My compression reading were 141 to 129 all 8 cylinders and I am running premium gas. Is it true if the lower compression and higher octane might cause poor firing in the cylinders, premature firing. I did charge my third battery in the back by the onan generator and I got the beast to run. So the gas is good enough for the generator to run. How does carbon buildup in the intake affect the gas flow and stalling? I have multiple issues with engine related stuff. thanks

budworks521 wrote on Sun, 09 August 2015 11:06

My compression reading were 141 to 129 all 8 cylinders and I am running premium gas. Is it true if the lower compression and higher octane might cause poor firing in the cylinders, premature firing. I did charge my third battery in the back by the onan generator and I got the beast to run. So the gas is good enough for the generator to run. How does carbon buildup in the intake affect the gas flow and stalling? I have multiple issues with engine related stuff. thanks

I suppose this will be unpopular but it is my understanding that high octane does pretty much nothing for an engine that does not need high octane. If the compression ratio is high or there is a turbo or mechanical boost that in essence raises compression pressures, then high octane will be needed for knock or pinging. The manufacturer will state this is the owners manual.

If my 455 is pinging I know it is due to too much spark advance, or perhaps the vacuum advance is not releasing during low vacuum times and not retarding the spark.

I have never heard of high octane fuel causing missing. I would look to other problems, generally with spark.

The rest assumes you have the HEI distributor. My '74 came with a regular distributor (wires coming out of the top of the cap) and that meant it originally had points opening and closing to make spark. I got rid of that distributor as soon as I could.

If it is missing it depends on when this happens. If it is missing, is it regular or is it hit and miss? If it is regular, did any of the plugs seem dark, oily or covered with carbon? You compression test is good enough that all cylinders can fire. There is enough compression for firing.

If it is one cylinder, missing regularly it could be a plug, the wire to the plug or the distributor cap.

Now if the missing is irregular, even at idle, I am thinking module or coil. This many times gets worse when it warms up. If irregular missing happens when you give it some gas (while driving), especially at low RPMs this can indicate weak spark (Again module, wires or coil) Opening the throttle lets in more air and increases the pressure in the cylinder. A weak spark just cannot make the jump in dense fuel-air.

To look, shade tree style, at spark, you can pull one plug (or the wire and use a spare plug), lay it on metal, connected to the plug wire. You should see a hot spark. An HEI distributor should be putting out a real zap, seen in daylight. WHen you do this, don't have the plug near the carb or and fuel smells. (Be careful pulling those wires off. Twist it back and forth or you might leave part of the wire attached to the plug.)

Of course, doing the above is not testing the plug but does test wires, module and coil to make sure the system is generating good spark.

I have heard of testing the spark regularity by hooking up the timing light and clamping the lead on different wires. You can't time unless it is on one (or six) but you should see regular flashes when idling. You might catch it missing and not seeing a light with some patience.

For advance, make sure the vacuum pot on the side of the distributor is fee and the lever coming out of it can move in and out. It should spring back. Vacuum from the engine pulls it out of the distributor, advancing the spark. If it does not return to the in the distributor position, something is wrong. It will be too advanced and when you climb hills it will ping and you may think you need high octane.

You ask about carbon. Carbon in the engine can cause pinging because it can get red hot and light the gas before the plug does. THis is often accompanied with running on... not stopping when the key is turned off if you have a carburetor. There should be no carbon in the intake. That would mean an intake valve is letting the explosion come back upstream, or it is running so lean that it is popping through the carb all the time. An intake valve leak would be bad, but your compression test show reasonably well sealing valves. Before rebuild I had 40 lbs on one cylinder and it would occasionally cough back through the intake.

I run the cheapest lowest octane fuel I can find, but that is just me. I am not "right". I have been known to put in a higher blend if I am going to climb a super pass but I rarely do this in the GMC. Midwest 85 octane seems to work just fine. But on a hot day and with a steep hill it will ping. (I have fuel injection and electronic spark control so it retards the spark automatically when it starts pinging.) Excessive pinging may mean it is time to time the engine.

I should add that an engine with knock sensor controlled spark may get worse mileage on regular than high octane because if it pings the spark is automatically retarded and running around with a retarded spark will for sure result in lower mpg. My Chevy Volt requires high octane (grrr) and using lower octane can diminish mpg. (I guess.)

Lastly, you mention stalling. That is one of two things... gas or spark. Again, I think spark. But fuel needs to be there as well. I talked about spark so much because I have chased so many "starving for fuel" problems only to find out it was spark.

Good luck. You will get to the bottom of this.