As posted previously, I finally put my PRH-ported Speedmasters on my 451 Dart, and drove it to the first-of-the-season West Plains Car Club cruise-in at Steak & Shake!

It seems I have a little work to do - the (Performer RPM) intake manifold must not be sealed properly so it idled rougher, less vacuum, and AFR leaner, and when I let off the gas on the highway and hit it again, a puff of blue smoke comes out the tailpipes. All of those are classic symptoms of intake leaks sucking oil from the valley.

I re-used the valley pan (once) and brushed on Permatex #2 for the sealant on both sides of the pan (Ultra Black on the ends).
The manifold feels just a little too high in the V and the bolts wouldn't start. I rat-tailed the holes in the intake just a few thousandths (barely removed the paint) and then all eight would go in. (The heads were cut .055 I think, but obviously Dwayne knows how much to cut the intake surface) Also retightened the bolts.

Another variable is the lash on my lightly-reground Mini-Express. Schneider recommends .018", so I set them to .012" cold, expecting the typical .006 growth of the aluminum head on iron block. Previously with the iron heads, I'd set the lash "a little" looser than the usual +.002 for break-in, but don't remember how loose, and never got around to readjusting the valves in, now 1370 miles.

So I'd expect some vacuum drop at least until the engine warms up, and maybe even a leaner AFR due to the increased overlap (charge dilution) with tighter lash? But valve timing wouldn't cause the puff of oil smoke... that has to be oil getting sucked in to one or more ports since the guides and seals are all brand-new... also, when I abruptly let off the gas after accelerating to 5200 rpm, I could actually hear an intake sucking sound...

My plan is to experiment with propane or carb cleaner. Luckily the big-block is an easy manifold removal. I'll get a new valley pan, and use a thicker sealant. Probably Ultra Black unless everyone says to use something else

thanks for any tips.
-Charles

Should be a streak on the surface where it is leaking. Pull it and see where it is and work on a solution from there.

Maybe thinner gaskets to let the intake sink down into the lifter valley further
It sounds like the intake needs some surfacing to make it fit properly.
If it was me I would take the intake and valley pan off and then sit the intake on the heads and use a feeler gauge to see how much clearance I had at each corner(all four) at the top and bottom between the intake and heads to see it fits square or not
I like to see a tiny bit (.0002 to .0015) wider at the top of the intake to the heads or dead even, not wider on the bottom surfaces
BTW, screw Ulrta Black sealant Gasket cinch on the fiber gaskets to seal them and maybe a tiny dab of adhesive silicone on the corners of the valley pan and block to head corners between the valley pan and heads and block
Last edit, I hope. If the intake ports are not shiny there is no oil, if they shine there is

Thanks for the tips. When I've recovered from this week's slog to get it back together, I'll start measuring, and looking for the location(s) of oil intrusion.

The problem is, in order to get the bolts started, I couldn't use any of the fiber gaskets. Just the bathtub valley pan of course, which has limited compliance (especially when already crushed once). This intake worked fine on the iron heads and I had no problem with leaks then. Can't take anything else off without removing metal.

In this area it may be difficult to find a machine shop that is capable of doing precise machining on the intake...

Did you have to rat tail the holes upward or downward to get the bolts in ? If downward a THICKER sealant is only going to make matter worse , you intake needs to be cut because it's not fitting properly , which is why you had to file the bolt holes.

If you had to rat tail them upwards then adding the thin paper gaskets between the pan and the manifold would have been the proper solution ... maybe on both sides of the valley pan if it was really low ...

Filing the intake bolt holes is not the right way to fit the intake.

I use thin paper gaskets glued to both sides of a new bathtub, with a thin smear of grease on them, and silicone at the china walls.
I've never had a leak of any kind as a result.
Surfacing of heads/block has usually been fairly minimal for me, but the head/intake fit gets checked and addressed if necessary, as others have said.

Downward - but the sealant will band-aid the problem if the gap is bigger at the bottom than the top (i.e. the angle of the manifold face does not match the angle of the head intake face). As already pointed out here, I need to take everything off and use feeler gauges to see where it's not mating properly.



Yeah, no [censored] Sherlock but I don't have a Bridgeport mill handy, nor a nearby automotive machine shop I can trust! And I wanted to get the car running again while the weather is nice and cruise-ins are frequent

i check the intake as Cab and others have said, and also use a smear of grease on the head side, and silly-cone one the china walls, big and small block alike.
when using the silly-cone of your choice on a small block, make sure the wall "guide pins" for the gasket are removed, as this will make sure your intake doesn't bottom out on the ends, thus not allowing it to fully seal on the heads.
just an old geezer spilling his experience with a small block intake leak he couldn't find. ............. [as my old man uset'a say : Back up to an A$$kickin' machine with a bag full'a quarters and start shovelin' 'em in !" ]

Interesting you should say that... earlier this afternoon I discovered what I think is the problem.

I pulled the carb and manifold and the only unusual thing I saw was a puddle of oil on top the valley pan.
No oil in the ports. Pulled the valley pan and it looked like some of the sealant had run down the pan, confirming a big gap.

I set the manifold directly on the heads and I immediately found that the "tips" were hanging up on the Chinese wall and I couldn't slide the manifold back and forth to line up the bolt holes without one end or the other hitting the wall. And when I had torqued it down previously, it deformed the valley pan there too like a sheet metal brake!

This is one of those "I shoulda thought of that"... with .055 milled from the heads, and my block has not been decked (it was really close when I built the short block a long time ago).

A couple minutes work with a die grinder and it then sat on the heads perfectly... So I put the valley pan back on with the bolt holes centered, and sealed the corners and the end rails. I also cleaned the Permatex #2 from the valley pan, as many people don't use sealant around the ports at all. Intake back on - only had to wiggle it a little to get all the bolts in and torqued to 25 ft-lb.

HOWEVER.
It still is running leaner and with lower vacuum, even after warmup. At least it will now idle without having to tickle the gas pedal, so that is an improvement. I waved an unlit propane torch around all the ports, especially underneath, but there was no effect on the idle at all. I didn't get a chance to see if the puff of oil smoke after decel is gone tonight.

Is it possible that my new, much better-breathing heads (made that much difference to the carb tune? I figure at idle, the throttle butterflies are the most important airflow limitation

Also the lash is tighter (set .006 tight for aluminum since the hot spec is .018), so maybe more overlap. That could definitely make vacuum drop, but from 8-9" Hg. down to 6-7"? I'll need to check one hot and see if it actually loosens up .006, as every engine is different.
Edited to add: I've been reading on other forums and it does indeed appear that it can make that much difference! I'll check one hot, but I think I'm going to run them again at .003 or .004 looser and see if my already-borderline idle manners come back

Any more thoughts? thanks.

tighter lash = bigger cam, correct
Murphy loves messing with us Hot rodders, especially Mopar lovers
Good find

Had the same problem with my Performer RPM. Had to grind all four corners. It's not an uncommon problem.

So I have learned! Thank goodness for the Internet
What threw me off is that the same intake didn't interfere with my old closed-chamber iron heads (but they had not been milled, the CR was just right as-is).