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Dollar bill trick to find a missfire

18K views 20 replies 8 participants last post by  4speed57 
#1 ·
I've heard about this old school trick so I thought I would try it. I have a missfire in my 400 sbc that I haven't pinned down yet. On the drivers side bank all was good. When I tried the passenger side, everynow and then the paper would get sucked up to the pipe. It was very inconsistent, not on every revolution.

Where should I look now as far as the miss goes? Is it something to do with an exhaust valve? Would any type of miss cause the dollar to be sucked back up to the pipe?

Any help would be appreciated,
 
#3 ·
4speed57 said:
I've heard about this old school trick so I thought I would try it. I have a missfire in my 400 sbc that I haven't pinned down yet. On the drivers side bank all was good. When I tried the passenger side, everynow and then the paper would get sucked up to the pipe. It was very inconsistent, not on every revolution.

Where should I look now as far as the miss goes? Is it something to do with an exhaust valve? Would any type of miss cause the dollar to be sucked back up to the pipe?

Any help would be appreciated,
Why don't you just check the exhaust temps on each runner of the exhaust manifold. the coldest one isn't firing.
 
#5 ·
Thanks for the replies everyone.

My engine has about 500 miles since a total rebuild. I recently replaced my cam because the first one went flat. I've watched all of my rockers while the engine is running and they all are operating as they should be. None appear to have less lift than the others. I also done a compression test and I got 200lbs per cylinder. My 406 has 11-1 CR so that is why I have such high numbers.

I've pulled off the plug wires one at a time and cylinders 6 and 8 seem to have less effect when the plug wires are removed. I've sprayed starting fluid on the intake and I haven't found a leak. I don't suspect a vacuum leak because the engine seems to be running rich. I've had vacuum leaks before and the engine would run lean. Not to mention it's hard to start and keep running.

F-BIRD'88, I'll try hooking a vacuum gauge to full manifold vacuum and pull the plug wires one at a time. One thing I may also try is lowering the float in my Holley 650 double pumper.

It seems I've checked almost everything except pulling the heads and looking at the valves and seats. I did a compression test and got 200lbs per cylinder. Could I obtain such high readings with bent or burnt valves? Would a leakdown test be necessary?

I've check my ignition system and everything looks ok. My plugs do appear to be fouling from running rich.

Thanks for your time,
 
#6 ·
Another "old skool" trick is a small amout on water on the exhaust manifold or header on each cylinder. You find out real quick which one isn't working very well. A spray bottle with water works good. It doesn't smell too good but spitting on the exhaust works too...and i know someone else has done this before...lol
 
#7 ·
Thanks RSM, I actually tried that trick over the weekend. I sprayed down all eight header pipes when the engine was cold. By the time I exited the car from starting it all 8 pipes were dry. The two suspect cylinders dry off the headers just as quick as the other six.

Thanks again!
 
#8 ·
The trick is to warm the engine up then spray them one at a time while its hot. Then you can see how fast the water boils off. That will usually give you an idea as to how each cylinder is doing. If you run across one that is way slower to boil the water...that cylinder should be checked...plug, plug wire, dist cap, valve adjustment...etc
 
#10 ·
4speed57 said:
I did a compression test and got 200lbs per cylinder. Could I obtain such high readings with bent or burnt valves? Would a leakdown test be necessary?
An exhaust lobe going flat will still give good compression/leakdown. A little less so w/an intake valve, but it'll still pump good numbers, just maybe not as fast or as high on the first compression stroke as a good cylinder- depending on how much wear has occurred. In the early stages, all will seem fairly normal.

Once you isolate the cylinder(s) you can check the secondary wire, plug, cap/terminal, etc. to pin it down to the ignition system. If all else fails, you can measure the valve lift on the affected cylinders, check for a pulled stud, or bad adjustment, bent p-rod, etc.
 
#11 ·
cobalt327 said:
If all else fails, you can measure the valve lift on the affected cylinders, check for a pulled stud, or bad adjustment, bent p-rod, etc.
I forgot to mention I measured the lift on the two suspect cylinders and compared the numbers to two good cylinders. my numbers were the same, so I think the cam is good.

