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400 Big Block High Speed Misfire


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52 replies to this topic

#1 kzooman83

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Posted September 10, 2011 at 07:00 pm

I am having a problem with the 400 big block in my Duster; it misfires pretty badly beginning about 3500-4000 rpm, and I am about out of ideas.

-It misfires worse under light throttle; at full throttle it only misses a little or not at all
-the misfire goes away when I disconnect the secondaries so they do not open; no change in off the line performance with the secondaries disconnected.
-I have run as little as 28 degrees total timing and as much as 38 degrees total timing with no change in the misfire

Engine specs:

-.030 over 400
-9:1 compression
-comp xe 275 cam 275/287 duration, .525/525 lift
-edelbrock performer RPM intake manifold
-Holley 750 vacuum secondary carb
-mopar electronic ignition, no vacuum advance (it is capped off, and not hooked up to a vacuum source)

I am just wondering if anyone can point me in the right direction. I am thinking about taking apart the carb to check for a problem but, truthfully, I don't really know what the problem is at this point.

#2 dana44

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Posted September 11, 2011 at 12:55 am

Is this something that just started, or has it been going on since the carb was put on the engine?

If it goes away when the secondaries were disconntected, it sounds like a lean condition with the secondaries, meaning one of two things. Something is in the secondary fuel plate that is plugging one or both the bleed holes up, or, if it has been pretty consistent since putting the carb on, adjust the secondary spring to a heavier spring to open slower (a spring kit from Holley is color coated soft to stiffer so you can tell which one to go to compared to the one you have in there now). Also, on some occasions, the secondary metering (fuel plate) gasket itself can shrink and deform, causing fuel not to deliver properly.

Key clue to look at is it goes away with the secondaries locked down and disconnected.

#3 kzooman83

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Posted September 11, 2011 at 08:11 am

It has come and gone periodically in the past, but lately the misfire has been pretty consistent. I am thinking that whatever the cause is, maybe it is getting worse? The carb was rebuilt at the same time as the engine, which was probably 4 years ago now; I went with a billet specialties vacuum diaphragm housing for the carb when I rebuilt it. This setup allows you to adjust the opening point of the secondaries without changing the spring; there is a screw-in adjustment needle (like the ones for the idle) which controls airflow through the vacuum passage to the diaphragm, so instead of changing the spring, you just turn the screw further in or further out. I do have a stock holley diaphragm housing that I can swap on, however, if I need to.

#4 dana44

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Posted September 11, 2011 at 02:00 pm

OK, that's good to know, and it is a whole lot simpler that the springs, I admit.

Now, do you know what the metering plates are in the back of the carb? there may be a piece of junk floating around that is plugging them, or is able to float around and change the running, like something that could float and alter its blocking of the secondary metering plate fuel bleeds, or gasket that has warped or buckled, fuel gets behind the bubble in the gasket and starts blocking it a little bit, making it intermittent and non-consistent in performance. You already islolated the location of the miss, it is a high speed secondary lean condition, gotta take her apart and see if you can specifically identify what is causing it, whether gasket or debris. Check to make sure the air bleeds in the top of the secondaries aren't plugged, too. They are the little brass plugs in the top of the carb (well, some are brass, but they are the little pinholes in the tops near the outer edge of the emulsifier tubes), When these get plugged they usually will run lean.

#5 kzooman83

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Posted September 12, 2011 at 05:57 pm

I took the carb apart today and used a wire and some carb and choke spray to clean out all the passages in it, including the air bleeds and booster venturi. I also checked the gaskets and the secondary diaphragm and accelerator pump diaphragm. I did not see anything that was very obviously not right, but the carb cleaner flows easily through all the passages now and the gaskets and diaphragms are all ok. I will put it back on the car and see what happens.

#6 dana44

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Posted September 12, 2011 at 10:02 pm

Make sure you also have a good clear passage for the vacuum signal from the base of the carb, gasket clear of its pickup location, gasket thick enough that change in temp and stuff like that isn't any possibility of causing signal fluctuation.

#7 kzooman83

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Posted September 14, 2011 at 06:30 pm

Got the carb back on the car today; it did the same thing. I then swapped on a stock Holley secondary diaphragm housing and the misfire is much better; only noticeable at light throttle and only at about 4000 rpm. Maybe I should stick with the stock housing with a slightly heavier spring? Or should the secondaries be all the way open at 4000 anyway?

#8 dana44

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Posted September 14, 2011 at 08:12 pm

The amount of opening is determined by the engine needs, not the rpm of the engine. You could actually be turning 5000rpm and the secondaries barely cracked, it is vacuum driven. I think your secondary meteringplate is too lean, so a heavier spring should help the situation, so in essence the secondaries are opening more than the amount of fuel available, shutting it down richens it up, heavier spring does that by reducing air to fuel suction volume. At least you proved it wasn't because of mechanical gasket or the likes causing it.

#9 mountainrich

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Posted September 14, 2011 at 08:19 pm

What kind of Mopar ignition?

