318 tired, compression test proves it...

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DQ81

'73 Dart
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Started trying to track down the reasons my 318 runs inconsistent below 1500rpm, the skips, misses. At a stoplight it feels like it's gonna choke one second and tries to lunge forward the next...

After using a vacuum gauge to set the carb up today I did a compression test.

Results not so great:

around 105 psi per cylinder, the lowest being 101 and the highest being 108

I also noticed my spark is really weak. So I'll check for ground issues asap, replact the cap, rotor, wires and coil with an internal resistor...

My charging system doesn't charge wel under 1500rpm...any possible link to the weak *** spark I"m getting?

Gettin' kind of bummed with this car, it's just one thing after the other, the money and time pit... With that kind of compression I'm just gonna look for a BB or a 360 to chuck in it for sanity's sake...
 
All within 7 pounds of each other, I don't think you have a compression issue. Even if overheated, at least a couple cylinders should be higher than the others.

Find something that runs good, a 318,and see what it has, using your guage.

I would look for a vacuum leak, lean idle mixture, and spark issues. Weak compression shouldn't cause that.
 
dodgetkboy78 said:
All within 7 pounds of each other, I don't think you have a compression issue. Even if overheated, at least a couple cylinders should be higher than the others.

Find something that runs good, a 318,and see what it has, using your guage.

I would look for a vacuum leak, lean idle mixture, and spark issues. Weak compression shouldn't cause that.
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I don't think the compression is the only problem but wouldn't you agree that the compression reading I"m getting is rather low? Especially considering I was planning on putting a cam in this thing...
 
Yes, Buuuut......

You need to make sure YOUR guage is reading correct.

try this, put a couple of Tbsp marvel in each cylinder, and do the check on each cylinder again.

I have two guages, one reads 35PSI lower at 100 than the other.

Your 318, if stock, should be around 130-135
 
My 318 was around 110 psi when I measured it before I took it out and that was at 7000 ft. elevation with mostly receded exhaust valves. Mind you the minimum cranking compression for a 318 as specified in the factory service manual is 100 psi. I think you should start worrying about compression if you get noticeable blowby out the tailpipe or out the valvecover if you remove the PCV valve.

EDIT: If I remember correctly your car had the crappy messed up stock Carter BBD carb. I'd first blame that and possibly retarded cam timing before I worried about low compression causing poor performance.
 
I agree with both of them. I dont think your compression has to do with it at all. My last 318 was a great motor and only read 110 on the highest cylinder. I think that is normal. At least according to my book.
 
Is there anyone around you that can tune an American V8 well? It has low compression, but not fatally low. My buddy's Jeep used to drive daily with 55psi... gear braking down a hill would produce 4' flames out the tailpipe..lol. That, was a bad motor. I'm not sure what you've done already as far as tuining. But the same stuf applies to stock parts... Timing curve, initial timing, idle speed, and idle mix are all critical.
 
dodgetkboy78 said:
Just tested one cylinder in a stock compression 318 I am building soon, 120psi.

Itis a 70 318 though.
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Hey thanks, how did you measure your compression? I'm starting to wonder about my methodology and my gauge...it crapped out on the last cylinder when I went to test that one a second time.
 
MOPEkidD-3 said:
My 318 was around 110 psi when I measured it before I took it out and that was at 7000 ft. elevation with mostly receded exhaust valves. Mind you the minimum cranking compression for a 318 as specified in the factory service manual is 100 psi. I think you should start worrying about compression if you get noticeable blowby out the tailpipe or out the valvecover if you remove the PCV valve.

EDIT: If I remember correctly your car had the crappy messed up stock Carter BBD carb. I'd first blame that and possibly retarded cam timing before I worried about low compression causing poor performance.
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That's right the ol' Carter BBD POS is on there. What kind of blowby do you mean? I just replaced my valve covers and gaskets and had a ton of oil blowby out the oildipstick when I didn't run a PCV, have one now and all is well.
 
I'd dismiss the compression theory, but i would consider the ability to get good spark to the engine and possibly give that carb a good bebuild or play a little shot put with the neighbor kids. It maybe something as simple as a ground issue or a faulty regulator or alternator. How about a vacumn issue?
 
Small Block said:
I'd dismiss the compression theory, but i would consider the ability to get good spark to the engine and possibly give that carb a good bebuild or play a little shot put with the neighbor kids. It maybe something as simple as a ground issue or a faulty regulator or alternator. How about a vacumn issue?
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Another good call.

