360 a miss

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Valiantsignet20

Valiantsignet20

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I have a dead miss that occurs through the whole rpm range. Not as noticeable at wot but still there. I thought vacuum and changed intake gaskets and visually inspected valvetrain and intake. All looked good.

Here is the engine specs.
360 .030 over bore.
Flat top pistons with slight dish
stock bottom end with windage tray
Double roller chain w/ tensioner kit
Comp cams .488/ .498 lift 274 / 286 .006 duration
Comp cams 867-16 anti pump up lifters
Comp cam cupped push rods
273 adjustable rockers
340 Valve springs damped
360 J heads w/ 2.02 valves
Weiand dual plane intake
Holley 600 Double pump w/ 2 corner idle 70 primary jets, 80 secondary, 6.5 power valve
msd 6al box
Mopar perf dist
8mm wires accel
Autolite plugs.
rotor and cap good
Stock fuel pump at 9-14 psi New reading is 4-6 psi.

Compression on my antique comp gauge is.
1 = 100
2 = 90
3 = 100
4 = 90
5 = 90
6 = 100
7 = 85
8 = 85
The engine was cooling off as I progressed.

I also ran a cranking vacuum check which the needle fluttered between 0 and 5 indicating a leak? I also ran a running vacuum at 2000 rpm. This showed a consistent drift between 19 an 18.5. While doing this I unplugged each wire and took note of a drop of 18 hg on every one with the engine changing sound on everyone. Not really indicating which cyl is a miss.

Short of tearing down to the shortblock I don't have an idea. Anyone else
 
Do you think it's possible that a little too much preload on those anti-pump up lifters could be slightly floating the valves at times? The fuel pressure looks a little steep, hopefully it's not that high at the carb, but if you were really overly rich i think you'd be able to tell. Also, what is your initial and total timing and do you use vacuum advance?
 
in my opinion , that fuel pressure is too high for that DP . most stock fuel pumps are already around 5.5 - 6 or so. you should be around 6-7 lbs. i would start right there if you are pushing 9 -14 pounds of fuel pressure. it may be pushing fuel past the needle and seats and dumping fuel. how do your plugs read?

my fuel is regulated at 5.5 for my holley 600.
 
I have thought that about the lifters. But I have adjusted these to a 1/4 turn after quiet. How would I test them for a pumped up lifter in the block? I have gone through pushing on the rockers and noticed some bleed down on the plunger springs and others are tight.

I don't believe my fuel pressure is that high. I was going by my pressure gauge that i just happened to notice isn't zeroing out. It's probably half that. I will take a reading with my vac gauge too see. It's not overly rich. A tad on idle but no smoke out the pipe at acceleration rich.

As far as timing. I don't have timing tape or a fancy adjustable light. i use the light to take it to about 10 w/ vac unpluged and throttle and hot start test. Then road test for knock. I do run vac but don't know when it comes in.
 
Took new FP reading. 4-6 psi. The plugs are new and still look it. Not enough milage yet. The old plugs where carboned from a previous intake vacuum leak sucking oil and over rich condition 750 with a warped TB.
 
I think as far as your lifter preload your fine. The ability to get a little depression by hand on some is more then likely not a issue. I think that's just dependent on the different positions in the bores, and that type lifter will bleed some anyway. Don't see anything with the timing that would cause it either. I am curious if the miss is the same with the vacuum disconnected? While we don't know your exact fuel pressure, as i suspected it's not as high as you thought. 14 lbs. and you'd have gas pouring out the bowl vents....lol. Have you run this in the pitch dark and observed if you have any arcing under the hood? I'll think on it some more and see if anything comes to mind.
 
Valiantsignet20 said:
Took new FP reading. 4-6 psi. The plugs are new and still look it. Not enough milage yet. The old plugs where carboned from a previous intake vacuum leak sucking oil and over rich condition 750 with a warped TB.
Click to expand...

4-6 is much better lol , you are fine then as far as that goes.

OldmanRick said:
I think as far as your lifter preload your fine. The ability to get a little depression by hand on some is more then likely not a issue. I think that's just dependent on the different positions in the bores, and that type lifter will bleed some anyway. Don't see anything with the timing that would cause it either. I am curious if the miss is the same with the vacuum disconnected? While we don't know your exact fuel pressure, as i suspected it's not as high as you thought. 14 lbs. and you'd have gas pouring out the bowl vents....lol. Have you run this in the pitch dark and observed if you have any arcing under the hood? I'll think on it some more and see if anything comes to mind.
Click to expand...

i agree on all accounts here. if it is a dead miss on one hole , it is usually a plug or wire . if it is a sparratic misfire on more than one hole, it could be in the ignition box or distributor advance doing weird things. it could also be a rich or lean mixture
 
73AbodEE said:
if it is a dead miss on one hole , it is usually a plug or wire . if it is a sparratic misfire on more than one hole, it could be in the ignition box or distributor advance doing weird things. it could also be a rich or lean mixture.

