New motor's killin me...

Had a new motor built, 500" 400 block, 10.8 compression, .629 lift, 263@.050, my Bulldog heads from old motor, MP M1 single plane, Holley HP 950, MSD Pro billet Dist and 6al and TTI step headers 2" - 2 1/8".

It runs but not well. We're pretty sure it's a vacuum leak at the intake. First we found one of the extra four carb mounting holes for AVS style carbs (I guess) was drilled all the through to the plenum. We fixed that but still seem to have a sizable vacuum leak.

We suspect the intake's not sealing to the heads well, looking at the gasket after pulling intake could see indents along top of gasket where manifold was sealed but nothing along the bottom. Used intake of course but it looked as if it was never on a motor before. Cylinders on even side seem to have the issue based on temps at header tubes. Odd side was showing ~200* (didn't run it long) even side (#2 specifically) ~85* We checked and we know we're getting spark btw...

Tried thicker gaskets and used a little sealer, better but still not right.

Looking for a way to track it down, other than spraying carb cleaner trick, we tried that and found nothing.

Might put old Performer RPM on tomorrow I think to see if it makes a difference but would like to try to get the M1 to work

Thanks

Were the chambers vacuum checked after the heads were assembled? I've had 2 sets of Bulldog heads where the seats were cracked or crumbling away. Always had a chamber or three that would not vac check.

I cheated and brought home the smoke machine from work for checking evap leaks. Showed me a lot of my issues from start up. You can put the intake on without the gaskets to check mating surfaces and angles. I think that .003" gap would be max before clean milling the intake to match the heads.

I ran into the same issue with my factory 6bbl intake. Block and heads cut and had vacuum leak as you described. I ended up finding this write-up on Hughes site. I did as described and it worked out well for me.

http://www.hughesengines.com/Upload/TechArticles/INTAKE_MANIFOLD_SEALINGJULY2015.pdf

Good article: Thanks for sharing

I have sat the intake on the heads, make sure all the flat surfaces are clean and ding free, to check the alignment and then used a feeler gauge to measure all four corners with a feeler gauge at the top and bottom of the intake to head mating surfaces. don't be surprise to see some at zero and some as bad as .015+
Write the clearances down for each corner and then take that manifold to a good machine shop and have them surface it to cure the machining differences, if the bolt holes in the manifold are above the ones in the heads you may need to have additional material removed to get the port alignment better, if below then add gaskets (stack them if needed ) to raise the intake up to align the manifold

Here's a little idea that we tried on my friends motor. With the intake off, stuff some rags down each intake runner on the heads. Run double sided tape across the intake face of each head. If your tape is not wide enough, make sure it just butts edge to edge and no overlap. Then stick some strips of sand paper on the tape, grit side up. Start with course grit and work to finer. Place the intake on this and slowly slide it back and forth to match the surfaces. Depending on how far out the intake faces are, this should not take very long. Once done, you should have perfectly matched faces of intake to heads. Caution, do not go overboard on this as to cause misalignment with the ports.

Thanks for the replies and suggestions.

We tried my Performer RPM intake that had been used with these heads before and had the same issue, also tried another carb I used on my old 470. Still the same.

After all was said and done we discovered it wasn't a vacuum leak at all.

It was the power valves. My HP 950 came jetted square with 79 mains and 6.5 PV's. We knew 6.5 wouldn't work, we dropped them to 3.5 and that's what was in when we thought it was a vac leak.

Finally realized 3.5 wasn't low enough and installed PV block off plugs.

Ran perfect, idling right at 1000 rpm.

Now, how much to increase jetting without PV's?

8-10 to sizes.

This looks like a street motor to me. As long as the power valve isn't ruptured it will have no effect at idle. If the carb is lean at WOT the power valve channel restrictions can be opened to richen WOT. This will allow a leaner mixture for cruise on the street. No way the power valve needs to be blocked for street use. This will just cause cruise to be dead rich.
Doug

Sledge!!! Now bring that pedal car to Mopars at Milan next month.

OK one more thing I remembered from last night and I think this may have been the culprit.

When I pulled the secondary PV it had 2 gaskets, can this cause it to dump fuel?

Seriously the car wouldn't even run until we blocked the PV's. I'm going to check vacuum today to see where it is and try PV's again.

It was impossible to check it before had to keep it over 1500 to stay lit and was missing badly.

Kathy, I'm hoping to...

The jet spread depends on the size of the pvcr's but a 2.5 or 3.5 rarely belongs in ANYTHING so you need to clean up the idle/transition circuits first then start tuning again. Cruise is where/why the p/v's work or not NOT idle and my Dart has 5 in. of vacuum at idle and I run a 6.5 but cruise at 12 in so when I crowd the throttle under load, it opens up giving me the enrichment I need accordingly..........That carb needs work and not just jetting but a complete make over and if u call me I can help ya out if u'd like.........

make the call,,,,you will be glad you did.

Put a vacuum gage on it today and has a steady 9in at idle with PV's blocked so at least I have a starting point. I have to assume that second gasket on the PV that we didn't know was there was the original problem

If you run a plugged p/v on the street you are doing nothing towards fixing the issues and creating more nasty oil fouling richness.........

I have no intention of leaving this way Dom, but it was the only way it'd run. Like I said, based on the amount of vacuum I have it must have been the extra gasket I accidentally put under the PV, other than changing the PV's this carb is just as it came from Holley.

Nothing else makes sense to me, unfortunately working out of town next week so it'll have to wait till next weekend, gonna start with a 4.5 PV in the primary and go from there.

Here's a small vid

https://youtu.be/qkr0uWS2gcE

The car sure looks great, Doug. I'm sure you'll figure it out soon.
These kind of issues are part of what led me down the path to air/fuel ratio gauges. I bet if you could grab one it'd give helpful directional info. Just a thought. Good luck.

I wasn’t there when you pulled the PV out, so I can’t say for sure...... but......

It’s unlikely you had 2 gaskets under the PV.
The PV gasket is double thick, two pieces glued together....... and used ones often split apart when you pull the PV out...... and it seems like there’s two gaskets there.

The correct thickness is about .065”.

Trust me there was 2, I had just swapped the secondary PV with a new 3.5 and missed the first gasket stuck to the metering block in my hurry to get it running (plus I'm old and half blind )

That new PV and gasket weren't in the carb long enough to become stuck like they do, how ever I guess it could have been 1.5 gaskets LoL I'll have to look when I get home Friday.

The guy who built the motor and I just sat there amazed I could do this when we saw it...

Thanks Rich, I have access to one but don't have bungs in the exhaust yet. For that matter I don't even have exhaust yet, just headers, LoL

I see you're finally showing the GTX some love too, looks good in the pic at the gas station.

Looks like you've done some body work...

I'm glad to hear it looks good....that's why I took the video at night!
Unfortunately it still looks somewhere between a barn find and a refugee from Mad Max. Maybe worse than ever LOL.

Shop/time/money/desire seem to be coming together and I hope to give it more attention pretty soon.

Meanwhile I've had it out driving around the last 3 nights playing around with the street tune up, which has gave me a push.