Need help tuning Quickfuel carb

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pjc360

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ive got a quickfuel super street series 680cfm vacuum secondary carburetor on my 300hp crate 360 magnum.
I’m running the Mopar Dual plane M1 intake and a 2 inch 4 hole carburetor spacer.
The issue I am having is a missfire that starts around 2500 rpm.
I’ve narrowed it down to cylinder 6 and cylinder 8.
At first I thought this was an ignition issue, I’ve replaced every last piece to the ignition and the issue still remains.
I’m rinning a brand new MSD Digital 6 box, a crane cams lx91 coil, brand new Taylor plug wires that have been ohmed for resistance and check out at 40 ohms per foot.
Brand new Ngk bkr5e plugs gapped at .040, a brand new firecore electronic vacuum advance distributor, new cap and rotor.
Timing is 18 initial and 34 total all in at 2500 rpm.
I’ve berified the accuracy of the timing
Mark on the balancer with a piston stop.
I’ve done a warm compression test on all 8 cylinders, I get 155psi across the board.
I’ve removed the passenger side valve cover and inspected the rocker arms and valve springs, no issues there everything is in tact and tight.
Since number 6 and number 8’s plugs are almost always carbon fouled I determined this has to be fuel related.
So I pulled the carb off and leaned everything out quite a bit.
I dropped the idle feed restrictors down from .031’s to .028’s.
Dropped the main jets down from 68s to 65s, dropped secondary jets down from 78 to 75.
I lowered the floats to the bottom of the sight glass. I replaced the accelerator pump cam with a white pump cam and replaced the squirter to a .028 from a .031.
All this seemed to help with the missfire a little bit but it didn’t solve it.
So I started calling around to different carburetor shops and ended up talking to a guy at AED performance carburetors.
He imidiatley asked me if I was running a carburetor spacer after I told him everything that I have found and what’s going on.
I said yes I’m running a 2 inch 4 hole spacer and he said to get rid of it for an open spacer because it sounds like a fuel distribution issue to him with how the plugs in cylinder 6 and cylinder 8 end up carbon fouled and the other 6 plugs look fine.
So I haven’t done that yet but I’m going too.
I was just wondering if anyone else had an idea for me.
I am at 3500 feet above sea level here in Montana, one thought I had is maybe I need to increase the size of the high speed air bleeds to delay the main circuit.
This thing idkes smooth and makes 20hg of idle vacuum, no missfires at idle, and it drives smooth below 2500.
Once you get to 2500 rpm it starts missing really bad, especially in 3rd gear, (automatic A-518)
So it seems when it has a load on it it’s just getting too much fuel and causing it to miss.
Idk what else to look at when the ignition is all brand new and everything checks out there.
And the compression in all 8 cylinders is great and I can’t find any broken parts in the valve train.
This has been stumping me for awhile now.
Carb is brand new too with less then 5k miles of run time on it and the engine is pretty new with only 30k miles on it.
 
Just going to throw some ideas out there:
Are 6 & 8 wires running parallel to each other for any distance?
Something wonky with the Digital box.
Maybe a bit too much timing at 2500, try it 32 at 3000 or 3500.

Go through everything on the carb. Check each bleed and e-hole has been drilled and there's no trash in any of the wells etc.
(unfortunately QC issues are common enough to make this a consideration; if tapatalks crappy servers are working, look in here for examples reported racingfuelsystems.myfunforum.org -)
Here's one: Q-850 metering block - racingfuelsystems.myfunforum.org

If its just the dynamics of the carb/engine and intake, you can experiment with some spivies on the boosters.
List-4668 - Page 5 - racingfuelsystems.myfunforum.org

