Twin carb conversion...a proposed cheaper way..

[wittmann]

Hi Guys,
I wonder if you could please 'Cast your eyes over this for me and tell me what you think ? The dual IDF Webers mounted on either sides pair of cylinders look superb and undoubtedly perform very well....But as we all know they are expensive, VERY expensive!!!
The other GoldWing carb conversions that I have seen on the forum involve a variety of centrally mounted single carbs, which often seem to come with long inlet manifolds causing fuel puddling and poor running. ( No criticism is intended there, the single central carb just have their own unique set of problems that have to be overcome)
The original carburation was 2 x32mm or 2 x31mm or even down to 2x30mm per side so ....my question is, has anyone thought of fitting TWO Double barrel 'PROGRESSIVE' carbs ( of differing sizes ) per side and so per pair of cylinders. A Weber 28/36 would probably work fine because the total size of the carb when open is 64mm and the Wing is 64mm or 62mm ( or 60 mm ) per pair of cylinders.
The price difference between the carbs is amazing..the 28/36 Weber is so much cheaper ! so you save money! , you still get the 'Twin Carb Look' and the bike should perform at least as good as standard due to the double barrel Webers. and of course of having a shorter inlet manifold
All you would need is a pair of inlet manifolds...does anyone have any idea where a couple can be got cheaply The fuel flow from the Weber would have to be split 50/50 per cylinder so the carbs would have to be mounted with the barrels in line and coming out from the main frame. Which in someways is better due to the linkages now facing one another and so the carbs are easier to 'link' together.
Your thoughts please gentlemen ?
Alb.

[Old Fogey]

Sounds like a good bit of thinking. Not sure that the the carbs would have to sit 90 degrees to the line of the frame, although that might have a slight bearing on the mixture. Have a look at this setup which has it longways feeding down to two outlets.

http://www.ebay.co.uk/itm/WEBER-28-36-C ... 231d8c1bfd

[gregh35]

A great choice for a carb, though a 28/36 isn't cheap either. Other progressives could get it done for less money with pefectly acceptable performance. I run a single DCD and 90% of my riding happens on the primary.
I understand what you're saying about the carb mounting, and for these bikes, would allow that particular carb to sit closer to the centerline of the bike. A large progressive carb is going to dictate what the manifold is going to be in the end. A "one size fits all" set of manifolds would be near impossible for something like this I think.
As to linkage, maybe a BMW cable could work.

[wittmann]

Hi Old Fogey,
...That is one of the Weber carbs that is currently in my watched list....I must confess that I looked at the lovely 90 degree bends and thought....Oh I bet that would get some unfavourable comments these days.....including on here if it was a single central carb set-up with another pair of arms on the other side.....
I would obviously prefer a set of the vertical DCOE..I cannot remember the name...I think that it might be IDF, although they may lack a choke.....BUT, I am NOT paying hundreds of pounds for a pair of Webers !!!!....I can also believe that the FOUR barrels MIGHT just be 'Over-Carburettoring' for the engine at lower rpm's.....so quite probably using 2 TWIN BARREL carbs may give a 'better and possibly' more progressive acceleration....Then its just the manifolds that are needed to support and attach the Webers.....
Realistically I would like to get the whole idea up and running for say £200 to £300.....and hopefully a lot less than that if I can just get the manifolds.....

[wittmann]

Hi gregh35,
Progressive twin barrel Webers are relatively common in Europe because they were a tuning option on LOTS of smaller 4 cylinder engines in the past before fuel injection became compulsory with the increased emissions laws !! I just missed one for about £11.....
.
The hardest part will be the manifold to allow the two cylinders to receive all the 'Mixture' from one barrel at low speed and then ALL the 'Mixture' from both barrels at higher speed.....I will look at some of the 4 cylinder car manifolds and see how they over come the problem.....Beetles will also be worth checking as well because I am fairly certain that you can get twin TWIN BARREL progressive carbs that supply a pair of cylinders each and THAT is exactly the current 'Flaw' in my idea,
Alb.

[gregh35]

...I would obviously prefer a set of the vertical DCOE...

Weber progressive 28/36 DCD downdraft is tuned as DCOE sidedraft would be.

