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resident gearhead
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20 Posts
Discussion Starter · #1 ·
Long story short version. The #5 cylinder is dead. I have checked just about everything i know to check, and i am very well versed in engine diagnosis and repair. I have to be looking over something simple. Carbs are fresh predators on a tunnel ram. Head gasket is good, no cracks in the head, block, piston, or valves. I have not done a compression test or leakdown yet, i will do that tonight. But the old finger in the spark plug hole, the #5 cylinder feels just like the other known to be firing cylinders. Meaning that it is not a complete loss and even if it is low, it would have to contribute some to the engine. Valves are not sticking, and they are opening correctly. Valves adjusted on a solid roller, no flat cam lobe. Ignition is good, getting good spark to the cylinder at the correct time. When it is running, and i pull the #5 wire there is no difference in engine idle or rpm. You can tell on the over transoms that there is a dead hole on that side, and the #5 header tube is no where near as hot as the other 7. What else can it be? I need some ideas, because i know that i am overlooking something simple. Hopefully somebody will chime in.
 

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Seriously off center
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2,379 Posts
Only way to tell for sure is a compression test. If the valves are good and no compression, you've got a ventilated piston. Burnt valves or blowing between cylinders (head gasket) should still show some compression.
 

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resident gearhead
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20 Posts
Discussion Starter · #3 ·
Only way to tell for sure is a compression test. If the valves are good and no compression, you've got a ventilated piston. Burnt valves or blowing between cylinders (head gasket) should still show some compression.
head gasket was good, i had the head off yesterday to check it out. that is what i thought at first too, but i was obviously wrong.
 

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Just Me
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4,104 Posts
The tried and true method of figuring out which of the 4 things you are missing would be the best route to go. It takes fuel, air, spark, and compression to make one run. You can check a lot of this by looking at the plug in that hole and figuring out if it has spark or not, too much fuel, ect.... If you get compression building when you stick your finger in the hole then it should be doing something when the plug wire is pulled. Hard to guess over the net, but I'd bet it's something like obstructed air path, or a bad wire. Easiest way to check the wire would be to move it to another cylinder and see if your dead cylinder moves with it. But if it's just no spark the plug should be fuel fouled when you pull it out. Good luck on the hunt.
 

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resident gearhead
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20 Posts
Discussion Starter · #5 ·
The tried and true method of figuring out which of the 4 things you are missing would be the best route to go. It takes fuel, air, spark, and compression to make one run. You can check a lot of this by looking at the plug in that hole and figuring out if it has spark or not, too much fuel, ect.... If you get compression building when you stick your finger in the hole then it should be doing something when the plug wire is pulled. Hard to guess over the net, but I'd bet it's something like obstructed air path, or a bad wire. Easiest way to check the wire would be to move it to another cylinder and see if your dead cylinder moves with it. But if it's just no spark the plug should be fuel fouled when you pull it out. Good luck on the hunt.
cylinder is getting fuel, and the plug is firing. i checked resistance all the way from the cap to the plug and it shows up good. as far as too much fuel, i dont know, but the other 7 cylinders are getting enough to fire. i was thinking about obstructed air path but i am having a hard time seeing that because i just had the intake and head off to look for obvious problems. thanks for the suggestions.
 

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Registered
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696 Posts
This is a dual quad motor correct?

Check your power valves, work the linkage by hand and make sure BOTH carbs are spraying fuel. This has happend to a motor I was working on before, the header was cool to the touch and the others were hot.

Wanna_be
 

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resident gearhead
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20 Posts
Discussion Starter · #7 · (Edited)
This is a dual quad motor correct?

Check your power valves, work the linkage by hand and make sure BOTH carbs are spraying fuel. This has happend to a motor I was working on before, the header was cool to the touch and the others were hot.

Wanna_be
yes dual quad, but it dosent have holleys. i am going back to the drawing board tonight when i get home. i am just going to start from the beginning and double check everything. i have overlooked something, and it is going to wind up being something very simple when i find it. the only joy i get out of this is knowing it isnt my boat, its my cousins. my boat runs just fine.:D
 

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383 Posts
tuliped valves ??

check your valves! if you already put the head back on, you can put a straight edge on top of the valve stems. they should all be sticking out of the head the same distance.
i only post this because it just happened to me!
your problem sounds like a dead plug!!
 

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resident gearhead
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20 Posts
Discussion Starter · #10 ·
check your valves! if you already put the head back on, you can put a straight edge on top of the valve stems. they should all be sticking out of the head the same distance.
i only post this because it just happened to me!
your problem sounds like a dead plug!!
looked at the stems yesterday just to be sure they werent mushroomed. and when the head was off they were all closed. a sticking vlave should have shown up on the vacuum gauge.
 

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Registered
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75 Posts
A wire can ohm out OK and still be arching out to a ground.

Take a spray bottle of water and mist the wires/cap/coil (secondary system) with the engine running and see what happens (if the engine is back together). This is a very easy intial check that I do whenever an engine is misfiring.
 

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resident gearhead
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20 Posts
Discussion Starter · #15 ·
A wire can ohm out OK and still be arching out to a ground.

Take a spray bottle of water and mist the wires/cap/coil (secondary system) with the engine running and see what happens (if the engine is back together). This is a very easy intial check that I do whenever an engine is misfiring.
i was running it in the dark, it isnt arcing externally. when i pull the plug wire off, it arcs out of the boot at the cap to the dist. base.
 

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resident gearhead
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20 Posts
Discussion Starter · #16 ·
Do a compression check, and not with your finger. You're wasting your time untill you do.
not true. there is no magic number that it has to have, but it has to have something. if it has enough compression to blow my finger out of the hole, it should be able to contribute to the engine output. but, i do plan on compression and leakdown checking tonight. i just didnt have my tools at the house for that last night. hopefully the brainstorming session can help me out. thanks for all of the suggestions so far.:D
 

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resident gearhead
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20 Posts
Discussion Starter · #19 ·
If you have spark, fuel, correct valve movement and timing, then obviously you have a shop rag stuffed in the #5 intake port.:)sphss
sounds so obvious, but i dont have anything in there. tonight i am going to start all over. hopefully i will be back tomorrow saying i fixed it and be able to tell you guys what stupid mistake i made. just waiting on 5 o'clock
 
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