Spivy - racingfuelsystems.myfunforum.org
 
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I’ve experimented with the timing, I’ve had it as low as 32 and as high as 36, I’ve tried different initial timing settings from 14-20.
I’ve had the carburetor completely apart and sprayed every passage out with carb cleaner and compressed air.
Never found anything un-usual.
I have noticed that some days it’s not as severe as other days which is really strange.
Another thing I noticed was my voltage regulator was going out and when the voltmeter in the truck would peg at 18v the missfire would get better.
I’ve also noticed that sometimes if I step on it really hard and then back off it and maintain a steady speed it will clear up for a little bit.
I don’t think it’s a vacuum leak, why would a vacuum leak be causing two plugs to carbon foul?
I’ve always been under the impression vacuum leaks make engine run leaner not richer, plus I have checked for vacuum leaks around the intake and carburetor and can’t find any.
My intake manifold gasket is fairly new maybe 5k miles on it and my carb gaskets are brand new cause I just had the carb off last weekend.
This msd box I’m running now is the second cd ignition box I’ve tried, originally I had a crane cams hi-6 cd box and I thought something may be wrong with it, so I bought this msd box and no change.
So this is the second ignition that’s been on the truck, I’ve literally tried swapping every last piece on the ignition And it hasn’t done anything at all.
I’m very certain it’s not ignition, it’s ether fuel or a mechanical issue in the heads.
But the good compression numbers are what’s throwing me off and the steady vacuum at idle.
Nothing has indicated an issue mechanical wise, I don’t hear any strange noises in the valve train, i don’t see anytning when I pull the valve covers, I’ve even let it idle with the valve covers off and everything appears to be moving as it should be.
This is a real stumper, I’ve been fighting this for months with no luck at all.
The only two other things I could do at this point is replace the heads or replace the carburetor.
I just don’t want to spend that kind of money without knowing for sure what is causing the problem.
Like I said this carburetor is brand new, very low miles on the carb maybe 5k miles.
A guy I’ve been talking to over on moparts is who steered me in the direction of an issue with my carburetor or my carburetor tune.
He only lives a couple of hours away from me and has a shop, I’m getting frustrated enough with this missfire that I Am seriously considering taking it up to him to check out.
Cause I’m just not seeing anything and it’s very frustrating.
I was thinking maybe a valve spring but I can’t see one that’s broken.
But the missfire is very prounounced like it misses hard and you can feel it in the cab of the truck through the floor and the steering wheel everytime the engine fires on the cylinders that are missing.
I mean it misses hard enough that for the longest time I thought it was a drivetrain issue or an engine balance issue.
But when I’m in park I can bring the rpm up to 2500 and it will start missing and you can hear it through the exhaust.
 
Everything except the fouling, sounds like a fuel supply issue. Which would lead me to agree with Teringer about the intake sucking a little oil. Tell us about your fuel system,from gas cap to carb.
 
I just installed a rebuilt 340 with a large cam and edlebrock heads in a Demon. It to had a misfire at engine speeds. We opened the hood outside the garage at night and saw sparks coming off of the carb secondary linkage. closer look and it looked like a three ring circus of sparks under the hood.

When installing the motor we installed a mechanical advance distributor and a MSD 6AL with a blaster II coil. We used the wires and cap that were originally on the car when it came in.

They were new Taylor wires . we replaced them with MSD wires from my Duster race motor and the miss was gone.

I always liked Taylor wires until I saw this with my own eyes. They cannot handle A hot coil and the multiple spark ignition. I put the Taylor wires on my stock 340 Duster with standard electronic and coil. No problems, No miss.
 
pjc360 said:
ive got a quickfuel super street series 680cfm vacuum secondary carburetor on my 300hp crate 360 magnum.
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Click to expand...


Magnum motors are notorious for misfiring without proper spark plug wire routing. There is even a TSB for it. Make sure your wires are routed as the Technical Service Bulletin specifies, check for vacuum leaks, verify engine condition and re-adjust your carb correctly.
 
One other thing you should know is changing air bleeds in the Quick Fuel carb is easier and has more effect than changing the jets.

Determining Power valve should be vacuum at idle divided by two. 12- 15 vacuum 6.5 power valve, 6-8 vacuum 3.5 power valve. under 6 is a 2.5 power valve. I have a box of power valves 6.5. That is what is usually in the quick fuels when new. Never worked out for me. but we usually need a lower number for big cams.
 
jbc426 said:
Magnum motors are notorious for misfiring without proper spark plug wire routing. There is even a TSB for it. Make sure your wires are routed as the Technical Service Bulletin specifies, check for vacuum leaks, verify engine condition and re-adjust your carb correctly.
Click to expand...
Wires don't know what motor they are on. Some wires are not non-conductive enough for high energy coils. If You got the right wires you should be able to run them side by side
 
Oldmanmopar said:
Wires don't know what motor they are on. Some wires are not non-conductive enough for high energy coils. If You got the right wires you should be able to run them side by side
Click to expand...

