Miss at WOT Only

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73Swinger18

73Swinger18

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Camarillo, CA
I took my '73 Dart w/318 out for a spin tonight and noticed I have a miss above 4,500 RPM only when at WOT. If I keep the throttle below 1/2 way, it runs fine at the same RPM range. I verified the fuel bowls and accelerator pump adjustment with a feeler gauge thinking it was a fuel delivery problem. It had no effect on it. Below is some info on the car. Any suggestions on what I should check next? Thanks!
  • Original 318 bored 0.020
  • Original heads with larger valves (1.88/1.60)
  • Classic Holley 600 (68 main jets, 4.5 Power Valve - 10" of vac at idle, #31 Pump Nozzle)
  • Weiand Stealth intake manifold
  • Comp Cam 20-223-3
 
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Do you have a choke on the carb , make sure it opens all the way. Had this problem not to long ago choke was stuck about 1/4 in. from fully opening , made a big difference at wot. Just a suggestion.
 
fatoldguy said:
Do you have a choke on the carb , make sure it opens all the way. Had this problem not to long ago choke was stuck about 1/4 in. from fully opening , made a big difference at wot. Just a suggestion.
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Electric choke. It was open all the way. Thanks for the suggestion.
 
Here are two things that are rare, but have gotten me before. One time I had a loose engine ground wire. During idle or regular driving, there were no problems. But when I stomped on it, the engine twisted and moved just enough that it disturbed the ground contact, and that caused a miss. Another time, quite recently, the positive wire on my coil was loose. At first, I only had a miss at full throttle, but eventually it got worse. At first, both times, I thought is was a fuel (carb) problem.
 
harrisonm said:
Here are two things that are rare, but have gotten me before. One time I had a loose engine ground wire. During idle or regular driving, there were no problems. But when I stomped on it, the engine twisted and moved just enough that it disturbed the ground contact, and that caused a miss. Another time, quite recently, the positive wire on my coil was loose. At first, I only had a miss at full throttle, but eventually it got worse. At first, both times, I thought is was a fuel (carb) problem.
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I'll double check them. I might try some different accelerator pump nozzles too.
 
Eliminate end shaft movement in distributor.
If there's any movement to speak of, harmonics at WOT will make ignition timing change as rotor and shaft reasonate and work up and down as they rotate.
This can happen because of the diagonal cut on the drive shaft for the oil pump and dist.
 
Darthomas said:
Eliminate end shaft movement in distributor.
If there's any movement to speak of, harmonics at WOT will make ignition timing change as rotor and shaft reasonate and work up and down as they rotate.
This can happen because of the diagonal cut on the drive shaft for the oil pump and dist.
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I pulled the distributor last night and it's nice and tight. I'll try the ECU with the spare I have. You don't think it's fuel related? I can maintain 5000 rpm at light throttle, but once I crack it all the way, it starts to stumble.
 
73Swinger18 said:
I pulled the distributor last night and it's nice and tight. I'll try the ECU with the spare I have. You don't think it's fuel related? I can maintain 5000 rpm at light throttle, but once I crack it all the way, it starts to stumble.
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Depends maybe on if you have vacuum advance. Timing plate swings on one pivot, off to the side, and if reluctor gap or points are set critically, the gap changes as soon as vacuum level drops when you go into it, and then you misfire. How the carb is ported, how much advance picks up in the canister, and the spring tension against the canister diaphragm all work together.
But I think your reluctor gap is too close or too loose.
 
Darthomas said:
Depends maybe on if you have vacuum advance. Timing plate swings on one pivot, off to the side, and if reluctor gap or points are set critically, the gap changes as soon as vacuum level drops when you go into it, and then you misfire. How the carb is ported, how much advance picks up in the canister, and the spring tension against the canister diaphragm all work together.
But I think your reluctor gap is too close or too loose.
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I disconnected the vac advance and it was the same. I did adjust the reluctor gap to the close side. I'll try opening it up a little as well.
 
Make sure the coil is the right one for the ignition. The wrong coil can cause a miss at higher RPMs. Ran into this problem with my mini-stock. The motor ran strong but started missing above 6500. Changed out the coil to one recommenced by the ignition manufacturer and it ran strong to 8000.
 
supertruck said:
Make sure the coil is the right one for the ignition. The wrong coil can cause a miss at higher RPMs. Ran into this problem with my mini-stock. The motor ran strong but started missing above 6500. Changed out the coil to one recommenced by the ignition manufacturer and it ran strong to 8000.
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It is a factory spec replacement. Same as NAPA's IC12, just the epoxy version. I will swap this one out with a known good Borg Warner I have in the trunk as well. It's going to rain here for a while, so may be a few days. Just want to get as many ideas before I start.
 