Many of the same parts.
DCD is essentially the downdraft version of the DCOE sidedraft with different sized barrels.
The small primary's are excellent for our small engines.

28/36 DCD is hard to get here in the States.
Tuning parts are hard to find as well.
No longer supported by Weber NA.

A hot set-up for the older RX-7's was a twin 28/36 DCD conversion. I've a manual somewhere on it.

[n5zhu]

A few years ago one of my co workers had a VW rabit with a small 4 bbl with 4 same sized bbl's aftermarket I'm sure, may be worth looking into. 73 Patrick

[wittmann]

Hi gregh35,
Sorry I have only just checked the Forum and have found your post......I was looking at the Weber DGV which is a very popular carb for Fords...but it has 2 diaphrams in and I am not a fan of those....So rather unbelievably I had looked around and was also now looking at the Weber 28/36 DCD .......Its old but its a nice little carb, it uses replaceable chokes and it also has a 'piston' accelerator pump like the DCOE..Both of the barrels 28+36=64 mm ...2 of the standard 1100 carbs are 31+31=62mm...so in theory the DCD is right on the flow rate.
Apart from spending the money to buy a pair of old carbs....I am more concerned about a pair of manifolds to match them to the GoldWing cylinder head inlet ports.....I still need to find a way to allow the mixture to be properly 'dispersed' so that it can be 'shared' between the two inlets.... Do I need an 'open-area' below the carb outlets as a 'mixture area' that both of the cylinders can draw from ??.....Is there any merit in the idea of mounting the carb at 90 degrees across the cylinder head so that each inlet port can see half of the carb and so both of the barrels. Instead of mounting the carb as it is traditionally done....(To try and make it clearer)..... rotate the carb through 90 degrees and have the longest side facing front and rear.....it would certainly look ODD.... but would it help in dispersing the mixture more evenly between the two inlets ??....I have stayed up way too late its 0315 Hrs here ...Time to go and look at it again with a fresh head in the morning !
Alb.
........MAYBE...just maybe this idea might actually 'fly' ....

[gregh35]

The inline barrels would be rotated 90 degrees to centerline with the DCD. I would face the float chambers forward. Doing this would allow the carbs as close to centerline as physically possible. A plenum would be required to distribute the mixture properly, ...and to mount the carburetor.

A DCD is highly tunable. Much more so than the common DGV's. No comparison IMO.

It was either Lamborghini or Ferrari (I don't remember which) that used triple DCD's (might have been DCH's, again don't remember) on one of their engines from the factory.

Old-style DCD's (what I am using) are no longer made. New 28/36 and 36/36 DCD's are available. A fella on ebay over in Italy professionally refurbishes old 28/36 DCD's (as well as other carbs). I know of only one new, old-style 28/36 DCD still around, and it is over in England. I'm still saving my dollars for that one, because it isn't cheap.

I've superb throttle response with my set up using a single DCD. One per bank would work beautifully I think.

[wittmann]

Hi gregh35,
I am glad that my turning the carbs through 90 degrees is NOT a silly idea ............I found this diagram of a DCD and it does show it mounted 'Crossways' on a manifold to a 4 and a 6 cylinder engine.....So obviously the 'Mixture' that I am concerned about is in reality a fairly minor issue that can be taken care of with a standard plenum chamber. The Arrow that is just visible in the photograph would indicate the front of the car so the float chamber Does look to be mounted at the front. Apart from the manifold issue these do seem to be an ideal pair of carbs to be used on our Wings instead of a Very expensive pair of the matched 'Down-drafts' and probably better than a single central carb as the size and flow issues are more easily managed with 4 'Progressive' barrels than with just 2.......If only 'Old Wings' were as cheap over on my side of the pond as they are in the US, then I could have a perfect test mule for the idea !.... ...unfortunately they are NOT ........Can I just ask what is the difference (s) between the 'Old Style' and the 'New Style' DCD..???

weber dcd.JPG

[gregforesi]

One of the design criteria must be access to the spark plugs. The plenum will have some volume to dispense fuel evenly from the different sized carb throats.
The throat spacing on the IDF/HPMX is 90mm because the float chamber is between the throats. That helps leave room for the plug wires. I can just sneak them out under the Randakk manifolds.
So there will be some minimum height (above the plugs) for the floor of the plenum. Volume will have to be created by going either front/back, or up. Going inboard - the shelter sides present a clearance issue.

