Dellorto pumper work on a ATK 604. The Rotax was actually quicker with a 36mm TM bored out to 39mm by Harry Lillie. Much quicker. Yeah I know way off topic.

The 35mm FCR pumpers work well on a street Norton 750 with some compression and a JS2 cam. I know people like to disable the pump, but I can't find anything negative about leaving it active.

Have you disabled the accelerator pump on the FCRs on your hot rod build?

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I just noticed that I never answered the last part of your post. Sorry about that. I did not disable the pumps on the FCR's. Didn't see any reason to. I've used them on other bikes and had no problems with the pumps. They certainly made the bikes easier to start.

Speaking of Rotax singles, I ran a pair of 39 mm FCRs on my Ron Wood road race bike, an SJ600 updated to the twin cam water cooled top end. Worked great. Slightly off topic, but here's a picture of the bike.



Ken

lcrken said:

I just noticed that I never answered the last part of your post. Sorry about that. I did not disable the pumps on the FCR's. Didn't see any reason to. I've used them on other bikes and had no problems with the pumps. They certainly made the bikes easier to start.

Speaking of Rotax singles, I ran a pair of 39 mm FCRs on my Ron Wood road race bike, an SJ600 updated to the twin cam water cooled top end. Worked great. Slightly off topic, but here's a picture of the bike.

View attachment 116203

Ken

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I forgot all about what I did yesterday, so thanks

That Rotax I would like to have mounted on my living room wall. Then again maybe sitting in the garage.

lcrken said:

I just noticed that I never answered the last part of your post. Sorry about that. I did not disable the pumps on the FCR's. Didn't see any reason to. I've used them on other bikes and had no problems with the pumps. They certainly made the bikes easier to start.

Speaking of Rotax singles, I ran a pair of 39 mm FCRs on my Ron Wood road race bike, an SJ600 updated to the twin cam water cooled top end. Worked great. Slightly off topic, but here's a picture of the bike.

View attachment 116203

Ken

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You must have some very unusual race classes. In Australia, the guys who write the class rules, usually start by deciding what they DO NOT want racing. However the guys who run the meetings always want more competitors. In the 1970s historic stuff, there is a class which has TZ Yamaha parts prohibited. It must be rocket science to allow air-cooled two-strokes to run methanol.
I would like to race that bike in the photo against a good Manx Norton, in a four-stroke singles and twins class.