Vacuum Leak? How to find?

[ewingr]

I have a 350 Engine and suspect I have a vacuum leak. Number 2 plug, and number 1 to a lesser degree, are running almost white. Other plugs are chocolate.

It idles rougher than normal.

It was running VERY lean altogether and exhibiting symptoms of running lean: surging, very cold natured; even would die while driving at 15 mph. I jetted the carb up and that richened it up. But those front pluts are still white, and a bit of a rough idle.

I have sprayed carb cleaner all around the carb and intake manifold, and it does not change the idle at all, which would indicate the source of a leak.

Anyone know of a way to idenity for sure if I have a leak? I hate to go to the trouble of re-gasketing everything if that isn't the problem.

Thanks for your thoughts.

[Shortbus]

Weak valve intake springs on 1 and 2 cyls maybe? Or flatspot on the cam lobes. If you have no vac leaks around the intake I would think it must be something internal. Strange that it is only the forward cylinder on each side.

Sounds lake you have done all the right tests so far maybe a compression test would give some more insight on an internal problem.

[ewingr]

Well, I got home tonight, and looked a little more, and have a little more info.

The back two plugs, on both sides are dark. The front two plugs are half white and half chocolate. On the passenger side, #2 and 4, the white is much more pronounced than on the driver side, number 1 and 3.

Also, while letting it run, the headers turn glowing red hot at the point where the four tubes gather, right above the collector. Sanderson (the header mfgr) tells me this is absolutely not normal, and their first thought was the engine was very rich.

I would buy this, as I had overjetted the carb when the symptpoms of lean drivability started, and based on teh back plugs.

I am about to decide that I have blown the head gasgets between the cylinders.

As for why that would happen: this engine has a small B&M Holley Street blower on it, and I got on it pretty hard recently.

I would prefer to have a way to validate that the head gaskets are blown before pulling the engine apart. I may call Holley and discuss this with them tomorrow. I don't know if it is possible to blow head gaskets and not have symptoms of water in oil or something like that.

[Shortbus]

I still think it sounds like maybe a stuck valve, If you got on it hard and floated the valves it could be possible. Not highly probable but still possible.

Like you I would have a hard time ruling on blown head gaskets without white smoke from the exhaust or coolant contaminating the oil.

[ewingr]

I'd think it would miss if it were a stuck valve. Although, I'd think it would miss wiht head gasket too. Maybe I can pull valve covers and see what the valves look like.

[HannabilRox]

Sounds to me like it isn't getting a proper spark. Sporadic or no spark will cause unburned fuel to go through the exhaust system causing it to glow. I know, because my ignition module went out on me, only two cylinders though, not all six. Car still ran, sort of, but had all the symptoms of what you are describing.

[ewingr]

Hmmm...that's an interesting concept. Particularly when you consider...

I recently put on new Accel 300+ racing wires. (In other words, I've recently messed with something that provides spark).

However, the car has run better than ever...which has resulted in my running the RPMs up higher than in the past. (Speed happy, I guess ).

At any rate, that also contributed to my considering blown head gaskets. I blew one in the past when I got on it particularly hard...but it leaked water into the cylinder that time.

But, one thing I can say is when I get on it, it runs really good. My 'troubles' are when I drive it easy (which frankly is most of the time); driving in low RPM range; and particularly while the engine is cold. I'm thinkin' that if it was spark, it wouldn't run so good when I'm 'on it'.

Since this post, I have spoke with Holley, and they recommended 'stagger jetting' the carb. That is, richen it in the front.

So, what I have done is put standard jets back in the rear, left the fron jets upsized by 2, and I have put in a new power valve.

I'm about to just bite the bullet and go ahead and spend a weekend putting new head and intake gaskets on. But I think I should scrutinize the ignition.

[comp]

have you put new plugs in ??? look at Vac. line's .... Then there are other "cheap" test's but you need to know engine's

[MagicRat]

Try a leak down test. If done properly, you can diagnose leaking intake and exhaust or bad rings,or a blown head gasket and where its leaking.

[ewingr]

Originally Posted by comp

have you put new plugs in ??? look at Vac. line's .... Then there are other "cheap" test's but you need to know engine's

Yes, new plugs went in with the new wires. Ran great for a while. (It was shortly after that that I ran it pretty hard and it started running this way...leading to my thoughts about the head gaskets).

By the way, my 'definition' of running it pretty hard would not be perceived as all that hard by most, I suspect. I ran it up to just under 6000rpm. But I have since read that Holly recommends not going over 5K with this blower on it. So, maybe that is the problem.

On the leak down test, I've had that recommendation. I don't have any personal experience with that. I have called a couple different shops that seem to have a good reputation in my area, and while I didn't ask specifically about a leak down test, they said they didn't have any tests they could run that would answer this question.