how to tell engine RPM? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

shelby987 · 08-25-2006, 03:30 PM

I am beginning to suspect my tach is not correct, I think it may be off by a couple hundred RPM, when I get my carb to idle the truck nicely, it says I am sitting at 1,000rpm, but shouldn't a nice proper Idle be around 750rpm? If I pull the idle screw out to get the tach to read 750rpm, it feels like the engine will die....is there any other way to check engine RPM other than my tach (Is my only other option to just go buy another tach?)

Jim_PA · 08-25-2006, 03:31 PM

You could have it checked with a timing light.

shelby987 · 08-25-2006, 05:21 PM

how do you do that?

68haywagon · 08-25-2006, 05:38 PM

Some timing lights have a tach built into them. If you have one all you do is hook up the battery terminals, the plug wire terminal, and start the truck and it will register the RPMs.

Mike

Jim_PA · 08-25-2006, 10:04 PM

Quote:

Originally Posted by 68haywagon

Some timing lights have a tach built into them. If you have one all you do is hook up the battery terminals, the plug wire terminal, and start the truck and it will register the RPMs.

Mike

Or just count the number of flashes for 1 minute

dh81k30 · 08-26-2006, 07:48 AM

Quote:

Originally Posted by Jim_PA

Or just count the number of flashes for 1 minute

and multiply by two

Blazer1970 · 08-25-2006, 05:59 PM

If you are talking about a factory tach, it is definitely incorrect. They all seem to read high by a few hundred RPM after all these years. I think some of the electronic components did not age too well.

*rage'nrat638* · 08-25-2006, 06:12 PM

Quote:

Originally Posted by Blazer1970

If you are talking about a factory tach, it is definitely incorrect. They all seem to read high by a few hundred RPM after all these years. I think some of the electronic components did not age too well.

yea and many pay extra for that!!!!!
unplug your dizzy vac hose and see if your rpm changes if it don't your vac advance diaphragm is shot...

LUV2XCLR8 · 08-25-2006, 06:06 PM

Although by no means am I any kind of expert

byrd · 08-26-2006, 08:50 AM

Factory tachs can be calibrated to be more accurate than that. Look at the back of the tach there is a hole in it with a small phillips head screw inside for just that, calibration. See if you can use that screw to get the engine and tach a little closer to being right. Makee samll adjustments and mark it before you move it so that if it doesn't help you can al least get it back t where it was before.

On another note I idle my truck as low as I can, around 550-600 rpm for one main reason, no power brakes and it is easier in traffic to keep it stopped,

08-25-2006, 03:30 PM	#1
shelby987 Registered User Join Date: Mar 2005 Location: Just outside Durham, NC Posts: 894	how to tell engine RPM? I am beginning to suspect my tach is not correct, I think it may be off by a couple hundred RPM, when I get my carb to idle the truck nicely, it says I am sitting at 1,000rpm, but shouldn't a nice proper Idle be around 750rpm? If I pull the idle screw out to get the tach to read 750rpm, it feels like the engine will die....is there any other way to check engine RPM other than my tach (Is my only other option to just go buy another tach?) __________________ 1969 SS 350 Camaro Sold 2008 1970 3/4T K20, stock height with 33's, 250 I6, 3 on the tree, and locked front and rear 4.10's 1980 Chevy Malibu 2 dr Sold 2007 1993 Olds. Cutlass Ciera Wagon Traded 2006 2003 Saturn L200 w/5pd. D/D My list spans 5 decades with One common thread.....GM!

08-25-2006, 03:31 PM	#2
Jim_PA Registered User Join Date: May 2005 Location: PA Posts: 2,696	Re: how to tell engine RPM? You could have it checked with a timing light.

08-25-2006, 05:21 PM	#3
shelby987 Registered User Join Date: Mar 2005 Location: Just outside Durham, NC Posts: 894	Re: how to tell engine RPM? how do you do that? __________________ 1969 SS 350 Camaro Sold 2008 1970 3/4T K20, stock height with 33's, 250 I6, 3 on the tree, and locked front and rear 4.10's 1980 Chevy Malibu 2 dr Sold 2007 1993 Olds. Cutlass Ciera Wagon Traded 2006 2003 Saturn L200 w/5pd. D/D My list spans 5 decades with One common thread.....GM!

08-25-2006, 05:38 PM	#4
68haywagon I'm watching you! Join Date: Feb 2005 Location: Levittown, PA Posts: 1,822	Re: how to tell engine RPM? Some timing lights have a tach built into them. If you have one all you do is hook up the battery terminals, the plug wire terminal, and start the truck and it will register the RPMs. Mike __________________ 2012 Chevrolet Silverado 1500 Z71 4WD 1993 Chevrolet C-1500 W/T - Blue - 4.3L V-6, 4 speed auto - Dad's (RIP) 1992 Chevrolet S10 - LS swap project 1968 Chevrolet C-10 longbed fleetside - sold

08-25-2006, 05:59 PM	#7
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,510	Re: how to tell engine RPM? If you are talking about a factory tach, it is definitely incorrect. They all seem to read high by a few hundred RPM after all these years. I think some of the electronic components did not age too well. __________________ Tim

08-25-2006, 06:06 PM	#9
LUV2XCLR8 The LuvShack Garage Join Date: Jan 2004 Location: Maple Grove, TN (West Side) Posts: 30,484	Re: how to tell engine RPM? Although by no means am I any kind of expert __________________ Owner/Op: "TN Classic Transport Carriers" The Toy: "Square Vette" 72 Hybrid Blazer Toy Barn: "LuvShack" 40 x 60 x 20 Shop Tow Piggy:"Maddy" 88 Silverado 3500 Hauler: "Feathers" 14 Aluma 8218T

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