Where would you look first (engine "miss" at idle)

[choppa]

guys. I have recently just gone thru my entire fuel system on the truck, from a new relined gas tank, new stainless steel in tank filter, all new fuel lines, new flexible fuel lines as well as NOS fuel pump and NOS carb.

all 100% new.

at the same time, I also installed my new pertronics ignition, new capacitor in distro, brand new plug wires, brand new spark plugs. all new there (didnt replace coil, seemed fine, still think it is but...) running the 24v origianl equiplent with waterproof wire.

here is my problem:

after getting it all together, it sure runs fine.. startes right up on first revolution of the motor. pleanty of power on a short run, on flat ground or up hills..

(Mind you I have NOT put on the carb air filter tube and air filter, thus none of the distributor to carb inlet vent lines are attached at the air intake. I did this so if I found any problems in the shake down run, I didnt have to remove all that to make changes.)

So, with truck at idle, I am getting a slight and ocational "miss" I want to call it.. Inice and smooth for a few seconds, then a fast "miss".
I can hear this at the exhaust pipe, hence not hearing it while driving.. if I go over to the passenger side and put my hand over the exhaust exiting the pipe I can hear it do this "miss" ...

My first thought was the NOS carb right out of the box (From VPW) maybe the fuel float was too high?

BUT.. after searching the older posts here, I seen a few posts sort of relating to my issue, and the reason given was perhaps the vent lines not hooked up?

can this cause the issue I am seeing? ( I am going to try this over the weekend when at the cabin.. the truck isnt here with me to play with and figure out)

a couple more points:

NO change to the carb air/fuel mixture screw changes anything with the miss. Its still there whether rich mixture or lean..
Neither does any changes to the distributor and timing change eliminate the "miss". From one extreme to the other extreme on the distributor adjustment, nothing changes that engine idle "miss"

Turning the idle up to a higher rev its still there too.

So, my first guess was that the carb float bowl might be too high. even though this is a NOS unit, perhaps it got "justled" a tad in shipping?

I was going to take the carb off and do this, but I am askign this question about the vent lines first.

anyone think not having the vent lines in place (from distro to air intake elbow) could cause a situation as I describe here? do these so called vent lines actually provide some vacuum or sort that affects this performance like this?

[ZGjethro]

I disconnected all my vent lines when I welded up my K&N style intake, and noticed no problems with them off. Now I have the two distributor lines and the oiler filler line vented to the intake, but not the brakes or gas tank. I can't see vent lines making a difference since they are before the choke plate. If they were after the choke I could see vacuum issues.

Are all the rotor contacts good? is the miss rhythmic, or random? Could you test for spark at each cylinder? I'm betting it is on the spark side of the equation, and not the fuel side

[choppa]

Random misses. I agree if it was timed I could see it being spark plug or wire fault as it would happen each firing sequence. Which doesn't seem to be the case. I got a new cap and new rotor as well in there. Not thinking this is carb float level related? Well I am here at cabin today so I am going to check it out. Argh.

[just me]

Run a compression test and a cylinder leakdown test. If base engine isn't solid, no amount of fuel or ign adjusting will stop it rom stumbling/missing. The intake design doesn't lend itself to equal fuel/air distribution to all cylinders and getting it "perfect" is more than unlikely . I can, for all practical purposes, tune the occasional miss away. And then the season changes and it goes out of balance and needs to be readjusted.

[choppa]

I did take off the cap and rotor and inspect. No issues there I can see. I cleaned it though while off in case of a possible carbon tracing I might not have seen?

No amount of distributor adjusting did a thing. nor did any amount of carb adjusting make much difference. (I have no timing light to check the timing. going by ear, how the engine sounds, etc )

ONE LAST THOUGHT / COMMENT : I keep wondering if I might have caused this when I pulled the distributer apart to replace the capacitor and install the pertronics kit.

I loosened the MAJOR distributer adjusting screw at one point, and when I assembled, I just assumed to tighten it at about the 1/2 way mark. wondering if it needs to be elsewhere?

I CAN get at this major adjusting screw, while its in the truck, correct? hopefully I dont have to remove the distro again to access the MAJOR adjusting screw?
I didnt try this as I ran out of time.

