Leaky DCNFs

sptcoupe

True Classic
I've been struggling with a problem on my 1986 X/19.

It's a great 1.5L motor built by the guys at Midwest. But we have had two issues that I can't seem to solve. The idle is rough and moves from 400 to 1500 rpms at will, it comes off idle with stumbling, and it backfires like crazy if you let off the throttle. The backfiring only occurs as the rpms drift down to around the 4000 rpm mark (the idle jet circuit). At WOT and once on the main/air corrector circuit, the motor is spectacular past 8000 rpms. So the issues are all in the idle jet circuit range.

I know what the problem is - after the engine reaches operating temp, the carbs begin to leak fuel through the aux venturis. You can see it pouring in at idle. And this will, of course, cause the issues and make it impossible to get it set up right. But I can't find a fix. When the engine is cold, it really is sweet all through the rpm range, and very strong (when I have the normal jetting set ups in the carbs).

This is what I've done over the past year:

- Have had 5 sets of new DCNFs on the car. I even had Weber in Spain involved.
- Have tried all manner of jetting set ups on both F24s and F36s, from way lean to way rich, swapped to brass floats and set them high and low, run 150 -200 needles.
- Have tried numerous fuel regulators and ensured I have 3psi
- Have tried 34 and 36mm chokes to try to reduce the vacuum signal.
- Have checked valve timing, and tried a wide range of initial timing settings, from very retarded to very advanced. Total advance is consistent.
- Have scoured the car for any kind of vacuum leaks and replaced the under carb gaskets and intake manifold gaskets numerous times- none I can find.
- And some other stuff as well.

The point is, every carb, no matter how it is set up, has the same leak. So the carbs are getting some sort of vacuum signal from somewhere, and no matter what I've done, they all leak, and the rough idle and backfiring haven't changed at all. So nothing I've done has touched these two issues - obviously because I haven't addressed the real issue - the fuel leaking into the bores through the aux venturi.

I think the issue is perhaps the cold start circuit leaking air, but I've had ten carbs on her, all brand new, and all with the same issue. I doubt that all ten of those carbs have the same issue with the cold start circuit (which isn't hooked up).

The engine is a mild street build. Mild cam, 10:1 pistons, header, valves - the usual hotrod stuff, but nothing radical. And, it really runs like a screaming banshee in the main/ac circuit, whether at WOT or cruising at steady throttle.

I'm no stranger to FIATS and dual weber set ups - been at it for over 50 years. But am always open to any advice or tips you guys can give me.
 
We had a similar problem on an air cooled VW powered Formula Super Vee. In our case the carbs were mounted on the heads and not together. What solved our problem was carefully adjusting the linkage so that both carbs opened/closed at exactly the same time. I guess yours are side by side and that makes things simpler, but I would at least check it before you dig into the carbs.
 
Yeah. They're carefully balanced and synchronized. But I don't think a balance/synch problems would cause them to leak through the aux venturis. Am I missing something?
 
Have you checked the float levels? It sounds like they may be set to high and allowing the fuel leakage you describe.
 
Seems to be correlated to when your engine is hot. You've ruled out a lot of issues, so perhaps it's fuel atomizing.

This post was informative

 
I've tried a wide range of float levels, from very lean to very rich and all in between. Both with plastic and brass floats. No luck.

The fuel pressure is well-controlled at 3psi. Is there anything else that could atomize the fuel? It only occurs at normal operating temps, and it doesn't seem to matter if its a colder winter day or a hot summer day.
 
Yeah. They're carefully balanced and synchronized. But I don't think a balance/synch problems would cause them to leak through the aux venturis. Am I missing something?
Not really sure. I see some info has been added about the leakage and causes of that, but after 5 sets of new carbs all with the same symptoms I tend to believe there is something external to the carbs that is causing the problem.
Of course, I admit to having been wrong before...
 
That's my thinking, too. I'm really at a loss. The last possibility of it being a carb issue that I can think of is the cold start circuit, but I don't know how to fix that.
 