Tell that to the Mopar engineers who went to the trouble of issuing a TSB for the misfire problems on Magnum motors, big guy.
 
Oldmanmopar said:
Wires don't know what motor they are on. Some wires are not non-conductive enough for high energy coils. If You got the right wires you should be able to run them side by side
Click to expand...
THANK YOU, I had a hole bit in my tongue.
 
The wires are routed as far away as they can be and in their own individual looms.
Carburetor power valve is an 8.5hg, idles with 20hg of vacuum in park and Idles with 15hg of vacuum in gear.
I just checked for vacuum leaks again tonight and I can’t find any, I’ve sprayed carb cleaner along the sides of the intake and listened for a change in idle, no change at all.
Sprayed around the carburetor, no change in idle.
The plugs in cylinder 6 and cylinder 8 are never oil fouled, they are carbon fouled, too much fuel, they have a dry sooty black substance on them like the inside of a chimney would have.
Like I said I had spoke to a guy at AED performance carburetors and gave him all the information and he is blaming it on the 4 hole carburetor spacer causing a fuel distribution issue and suggested trying an open spacer.
I can get an open spacer for 20 bucks at my local part store so even though I am skeptical of it being the problem I’m going to give it a shot and see what happens.
I’m also going to wait till it’s pitch black out and fire it up and have my wife bring up the rpm’s to where it starts missing and watch for spark scatter around the plugs, because this is about the 3rd time I’ve been told the Taylor wires suck and leak spark around the boots causing missfires.
 
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Maybe look for a defect inside the distributor. If you're all in at 2500 and that is where the problem begins, maybe the weights are making contact with something. I have chased loose internal distributor parts on 2 different occasions and both times were a challenge.
 
Does your intake have the holes drilled at each end to accept the pins that are pressed into the front and rear china wall of the block or did you remove them?
 
The Demon that was here had the wires run perfect in moroso looms. You could barely see the arcing. My son saw the spark jumping while holding the throttle open on the carb. We fought this problem for a while, Even changed plugs thinking we had a bad set. It ended up being the wires. It was almost acting like a 2 step. Try the wires or put a stock coil on it.

This was a customers Demon. It was stuck here for a month until we figured it out. Everything was new under the hood. The only thing we didn't change was the wires because he just had the car tuned before we did the motor. It came here for a popping in the carb . It was the Cam wiped.

One thing led to another and the complete motor was done. We added the MSD ignition with the new motor. If we would have left the stock ignition in it the wires would have been fine. I put them on my Duster I am selling , It has a factory ignition . They were new dark blue taylor wires. Like I said my car does not have a problem with them.

If you have a dual plane intake it cannot be fuel . #6 and #8 are on a different plenum. I don't think it would be the 4 hole spacer. One other thing you could check is lifter pre-load. Oil pressure could be holding a valve open at a higher RPM if the pre-load is more then a half turn.
 
jbc426 said:
Tell that to the Mopar engineers who went to the trouble of issuing a TSB for the misfire problems on Magnum motors, big guy.
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What does the magnum motor have that a LA doesn't? Cheaper Mopar wires sold at the time of the install? I have been building motors and tuning them for a very very long time. Never did I see wires act different because they knew they were on a certain style motor.

Higher voltage coils cause wires to fail. If you can't afford good wires stay with a factory coil. All motors have non-conductive wire looms to stop them from rubbing through and allow the use of cheap wires. It is kind of hard to keep them from crossing paths at the distibutor with a firing order of 18436572. The same on all mopar V-8's. LA's and Magnums. The TSB for the misfire problem was for the wires running to the distributor stuffed under the firewall of the Rams. They had a hotter coil and cheap wires . When wet the wires would cross feed if to close. Been there.
 
dano said:
Does your intake have the holes drilled at each end to accept the pins that are pressed into the front and rear china wall of the block or did you remove them?
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This could be a problem. But most know that the wall separating the ports need to line up with the center of the Valve cover bolt. Scribing a line in the surface of the intake directly above the separator is a good way to make sure your intake is port matched. This should be done on all intake installs.

