I’ve been chasing my tail all season on what sounds like a misfire going down the track. Sometime it happens some and clears up, sometimes it is more severe, and sometimes it doesn’t do it at all. I’ve replaced the pickup, coil, wires, and plugs, cap doesn’t look burnt inside, rotor looks fine.

I was thinking for a while it was tires or suspension setup, but this weekend I backed timing down from 36* to 28* which killed a tenth just in the 60 and made the 3000 lb car go 6.40s. I just don’t see spinning being the issue anymore.

Does anyone have any thoughts??? I’m lost here!

What is different/new about the combo this season compared to last? Describe exactly what it sounds like, feels like, at what rpm, in what gear and where on the track.

Also what carb, what jetting.

quote:

Originally posted by 1320racer:
What is different/new about the combo this season compared to last? Describe exactly what it sounds like, feels like, at what rpm, in what gear and where on the track.

Also what carb, what jetting.

Only thing different this year vs last is plugs and wires (same brand/head range plugs). If you have ever heard a normal street Harley hit the rev limiter that’s about the speed of the miss (it’s not as fast as a two step sound). Happens intermittently after launch maybe 5500 rpm and up in both gears.

It happens less at a good track vs a mediocre track but it still does happen (usually in high gear only at a good track)

468” bbc with iron 990 heads
670 lift cam
950 alky KJP carb with 148 jets (although I think it needs to go even smaller it doesn’t build heat during the run)

14.5w x 32 Tires feel good and soft. They get sticky.
Ladder bars with strange single adjustable shocks on the rear set at 5 clicks from tight (10 settings total). Front has qa1 race Rs set at 0.

The only thing I can think of left with suspension is the rear springs being too heavy. Previous owner had an aluminum small block in it. And I suspect it had 350 lbs less on the nose than my iron headed bbc.

Seems like week to week I go back and forth between possibility of tires/suspension to thinking it’s ignition issues.

What brand and heat range plugs? Too much alky will show as a miss down track from my junior dragster days and pig rich will shut the engine off.

Ngk plugs 8 on the heat range

Just about every miss I’ve ever had has ended up being a fuel/air issue after spending a lot of time looking at ignition parts.

A power valve that was too strong sounded pretty close to going on the 2-step.

Also has a small crack in the intake manifold causing a lean condition at high RPM.

quote:

Originally posted by Lenny5160:
Just about every miss I’ve ever had has ended up being a fuel/air issue after spending a lot of time looking at ignition parts.

A power valve that was too strong sounded pretty close to going on the 2-step.

Also has a small crack in the intake manifold causing a lean condition at high RPM.

I’ll check for intake leaks today. The metering blocks on this carb don’t have a power valve in them.

The hottest plug I would run is a -9. Many on alky are running a -10

quote:

Originally posted by Holytown:

Only thing different this year vs last is plugs and wires (same brand/head range plugs).

I’d try putting your old wires back on and another new set of plugs.

what kind of ignition do you have on it? I got burned by a "weak" MSD 7al2, it seemed to work fine on the ignition tester but would miss sometimes going down track. what i didn't notice was the spark while on the tester was white not blue. If you can borrow or have a spare box, might want to swap it out for a test.

Joe

suggestions;

*The fact that the miss developed after changing plug wire raises a flag. Did you keep the old set, and could you reinstall them for a test? Where the wires terminated at the factory, or did you terminate one end? It is possible to make a mistake with the terminals unless you have the proper equipment.

About jetting, as a reference, I have a BLP 4150 series rated a 960 CFM that is jetted at .166

First thing I would do is to change out the ignition box with a known good one. I have had 3 of the 7AL boxes go bad through the years and all three tested good with the MSD ignition tester. I have several spare boxes now and if I have any issues will change it out first before I start on a wild goose chase. Note, if you call MSD and ask them they will tell you the box is good and to basically change out everything BUT the box. One of my boxes that failed sounds like it did something similar to yours. It would start and run great but as RPM got higher going down the track it started missing very light and just seemed like it did not want to go faster. Another box I had failed car would run 5.04 clean and nice, next pass 5.49 banging and popping. I went through same deal you are changing everything and calling MSD and they kept saying box is good, blah, blah, and led me on the wild goose chase. Do not fall in that trap, change the box first, if by chance that does not fix it then I would start looking elsewhere.

Do not think you said what box you have but I would change it out first. Then you need to verify that it has good power to the box. Garbage in Garbage out. If you do not have enough good clean power to the box, coil etc. it will get worse as RPM goes up. I think it requires around 9? volts minimum but you really want 12+ volts or more all the way down the track.

From my experience missing or popping it usually ignition related. If it is a fuel issue it will often just lay over. If your fuel system is not right the ignition has to work harder to fire it so it may show up ignition problems quicker.

quote:

Originally posted by Holytown:
I’ve been chasing my tail all season on what sounds like a misfire going down the track. Sometime it happens some and clears up, sometimes it is more severe, and sometimes it doesn’t do it at all.

Does anyone have any thoughts??? I’m lost here!

You stated you have a crank trigger. Make sure the cable to crank trigger is not bundled with other wires causing EMI. Separate all the MSD wiring from your other wiring.

I'll bet it's an electric fuel pump and the tank is in the back, on methanol.

One of the fittings is bad, and/ or it has china garbage black fuel lines.

Alky is harder to light than gas, you need a good ignition...Ed mentioned heat range on plugs....I'm running a -9 on my stuff, 16.5:1 compression....and alky seems to like more timing on most combos...Mine wanted 4 more degrees than it did on gas. Make sure you're getting plenty of fuel to the carb too.....

I chased miss for 2 years on new motor. Ended up being new defective weak valve springs. Put new set in, no more issues.

Did you put a second set of wires on? You said the only thing different from last year was plugs and wires. You also stated all the things you have changed and it included wires. If you aren’t on a 3rd set I would absolutely revert to your original wires as a cheap test before throwing a ton of money at it.

Couple things here.
It could be valve springs.
I agree with running a hotter plug.
If only thing you changed was wires then like CURTIS said put the old ones back on to see. I do think that good plug wires are critical and I always run the Smileys Racing which are Moroso Blue Max Ultra wires with Smileys name and $90 cheaper. They even come in same box.

From my experience my money is still on bad ignition box. Especially if it is a 7-AL.

Thanks for all of the replies. I have been fighting this issue on and off for a while. The reason for changing the wires over winter was due to the miss. I have been ohming the summit brand, JEGS brand, and a couple msd. The engine came mixed and matched. The summit I bought over winter. Anyway. The summit brand was supposed to be 50 ohms per foot. Turns out they’re 230 or so per foot. Same with the JEGS wires.

Msd were 50 like they were supposed to be. 7al2 box btw

Could someone explain to me why the colder plug would be better? I don’t understand plug heat ranges. Just used what I was told and am trying to learn.

Thanks for everyone’s replies!

This message has been edited. Last edited by: Holytown,

I’ve experienced the same issue and chased everything. For me it was an issue in the distributor cap, allowing arcing to jump around at higher RPMs. Vent was open however o removed the little plastic tab and also drilled a 1/4 inch hole to allow extra venting. Problem went away. This was on a MSD crank trigger distributor, sbc.