I'm plagued with a strange misfire that makes my stroker idle very smooth at times yet a few miles later, you'll feel it with an erratic misfire at a stopsign (auto trans). It's not a steady miss, like a bad wire or plug.

I put new plug wires (taylor spiropro 8.5's) last time I was chasing this misfire and cleaned up the plugs. yesterday I put new plugs in it, and had no change.

-BUT- two of the plugs looked lean. #3 and #5 were MUCH leaner looking than the rest, which looked to be firing normally - if not a bit rich.

The distributor is a new Mallory HEI and the carb is a (even newer) Edlebrock AVS Thunder 650. Out of all the carbs I've had on it this one runs the best. I put the dist in about 5K miles ago but it had no effect on this misfire.

So I'm looking for a vacuum leak. Problem is that I can't detect it by spraying, either at the carb or the intake manifold. I've tried spraying Chemtool and WD40 both and can't detect any vacuum leak.

And a good part of the time, the thing idles fine. So it really is a mystery to me. When it's hot, I can set the idle up on the fast idle cam and let her run at 1500 or so, and if you let it run like that long enough you'll hear the engine speed drop a bit every now and again like it's losing a cylinder for a second but I can't put my finger on it.

Just looking for suggestions here. I've got lots of engine work and tune up under my belt in my years as a mechanic so it's really rare for me to post about a driveability issue I can't diagnose. Hoping for some suggestions though

-Thanks Paul

 

This really sounds like an intermittent ground problem. Years ago, I was taught the following highly sophisticated trick for finding these.

Put the car in a place, like a garage or outside in the early evening, when you can make your surroundings a little bit dark. Take a spray bottle of water and start tracing through your high voltage electrical system from the distributor to the individual spark plugs. If there is a spark grounding somewhere, it'll light up like a Christmas tree.

 

ICM
wires bleeding because there to close (Cross fire)

 

Intermittent failures are a PITA. Sounds like you eliminated the obvious. Wish I had better ideas. All I can offer is encouragement at this time.

 

Nope I eliminated that possibility. Checked the wires for damage and they are still perfect.

 

Do you have an EGR or PVC valve?

 

Yea there is, but the TVS is disconnected and there's no tubes to the EGR. It was connected, when I was using a QJet carb and it worked fine. But when I put the AVS carb on it, that required using a spacer/relocator wedge under the EGR (available from EDL for the Performer manifold) and when I got through with all that, the vacuum lines were too short to reach it. So I capped it off at the carburetor and left the EGR and the TVS both disconnected.

But the plugs that showed leaner, were on the other side of the engine - they were #s 3 and 5. All the rest appeared to be OK if not a bit on the rich side.

 

i do that all the time but never heard of it done with water. it works fine without it. the last time i was chasing a mis like this i found i had missed putting the ground wire from the dist. back on, and it would flop around on the manifold giving the erratic miss. most electrical problems will be a bad ground somewhere

 

Tell you what drove me nuckin FUTZ for a long enough time....

My '72 SBC here was driven only intermittantly, and for short trips, and so it's not the only thing on the plate....

the #5 and #6 plugs would ground out only when hot, against the header tubes.....but only when HOT...not cold, I tried all sorts of things on and off, and it drove me crazy....
finally had #5 spit a spark at me in bright sunlight one day as I was banging my head on the fender....literally....

POS, SOB, MUVVA, rotten KSer....freakng name it....

Jacobs ceramic boots, all 8.....do THAT again you dirty rotten illegitimate cheap KSer.....

 

You gotta love this hobby.

 

Bigbird is right. This made me laugh out loud. Right too, it's amazing how often we ehd up chasing thigs like this and solving them with overkill once identified. Things like grade 8 bolts on valve covers to fix a leak......

 

On my new-ish Goodwrech 350, at idle only, a small spurratic but noticeble miss turned out to be a wiped lobe on #4 exh of the stock cam . This may not be your case but something to be aware of. Only test is to measure vlv lift - something i have never been able to do with the vlv train assembled (eng running) - or visualy inspect the cam.

