DCNFs on a T/C, advice needed please.

[nickp]

Hi Guy,

After finally getting my freshly built engine in my Lancia Montecarlo run in, I'm having a bit of bother with one of the carbs. Basically, at tick over one carb (cyls 1&2) is running richer than the other. After ticking over for a while the plugs are sooty on 1&2, whilst 3&4 are a nice colour. I should be able to 'cut' each cylinder by winding the idle mixture screw right in to stop the fuel, but only 3&4 will cut completely. The carbs are 42DCNFs and have been rebuilt with new gaskets, seals etc. They are fed by a Facet silver top pump through a Filter King regulator set at 3psi. I've restripped the carb and checked the float level, but can't see any obvious problem.
It seems that there must be extra fuel getting in as well as via the idle mixture screw, although nothing can be seen down the carb when ticking over (it was dripping fuel in until I fitted the regulator). Logic says that it's something common to the whole carb rather than the individual chokes of the carb, so that kind of rules out the idle screws / jets etc I think.
Can you offer any advice please? The car's ready to go for setting up on the rollers, but I don't want to take it yet with this known problem.

Thanks in advance,
Nick

[Guy Croft]

Buy a Colortune (see links) and have a good read of the instructions.

Seriously! Buy a Colortune and you will be able to answer many of your own questions very quickly.

GC

[nickp]

Thanks for the quick reply Guy, I think I will.
If it does shows cyls 1&2 still running rich even with the idle screws right in, is there anything obvious you can think of that might cause this in the DCNFs?

Thanks again,
Nick

[Guy Croft]

Nick hi,

carbs out of balance can do that, where the throttle plate on the carb running rich is tood far open and the proggression drillings are pulling fuel.

A dripping accelerator pump jet can do it (have a look down the barrels with engine running, wear protective glasses in case of backfire though) as can wrong float level which should be 50mm from the cover to the bottom of the float, cover held vertical with float arm in light contact with ball of the needle valve.

The other cause, not so rare, is an exhaust system with primary pipes to short.

Naturally the idle jets should all be the same size.

GC

[nickp]

Guy,

I reckon your first theory could be right, and now you've said it I might know the cause. There's a little 'spring loader' type thing on the linkage where the 2 carbs join. Maybe this is causing the carbs butterfly to stay open enough to draw the fuel as you describe. I'll check this and hopefully adjust the springs effect out with the adjuster.
I did have the dripping pump jet problem before I fitted the regulator, even though I only have the lower rated 'Road' silver top pump (4.0-5.5 psi). I've looked down the carbs at tickover and it's bone dry above the butterflys, which can only mean the fuel's been pulled in below I guess.

Thanks again, you might just have jump started my grey matter.

Nick

[Guy Croft]

Good man!

Did you replace the needle valve assemblies? I guess so but if not do so, they should not lift-off on a Silver Top pump even with no regulator.

GC

[nickp]

Hi Guy,

I'm back to scratching my head again here!!
Yes I did put new needle valves in, and even tried an old one back in there just in case. I was surprised too as I'd read on here (and elsewhere) that the lower spec Silver Top should be OK with no regulator and that was one of the reasons I went for it. Either way, the regulators on now and all's fine there.
I got the Colourtune and have been tinkering this afternoon. I backed off the linkage and wound off the throttle stops so both carbs were nicely shut. I can dial all cylinders to a nice 'bunsen blue' but as I wind the mixture screw in on 1&2 it starts to flash intermittently orange before cutting completely as it should. Winding out the air bypass screws still gave the same syptoms. Cyls 3&4 go lighter (leaner) then cut as they should. So now with all the cylinders set I've got what seems to be a pretty decent idle at about 1000rpm, but when I pull the leads off 1&2 it barely affects the idle. If I do the same on 3&4 the engine virtually cuts out. It seems that even though they've got a good mixture burn now, these 2 cylinders are just along for the ride. The tickover won't go below 1k, which seems strange if 2 of the cylinders aren't doing much doesn't it?
I know the carb is the common factor here but it's had me doubting the engine itself. So I did a compression test on all cylinders and got 180-190psi across the board, which seems about right as I have std CR and cams. The valve clearances are OK (0.40-0.50mm). I've also swapped plugs and leads around and fitted a different distributor cap just in case the 2 pots are getting a weaker spark, all to no avail.
The car hasn't been driven hard yet, but seems to pull cleanly with no misfire. I'm worried that if I take it the rollers that it'll make no power due to this fault, where it should hopefully be making a pretty lively 150+bhp with the mods I've done.
Please help before I scratch away my already thinning cranium!!

