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Need info on DCOM carbs


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Yes, DCOM, not DCOE. I recently bought a pair of DCOM 40's here on the FAQ. I have books by Braden, Passini and Haynes, and none of them discuss the DCOM. Shakespeare, D. H. Lawrence and Dr. Seuss weren't much help either. I can't find an exploded view or parts listing for the DCOM on the vast internet. Pierce Manifolds doesn't carry parts for DCOM's, and I've not received a reply to an email inquiry I sent to JAM Engineering.

I'm curious as to whether I'm missing some parts, or whether the parts are unnecessary. I'm also curious as to the function of some of the parts, as they are not on the DCOE's that I currently have on my car.

Here's a particular case: https://picasaweb.google.com/104033389162157892781/DCOMMissingAScrew#5638567255527959922

I've found on the internet images of DCOM's wherein the pair of thin tubes in the image linked above are connected by a hose, seemingly to close the circuit and cancel whatever function the tubes are intended to perform. While this is clearly one way to skin Schroedinger's cat, I'd prefer to know the purpose and functionality of the cat rather than to simply kill it.

Additionally, there's a tube exiting the bottom of the carb that I suspect is a fuel return. https://picasaweb.google.com/lh/photo/i1OyoyeHUFHnrJCRTsLeAg?feat=directlink

Is this the case? If so, where would I connect this fuel return, or should I block it off as well?

Many thanks for your assistance!



'76 2002 "Verona" / '12 Fiat 500 Sport "Latte" / '21 Toyota 4Runner TRD Off Road Prem “The Truck”


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I beleive the top 2 are vacuum take offs to sync your carbs.

The lower vaccum tube is for a dist. vacuum advance.

Take a fine wire and poke through the tubes and they should exit into the bore of the carbs. The top 2 should poke through on the engine side of the throttle valve and the side tube should poke through on the air cleaner side of the throttle valve.

Its been a while since I putzed with the DCOM's, but I beleive thats what they are for.

The DCOM is supposed to be the "improved" version of the DCOE.

I dont know where to get the screw to block off the sync port, but I beleive the jets are the same as the DCOE.



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Be yourself and be free with your thoughts because those who matter don't mind and those who mind don't matter..

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The DCOM was to be a replacement for the DCOE, but not a direct replacement.

It was designed to be more emissions friendly, consequently, it is more difficult to tune.

The most notable difference is that it has a diapram type accelerator pump instead of the DCOEs piston type.

It has vacuum take-offs for the dizzy as well as for other emissions appliances.

It has not been produced in huge numbers due to the predominance of EFI.

You will find that it is jetted to be much leaner (for emissions purposes) and if using it on an M10 motor, you'll likely end up going 3-4 steps up in both the idle and main jets to get satisfactory performance.

Set up properly, they are possibly slightly 'better' than the DCOE in performance. But they were designed to be tuned with an exhaust gas analyzer as opposed to a flow meter. The goal being to achieve favorable CO, HC and CO2 values rather than max performance.


1976 BMW 2002

1990 BMW 325is (newest addition)

1990 Porsche 964 C4 Cabriolet

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Amen. You'll need to put a vacuum cap on the "fuel return" bottom.

You'll need a random screw to plug the sync port. Prob came loose in the box.

Without plugging these ports it will be as if there's an air leak and a high idle condition will result.

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Awesome! Thank you for your help. I'll get these bad boys on my car soon enough and see how they work relative to the pair of mismatched DCOE's I've been running.



'76 2002 "Verona" / '12 Fiat 500 Sport "Latte" / '21 Toyota 4Runner TRD Off Road Prem “The Truck”


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agree - the 'taps' with one missing a cap screw is for

vacuum gauge sycronizing.

Buy 4 cheep vacuum gauges from any NAPPA store,

4 equal length vacuum hoses to connect the 4-gauges,

and adjust your idle mixture, idle speed, and sync

the carb connecting linkage by matching your 4-vacuum

gauge readings.

Try your local ACME hardware store for a tiny metric

cap screw - use a tiny copper washer under the screw

head to keep it there.


'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

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