Hi all-

First post hope its in the correct place.
My 68 400 began to develop a light mis/ surge only at light to mid throttle and only once warmed up.  If I leaned into it, she would pull nicely wthout a mis.  Thought it was maybe runnning a bit lean in the primaries.  But then last night after an easy 60 mile ride it began to really mis/ surge.  barely made it home unless I reaaly drove it hard, and then it seemed to pull well.  Checked for spark with the engine running by pulling the wire at the plug and found that cylinders 3,5 on drivers side are getting spark (i can hear the "crack" when i ground it), but are not firing.  The pass. side is difficult to see b/c it a 68 Firebird with air, but by pulling the wires off at the cap i can tell 2 other cylinders are not firing not sure which though--getting dark now.  
68 WT with 16x and 5k miles, RAIII cam, HEI conversion, old 750 vac sec Holly on a Performer manif.
car did run hot today 200-210 with a 230 spike fro 10 mins in traffic. No water in oil or oil in water.  Timing is 12 BTDC.  NO noises under hood.
Head gasket on both sides?  Bad valves?
If I do a compression test how do I tell what is wrong?
Thanks in advance.
Rich  

You and I have same problems. 70 400 in an 81 *** ebrock perf intake, carb, hei, 6x heads. I think mine is an intake leak. 3 & 5, 2 & 4 plugs are darker. Mine runs hot too. Stumbles under acceleration until about 3000 rpm, then comes to life. If the leak is real bad you can find it with a propane torch. Start the car and open the gas around the intake gasket. Idle will bump when you find where the leak is. Just like with ether, just a little safer. Still use caution.  

Thanks so much!  Sounds like the same issues.  Cant wait to try it this weekend.  I was thinking head gasket   Hope its just intake manifold gasket!!  Ill let you know.  Thanks again
Rich  

Guys, common issue with Edelbrock Performer and square pattern Holleys is the area where the secondaries of the manifold widen out leaves very little material left for carb gasket to seal. Buy a 1/16" carb spacer plate and extra gaskets, mount it between the carb and manifold. The carb will seal to the plate and the plate will seal to the manifold, no more vacuum leaks!
Steve  

Tried the propane trick, but unable to notice a difference in idle quality or speed.  
If there is a vac leak by the secondaries, wouldnt ALL cylinders have a problem, not just the center four??  As i typed that I wonder if the center four on the manifold are lined up exactly under the secondaries.  i will look into this!

Rich  

Hello,

I also have a 76 400 with 6-8 heads, holly street dominator intake, holly 750 dual feed, and RA3 type cam grind. Engine is newly rebuilt and has had a similar light throttle intermittent mis-fire since I started it. Swapped out HEI distributor, wires, and plugs. Also dialed cam and replaced intake gaskets. Checked compression, 140 on all cyl. Engine pulls great WOT and plugs appear good. I have often thought of vac leak. I am going to look into carb to manifold seal. Thanks in advance for any advice and I will keep in touch.

Dave  

Barneyformula-  You the man!!!

listen up all you mis fire people.  Try the simple stuff first.  Listen to Barneyformula.  Compression for me was 165-185 all cyl with 16x heads, RAIII grind... so i was rlieved.  Just snugged up the 4 bolts on the old worn Holly and PRESTO!!  No more light throttle miss.

Time for a full rebuild on the old Holly after the season.

Thanks again all!

Rich  

Rich, glad I could help, I learned that one way back in the early 90's, forgot it, and had to relearn the same problem last year when I put a freshly rebuilt Q-jet on my factory 4bbl/egr intake. The base gasket in the kit was wrong and there was only 1/32-1/16" overlap where the manifold is milled out around the primaries for the egr. It ran like crap until I got the right gasket, now it idles great and has no hesitation at all.
Steve  

My cam was advanced by a tooth and my vac advance was giving me over 25 degrees of advance. After correcting cam and replacing and adjusting new vac advance for 14 degrees the mis-fire is gone. I,m still not real happy with low rpm response, I,m thinking a dual-plane manifold is what I should be running.

Dave  

Dave, with the relatively low c.r of your 400 and 6X-8 heads(7.6-8.0:1 cr) and the RA3 cam (either 068 or 744) you would be better off using either a stock 4bbl intake or an Edelbrock Performer dual plane (not the RPM). You will get far more low speed throttle response and likely better idle and mileage to boot.
Steve  

we almost have the same situation, got misfiring extremely bad on my front four cylinders after driving it for hours. then got new firdbird parts like new plugs and an inlet manifold but nothing worked out. I was assuming of it to be an egr valve problem.