Dang, Had a bad set of plug wires on my truck once that would miss under load once the motor got warm.
 
340doc said:
Dang, Had a bad set of plug wires on my truck once that would miss under load once the motor got warm.
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I just put these wires on last week. Used looms to keep everything separated too. I just got off the phone with Holley tech support and they said it sounds like a rich condition when the secondaries open since it pops out the exhaust. My Holley Classic 600 has a secondary metering block without replaceable jets, so all I can do right now is try to bring the main jets down a bit and and the next stiffer secondary spring to see if the carb is the cause. I've got a whole list of things now.
 
That doesn't sound right to me. There's no way a 600 is too rich unless there's a major problem in there. Disconnect the secondary rod from the throttle shaft and wire the secondary's closed and go for a blast before you make any changes.
 
So to recap; Engine will rev to 5000 at part throttle, but not past 4500 at WOT.If slammed WOT at that 5000, then it stumbles and complains by popping in the exhaust pipes.
It's the same hot or cold. The wires,coil,and dizzy are good,and,the choke is known to be off.And it's like a switch.
It has 68MJs in the front with an as yet to be identified secondary plate.

So we know it IS NOT actually rpm related.And
we know that IT IS load related, right so far?

While the 68s might be a tad rich for that cam, I don't think they are the source.I think the 10" idle vacuum is telling you something. That cam is a 268/280/110 and IMO should idle with higher vacuum.
So I have three questions;
1) How old is the gas in it,what grade is it or was it, on the day it went in, and?
2) what is your TDC verified,all-in timing,not including the v-can advance, and?
3) How come you haven't told us what the plugs look like?

Depending on your answers to the above; I would guess that;
A) the plugs are toast, or
B) the gas is stale, or
C) you got too-much power-timing, for the octane gas you are using, or
D) the secondaries are slamming open a tad too fast,or
E) the secondary plate is too lean, and
F) Popping in the headers is usually an air leak into them, but at WOT it could be other
 
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AJ/FormS said:
So to recap; Engine will rev to 5000 at part throttle, but not past 4500 at WOT.If slammed WOT at that 5000, then it stumbles and complains by popping in the exhaust pipes.
It's the same hot or cold. The wires,coil,and dizzy are good,and,the choke is known to be off.And it's like a switch.
It has 68MJs in the front with an as yet to be identified secondary plate.

So we know it IS NOT actually rpm related.And
we know that IT IS load related, right so far?

While the 68s might be a tad rich for that cam, I don't think they are the source.I think the 10" idle vacuum is telling you something. That cam is a 268/280/110 and IMO should idle with higher vacuum.
So I have three questions;
1) How old is the gas in it,what grade is it or was it, on the day it went in, and?
2) what is your TDC verified,all-in timing,not including the v-can advance, and?
3) How come you haven't told us what the plugs look like?

Depending on your answers to the above; I would guess that;
A) the plugs are toast, or
B) the gas is stale, or
C) you got too-much power-timing, for the octane gas you are using, or
D) the secondaries are slamming open a tad too fast,or
E) the secondary plate is too lean, and
F) Popping in the headers is usually an air leak into them
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I just filled the tank prior to this happening. It was a brand new empty tank filled with 87. As you know, I recently installed a fuel return line and disconnected it thinking it may be causing it, but it didn't change it at all. The plugs have light coloring almost around the entire base ring and the porcelain is clean with no specks or other abnormalities. There is a definite color change at the bend of the ground electrode. I have not killed the engine at WOT and rolled to the side yet to read the plugs. Initial timing at 18 and total mechanical at 36 by 2800 RPM. I disconnected the vacuum advance and left disconnected while troubleshooting. Tried reducing initial timing to 10 and mechanical to 28 with no change. Secondary plate PN 34R9716-6 (0.070 hole). I'll check the headers. I recently removed the driver side for a power steering gear replacement.
 
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Ok now we're cooking. That cam,268/276/110 in at 106, and with a probable Scr of under 8.5, will have a Dcr of under 7.0 and less than 135psi cylinder pressure. That means fresh 87 will do just fine. It also means that it is a very low C/R engine and may not like 36*. But you tried 28* with no change. So you have ruled out the major players, items B and C.
So next I would be looking at D and E.
F was FYI only, and A is still on the table.
But the lack of coloring on the plug is more telling, so one more question,
4) what heatrange are your plugs and are they conventionals?
At this point I'm thinking very lean or overheated plugs, so, next; I would disable the secondaries (like Doc said in post 21)and retest, unless you tell me the plugs are too hot a heatrange, then it would be plugs first..

Oh and one more. Please don't tell me this just started after the last fill-up, or that it was fine on the previous tank,cuz that would throw a monkey wrench into my thinking.
 
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