Dellorto 36 DLRA on a 2L -4cyl
These were rebuilt by a local 914 shop about 3000 miles ago after about 15k of great no problem running. Rebuilt due to gas seeping under the gaskets and onto the carb, and the float was out of adjustment.

After the rebuild they ran great and no issues until today. (NOT banging on the shop,,,this is a normal something to do I'm sure)

idle TO 2000 rpm cyl #4 pops through the carb with a little little gray smoke. After 2000 rpm it runs fine. Carefully looking into #4 there's fire showing in the barrel. See the picture. Put a snail on all 4 carb barrels and it's at 6.5 where all the rest are at 7 at idle...kinda ok.

Bent, worn, or crushed needle letting too much gas in?
Torn gasket?
Air bypass adjustment?

Thanks for your thoughts.

When I ran Weber's (Dell's really not that different) it was more often issues with ignition and timing that it was with the carbs. I ran them for the better part of 10 years. I eventually began to check ignition 1st.

Also real easy to pull up the jets and emulsion tubes to see if there is any cloudy gunk or anything that would indicate the carbs are a place to look at closer. If clean I'd move on to ignition as my first places to check.

Timing set properly? Cap and rotor contacts clean? Points clean and gap set properly? This stuff is way more prone to mechanical wear than carbs.

YRMV.

Normally you are right we are the timing and valve adjustment first. The reason I’m looking to the carbs is it is only one throat out of four

Take this as a thought exercise.

When I have engine issues I always follow this order in troubleshooting
1) Compression - do I have it? In all honesty, I usually skip this under the assumption that things are fine unless there is reason to question it.
2) Ignition - do I have spark and at proper time
3) Fuel - is mixture right?

Spark is weird on these old cars, it's not like coil on plug in modern cars that is high intensity and precisely controlled. Coils get weak, some wires get high resistance and can cause 1 cylinder to spark earlier or later than the others, distributor lobes wear, etc. I once had a distributor slip the timing over a period of time because I didn't tighten the holding bolt enough.

A slightly stretched intake valve or a valve adjuster vibrated loose can close up the valve clearance leading to a slight intake leak that either doesn't get good compression or allows the combustion flame front to leak by the valve slightly (Only needs to be minuscule since combustion event is at high pressure) and get's access to the otherwise incoming fuel charge in the manifold and causes backfiring in just one cylinder. Highly unlikely, but could account for a 1 cylinder variation.

Moral of the story: If you haven't checked valves and ignition don't discount the possibilities just yet.

I always kept records of what my carb idle adjustment screws and air bypass screws were set at. That way you have a know "good" setting to return to if there is any doubt about whether a carb adjustment backed out over time. I always documented them as # of turns from full seated position. That way you can screw them in and reset without much risk of disturbing the settings.

Likewise I always kept record of my valve adjustments over time - if you keep having to loosen a valve clearance, it's stretching. Usually happens with exhaust but again could be either.

I personally found record keeping to be important. Just kept it all in a Moleskine type journal before Moleskine's ever became cool.

Click to view attachment

If you place your hand, or something else, covering the offending veturi with no effect on idle, check the idle jet. Or, if gently closing that same venturi idle air mixture screw has 0 effect, check the idle jet.

Dirty idle jets. You can try to clean them out by putting your hand over the velocity stack as you accelerate to try and remove the junk. If that fails, take the idle jets out one by one. Do it very slowly and softly, because as you place the screwdriver or jostle the jet, the crud can fall back down and make it seem as if the idle jet was clean to begin with. When you restart the car, the crud can go back into the jet. This will drive you nuts.



Look at them through the light before blowing them. Once you find the culprit, clean it out and youre done. Don't mess with anything else. It was running perfect, the day before right?

I've not worked on Dells enough to give advice on those, but on Webers if you suspect a clogged idle jet, all you have to do is loosen the jet carrier about 1/4 turn and if the idle changes you need to clean that jet.