RPM Help - JeepForum.com
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post #1 of 14 Old 09-18-2017, 04:35 PM Thread Starter
CJPondScum
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RPM Help

Hello, I have an 83 CJ7 with an AMC 390. Sporting 4.56 gears, a Turbo400 and 35' tires. At about 50mph I am pushing 3500 rpms. What can I do to get about 60mph with a safe rpm range?

Thanks, Kev

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post #2 of 14 Old 09-18-2017, 04:52 PM
Fourtrail
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Up shift. At 50 mph at 3500 rpm is still in 2nd gear with your setup. Make sure you have the vacuum hooked up correctly to your shift modulator. With no vacuum a TH400 will only shift at WOT/redline and might be why you are not shifting into high/3rd gear.

This is under consideration that you are mapping speed by gps and not looking at the speedo which will be way off with the gear and tire size you currently have.
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post #3 of 14 Old 09-22-2017, 09:30 PM Thread Starter
CJPondScum
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I'm pretty sure it's shifting through all 3 gears but I will definitely take a look at the Vacuum modulator. I sure hope that helps!
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post #4 of 14 Old 09-23-2017, 04:28 PM
skizriz
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You definitely got something wonky going on. At 50, mine probably cruises at around 2200 rpm or so with a TH400 and 4.88 gears.

If it shifts through 1-2 ok, it's probably not your modulator, that should show up through all shifts if it was the problem. With a non working modulator, the 1-2 shift is about enough to give you whiplash, I drove mine 30 miles home after crushing my modulator with a broken driveshaft. Not a fun drive.


Sounds like a sticky valve in the valve body to me.

Check your kickdown switch. Make sure it's not getting power all of the time. Only at wide open throttle.
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post #5 of 14 Old 09-23-2017, 06:56 PM Thread Starter
CJPondScum
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Thanks I will check the kickdown switch as well.
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post #6 of 14 Old 09-24-2017, 02:43 PM
LumpyGrits
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Cj

Check your speedo against a GPS first.
LG

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post #7 of 14 Old 09-24-2017, 07:56 PM Thread Starter
CJPondScum
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The speedo is correct... Need to verify the RPM gauge is correct. But my Jeep is up on Jack stands right now with the front carrier out maybe next week!
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post #8 of 14 Old 09-25-2017, 05:02 AM
jp360cj
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Quote:
Originally Posted by CJPondScum View Post
The speedo is correct... Need to verify the RPM gauge is correct. But my Jeep is up on Jack stands right now with the front carrier out maybe next week!
Are you using a factory tach or aftermarket? An 83 would have came from the factory with either a 4cyl or 6 cylinder, so the factory tach won't work unless it's swapped out with a V8 tach from an earlier CJ.

If it's an aftermarket tach, make sure it's set up for 8 cylinder. If you "know" it is, double check...I bumped the switch while install a tach one time. Had me stumped for a while.

77 CJ-7- AMC 360, Edelbrock cam, MC2100, Fenderwells, CJ T18/D20, Chevy D44/EB 9" rear, Trutrac f/r, 4.11, SOA, 1.25 lift shackles, 35x12.50 BFG KM2s, Shackle Reversal
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post #9 of 14 Old 09-25-2017, 10:11 AM
Fourtrail
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Originally Posted by jp360cj View Post
Are you using a factory tach or aftermarket? An 83 would have came from the factory with either a 4cyl or 6 cylinder, so the factory tach won't work unless it's swapped out with a V8 tach from an earlier CJ.

If it's an aftermarket tach, make sure it's set up for 8 cylinder. If you "know" it is, double check...I bumped the switch while install a tach one time. Had me stumped for a while.
Possibility for this. Running on the 6 cyl mode for the tach, 3500 rpm would be reading 10500 sparks per minute based on 3 sparks per revolution, then converted to 4 sparks per revolution for a v-8 gives 2625 rpm which calculates out to 53mph with a true 35" tire so roughly 50 mph with a marked 35" tire.

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post #10 of 14 Old 09-25-2017, 05:06 PM Thread Starter
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Hmmm that may be it!! When I get home I will have to check. I'm pretty sure it's the stock tach but it was there when I purchased the Jeep. I will investigate.

Thanks!
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post #11 of 14 Old 10-14-2017, 09:46 AM Thread Starter
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Finally had a chance to pull my Tach... You were correct!! It's the original 6 cyl tach with the blue casing. Looks like I will need to pick up an aftermarket.
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post #12 of 14 Old 10-14-2017, 11:04 AM
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Quote:
Originally Posted by CJPondScum View Post
Looks like I will need to pick up an aftermarket.
Don't bother with the Crown or Omix Ada versions. They don't even work well as a paper weight.

Speehut makes a nice gauge from what I've heard. Im sure there are other manufactures with decent offerings, Just make sure it fits in a 2-5/8" hole.

Or, you could search for a used OEM tach. Unfortunately, the V8 version is kinda hard to come by.

Good luck,

Matt
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post #13 of 14 Old 10-14-2017, 04:47 PM
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Quote:
Originally Posted by CJPondScum View Post
Finally had a chance to pull my Tach... You were correct!! It's the original 6 cyl tach with the blue casing. Looks like I will need to pick up an aftermarket.

That's funny as *****. I was about to agree with fourtrail and the grimmjeeper gear ratio calculator was only off by a mile or two per hour according to your OP so his original thought was right on track.

This tach deal reminds me of a story back in my high school, modded VW days, only in reverse. 3 of us routinely shifted at 6,000 but Brian was scared to go past 4,000. As kids do, we called him all the names that you can imagine but, as it turns out, his aftermarket tach was pre-set as a 6 cylinder so who knows what he was actually spinning, lol.

I'm glad it was a simple fix.


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post #14 of 14 Old 10-16-2017, 02:53 PM
LumpyGrits
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Cj

These folks put out some great gauges.
http://www.marshallinstruments.com/

LG

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