Popping carb issue

Is it occasionally popping, or constantly? Does it pop throughout the entire throttle travel (like reving up the engine) or just at idle speed?

Take the carb cover off and check float height. Check also the needle valve to see if it's stuck or maybe something is blocking the fuel inlet.

Are the little O-Rings in place where the idle jets screw into the carb body? If they're damamged or missing, they could be letting excess air by and leaning out the idle mixture.

 

Did the carb rebuild include restaking the lead plugs?

Can you get any change in cly 5/6 by running the mix screws in and out?

 

OK, I had exactly the same issue on 5/6 just after rebuild of top/ bottom end.

It has to be either:

1) intermittant ignition issue on 5/6 - manually rotate the crank with spark for 5 or 6 out when it fires on cylinder 5 and 6 place a mark somewhere on the harmonic balancer for each that is visible. Start the engine, put a timing gun on lead for 5 or 6 , watch the mark as the engine runs, when the spit occurs see if the mark moves back or forward, if it does then you have an ignition issue - leads dizzy or rotor sparking too early - assuming not goto 2

1a) Make sure the idle is running at AF6 - use a timing gun on cylinder 1, if it is too advanced you could get some spit back.

2) Inlet valve issue on 5/6 - Assuming no issue with the above, take cylinder 5/6 carb off, disconnect fuel pump (remove fuse), get someone to sit in the car, while you observe inlet valves. Spray something like WD 40 into the inlet valve port, watch to see if there are any significant bubbles/ blow back when the engine is cranked over - if yes then you have an inlet valve issue - bad seal most likely, the worst possible outcome - If this doesn't show anything, then it has to be a Carb set up issue - the best outcome....

3) It has to be something simple like idle mix issue, crud in the idle circuit ( if both 5 and 6 spit, a real possibility), or air leaks somewhere. Possible checks.

a. Spray carb cleaner around (not into the carb) the carb body with engine running, see if the ignition changes or engine studders, then you have an air leak somewhere.

b. Take out idle jets and clean.

c. Blow dry compressed air through the idle circuit.

d. Remove idle mix needle valve - this can be the source of many issues- it is not always the case that 3.5 to 4 turns is about right, sometimes it can take 5 or 6 turns.

e. Place the carb on a very stong white light, look into both cylinders see how much light is coming through the butterflys when fully closed, if one is signifincatly different than the other, that could be your issue - butterfly could need reseating/ positioning.

It is most likley you are running lean. The best way to sort that out is individual sample pipe for each cylinder to get the CO at approx 3.5%, best way to ensure you are running OK.

Let me know what you find. In my case it tuned out to be the idle mix needle valve. Colortune spark plug was misleading, thought it was tuned properly due to colour of cylinder flame, but was running lean, not enough fuel at idle and also idle speed was too low when warmed up.

All of the above of course assumes you carbs were balanced perfectly to start off with, whcih takes time..






3) Fuel/ air mix issue

 

swap the idles jets with another carb that is working.

 

I've found that with today's gas the idle jets need to be 2 to 3 jet sizes big to run right. I've had this problem with a lot of DCNF and DCOE equipped cars.

 

Tom's 3)a suggestion should be tried. I had a similar issue and it turned out to be leakage in the manifold. Propane will also do the trick. BTW, the manifold gaskets are not easy to remove, but the engine is now very smooth.

 

Did those Webbers come with brass or Plastic Floats? If they are Plastic they could be saturated with gas internally,and you'll never get it adjusted.

 

I believe they came with 55 idle jets, increasing to 60 will get some adjustability back, since normally correct idle mix is achieved on these with about 4-5 turns as someone has already stated. ANY flow restriction in the idle passage will lean it out and cause the spit-back due to the slow lean-burn effect. Be advised messing with emission calibrations is frowned upon if you are in a smog check state. More important to get your car running right. If modifying one carb, do them all. Often, changing from the stock exhaust to an aftermarket muffler would create this condition, what do you have?
If the distributors are set up correctly it is unlikely an engine or ignition problem just affecting the one carb.

 

Re the idle mix screws don't change any behavior at idle on the #6 cyl or any of them?

 

One other possibility......

Have you checked the clearence between the bucket/ shim and cam. There is a possibility the distance has tightened and you don't have enough clearence, which could cause the spit back you are finding, especially if clearence is marginal.

 

valves bed in in rebuilt engines, clearences tighten, if the shims on rebuild were close to the lower limit already this could be the issue, only way to be sure is take the front cam cover off and use a feeler guage to check the clearence.

 

The two are not related at all,

popping is either a carb/ fuel issue or a valve closure/ seating issue.

You seem to have completed a lot of work to rule out the carb issue......

Now check the other possible source.........

 

Fave,

What are your float heights set at and what are the suffix numbers on the carbs (i.e. DCNF40 /45,46,47,48?

Bill

 

Fave,

The reason I ask is because I replaced the DCNF40 /75-78 on my 78 GTS with earlier, new DCNF40 /45-48 carbs, OEM to GT-4 USA models I believe. When checking float specs in the various 308 Owners Manuals, it turned out the /45-48 carbs had a slighly different float height specified IIRC. For instance, the 75 Euro GTB carb specification /57,58,59,60, calls for a 50 mm setting to the base, without gasket. Your carb series should be stamped into the base or exterior housing somewhere, if you still have the 5-bolt OEM carbs with the removable air horns.

Are you saying that when you adjust the mixture screws on the 5,6 carb, even all the way in (closed), there is no effect?

What is your idle rpm?

One thing you might try is to increase the flow a little bit through the 5,6 carb and see if it will then respond to the mixture adjustment. It turns out that there is a minimum air flow required to set the mixture properly. Too low an air flow and the mixuture adjustment will not work. I discovered this one day when trying to set the idle mixture adjustment on one of my carbs usng a Color Tune. What the Color Tune showed was a partial white flash (at idle) on the affected cylinders, instead of a steady burn with a color. I increased the air flow rate and was then able to set the mixture.

I hope this helps.

Bill

 

If you are not getting any change in engine speed when you screw the idle mixture screws in or out then cylinders 5 and 6 are not firing consistently or at all when the engine is idling. Many things could be causing you to have intermittent or or complete misfiring but the most obvious and likely are spark problems (wrong timing or too weak, bad wires to those cylinders), an air leak at the affected cylinder(s) or a poor balance between that carburetor and the other carbs. While all of the causes that people have brought up are indeed possible, applying Occam's razor, the simplest and most straightforward cause is the most likely. Focus on these three things first as they are the most likely. While possible, it is unlikely that you have dirt in the idle circuit for any number of reasons- you just cleaned them, they don't get dirty easily in my experience unless you are just careless (which I highly doubt), especially as you are only getting the popping at idle.

I would disconnect all of the linkages and let the engine run. Still get popping? How are your vacuum hoses connected? Do you have a different routing to/from carb 5/6 than the others? Replace the plug wires to cylinders 5 and 6 and see what happens.

Hope this helps. You are right in that you have a fundamental problem if manipulating the 5/6 idle screws does not alter the engine speed. You MUST get the engine to falter when you screw in the idle screws past a certain point. If it doesn't then it ain't right and you need to keep looking.