Who has done a DCNF TB swap?
As I read your posts I was thinking to myself, man I'm glad I gave up carbs years ago and only do FI now. Not being sarcastic, and I know it doesn't resolve your issue, but this sort of thing was so common with carbs - particularly multiple Weber's.

Wondering, did all of the new carbs come from the same source? I know there has been some major issues with the quality of Weber's over the years. You mentioned involving Weber in Spain, is that were they came from? I'm wondering if there is a production issue that affects all of the ones you've tried? Perhaps they warp when hot and allow a vacuum leak? I know warped Weber's has always been a problem, but usually on older used ones. However it might be an issue with the new supply as well? But I have to agree that isn't very likely, I'm just throwing it out there as it sounds like you have checked all of the obvious things.

Please let us know what happens. And keep thinking how much better it will run with EFI. :p
 
Ha! I hear you on the Webers, but I'm a carb guy and all my FIATS have dual carbs except the "81 spider. And they all run like your daddy's Caddillac with little to no maintenance once you get them right. This is my first experience where I am totally stumped.

I also wonder if DCOEs might be a better carb for the SOHC with its long rods. Most of the cars I've set up are DOHCs with IDFs.

I'm just wondering if I would have the same issue with FI, and if so, what the symptoms might be.
 
but I'm a carb guy and all my FIATS have dual carbs except the "81 spider. And they all run like your daddy's Caddillac
I understand, I loved multiple Weber setups for decades, while I resisted the move to FI. But ha-ha, my dad had Cadillacs and I would NOT want any of my cars to run like them. :p

Joking aside I am curious to find out what's going on with your engine. Sadly it has been too long since I was well versed in Webers to really offer any help, sorry.
 
Fuel spilling from the auxiliary venturi when idling is symptomatic of a blockage / restriction in the idle circuit somewhere... or an air leak into the idle fuel circuit somewhere...the third reason I've seen would be low manifold vacuum (but not seen it on twin carbs)

If you have tried multiple sets of carbs and the problem is persisting then we can sort of rule out the air leak into the fuel circuit, sometimes the lead or brass plugs that are factory installed to close off holes left by drilling a passage can leak... this lets air in and stops the fuel getting drawn into the circuit properly - like a tiny hole in the side a drinking straw -but it seems unlikely this would be the case on multiple sets.

the blockage / partial blockage... that can be from the idle jet itself. you haven't mentioned trying a range of sizes in the idle jet?

there is also an idle air bleed / bypass screw, where is that set?

and there is also an idle circuit air bleed (emulsion) hole ... these can get blocked , but multiple sets of carbs unlikely to be an issue...unless you're using the wrong gasket over and over again and the gasket is blocking these holes?

Low manifold vacuum could be a cam timing issue, you mention you checked valve timing - but then mention total advance in the same sentence so I think you meant ignition timing?

SteveC
 
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I have had similar problems with dual IDFs and the reason was a rough idle causing the motor to rock and the accel pumps would self energize. Fine tuning of the idle solved that. Also gas will dribble out the aux venturi on any carb if the car is actually idling on the main circuit rather than the idle circuit.

Dribbling cold starts won't feed the gas through the aux vent, the cold starts have their own port at the base of the carb. I plug these with grub screws.

Having the same problem with five sets of carbs is what's confusing. It must be something external to the carbs. I have found dual Webers to be incredibly forgiving to wrong settings but fiddly to get just right.
 
Steve - very helpful info.

I agree that multiple sets of carbs from different sources rules out blockage/air leak issues. When I mentioned that Weber Spain got involved, it was the result of Pierce Manifold being so helpful trying to solve the issue. They thought that perhaps they had a bad batch of carbs after sending two sets to me (four, actually, when you count the first two sets that Midwest tried on the car). I sent Spain a pair of them to look at and they found no issues at all in terms of blockages/leaks or warpage. After a fifth set of carbs from Redline with the same results, I ruled out those scenarios.

