sredmyer Posted August 11, 2018 Share Posted August 11, 2018 So I got my silver SS454 out to go to a show today. The car has never (at least by me) been driven more than just around my local area. Furthest I have driven it (prior to today) was probably 30 miles. Anyway, the show I went to today was a bit further away. Everything was fine on the drive there for the first 20 miles or so but then it just started missing really badly and backfiring in the exhaust (not through the carburetor). It seemed to run mostly ok as long as I wasn't really under any load but as soon as I had to give it a little extra gas to make it up a hill (or go over 50mph) it would start the sputtering and backfiring. I was nearly to the show when this happened and it is a hot day so I thought maybe it just got hot so I limped onto the show keeping it under 50mph. I reasoned that maybe having several hours at the show to cool down might help it. Well when the show was over it started right up and seemed to run fine. Unfortunately as soon as I got out on the road it started doing the same thing so I limped it home again keeping it at or under 50 mph (which was difficult on the freeway). So here is what my software engineers brain (not a mechanics brain) has made of the situation thus far. It is back firing out of the exhaust not the carburetor so it is likely that at least one cylinder is not firing and is just pitting its gas out the tail pipe to be ignited in the exhaust. It seems to run fine (not missing noticeably) at idle so the problem is likely electrical...spark to week to fire plug under load conditions. It seems to run ok when not under load that requires giving more gas (ie. going up a hill) so the problem is likely when the carburetors secondaries are opened up As mentioned, I am a software guy not a mechanic so it is very likely that all of my conclusions are completely wrong. Can anyone help shed some light on what might actually be going on? Thanks Quote Link to comment Share on other sites More sharing options...
MCfan Posted August 11, 2018 Share Posted August 11, 2018 Steve, really sorry to hear about this episode, especially when taking her to a car show (hope she got a lot of looks and love there, BTW). As you know, I am NOT a software engineer, but your diagnosis sounds pretty good to me. Since it came on suddenly and behaves as you described under the conditions you described, I'd guess that either a plug wire fell off (may be still on but not making contact) or a plug wire burned through and grounded out. It is amazing how much a single "dead" cylinder affects performance. Have you inspected and pulled all of the wires and re-plugged them at both ends? I changed the plugs in my 408 one time and took her out for a test drive. Everything was wonderful until it suddenly started behaving as you described. It was a typical hot Florida day and I had just gotten on the loud pedal a couple of times so I thought something serious had broken in the engine. After I limped home and let her cool down, I checked all of the plug wires and found one connector was unseated although the boot still held it on the plug. Re-seating that plug connector was all it took for the problem and symptoms to completely disappear - to my great surprise and relief. Another possibility is that a rocker arm suddenly broke as happened on my first stock 402 back in 1974. It ran rough and lost power then also, but I don't remember it backfiring. It probably exhibits different symptoms when it is an intake vs exhaust rocker that breaks. I found it by simply pulling the valve covers and there lay the stock rocker arm - completely broken in two pieces. Simply replacing that rocker arm and re-gaping the valve solved the problem. I'm sure other members will chime in with ideas from their experience. I'm guessing and hoping it is not a serious problem and has a simple fix. Good luck. Quote Link to comment Share on other sites More sharing options...
A_Rescue Posted August 11, 2018 Share Posted August 11, 2018 Im thinking spark also, the coil probably, easy/cheap change... as the AFR decreases (more fuel/enrichment) in the cylinders the spark plug spark gap dielectric resistance increases and the coil cant support the energy... coils can short on the primary one winding at a time and its progressive until the missing starts... heat accelerates this and a shrink tubing gun on the coil could help if the misfires are the coil and thermally aggravated... check the coil/plug wires... 1 Quote Link to comment Share on other sites More sharing options...
sredmyer Posted August 12, 2018 Author Share Posted August 12, 2018 I don't see anything obviously wrong with the wires. I also turned all the lights off in my building and ran the engine and saw no evidence of spark jumping. That said, the wires that are on the car are quite long and look like they might get tangled in the steering so I would like to replace them with a better fitting set. Unfortunately, I am not sure how to buy new spark plug wires for the car...the car has been converted over to GM HEI ignition so I can't just go tot he local parts store and say I need a set wires for a 1970 Monte with a 454. Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted August 12, 2018 Share Posted August 12, 2018 Pull the plugs see if any are fouled. Quote Link to comment Share on other sites More sharing options...