Thanks again,
 
#12 ·
A Couple More Old School Tests

Hey 4SP57, Here's a couple of misfire tests I sometimes use. Connect timing light to 12v supply, clip pickup on one wire at a time. Watch the flash in the palm of your hand, blink-blink-blink steady no problem. Blink-blink-blink-pause-blink-blink-pause-blink-pause random=misfire(bad plug or wire) if all the cylinders have some blink pause=multiple misfire(bad distributor cap or rotor,bad coil or coil wire).
Another way is with a old handheld dwell-tachometer, same setup. Clip on one wire at a time, the missing cylinder will read lower rpms or needle will wander up and down. Again, if all cylinders wander up and down the problem is in the distributor. I call it the poor mans oscilloscope. Found three bad plug wires on a friends car one time, that visably looked perfect. Had a big block with brand new MSD distributor and setup once with random misfire. The problem turned out to be a smushed rotor blade, bent it up to make good contact on the button, problem solved. These tests will eliminate the ignition system.Try it, it works for me. If all is good with the ignition, then you can focus on fuel mixture/valves/cam,etc. Happy hunting. :thumbup: olnolan
 
#13 ·
Old school, huh? Here's one:

Try opening the hood at night w/the engine running when it's pitch black outside. Sometimes, it'll look like there's fireflies on coke running amok under there! If not, then take a spray bottle filled w/water and spritz the wiring and around the distributor cap and see if that incites any fireworks.

You can also use a non conductive piece of material (short stick) to lift the plug wires away from where they're laying to see if there's a breakdown of the insulation- you'll see the current popping to ground if there is. Just watch out for the fan, belts, etc. You may be surprised at what you see.
 
#14 ·
Thanks for the replies and help guys.

I'll go out later today and try those tricks and see what I find out. One thing I do know is I don't have any dead cylinders. There are a couple of cylinders that seem down on power compared to the rest. I did the vacuum gauge test that F-BIRD'88 suggested. Most of the cylinders dropped the vacuum 4lbs or more when pulled. A couple of them dropped it less, but there was a drop.

I don't know if it helps or not, but I'll describe the miss. When you speed the engine up, say at least 2000rpm and more, the engine sounds like the choke is half shut. It has a lope sound to it. It has a lope at idle because of my cam, but the lope never goes away after you speed it up. The engine isn't flooding out, I have no choke, the float level is good, and there is no raw fuel being dumped into the engine. Also if it was flooding out I would have black smoke rolling out of the tailpipes and I don't.

Thanks for your time and help everyone
 
#15 ·
What do you have for an ignition system? I'd be inclined to look at that. Since you dont have any smoke out of the exhaust it's not flooding so fuel doesn't seem to be the issue. Electrical is some way seems to be the problem. I have seen coils break down at higher rpm just like u describe. Thats where I would take a look...just a suggestion.
 
#17 ·
How about the voltage to the HEI? Is it full 12+ volts at the rpm where it starts running bad?....without actually being there it's hard to say whats wrong. I'm sure someone will come up with something that makes sense. You have a Merry Christmas and I hope you find the solution to your problem soon!! :thumbup:
 
#18 ·
Thanks RSM, Merry Christmas to you also.

I did check for 12 volts at my distributor and it was getting the full amount. I didn't notice a reduction with the engine running. I need to double check that again.

I'm worried it's something to do with my heads. I bought my Dart heads used and I only took out a couple of valves to look out. I should have went thru them. The engine doesn't smoke so there is no valve seal or guide wear. I'm worried I could have a bent or burnt valve.

Thanks again,
 
#19 ·
One way to know about valve problems is to run a compression test on all cylinders. One thing it will do is show any low cylinders and it gives you a general idea as to the condition of everything. Sometimes a compression test will show you a bad head gasket. If you get 2 cylinders next to each other that are low...thats a possible blown head gasket (not saying that is what you have). Check your voltage to the HEI with the engine running at high rpm to make sure you have 12+
 
#21 ·
I did get 200lbs in each cylinder when I did a compression test. I guess with a bad valve the readings wouldn't be that high.

I have my plugs gapped on .045. I thought about gapping them a little more, but I heard to much could burn up the coil.

Thanks guys!

Happy Holidays!
 
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