I recently installed a 355 in my chevelle. 12.65:1 compression, .574 , .597 lift @ 248 duration, msd 6AL w/ blaster2 coil, 750 mighty demon, GM HEI that has been recurved so I'm running 18degrees initial. It should be at least 450 hp and I'm using a 3500 stall convertor and a th350. In park or nuetral the motor will rev through the rpm range with no problem, while driving the car above 3500 I pick up a bad miss. I can also power brake to 3500 and it still misses beyong that point. Put it back in park /nuetral and it will rev to 7000 no problem. ANY SUGGESTIONS?

Ive heard that stock HEI ignition modules are only good to around 4-4500 RPM(under load)
MSD makes a direct replacement that lets the thing rev higher. MSD (http://msdignition.com/)
Type 'ignition modules' into the search box.
I know you can buy the pictured module separate for around 60-70 bucks.

I had this same problem, above 3500 missing badly. It turned out I had some bad engine grounds. You might also want to check the vacuum advance canister. I had a bad one and it caused some missing under a load as well. Hope this may help.

Check your valve springs and lash

What rev chip is in the MSD unit?
The Stock HEI module is fine, as long as it's a GM part. In fact I was going to offer some diagnostic tips till I re read that you MSD-ed the whole thing now you are going to have to rely on the tech line Or others here who are more knowledgeable than I am.
my 2 cents is that you get a good HEI distributor from the junkyard and at least stab it in for testing, I strongly dislike any HEI replacement ignition systems. A properly operating HEI will operate ANY normally aspirated gasoline motor. PERIOD I run one in my 13.6:1 bb. THEY Work Period. Those wonderfull Red boxes do nothing more than complicate tuning and diagnosis!!
My 2 Cents again!!
Other than that how many miles on the motor?
Possible cam going flat?
good luck
Milan

I like to start with the easy stuff when it comes to a mid to high rpm miss under a load only with no misfire when not loaded in neutral.

When i have encountered a misfire under a load at mid to higher rpm's 9 times out of ten one or a mixture of the following have been the problem.

I will leave out you'r stock GM HEI for now because they will rpm fine for your setup and i have never had one only misfire under a load because they don't to seem to act up in that way when they are bad so i am assuming it's good at this time.

* fuel/carbon fouled plugs (they can be new but if your running too rich or have any oil issues etc they can get dirty/partially fouled very quickly and misfire under a load)

* lean cond that caused the plugs to get those little spotty particles mealted into the porcilen on the plugs which will cause mosfires.

* bad plug wire connection/s or 1 or more bad wires. (this is esp a problem with the cheaper
carbon wires when used with headers so don't rule out a fairly fresh set of wires being bad either.)

* cracked and or loose dist cap/rotor.

* bad coil.

* if it's the GM HEI with the coil in the cap look for a melted cap or rotor. (I have seen high output coils in this application burn/melt through the cap/rotor and cause misifre and if contineued to run eventually die altogether as more miles pile up.)

* float level too low or fuel pump not up to the task when under a load. (If suspected carte rmakes a nice street/strip pump and also run the lighter hollow pump rod for added insurance to eliminate any chance for pump rod float at higher rpms. They are recommended for 6k+ rpm's but i have found them usefull/needed at above 5k rpm's in some setups so keep that in mind.)

* weak/broken valve springs.

* if hyd cam possibly you have way too much preload. (even with a full 1 turn past zero lash i have easily turned 5k rpms + with any sbc/bbc with stock hyd lifters with a mild perf cam so check you'r valve adj. I would suggest 1/4-1/2 turn past zero lash for a hyd perf cam,some people use 1/8 turn past zero lash but that will often give you some lifter noise at startup.)

If all the above checks out ok then get back to us here on T/C so we can get deeper into helping you solve your problem.

Let us know how you make out.

Scott

I have replaced the module inside the dist,with I beleive to be a 2-pin module. This whole ignition was working fine on the previous engine which was a ZZ4. The only thing I've done is have it recurved. The engine has only about 300 miles on it, I really think all od the internals are in good shape. The only thing I haven't rechecked since the install are the grounds, and I've been told that may be the problem. I'll try that tonight. Thanks!

Scott, Thanks for the reply. I had the plugs out and they didn't look bad, I cleaned them and put them back in. A new msd cap and rotor, blue max wires, same result. I am running very cold plugs since I was running NO2, although I haven't with this motor yet. I'll put an older set of hotter plugs in and see how they work. The last few caps I've had on, when removed, had more wear on some of the terminals than others. I verified all of my connections, and everything seemed o.k.

One more sugestion,you stated the dist worked fine in the old motor but you did since have it recurved before placing it in your current motor.

This is a long shot but you say that the misfire happens at approx 3k-3500 rpm's and up so i wonder if your distributor phasing off. This can cause a weak/unstable ignition at higher rpms esp when the full mech anvance cuts in at appro 3k+ rpm's. This is becasue the contact on the rotor mabe be just on the edge of missing the contacts in the distributor and then when the full mech advance cuts in the rotor's position is pushed past the wire terminales in the cap thus causing a misifre due to a weaker ignition having to arc over the each dist terminal to fire each plug thus lowering the juice to each plug at that time which will show up under a load and not when in neutral as you stated so check this out too. This could also cause the spark from the rotor to ark over to the wrong terminal/s at times making it backfire etc.


Scott