As far as idles, I ran a series of set ups on both an F24 and F36 Etube with an idle jet range from very lean (45s) to very rich (57s) changing one jet at a time. For each idle jet set up, I changed the mains in a range from 120 to 140, changing one jet at a time. Painstaking work, and while I got the expected results (I am running an AFR meter on the car) none of the combos changed the symptoms (leak and backfiring) in the least. So I've ruled out the idle circuit jetting.

As far as cam timing, I meant that with #1 at TDC, all the marks lined up perfectly, so the belt hadn't skipped a tooth. I also tried a range of ignition advance from 4 degrees BTDC to 14 BTDC, with each of the jetting set ups (many, many combos). Again, no affect on the symptoms at all. I am running in the initial advance at 10 BTDC.

I've changed the manifold gasket, under-carb gaskets and cab top gaskets several times, and I've never had any indication of a leak.

The air bypass screws are set by lightly seating them, tightening the screw on the best drawing ore, and backing out the other until the flows are equal (balancing them), the I synch #2 with #3 to synchronize them. I also tried leaving all the air bypass screws fully seated and synching #2 with #3- no affect.

My last thought is that somehow air is getting by the enrichment circuit. Would a plate blocking that off help? I'm not using that circuit.
 
the result of Pierce Manifold being so helpful trying to solve the issue. They thought that perhaps they had a bad batch of carbs after sending two sets to me (four, actually, when you count the first two sets that Midwest tried on the car). I sent Spain a pair of them to look at and they found no issues at all in terms of blockages/leaks or warpage. After a fifth set of carbs from Redline with the same results, I ruled out those scenarios.
That was my earlier thought, maybe a bad batch of carbs. But it sounds like that wasn't the case. ;)
 
If the Weber Spain and Pierce Manifold can't help you I doubt I can be very helpful, I'm not an expert.

I've got a pair of 40 DCNFs on a 1500 and they run great. They've been running good for years. So, I know it's possible to make them work.

My take is that when you have a problem like this it's time to go back to basics.

The DCNF has a funky "choke" mechanism, and it's prone to leak. I've never even connected a cable to them on my car so I've never used them. So disabling the cold start mechanism isn't a problem for me. I think Carl's suggestion to disable them is spot on. I wouldn't use a grub screw though, I'm not sure that mod is easily reversible. Some folks cram a hunk of lead in there, that's more reversible. But there are blank starter jets available. These folks offer them on eBay: 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// Installing these on your carbs would be a good (reversible) way to eliminate one know problem area with these carbs. But...I don't think that's your problem.

If your problem is the main circuit is engaged at idle you have to look at why it's adding fuel to the intake charge when it's not suppose to. I can only think of two ways that would happen.

One - the fuel level in the float bowl is higher than the aux venturi feed. So the fuel feeds out by gravity.
That could happen when you have a misadjusted float, a leaky needle/seat, and/or the carb is mounted at an extreme angle.
I'll have to assume that the float level has been checked.
Leaky needle/seat - could be that the needle/seat isn't controlling the flow of fuel adequately. Could be a bad needle/seat (NOT LIKELY), or it could be that the fuel pressure is too high. (the fuel pressure would be common to all five sets of carbs.)
If the carbs are mounted at an extreme angle you could have fuel in the float chamber higher than the feed for the aux venturi. What manifold are you using? If it's the PBS (or a copy) the carb mounting angle is not ideal to begin with. If you have bad motor mounts that could cause a problem. As I assume the manifold would be common to all sets of carbs you've tried. That means the manifold, and/or fuel pressure are suspect. (But easily checked.)

