36 DCNF Questions

JTBernhardt

JTBernhardt

True Classic
I'm looking at buying a set of 36 DCNF carburetors for my '75 X1/9. I had originally planned on going with 40's, but I'm of the thinking that without more modifications and a compression bump the 36's will be a better compromise. The engine is running a stock 1500cc engine with a slightly ported and polished head with an Alquati A16 camshaft and an ANSA 4-2-1 header and dual tip muffler.

So my question about the 36DCNF's I've found is pertaining to ported vacuum. The set I found appear to have extra vacuum ports and I'm curious as to whether these would allow for the stock Bosch electronic ignition set ups vacuum advance to work properly? Pictures below. Notice there is a single port on the rear of the carbs and dual ports at the front base of each carb.

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they are misrepresented in the advert, as they are 36dcnfA carbs... that last A makes a difference.

original fitment to some Matra / talbot, so you'll have to search the internet for specs, but I can tell you they differ to a DCNF.

There is no idle air corrector screw to balance the carby throats against each other, and the supplied linkage will be too long, as the talbot/matra engine sits the carbies further apart than a sohc.

the ported vacuum point is the single fitting up high on the fuel mixture screw side, just like on the dcnva/dcnvh.

what the four small vacuum fittings on the float bowl side of the carby do / were originally fitted to I have no idea, you will need to figure that out yourself, as I've never seen a car with these fitted standard to see what they hooked up to

SteveC
 
So what I'm deducing from this is that these carbs aren't the best choice for my engine, since I won't be able to balance the carb throats. It's a shame too since they do have ported vacuum and would work with a stock Bosch ignition...
 
I don't think running dual carbs without vacuum advance is much of a hardship, particularly if you are using a Bosch distributor from an X FI engine. Their advance comes on much sooner than the carb version. Not sure what your 75 originally had in that regard but the 74 cars actually had vacuum retard, not advance, for emissions control.
 
Given the engine you describe the 40s would work fine. I have used 40s on both 1300s and 1500s of similar configuration.

I found that the vacuum pickup was not necessary. Simply set full advance at the desired number, likely in the range of 36 degrees and let the base advance end up where ever it does. Set the idle accordingly. You'll be fine.
 
JTBernhardt said:
So what I'm deducing from this is that these carbs aren't the best choice for my engine, since I won't be able to balance the carb throats. It's a shame too since they do have ported vacuum and would work with a stock Bosch ignition...
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Not really, I was just pointing out that you wont be able to adjust the individual idle AIR trim, as there is no adjustment for it on these carbs like there is on a DCNF ... whch you probably didn't notice until I mentioned it.

but they are used like this on a factory fitted setup, and there are other makes / models of car/weber that also do not have idle air trim screws (late model emission alfa romeos come to mind) so they will function, but you loose (what I think) is a quite imortant point of adjustment for an individual runner setup. It could be that the four ports at the fuel bowl side connect together in some air balance assembly for this purpose, but as I said I'm not sure.

Given your engine is quite standard, how about thinking a single 36, rather than twin carbs.

SteveC
 
If i remember correctly these were advertised as a matched pair.
Does This negate the need for individual tweaks?(limited experience with multi set ups)
Kev
 
no, youre not balancing the carbs to each other, you're trying to balance the draw from the cylinders to each other... a variance in lash for example will equate to a variance in cam timing, which equates to a differnce in compression in / vacuum from that cylinder, and having an idle air bypass helps to balance that and get a smooth idle.

SteveC
 
@JTBernhardt I had a look online, and the DCNFA actually comes in both types... with and without idle air bypass, I didn't realise this and had assumed the A suffix had something to do with this...but maybe not

I believe the difference in design is to do with the manifolding originally used... some vehicles have a balance port in the manifold, connecting the runners, so this would negate the need for any sort of idle air trim.

I found the ad you took the pictures from, and these are in fact 36 DCNF18's and do have the idle ar bypass, so will be perfectly fine and able to be tuned for a smooth idle on an individual runner manifold
s-l1600.jpg

what had me confused is the same seller has several different ads for various 36's, all called DCNF in his listing titles but some are actually dcnf A' s

so back to your original question, yes you could hook up your distributor vacuum advance to the fittings on the mixture screw side, the fittings on the other side could possibly be useful for using a manometer to balance them, or fitting a vacuum gauge

SteveC
 
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Getting a vacuum advance signal from multiple carbs is tricky. Just using one venturi makes for a heavy pulsing vacuum signal that won't work. Several folks running dual IDFs on spiders have run four separate vacuum hoses to a connector manifold and then one hose to the dizzy. Not a very pretty setup. I have dual 40 IDFs on my internally stock 1500 and the stock electronic ignition. I run no vacuum advance and the car runs just fine.
 
@carl But IDF's don't have a ported vacuum point above the biutterfly plate, the only place I've seen people connect a tree of T's to is the (usually plugged) point that's supposed to have the manometer screwed into for balancing, which is under the butterfly plate, i.e. the wrong location to get the right sort of vacuum signal to drive a vacuum advance module on a distributor.

These DCNF18's have a ported vacuum point above the butterfly plate.

SteveC
 
Dual DCNF's 01.jpg


I have dual 36 DCNF's on my 87 with a 1300cc motor.
While your carbs do not have the air bypass adjustment screws, I have all four on my carbs closed and have good vacuum balance for each carb. I never had to adjust them.

I have blocked and do not use the vacuum advance and had had no issues.

I blocked the ports on the base, again no issues.

I strongly recommend eliminating the startup circuit as this makes adjusting the idle much easier.
https://www.ebay.com/itm/COLD-START...-45-DCOE-40-44-48-IDF-DCNF-EMPI-/302788615337 And, it looks like they are even cheaper on their website: https://www.carbparts.eu// (Thanks to MikeHynes).

I've been running these for about 8 years now and I love 'em!
 
I have seen all manner of tuning guides which seem to take different approaches to those bypass screws. Some guides say close them all and don't touch them, some say close them all and adjust as needed to match flow on the two venturies on each carb and I have seen guides that say set them a set amount such as one full turn out as a base line. I have never really needed to use them to properly balance Webers so I wouldn't be too afraid of a set that didn't have them.

Steve, the point I was making was to get proper vacuum advance you would need a lot of external plumbing.
 
carl said:
Getting a vacuum advance signal from multiple carbs is tricky. Just using one venturi makes for a heavy pulsing vacuum signal that won't work. Several folks running dual IDFs on spiders have run four separate vacuum hoses to a connector manifold and then one hose to the dizzy. Not a very pretty setup. I have dual 40 IDFs on my internally stock 1500 and the stock electronic ignition. I run no vacuum advance and the car runs just fine.
Click to expand...

Yeah, that's a problem but one that can be easily solved. This is the ITB setup on my MR2. Those are 43mm throttle bodies and that's a vacuum manifold mounted on top. It has 4 lines running from the individual intake runners (there are ports machined into the mounting flange), one line running to the fuel pressure regulator, one running to the MAP sensor (the larger hose on the far left) and the largest hose running out the right end is for the brake booster.

The manifold volume helps dampen the individual pulses and provides an improved signal to the MAP and FPR.

Looking at live data the signal is a little noisy but then its a 14:1 compression motor with 306 degree cams. Fuel pressure is pretty stable at idle. I use the TPS for accel enrichment at tip-in so the signal lag isn't significant. I had one the simple plastic line splitters on the X1/9 and it worked OK. At some point I switched to one of these.


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