I mainly keep leaning toward a ignition problem, although not sure where. VS20, by dead miss do you mean a steady, repeatable, rpm consistant miss? Also, is this a recently developed problem and did it correspond with any work, mods, event? Also, is it something you can hear at the tailpipes? Meaning left, right, both.
Click to expand...
 
by dead miss do you mean a steady, repeatable, rpm consistant miss?

Yes.

Also, is this a recently developed problem and did it correspond with any work, mods, event?

I had it slightly before and always assumed it was the cam grind. I have a OD tranny and a 3.23 8 1/4. The engine hardly ever sees above 2k in normal driving. I assumed it was chugging because of cam lope.... I recently had a drive-shaft failure that wiped out my bell-housing and clutch package. After replacing those it became more pronounced.....

I then replace my worn distributor and plugs, checked valve lash re-replaced intake gasket with out end seals assuming another internal leak.

I am going to go through my plug wires with a good one in a minute to see if that could be a culprit.

Also, is it something you can hear at the tailpipes? Meaning left, right, both.

Yes, It sounds like driver side
 
Valiantsignet20 said:
by dead miss do you mean a steady, repeatable, rpm consistant miss?

Yes.

I recently had a drive-shaft failure that wiped out my bell-housing and clutch package. After replacing those it became more pronounced.....



Also, is it something you can hear at the tailpipes? Meaning left, right, both.

Yes, It sounds like driver side
Click to expand...

if you recently had a driveshaft failure, were you turning high RPM's? you could have hurt something with the high revs. if you have it narrowed down to the left bank, it should be easy to pinpoint the cylinder. by taking the plug wires off one at a time with it running. you may have done this already, but i would try it again.
 
if you recently had a driveshaft failure, were you turning high RPM's?

I was cruising on the interstate at 1900 rpm. Ujoint cup came out of the front yoke.

by taking the plug wires off one at a time with it running.

I am going to do that after I post this
 
Seeming how you hear a difference in the left side, then 73AbodEE's double check is a good idea. When i saw clutch replacement, a balance issue popped into my head? Although that's not likely.
 
Just got through with unplugging each plug wire and noting a change in vac and rpm in all 8 of them....miss still evident no matter the combo. The vac gauge is rapidly moving between 19 and 18 when the miss occurs. I am suspecting a valve problem.

My next trick will be compressing air into each cyl and check for a leak down and hiss. It will be a day or two before I can do that. I need an adapter for the plug hole.
 
Please let us know if and what you find. This will be one for the memory bank. Good luck.
 
Valiantsignet20 said:
taking the plug wires off one at a time with it running.
Click to expand...

Valiantsignet20 said:
Just got through with unplugging each plug wire
Click to expand...

The IDEA of this test is good but do NOT do it this way. NEVER pull plug wires like this, especially with electronic ignition. It can "kill" the box.

Also, pulling wires causes crossfiring which confuses results.

The RIGHT way to do this is to SHORT each cylinder

I do this in a couple of ways.

Sometimes I use a grounded probe, loosen the boots at the dist, and loosen the wires so they come out easily. Stick the grounding probe down beside the wire as you pull it out, and as the wire comes out, stick the probe into the dist. cap so that tower is grounded, and not crossfiring to other cylinders.

Another good way to do this is to get small brads/ nails, and slip them down into the dist. towers beside the wires. Then you can just take a grounding probe and go around the cap.
 
The IDEA of this test is good but do NOT do it this way.

I didn't know that. I was grounding the wire to the exhaust man after unplugging. Hopefully no foul.
 
That would be good, so far as telling you what the cylinder is doing, but it can still "kill" an ignition.
 
Done a cylinder leak down test today. #5 is whistling dixie through the carb at 13 psi. Now to pull the head and see if it is a burnt or bent valve or ?.... What do ya'll think the chances of replacing the valve will fix it. Or redo both heads completely.

I had a problem with that valve when I had the heads done 5 years ago. The guide was too tight and the valve stuck and bent it up nicely.
 
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