HELP w/ 340+6 T/A Engine dropping a cylinder under load

[Cuda Cody]

This is a stock 340+6 T/A engine with a Mr. Six Pack Cam.  @ScottSmith_Harms  built the carbs and the beast starts and runs fantastic.  9 to 1 compression and using headers to break in so we can get temp readings.  Sounds good and everything seems fine until we put it under load.  That's when we lose the 8 hole.  It stops producing power and drops offs completely.  Look at the video and watch the far right graph of the exhaust temps.  You can see it stops firing and the excess fuel cools the cylinder down thus dropping the temps.  I 8 cylinder just SHUTS off.     Checked compression and we have 185.  Really good!  We adjusted the valves several times to make sure that was not it.  Swapped the plugs around and nothing changed.  It's always the #8 hole that stops working.  If the engine is not under load it works fine and the #8 hole keeps firing even at higher RMPs.  We did make 1 pull on the dyno and as soon as we noticed it wasn't firing on the #8 hole we shut it down.  It made 272 Horsepower on 7 cylinders.  Well short of the stock 290 number.

We're thinking it might be a bad lifter?     They are new Johnson lifters.  If I replace the lifters in the 8 cylinder do I need to replace the cam?  I can use new lifters and it should be fine, right?  @Chryco Psycho


Watch the bar graph in the video on the lower right hard side of the screen.

[video]https://forum.e-bodies.org/gallerypics/videos/1_08_03_17_3_10_19.mp4[/video]

[Brads70]

Electronic ignition box? Bad plug wire? 

[RUNCHARGER]

New lifters will work fine on a used camshaft. Did you check the spark plug lead?

[ScottSmith_Harms]

Single dead hole with fuel (cool temp) would point to ignition. Id start looking at the plug, plug wire, or distributor cap.It may be a partial failure that only shows when under load.

[Roadman]

Electronic ignition box? Bad plug wire? 

                                              plug wire.   

[HP_Cuda]

Definitely ignition, check the resistance on your #8 wire and you can also pull the plug on #8 and see if it fires while cranking.

I'll bet if you swap a wire with #8 you will probably move the problem.

[Chryco Psycho]

 
new lifter on old cam is OK
the lifter can be swapped out with a magnet by just removing the valve cover & rocker , the intake lifter is not sensitive to engne load it opens after the pressure has exited , the exhaust lifter could be collapsing as it has to open the valve into a pressured cylinder however I doubt this is the issue as even with the lifter bottomed out it will still open the valve some .
What about a vacuum leak the power brake booster hose attaches to one of the rear runners 7 or 8 it could be going leak but that would also increase heat before it quits firing .
Worth checking for a bad reluctor & reluctor gap , dist cap or wire , even if the wire looks & tests good if it is run to close to metal & has bad insulation it will always take an easier path & that is definatly affected by engine load as the plug requires a lot more voltage to fire under load .

[RUNCHARGER]

I think he should just tune for another 3HP and be happy.

[fc7cuda]

I know you swapped out plugs, now replace the wire.  Also, as CP mentioned, replace the one lifter by fishing it out.  It may be weakening under load. 

[Cuda Cody]

At the dyno right now.....  Problem solved.  I'll give the details as soon as I get home.  But it was nothing I would have ever guessed.  Going to start tuning now...  But the first pull today with it all working is the black line, yesterday with the problem is the red line.

[blown motor]

 

[Roadman]

[Cuda Cody]

I need to talk to someone first before I share my findings.  But the difference in the engine was night and day once we found the issue.

Ended up making 349.5 HP and 377.9 Torque.  That's super impressive for a stock 340 at 9 to 1 compression and pump gas.  That Mr. Six Pack cam is the BEST!!!!  If anyone wants a great cam contact Mr. Six Pack (Bob):

Bob Karakashian (Mr. 6 Pack)
248-477-777six days
248-489-407six eves
mr6pk (the at symbol) hotmail (dot) com

Here's the video right after we found the issue and fixed it.  Only took it up to 5,200 RPM this time, but it was very clear we had fixed the problem and it was firing on all 8.  We peaked on a later run around 5,500 RPM.

[video]https://forum.e-bodies.org/gallerypics/videos/1_09_03_17_1_25_52.mp4[/video]

[RUNCHARGER]

Ha! Okay you're allowed back on the forum now Cody, it was sketchy there yesterday.

[MoparDave]