Daytona Carb problems

[Steve Meltzer]

I’ve been struggling with an erratic idle on my Daytona since i got it. It’s impossible to set the carbs as, the idle won’t remain stable or consistent enough to do so. Sometimes it will idle fine and then just die. Dies are nearly every stop light. The first thing I did, months ago, was to check the floats, blow out all jets, new fuel filter...no go. The distributors were checked on the machine by Dyke Ridgely, so I know they’re not the culprit. New plug wires and brand new MSD boxes. (The entire exhaust system was completely replaced by the previous owner.) Still no success.

The car is an American Daytona, so I think it has the 40 DCN 21A carbs, tho’ I can’t find that designation anywhere but in a book about Ferraris. Thinking that it might be the needle/seats, I bought kits from Pierce and replaced them, the filters and the accelerator pumps. I put the tops back on the carbs (I didn’t do a complete disassembly of the carbs, only the top portion). With the fuel pump on, but no ignition, carbs 1,2, 3 and 6 poured raw gas into smaller, center secondary venturis, eventually filling the main venturis with raw fuel as well.

Despite checking the floats several times, once by a professional mechanic, are these just set wrong and don’t shut off the fuel flow? (58.5 mm at the bottom and 50 mm at the top where it would shut off the needle and seat) I believe all of the floats are fine; if you shake them, there’s no evidence of fuel within, but I didn’t note on everyone of them, a place on the side of the float where the brass had worn off....in the same place (the inside portion that faces the venturis). When you install the floats, there’s no evidence that any float touches the side of the bowl to account for these worn spots. I’ve enclosed a photo of one of the floats, but they all look the same.

Open to all suggestions. thanx. steve

[DWR46]

Steve:

What is the fuel pressure? If the original Bendix pumps have been replaced with higher pressure versions, you will have the condition you describe.

You have decribed the correct float settings for the U.S. Version 40 DCN 21A. I use the European settings on my car, which is 48mm height and 56.5 mm drop, without ever having fuel overflow.

If you received new needles and seats, do they have the spring loaded needle? If so, make sure you set the float height with the ball NOT compressed.

Did you ever verify the front carb fuel fitting has the small orifice to vent fuel pressure after the engine is shut off?
Dyke

[Steve Meltzer]

Thanx for the thoughtful answer, Dyke. I reset the floats at about 51mm and the drop at about 58.5/59. I gently set the top of the carb in a vise so that the float was hanging so I don't think it was compressing the needle's tip. I got the deluge of fuel to stop in every carb. But they must be very sensitive, as I left the car for a few hours, planning to drive it tomorrow; when I went out to see if I'd really solved the problem, the #4 carb filled with raw fuel again (pump on, no ignition). Maybe that float needs to be recalibrated?.

The car seemed to idle OK, steady, but there were occasional "pops" coming from most (?all) of the venturis, from time to time. The popping occurred both at idle and under acceleration, but seemed some better once I enriched the mixtures. What, if anything, do you make of the eroded brass area on the float, depicted in the photo? thanx again, Dyke. steve

[Jimmyr]

Steve, looks like you checked all the normal areas, but there is another that seems to be a trouble spot. If the carb bases and insulator blocks are not flat and true, air (vac) leaks can be a bear to try and correct by any other means. Try the old spray some carb cleaner or WD-40 at the bases and see if there is any change. Since the DCN series are 3 bolt vs the 4 bolt; they are more subject to warping and the over torquing that can happen in their life. Jim

[Steve Meltzer]

Hey Jim, thanx for your response. I neglected to say that I've checked 3X with a propane torch as I was also concerned about a vacuum leak, but I was more worried about the throttle shafts. In any event, I've never found any evidence of the dreaded vacuum leaks. Here's an update: I reset all of the floats from 50mm to 51 or close to it and the overflow stopped completely, until I rechecked them (ignition off, fuel pump on) and found the #4 was at it again...filling up with raw fuel, so I reset it's float again!

Fixed the #4 float and indeed the venturi was dry (key on, but not ignition), but now the #6 overflowed with fuel! Pulled the top off the #6 carb and noted debris in the bowl. This had been previously seen, in particular, when I first got the car in the early summer. (All of the bowls were full of crap when I first got the car.) Trying to be sure I wasn't hallucinating, I checked the #5 bowl (by now, I can take the top off a 40DCN with the best of 'em!...not something to brag about!) Same: small particles of debris, easily seen on the piece of white gauze I used to swab out the bowl.

