70 K10 engine stumbling ???

[tkeduk8r]

Well for the first time in nearly 10 years, old reliable isnt acting old relaible anymore... during the past few weeks, my 70 K10 350/sm465/NP205/3.73 is acting up... under load the engine will begin to stumble and even an occasional pop through the carb... the problem is not repetative in that it doent seem to follow a pattern as to when it happens, or the severity of it... the timing is set at 12 deg initial and i am running a pertronix ig module in stock dist. The carb is a holley (todd-68c15 or shawn-71swb4x4 would know) 4150? 600 cfm... this is the third diff tank of fuel and third diff station for fill up.. also the problem has been getting worse as time has gone on, starting with a minor miss (i thought arcing ignition wire) to now a 'might be bad for the motor if i keep driving" issue... any of you guys had similar problems??? thanks in advance...
--Mike

[mrein3]

Do a compression test. You may have a flat lobe on your cam.

[1STtimePRo'71]

Mines missing too. I was told to do plugs, wires, cap, & rotar. Although mine isnt as bad as yours, good excuse to put a new cam in!!

[mrein3]

Originally Posted by mrein3

Do a compression test. You may have a flat lobe on your cam.

Ooops! I answered too fast. One time my engine was running as you described. It turned out I had a bad valve. A compression test will tell you if that is the problem.

Another time I had it run like you said and it had a bad cam. To diagnose that I simply pulled valve covers and watched one rocker not move as far as the rest. That bad cam is still in that engine...
sittin on a stand in my shed.

[VetteVet]

I think you may have bad plugs as they tend to miss under load while they might run fine normally. A bad plug wire is also a possibility. Here's a tip- wait till after dark when there is almost no light and start the motor and look under the hood and see if you can see any arcs around the plug wires. I found my miss like that. Did this happen after you installed the pertronix module or have you had it in for a long time. I bought one for my vette distributer and they told me to be sure and get the matching coil or the hotter spark from the module would burn up the stock coil. I haven't gotten the vette back together so I can't comment on that although I did buy their coil because I needed one anyway.
A weak fuel pump will cause the engine to cut out under load also.
When I bought my 67 step it had a flat lobe on the cam and the noisy lifter was very apparent but it was bad enough to kill that cylinder. I had a 70 longbed once that had several lobes worn and it ran like sh!t so you may be headed in that direction. Plugs and wires and a overall tuneup. Is the place to start. Keep us updated.

VetteVet

[tkeduk8r]

yeha im gonna hve to get the old lady in and do a compression test... however if it was a spun cam lobe wouldnt i be able to hear a timed repetative miss on every revolution??? this is not a all the time problem.. i can be going 55-60, running fine and then no power, bad vibration, and it almost sounds like a jake brake on a diesel... thn i back off and feather it and were good to go again.... i did do the nighttime in the dark plug wire check and no lightning, either by the coil, cap or wires to block... and another stange phenomena is that my oil pressure used to read about 52psi at 55 and now its down to 42psi@55... i cheked the oil and no shavings, but i havnt gone so far as to drop the pan or anything yet... the motor has about 35K on it since being put in in 1991... thanks for all the replys and keep em comin if you have any other ideas!!!...
thanks
mike

[Perchjerker]

Holley carbs tend to bust their power valve when they backfire through the carb. I would check that as well.

[curbyk]

I have a 95 suburban w/ 350, changed plug wires along with plugs, rotor cap and button, ran fine for a while then started missing occasionaly usually under load or acceleration, kept getting worse, put old plug wires back on...no more problem for over a year.

Good Luck