su needles for 1-1/2 SU's on 1275

M

maxiblu

Active member
I'm looking for some info for needles for 1-1/2 SU's on my 1275. I know every engine is different but I would like to get close. If anyone is runniong a 1275 with full race cam, roller rockers, 11 to 1 compression or anything close I would like to here what needle numbers you use. I do know #5's are way too rich.

Thanks for any help.

Keith
 
Don't know what you need but i have a fair selection on needles ranging from lean to rich around 5s. i bought them i changed engines and used them to get mine right. Even have charts on differences. if you need something contact me.
 
Keith,

1 1/2" SUs may be a bit too much carb for a 1275. They don't provide the velocity that the 1 1/4s do. They work well in an MGB though.

RJS
 
I think 1-1/2" SU should work well on a 1275. I ran 1-1/2" SUs on my 1098 for a while and had better performance on the track than I did with the original 1-1/4" carbs. Driving around in the paddock area was not easy but it did work. When the rules changed allowing opening the throttle plates on the 1-1/4" carbs in GP, I eventually switched to that setup. The 1275 should draw enough air to gain performance with the 1-1/2" carbs over the 1-1/4" carbs.

As with any carburetor, you do need to get the A/F mixture correct. The #5 needle is rich but I actually used that at one time. However, that was before I started using a dyno for tuning. I could go back an take a look at my dyno records to see what needles I ran with the 1-1/2" carbs but I am not sure that information would be useful for a 1275.
 
The engine that is described, either as one that exists or as one to be built, is / (would be) a strange collection of parameters.
For a 1275 cc engine, I assume that a "full race" cam would have a duration near 300 degrees at 0.020" of lifter rise. If the roller rockers are 1.5:1, then the effective duration is considerably greater, probably near 315 degrees.
For a compression ratio near 11:1, the cam has too much duration, resulting in low compression pressure and lower than optimal horsepower.
Twin SU carbs with a diameter of 1.5" might be too large, depending on the intended rpm range.

Every engine is somewhat different, but many years ago I had a 1098 race engine with 12.5:1 compression ratio, cam duration of 295 deg, stock rocker arm ratio of 1.25:1, SU 1.25" twin carbs prepared by a recognizable speed shop, and used #7 or #MME needles. Top speed was 115 mph.

Charlie T.
 
I think the 1-1/2s are to big. I have a street Turner 950S, now 1275S, ratrod kinda toy I built with 260 Elgin cam, 10:1cr, HL rollers, and because I had them, a pair of 1-1/2" SUs originally on an MGA. 3s worked but the best needles I have found are I think AAA (can check but they are in car and can't find notes). Springs are red. Engine runs great, makes good power, is still pulling very hard when I bail out at 7000rpm, but only with up to about 85% throttle opening. In mid range it pulls hard at full throttle but no harder than at 3/4 throttle. Accelerating at high rpms it falls on it's face at full throttle but again pulls very, very strong at 85%. Sorry I have not had it on the dyno, nor had an 02 sensor in it yet so can't say if it is rich or lean at full throttle.

Be sure to post if you figure it out as I am not to proud to copy !

Edit/add: Duration measured at .050 with the HL rockers installed
 
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