high rpm miss

Hooked up to the sled for the first time Friday night took the motor up to 6500 and slipped the clutch out and got the tires spinning (everything is great at this point) next I give it some throttle and about the time the tires start to hook up and put some load on the motor I get a miss with some fire shooting out of my upright headers. It was also evident that it did have the hp it should. This motor sounds great without a load on it. I had a little time to start checking things,timing is @ 33 deg. @ 3,000 , the 2 gel cell batteries test out fine, the msd 6al is ok, the module in the dist is tests out good, checked the ohms on the spark plug wires there ok, plugs are just a tic on the lean side. I will spend some time on the fuel system today and see what I find, only bad thing is I wont be able to test it out under a load till the next pull so I hope to find something that sticks out to me. If anyone has ever experienced something like this or has a idea of the problem let me know. I will report back with anything I find. Thanks guys

I'm thinking that if this was a fuel delivery problem (going lean), you probably wouldn't have flames from the headers. That indicates plenty of fuel in the pipes. I'd look for ignition troubles. I don't know how you checked out the 6AL, but they can be tricky to diagnose. To really rule it out, I'd swap in a known good box for a test. BTW, have you added any retard functions to your system? What's your rev limiter rpm....and were you anywhere near that rpm?

Also, valve train problems (i.e., valve float) can give you this kind of problem. Are you sure your heads (springs) are correct and in good shape?

I will swap the msd box out and make sure (good idea) I do not run any retard functions on it, the limiter chip is a 8,500 and the most rpm I ran was 7,000 I like to run it 7800 to 8,000. The valve springs are brand new but I guess one never can tell if you dont check them yourself. thank you

Considering the revs you're seeing, the FIRST thing I'd check out is the valve train. If you're not 100% sure it's right, you're flirting with disaster. It sounds like someone else set up your heads for you and you have some doubts. Even if you find no problems in there, the peace of mind is priceless.

Then continue with your ignition system checks. Good luck to you.

Your right on the piece of mind deal I did set the heads up but I do not own a spring tester so I have always had the local shop test them out for me and they have never done me wrong, but there is always the first time. Its time for me to buy my own!!!!!

Is there fire out of both headers or just one side?

Do you have any video?

We chased a miss on my buddy's altered for almost a whole season, it just went away, we didn't know what fixed it. We replaced everything and it kept missing, then just poof! gone.

Bill

get rid of the 6al box. at that rpm they are not that reliable. yes, I know you probably have a lot of buddies that use it just fine but you need to step up to a 7 box. I had to make the jump to an 8 box but I have boost. mine with a digital 7 box would miss around 8500rpm. put the 8 box on perfect.

Your problem might be something else but you have a street ignition on a high rpm pulling engine. that is just asking for trouble later.

I have fire coming out of both pipes, and I guess I got to just keep replacing and testing everything till I get rid of it. I will step up to the 7 box Whatbumper I was thinking about upgrading anyhow but I have not because the 6 has not ever gave me a problem. It ran well last year and I didnt do any major changes to the motor just a fresh set of bearings , a kaase oil pump , new oil pan and valve springs. thanks for the ideas

My first guess would be (if not ignition) that the valve spring pressure isn't enough and your getting a bit of valve float. But if it lets you get the rpms up higher that wouldn't be it. Maybe your not burning up the A/F mixture fast enough at high rpms so its coming out the headers and you should run a hotter spark plug.

I may have found something that is at least contributing to the problem, I can move my rotor at least 5/8 of a inch back and forth when installed. pulled the dist. and found a lot of wear on the bronze gear. This gear has a some years on it, I hope this is at least part of the problem, new gear on the way.

I had a similar problem with a drag car a few years ago...It ended up being an intake gasket leaking.
It sounded great until you got up in the rpm with a load on it.....just something to check.
Good luck...
Jeff

on my MSD 6al box. the lead to the dizzy, the one male pin pushed out, even though it looked like it was still in. I found this by luck...had bought a new box, and found the problem when I started unplugging the leads....pushed it back in and fixed the problem. I had a similar problem to yours...the car would rev to the moon in park or neutral, soon as I put into gear and put a load on it it would miss.....check all your connections...just a thought!

Could be a lean miss. fatten it up a little and see what it does if the new gear doesn't fix it. Good Luck Brother

I thank you all for the input!!! I have implemented most all of the suggestions that were given, now I will find out Saturday night if it runs like it should, and hopefully I will will wonder what was the one thing that did it. It doesnt matter though as long as it runs well that is what really matters!

After many hours and dollars I have found the problem. Yesterday we a pull and I hooked up to the sled and same crap, backfire under load took it back to the trailer and put on a buddies old back up dominator and made one more hook at the end of the day. I took it up to 6500 and started rolling and ran down the track without a miss, fart, or burp!! Boy its nice to have that monkey off my back.

What's wrong with the carb. you had on there to begin with that makes it miss under load ?


Dan

Make sure you are using the right meetering block gaskets, I had a high r.p.m. miss, and I had three circuit gaskets on a two circuit carb, right gasket, miss was gone. just a thought..

I have not pulled the carb apart yet to find out but I will. The carb was rebuilt before last season started and ran great all year long, pulled it and the end of the season drained the fuel and bagged it up.

sounds like trash.that's what happened to maverick.