But am thinking if I got the MAJOR setting set incorrectly perhaps I cannot get to the sweet spot with the MINOR adjustment. where should the MAJOR adjustment be set? the middle of the adjustment throw sound about right or??

Ill try the compression check next. Problem was, I didnt have a compression check kit with me. will have at the next trip there.
Ill figure this out sooner or later!

S**cked that I missed the Clayton, OK homecoming / 4th of july parade as it wasnt running yet.. oh well. next year I guess

[just me]

John at MWM makes and sells the timing adaptor.

[hbb]

The coil still might be an issue, the vent lines are NEEDED to keep the coil from over heating and breaking down. I would suggest putting air pressure through the lines making sure they are clear of any blockage, the fittings at the intake are designed to draw the air flow through the lines. You cannot visually check a coil. I forget what the resistance reading should be but i am thinking around 4 to 6 ohms but that may be wrong. The new coils that are on the market have a high failure rate ( Chines crap ) if you can find a good used or nos is the best way to go.
The next thing you might look for is a vacuum leak at or around the base of the carburetor and intake manifold to block gaskets. timing will not cause a misfire, timing will effect power and starting, if it starts right up and runs it is timed close.Have you adjusted the Idle mixture screw? If not screw it in until it seats then back out 2 1/2 turns, that is a base setting. Now start and let idle turn the mixture screw in until it stumbles and then back out to adjust to the highest idle, if the Idle setting can be lowered bring it down and recheck with the mixture screw. This should all be in the TM's if you have them.
I hope this might help you.
Hb

[PoW]

I've seen them burble a bit when the PCV valve is acting up.

Pull it down, give it and all the piping a soak in carb cleaner.

Amazing how gunky they can get....

DDG

[MSeriesRebuild]

Good advice to check compression now. I've seen people spend hundreds on tune-up stuff only to find a low compression issue that only an overhaul will cure. Also good advice on vent line hook up, coils, no matter where original type or replacement will die a quick death without ventilation. If you have a Pertronix unit, same thing applies, it will overheat and die without venting. There are some crap coils out there, but are also some decent quality replacement ones. They have a good track record and few failures if you look at the numbers of them that are out there these days. The radio interference capacitor in the distributor serves no purpose unless you are dealing with an interference noise issue. If you don't need it, remove it, and install a new lead in wire going directly to the Pertronix module, one less component to give trouble. Back to vent lines, BE SURE the proper fittings are installed in the air cleaner tube, if not, there will be no forced air circulation, thus vent lines are serving no purpose with no forced air. Ventilation system is not hooked into the vacuum circuit on M37's, totally different system. The default idle screw setting is at approx. 1 1/2 turns open, however it is highly doubtful that it is the cause of your "miss" issue. Drop the thought that your ears are a timing light, it simply is not the case; get a good quality timing light and set timing at 4 degrees BEFORE TDC; then get the carb properly tuned. If you are dealing with a NOS carb that you think should be right out of the box; stop dreaming, tear it down and check everything in sight, build it back with a new, CURRENT production rebuild kit. Follow good advice and clean that PCV valve too, check for vacuum leaks around the intake, they can be minor and hard to detect, but cause very irritating problems. If your compression readings are not 105 PSI or above, get suspicious; if any cylinder reading is as low as 100 PSI, you can expect some intermittent skipping to happen. If any readings are down to 90 PSI, unless retesting determines that valves are the problem, start pulling the engine, it's rebuild time. By the way, unless it has been rebuilt, that NOS fuel pump does not have a diaphragm that is compatible with today's gasoline. The diaphragm will fail, dumping gas directly into your oil pan causing oil dilution and engine ruin in a very few minutes. Best solution is to install an in-line electric pump with a 4-5 max PSI rating. If you insist on running the NOS mechanical pump, you better be sure you get a new production diaphragm in it sooner rather than later. I can't imagine anyone would even sell a NOS pump, they have to know the danger. Many buyers think NOS spells GREAT; some vendors who only see $$ signs take full advantage; buyer BEWARE!