Twin Weber 28/36 DCD Bridge Port 12A
 

I have 12A that is bridge ported on the two outer intakes. The Carbs are two Weber 28/36 DCD as developed by Rotary Engineering. The intake manifold is Rotary Engineering. The engine is very hard to start and fouls the spark plugs fairly quickly to the point the engine will not start. Clearly the engine is flooding. The fuel pressure is 3psi. The engine timing is set at about 24 BTC. The floats are at factory spec and operate against gross jets. I have reduced the main jets from 135 to 110. The air bleed jet was already a 260. Still the engine will not idle below 2000 RPM and is very hard starting. I do not use a choke. Does anybody know what I need to do to get this engine to a reasonable idle. Stop fouling plugs and start easier.

Your timing needs to be at 0 D.

timing is kind of high, but it should idle. stock timing at idle is TDC, for leading. if your distributor is locked, i would run something more like 18BTDC, it will idle there, although not as well a if you had advance.

for the carbs, they both have an idle circuit, and a main circuit. the idle circuit supplies fuel until the throttle plate is about 20% open, and the main circuit supplies fuel based on air flow.

28/36 DCD

so basically if you are having problems with idle, you probably need a different idle jet. also you should adjust the idle speed, i would imagine you could get it to idle about 900-1000, possibly lower

With carburetors that small, you may be trying to idle with the throttle plate far too open for it to run on the idle circuit. 1000rpm idle with a bridge port takes a lot more air than 1000rpm idle with a stock port.

I tried a Nikki on my peripheral port, it ended up needing the throttle almost a quarter of the way open to run at 1500rpm. Of course with this much opening, it was running on the mains and not the idle circuit...

I don't understand what is meant by my idle needs to be O.D. The rotary engineering catalogue suggested 24BTC for bridge ported engines. Maybe it should be 18BTC because this engine is only partially bridge ported? I have attempted to get smaller idle Jets but so far no success. Suggestions welcome. there is no idle below 2000 RPM. I believe the idle adjustment screws are adjusted to that idle KINDA. Turn those screw out and the engines dies.

Thanks Guys at first I thought no one would pay this post any attention. Yes the dist. has no vac. advance or mechanical advance. This was the way the engine was run with a Dellorto Carb. Un luck I do not have a manifold to use with that carb.

28/36 DCD

if you have the idle screws all the way in, it needs less fuel, or more air. is it possible to crack open the secondary throttle a little?

The engine is getting to much fuel. that's clear. The idle screw are not all the way in. The are screwed in enough to provide the only idle the engine will tolerate. This so I can take my foot off the gas get out of the car and make adjustments. That idle is at 2000PM and it is not steady, drifting between 1600RPM and 2000RPM. Right now I will not be able to start the car again until I get some new spark plug. After 3 starts and thee run they are fouled.

cant you clean them?

Some one has suggested to me that I add another progressive drilling in a air bypass circuits. The purpose of this is to lean the idle mixture. Sounds interesting but I have no idea how to go about this. any body have knowledge?

i think drilling anything would be a last resort.

step one is to actually tune what you have.

and you need to start from scratch. carbs have two circuits in them, the main circuit and the idle circuit. you need to tune the idle circuit.

the idle circuit uses a jet, and if you're too rich at idle, it is probably too big.

if you have the idle cranked up to 1800-2000rpm, the idle screws wont work, because you have the throttle open past the idle hole.

for this stage of tuning i like to use the autolite plug, for an 81-85 Rx7, they are junk, but they are cheap, so you can foul a set or two and its no big deal.

The Autolite plugs are a great idea and I know just where to get some of the crappy things. I've tried to find smaller idle jets for this carb. But all I accomplished was wasted money. Any leads would be appreciated. I'm told this carb is very similar to one used on the Ford Pinto

what size idle jets are in them?

Yea, pierce manifolds is the best source for parts,

but..... I am running these carbs and I have a stash of spares, I may be able to help with some jets.

The idle jets are 45. I tried to get 40s and failed from Europart Direct in NY. What else can you tell me about Pierce Manifold?

i'm a couple hours away, so i get 2 day service?

Is there no one who has experience with this Rotary Engineering Twin Weber 28/36 DCD carbs on a bridge ported engine? I would have thought this would have been a common occurrence . Looks like I am wrong. A wise man named Albert once define insanity as doing the same thing over and over again and expecting different results. I'm at that point now. Nothing I do seems to result in an idle lower than 1600 and nothing makes the engine easier to start while it continues to foul spark plugs. So does anyone know why I should not try one of those Racing Beat modified four barrel Nikki carbs on a bridge ported engine?

28/36 DCD twin Webers on bridge port, engine
 

Does anyone have experience with using Rotary Engineering manifold and 28/36 DCD weber carbs on a bridge ported engine?

so it is time to do something different. did you change the idle jet? have you taken the carbs off to see where the throttle plates are in relation to the idle port? have you looked into the boosters to make sure the floats aren't too high, and leaking into the engine?

i'm sure these carbs can work on a BP engine, but you will have to try a bunch of things and see what works