Kevin Wiles Posted June 27, 2011 Share Posted June 27, 2011 427 *TALL* big block 427 block with 454 heads and crank. This is a tall deck truck style motor. Engine is running rough and backfiring, seems like its running on 7 cyls..... When at idle, we found that pulling the #4 plug wire has no effect on engine. Other plug wires, when pulled, nearly stopped the engine from running. So the culprit is cylinder 4 for sure.. We discovered that plug #4 IS getting fire. Pulled the valve cover and the valves appear to be moving fine, like the other cylinders. Results of the compression test on cylinder 4 - 135 Lbs. Did a compression test on the neighboring cylinders 2 and 6... results 125 lbs each. What have I proved that is good? What have I proved that is bad? Surely this can be diagnosed with the testing I've done above. What does all this point to? Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted June 27, 2011 Share Posted June 27, 2011 how about a cracked plug Kevin? Put a heat gun one each header tube and see if they're even.... Quote Link to comment Share on other sites More sharing options...
wallaby Posted June 27, 2011 Share Posted June 27, 2011 Well, there is some good news...it sounds as if the compression is OK in that cylinder: that would indicate that the rings, piston, valves are in good shape there. Now you can focus on what else it might be. The important consideration now is to realize that the engine faces different demands at different speeds and loads. Things that are good enough at the 100rpm cranking speed might not seem so good at the 500rpm idle speed and above. Start with the simple, common stuff. A vacuum leak perhaps? If that cylinder isn't getting fuel it will act dead. A cracked distributor cap is another thing to look for: it may send spark down the wire when you crank the engine, but under the load of the engine actually running may want to send the spark elsewhere. A bad spark plug or wire could be the issue...you can swap plugs/wires from another cylinder to find out if the problem moves. The more complicated stuff comes next: a bad cam lobe can still give good compression readings at cranking speeds, but not open the valves enough to flow air at operating speeds...you'd probably notice lesser movement of the rocker arms in this case, but not always. ...And I'll forever be stuck on the idea that it could be a broken valve spring. I've had 2 break on me; at different times. Each time I was baffled as to what the problem was. Like your situation, both times a compression test at cranking speed showed good numbers, but once the engine was running the cylinder acted dead. Both times the broken spring was good enough to actuate the valve at cranking speeds, but too weak to function at idle speed or above. Both times the spring broke not in the middle, but close to one end...making the break hard to see even when I looked. It took close inspection all the way around the spring to finaly see the darn thing was indeed broken. Hope that helps you some. It will give you something to do anyway. As usual, the process involves finding out what it ISN'T. Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 27, 2011 Author Share Posted June 27, 2011 Here is some additional information, after reading these responses, thought it would help diagnose We switched the number 4 plug wire with the known good one from number 8, just to see if the problem would move. It did not. The problem stayed on 4, proving to me, not a plug wire. We pulled the cap and looked inside. There was no evidence of any arcing or deposits of any sort. Looked totally normal and had no cracking. It is relatively new. The plug is brand new. And 2 new ones have been used for testing. I am going to pull the valve cover again and look at the exact travel of the valves. I know they were moving..... and appeared to be moving the same as the rest. But I will inspect it more closely tonight. I will also look for a broken spring. Any and all other replies are very appreciated. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted June 27, 2011 Share Posted June 27, 2011 check for a vacuum leak at the intake port, that could lean that cylinder out enough to cause a misfire Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 27, 2011 Author Share Posted June 27, 2011 how do you do that? Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted June 27, 2011 Share Posted June 27, 2011 I spray starting fuild on the port in question to see if I get a rise in RPM, I'll also pull the PVC and spray it in the valve cover to check the bottom side of the intake, you could also close the choke to see it the cylinder in question starts firing Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted June 27, 2011 Share Posted June 27, 2011 Kevin, do you have single or dual valve springs? I've never had a broken spring (especially if it's just an outer), a vacuum leak or a dead cylinder cause a backfire situation. Sounds more like a valve/timing issue to me, either the cam or lifter like you said, timing chain or something in the distributor (check the gear)....JMO Quote Link to comment Share on other sites More sharing options...
Mo's70MCs Posted June 27, 2011 Share Posted June 27, 2011 Surely this can be diagnosed with the testing I've done above. What does all this point to? This sounds like a test to me? My money is on a new mechanic. Quote Link to comment Share on other sites More sharing options...