Nick

[nickp]

Hi Guy,

Is it possible that I'm getting the right ratio of fuel/air into 1&2, but just not enough of either at tickover to give a good 'bang'? Hence the weaker cylinders? Again pointing back to the carb. The carbs are fed by a t-piece input to 3&4 leading on to a single input on 1&2, could this cause any problems?

Nick

BTW - My Dad's a real old school mechanic who's worked on most 'retro' things in his time, but this has got him baffled too.

[Guy Croft]

You have to balance the carbs - and I think the dcnf have a barrel_to_barrel air bleed screw too, the carb drawing the most air must have its bleed screw (allows air to bypass the throttle plate) closed.

If you don't have a carb balancer you need one - Gunson make a reasonable low-cost one - because the balance is affected by the power each cylinder is producing - which is a function of the A/F ratio at idle. All the idle mixture tests must be done around 850 rpm and if you can't get it to run slower then it's either out of balance or the throttle plates are not actually closing - or - there is an air leak on the inlet manifold or betw carbs and manifold.

I'm interested in the type of ex system and muffler, because high back pressure from too much baffling or primary pipes too short can cause very erratic idle mixture. As can having a servo line without a non-return valve - although the latter only affects the cylinder that hose is hooked up to.

GC

[nickp]

I hear what you're saying Guy, I'm focusing my attention on the problem been at the weak cyls 1&2. Where really it may be that cyls 3&4 are just pulling too much air, even though I have the air bypass screws right in on this carb. I'll turn my attention to this carb and check again that it is definately shutting properly, maybe that's why I can't get the tickover down as you say. I don't think these carbs have a barrel-barrel screw, just an individual one per choke.

The exhaust manifold is an Ansa item, and the primarys seem pretty long (at least compared to the other one I have). The silencer is a Stainless straight through item provided by PD Gough. The inlet maifold has a take off on cyl 2 but I've got this blanked off.

BTW- my fuel feed is effectively the same as the one in your picture, although yours does look a tad posher!!

Thanks again,
Nick

[Guy Croft]

OK, you can rule out the ex system as a culprit.

G

[Guy Croft]

BTW - Common problem is that the master carb doesn't have a throttle return spring strong enough to close the throttle, esp on the MC with that very long cable.

The adjuster screw on the balance quadrant (that spring loaded assy betw the carbs) only controls the posn of the 'slave' carb relative to the master.

Before balancing make sure the master carb throttle pair are closed effectively against the idle speed screw on the side of the master carb. If they are standing off from that screw it's because a throttle plate is misalinged in its bore or the spindle is sticking (usually caused by the balance quadrant rubbing against the carb spindle bearing housing) or the cable is holding them off it. It should always be possible to loosen the idle speed (control) screw so far that the engine actually stalls.

GC

[nickp]

Good info, thanks again. One thing I did notice from the photos is that on my balance quadrant the spring and the screw are the opposite way round. Which thinking about it now has got to be wrong hasn't it? The slave carb has effectively got spring loaded operation. I thought it strange that I had to struggle to adjust this from the back of the carbs. I don't think this is part of the tickover problem as I tyrapped the spring in place so the quadrant wasn't touching at all and I still had the same problem. Hopefully it should be easy to swap them over.
I'm chomping at the bit to get this sorted, but unfortunately work always gets in the way of play!!

Nick

[Guy Croft]

It doesn't matter which is master/slave..

G

[nickp]

Is the 'master' always the one that the throttle cable connects directly to though? Mine looks the same as the pic you've posted (master on left, slave on right) but the spring is where your adjuster screw is so the spring pushes on the actuator of the slave carb instead of the screw. Which can't be right can it?

Nick