The Stovebolt Site Forums The Shop Area Engine Shop Idle RPM for 65 283

Hey there shipmates, how about somebody weighing in on the idle RPM for a 1965 283. Recent stock rebuild w/ hi-torque RV cam.

It's in my 58 Apache, stick shift, manual choke Holley 4BBL carb, no AC, no PWR brakes or steering.

I've got it at approx 750 or so but it seems a little rough. I referred to the 58 shop manual to set the timing at approx 4 deg advanced. It smooths out if I advance the timing to about 6 deg but it kicks the RPM up to about 1000.

At its present setting it idles OK but has a bit of fluctuation. That is, it idles OK then the idle seems to slow for ahort period then it comes back up.

I don't know what I'm shooting for but I want to get it right so I can decide if I have to tweak the carb.

As always, any suggestions are appreciated.

Many thanks.

What's the timing and lift numbers on the RV cam? If it's over a 260 degree total duration, you'll probably experience a little idle roughness. I've been able to smooth out some fairly aggressive can profiles by increasing the initial timing, but it's necessary to limit the centrifugal advance by the same amount, or it will over-advance. For instance, if the specified initial timing is 4 degrees before top dead center, it will idle (and accelerate) better with an initial timing setting of 12 degrees, but the centrifugal advance must be reduced by 8 degrees to prevent an over-advanced condition at cruise speed with its resulting power loss and engine damage.

Some dyno tests we did back in the mid-50's showed that over-advancing ignition timing by one degree after maximum power was achieved resulted in as much power loss as 5 degrees of spark retard from the max-power timing. If you're going to err, do it toward too little advance, rather than too much!
Jerry

Thanks HRL.

I think I'll skootch it up to about 6 or so advanced & take it to a local shop & put it on a dyno. Don't know why I didn't think of that before! darn, I hate being stupid. It slows down the whole process!

Hey red58 -

you got a 283 in that Fleetside of yours, dontcha? Where you got your timing set and what's your idle RPM?

hey Bob! didn't weigh in cuz I have no idea what the rv cam would change, but in my experience, 8° advance for a start point is what works for the 283, and any I've had work ok around there - idle should be able to adjust down to 450 - 500rpm when well warmed up, but I keep mine a bit higher warm so that it stays high enuff cold w/o using the choke [2bbl] - maybe 600 - I don't have test equip, even had my timing light stolen awhile back - I just tweak a bit to see if it talks back and listen to what it tells me! try 8 and twiddle the idle and mix screws to smooth the idle to about 600 - wandering idle maybe slight vacuum leak?

Bill

Bill - thnx for the reply.

Would having an HEI distributor affect the timing setting?

I advanced the timing to 6 & it smoothed out, seems real smooth at 8 but I backed it down 'cause of HRLincoln's warning.

BTW HRL - your advice is always helpful and always appreciated but ya gotta remember, I'm a danged hillbilly shade-tree mechanic & sometimes I only understand about half of it (OK so I get about 30% on a good day!) I often wind up doing more research AFTER I read your advice than before! I gotta be careful here or I might wind up a mechanic!

Anyhow, the idle is smooth, but smooth at about 1000 RPM. I still have a wee bit of fluctuation but significantly less now. Vacuum seems to follow the idle rather than vice versa. I checked all the vacuum fittings & all seems good. I'm going to try re-tightening the intake manifold (darn, I sure don't want to take THAT booger off again!) but everything inside, above, and bolted to the block is new so I'm not too concerned about that. Of course, I did bolt 'em all on myself and operator error is a far too common occurance when I pick up a wrench!

I'll tell ya'll this though, if it weren't for ya'll and this BBS, I'd still be unhooking the old girl from the wrecker!

THX.

I smoothed the booger out some more by adjusting the idle mixture, smoothed out the fluctuation too. It seems better at 8 deg advanced so I put it back there for now. Still idling a bit high, near 1000 (the old tach in the booger just ain't that accurate).

Do either of ya'll have a feel for what the vacuum pressure ought to be? (is vacuum pressure a contradiction of terms?) it's abot 15 in now. With the mixture adjustment there is almost no detectable fluctuation. I might even be immagining it now 'cause I'm listening so dang hard!

Also, it's dieseling when I shut it off hot now. Is this a timing issue or because I leaned out the carb or none of the above?

I'm gonna keep tweaking the mixture until I can max out the vacuum - hadda take a break to grab a glass of JD, it's freakin hot out there!

I'll still be taking it to a shop to put it on a dyno but half the dang fun of owning the old girl is screwin' with her!

Boy, hope my wife doesn't read this she'll think I'm talkin' about her!

THX.

Try different things with your vacuum advance. Try it with it hooked to ported vacuum (no vacuum at idle) or manifold vacuum. Sometimes a vacuum advance with lower starting points works better with a cam. An adjustable one or one of the Corvette performance models. I take it you disconnect the vacuum advance while you set the timing. 15 in. of vacuum at idle is alittle low.

OK, first things first- - - - - - -Can you back off the throttle stop screw any more, to close the throttle plates? I'm assuming you can't, or you would have done that already. Some of the Holley carbs have holes drilled in the primary throttle plates to allow more air to flow without uncovering the transfer ports where the midrange mixture system kicks in. If so, you need to install primary throttles without holes in the plates.

You can slow the idle down by turning the mixture screws in enough to lean it out, but this might cause undue roughness in the idle. With the vacuum gauge connected to full manifold vacuum, turn each idle screw clockwise until the engine tries to stall, and then back it out only enough to keep it running. Once both idle screws are as lean as possible without stalling, back each one out until the vacuum stabilizes, but don't let it get over-rich. You SHOULD be able to slow the idle down with the throttle stop screw, unless the carb is just too big for the engine. If you've got more than a 600 CFM carb on that 283, the carb is too big! Your dieseling problem on shutdown is idle speed, plain and simple! Do whatever it takes to get it down to 550 RPM or so, and the dieseling will stop. Also, the vacuum advance should receive NO vacuum at idle!
Jerry