Hey all,

Now that I’ve gotten a few other things straightened out, I’m trying to get the 390 tuned up right. I had a leaking intake gasket that caused a lot of oil burning but that seems to be fixed now. Trouble is, I still can’t quite get my idle tuned right. Vacuum seems low and fluctuates too much. This is a fresh rebuild with darn near new everything. There’s a list of goodies in my thread on the intake gasket over on fordfe, here, but here’s the list of parts again:

Block .060 over
H395p cast pistons
Hastings moly rings (at least, they are supposed to be - bought from machine shop as part of rebuild parts set) - I don’t recall the gaps exactly but they were all consistent and what you would expect for a street engine; I believe around .015 but I’m not positive. I wrote it down somewhere; I’ll look for it
C6AE-R heads (rebuilt by machine shop)
Performer RPM intake
Howards 272/278 hydraulic roller (specifically this one)
Factory distributor with Petronix I and no vacuum advance per engine shop (I am still not sold on this idea but that’s a topic for another thread)
Holley 600 vacuum secondary
Sanderson FF427s into true duals

And here’s a short video of my idle as it is right now. This is about as high and steady of a vacuum as I could get.
Timing is set at 15 degrees right now. It will idle at a lower RPM, like down into the 500s, but only barely. The lower the RPM the worse the idle seems to be, and the vacuum fluctuates more. The engine tends to kind of shake a bit like it’s misfiring as well, unless you raise the idle RPM towards 1,000.

The engine will die if you turn either one of the idle mix screws all the way in.

The cam is really not that big (219/225 intake/exhaust at .050, with a 112 degree LSA), so I don’t think this is a normal idle for this engine.

Thoughts? This is my first engine build so I’m rather green on all of this. Thank you guys!

 

I have a similar cam in my 447. Idle is a little rough at 750 so I don't think you should expect your's to run too smooth at that RPM. My engine builder, who works almost exclusively on FE's, strongly recommends keeping the idle up at 850 for adequate oil splash to protect the rollers. He also recommends running high zinc oil with rollers in conversion applications - so I do as he suggests. I am running a Holley Sniper and can set mixture and timing wherever I want at idle and nothing I did at 750 would get it to smooth out. My vacuum does jump around at 750 and is more stable at 850. With a 390, the effects of this cam will likely be more noticeable than on my 447.

What are you using for rockers? The only thing I can think of that would influence vacuum other than the cam is a preload that is too high.

I did expect mine to idle a little smoother but have gotten used to it. Above idle it runs smooth and strong. I am running a 4R70W transmission with the truck low-stall converter. At 850 RPM idle I definitely have to keep my foot on the brake at a stoplight.

Keep the revs up to keep the lifters safe and enjoy it.

 

I watched the video and it looks pretty good to me. Mine idles best with about 22 degrees advance at idle. If you use the full vacuum port on your vacuum advance you might get a little smoother idle. With the Sniper I can set a cranking advance separate from idle advance. A little harder to do with a carb. I am running BBM heads and have WOT timing set to 30 degrees which is the sweet spot for those heads. What does your full mechanical give you.with base timing at 15? I would think that with those heads about 34 or 35 might be safe.

So how does it run when you open it up?

Note on the Summit listing for this cam - Fair idle, High performance street, Good mid-range torque

In cam lingo, fair is not smooth - slightly rough is probably a better description

 

Was thinking the same thing exactly. But I wonder where their "fair" crosses over and becomes "lopey" in their lingo?
Or maybe this is a lopey idle cam, but with so many people asking what lope and lopey mean in the last 15 years, maybe they just got tired of explaining it and decided to use different wording?

Also agree that the preload is suspect, but just barely. Did you do the valvetrain setup Sparky? Or was that all assembled and done at the machine shop?
Very easy to misjudge zero-lash the first time.

And I also agree that you were correct in questioning the value of vacuum advance. On the street you almost always want vacuum advance.
But maybe the machine shop knew something we didn't. That the vacuum signal was going to be a bit wonky and cause some trouble. Otherwise it's just the typical engine-builder's race history talking. After all, racers don't use vacuum advance, therefore it must be bad. I know scores of people that think that way about it.
But I wonder then, why the acceptance of a small cfm, vacuum secondary carburetor? I would question the 600 vacuum secondary carb myself. And maybe even the size.
Unless you have changed the springs, your secondaries may be partially opening even at idle. This could also cause your erratic idle and strange vacuum readings. This carb must be tuned for this engine even more so than if it was being put on a stock engine.

