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su carb choice

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healeyboz

healeyboz

Jedi Knight
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Ok,
I am finally to that point in the build!! /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
I have a bunch of SUs in the garage and I need to figure out which ones to use with the old 1275. Am I right in looking for 1 1/4" diameter? What else should I look for. After they are found I will probably be calling Hap for the number of the guy in SC.

Thoughts??? Part Numbers???
 
The 1275 used HS2s where the 2 indicates the number of eighths of an inch over one inch, so that makes them 1 1/4 inches.

Unless you have some stuff from WAY back, anything that is of that size is likely to be an HS2.

Here is a picture of ncbugeye's HS2s... it's a pic I have posted before but it shows what you should be looking for.

btw The 948 fitted to the Bugeye in the factory used H1s, ie 1 1/8 inches.
 

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However HS-2s work just fine on a Bugeye as well, best if you have made a few mods to the engine like head, booring, cam.

By its self, with its intake manifold which is a bit different, should give another 3 HP on a Bugeye.

On a 1275 it is my understanding that any of the 1/14 inch HS-2s are fine. Pick the best ones you have and rebuild them.
 
THanks fellas. Chris, I notice that you do not have a pgr valve on the top of your mainfold. How did you vent the timing cover? I have a manifold that does not have any of the added holes on the top of it and would like to use if it will work. I believe it is 1 1/4
 
Boz, I'm old school, so I go with the early nonsmog HS2s, the one pictured above are the smog model carbs, notice the hose running between the two carbs, the early one don't have those and also have non poppet valve butterflies. Oh and that awesome carb rebulder in SC, also has a boxes full of SU carbs, probably a hundred plus carbs, and he'll take your smog carbs in on trade if he's doing the rebuilt on your carbs, call me when you get ready.
 
I have a similar question regarding carbs.
While Waldrop is hanging around, you seem to have a lot of experience with setups. I'm building my 10CC +40 flat tops with a ported and chamber flowed 12G295 with 1.22 in and 1.06 ex valves, Kent 276 cam and LCB. I was wondering what carbs would work best. I have a lot of carbs hanging around including a set of 1 1/4 duels, a set of 1 1/2 duels on a shortened MGB manifold, and some HS6 and 1 3/4 Strombergs. I don't have, but could buy or make a single 1 3/4 manifold.

According to Vizard I should need 2 to 2.2 cfm per hp and I would expect about 75 hp (Is this completely wrong?) making about 150 to 165 cfm? That would mean according to his diagram number 7.1 a single HS4 would not be enough and a single 1 3/4 would easily be enough. But what about the duel 1 1/2 ? Since each individual cylinder only feeds from one carb, and each carb is only used half the time, I don't see an advantage of using the duel carbs. With the exception of a little flow through the equalizing tube. Would I be better off just using a single 1 3/4 ? What's your experience?

Donn.
 
""""""10CC +40 flat tops with a ported and chamber flowed 12G295 with 1.22 in and 1.06 ex valves, """""

I assume this is a race motor?

youll end up with over 11:1 cr and more with a smaller chamber.....also there will be a packaging problem with those valve sizes in that 2.580" bore.

Let us know breifly what the car will be used for ......

Boz....

I would suggest a PAIR of AUD136 or 266 carbs....a matched pair so you have a front and rear.( Fixed needle ..start w/ an AN needle wwith stock compression and cam or H6 with a bit more compression......and no spring in the throttle blades). Also a ported vacuum nipple on the front carb. the intakes are pretty much the same. Just use one with a manifold vacuum port for PCV.
 
Hap, I'm not sure whether I should be worried when you say I have the "smog" carbs. Are they a bad thing? Should I be doing something with them to make things different/better?

I am just about to start on a complete revision of the fuel system, so I am up for suggestions. I was thinkling of shipping the carbs to Joe Curto but if there is a recommended alternative in the Carolinas I'd go for it.

This weekend I'm starting this project from the back with the fuel tank. I found a place that boils them out etc quite near home.
 
Hap's a bit circumspect here. He's the source in the Carolinas for those who haven't figured it out. His shop (ACME Speed Shop) is now doing carb rebuilds. He'll be getting mine in a little while, once I get to that point.
 