I'm not saying it's the compression that's causing it to run the way it does, (which btw is a lot better today after I cleaned my plugs and set up the carb with a vac meter)

I've a got a new coil, distr. cap and rotor, wires and plugs to put in. The cap looks particularly worn. Once the spark is nice and bright blue/white as opposed to yellow as it is now I'll put the 360 DP manifold on with the rebuilt TQ I have on hand, getting them painted pretty today :)

I just wonder if it's worth putting in the cam I have with an engine with this kind of compression. Although I must admit I didn't take out all the plugs, but I have a high-torque starter so I figured iti didn't matter. I also neglected to keep the throttle at WOT so I'm back out there right now, with a new gauge to do it the right way...hopefully I'll get better results.
 
I just went out this morning and tested a '72 318 that is in my garage and I got an average of 110-114 being the highest with both of my gauges. I hope that helps you. I dont know if the compression was better throughout the years but there ya go.
 
Thanks for all the input.

I realized after a pm from mikesdart (thanks dude!) that I had stupidly left out a critical step in my compression test. I usually have a helper but was on my own this time.

I forgot to hold the throttle at WOT while cranking! DOH!

Difference is considerable compared to my first testing! Where before all cylinders measured around 105 psi with very very slight variation I now have the following compression results:

1- 138 PSI
2- 136 PSI
3- 138 PSI
4- 139 PSI
5- 146 PSI
6- 125 PSI (is this difference detrimental?)
7- 144 PSI
8- 145 PSI

It's worth noting that I tested all cylinders without holding the throttle at WOT also today and that yielded the exact same results as yesterday, just goes to show, make sure to follow all the steps to a T.

These results put a smile on my face, although cylinder 6 worries me slightly. Should it?

Get all the new ignition components in tomorrow. Hope that helps, if not it can't be anything other than that crappy Carter BBD carb as Moper noted. I just want to be sure everything is up to par before I try to dial in a new carb.
 
I wouldn't be too worried about cyl 6. Should be alright. The stock Mopar alternators suck at low RPM's although I would check how tight the belt is.
 
goldduster318 said:
I wouldn't be too worried about cyl 6. Should be alright. The stock Mopar alternators suck at low RPM's although I would check how tight the belt is.
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Sweet thanks,

Now this mechanic I know, a bit of a negative Nancy, is trying to tell me that my most recent readings are too high, thus not "possible"....any sense in that? Or is this guy just talkin' smack?
 
He's mildly confused. You verified it by doing it twice. A small cam engine with good rings and valves will have pressures like that. It's not a problem. If you're nervous... do it again.
 
moper said:
He's mildly confused. You verified it by doing it twice. A small cam engine with good rings and valves will have pressures like that. It's not a problem. If you're nervous... do it again.
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Lol, no I'm not worried, I think he's just lookin' for business haha.

I'm sure of my work, I tested every cylinder at 10, 20 revolutions, then also checked by watching the gauge max out and stopped there. Did that twice on every cylinder. I'm not worried. Just sometimes being a relative noob all the conflicting information can be confusing, that's why I like this place, on the basic stuff everyone agrees.

You guys have been a great help! Thanks a million!
 
goldduster318 said:
Its the design. Its just not going to perform well at low alternator rpm's. So if you made it spin very fast at idle, you'd trash it when you take the engines to high rpms.
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The pulleys use a regular v shaped belt.

Anybody ever put a different alternator on with an internal regulator? I saw a guy recently who put a 90Amp Volov alternator on his chevy...works well.
 
Those compression results look very good. I would not be concerned about it. Your problem is isolated to the carb or the ignition. Work through both systems and you'll probably find the problem.
 
Beware of the lure of higher amperage alternators. The ammeter for your 73 is going to be set up for +/- 40 or 60. If you have a charge situation that exceeds the gauge by a good bit, it will fry the gauge. When that happens, the car will start and then die as soon as the key is released from the start position. If that happens, put both leads to the ammeter on the same post, by-passing it. This will allow the engine to run but remove the ability to monitor the charging system.

My 73 has the lower capacity of the two alternators that were available on the 73s. It does not charge much until you are running more than 1000 rpm. Headlights and interior lights get dim when stopped at a light.

I agree with most that the irregular running sounds like a vacuum leak. One source on the carb that gets overlooked is the throttle shaft. If it wobbles around, it's a problem.
 
72 Demon said:
I agree with most that the irregular running sounds like a vacuum leak. One source on the carb that gets overlooked is the throttle shaft. If it wobbles around, it's a problem.
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I'll second that, I'm pretty sure it was part of what made my 318 act the same way. It was weird how I could never get above 14 in-Hg of vacuum at idle no matter how much I fiddled with the idle mixture screws.

On another note if you do end up swapping your timing set, I HIGHLY recommend you remove your oil pan, pump, and pickup and clean them out of all the debris that I am 95% sure will be clogged up in there. Before I put my oil pickup back on I washed it out and was amazed by how much crap came out of it.
 
Just completed a compression test on my 318 69. According to a manual on hand, the difference between your hi and low reading is within limits.
 
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