And what distributor are you using? You said stock, but is it original? How many miles on it, and did you check it for wear? With no vacuum, then I assume you simply disconnected it? But your mechanical timing could very well be optimized for working with the additional vacuum advance.
If you have not already, I would document the curve with your light and tachometer and timing marks. See where the advance is for every 500 rpm or so. Takes a little time, but it could be worth knowing what you're dealing with.

Personally, as it sits I would raise the base timing to as much as it will tolerate, then see what happens to the other parameters. If you can set it at 20 degrees and the starter does not struggle and there is no pinging under heavy acceleration, you might find that a higher-than-average advance setting is called for. It's an easy experiment and totally worth seeing what your engine likes now that it's been modified so much from stock in some areas, but not others.
Any engine that's .060" over and has a moderately big cam in it (compared to stock) could use some more experimentation. The more advance the cooler it will run generally speaking, and with .060 over you need to keep the heat in check. Thinner cylinder walls can easily lead to hotter coolant, for multiple reasons.

Paul

 

I had the machine shop make pushrods for me, so hopefully they are correct!

He seemed concerned about introducing too much timing at cruise and causing detonation, which I get, but it seems to me that by using an adjustable vacuum advance canister this could be avoided.

I chose the 600 based on what I had read about calculating carb size based on displacement and expected RPM. I’m open to other choices of course, and some folks told me a 750 would be fine but I was worried about over-carbing it. It might be worth picking up a 750 and seeing what it does.

I’m not sure whether it’s original to the truck or not, but it’s a Ford unit that came with the truck when I bought it. I had the shop rebuild and recurve it for me, so I would assume that there wouldn’t be enough shaft wear to cause erratic ignition. I’m not sure though. What’s the best way to check that?

Will do!

It does seem to like timing. I’m giving consideration to adding back vacuum advance and connecting it to manifold vacuum to add idle timing without having crazy high total timing. That seems to be the advantage to using manifold vacuum.

If I had a spare carb, distributor, and coil, I would try swapping those out one at a time to see if any are the problem.

 

I really don't think you are having a problem. It won't idle smooth with that cam so tune it to give best vacuum and drive it. I'll see if I can figure out how to make a video of mine at idle for comparison. Depending on your rev limit, the 600 will probably be fine. I would try a few different springs to see what the engine likes. If it has a purple spring in it now, go lighter until you find a bog and go back up one spring. If you have a bog now, then go stiffer.

 

But did they install them too, or did you make the final assembly?

Good for you, on being concerned about over-carbureting. That was always a big problem with the bigger-is-always-better crowd.
But I think you're leaving something on the table with a 600 and your engine.
However, I would run it and see if it needs any fine-tuning of the secondaries and maybe even the squirters (accelerator pump nozzles) to best throttle response. A 600 should be able to feed a pumped up 390 on paper, but in the real world it's best place is probably in the low to mid-range power areas. Which actually can be a good thing, because most of us spend precious little time at wide open throttle and high rpm levels.

You're right. A rebuilt distributor should be nice and tight.
Easy enough to do the basic tests though. With the rotor removed so you can see the top of the shaft, move it side to side to see how much play is there, then move it up and down.
Side-to-side should only be a few thousandths of an inch. Hardly any movement at all. Up-and-down is usually more, but I don't remember the specification. Someone will know I'm sure.
The other thing to check by hand (even when it's all supposedly new, or new-ish) is the advance mechanism's general smoothness of operation. Put the rotor back on and twist it clock-wise. It should be smooth and move easily against the spring pressure of the weights and springs. Then it should snap all the way back. There should be very little to zero "slack" in it where the movement is uncontrolled.

I'm on the side of the fence that is basically in the middle of the fence! I say some engines like full vacuum, while others really like ported vacuum with the way they're tuned.
Either way, you will adjust initial timing to suit, and re-adjust the carburetor's idle speed as necessary. Usually full vacuum raises the idle speed somewhat (different engines, different amounts of rpm rise) to you get to fiddle with things some more.
But it's all a good learning curve.

Remember though that your distributor has supposedly been re-curved for no vacuum advance. So if it turns out that it does not like vacuum advance added, well then at least you tried. If it can tolerate it though, you're likely better off.
And only one way to find out!

Yep, can be very helpful. But you just gotta' use the tools you've got.
But remember too, even in all this fooling around, you may not actually have a problem. Just like GPatrick was saying, it might be perfectly normal.
Doesn't mean you should not tweak it to work the best for you, but you may be stuck with an idle that requires more speed, and an engine that likes more timing.

Every case is different even when all the same parts are used.
And speaking of that, what altitude are you at? Are you using vacuum assist brakes or accessories?
Auto transmission with a vacuum modulator by any chance?