Good luck Paul.
cardo0

 

Holy cr_p. How many miles?

 

Thanks Cardo - yea I've thought about that a few times. The cam is a Crane and it was certainly broken in correctly. Never had a drop of anything but Valvoline Racing Oil since the engine was first fired. But yea unfortunately these days that is a possibility. Engine is four years old now, and has a good 20K miles on her.

And when I drive it I flog it. I've always driven my hot rods hard and this one is no exception! Drives the wife nuts.

"Why do you floor it in low gear just to slow down for a stop?"

"Why do we have to take mountain roads so damn fast?"

She does kinda dig in and love it though when I get on the freeway, redlining it in low gear and gettin' scratch when the reworked Turbo 350 pumps into second gear though. And she certainly digs it when we travel at 110MPH on the freeway at half throttle!

 

I chased a simular problem for a month....How new is the Mallory? When did your problem start? What was the last thing you worked on before it started?

 

The ign system is about a year and half old - maybe under 5K miles. I looked under the HEI cap and all was fine.

This miss began before that. I had a bout with a few Quadrajets... I sent one to Lars and used one of his loaners for a while. The misfire was there then, but it was mixed in with a couple of standard issue Qjet problems at the time. I gave up on it, and went to an Edlebrock carb that I bought rebuilt (against my better judgement, but it was worth a try.)

Using that regular old Edlebrock carb the erratic misfire became more prominent. And that's when I started noticing that if I kept the RPM up with the engine hot, either by holding the throttle a crack open, or manually kicking in the fast idle cam, I could hear the RPM vary a hundred or so every one or two minutes. Not a regular misfire like dropping a cylinder due to a plug wire grounding, but much more irregular. And the exhaust stunk too much more than normal.

So I tossed the rebuild Edl 600 and bought a brand new AVS Thunder 650. The stroker runs better with this carb than with any of the quadrajets, or the other Edlbrock. But that odd misfire is still there and it will still be very noticeable if you hold the RPM up with no load on the engine.

When I pulled the plugs two days ago, six were slightly rich while 3 and 5 looked lean. New set of Accel Shorties had absolutely no effect. It runs exactly the same.

I ran over every inch of every plug wire (Taylor Spiro Pro 8.5's) and found no ground spots or evidence of crossfire. Lars says these AVS Thunder series carbs run very lean and that I need to richen it up. I'm willing to give that a try but this situation 'feels' a bit odd to me.

I did tune up work for 15 years and engine builds/heavy line work for another ten before bringing my toolbox home. So I'm certainly familiar with what's under my hood...I've chased odd misfires all my life.

But I always FOUND 'EM on the customer's cars! DAMMITALL! :o

Oh yea one more thing: Regular part-throttle performance is great! Passing someone around town only requires a small goose and if I am anxious and give her half-throttle, then the secondarys open a bit and it will jerk your head back even if you don't cause the trans to downshift. And if you run it hard, redlining it in first gear it will still get a bit of scratch when the reworked trans kicks into second gear. The trans has a B&M kit in it, and a 2800RPM stall convertor. It will still do hole shots like it did when brand new. No lack of power at all. But it drives me nuts having this misfire that I can't diagnose, and also just *knowing* there's something wrong under the hood of course ....

 

Hey Boss, sounds like you've done just about everything I could think of and more. At this point I would try to get a second set of eyes. Good luck.

 

Wow, I chased all the same S*&^, after roughly 1000 mine started,not all the time, under heavy load,if I ran the R's up slow it was fine. Got worse as she warmed up, but no pattern to the mis,all over the rpm range. My ignition is MSD though. This may sound elementary,for whatever reason, I had hooked the secondary 12v source to a voltage dropping line,oopps. I checked the source before doing it,she showed 12,but after a few minutes dropped down and this did not effect it for at least 1000 miles. Sounds like you know what your doing, a second set of eyes may do it, good luck..

 

Paul, You did run a new wire, right?(Have to ask)

 

Another set of eyes? That's why i'm here on the forum! HUndreds of eyes!