I don't know where to start... I guess I'll say I think the intake gaskets are leaking on a small block that I have. I don't get more than 3-4 PSI of vacuum at idle, the motor won't stay running once I let off the throttle, if I throw any amount of timing advance (more than 0*) at idle it will backfire out the carb, plugs are fouled.

The motor has new oil, significant amount of gas in the oil (as in I notice it on the dipstick)... kinda hard to adjust air fuel ratio with it backfiring. Everything screams common symptoms of a bad vacuum leak and rich fuel ratio.


I was wondering if this could be related or not; the plugs in cylinder 1,3,5,7 all look like they have oil (remind you right now my oil is new and not at all dirty but this stuff looks like brown oil) around the spark plug threads. Not on the plugs though. The valve cover gasket doesn't appear to be leaking and I can't feel any oil on the outside of the spark plug holes. THE PLUGS ARE DRY FOULED.


Could this be a really bad vacuum leak on the driver's side of the intake?

 

I forgot to mention I didn't do a leak down test but I did to a compression test and they vary from 190-210 psi across all 8 cylinders.

 

If you have gas in your oil, you are probably flooding the cylinders with gas. See if you can adjust the IMS on your carb to lower the amount of gas being pumped into the intake manifold and carb. Have you tried the carb cleaner trick? Just spraying it around the intake manifold gasket to see if your idle goes up? Then again that may not work if you're running excessively rich.

 

That dry fouling: are you burning oil? I have a leaky valve guide seal and that plug always had significant buildup around it from that. But I'm not sure that's what is going on with yours. Pictures?

 

Carb cleaner usually doesn't work on intake gasket leaks from past experience. It looks like it is fouling from fuel not oil if I had to guess. What are the chances that one side of the motor would have bad valve seals across cylinders 1,3,5,7 all in the same bank and not the other side? But still have a very bad vacuum leak?

The fluid is not on the plugs themselves but around the threads. I'd swear it is oil but my oil is still very clean (fresh oil change). It's a high zinc oil that is quiet clear compared to normal motor oil. It still shows up pretty clear on the dipstick but whatever around the plugs is not the same color/consistency (possibly unburnt fuel?).

 

The only time I've seen oil on the threads (and I don't have a whole lot of experience with this stuff) is when I had to break in a crate 350. I don't know if it was because the rings weren't set, or if it was the excessive fuel that I had. I was using an electric fuel pump and just couldn't get the fuel pressure low enough for that carb I had. When I pulled the plugs after that, there was obvious oil on the threads, but all of them. It's quite odd that it's only on one side. I would pull that intake manifold and check it out. I still think you have excess fuel too, but that might be second on the list to sort out. At least it has a carburetor and a little easier to take off the intake manifold. I don't have enough experience to be able to guess why one side has oily spark plug threads, only that I remember having an issue like that.

 

Ok well I just resat the intake today and noticed something odd. The timing indicator on my timing chain cover is about 10* prior to the TDC when the motor is at TDC on the exhaust stroke (I know this is technically 180* out but it's the point where the intake rocker shuts and the exhaust opens or vise versa).

I did check to see if the harmonic balancer might have twisted. Center of my marks shows the TDC mark. How can I figure out where the timing tab (on the timing chain cover) is supposed to be?

 

Did you build this engine yourself, or was it a create engine? I know many create engines don't come with the harmonic balancers or timing tabs, but those are usually keyed anyway so it would be difficult if not impossible to put the hb on wrong. I would say try to check the valves and when they open, but you are probably using hydraulic lifters. Try moving the tab to indicated tdc and see what happens with your vacuum/idle. That's the only thing I can think of to do. Essentially, you would be advancing the timing ~10*. Maybe that's why it was backfiring on you when you advanced it...?

 

Did you try and fire it up after checking the intake manifold seal?

 

3-4 inches of vacuum would have to be a huge leak. More likely some other issue. I assume you've connected a vacuum gauge and that the needle isn't bouncing, as in a bad intake valve. It's hard to judge valve timing by the overlap side without the cam card in your hand and a degree wheel and dial indicator. Check for timing chain slack by rolling the crank back and forth while watch the dist shaft for delayed reaction.

Are you sure that you were advancing the timing and not retarding it when you got the backfiring? I usually see backfiring when the timing is too retarded. Remember dist shaft rotation on the SBC is clockwise. One thing that will cause backfiring through the intake is one or more exhaust valves not opening sufficiently. I've found pulled out rocker studs that caused this. How about EGR? Does this engine have EGR, and is it stuck open?

One side having a large buildup of oil ash on the plugs could be a plugged oil return in the head. Sounds like you need valve stem seals anyway.

 

It was in fact the intake manifold not getting a good seal on the passenger side. Drivers plugs were getting the bulk of the fuel, what it couldn't burn I assume it was carbon fouling and making me think it was oil. All better today. Also found out the previous owner had the wrong timing tab when I resat the distributor.

 

He actually said 3 - 4 PSI, I presume he meant inches but maybe not.