[JOHN WILSON]

Guys, I need to get some thoughts to help make sure I'm not overlooking something obvious. My motor seems to have developed a high rpm miss. It happens at ~6800rpm. It will sometimes occur in 2nd gear, but always happens in 3rd gear just past the 1000' mark.

Looking back, its possibly been going on some time. On previous track trips the car would lay down just a hair going thru the traps @71-7200. I can feel the front end drop down just a bit and the motor feels like its on the rev limiter.

Last night it started showing up in 2nd gear on 3 out of 6 passes, and in 3rd gear on all the passes.

It also did this last week on the chassis dyno during the last pull. It didn't occur till right at 7k and was just a blip just before I let off. On the 2nd out of 4 pulls the motor ran clean to 7300 without a problem with only a few hp drop from its peak @6900.

My first thoughts are:
1-Valve springs
2-ignition (wires, rotor, cap, coil)

I don't believe its fuel related. Today I checked fuel flow out of each side of the regulator independently and together. Both hoses together will push out 2 gallons of fuel in 20 seconds. Each side alone will push 1.75 gals in 20 seconds. So, there appears to be plenty of fuel.

Next- I plan to check seat pressure on the springs. If nothing turns up I'll move on to the ignition. The wires, cap, rotor, and coil are all around 3yrs old.

Am I overlooking anything simple or obvious that you can see???

TIA.

 

[Bob West]

MSD boxes sometimes do strange things????? of course thats ignition and you havent got there yet.

 

[BB485]

I had a the same problem a few years back. Put new plugs in and a closer gap 030 fixed the high rpm problem

 

[70_FathomBlueMalibu]

Well, there would be no load on the engine/fuel pump while checking it with the engine off. In high gear they would see the greatest amount of load. So, I wouldn't completely discount it as being fuel yet. Also, do you have any rubber fuel line that could be collapsing?

plugs
wires
cap
rotor
coil
ignition box
fuel pump
fuel pump regulator
fuel lines
carb
springs
lash


Obviously, check and change the cheapest things first, before spending any unnecessary cash. I'm sure you'll figure it out.

Keep us informed.

 

[JOHN WILSON]

Thanks for the replys.

Bob- I'm hoping its not the box, but its on the list.

BB485- The plugs were new, installed Friday night, gapped at .045".

Justin, would checking the fuel flow with the bowls attached to the hose ends better simulate a "loaded" condition. I was actually thinking the flow would go up if I were able to check it with the alternator charging rather than just battery only. If checking with the bowls attached is better, what kind of volume/time should I be looking for as opposed to free flow #'s. Thanks!!!!

 

[10secBu]

My experience with MSD boxes is either they work or they don't.

When my car has ever had a slight miss on the second half of the track, it was almost always from the carb jetting being lean. Usually two jet sizes front and rear larger fixed the problem. Might be worth a shot.

Maybe try to think back to any work you performed between before the miss, till now and maybe find something very simple that changed or was worked on.

Maybe check the needle & seat for any dirt, verify float levels, check the pump for any debris, check the filter, etc. On the electrical side as some suggested, plugs wires, cap & rotor.

What about the battery...maybe swap the battery for another one. I have heard rare cases where a bad or shorting cell could do strange things to how a car ran. Check the alternator for proper charging, check all grounds.

 

[JOHN WILSON]

My experience with MSD boxes is either they work or they don't.

When my car has ever had a slight miss on the second half of the track, it was almost always from the carb jetting being lean. Usually two jet sizes front and rear larger fixed the problem. Might be worth a shot.

Todd, jetting was my first thought last night. I bumped the jets from 81/82 to 83/84 and saw no change in performance and the problem was still there.

Thanks for the other ideas.

 

[zeke67]

With this type of problem, I usually start with the ignition system first as many of the parts are wear parts. Plus, at high RPM, the ignition system has to do it's work faster.

 

[Bob West]

I'd close the plug gap, increased compression or a power adder needs a gap of .030-.035

 

[70_FathomBlueMalibu]

I have no idea on what flow figures you'd need, John. My remark was almost more of a question.

Obviously at idle, the engine brings in a certain volume and density of air to fill up a certain sized cylinder. At high rpms with the "inertia ram", the volume of air is still the same, as it's filling the same size cylinder as before. However, the air is much denser, due to that cylinder packing (inertia ram). Anyway, you'd need more fuel. So, the pump must keep up with this. Or at least that's the way I look at it.

Todd brings up some good points about the battery, etc. Hopefully, you should be able to come up with a donor battery from a friend to swap out and check some things. That way, no cash spent.

FWIW, my MSD Digital 6 box has a crazy stutter to it when I try to enable the rev limiter. Something in that little circuit is burned up according to MSD. I've never gotten it fixed, as the box is fine otherwise. So, the boxes miiiiiiight do something crazy and work fine at other times. Can't discount that. I think sending the box in to MSD for a checkup would be the last thing I would do, since it costs at least $65+, plus shipping, plus downtime. Now, if you could find a friend with an MSD box to swap out for a pass, I'd try that.