Mikstudie Posted June 27, 2011 Share Posted June 27, 2011 replace the simple stuff first,plug,wire,cap Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 27, 2011 Author Share Posted June 27, 2011 plugs wires and cap is relatively new even before i started trying to figure this thing out.... I have tried 2 different new plugs, and tried different wires, all with the same old result. The cap is in mint condition with nothing that i can see wrong with it. it has less than 200 miles on it. Quote Link to comment Share on other sites More sharing options...
Mo's70MCs Posted June 27, 2011 Share Posted June 27, 2011 Stupid question. is it standard ignition? Quote Link to comment Share on other sites More sharing options...
FastClass Posted June 27, 2011 Share Posted June 27, 2011 Have you tried the ol` starting it up in a dark garage trick? That will show any arcing, bad insulation, cracked cap, that is`t evident in the daylight in a hurry. My .02 for a quick, free test. I now return you to the Wrenches that actually know what they are talking about. Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 28, 2011 Author Share Posted June 28, 2011 HEI ignition.... and it has been run in a pitch black garage with no visable arcing. Starting to hope that its a vaccuum leak around the intake..... I'll know more tonight when I return home Quote Link to comment Share on other sites More sharing options...
Mikstudie Posted June 28, 2011 Share Posted June 28, 2011 Theres a rubber washer looking insulator under the cap and a spring loaded button that make contact lots of problems with those pieces check them. But most of the time they cause a multiple misfire.. Quote Link to comment Share on other sites More sharing options...
Jared Richey Posted June 28, 2011 Share Posted June 28, 2011 The only thing I still want to do to be safe is try another Cap and Coil, maybe change the electronic gismo inside the HEI dist just to be sure its not somthing simple. Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 28, 2011 Author Share Posted June 28, 2011 I just got home and after talking to someone today, they said place a rag on top of the carb and see if it clears up.... Well, I placed a soft rag gently on top of the carb with the engine running and .... IT RUNS LIKE A CHARM!!!!!! So we have a vaccuum leak !! I'm trying to pinpoint the vaccuum leak with some carb cleaner now.... I'll be back in a few to give you all the results of that. Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 28, 2011 Author Share Posted June 28, 2011 i sprayed carb cleaner all over the front back and sides of the carb with no improvement in idle. i also sprayed it all around the edge of the intake manifold with no improvement in idle. could this be due to a vac leak INSIDE the carb? is it safe to say i need a new carb? of course, spraying the carb cleaner into the carb helps the idle..... wish i could just switch the carb from my other monte, but it has a side loader fuel line, whereas this coke car has a front loading line. if i have to get a new carb, will i need a new line? or do they make an adapter? i'm googling around now.... Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 29, 2011 Author Share Posted June 29, 2011 anyone else got any ideas? i'm gonna try a couple more things tonight and if i can't figure it out, i may have to pull the intake and look at the gasket. the gasket could be messed up on the block side, where no carb cleaner will get to it. or perhaps the carb is leaking vaccum inside somewhere. still baffled here.... Quote Link to comment Share on other sites More sharing options...
FastClass Posted June 29, 2011 Share Posted June 29, 2011 Sounds like time for a carb rebuild. Quote Link to comment Share on other sites More sharing options...
Andreas Posted June 29, 2011 Share Posted June 29, 2011 I would take the carb off and carefully inspect it.... Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 29, 2011 Author Share Posted June 29, 2011 ok brace yourself everyone, i'm about to ask a really really dumb question. Is it ok to run an open spreadbore gasket on a regular 4 hole intake with a quadrajet? I'm assuming the answer is no. *runs and hides* Quote Link to comment Share on other sites More sharing options...
Andreas Posted June 29, 2011 Share Posted June 29, 2011 I did not know that you can even bolt a spreadbore carb to a squarebore manifold? We are breaking new grounds here.... Quote Link to comment Share on other sites More sharing options...
Kevin Wiles Posted June 29, 2011 Author Share Posted June 29, 2011 maybe i am using the wrong terminology. here is what my intake looks like: and here is my carb and what I BELIEVE COULD BE whats causing me these problems. It is an open gasket, as opposed to a 4 hole gasket. My question is, does this matter? Quote Link to comment Share on other sites More sharing options...
Andreas Posted June 29, 2011 Share Posted June 29, 2011 That should not be a problem, Kevin Quote Link to comment Share on other sites More sharing options...
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