400 Big Block High Speed Misfire
Posted September 10, 2011 at 07:00 pm
-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
-.030 over 400
-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.
Posted September 11, 2011 at 12:55 am
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.
Posted September 11, 2011 at 08:11 am
Posted September 11, 2011 at 02:00 pm
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.
Posted September 12, 2011 at 05:57 pm
Posted September 12, 2011 at 10:02 pm
Posted September 14, 2011 at 06:30 pm
Posted September 14, 2011 at 08:12 pm
Posted September 14, 2011 at 09:20 pm
Posted September 15, 2011 at 12:52 am
Posted September 15, 2011 at 09:17 am
Posted September 15, 2011 at 11:11 am
Posted September 15, 2011 at 12:45 pm
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.
Posted September 15, 2011 at 03:12 pm
Secondary metering numbering system is different than primaries. Start with adjusting the speed (slow it down) at which the secondaries open first.
Posted September 15, 2011 at 07:25 pm
Posted September 15, 2011 at 10:09 pm
Posted September 16, 2011 at 06:35 pm
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?
Posted September 16, 2011 at 06:51 pm
Posted September 16, 2011 at 11:07 pm
How can I tell if I should fatten up the secondaries? Would allowing in more fuel allow them to open earlier without running lean?