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MG MGB Technical - Weber DCOM 40?

I'm considering moving to a Weber sidedraft setup, and finding a very cheap manifold on eBay has sealed the deal. I'm now mulling over my carb choices. My engine is straight stock 1969, and I have no plans for other performance modifications aside from (maybe) an aluminum head. I will not be replacing the dizzy or any other major components for this build, and I am not looking to start a major debate on this topic as it's been done several times in the past from what I can see in the threads (so please, no "keep the SU's" comments). I've found a Weber DCOM 40 carb that seems like it might do the job. Particularly interesting is that this job seems to have a ported vacuum advance, like the rear SU carb on my current setup. I know that a 45 would better accomodate future upgrades, but since I'm not planning any then I'll probably need a 36mm venturi (am I right about this?) so a DCOM or DCOE 40 should suffice. I'm off to the bookstore to buy some study material, but any advice that can be offered would be appreciated - there doesn't seem to be much info on the DCOM at all on the web, aside from that it uses the same jets and venturis as the DCOE.

Adam
Adam Birnbaum

Adam,
What's wrong with SU's? You don't want those crummy old Italian side suckers on an MG! Just kidding. Having spent a great deal of time chasing DCOE ghosts, the best advice I can offer is to call Pierce Manifolds and talk to Mike Pierce or Doug in the tech department. They have been down this road many times and are always willing to help out. I just about went nuts trying to figure out the right combination of chokes, jets, etc. on my crossflow car until I called Mike. He set up a pair of 45's for me and they are sweet as can be. My car idles like it has well tuned SU's and revs like mad. Two things to keep in mind with Webers. One is that similar results can be attained with several different combinations of bits. Two, remember that the "40" or "45" is just the raw internal diameter of the carb passageways and one the primary keys to good results is the relationship of carb size, choke size and the engine's demand. The increased air velocity created at the choke tube constriction is critical to creating a good carb signal and is dependent upon all three variables. If the choke tubes are too large for the application you will be very disappointed in your low and mid range power. Good luck and keep us posted.
David
David Gable

DCOMs are updated DCOEs basically and eliminated some of the problems associated with DCOEs. I considered changing to webers a few years back on an engine I was building and did some research on the difference but you should do your own research too and not believe me.
Mike MaGee

Mike,
I know that sometimes we get added complexity for not much improvement - witness the HIF series. Did you find that the DCOM was an overall improvement over the DCOE, or did it seem like it wasn't a worthwhile upgrade?
Adam Birnbaum

I thought they weren't more complicated just a better design. If you're still interested go on eBay as there's a guy in Italy who sells them used for cheap. Well I didn't buy the weber setup for the car as when all was said and done it was too much $$$$ for not enough HP over my SUs. Tuning and buying jets, emulsion tubes, air bleeders etc is an additional and possibly expensive cost. It was for a highly modified, bored, stroked, ported Datsun Roadster motor which I had no baseline for. I find SUs better for the street over sidedraught webers as they generally have more low end grunt. Webers are great carbs but for higher rpm driving where they are strongest. They must also be setup correctly to get the most from them and generally most people don't know squat about that including me. I had a 240 w/triple webers and I ended up taking it to a shop that specialized in Italian cars as they are/were fairly common on Alfas and of course Ferraris. They worked 100% better after they were finished tuning them and even more so when I took it back to them for some additional tweeking. One more thing - Joe Curto's shop is close to me and he helped me with the needle selection - but he busts my _alls about the Jap SUs :)
Mike MaGee

More info:
Called several places today about rebuild kits for the DCOM. Most said they simply didn't have them/didn't know what they were, and one place said, "Those carbs never made it to the US, and you will never be able to find a rebuild kit for them."
And then I found this site:
http://www.bmw2002faq.com/store/ramflo_weber.html
Towards the bottom, we have...rebuild kits for the DCOM 40! I wish I'd taken a bet from the guy who said there was no such thing to be found.
Adam Birnbaum

Adam,
You're a man on a mission I can tell. FWIW,and speaking as someone who has been there with DCOEs (and still is) to avoid disappointment make sure you have access to a rolling road and budget to spend as much again getting it set up. The pain and cost of the Weber is in the setting up and it really does not lend itself to the home mechanic.