The 1098 is to be used for street and occasional hill climbs. Not what you would consider a race engine. However with the Kent 276, Vizard claims that the lumpy idle is diminished due to the long stroke. No valve interference. The engine has been running with these valves, I'm just switching the casting. 11:1 cr would be fine (we can still get 98 octane leaded gas here) but I haven't cc'd the chambers yet so I really don't know.

The main query though is the difference between double 1.5s and a single 1.75. For the 1.75 I think I would have to make a manifold. The Mini manifolds are too short and possibly mount the carb too high.

Or should I just go with the Weber? But on an 1100! Isn't that a little too much?

We have strange rules here concerning modifying and changing engines. I can modify the original (or apparently original) engine as much as I want, but if I replace the original with a different type (1275), and the power increase is more than 20% (of the original 46hp = 55.2hp), I would have to pay registration tax on the car again. That is 180% of the value of the car*. And I would have to find an engine, which is very rare here now. It suddenly becomes very expensive to run a 1275. Building something that looks like the 950 (with id plates removed) up to 75hp costs nothing in taxes.

*The relevant tax department makes the evaluation of "value" and there is no appeal. The tax could easily come up to $15000.

btw I used to live in Raleigh... back in 1960.

Donn.
 
Donn, I run a very similar setup as you are building, but with a hotter cam. 92 HP at the flywheel, and pulls like a freight train up to 7400 RPM, on a set of HS2's.
I have a 948 in the race car, that has about the same HP, but at higher RPM, that also lives on HS2's.
Jeff
 
Yes for a strong 1098 street motor, I would think twin HS2s are still the best answer for performance. AS for twin HS4 on a street 1098, well they proved to be almost too much carb of the racing 15 to 1 compression ratio 1098, most of the SCCA 1098 racers now are using a enlarged butterfly that measure aprox 1 3/8" in thier racing HS2 thru bored carbs, so the stock HGS2 will deliver plenty enough fuel for your engine.

Chris, yes guilty as charged, I'm the SC carb rebuilder. We always built carbs for racing, thru bores, when the SCCA came out with the "Limited Prep" engine rules which require stock carbs, we started building stock carbs for those guys. Last year about this time I was in Dallas, Texas at the BMTA conference and met Joe Curto in person, he and I shared a few beers and he told me I should be doing SU carb rebuilding for the street, he's looking to sell more parts to rebuilders like myself and slowing down a bit on the amount of rebuilds he does, he thought adding street SU rebuilding to my shop's job could be a good thing, well he was right. We plugged into Joe system and off we went, we built about about 35 sets of carbs last year, now we're trying to get 2 sets a week out in the shop, so who knows maybe we build 75-100 sets this years, I literally have 4-5 sets of carbs for customer at any given time. Joe kids me and tells me I'm building the world's stock of HIF4s for the MGB crowd , for sure we doing a bunch of them. I put alot of effort into the cosmetic restoration of the carbs as well, and I think that has helped my carb rebuilding business. So in short, we got into the carb rebuilding business in a big way, Joe Curto is the top SU man in the country in my opinion and he has helped me greatly with good parts and technical support, I really glad I listened to Joe and made the jump into carb rebuilding. Joe has became a good friend over the last year.


Chris your carbs are fine, the later HS2 smog carbs, have the side tubes like in your picture, if you're not running smog and it looks like you not, get rid of the hose in between, and plug the brass tubes off, also these carb have the poppet valve butterflies, so in a rebuild we can get rid of those and istall solid butterflies which inproves flow.

This a complete HIF4 set up for a MGB, I don't have a picture of a HS2 set up, but this picture should give you a idea of my work.

https://www.acmespeedshop.com/_images/stewartshif.jpg
 
Those are a work of art, Hap!!! Beautiful!! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
 
Oh yeah Hap!
Those carbs look NEW. I must assume they work just as well as they look and probably better.

I suppose I definately have to listen to BE58. If he can get that kind of power from HS2s then that's the way to go.

I had used HS4s on my FP 1275 car back in the 80's
There are a couple pictures of the car here
https://www.eccentrix.com/members/donn_engler/
(and some other stuff)

Somebody at the local garage club suggested Megasquirt
https://www.megasquirt.info/
I must admit it is tempting, but probably too much electronics for an English car.
 
Donn, I built that 1098 I spoke of about 10 years ago, and it's still going strong.
<u>Reliable</u> horsepower is a <span style="color: #3333FF">good</span> thing.
Jeff
 
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