As near as I've been able to figure, I'm running about 430cfm on the 1 liter motor. It isn't over-carbed. When I changed to smaller throats at about 390cfm, the top end dropped way off. It didn't pull near as hard when it got over about 6000rpm. The 28/36 DCD is rated (depending on who you want to believe) at 224cfm.
The simple math says the engine will go through 150cfm at 8500rpm.
And that is the conundrum.

Either way you'd still have to paint them black (or wrap them in rubber or something).

I believe the vertical DCOE is the IDA. Both have adjustable mid-range jetting. The IDF is a simpler carb and there is no true mid-range circuit.

Greg (I've kinda been through this) Foresi
The purple things are the nitrous nozzles.

Right side DCOE.jpg

[gregh35]

Ideally (as I've thought about this before) the plenum for the VW type /3 and /4 could be duplicated for the dual carb set-up. Instead of the four outlets, a single outlet per side of the same dimensions would work. Those Empi manifolds have excellent shapes to them internally and would be difficult to reproduce in sheet. Not knowing the VW stuff very well, I have wondered if there is already something made that could be adapted. Probably not.

I'm slowly gathering parts and supplies to run nitrous myself. Obviously a wet system, but not certain whether I'm going to plate it or fog it at each head flange. If I plate, it will be hidden, and that I like. Fogging the ports hides nothing and I'd rather not advertise it.

Another carburetor I'm fond of is the 32 DFT progressive. The float chamber is centralized. May be a good application here as well. Not as tunable, but tunable enough. Readily available new and inexpensive carbs. They are a 32/32. Smaller base flange than a DCD, so standard DGV style flanges will not work with modification. One-off plenums/manifolds are going to be made anyway, right?


I like both carbs but prefer the DCD for a variety of reasons.

[wittmann]

Hi Greg,
I thought that you might have looked at something else before deciding in the Randakk kit ......Please can I ask the flow figure that you quote for the 28 / 32 DCD of 224cfm is that each or a total figure for the two ( Pairs ) of carbs ??..single would make sense, as two would then equate to 448cfm which would be potentially similar to your current set up....But are yours 40 IDA or IDF...I MUST find my books on Weber Carbs, but at the moment they are somewhere safe !!
I started out looking for a CHEAP way to get around the problems of original 'messed-up' carbs and a twin set up seems to have less 'issues' to sort out than the single central carb. I like the idea of a Single carb but 2 x Twins seems easier and more likely to retain Full Power , through out the rev range, especially 'progressives'. Nitrous is an awesome idea especially if it is effectively 'hidden', I will be very interested to see how many miles you then get from a rear tyre
.....I would like to use the 40 DCOE since I am so familiar with it but since they were fitted to a Lotus Twin-Cam engine of 'only' 1558cc, they seem a bit too big for only a 1000cc engine..... . Your image of them fitted via that manifold is a work of art......Heavens knows what 2 of those would cost to make....and then do they face inwards or outwards ???...Decissions, decissions.......

[gregforesi]

The CFM for the DCD is per carb so 448 total. When jetted correctly it should be fine. The reason I went to 26mm d'ellorto throats was because of a persistent backfire I had at constant throttle at around 3800-4200 rpm. I was trying to speed up the air through the carb venturis thinking that I was over-carbed. It turned out to be a vacuum leak at the throttle shaft seals on the carbs so it was a lean condition (but only at high vacuum signal). That was a really hard bugger to find.

I also thought about a 450 Holley dual pumper as a single carb setup. They make the small ones for boat engines. In the end I really liked the mechanical look of the 2bbls sitting outboard.
I got 6500 miles out of the first rear Pirelli. They are pretty soft but that makes for high confidence when flipping it back and forth.

Mine are IDF's. The IDA's are physically quite large in comparison. Randakk's manifolds are designed for the IDF.

My gas mileage is normally around 34mpg. When it's down around 27-28 I know I've been having big fun.

[Whiskerfish]

SNIP
When it's down around 27-28 I know I've been having big fun.