Ignition: 289 inconsistent idle return, looking at distributor advance

[Stunt]

I have a '61 Hawk with a supercharged 289 - R2 style supercharger and carb, though a '61 build other than that, plus Pertronix ignition. When coming to a stop, the idle stays quite high unless I gun the engine, maybe even two or three times. The accelerator/carb linkages look fine; the idle screw sets solidly against its rest, the choke works properly, and the fast idle cam is all the way off when warm, so I'm pretty sure it's not that.

I'm thinking it might be the distributor. For one thing, the advance timing is quite high - like 20 degrees, even when at the lowest idle, around 700 RPMs. No knocking or pinging when going up hills or anything though, so I guess it can take it? I tried lowering it but when it got to around 5-8 degrees, the engine would sputter and die, even if I raised the idle screw, or it would backfire if revved. The total timing is around 50 degrees. That seems like a lot, too.

The vacuum advance is getting 7 pounds of pressure at idle coming from the carb, and up to about 20 at speed. I'm not sure if that sounds right or not, and if a sealed carb makes any difference in vacuum advance setup or not? If that IS okay, then I'm looking at the distributor mechanical advance being out of whack. I haven't yet opened up the distrib to have a look, but wanted to see what any of you think, if I'm heading in the right direction here or any other thoughts before digging deeper.

And if a distrib recurve is called for, does anyone know anybody in the Pacific Northwest who does that?

Any thoughts on this stuff would be much appreciated!

Dave

[Dan Timberlake]

When it is idling fast does the idle screw still "set solidly against its rest?"

How are you testing "total timing" ? The vacuum advance should be disconnected and plugged for that.
While the vac advance is disconnected I'd rev the engine and watch the timing marks with the timing light. The timing should advance smoothly as revs climb, and retard smoothly as revs fall, and be steady whenever the revs are held constant.

[Stunt]

I did forget to disconnect vacuum advance before checking timing, so thanks for that. And I can never remember, do I plug the hose where it disconnects from the carb, leaving the carb vacuum open, or plug the hose where it was pulled off the vacuum advance, leaving the the port on the advance module open?

[Stunt]

So with the carb vacuum plugged, timing at idle is 16 degrees, with the vacuum advance plugged and the carb port open, 18 degrees. Either way, it goes up to 44 degrees when revved up to a couple thousand RPMs. When connecting carb and vacuum advance module, idle goes up and timing goes to 20 or 22 degrees. Timing mark moves smoothly up and down, and stays steady at constants revs.

That's a little better, but idle timing should still be lower. I've heard supercharged engines should be about 4 degrees, and this one won't go there. Plus it still doesn't easily return to low idle speed.

[Mike Van Veghten]

Normally, inconsistent idle is from loose throttle shaft to throttle shaft bore clearance.
Also normally this inconsistent idle speed drop has nothing to do with the timing. That's not the way things work in the distributor.

What happens is that one time the throttle closes, the loose shaft lets the butterflys hit the throttle body rather than than the idle stop screw and not close fully. Then...you give the throttle a little whack, and the short quickness will normally reset the throttle shaft back into the center of the bore and the blades will clear the throttle bore and the throttle stop, will stop the butterflys. Then the next time you back off the throttle for a stop sign, the routine starts all over again.
Every now and again...the shaft will stay centered and allow a proper closure, but about 75% of the time...you'll have to give the throttle a light whack to recenter the shaft in its bore to get the as adjusted idle speed.

Time for new bushings (maybe new shafts, depending on their condition) in the throttle shaft bore of the carburetor base. Nothing new, these are OLD...used up parts..!
And a "normal" rebuild will not do a thing for this problem. New shaft bushings are required.

Mike

[BRUCESTUDE]

I agree with Mike-I had the same issue with the 2 bbl in my Lark 259; the throttle lever on the end of the throttle shaft was loose and required kicking the gas pedal to have the idle return to normal. I tried peening the end of the shaft and it helped for a bit, but the issue returned. For the cost of a pro-type rebuild (including new shaft bushings and throttle shaft) I'll get a new Edelbrock 4 bbl....been looking for an excuse to do that anyway!!

[Stunt]

Okay, thanks for these comments. A friend had mentioned that possibility, but the very high ignition timing was making me think it could be in the distributor. The symptoms you guys are describing does sound like what it's doing though.

Anyone have any thoughts on why my timing has to be in the mid/high teens for it even run to run? And does 4 degrees sound more like what a supercharged motor should be timed at, as someone told me?

