Weber for a 1275 cc engine?

[twas_brillig]

I've got a 40 mm DCOE Weber on our 948 cc engine, and have a 40 DCOE and manifold that I'd like to use on the second Bug-Eye, but with a 1275 cc engine.

THis is a carb that I bought used in a parking lot, so I expect that I'll have to chase down choke and jets and such-like.

The Haynes Weber manual talks about the 948 and and 1098 and the Mark IV Sprite, but I'm concerned that that is the 1500 Triumph.

Can anyone provide advice as to the choke/venture/jets for the 1275? And we're at about 3000 ft elevation, but I figure that's a bit minor.

Thanks!

 

[SaxMan]

Mark IV Sprite is a 1275 motor. A Mark IV Midget has the 1500.

Sprites were always one mark ahead of the Midget.

Mark II Sprite = Mark I Midget

Mark III Sprite = Mark II Midget

Mark IV Sprite = Mark III Midget

Hope this helps!

 

[bugedd]

I did the same thing, picked up a used DCOE40 off ebay, it was off a race 1275 so I knew it was way too fat for my stock 1275. I contacted the folks at Redline in Torrance, CA, and they set me up with everything I needed....car ran great!

 

[matmire]

I contacted the folks at Redline in Torrance, CA, and they set me up with everything I needed

Interesting, did you take it to them for tuning, or did they just recommend and supply the components based on your car setup? I ask because I plan to tune, or get tuned, my DCOE 45 at some point soon.

 

[Tybalt]

These siamesed inlet port engines complicate the calculation of venturi and jets to select for the DCOE carbs. As a general rule, the Webers that act as multiple single barrel carbs sharing float bowls such as the DCOE, IDA and such are tuned with the assumption that each cylinder has its own inlet port and that you have one venturi per cylinder. Many of the normal calculations get turned on their heads once you deviate from that model such as on the BMC A and B series engines. Here's a little something that I ran across that might help since it deals specifically with BMC A series engines:

www.ahsdc.org

One item I noted was regarding emulsion tubes. Initially the callout is for an F2 emulsion with more to come later in the write up. If your intent is for the car to be streetable, you should probably start off with an F7 or F8 as discussed in section "5" in the main jet system. This would also be in keeping with the John Passini Weber "bible."

If I were setting this up, my best guess at initial calibrated components for street use would be as follows for a 40 DCOE on a 1275 A series engine:

Main Venturi - 32
Main Jet - 140 or 145
Emulsion Tube - F7
Air Correction Jet - 185
Idle Jet - 45f9
Pump Jet - 50
Auxiliary Venturi - 4.0

 

[matmire]

That's a great resource! Any ideas on how you would have to adjust for things like headers?

 

[Tybalt]

That's a great resource! Any ideas on how you would have to adjust for things like headers?

Maybe go a bit richer on the idle jet and main jet but it really comes down to how much extra intake charge can be used if there is sufficient improvement in exhaust extraction.

Before getting too involved in trying to figure out what to do, two things I would recommend that you consider getting is the reprint of the John Passini books covering Weber carbs that are combined in one volume and/or the Des Hammill book on Weber DCOE and Dellorto DHLA carbs. There are good explanations regarding the operational theory behind these units. The Passini book covers many Weber models from post WW II and is heavy on theory and the specifics of each system while covering the various models of carbs in that section. Lots of good information but a bit of a slog to go through. One of the complaints I saw in the reviews about the latest printing is that the book has been shrunk and as a result the font is rather small. I have the earlier "green" release and that is not a problem. I'm not trying to push you to Amazon, but they are one of the easier ones to do a quick look and see in providing information on books. You can always go local or someplace like Powells or a chain like Barnes & Noble as well.

https://www.amazon.com/Weber-Carburetors-Tuning-Tips-Techniques/dp/1855207591

The Des Hammill book is newer and concentrates on the DCOE along with some information on other DCO_variants and the DHLA carbs. It has much of the operational theory that you find in the Passini book, but I think that it offers up the tuning information in a better format than what you find in the Passini book. Then again, when I bought the Passini book, there really wasn't anything else out there that had all of the operational theory that he included.

https://www.amazon.com/Build-Dellor..._UL160_SR129,160_&refRID=17BKG0338Q9CRVFS9RP8

There's also the Pat Braden book which slots somewhere in the middle of the other two to me. It's what I used as reference source along with the Passini book for years and I still reach for it from time to time for comparison purposes, but Passinin and Hammill are my first "go to" references.

https://www.amazon.com/Weber-Carbur..._UL160_SR126,160_&refRID=11V0A5QB4T69N05FAAEC

Since we are talking BMC 1275 A series engines something else that might be of interest:

https://www.amazon.com/1275cc-High-...8&qid=1454736209&sr=1-10&keywords=des+hammill

https://www.amazon.co.uk/Tuning-A-Series-Engine-Definitive-Performance/dp/1859606202