MCfan Posted August 12, 2018 Share Posted August 12, 2018 Unfortunately, I am not sure how to buy new spark plug wires for the car...the car has been converted over to GM HEI ignition so I can't just go tot he local parts store and say I need a set wires for a 1970 Monte with a 454. I believe from '75 on all Chevy 454s had GM HEI distributors. If you go to the Summit website and search using the Make/Engine (not Make/Model) you will find 323 choices of wire sets for a Chevy 454. Find the brand, color, type of wire, etc. you want and check the specs of that model for "Distributor Terminal Ends". If it says "Male/HEI" you have the right set. Also check the the "Spark Plug Boot Ends" to see if they will work with your exhaust manifold/headers. Many are "Eight 90 degree" so you know they're all right angle boots which generally work with stock exhaust manifolds unless you want something else. I picked up a full wire set for a BBC already in chrome looms at a swap meet and they all have slightly angled 135 degree boots which work well with my Patriot headers. Quote Link to comment Share on other sites More sharing options...
sredmyer Posted August 12, 2018 Author Share Posted August 12, 2018 Well, I feel silly! I bought a new set of wires at the local AutoZone (before seeing Dennis's last post) only to find out that the straight boot on them made the wire for cylinder #5 rub my steering shaft. But that's not the part that made me feel silly...while looking at this situation I noticed there was a wire off the distributor cap. Put that wire back on and she runs like a dream again. I can't believe I missed that...I had convinced myself it was a spark issue so I can't imagine how I didn't see that. I guess I was so focused on finding a broken or burnt wire having one just fall off never even occurred to me. Oh well, it is all better now but I still think I want to put on a better fitting set of wires. Thanks all for the help. 2 Quote Link to comment Share on other sites More sharing options...
stangeba Posted August 12, 2018 Share Posted August 12, 2018 (Never mind, didn't see your last post) Carbon track inside the cap? The darkness looking for stray sparking is a great idea! Add a light mist of water to the wires and brake torque the engine a bit (requires two or three people) so it's under a load. (Don't get run over) I remember the first time I did that with my dads Olds 98, it was like fireworks they all light up! Good luck, Bruce Quote Link to comment Share on other sites More sharing options...
sredmyer Posted August 12, 2018 Author Share Posted August 12, 2018 Can anyone with a big block and stock exhaust manifolds post a picture of how a 90° spark plug boot fits. My other cars both have the more original wire style with the small straight boots. It seems everything available these days (especially for HEI) have the extremely large spark plug boots like pictured below. In the attached picture, the two boots on the top are what I am finding available. The bottom one is what is on my other cars. Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted August 13, 2018 Share Posted August 13, 2018 I love an easy fix, we've all been there I had a plug wire come off a plug once too. I didn't find it until I got to #6 pulling each plug and checking them one at a time Quote Link to comment Share on other sites More sharing options...
Paul Bell Posted August 13, 2018 Share Posted August 13, 2018 I'm glad you found the problem! I would recommend Moroso Ultra 40 wires. They're very good quality. Here's a list to the 1975 big block sets with HEI ignition cap: https://www.summitracing.com/search/year/1975/make/chevrolet/model/monte-carlo/department/ignitions-electrical/section/spark-plug-wires-and-accessories/engine-size/7-4l-454/engine-family/chevy-big-block-mark-iv/brand/moroso/spark-plug-wire-resistance/40-ohms-ft?N=4294950311%2B4294951398%2B4294950342%2B4294951516%2B4294943637%2B4294951383%2B4294951380%2B400105%2B4294565108&PageSize=100&SortBy=Default&SortOrder=Ascending Whatever you decide, certainly stay away from Taylor because they fall apart and any wire with very high resistance. Quote Link to comment Share on other sites More sharing options...
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