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By unibodyboy - 16 Years Ago
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OK, guys, i've tried everything I can think of and nothing is working. Here's to hoping someone can point me in the right direction....
It all started when I developed what I thought was a vacuum leak in the intake manifold. Pulled the intake, found major tear in the gasket, replaced.it. Put everything back together(with a new BG Demon), the engine ran smoother but had a bad pop(sounds to be a crossfire, IE ignition) so I pulled the cap and rotor to check. Found the distributor was virtually rusted to pieces(nothing else in the truck is rusted--anywhere), so I promptly sent it back to MSD. They have since told me that the distributor was indeed bad, fixed it and sent back. However I have another MSD distributor new in the box, so I used it and nothing changed. This problem exists ONLY when the engine is warm to hot, in the 155 to 185 and up range. When it first starts up, it runs perfectly and once it is hot, I can't stop the popping. I've tried moving the timing all over the place to see if the marks slipped, but to no avail. Not sure exactly where to go from here.
292, rebuilt 30 over, 3000 miles on the rebuild with zinc supp
G heads, rebuilt, heavier springs hardened seats 1:5:4 rockers
Stock cam
Blue thunder
MSD Pro billet, 6AL, Blaster coil, MSD wires, AP45 plugs (and yes, they wires are in the correct order on both ends)
Reds Headers, no crossover straight out the back
Road Demon jr 525, or Holley SA 575
195 degree thermostat, temp reads right about 185
Compression checks at about 110, ran the valves, 19 thousandths.
No vacuum leaks.
Spark at all cylinders
All wires have been check for resistance
Anyone want to take a stab??
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By unibodyboy - 16 Years Ago
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Someone has to have an idea!
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By PF Arcand - 16 Years Ago
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When you say "popping" do you mean a backfire? Or popping at the exhaust?
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By Ted - 16 Years Ago
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Compression sounds low. Was the throttle in the wide open position when the compression test was performed? Did you run the compression test on all cylinders and if so, what was the range of the values. Generally accepted rule for compression test variance is 10% from lowest to highest reading. Is the vacuum advance to the distributor hooked up or unplugged. If hooked up, is it to direct vacuum or ported vacuum?
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By John Mummert - 16 Years Ago
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You might check your valve lash. If a valve is sinking into a seat it can tighten up to the point of misfire. We've also seen problems with unleaded fuel turning to gum on the valve guides if the car is stored too long. You would probably also hear some rocker noise if this was the case.
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By 55vickey - 16 Years Ago
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Noticing somewhat of the same thing with my 272, if I hold the rpm's around 1200 in park it will be so sweet and then a miss, tach jumps, can actually feel it in the car, then back to smooth again, then another miss and so on. I can't see a true pattern, just random misses.........hot or cold. Hate to make any adjustments as it starts and funs so good, just a little quirk that I'd like to have disappear. Gary
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By gentz - 16 Years Ago
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that there sounds like an ignition miss... possibly bad dist cap.. sounds more in primary side of the igntition..
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By unibodyboy - 16 Years Ago
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OK, some updated information. I ran the compression again with the throttle wide open, and the engine luke/medium warm. Compression is as follows.
149
156
155
160
165
165
165
157
161
It doesn't seem to matter where I set the timing or where the fuel is adjusted to. This only happens when the engine is hot, but once it is hot it will pop from the exhaust only (nothing coming up through the carb) and the RPM doesn't seem to matter high or low it happens. Since the distributor is completely new, including cap and rotor, I will bypass the MSD 6AL ignition box just to make sure it isn't the culprit. Also, I am running the ported vacuum.
I ran the valves less than 100 miles ago, however there is a possibility I made a mistake and will re check them tomorrow. I am concerned that when I adjusted the valves the last time the engine wasn't hot enough, how hot is hot enough? Also going to run a dial indicator on the cam to check for a wiped lobe, but with these compression number I don't see that as much of a possibility.
Ted, when we spoke you mentioned using a vacuum gauge to set the idle mixture, could you possibly re explain that process for me so that I have it on paper? Thanks all.
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By Ted - 16 Years Ago
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Greg. Sounds like you are touching all the bases. For the vacuum gauge, hook it up to the intake manifold or a direct vacuum source. With the idle mixture screws on your Demon carb initially set the same, then slowly adjust the screws in equal increments until best vacuum on the gauge is achieved. A good starting point for the Demon carb is with the screws each out at one full turn. If the vacuum decreases while opening up the idle mixture screws, then go the other way. If your particular carb has four idle mixture screws, then you need to be working with all four simultaneously even though two of them are on the secondary sides of the carb. You should be able to kill the engine by turning the idle mixture screws in all the way. If not, then the carb needs to be looked at more closely. Another suggestion is to take some carb cleaner and point the tube into each of the air bleeds located around the venturies and be sure all those are clear.
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By Brodie - 16 Years Ago
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I've been following this post for awhile and thought I'd chime in. It sounds to me like you have a "burnt" or sticking exhaust valve. Once hot and at idle, take a light cardboard sheet and hold it up to the tailpipe tip. When the popping occurs, the cardboard will most likely be drawn toward the tailpipe (suction). If you have dual exhaust, you can zero in on the bank that's causing the problem. I would suspect the cylinder that has the lowest compression reading. Might be a weak valve spring or more likely an exhaust valve tight in the guide that shows itself when the engine gets up to temperature. Just my two-cents.
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By unibodyboy - 16 Years Ago
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Thanks for all the suggestions guys. Problem is solved, it was actually running too lean. I ended up at 2 1/4 turns from closed on the mixture screws for the best vacuum. Ted, great advice on tuning the carb with a vacuum gauge, worked wonders. I am running a very large initial advance (reading of 16), but think the timing may have slipped a slight bit and so I set the initial with a vacuum gauge from the distributor as well. The things I can learn on this forum from some of you are limitless.
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