The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1967 - 1972 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 08-25-2006, 03:30 PM   #1
shelby987
Registered User
 
shelby987's Avatar
 
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!
shelby987 is offline   Reply With Quote
Old 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.
Jim_PA is offline   Reply With Quote
Old 08-25-2006, 05:21 PM   #3
shelby987
Registered User
 
shelby987's Avatar
 
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!
shelby987 is offline   Reply With Quote
Old 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
68haywagon is offline   Reply With Quote
Old 08-25-2006, 05:59 PM   #5
Blazer1970
Registered User
 
Blazer1970's Avatar
 
Join Date: Dec 1999
Location: Old Mission, MI, USA
Posts: 2,507
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
Blazer1970 is offline   Reply With Quote
Old 08-25-2006, 06:06 PM   #6
LUV2XCLR8
The LuvShack Garage
 
LUV2XCLR8's Avatar
 
Join Date: Jan 2004
Location: Maple Grove, TN (West Side)
Posts: 30,468
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
LUV2XCLR8 is offline   Reply With Quote
Old 08-25-2006, 06:12 PM   #7
rage'nrat638
Account Suspended
 
rage'nrat638's Avatar
 
Join Date: Mar 2003
Location: ** THE FALL GUY **CHICAGO IL
Posts: 5,883
Re: how to tell engine RPM?

Quote:
Originally Posted by Blazer1970 View Post
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...
rage'nrat638 is offline   Reply With Quote
Old 08-25-2006, 10:04 PM   #8
Jim_PA
Registered User
 
Join Date: May 2005
Location: PA
Posts: 2,696
Re: how to tell engine RPM?

Quote:
Originally Posted by 68haywagon View Post
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
Jim_PA is offline   Reply With Quote
Old 08-26-2006, 07:48 AM   #9
dh81k30
Junior Member
 
dh81k30's Avatar
 
Join Date: May 2006
Location: W-S, NC
Posts: 211
Re: how to tell engine RPM?

Quote:
Originally Posted by Jim_PA View Post
Or just count the number of flashes for 1 minute

and multiply by two
__________________
Have fun, Don
dh81k30 is offline   Reply With Quote
Old 08-26-2006, 08:50 AM   #10
byrd
Union Electrician
 
byrd's Avatar
 
Join Date: Feb 2005
Location: Louisville, Ky.
Posts: 3,354
Re: how to tell engine RPM?

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,
__________________
69 Custom-10 SWB FLEETSIDE 350/350 TH Dropped 3.5/5.5"
Help support the board->HERE
Board vendors -> Vendors list
Zip:40272
Big Sandy Survivor...

Quote:
We cannot choose the way we die, but we can choose the way we live
Quote:
If you can't fix it with a hammer, you've got an electrical problem.
Quote:
I think Congressmen should wear uniforms like NASCAR drivers so we could identify their corporate sponsors
byrd is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 09:29 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com