high speed miss

[dietz_diggler]

Hi guys, I recently got a 318 duster. You can see the car in the Welcome Wagon http://www.forabodiesonly.com/mopar/showthread.php?t=50051

I got the car from a guy who got it from a guy. He didn't know anything about the car, and I've yet to track down the PO before him. Anyways it has several problems, the first one I'd like to fix is a high speed miss.

The engine has a slight cam (340ish/RV type) I don't know the specs yet. Iron manifolds dual exhaust. Edlebrock Performer intake and a Holley 4160 vacuum secondary 600 cfm carb. (list# 80457-7)

when I first got it the vacuum advance was plugged off and the choke disconnected. Also the vacuum canister was pointing directly at the back of the intake so the vacuum line had a sharp bend to it to get it on. also the plugs had a good color but to the lighter side of tan, but the gap was .025 to .030 so I re-gapped them .035 to .040 also re-set the gap in the distributor between the reluctor and the pick up to around .008 to .010. (it was closer to .020 when I checked it) I also replaced the fuel filter and I'm using 92 octane gas.

Its got electronic Ignition w/ a chrome 5 prong ECU. The #3 prong/green wire is not being used. It also has a 4 prong ballast resistor but is only using one side of it, the 1.2 ohm side. votage measures 3.5 here and drops as rpm's increase. (dropped to 2.8 before I let off)

The car has a high speed miss at around 3 grand (I dont have a tach.) it starts right before the 1-2 shift w/ a stock feeling converter. once it shifts to second and the RPM's drop the miss goes away until the RPM's rise again then its sputtering badly and I have to back pedal on the fuel for it to shift to third. Then its running smooth again. till the RPM's come up.

you can floor it and it sputters instantly, you can stay on the gas and it never clears up. let off a little and its ok, but you cant go back up or it will start up again. The engine never dies as if its starved for fuel though.
also, I put a paper clip on the rod going to the secondaries diaphram and took her for a spin, and it never moved so they never opened. but it will miss reving it in park so the problem may be 2 fold. And I'm not sure the secondaries staying closed would cause a stumbling/missing motor.

sorry if this post seems like "War and Peace" I just thought I'd tell it all to speed up the trouble shooting part. I've been fooling w/ it for a couple days with no luck. To those that read all this ... Thanks!!!
Dietz

 

[George R]

Retard the timing and try it again. I just went through all this with my car. Turned out to be a problem within the dist that would keep the advance weights in the advanced position ALMOST always.
Give it a shot. Set the initial timing to about 5-7º and see how it runs.

George

 

[dartman340ta]

have you a compression tester ,pull a valve cover to see if you have any springs bad . almost sounds like a sett of bad valve springs (weak) like 340 cam an 318 springs . need a bit more details . ck as see if valves with a straight edge (remove rockers top of valves)all are touching the straight edge . just a couple things to look at for now

 

[dietz_diggler]

Retard the timing and try it again. I just went through all this with my car. Turned out to be a problem within the dist that would keep the advance weights in the advanced position ALMOST always.
Give it a shot. Set the initial timing to about 5-7º and see how it runs.

George

I have played w/ the timing a bit, and its about the same anywhere I put it as far as the miss goes. Also you can see it advance and come back w/ a timing light and giving it gas. I did give it another go today but no luck. Thanks though

have you a compression tester ,pull a valve cover to see if you have any springs bad . almost sounds like a sett of bad valve springs (weak) like 340 cam an 318 springs . need a bit more details . ck as see if valves with a straight edge (remove rockers top of valves)all are touching the straight edge . just a couple things to look at for now

I had thought about the springs, haven't checked yet. Starting to worry about the cam being wiped out also. I've never had one so I'm not sure how they act when this happens and I have no clue how it was broken it, or if they used oil w/ zinc in it. which brings me to another ?.. the oil has gas in it, I figure from the miss but I'm not sure. It was there when I got the car. thanks for the input so far guys.

 

[crackedback]

CHANGE THE OIL NOW! How old is the fuel pump. If it's busted the diaphram, it may be leaking fuel to the crankcase.

Get the baseline carb adjustments set. Float level, 1.5 turns out on mixture screws.

Disconnect and plug everything vacuum at the carb/manifold. You don't need the vacuum advance at this point. Plus, this gets rid of potential vacuum leaks.

What's your initial timing?

Could be any number of things. Start with getting a good baseline timing set up. Say 10-12 minimum before TDC. If you have a vacuum gauge set it to the highest point advancing the distributor then backing it off one inch, while keeping the idle RPM constant.

Do you know how much mechanical advance is in the distributor.

The internal distributor plates are known to move around a bit. If they aren't solid and the air gap is changing, it could cause a miss. Bad cap/rotor, rotor phasing.

Since you say you have to back out of the throttle, what exactly happens. Can you roll back into the gas and it picks up. Does the thing basically shut off at 3K?

 

[dietz_diggler]

CHANGE THE OIL NOW! How old is the fuel pump. If it's busted the diaphram, it may be leaking fuel to the crankcase.

Get the baseline carb adjustments set. Float level, 1.5 turns out on mixture screws.

I've got the idle screws 1.5 turns out, the float level on this Holley can't be adjusted w/out taking it apart, so I've left it for now. I am going to change the fuel pump (it looks new but...)

Could be any number of things. Start with getting a good baseline timing set up. Say 10-12 minimum before TDC. If you have a vacuum gauge set it to the highest point advancing the distributor then backing it off one inch, while keeping the idle RPM constant.

I have it set at 10* BTDC right now. I've been everywhere with it, no change in the miss

Do you know how much mechanical advance is in the distributor.

I don't know and don't know how to tell.

The internal distributor plates are known to move around a bit. If they aren't solid and the air gap is changing, it could cause a miss. Bad cap/rotor, rotor phasing.

I did set it to .008 to .010 but only on one tooth. rotor and cap look new, I didn't see any carbon tracks on the cap but cant rule either one out yet.

Since you say you have to back out of the throttle, what exactly happens. Can you roll back into the gas and it picks up. Does the thing basically shut off at 3K?

It basically shuts out at 3 grand. if you let off its cruising but as soon as you give it more it starts cutting out again. Like a governor. Thanks for the food for thought. I'm gonna get a fuel pump first thing. We'll see what happens from there. again Thanks.

 

[crackedback]

I'm gonna guess it's carb related.

You need to get a handle on the float levels.

The secondaries won't be opening at 3K on your car with anything less than the weakest spring, then, just barely.

 

[ramcharger]

Sounds almost as though a jet fell out. Pull that carb.

 

[dietz_diggler]

I'm gonna guess it's carb related.

You need to get a handle on the float levels.

The secondaries won't be opening at 3K on your car with anything less than the weakest spring, then, just barely.

Sounds almost as though a jet fell out. Pull that carb.

Thanks and Thanks, I'll give an update sometime after Christmas... Thanks for your time guys.
Dietz