Shdggsdv

So I've got a freshly built 350 which should be getting 400+ hp and is only getting around 200 or a little less that I've been troubleshooting for months now. Promaxx 64cc aluminum heads, Comp XE275he, Assault air gap aluminum intake, Holley 670 Street Avenger, MSD Streetfire HEI, some Comp full-roller rockers adjusted and rechecked. All brand new stuff too.

For the life of me I can't seem to tune the damn thing! I've spent lots of time finding vacuum leaks and I should be past all of them judging by how steady the needle is now, but I'm not sure what's causing these fluctuations and I assume it might have something to do with my performance issues. Also, it definitely has an audibly fluctuating idle RPM. I have it set around 800 which is supposed to be the sweet spot for this cam. I'm getting a perfect 7psi ahead of my Holley.

What do you guys think of this vacuum reading? I can't really tell since it seems to switch between slow fluctuations and rapid ones.

HeadsU.P.

What I would consider slow then rapid fluctuations would be 10 inches then 20 inches then 10 again. I'm not seeing that. However, there maybe a valve or two that is a little tight on the valvelash perhaps.

We know now that a powervalve has nothing to do with this scenario. It has nothing to do with the idle circuit. But.
What's your situation on the Vac Can on the dizzy? The Vac advance might be very confused as to where it is supposed to be at idle.

Shdggsdv

Ohhh, huh. Never thought about the vac advance. It's hooked up to the correct spot on my carb as indicated by my Holley's manual

When I set my ignition timing per the Lars papers, my advance seemed to be working fine. I assume any issue would be immediately apparent with a timing gun, right?

HeadsU.P.

Well, I am questioning if you have the proper Vac Canister. At what point does it begin to move (advance) and at what point does it max out? You have really low Vac due to a cam with lots of duration I assume.

And I assume the dizzy is routed to Full Manifold vac.

suprspooky

I'm not sure from your post how you are measuring the HP but, to be down 1/2 expected hp with all cylinders are firing, I'd be looking at exhaust restriction or fuel mixture problem (provided the timing is close to the recommended total advance). The vac. advance has little to do with max HP particularly if you have it connected to a manifold vac source vs. carb ported source, it is for part throttle cruise when connected to a carb ported source, centrifugal advance is the critical thing for WOT power. I don't remember what my sweet spot timing was on my 355 Jet Boat Motor (think engine dyno, pump sees same load all the time) but I can tell you that to be down half power I'd have to retard or over advance (major pinging) so much that a restart would be near impossible. (your mileage may vary). I have a big block 460 currently in my 68 and it likes about 36 total (engine dyno pull determined), my cam is fairly mild (hyd roller) and I see about 10" vac at 900 rpm idle (not much steadier than yours). You can buy canned (spray) smoke to help find vac. leaks.

Hammerhead Fred

That's really low - I'd be thinking a intake manifold gasket leak.

HeadsU.P.

Another good vacuum test (maybe you already did this) is to unhook EVERYTHING off of the back of the carb & Intake Vac Port. Then hook up your gauge to a known full vacuum port and take a reading. This will isolate any leaks down-stream of the main Vac port.

Hammerhead has a point about the intake gasket. They don't always leak at the top of the gasket. Sometimes they leak at the bottom and suck oil mist out of the lifter valley. That results in fouled plugs on one bank though.

A leaky intake should still have a somewhat steady needle on the gauge unless just one intake port / one cylinder did not seal correctly. That usually shows up on the gauge with a jumpy needle. You don't have that either. Your gauge acts like its searching constantly; up & down, up & down.

Shdggsdv

I haven't messed with it in a while, but it should be set to 12 degrees initial and 36 total. It begins to move as soon as you start touching the throttle last time I checked, but I'll take another look at it all and report back here later. I shouldn't have too low of vacuum. This cam is rated for 11" at 800rpm and I'm only 2,000ft above sea level so I should be around 10". I misstyped the cam as it's the 274he with the duration stated as this - Duration @ 0.006": 274/286 - Duration @ 0.050": 230/236

I'm measuring my power by guesstimation. I based my engine build off of Hot Rod Magazine's budget 440hp Vortec, but I used better flowing aluminum heads, a slightly better intake, and a 670 instead of 750 cfm carb. That engine dyno'ed 442hp @ 5900 RPM and 452lb/ft @ 4400 RPM so I shouldn't be too far off the power figures of that engine. The power I'm estimating as half because it feels less powerful than my Nissan Maxima which is only about 250lbs heavier and about 240hp as stated by the manufacturer. I also can't really seem to get past 4k RPM, but my plugs don't seem to indicate a lean condition.