I'm guessing the tank needs to be taken down and be cleaned and/or replaced. Do you agree with this? Wonder if the debris was intermittently obstructing the needle/seat? I'd put a new fuel filter on during the summer, but was surprised that the one I was replacing looked good to my eye. Not sure why.

One other puzzlingly thing, at least to me, was this: with the top of the over-filled #6 carb off, the key "off", and both venturis (on the #6) fairly full of fuel from the overflow, you could see air bubbling up in the venturis, both sides of the carb. You could see this percolation even 20 minutes later! Is this normal? Seems like there should be no air bubbling up from the bottom of the carb.

Do I drain the tank? Pull it out? Suggestions? Do the carbs need to come off the intake and be soaked? Thanx again for your interest. s

[tyang]

Hi Steve,

Sorry to hear about your struggles. When I have had a lot of problems with the needles and seats, I've checked to see if the needles were seating properly simply by blowing into the inlet while operating the float mechanism. Sometimes an imperceptably small piece of trash can cause the needle not to close against the seat. You should be able to feel your breath stop against the seat as the needle closes.

It definitely sounds like debris is one of your problems. The fuel filter should catch the crud, but they may be so full or damaged, that things are getting past. Have you pulled the filter and checked the element?

Tom

[Steve Meltzer]

Thanx for your note, Tom. When I got the car, I replaced the old fuel filter, though I thought it looked fine. I've not rechecked this one since I installed it. Can't have more than 20 miles or 2 hours on it, however. These are all new needle and seats and it would seem unlikely that they're all defective. Dyke suggested checking the fuel pressure, which I plan to do this PM. thanx again. steve

[Steve Meltzer]

Measured the pressure from the fuel pump....perfect @ 4 PSI. I looked at all the bowls and have included a few pictures of the debris. Could the debris have come from the worn spots on the floats? (they didn't really peel easily with my thumbnail). What next? R&R the tank doesn't really sound much like fun. Thanx. Steve

[Steve Meltzer]

oops i forgot the other picture. s

[tyang]

Hi Steve,

Yeah, trash like that will definitely give you stuck floats. You may want to see if the filter is seated properly on the top and bottom rubber gaskets and not allowing debris from the tank to get to the carbs.

Very few gas tank R&Rs are enjoyable.

Tom

[Jimmyr]

Steve, another thought; there has been a batch of really poor fuel line replacements and their insides flake off with our junk gas. Analyze the stuff you pull out of the bowls, is it rubber, rust, brass, other? After you find out the material get rid of the source. Did you find any material in the glass filter bowel, if not look at the hoses as they are after the filter! Jim

[Steve Meltzer]

I'll have to look at the debris, but here's some data that might be worth revisiting: When I got the car, it ran like crap and there was crap in the bowls, before I ever put in gas. When I changed the filter, I noted that the old filter still looked great, tho' I did put on a new one. This would suggest that the junk is coming after the rear filter, but why don't I see it in the little filters built into the carb's top? I wonder if it would be safe to put a boroscope into the tank? (This would be the ideal diagnostic test, for sure.) This car has no glass bowl like my 330 GTC (too bad, eh!). It seems like the first step would be to replace the fuel lines, and if there's still junk, go after the tank. Do the carbs need to come off and be soaked or can i removed all of the jets and emulsifiers and blow everything out with compressed air? I appreciate everyone's interest and suggestions here. steve

[Steve Meltzer]

Made a quick trip to the garage to see my baby and upon closer inspection, there does appear to be a bit of trash in the filters of the carbs but it's minimal compared to the debris at the bottom of the bowls. I'll take some samples to work and check 'em under my microscope....Louis Pasteur meets Eduardo Weber! s

[Jimmyr]

Steve, just to check, pull off one of the accelerator pump diaphragms on the side of the carb. If it is one of the old orange Fispa like materials they can crack and leave stuff in the jet. Note that they are after the fuel bowl, but are still trouble spot. Trash also builds up in these pump wells so if there is junk in the float bowl there will also be stuff in that pump area. Jim

[Steve Meltzer]

Thanx again, Jim. I replaced all of the accelerator pumps and none were cracked and there was no debris in the AP well.
steve