If using the pins tapping down on the intake after alignment is the way to mark where to drill them. This makes the intake specific to that motor. With no pins you can slide the intake front and back to get them closer before torquing if the alignment is off.

This would be the only other thing that would connect #6 and #8 together for his problem. A head gasket would show compression loss. Good call.
 
I think oldmanmopar is on to something with the wires.
What’s funny is I had this issue and I bought another set of Taylor’s to replace the ones I had, the same exact set wires in fact the Thundervolt 8.2mm Black Taylor’s.
The idle smoothed out for about a week when I put the brand new set of wires on and I’ve noticed it doesn’t idle as smooth anymore.
I’m going to watch it in the dark tonight while my wife holds the throttle where it starts missing and watch for spark scatter around the boots.
It really does feel like ignition, but I just crossed wires off the list cause they are only a few months old.
I used dielectric grease on the boots too maybe that’s adding to the issue?
The reason I think he could be right on it being the wires is number one it feels like ignition, it really does.
Number 2 the missfiring would clear up a little bit when my voltage regulator was going out and the voltage would red line at 18 on my gauge.
Everytime the voltage redlined it felt much better but was still slightly there then when the voltage regulator would start working again and the voltage would drop back down to 14 it would gt rough.
Number 3 this morning I decided to turn on the plug boots with my hands before leaving for work and I noticed it felt a little better on the way to work this morning.
Another reason is this carb is brand new and I think I was fouling the 6 and 8 plug because I had the floats too high, ever since I lowered the floats And leaned out the idle feed restrictors a little and the jets I haven’t fouled any plugs.
I am at 3500 feet above sea level so I know that affects the carb tune quite a bit and out of the box settings were a little too rich.
If I can see spark scatter then I’m going to call rick at firecore and get a set of his custom made wires, I’ve heard great things about his wires.
It’s not the distributor by the way I’ve had this distributor apart changing advance springs and everything is solid, this firecore distributor is actually a really nice piece too.
Plus I have a modified mopar distributor that Dan at fbo systems put together and I’ve installed that distributor and it still does it.
I have spare distributors and spare ignition boxes and coils because I’ve swapped so many things out trying to figure this out.
The one thing I haven’t changed brands inn is the plug wires.
I’ve always ran these Taylor’s and I’ve read a lot of reviews online of other people having missfiring fits with Taylor wires and spark scatter.
They don’t have a good reputation right now that’s for sure.
 
Oldmanmopar said:
What does the magnum motor have that a LA doesn't? Cheaper Mopar wires sold at the time of the install? I have been building motors and tuning them for a very very long time. Never did I see wires act different because they knew they were on a certain style motor.

Higher voltage coils cause wires to fail. If you can't afford good wires stay with a factory coil. All motors have non-conductive wire looms to stop them from rubbing through and allow the use of cheap wires. It is kind of hard to keep them from crossing paths at the distibutor with a firing order of 18436572. The same on all mopar V-8's. LA's and Magnums. The TSB for the misfire problem was for the wires running to the distributor stuffed under the firewall of the Rams. They had a hotter coil and cheap wires . When wet the wires would cross feed if to close. Been there.
Click to expand...

I have a factory JTEC that's been reflashed running my Magnum. The reason I mentioned the issue with the spark plug wire routing is that when I first got the motor running, my Matco Scanner was showing the computer detecting hundreds of misfires. Everything was brand new from rotor to plugs. It had fresh MSD wires, an aftermarket coil and the stock distributor with new electrical parts in it.

I had run the wires like I have done in the past on hundreds of different motors without issue, using wire separators and keeping them off anything hot. Never had an issue with a miss associated with wire routing before. I've had bad wires in the past that looked like a Christmas tree at night like mentioned above, but with my motor, I couldn't account for all these misfires my scanner was indicating.

