The timing on my 390 is somewhat unstable. Initial timing is 10 degrees BTDC. Idle speed is 800 rpm. At idle the timing will stay steady for several revolutions, then it will "jump" a number of degrees (haven't been able to measure the jump, but it is noticable). Idle stays steady at 800 rpm. At first I thought it was my 625 cfm Road Demon carb, but it has acted this way since the engine was installed in April (was running the stock Autolite 4100 carb at the time idleing at 750 rpm).

My cam is slightly hotter than stock, being an Elgin "Pro Stock", E-966-P. The specs are 204/214 duration at .050 with valve lift of .486 (intake) and .512 (exhaust). Lobe centers are 107 (intake) and 117 (exhaust) (have no clue that that means), timing at .050 is -5 BTC, 26 ABC, 47 BBC, and -3 ATC (have no clue what these numbers mean either). It seems to have a fair lope, accentuated by the unstable timing. It was installed by the engine remanufacturer when they built the engine.

Intake is an Offenhauser 360 single plane currently running a 625 cfm Road Demon carb (recommended for anything less than 220/220 duration at .050).

Everything seems to be OK with the distributor (remanufactured stock dizzy). I am running a Pertronix Ignitor and Pertronix coil so no points. Everything seems tight with no slop. The rotor was a little loose so I replaced it but made no difference.

So is this a problem or just how the engine should idle? Above idle the engine runs strong and smooth all the way up to the redline (5000 rpm) with no misses, flat spots, or other problems. Any help would be appreciated. Thanks in advance.

 

The only thing I can think of, is maybe the timing chain is a little loose? Or maybe all pertronix ignitions do this?

Did you check the pickup of the Pertronix for dust?

 

The engine was newly remanufactured in March of this year and installed in April.

The Pertronix was on my original 390 and did not behave this way. Maybe the Pertronix is bad? But I would think it would just stop working.

 

bluesky636, at what rpm does your centrifugal advance begin? How many rpm above your idle rpm? If the rpm's are close with cam lope you might be floating on centrifugal advance. Has the distributor been rebuilt, cleaned, polished contact areas and lubed? Ii it a good core to start with? Bushing clearance and shaft end play? Wear on the advance weights? How much cam end clearance? I'd say the chains tight as you said new engine. Vacuum advance unit or linkage loose? You'll get it, start looking. Carl....o&o>...........

 

The distributor is also a "brand new" remanufactured unit. Everything seems tight. It is totally stock. Have no idea where the vacuum or centrifigul advance starts other than what it says in my service manual (which is out in the garage right now).

Come to think of it, I did notice that the part of the mounting plate for the Pertronix that is screwed down for adjustment purposes was awfully close to the vacuum advance lever in the dizzy. May be worth filing the Pertronix mount a little.

One other thing. When I set the intial timing the other night, I did it with the vacuum advance disconnected and the port at the carb plugged. Timing was rock solid. Does seem to point to the vacuum advance unit, wouldn't you think?

 

Put the timing light back on it. Dosconnect the vacuum advance and watch the flashing, then reconnect the vacuum and see what it does. That 800rpm idle speed is a little high (realize you have a cam) and maybe your centrifical advance is right on the edge of kicking in. Also, at 800rpm your throttle plates may be open just enough to start kicking in the vacuum advance too.

The jumping timing mark is also an indication that the rotor needs phasing with the cap but does the Pertronix allow you to adjust the phasing?

 

When I set the initial timing the other night, I had the vacuum line disconnected and plugged at the carburetor. Timing was rock solid at 10 degrees BTDC. No jumping around. It didn't start jumping around until after I reconnected the vacuum line. The main reason I have the idle set at 800 rpm is because of the jumping timing. Any lower and the engine runs even rougher and the idle starts to flucuate very slightly. If the timing were solid, I believe I could drop the idle rpm to around 700.

What is "phasing" and how does one do it?

 

Phasing is the relationship of the rotor tip with the spark plug tower when the spark occurs. When the phasing is off the spark has to jump a wider gap inside the cap if the rotor is too far away from it's intended tower which can make the timing erratic. To change the phasing, you need to be able to rotate the pickup cw or ccw relative to the breaker cam. Does the pertronix allow you to do this or is it located by a pin and therefore locked in?

 

Ummm, the only adjustment for the Pertronix unit is the gap between the sensor and the reluctor. So I guess it is effectively locked into place.

 

Ok, that eliminates that as a suspect.

I've nere run a Pertronix, just give advice on it.

 

OK. What next Master Yoda?

 

Well, from everything you've described the problem seems to be pointing to the vacuum advance so that's where I'd focus on for now. Do you have a vacuum gauge? See if there's vacuum on the hose when it's idleing.

Also, do you still have the original ground strap installed? It's that braided wire with ring-lug connectors.

 

Maybe there is a vacuum leak in the hose to the advace pot? A vacuum guage like Barry said can tell you a lot of things. If the needle isn't steady there is a leak somewheres.

 

bluesky636, is the vaccum unit like chebby with only a allen wrench to adjust?
I favor and still run the old style advance unit, removeable end nipple to change shims and plunger. I rat packed new ones years ago. You can adjust at what inches to start, full at inches and how many degrees total. Install a vacuum gauge, lower the idle, note when the carb starts vacuum to the dizzy, what's the increase in rpm? Have you adjusted the vacuum unit? Whats your centrifugal curve, same with vacuum ( use a hand vacuum pump)? After you do this it's the carbs turn ot tune. Fun ain't it. Go out and test. Carl........
.................................................. .........................................o&o>..... ..

 

How old is the dizzy in this engine ? If the bushings in the distributor are worn badly, it can make the timing do some weird stuff. DF, @ his Dad's