[StudeRich]

Are you running an R2 Camshaft? The R2 Timing Mark Plate goes to 24 Degrees for a reason.
Also inconsistent idle is often caused by incorrect, missing or weak return springs, BOTH of them.

[Stunt]

Are you running an R2 Camshaft? The R2 Timing Mark Plate goes to 24 Degrees for a reason.
Also inconsistent idle is often caused by incorrect, missing or weak return springs, BOTH of them.

No, not an R2 cam, though the timing mark plate does go to 24 degrees. I was under the impression that supercharged engines don't like a lot of timing advance, but is that not true?
And is there a way to test the return springs?

[TWChamp]

It's true that a supercharged engine uses less advance. My turbocharged Corvair Spyder has a pressure retard rather than a vacuum advance. With more air packed in the cylinder it would knock and do damage if regular timing was used. You mentioned 44* of advance, and with that much I'd expect your engine to sound like a popcorn machine and have severe piston damage.

I agree that the throttle shaft wear is often the cause of inconsistent idle speed. You can pull the throttle open and closed from under the hood to see if the return spring feels weak. My 1950 Champion had two return springs, but I removed one and it still returns fine. It's nicer this way for long drives. Also make sure you don't have a rubber mat or carpet interfering with the throttle return.

[Stunt]

It's true that a supercharged engine uses less advance. My turbocharged Corvair Spyder has a pressure retard rather than a vacuum advance. With more air packed in the cylinder it would knock and do damage if regular timing was used. You mentioned 44* of advance, and with that much I'd expect your engine to sound like a popcorn machine and have severe piston damage.

44 degrees is the total advance, initial being about 16-18. No knocking yet, which is a little surprising. But I can't understand why the engine just WON'T take less advance.

I agree that the throttle shaft wear is often the cause of inconsistent idle speed. You can pull the throttle open and closed from under the hood to see if the return spring feels weak. My 1950 Champion had two return springs, but I removed one and it still returns fine. It's nicer this way for long drives. Also make sure you don't have a rubber mat or carpet interfering with the throttle return.

Throttle return spring is definitely okay; it's fairly stiff and I can see that the idle speed screw is pushed all the way against its rest, no matter what the idle comes down to after revving it. But yeah, could be the throttle shaft wear, that people have mentioned.

[345 DeSoto]

How about a vacuum leak?...

[Stunt]

How about a vacuum leak?...

Hmm, that's a thought. Though the speed at which it idles before kicking the accelerator so it comes down - maybe 1500 rpms, very roughly - might be a little extreme for that. And not sure that would explain why revving it back up would bring the idle back down. But I'll spray some carb cleaner around the carb and see if it reveals any leaks, thanks.

[karterfred88]

Sounds like vacuum leak, throttle shaft wear usually only makes erratic idle by maybe 50 to 100 rpm, not 1500. Do you have a dashpot installed instead of a vacuum idle kicker? Sure your rod to the throttle bell crank is not hanging up (if it is one with the built in spring to allow overriding the choke lockout, it may be sticking and sometimes releasing, take it off the carb end and blip the throttle by hand-same results or is it more consistent. Timing light and pointer may not be correct. Pull number one plug and rocker cover. Rotate engine till the crank mark is near TDC on the indicator, Use a wooden dowel to touch the piston top and by wrench turn crank to see if the marks are right when you get to TDC. Then rock engine slightly back and forth by the crank bolt-dowel should not move if it is really TDC. If it is off find true TDC and remark the crank or the timing plate. Since it is not a factory R2 whoever put it together could have mismatched the timing plate to the vibration damper. A true 44 degrees, even with low compression would make it ping badly.
Opps almost forgot, make sure the vacuum advance arm is not catching or rubbing on the plate and moves it freely and the internal spring is working, pushing it back to zero advance. Could be hanging up leaving too much idle advance one time and not the next. Check distributor rotor position at TDC and its relation to the cap and see if it lines up like the shop manual shows--Good luck.

[Dan Timberlake]

"So with the carb vacuum plugged, timing at idle is 16 degrees, with the vacuum advance plugged and the carb port open, 18 degrees. Either way, it goes up to 44 degrees when revved up to a couple thousand RPMs. "

The condition of the test should be vac advance line plugged on carb side.
So when revved up to a few thousand rpms in neutral the vac adv does not change the timing? That ain't right.
How are you determining the ignition timing? With a timing light with dial back feature?
I'd be checking the TDC mark on the damper in the car, using a positive stop in the #1 spark plug hole.