High RPM Miss 340

[74 340 4speed]

My 340 has been running great until recently.  I took chryco's advice and plugged off the vacuum cannister on my distributor because it seemed to be missing just slightly at cruising rpm, which in my case is 3200.  Now, my problem is a high rpm miss.  When the motor spins over 5,000 rpm, there is a miss that I cannot get rid of.    I have ran autolite 63 plugs plugs in the past with great results.  I had them gapped at .045".  I changed the plugs to see if it would make a difference, and it didn't.  I tried running NGK plugs this time, part #BPR7ES, or the new NGK part#2023.  These crossed over directly to the autolite 63s.  They are gapped at .045" as well.

Here is what I have on my car now
NGK BPR7ES
Stock rebuilt electronic ignition distributor
new cap
new rotor button
new stock ignition module
new engine wiring harness
new accel 8.8 300+ race spark plug wires
new stock coil
new ballast

I've thought about it, and wondered if the ignition module might be going out.  Would it be worth it to upgrade to the mopar performance orange box?  I haven't seen many reviews on these so I don't know if they are much better than stock components or not.  Motor goes to 6500 rpm often so I want something to support that.  I didn't want to start changing out the coil and ballast because I just got a brand new engine harness and I don't want to cut and splice it.

[MEK-Dangerfield]

The only 2 things I can think of here are ignition module, or ignition coil problems. Are you using the stock Mopar ignition module now?


   Mike

[cudabob496]

If you have a stock distributor, you may need to gap at .035 inches.  Also, on my Cuda I eventually went with an MSD 6AL setup, MSD wires, and MSD billet distributor. Boy what a difference!  Also, it lets me gap my plugs at .060.

For now, also check if you have any crossed plug wires, put silicon in the spark plug boots, and maybe see if it goes away with higher octane gas. And sometime the ignition box performs poorly if too hot, like too close to the headers or exhaust manifold.

[426HEMI]

There is several things that this could be.  Might have a camshaft that is worn or a Vaccuum leak.  Dont forget the basics.  I would recheck the dist cap and rotor.  Could be that the bushings in the dist could be worn.  Just a few things to recheck.

[74 340 4speed]

The only 2 things I can think of here are ignition module, or ignition coil problems. Are you using the stock Mopar ignition module now?


   Mike

It has the stock 5 pin module on it.

[74 340 4speed]

If you have a stock distributor, you make need to gap at .035 inches.  Also, on my Cuda I eventually went with an MSD 6AL setup, MSD wires, and MSD billet distributor. Boy what a difference!  Also, it lets me gap my plugs at .060.

For now, also check if you have any crossed plug wires, put silicon in the spark plug boots, and maybe see if it goes away with higher octane gas. And sometime the ignition box performs poorly if too hot, like too close to the headers or exhaust manifold.

I thought that I could gap them at .045".   I'll close them back to .035 and see if it helps.  Plug wires are good.  It just started doing this within the last 2-3 times I drove it.  I haven't messed with any of the wires.  I have shell v-power 93 octane gas in it now with a bottle of pro-blend "nitro power" octane boost.  The ignition module is on the firewall in the stock location.  I have an MSD blaster coil, but you can't use those with the factory ballast, correct?  Since my car is a 74, the harness has connectors for a 4 pin ballast.  How would you correctly wire in the low resistance ballast in my application?

[74 340 4speed]

There is several things that this could be.  Might have a camshaft that is worn or a Vaccuum leak.  Dont forget the basics.  I would recheck the dist cap and rotor.  Could be that the bushings in the dist could be worn.  Just a few things to recheck.

I'm pretty sure the cam isn't worn and there aren't any vacuum leaks.  I have all new vacuum hoses and caps.  I also put a new set of intake manifold gasets on it and sealed the front and back part of the block with silicone in the process.  I'm thinking it could be the distributor since it is a rebuilt unit or the module.

[Oldschool]

Sounds like another possibility might be fuel delivery at high rpm.  If you have trash blocking a fuel line, or a weak pump, then it could be starving for fuel when the demand gets high.       I hate to add another thing to check,      but it wouldn't hurt....       

[426HEMI]

there is just a lot of things that could do this.   

[74 340 4speed]

Sounds like another possibility might be fuel delivery at high rpm.  If you have trash blocking a fuel line, or a weak pump, then it could be starving for fuel when the demand gets high.       I hate to add another thing to check,      but it wouldn't hurt....       

I can check it out but I doubt it there is a clog in it.  I have a new gas tank, line, fuel pump, and carb.  I've got a clear filter on it so that I can see when it gets clogged up.  It looks pretty clean right now.  Thanks for the info.

[Bullitt-]

There's always the ole vapor/vacuum lock issue when it comes to fuel delivery, your '74s tank was designed to be vented through the carb & most folks block off those lines causing vacuum in the fuel tank which can restrict flow.

[74 340 4speed]

There's always the ole vapor/vacuum lock issue when it comes to fuel delivery, your '74s tank was designed to be vented through the carb & most folks block off those lines causing vacuum in the fuel tank which can restrict flow.

Good point.  I'll definately take a look at that.  I didn't replace that line on the car, but it is hooked up to the gas tank.  It isn't hooked up to the carb though.  It runs to the back of the engine compartment and stops.  I'll check it for blockages, or remove it and put a short vapor line on the back of the car.  I have noticed that the car tends to vapor lock when I park it for 10 minutes or more and try to get back in it and crank it up.  I usually have to give it a pump or two of fuel before it will fire back up.

[Bullitt-]

Easy way to check is to remove your gas cap & test, if problem goes away then you've found the problem.....Couple of pumps even when hot is pretty normal I think.

[Chryco Psycho]

if you upgrad the ECU go to the Chrome box
 might be worth checking the wires , under higher load they can break down especially if the iwre has been pinched or pulled or burnt

[cudabob496]

You could check the resistance of your wires to make sure they are good.
Also, my limited experience has taught me that the original Mopar distributors weren't the greatest above 5000 rpm, but don't tell anyone I said this.