Sadly, there are alot of things to check, so I'd sit down with a list and figure out what takes the most time/money and go in order. I do that, cuz I'm always tight for money. If money's not a big deal, just start throwing the cash at it.

Again, I'd love to hear what it was.

 

[JOHN WILSON]

Justin, thanks for the reply. While throwing cash at it is always an option, I won't really learn anything from it.

Can you describe your rev limiter problem a little more. This problem sure feels like the rev limiter is kicking in.

 

[Bob West]

John, a friend on mine was at the track friday night, running a big block 4 speed Nova(high 10 sec. car), he was having a cutting out problem also, he changed chips and it still was cutting out, he then pulled the chip out, went back for a 3rd pass, no cutting out in the burnout box and was revving clean as he pulled to the starting line, took it to 5k and broke an axle on the line. He had been running slicks on the car too, was his first attempt at using MT drag radials, I guess they hook

 

[UDHarold]

Just curious, what cam grind?

UDHarold

 

[EMcIllece]

That was my thought too, is it a hydraulic cam? Maybe valve float?

 

[forcd ind]

.045 is pretty wide on a car with that rpm-we had a car pick up 100 rwhp by closing up the gap-the biggest diff was when he sprayed it with a 300 shot
the flame on the plugs was trying to get blown out, reducing it from .045 to .030 did the trick

 

[DragRacer]

John,

What was the car running ET/MPH wise? Did the miss show up in back half performance?

What plugs are you running? I would reduce gap to .035" to start with.

I would also check resistance on the plug wires and inspect closely. If resistance is high on any of the ignition components, the spark is going to be "looking" for the path of least resistance and arc to something other than the plug.

You should have PLENTY of fuel provided the pressure is there.

It's alway good to take a look at the valve springs.

Let me know if you need to borrow some ignition components for troubleshooting.

 

[JOHN WILSON]

Bob, mine are sitting in the garage awaiting duty. Just waiting for the Hoosiers to start going away.

Harold- Its a Cam Motion solid roller. .689/.673 (gross lift) 273/281 107lsa 104icl.

Forced- I would expect that on a hit of nitrous, seems like on a 12.45:1 compression motor .045" would be ok. None the less, I'm gonna tighten them up to .035" and see how it goes.

Jason, I ohm'd the wires back in June before I used them to dyno the 427, they checked fine. I'll check them again tho. If closing the plug gaps doesn't work, I'll go ahead with new wires, cap and rotor. If no luck, I'll check the coil next.

The alternator seems to be charging fine, as well the battery in good condiditon. I already have another battery on hand that I can try.

Most of the passes I aborted after it started to break up. The 1/8mi times were mid 6.5's. The last pass I stayed in it despite the breaking up and it went [email protected] Only picked up 22mph on the back half. Usual is 24+.

 

[540Hotrod]

Don't you just love this stuff?

Do you have fuel gauge you can see while on a pass?

After that was eliminated I'd try the plug gaps. If that fixes problem when it was seemingly Ok before, I would look closely at the rest of ignition system. I can't remember..is it a crank trigger? Any chance rotor is out of phase or slipped? Still show well on timing, but cause issues at high rpm if drastically out of whack.

For sure check grounds...maybe even run a separate one to box and motor.

It wouldn't take much for a wire to arc to something, but you would think with plug changes it wouldn't just keep happening.

Springs don't usually act intermittent, but I've found that when I lost performance without any popping, a new set of springs had me back in business. Hey, as a side note, I just checked mine the other day when I was reinstalling the roller cam and they are still dead on. Lots of passes and street miles over the last couple of years plus a 2000 mile road trip to Ky with a .731 lift cam! I was actually pretty surprised!

Since you run an alternator, I would think it could cover battery issues. No chance belt is slipping and voltage is dropping at high rpm maybe?

I'll keep working on it..let us know what you find....


JIM

 

[ejrempel]

End play on your distributor. If it is too loose, high speed miss. On an HEI, it needs to be about 25 thou. I even think mine might be a tad loose. I'm gonna pull it and check again.

 

[70_FathomBlueMalibu]

John, my MSD stutter is a strange one, but shouldn't apply to your situation.

If I put the rev limiter on 6000, the car stutters at 2200.
If I put it at 6100, the car revs all the way out and beyond, as if the limiter is disabled.

If I put it at 6200, the car stutters at 2200 again.
At 6300, the engine revs out and acts as if it's disabled.

And so on. Absolutely crazy. I asked about it over the MSD tech board and they said something in that circuit fried, send it in and it would be $65 or so. Plus, they would put any little upgraded electrical components in it for a "freshen".

Anyway, you have some detective work to do.