#10 dana44

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Posted September 14, 2011 at 09:20 pm

You can go with stock to save money, it is a good system, well proven. Aftermarket Unilite Mallory which fits right in the stock distributor and has three wires is nice, Accel, Mallory billet distributor, heck, even Pertronics has been proven to be reliable,

#11 kzooman83

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Posted September 15, 2011 at 12:52 am

I am running a Mopar Performance electronic ignition with the chrome high rpm ECU and a dual ballast resistor. The distributor is the Mopar Performance unit that goes with the mopar ignition; it has an adjustable centrifugal advance plate and vacuum advance canister. Currently, I am running about 18 degrees initial advance with total mechanical advance at 34, all in by 3000, and no vacuum advance; canister is disconnected and capped off.

#12 mountainrich

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Posted September 15, 2011 at 09:17 am

Maybe try heavier spring for centrifical, (to lengthen ramp time to max advance), and use the vacuum advance part of dizzy. If engine's going lean w/ secondaries, vacuum's probably dropping quickly w/ secondaries. W/ vacuum advance hooked up, it'll see the vacuum drop also and retard timing a bit.

#13 dana44

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Posted September 15, 2011 at 11:11 am

Total advance at 34 is good, initial at 18 may be a little bit high, 14 is about max for best results even though it may feel good. No, I don't think the problem is the advance or anything like that, you seem to have too much pull for the amount of fuel you are getting when the secondaries open. There are different sized metering plate volumes, they are the tiny holes at the bottom edge pointing down on the metering plates. See how the power is with a stiffer spring, or slower opening of the secondaries. If it is adjustable (the one you have), make it open slower, and if the power drops off too much, need a larger secondary metering plate to make up for the lean condition that is occuring. You have proven it is not the gaskets, that it can be cured by keeping the secondaries closed, so the two logical things are, slow the vacuum signal to reduce secondary opening, (lean condition is a miss condition, not enough fuel to the amount of air), and if more power is needed, fatten the metering plate up to compensate. If it really is leaning out this much, you would want it fatter so things don't get too hot and melt anyway. Would hate to see you break something from running lean at high rpm. You may want to look at the Holley website, I think there is actually a metering plate that has the ability to change the jetting on the plate. And also remember, the heavier springs in the secondary are by design designed to correct this exact problem. A low performance engine would simply have this problem as a stumble, a higher performance engine (your cam) would turn it into a misfire because more power is made instead of a stumble with a lower compression engine (9.5 compared to 8.2)

#14 kzooman83

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Posted September 15, 2011 at 12:45 pm

I will try opening the secondaries a little later tomorrow, and see if that helps without killing the power. If not, I know that Holley sells a metering block kit for this carb; maybe I will look into that. If I am running 72 jets on the primary side, what should I start with on the secondary side?

Also, I am running that much initial advance because the engine is weak below 1500 with any less; low compression with a big cam y'know....this was a recommendation from a local speed shop.

#15 dana44

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Posted September 15, 2011 at 03:12 pm

What you want to do is slow DOWN the secondaries opening, not opening faster, keep the vacuum up to pull the fuel into the carb, the high speed miss is a lean condition that the secondaries are opening too fast, vacuum pull drops and the needed fuel is too little. Slowing down the secondaries will make the engine pull the fuel harder.

Secondary metering numbering system is different than primaries. Start with adjusting the speed (slow it down) at which the secondaries open first.

#16 mountainrich

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Posted September 15, 2011 at 07:25 pm

Dana- "(timing) all in by 3000" is a very quick ramp.

#17 dana44

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Posted September 15, 2011 at 10:09 pm

True, but if it is working without pinging, then it is OK, he is going to 34degrees total, which is probably some of the saving grace, and at the same time it has nothing to do with the lean condition with the carb at that rpm, being above 4000rpm, it should be maximum advance and not alter the fact that the lean condition is happening from the engine at that point be pulling max vacuum and more than the carb can handle (more likely more air than the amount of fuel it can pull, so, the reason there are adjustment springs is to prevent this from happening, he just has too light a spring right now, period).

#18 kzooman83

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Posted September 16, 2011 at 06:35 pm

I put the quick fuel diaphragm housing back on the car today and did some testing. I found that with the adjustment screw on the housing 1/8 turn open, the misfire went away. Power seems to be okay; it pulls hard up to about 5500 then starts dropping off a little. The cam is supposed to pull to 6000, but I think the stock manifolds and only 2 1/2" pipes might be limiting it a bit.

Unfortunately, the screw will not stay where I put it. I set the thing a 1/8 turn, and after a lap around the block, it was open more like 3/4 turn. The screw working its way out could be what's causing the problem, since it then allows the secondaries to open sooner.

I think I am going to pickup a quick change cap for the stock holley diaphragm housing and try some different springs in it, rather than keep going with the Quick Fuel piece.

How do I tell if the secondaries are opening fully?

#19 dana44

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Posted September 16, 2011 at 06:51 pm

If you take the throttle and open it all the way and then mechanically ensure you can physically open them all the way, then with them still open, close the throttle and make sure they aren't sticking. Vacuum operated secondaries open the amount the engine wants, not necessarily all the way all the time, it is engine demand opening. You could put a drop of Loktite on the thread, or try teflon tape, stay away from the tip two or three threads to tighten it up.

#20 kzooman83

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Posted September 16, 2011 at 11:07 pm

I hadn't thought about teflon tape...I think I will try that before buying any other parts.

How can I tell if I should fatten up the secondaries? Would allowing in more fuel allow them to open earlier without running lean?


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