Sbc idle rpm

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Idle RPM for 65 283

#21290Sat May 14 2005 10:27 PM

B

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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.



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Re: Idle RPM for 65 283

#21291Sun May 15 2005 12:48 AM

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'Bolter

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



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Re: Idle RPM for 65 283

#21292Sun May 15 2005 03:03 AM

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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!



Bodega Bob
http://www.submarinestore.com

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Re: Idle RPM for 65 283

#21293Sun May 15 2005 03:20 AM

B

OfflineOP

Member

Hey red58 -

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



Bodega Bob
http://www.submarinestore.com

Where's my tin foil hat?!?

Re: Idle RPM for 65 283

#21294Sun May 15 2005 05:39 AM

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Extreme Gabster

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! grintry 8 and twiddle the idle and mix screws to smooth the idle to about 600 - wandering idle maybe slight vacuum leak?

Bill


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Re: Idle RPM for 65 283

#21295Sun May 15 2005 09:11 PM

B

OfflineOP

Member

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!) winkI 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.


Bodega Bob
http://www.submarinestore.com

Where's my tin foil hat?!?

Re: Idle RPM for 65 283

#21296Sun May 15 2005 10:14 PM

B

OfflineOP

Member

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.



Bodega Bob
http://www.submarinestore.com

Where's my tin foil hat?!?

Re: Idle RPM for 65 283

#21297Mon May 16 2005 10:00 PM

W

Offline

'Bolter

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.



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Re: Idle RPM for 65 283

#21298Mon May 16 2005 11:14 PM

H

Offline

'Bolter

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



The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!


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Chevy 350 idle at 500?

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Re: Chevy 350 idle at 500?

Kind of Recently I have noticed Starting somewhere around 06-07 mercruiser has lowerd there idle rpms on MPI engines from a normal 600 rpms down to 540rpms when warm. Now I was told it might have had something to do with emissions And saving fuel but I am not 100% sure of that, I do know the engine manufacturer's had to do other stuff to engines coming into CA to meet CA new emissions laws to get there 3, 4, 5, star rating system with in spec's so that could be it. So now I am curious Has any other boat service tecks out side of CA noticed the lower rpms on newer MPI engines while looking at A scan tool??? And if not then next time you get a chance to check it out while running a engine out with a scan tool could you let us know what you find, Thank's:)
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Cobra Club Forums > Tech Tips > General Tech Tips & Questions > idle speed for chevy 350


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Karl.davies

11-07-13, 07:54 AM

Can anyone suggest the best/recommended idle speed for a chevy 5.7 350


Lloyd Barnes

11-07-13, 07:59 AM

around 7-800, any more and it can run on when you turn off the ignition.


around 7-800, any more and it can run on when you turn off the ignition.

Yes, mine runs better at 800rpm. At 700 the charging red light flickers on and off.
Dek


Karl.davies

11-07-13, 08:24 AM

thanks for that


Probably doesn't affect you, but if you have an auto gearbox (everyone starts spitting at me now) the idle has to be set with the box in "D". If you don't as soon as you drop the box into D it drops the revs and usually stalls the car. This can though lead to the small problem of overrun when turning the engine off if you have it set too high.


Sidecarbod

11-07-13, 09:26 AM

Idle speed will be different for different engines, a standard SBC will tickover at a lot lower speed than one with a hairy cam in it. You just need to keep adjusting it until you get the lowest tickover speed that still allows the engine to run smoothly even when it is under load due to the fans and the lights being switched on.


steve_h

11-07-13, 01:30 PM

mines just under 1000 still feels a bit lumpy with the cam 800 revs its feeling like its gonna cut out


chillidoggy

12-07-13, 06:46 AM

Idle speed will be different for different engines, a standard SBC will tickover at a lot lower speed than one with a hairy cam in it. You just need to keep adjusting it until you get the lowest tickover speed that still allows the engine to run smoothly even when it is under load due to the fans and the lights being switched on.

Yup, totally agree. There's no hard and fast rule. It's whatever the engine (and the owner) is comfortable with.


kdavies3

12-07-13, 07:14 AM

If your cam will allow it you might get down to 650 ish.
then you get the almost like two 4 cylinder engines, one on each bank assuming side pipes.
It's quite weird. ;)
My BBF has a real gutsty idle but too fast when hot at the moment as I get a bit of run on when I shut it down hot, I need to tweak it.