So HeadsU.P. are you saying that my vacuum gauge doesn't match a leaky intake? I had one before I built the engine and it showed up as a rapidly fluctuating, almost vibrating, needle on the vacuum gauge. So should I still be looking at vacuum leaks? I can't really imagine that would be the issue considering the needle isn't fluctuating at a high rate and that I've gone through pretty much every possible source of a leak aside from the intake gasket

(Also, thanks to all of you for the advice so far!)

HeadsU.P.

I doubt its a intake gasket. Although your Vac gauge needle is not steady, its not jumpy either. Which is usually what happens when a intake port lost its seal.

You need to hook up a Vac pump tool (Harbor Freight) to the dizzy. You need to remove the dizzy cap. You need to know at what point (inches) does the advance arm begin to move. If the tool shows that it took 12" just to move the arm and you only have 11" available, well, there is problem number one.

Next: And you need to isolate other vac issues by running the engine with NO vac-lines hooked up. EVERYTHING plugged off. There are dozens and dozens of vac lines going everywhere, dash, tranny, dizzy, brakes, headlights, vapor can. Who knows how many leak? Shut them all off at the carb. Now what does your gauge say?

mikem350

Go back and verify TDC on balancer is really piston #1 at the top. You need to pull plug#1 and feel the piston top at TDC. Have seen the marks off MANY times!

My 350 is very same as yours, cam is 225/235, vac is 13 at 850. This big a cam needs lots of advance at idle, like even more than your 12 degrees. I had to limit mechanical advance to 18 degrees with 16 initial.

Also, verify cam timing, easy to be off a tooth

Shdggsdv

Ran the engine with no vacuum lines. Wanders less, but still isn't quite right.

I took the driver's side valve-cover off to locate TDC and when turning the engine by hand, I'm noticing a hissing sound like air is escaping somewhere in the engine. This doesn't feel normal, is this normal? Not really seeing anything about this on Google

HeadsU.P.

H-m-m-m-m-m-m-.
Statement 1. Vacuum gauge needle is more consistent. So now you know there is a slight leak "downstream". You will have to chase that down in sections. Its easier to apply vacuum using a electric accessory pump instead of using the engine. You should be able to hear something, somewhere. Another way is to apply compressed air with a blowgun nozzle at the main vac line behind the carb. Again, listen for air escaping.

Statement 2. Hissing while hand cranking slowly. That's normal. Just compressed air in the cylinders that you will never hear with the starter cranking. Or a snake hiding in the frame. Or a loose sparkplug. Or a snake in the cylinder without a sparkplug.

And by the way. Do you know of the condition of the carb base gasket?

Shdggsdv

Carb base gasket is totally fine hopefully! I have a spacer on that could be suspect, but I'll have to order a different spacer to test that out. My intake manifold has a weird layout which intersects the bowls of my Holley with the manifold vacuum T so I need a spacer to keep everything working.

Good to hear the sound is normal!

A friend of mine found a rattlesnake in his Datsun when he opened the hood a few weeks ago. That was quite a different sound

HeadsU.P.

Rechecking post # 1 you said you installed aluminum heads. Was the pushrod length verified with a checking tool? I also run the ProMaxx 64s and they, like most aluminum heads require a longer pushrod. This is usually due to the rocker stud machined pad being a little higher or farther from the camshaft lobe. If you or your builder used the stock length pushrods, you are leaving some H.P. behind. I needed 0.100 longer. Almost an 1/8 of an inch difference.

Shdggsdv

Yes, I checked with a pushrod length tool from Comp and then checked again after one of my head's studs failed (Thing just snapped in half at idle with perfect valve geometry. Replaced them all with ARP studs). I don't remember exactly, but I believe I came out as 0.1" longer than stock as well

Its_hers

Did you degree the cam or just set the timing set "straight up"?

Shdggsdv

Set the timing straight up since the cam has an advance ground into it already. Comp recommended the default positioning on the cam

suprspooky

In one of the posts you state, can't get past 4000 rpm. Is that true when snapping throttle out of gear or only under driving load?

suprspooky

I think Mikem350 is spot on, it's time to sanity check the cam timing. Get to TDC (piston stop), put some timing tape on the balancer if needed and verify valve event timing. The 4000 rpm limit is pointing major problem of some sort, vac possible, hence sanity check suggestion.

Shdggsdv

It revs up without issue out of gear.

Hm. Could this potentially be an auto transmission issue? I was planning to get the thing pulled here soon because it leaks like crazy from the top and I need to install a new torque converter to match the engine.