Paul

 

The one thing that was not discussed was how far is the throttle stop screw turned? If it is more than 1-1.5 turns then the cruise circuit is cutting in. On a Holley you can slightly open the secondary side to compensate or start drilling small holes in the primary throttle plates one size at a time until the stop screw is where it should be. A 1/32” hole would be a safe start point on the front of the plates.
Then try ported vacuum.
I did the drilling method on a boat and really got the idle dialed in.

 

I know of no situation where vacuum advance is not a benefit on a street engine. Part throttle fuel economy and reduction of coolant temps are the two big benefits, with a possible 3rd being additional timing at idle but not during cranking, if the engine wants more at idle. I'm gonna guess your engine will benefit from at least 18 degrees at idle and probably more. I say that based off the idle quality you report and your cam size. Advancing idle timing speeds up the idle, which causes you to back off the idle screw, which in turn raises vacuum. Increased vacuum tends to keep fuel atomized as it should be instead of puddled on the floors of the plenum and runners, as liquids evaporate more easily in lower pressures.

 

I hadn’t really thought of playing with the secondary spring, I’ll give that a try.

Final assembly was done by them initially, but I have had the intake off since then to fix a leaking intake gasket, so as they are now, they’re self-assembled.

Honestly, in the future I may end up getting a 750 and seeing how it works. Or if a good deal on an EFI kit comes along that’s possible too... haha

I’ll give this a try tomorrow. I have twisted the rotor to feel the advance before and I don’t remember it having any bind or roughness, but I’ll check.

Yeah, I am thinking about putting the canister back on the distributor and seeing what happens if I give it a bit of manifold vacuum. Only after I verify what the timing curve is, though.

I’m around 350ft, so close to sea level. I do have vacuum brakes but that’s it as far as anything being connected to vacuum (except PCV). Which, I should mention, isn’t the original valve; since the Holley doesn’t have a huge 1/2” vacuum port on the back for PCV, I bought one at NAPA that matches the carb port, and is a 90° valve as well. The part number is 39210 as I recall. And, the truck is a 4 speed manual (T18 to be specific).


The idle screw is opened a good bit. Not a huge amount but probably too much. I know some people open the secondaries slightly to add air at idle without opening the primaries too much, but I’ve never tried that. Thoughts? Would a larger carb achieve the same effect without having to open the secondaries at all? It kind of seems band-aid-y to have to crack the secondaries at idle if the primaries aren’t feeding the engine what it wants on their own.

Yeah, that sounds like just about everything I have read/heard about vacuum advance except for what the engine shop guy was telling me. It seems to me that, barring any pinging problems, there really isn’t a downside to using it. Sure it’s useless on cars that live at full throttle, but this ain’t one of those! Depending on whether or not there is notable wear on the distributor, I might just do a Duraspark conversion anyways, which would of course have vacuum advance. Davis seems to make nice ones

 

Here is a short video of my engine at idle. It was just approaching normal operating temperature. Note the RPM on the Sniper handheld and in the second handheld section note the MAP (manifold air pressure). It dances around 42 to 46 kpa which is roughly 13 inches of vacuum. I'm in Denver so our manifold vacuum values are a little lower than at sea level. The refresh rate of the handheld is slower than what you can see on your gauge but it is not stable. My cam is 218/226 which is similar to your's but I'm running .597" lift. I haven't uploaded a video before so I've provided a Youtube link and hopefully it works.

 

Worked great. Thanks for that.

Paul

 

Thank you very much for taking the time to record and post that! I guess I really shouldn't worry too much about this then, other than getting it dialed in as best I can. Your engine sounds great by the way.

Also, how are you liking the Sniper EFI setup?

 

I installed the Sniper about 1-1/2 years ago on my tired 390 so I could get it sorted out in advance of the new engine. I stared with just the Sniper without timing control. When it was running good, I installed the Holley/MSD dual sync to introduce timing control. The transition to the new engine just involved installing a steel gear on the distributor for the roller cam. Other than a bonehead move of not updating engine displacement, it started and ran right away.

The only time it let me down was when I installed the Hyperspark box and coil. I started to get EMI/RFI interference which is the downfall of having the computer in the throttle body. I changed to an older drop base air cleaner and the interference is temporarily solved. The air cleaner is/was acting like an antenna to the noise.

Had it been around a year earlier I would have gone with the Terminator with multi-port injection because I could also use it to control the 4R70W. More complicated than the Sniper but compared a carb, the Sniper is hands-down better and is a good choice. When temps would go over 80 degrees here in Denver, I couldn't stop the fuel from boiling over with the carb. No problems with that with the Sniper. Drop me a message if you get closer to moving toward FI and I'll take you through everything I did to get there.