I have a strange problem (at least to me).

I have a ZZ430 (clone) in my 69. It has recently developed a "burble" at idle and steady cruise speed. Intermittent misfiring. Seems like a vacuum leak but can't locate one yet. 3k miles on engine (new GM cate). The engine has GM aluminum fast burn heads (vortec) with the GM hot cam and 1.6 roller rockers.
Here's what I've checked so far:
Spark is good. Wires are good. Plugs have slight gray color. (except #4).
During troubleshooting efforts I measured the exhaust temps at the headers trying to identify which cylinder is missing. All come up to 500°+ quickly except #4. It stays below 300°F until some throttle is given. It seems like it's not getting fuel at idle. This plug is pretty white so it seems to confirm lack of fuel. When I accellerate it smooths right out.
I've sprayed WD40 around all the usual locations to locate a leak and nothing changes. I can pull the plug wire off #4 at idle and no change in idle so I'm sure it's the culprit.
Any suggestions or history o what where to check next?

Thanks for the help.
joe

How you been Joe? If this is a carb'd engine I can't see how one cyl would be getting less fuel than the others since you can't find any vacuum leaks to lean the hole out. It sounds to me like a cracked plug or bad wire.

A quick test is to swap plugs and see if the symptom moves with them.

Joe it's really too new a setup to have anything worn out. With the dual plane manifold and the fact it didn't do it before it almost has to be a vacuum leak at #4 intake port. You going to Clayton tonite if it doesn't rain?

I vote for a #4 port vacuum leak, ie, changeout of intake manifold gasket is coming soon !!!!

I vote for a #4 port vacuum leak, ie, changeout of intake manifold gasket is coming soon !!!!

The leak could be on the underside pulling crank case vapours into the runner on #4. Unfortunately, there is no easy way to find those short of pulling the intake.

dont forget the vacum modulator on the tranny!

good place for a vacum leak that everyone overlooks.

On the dual plane manifolds, the left plenum feeds cly 1, 2, 4 & 8, if all dual planes are equal so that eliminates the carb fuel distribution other than having a weaker vacumn signal b/c carb is further away from manifold floor. So you are not having problems with other clys? If not ,, go run a compression on #4 and see what you have compared to 8. Inspect the valvetrain parts, if all is in order then you have to be having problems with ingition , like the gentleman before me suggested. Be sure to check your wire resistance.

Thanks for the quick replies.

I had already swapped out plugs and wires; no change. Wire resistance is about the same as the other wires.
I did a compression test also; 175# and solid. Close to the other 2 I checked. I only checked the easy ones to get to (headers on a small block are a pain)
I would think if it was a vacuum leak,it would be lean and the exhaust would run hotter, not cooler, but maybe it's big enough to back out the fuel? Maybe leaking internally?

The manifold is a GM dual plane; carb is Holley 4150 750 VS. (#08770).

Stange......


Mike, it looks pretty wet out there...........
We'll be at Howies Friday.

How about a bad #4 terminal on the cap Joe? You might also check valve adjustment on that cyl...

You keep saying vacumn leak, is your booster leaking internally? can you eliminate all vacumn ports and see any diff?. The main vac fitting in right on same plenum that #4 is . Just a thought. From the looks of this part # on your carb,, its a single idle circuit I presume.

Joe I got right to the edge of town and it started pooring. See you Fri.

Quote:
I would think if it was a vacuum leak,it would be lean and the exhaust would run hotter, not cooler,

Nope, if no fuel is being distributed into the cylinder at idle, it ain't gonna heat up. Rev the engine up, more air gets distributed, and fuel comes in, cylinder gets hot.

Quote:
I would think if it was a vacuum leak,it would be lean and the exhaust would run hotter, not cooler,

Nope, if no fuel is being distributed into the cylinder at idle, it ain't gonna heat up. Rev the engine up, more air gets distributed, and fuel comes in, cylinder gets hot.

That seems to be happening.
I've disconnected (and plugged) the vacuum hoses with no change. I haven't removed the threaded fitting and plugged it yet.
I'll be pulling the valve covers to verify actual valve movement tonight. then it looks like pulling intake next.
The carb is single idle circuit.

I thought about a bad cap connection, but I am getting spark........Maybe I'll change caps just to be sure.

Thanks
joe

Found the leak. Guess it got big enough. Intake gasket at #4. Looks like time to pull it.
Thanks for the help.

joe

If this is a new motor, since I know little about the ZZ series engine, does it have a flat tappet hydraulic cam, or a roller?

My bet is that if you have a flat tappet cam, you have lost one or more lobes on cylinder #4. Pull the VC on that side of the motor, fire it up and watch to see if the rockers are moving the same amount as compared to those on opposing cylinders.

Let us know what you find.

It's a roller cam. All the rockers move OK.

The leak is just above #4 cylinder at the intake. I just got the upgraded felpro gasket set; MS98000-T that's supposedly the "fix". Hope so; it's an expensive set. About $80. has a bunch of pieces I don't need but it's the only way it comes.
I'll be back..............

thanks
joe

$80.00 for an intake manifold gasket set?

I got mine for $75. They come with some pieces you don't need (for injected motors) but they seem to cure the leakage problems of the factory and earlier Fel-Pro stuff.

Did you install the engine using a plate that bolts to the carb flange? The vortecs only use 8 intake bolts instead of 12 - none in the center. It's possible all of that weight hanging from the intake dislodged the gasket. (If you used the carb plate)

Another thing just FYI. My fastburn heads (and others here too) had no sealant on the intake rocker studs that thread into the intake port. A fair amount of oil was drawn in through all 8 of them and left a gooey gunk on the back sides of the itake valves. Since you'll have it apart, might be a good idea to pull the intake rocker studs and use a good thread sealant on them.

Yeah, I hung the engine/tranny from the carb plate. I was thinking the same thing about the leak being self induced............Won't do it that way again. That's alot of weight hanging from aluminum.
I pulled one rocker stud; it had sealant so I left the others alone.
The car runs good again. It's had an annoying stumble off idle since I installed the engine last summer. It's gone now so I probaby have had a small leak since the install; it finally just got big enough to identify. The stumble is gone now!

$80 for a gasket kit (with a bunch you don't need) is pretty pricey. But it is a real well made set. Built like an oil pan gasket.

Thank for the help.
joe

Joe,

Sorry I did not see this sooner, but I had exactly the same thing happen on mine but on 1-3 cylinders. The engine was not lifted with an intake hook.

I'm convinced that it was post installation error. I went back after 5-6 hours of driving and re-torqued the intake. That's when the problems started. After reparing the gasket with a new GM part, following the instructions to the letter, and not retorquing after installation, it has been perfect.

Thanks Carl. I did retorque the original installation. Maybe I'll just leave this one alone. 132 in/lbs isn't very tight to begin with.

joe