I searched Google, and came up with the bulletin. Skeptical, I followed the TSB routing recommendation and rescanned the motor. All the misfires were gone after the rerouting recommended. I was surprised at the result, because I used to think of plug wire routing in a similar way as you mentioned. The difference from the way I had initially routed the wires and the way recommended in the bulletin was pretty subtle.

Here's a link to the bulletins from Magnacor's site. There are dozens more search results about the missing problems associated with these motors. Maybe the computer controlled EFI has something to do with it?

Magnacor has some expensive wires, but I've never used them. As they mention, better wires are less prone to the issue, but they still recommend following it, as it is likely prudent. Telling someone searching for a mystery miss and to discount this type of well-documented, easy fix, just because it doesn't matter what motor their wires are on just does not sound like prudent forum advice.

MAGNECOR Race Wires: Dodge "Magnum" technical bulletin

I'm just trying to help the guy with my actual experience. I know the wires don't know what motor they are on, but they do care. Ever seen how unhappy nice wires look on a Chevy or Ford?
 
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I appreciate the link, I will do my best to follow the reccomended route.
I might have to buy some more clips and holders.
I have heard of the issue you are describing and seen the tsb online.
I think your right in thinking the computer has a lot to do with it.
Because this was never an issue On the older stuff that I know of.
And my set up is copying the old style other then the msd box of course.
My engine is as simple as it gets, an alternator and a ps pump, an intake and carb and distributor.
No major electronics other then the msd box and no emissions crap anywhere.
No major vacuum lines other then the brake booster and the carbuetor and the vacuum advance.
I’m going to take the truck out in the dark and have my wife hold the throttle at 2500 and I’m going to watch the wires close around the boots.
I just bought these wires back in July or August so they should still be under warranty and I still have the original box and reciept.
If I see that they are spark scattering I’m calling summit and demanding a refund or calling Taylor directly which ever.
Then I will put that 80 dollars towards a better set of wires like Firecore 50s or magencors or moroso ultra 40s.
I believe the Taylor wires come with a 1 year warranty.
 
I don't know which Digital 6 you have, but here's a little info from the digital 6 plus.

SPARK PLUGS AND WIRES Spark plug wires are very important to the operation of your ignition system. A good quality, helically wound wire and proper routing are required to get the best performance from your ignition, such as the MSD HeliCore or 8.5mm Super Conductor Wire. Helically wound wires provide a good path for the spark to follow while keeping Electro Magnetic Interference (EMI) to a minimum. Excessive EMI, such as the amount that SOLID CORE wires produce, will interfere with the operation of the MSD. SOLID CORE spark plug wires cannot be used with an MSD Ignition.

You might also want to try a basic plug gap, like .035"-.036" and go bigger from there.
Hope this helps.
 
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Oldmanmopar said:
One other thing you should know is changing air bleeds in the Quick Fuel carb is easier and has more effect than changing the jets.

Determining Power valve should be vacuum at idle divided by two. 12- 15 vacuum 6.5 power valve, 6-8 vacuum 3.5 power valve. under 6 is a 2.5 power valve. I have a box of power valves 6.5. That is what is usually in the quick fuels when new. Never worked out for me. but we usually need a lower number for big cams.
Click to expand...


This was wrong 45 years ago, and it's still wrong now.

Passing along bad information for decades won't make people any better at tuning.

The power valve doesn't affect anything at idle, unless the valve is defective. If you set the power valve open at half of idle vacuum you will have to crutch the tune up to cover the hole in the fuel curve on tip in.

Set the power valve opening a couple of numbers below cruise vacuum. Mark Whitner posted a great video on you tube of power valve opening at idle and proves the long told tale of half idle vacuum dead wrong.
 
Those two cylinders on a dual plane get fed from opposite sides of the plenum. So, it's likely not a carb issue.

Sounds like a sealing issue at the manifold/head. That's where I would concentrate. Are the plugs fuel fouled or oil fouled?

You could also swap some wires from the non fouling plugs and see if the issue resolves itself on 6 & 8. Route the 2 wires far away from each other to minimize potential crossfire/jumping spark.

Change one thing at a time and observe the results.
 
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