SU carb - too small?

[fiatfan]

I have been offered a SU carb that is restored and set up for a Triumph 650, it´s a 32mm which would make it a 1 1/4" if my math is correct? It has never been used. I have a standard 850 -73 engine. The most common SU used on Nortons seems to be the HIF6 which is 1 3/4" or 44.5mm. That´s a big difference. Is there knowledge out there about this, could it work?

 

[concours]

I have been offered a SU carb that is restored and set up for a Triumph 650, it´s a 32mm which would make it a 1 1/4" if my math is correct? It has never been used. I have a standard 850 -73 engine. The most common SU used on Nortons seems to be the HIF6 which is 1 3/4" or 44.5mm. That´s a big difference. Is there knowledge out there about this, could it work?

Of course it will "work".

Reference the endless spirited debate about single Mikunis.
Most put on 32/34's.
Very rare to see a 44VM mounted.

If you parade ride the bike, rather than riding it hard (as intended), the small single will work.
If you want it to run strong, as designed, keep twins on there.

 

[baz]

I have been offered a SU carb that is restored and set up for a Triumph 650, it´s a 32mm which would make it a 1 1/4" if my math is correct? It has never been used. I have a standard 850 -73 engine. The most common SU used on Nortons seems to be the HIF6 which is 1 3/4" or 44.5mm. That´s a big difference. Is there knowledge out there about this, could it work?

Don't forget a CV carb doesn't flow it's full choke size
Hence the large diameter

 

[kommando]

The SU is a CV carb, so the slides opens based on pressure difference not the hand on the throttle. I would be looking for a an example of an engine that during its production went from direct connection to slide card to CV carb and note the change in size.

 

[baz]

Of course it will "work".

Reference the endless spirited debate about single Mikunis.
Most put on 32/34's.
Very rare to see a 44VM mounted.

If you parade ride the bike, rather than riding it hard (as intended), the small single will work.
If you want it to run strong, as designed, keep twins on there.

Careful you are on dodgy ground there
Nobody needs another single verses twin carb debate

 

[concours]

Careful you are on dodgy ground there
Nobody needs another single verses twin carb debate

I included a general, abbreviated bibliography....

I especially love the mystique, the folklore, the black magic oft repeated about CV carbs. Up to and including "altitude compensating"

 

[baz]

I included a general, abbreviated bibliography....

I especially love the mystique, the folklore, the black magic oft repeated about CV carbs. Up to and including "altitude compensating"

Yeah but what damping oil do you recommend?

 

[dobba99]

I have been offered a SU carb that is restored and set up for a Triumph 650, it´s a 32mm which would make it a 1 1/4" if my math is correct? It has never been used. I have a standard 850 -73 engine. The most common SU used on Nortons seems to be the HIF6 which is 1 3/4" or 44.5mm. That´s a big difference. Is there knowledge out there about this, could it work?

Does it have the manifold to fasten to the engine? If not, you are in make your own manifold territory, The Phoenix modified SU's had the dashpot height shortened to fit under the frame tube.

 

[dobba99]

Yeah but what damping oil do you recommend?

20w50 engine oil as per the Phoenix instructions

 

[concours]

Yeah but what damping oil do you recommend?

Maiden Oil.

 

[DennisMo]

SU originally specified a 20W oil

 

[Fast Eddie]

I have zero experience with SUs on a Commando, but that sounds too small to me.
But whatever the size, the lack of space to fabricate a decent manifold means that’ll be the bottleneck anyway I reckon.
If you cut the gusset plate away, and juggle things around in the battery department, you could create space for a decent manifold. But if I was going to that much trouble I’d def want a bigger SU.

 

[Matchless]

I fitted a Phoenix kit to my 850 years ago & it ran very well, providing I didn't want to go over 85/90 mph, which I did! It was however extremely economical. The manifold is the problem, as it is designed to fit in the limited space, but if you're going to use a 1 1/4" size SU I think it will reduce the performance quite a lot.

 

[concours]

I fitted a Phoenix kit to my 850 years ago & it ran very well, providing I didn't want to go over 85/90 mph, which I did! It was however extremely economical. The manifold is the problem, as it is designed to fit in the limited space, but if you're going to use a 1 1/4" size SU I think it will reduce the performance quite a lot.

I rode cross country (4000 miles) along side a (1978) Harley Sportster. That bike used consistently 25% less fuel than my XS1100.
I was very impressed.

 

[trappnman]

Never ran a SU on a Norton, but have run one on my 73 Ironhead for almost 4 decades. I did replace the slide and bell a few years back. Im running a SU Rivera Eliminator carb- and the instructions state to NEVER put oil in the dampner. Beyond the 1st couple of years when I was told I "had" to put oil in the dampner, I haven't use a drop since. Only SU carb Im familiar with, so others might be different.

 

[dobba99]

I fitted a Phoenix kit to my 850 years ago & it ran very well, providing I didn't want to go over 85/90 mph, which I did! It was however extremely economical. The manifold is the problem, as it is designed to fit in the limited space, but if you're going to use a 1 1/4" size SU I think it will reduce the performance quite a lot.

I fitted the phoenix SU kit to my first Mk3 back in the early eighties and when i got back into bikes and commandos in 2014 i wanted an SU on Mitzi.
She will still do 100MPH plus, but when ridden like a granny i've acheived 90mpg, Usually its in the region of 70 mpg.
Granted the manifold isn't the best (Phoenix had to squeeze the manifold into the available space so it isn't optimum) but it is worth bearing in mind if production had carried on into 1976 and beyond the bikes would have been fitted with SU's. so maybe the frame and manifold would have been modified to a better shape. Bernard Hooper disliked the Amals

 

[Lineslinger]

I used ATF in my Austin Healey 3K. Twin SU's. Really enhanced the response. Thus my experience with SU's on motorcycles.

Using that size SU equates to using a straw for a snorkel

 

[Matchless]

I fitted the phoenix SU kit to my first Mk3 back in the early eighties and when i got back into bikes and commandos in 2014 i wanted an SU on Mitzi.
She will still do 100MPH plus, but when ridden like a granny i've acheived 90mpg, Usually its in the region of 70 mpg.
Granted the manifold isn't the best (Phoenix had to squeeze the manifold into the available space so it isn't optimum) but it is worth bearing in mind if production had carried on into 1976 and beyond the bikes would have been fitted with SU's. so maybe the frame and manifold would have been modified to a better shape. Bernard Hooper disliked the Amals

It might well do 100 or a bit more, but wouldn't get there anywhere near as quick as twin Amals or Keihins, at least not on my bike.
I do think that with a little more space for a decent manifold it would perform as well as twin carbs. I seem to remember someone on this site running a 40 or 42mm Mikuni, with the frame gusset plate chopped to make room.

 

[dobba99]

It might well do 100 or a bit more, but wouldn't get there anywhere near as quick as twin Amals or Keihins, at least not on my bike.
I do think that with a little more space for a decent manifold it would perform as well as twin carbs. I seem to remember someone on this site running a 40 or 42mm Mikuni, with the frame gusset plate chopped to make room.

In this day and age it is a bit of a moot point, getting quickly to 100mph will attract the attention of Plod and or a speeding fine. Its one of those things you do once, then revert to riding normally. Its a shame they designed the frame the way they did, it does hinder space for the carb/s

 

[Matchless]

In this day and age it is a bit of a moot point, getting quickly to 100mph will attract the attention of Plod and or a speeding fine. Its one of those things you do once, then revert to riding normally. Its a shame they designed the frame the way they did, it does hinder space for the carb/s

That depends on where you live