Sidecarbod

12-07-13, 07:46 AM

Getting the idle mixture and the ignition timing sorted out at idle makes a big difference to how low you can get the tickover to be. I have found that generally most of the engines that I have worked on have had the idle timing set too retarded, if the timing was then advanced the idle speed went up and this then allowed the tickover screw to be backed off in order to bring the speed back down. As a result the idle vacuum then went up and the motor would run at a lower idle speed. I guess this is what people mean when they say to twiddle about with the settings in order to get the highest vacuum reading. Obviously you can not just go winding on more ignition advance without checking what the all-in figure will be.


With regards to the pilot screws on the Eddy carb I have found that starting with them rich then winding them in slowly until teh engine just starts to slow down is a good menthod of finding the correct setting. (This setting being 1/4 turn out from the 'drop off' setting). Edelbrock suggests a different method which in my humble leaves the motor running a touch lean.


chillidoggy

12-07-13, 01:06 PM

I set my idle timing at 14 degrees, and then jigged around with the idle screws. It did work out to be the best scenario, and the idle was a smooth as I could get it for the very modded engine. I think the HEI has about 20 degrees advance in it, which would total 34, but I stand corrected on that. I don't recall any running on issues, but if the timing is too far advanced at idle it generates additional heat in the chambers, and I imagine running on could then ensue. An exposed spark plug thread, or maybe the edge of a combustion chamber could act as a glow plug in that case.


Sidecarbod

12-07-13, 01:25 PM

I set my idle timing at 14 degrees, and then jigged around with the idle screws. It did work out to be the best scenario, and the idle was a smooth as I could get it for the very modded engine. I think the HEI has about 20 degrees advance in it, which would total 34, but I stand corrected on that. I don't recall any running on issues, but if the timing is too far advanced at idle it generates additional heat in the chambers, and I imagine running on could then ensue. An exposed spark plug thread, or maybe the edge of a combustion chamber could act as a glow plug in that case.


I agree that if the ignition is too far advanced it will generate additional heat in the chambers but in full this statement should be "if the timing is too far advanced from the ideal setting required for idle the chamber will get too hot".

I have found that the more an engine is modified (hot cam, large bore headers ect) then the more the idle advance needs to be. My RV8 lump runs 20 degrees at idle as does Noel's highly modified SBC (427).

Finding what the idle setting should be at idle is not too hard with a well 'tuned' ear, the problem can then be that the all in figure is too high. Some people then just give up and reduce the static advance but really the dizzy curve should be modified to sort out the issue.


Retarded timing is what can really make things glow in the combustion chambers, this is because the burn rate is lower and because the charge can still be burning when the exhaust valve opens, the valve ends up getting very hot.

Some engines run with the timing retarded at idle in order to keep the exhaust and I guess the CAT hot.


Sidecarbod

12-07-13, 01:59 PM

Page 3 of this PDF has a graph of exhaust temperature Vs ignition timing...

http://rescomp.stanford.edu/~efroeh/papers/RDH_Engine_Performance.pdf


chillidoggy

12-07-13, 02:24 PM

:rolleyes:
I agree that if the ignition is too far advanced it will generate additional heat in the chambers but in full this statement should be "if the timing is too far advanced from the ideal setting required for idle the chamber will get too hot".

I have found that the more an engine is modified (hot cam, large bore headers ect) then the more the idle advance needs to be. My RV8 lump runs 20 degrees at idle as does Noel's highly modified SBC (427).

Finding what the idle setting should be at idle is not too hard with a well 'tuned' ear, the problem can then be that the all in figure is too high. Some people then just give up and reduce the static advance but really the dizzy curve should be modified to sort out the issue.


Retarded timing is what can really make things glow in the combustion chambers, this is because the burn rate is lower and because the charge can still be burning when the exhaust valve opens, the valve ends up getting very hot.

Some engines run with the timing retarded at idle in order to keep the exhaust and I guess the CAT hot.


Yep, and without the right mechanical advance in the distributor, you're potentially knackered. Or rather the engine performance will be. Trouble is, unless an engine is built to the exact same standard, as soon as the modifications take place (like most, I'm guilty of that), then the timing book goes out of the window, and it's back to trial and error. And with 4 BBL Holley carbs not exactly renowned for fuel economy, it makes it even harder to get mixture right throughout the range.

Oh, for EFI, and programmable ignition!
:rolleyes:


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Re: At what RPM does your 350 SBC idle?


550rpm in drive, very stable, 10inHg.

Any lower in rpm and it's pretty "iffy."

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Idle rpm sbc

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