got my dual dcnfs...questions, linkage, spacers, jets, etc

So is that the square devices on the backside with cable hookups? Called enrichment thingamabob? I'm guessing those hook up like a choke knob or something, and prime the carb, instead of having a choke?
I've got mine connected to the manual choke cable that 74s came with. They actually provide the same function as a choke without impeding the air flow like a choke butterfly would. Never had any trouble with mine.
 
intersting, so they operate just like a manual choke tho, right? Dunno, if I will ever need one, I'm in Texas...altho, I like driving it in the cold ass weather, as long as the roads aint Icy...well i take that back, it actually handles great in that too, but the other morons on the road are dangerous, lol
 
Yes, just like a manual choke.
I had mine hooked up, but with DCNFs you really don‘t need them.
 
Other things I'm pondering...will my little facet type pump supple enough pressure? I notice no return lines on these. Wonder if I should even use the fuel return. Are 2 carbs going to require more pressure? For some reason, I'm wishing I had paid more attention in high school physics class....
 
Low pressure Facet "cube" pumps work just fine for dual Webers, the pressure requirement is the same regardless of how many Webers you run.
 
I would run a return line with a restricter to ensure you don’t over pressure the carbs and cause the floats bowls to overfill.
 
Regulating pressure with a return bleed is pretty hard to do.
I would recommend using a fuel pump that supplies the correct pressure in the first place.
A return line is optional.
I have never run one and so have others. But there are arguments for both choices.
 
spacers definitely make a big difference for my setup
So docpan...what are the dimensions on them there spacers? I like the oversized thing going on there...was going to reference you in an email to the fellow who makes them if you don't mind
 
Here are the dimensions. I told him others would be calling.
 

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I ran my DCNF's and now my DCOE's with a fuel pressure regulator without a return line and never had a problem. In the picture, it's the round thing close to the (solid) dog bone. It's a pretty common model . Base adjustment is usually 3 or 3.5
20171013_195436.jpg
 
The enrichment circiut on the idf/dcnf is motorcycle esque and routes fuel from the bowl to just below the throttle plates or such to aid enrichment when the pistons in the bores are lifted up by a lever hooked to a choke cable or linkage of sorts. I never use mine and they are not hooked up. Usually on used carbs of this type the piston corrodes in the bore and can't be removed without destroying something in the carb. I haven't installed plates and screws to block the passages but just don't use them. On my dual Idf's i do have the cable hooked to the choke on my spider and during cold weather ops I have used them on occasion. Linkages are varied and usually not very geometrically correct. The best ones I have used are from dual carb sets I have bought from Europe. The Pierce center quadrant leaves something to be desired in the
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geometry department.
 
little facet cube pump will not supply enough fuel volume (flow) if the engine is destined to make any sort of horsepower... you need a pump that is capable of around 90 litre / hour ... 1.5 litres a minute ... the little cube pump falls well short.

I like to route fuel from the pump to a regulator (malpassi or filter king type with built in filters are nice) and then to a three way distribution block... one separate line to each carb from the fuel block, and the third outlet restricted down to 1mm and plumbed to the return fitting on the tank.

If the engine is destined to make good power (like 100hp at the wheels plus) there is a need for large 3/8 fuel lines ... 5/16 lines the flow gets marginal.

20180122_130505.jpg


this was the start of plumbing up the last 1600 I built... 3/8 lines from three way distribution block to the carbs... have more pics somewhere of the finished job, just need to find them.

SteveC
 
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little facet cube pump will not supply enough fuel volume (flow) if the engine is destined to make any sort of horsepower... you need a pump that is capable of around 90 litre / hour ... 1.5 litres a minute ... the little cube pump falls well short.

I like to route fuel from the pump to a regulator (malpassi or filter king type with built in filters are nice) and then to a three way distribution block... one separate line to each carb from the fuel block, and the third outlet restricted down to 1mm and plumbed to the return fitting on the tank.

If the engine is destined to make good power (like 100hp at the wheels plus) there is a need for large 3/8 fuel lines ... 5/16 lines the flow gets marginal.

View attachment 12778

this was the start of plumbing up the last 1600 I built... 3/8 lines from three way distribution block to the carbs... have more pics somewhere of the finished job, just need to find them.

SteveC
Thank you for the detail. Do you mind telling us which needle and seat you recommend for this set-up?
 
When we first dynoed the 1600 the owner had fitted a little cube facet pump as the original pump had died between engine fitting and dyno day (1000km of running in)... AF spiked to the 16's at 4500rpm on the first pull so we shut it down fast... pinched off the return line... made no difference. Upped to the facet red top pump (rated at 85litres/hour from memory) and the AF was still in the low 15's at 5500rpm... upped the jets from 125 to 135 then again to 145's and the AF began to look more healthy... but it still looks a bit like a hilly terrain above 4500 or so... I suspect this might be the needle and seat struggling to keep the fuel bowl full at high demand...it had 1.75 N/S and 32mm chokes ... next dyno run is with 36 chokes and 200n/s (with 2.50n/s on standby) and hoping it at least tops 110 at the wheels with just these changes ... ignition tweaks, cam tweaks, cold air box, bellmouths... I want to see 120/125 at the wheels with a flat broad torque curve. These engines certainly crave fuel and air when you twist them.

bottom line is you want to run the smallest N/s you can to keep the fuel flow steady to the engine... a 1.75n/s is plenty to feed a pair of 125/130 mains and 32 chokes... but I think a little too small to keep up with a pair of 145's ... with choke change and bellmouths I have sets of 145/150/155 mains on standby.

SteveC
 
well wow...been actually using a facet clone, by Mr gasket....2-3.5 psi....just found out the flow rate is higher than I would have thought, like 159L/hr. has been working great with my single 36datr. Wondering if I should have had a regulator on this pump all this time...I do have a hot start issue, as I have to crack the butterflies open to get it to start when hot
anyways, this is the pump I've been using, and it has worked quite well for last couple of years
https://www.summitracing.com/parts/mrg-42s
 
OK, new issue...Man, how I don't pay attention to things....obviously there are many styles of this carb. I noticed this when pursuing this avenue, but had no idea of how many different styles there must be. the fuel inlet on these is anemic....almost looks like these carbs were some kinda "stock" version on some kinda car, so they could "advertise" dual webers- haha. Or almost like these were made for a much smaller engine, like a 1+ liter.... The inlets must be 5/16" (8mm?) and no kind of banjo hookup like all the others I have seen here. The casting is there for banjo fitting to be tapped...or maybe a 3/8" threaded elbow, and then I could use the existing fuel inlet for a return, or just cap it off. Ugh, I really don't like tapping fragile stuff like this. Suppose I'll need a shallow tap
weberfuel inlet.jpeg
 
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