Two - The other reason you would have fuel feeding out of the aux venturi would be that the air pressure is lower at the aux venturi than in the float bowl.
I think that's very likely to be the reason you're having the problem, mostly because - that's how it's suppose to work. (The reason all five sets did it.) The air pressure at the aux venturi is lower than the air pressure in the float bowl only when there enough is air flow though the aux venturi. (Assuming the bowls are vented well.) There should be very little air flowing past the aux venturi at idle. Certainly not enough to create a pressure low enough to engage to main fuel circuit. If you adjust the throttle blades such that they are open too far at idle, you'll have problems as you are experiencing. The easy way to diagnose this condition is - you have fuel feeding from the main circuit at idle. If this is the cause of the problem it's not difficult to verify. Take the carbs off and see if they are adjusted such that the transition/progression holes/slots are uncovered by the throttle blades at idle. If you can't get the engine to idle without opening the throttle too far (running on the main circuit) then you need to concentrate on why it won't idle as designed. Does the engine get too much fuel at idle, as it would with leaking cold starting device? Or does it lack for fuel as if it have plugged idle jets/passages, or was simply misadjusted? Is there a vacuum leak?

Since you have experienced the same problem with quite a few otherwise "good" carbs I can only guess that something that is common to all five sets is the issue. Or, that the installation/adjustment that you do is the issue. My best guess is that you have the idle set too high (for some reason).

That's my best guess. Hope it helps. Good luck getting things sorted out.
 
I realize I miss spoke about rocking of a motor causing the accel pump to work and push gas out. It does but the gas would obviously come out the accel pump nozzle, not the aux venturi.

I discovered the problem of leaking cold starts when I could completely close a mixture screw and that cylinder was still idling...because it was still getting gas from the leaking cold start. I don't believe a leaking cold start would cause gas to come out the aux venturi, the cold start is not connected to the main circuit, it is totally independent.

As suggested above, you need to absolutely verify you are idling on the idle circuit and not the main circuit.

Not many of us can attain five sets of dual carbs to play with! Have you considered installing a single 2-barrel carb for test purposes to verify the problem is with the carbs and not something else?
 
Steve - very helpful info.

I agree that multiple sets of carbs from different sources rules out blockage/air leak issues. When I mentioned that Weber Spain got involved, it was the result of Pierce Manifold being so helpful trying to solve the issue. They thought that perhaps they had a bad batch of carbs after sending two sets to me (four, actually, when you count the first two sets that Midwest tried on the car). I sent Spain a pair of them to look at and they found no issues at all in terms of blockages/leaks or warpage. After a fifth set of carbs from Redline with the same results, I ruled out those scenarios.

As far as idles, I ran a series of set ups on both an F24 and F36 Etube with an idle jet range from very lean (45s) to very rich (57s) changing one jet at a time. For each idle jet set up, I changed the mains in a range from 120 to 140, changing one jet at a time. Painstaking work, and while I got the expected results (I am running an AFR meter on the car) none of the combos changed the symptoms (leak and backfiring) in the least. So I've ruled out the idle circuit jetting.

As far as cam timing, I meant that with #1 at TDC, all the marks lined up perfectly, so the belt hadn't skipped a tooth. I also tried a range of ignition advance from 4 degrees BTDC to 14 BTDC, with each of the jetting set ups (many, many combos). Again, no affect on the symptoms at all. I am running in the initial advance at 10 BTDC.

I've changed the manifold gasket, under-carb gaskets and cab top gaskets several times, and I've never had any indication of a leak.

The air bypass screws are set by lightly seating them, tightening the screw on the best drawing ore, and backing out the other until the flows are equal (balancing them), the I synch #2 with #3 to synchronize them. I also tried leaving all the air bypass screws fully seated and synching #2 with #3- no affect.

My last thought is that somehow air is getting by the enrichment circuit. Would a plate blocking that off help? I'm not using that circuit.
If you've tried all those idle jets, doubt the problem in the carbies, I would be checking cam timing and measuring manifold vacuum.

Changing emulsion tubes and main jets and air correctors is a waste of time and effort at this point as you don't have a problem with the main circuit, it's the idling circuit that is the issue.

and yes, Mike raised a good point, is the manifold correct for X19 or is it a "split the difference" manifold?

Double check that the idle air bleed bush is not being blocked off by the carburettor top gasket.

What model DNF are we talking about?

SteveC
 
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