how to test tach

[scottkanderson]

Hi, I have a 1986 4.3L gm motor with a stringer 800 outdrive. I just bought this boat, and the guy I got it from told me that the tach was off by 600 or so rpms. He said he was getting about 35 mph at 3600 rpms WOT at an altitude of 5400ft with a 15 pitch prop. That would be 4200 rpms according to him, which is the bottom of WOT 4200-4600. <br /><br />With the same prop, I ran it at a lake at 7500ft and got about 23 mph and 2600 rpms WOT. And then I took it to Lake Powell at 3650ft and it got 32mph at 3200 rpms WOT.<br /><br />What I am wondering is why am I getting less than the other guy did at an elevation 1800ft less than he was at.<br /><br />I did get new points and plugs, wires etc.. How can I check what the real rpms are. I want to know before I put on a 13 pitch prop to run It at the lake at 7500ft by my house. <br /><br />Will an inductive tach meter give me an accurate reading? In all the posts about rpms it says to test them. How can this be done?<br /><br />Thanks,<br />Telluride

 

[Bondo]

Re: how to test tach

If you've got a Timing Light,.. With the built in Tach,....<br />Just Compare the 2..........

 

[Boatist]

Re: how to test tach

If you have a Dwell meter with a tach you can also compare. Check the back of the tach and make sure it is set to 6 cylinder. Call your dealer and ask how much it would cost to have tach checked. Tell him you are looking to buy a new prop but want to make sure the tach is right first. he may give you a good deal hopeing to get the prop sell. <br /><br />Even a Cheap Auto Dwell meter usually has a tach as well.