You'll hate me for this but you may also want to reconsider the "no other mods" policy to get real fun out of the Weber. It's great at the top end as has been said, but that's not where a stock B series engine usually likes to play and that's not because of SUs. Just a few more dollars on the exhaust, cam, dizzy, K&Ns ....... go on, you know you want to!
Steve Postins

Steve,
Thanks for the comments. Of *course* I really want to do those things, but alas student loan payments, health insurance, etc. all rate more attention at the moment. I understand that the Webers are more difficult to set up at the outset than SUs, but I figure that I'll take a crack at it in my garage, and if I can get no joy then I'll head down to the local Alfa/Fiat repair shop and ask them to have a try. I'm not looking for something to take down to the track - if I were I'd go forward on my Nissan 240SX engine swap ;) But the SU's are driving me nuts, and I never, ever want to touch my carbs again after this. Oh, and the whole Weber setup is costing me less than $200, so that helps.
Adam Birnbaum

Adam said " "Those carbs never made it to the US, and you will never be able to find a rebuild kit for them."

I went to a Nissan dealer who said the exact same thing about my 66 1600 Datsun. He continued to argue with me even after I showed him the factory parts book which specifically mentions the USA. Young know it all jerk.

So don't get pissed about the carb - it's better than finding out your car was never imported here :)
Mike MaGee

I've never encountered a DCOM carb so I cannot
comment about them.

Actually, a 40mm sidedraft should work OK on
your car as DCOE carbs on MGB's are routinely
choked down to 34mm (street) or 36mm (on
all-out racers).

I would imagine that there would be less gas flow
turbulence when the intake air transitions through
the smaller 34mm-36mm choke sleeves mounted in a 40mm carb throat, ranther than a 45mm carb.
Daniel Wong

H
bob

Adam;
One thing not mentioned here is what is wrong with your SUs? They are one of the simplest most trouble free carbs ever. Once set correctly they just go on and on. Before I went down the Webber rode (since your not looking for performance) I would take the car to someone who knows SUs. Bob Thompson/International Auto
Bob Thompson

Bob,
I started writing a long response detailing my experiences - in short that 80% of all breakdowns I've suffered have been directly related to the carbs - but then I looked back at my original message:
(so please, no "keep the SU's" comments).

I'm not going to go into why I think the dual-SU setup is a royal pain in the neck, or my specific critiques of the SU design. I am sure that some people have great results with them, but my main concern is not breaking down on the side of the road (as much as this is possible, given our cars). In my experience, the SUs have been a horror, and they're being 86'ed so I can try something different.

I've found a fairly nice, modern carb for very little money, so that will be my experiment of the month. So the next question is - what venturis and jets should I use here? I've done the calculations, and it seems like a 34mm venturi is the best for a stock MGB engine. The rest - well, the Weber book hasn't arrived yet from Amazon.
Adam Birnbaum

Perhaps your success with the webers will be good, perhaps not. But had you used the same all-or-nothing approach to fixing the SU's you might not even be considering the Webers. For you to have breakdowns that strand you it seems there must be a significant problem with the fuel supply, either too much or too little, and in either case no control. Assuming you replaced the float valve, float, jet & jet tube and bowl gasket and set the float level correctly, a continued problem indicates trouble with the fuel line, pump, tank, or electrical system and even perfect carbs can't cure that. Just be sure your carb replacement will cure the problem or you'll be down $200 and have a more complex system to tune, as well as still having the breakdowns. Also assuming the webers are used there is no guarantee they will not have problems equal to or worse than the SU's you are removing.

The fuel pump on the MG and its electrical feed are notorious for causing breakdowns. Just a warning.

Jim
Jim Blackwood

There is not much that is any easier to figure out than an SU carb. If SU problems are proving difficult to solve then a blind leap to a used Weber is most likely going to be folly. Trust me, I know. Boy do I know.........

You keep asking what components to use. I'll say agin that you need to call Pierce or someone else with experience using DCOEs on MGB engines unless you just enjoy misery and wasting time.
Cheers,
David
David

This thread was discussed between 03/12/2003 and 07/12/2003

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