Has anyone had this problem??
Thread Starter
Intermediate
Joined: Aug 2009
Posts: 48
Likes: 2
From: Atlanta GA
Has anyone had this problem??
I have a '79 ... when I increase speed, as I approach about 40 to 45 mph, the rpms on my tachometer also increase accordingly. I typically don't go any faster than 45 to 50mph (under 4000rpm) until I figure this out. I ran at 60mph the other day just to see what the rpms read and it sat on 5000 rpm. If I continue to increase speed, the rpms would continue to increase.
Note: I dont think the tachometer or speedometer are malfunctioning. I had a vacuum leak that I recently fixed and no impact on this. The transmission does not seem to be malfunctioning but obviously could be an issue. The carburetor was replaced a few years ago and the car sat for several years afterwards. I'm not sure if this was an issue before the car sat or the carburetor was replaced.
Any thoughts?? Thanks.
Note: I dont think the tachometer or speedometer are malfunctioning. I had a vacuum leak that I recently fixed and no impact on this. The transmission does not seem to be malfunctioning but obviously could be an issue. The carburetor was replaced a few years ago and the car sat for several years afterwards. I'm not sure if this was an issue before the car sat or the carburetor was replaced.
Any thoughts?? Thanks.
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Sounds like your tach board is faulty. My tach would read fine but just didnt seem to go with how the engine sounded.
At 60mph, I would have thought turning 5k rpm to be high.
What rear end and tyres do you have, also, was that in top gear?
Assuming a 3.7 rear, 27 inch tyres and a 1:1 ( top gear ) you should be around 2700rpm.
Check this out,
http://www.corvetteactioncenter.com/...gearcombo.html
as you can see from this vid, mine was reading about twice what it should have.
At 60mph, I would have thought turning 5k rpm to be high.
What rear end and tyres do you have, also, was that in top gear?
Assuming a 3.7 rear, 27 inch tyres and a 1:1 ( top gear ) you should be around 2700rpm.
Check this out,
http://www.corvetteactioncenter.com/...gearcombo.html
as you can see from this vid, mine was reading about twice what it should have.
Last edited by jotto; Jun 25, 2010 at 05:37 AM.
Thread Starter
Intermediate
Joined: Aug 2009
Posts: 48
Likes: 2
From: Atlanta GA
Thanks for the replies. The vacuum line to the transmission vacuum modulator seems to be OK. I am going to try replacing the transmission vacuum modulator itself and see if that has an impact. After reading a few other threads about similar topics, replacing it is a good place to start.
Thread Starter
Intermediate
Joined: Aug 2009
Posts: 48
Likes: 2
From: Atlanta GA
I replaced the transmission vacuum modulator and that didn't resolve the problem. Inexpensive modification and one thing I can check off my root cause list. Thanks for the suggestions. Any other thoughts? :-)
Thread Starter
Intermediate
Joined: Aug 2009
Posts: 48
Likes: 2
From: Atlanta GA
MotorHead, the tran is an automatic. (My Mustang-loving father-in-law reminds me of it often.) It does appear to be shifting, just at rms way too high.
Roger, I will check the downshift cable. Any tips or recommendations with that. I have never had to adjust it.
I also need to check the tack as jotto recommended, just to be sure.
Roger, I will check the downshift cable. Any tips or recommendations with that. I have never had to adjust it.
I also need to check the tack as jotto recommended, just to be sure.
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Thread Starter
Intermediate
Joined: Aug 2009
Posts: 48
Likes: 2
From: Atlanta GA
And the verdict is ... problem was the tach. It was reading double exactly like jotto's video. Replaced the tach and its good to go. This cleared up several questions but better yet, I can ride faster than 45mph!!.
Thanks everyone for your help. I should have asked this question a year ago! These forums are outstanding!
Thanks everyone for your help. I should have asked this question a year ago! These forums are outstanding!