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Unread 05-06-2013, 05:51 PM   #1
CJKevin1978
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1984 CJ7 
 
Join Date: Apr 2013
Location: Pilot Mountain, NC
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No high RPM power

I just got this Jeep (258, 3 speed auto, weber 38, HEI) and it seems a little flat at high RPM. It also stumbles a little as the secondary opens. I can hear it drawing lots of air but it doesn't make much more power than it does just before the secondary opens.

I am a rookie with webers but I am at 2.25 out on the idle mixture and barely touching on the idle speed screw. I have 160/170 on the air corrector jets, 75/60 idle jets and I see the main jets on the exploded drawing but I don't know where to find them. I have some carb experience with Mikunis on PWC but very little automotive carb experience. So I am no expert but carb basics are not new to me.

Having said all that. My concern is that my ignition is not remaining advanced. The last time I dealt at all with vacuum advance was over 20 years ago and never had to troubleshoot it even then. When the Jeep is parked I can operate the throttle by hand while watching the advance mechanism. Immediately when I open the throttle, it will advance but then it fall back while the engine is still accelerating. My theory is that the same thing happens on the road causing it to be flat above ~3K RPM. The vac line is currently connected at the base of the carb and I have not yet tried to adjust the advance curve. I wanted to see if the advance movement I have is normal.

This thing is using way too much fuel to make such a small amount of power. But there isn't a lot of smoke that would indicate overly rich operation. Open to all ideas other than replacing the carb.

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Unread 05-06-2013, 05:58 PM   #2
WindKnot
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The power band of a 258 is at the low end. If your Jeep with that set up can do 65MPH, that's about right. Mileage? It's lousy

EDIT

Whoops, mistook the name for the year.
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Unread 05-15-2013, 04:41 PM   #3
CJKevin1978
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after doing a little more searching, I am thinking that I need to confirm that the ECM isn't screwing me up. I had heard of the nutter bypass but assumed that it involved jeeps newer and more computerized than mine.

Also I bought a vacuum gauge which shows me that its low(near zero) most of the time.
I have -15 when the choke is closed at idle, very little at warm idle/choke open.
-12 to -18 at very light throttle openings
back to weak as the carb's secondary opens until WOT.

I wish I had the means to measure airflow,, my guess is that it does not increase much beyond 1/2 throttle. Exhaust pressure test ahead of the cat-converter is next (after the nutter is either confirmed or applied).
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Unread 05-15-2013, 06:00 PM   #4
RamblingCJ
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Quote:
Originally Posted by CJKevin1978
after doing a little more searching, I am thinking that I need to confirm that the ECM isn't screwing me up. I had heard of the nutter bypass but assumed that it involved jeeps newer and more computerized than mine.

Also I bought a vacuum gauge which shows me that its low(near zero) most of the time.
I have -15 when the choke is closed at idle, very little at warm idle/choke open.
-12 to -18 at very light throttle openings
back to weak as the carb's secondary opens until WOT.

I wish I had the means to measure airflow,, my guess is that it does not increase much beyond 1/2 throttle. Exhaust pressure test ahead of the cat-converter is next (after the nutter is either confirmed or applied).
The ECM should already be "nuttered" if running the weber. I'd check for any intake leaks and give it a tune-up. Where are you hooking the gauge up?
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Unread 05-15-2013, 06:03 PM   #5
RamblingCJ
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Sorry, I'm on the iPad and can't see your profile. What year?

If you do have an ECM, just trace the wires from the distributor and see where they go. They should go directly to the ignition module.
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Unread 05-15-2013, 06:38 PM   #6
CJKevin1978
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1984 is the year. I am away from home now so I can't look at anything for a while.. probably Saturday morn.

I doubt its related but my tacho goes a little goofy occasionally. It reads about 1500 when at a smooth but comfortable idle.

The frustration is that it runs so good at low RPM and low load. I can push into the 70s on the highway as long as I can do so without using a lot of throttle. I found the carb linkage loose and mis-adjusted so that WOT on the pedal was about 1/2 at the carb. I think I know why now.
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Unread 05-15-2013, 06:43 PM   #7
swatson454
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It sounds like you may need to take the cap and rotor off and have a peak at the advance mechanism to make sure it's not welded shut or otherwise messed up.


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Unread 05-15-2013, 07:05 PM   #8
davbytrace
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google vacuum readings in the internet. there is a really good tutorial on interpreting vacuum readings out there.
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Unread 05-16-2013, 09:57 AM   #9
RamblingCJ
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Quote:
Originally Posted by CJKevin1978
1984 is the year. I am away from home now so I can't look at anything for a while.. probably Saturday morn.

I doubt its related but my tacho goes a little goofy occasionally. It reads about 1500 when at a smooth but comfortable idle.

The frustration is that it runs so good at low RPM and low load. I can push into the 70s on the highway as long as I can do so without using a lot of throttle. I found the carb linkage loose and mis-adjusted so that WOT on the pedal was about 1/2 at the carb. I think I know why now.
If WOT only had your carb 1/2 open due to the linkage would definitely be a reason why you can't get up and go.

Honestly, what you're describing sounds what many cant achieve. You might look into the "team rush" thread if you're looking to open up the RPMs of that 258. I noticed a significant change. Also upgrading your ignition and coil will help to. But all in all, it sounds very much how I was running before doing the nutter, team rush, and a Howell TBI.
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Unread 05-16-2013, 09:59 AM   #10
RamblingCJ
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Ditto on what swatson454 says too.


Also, your 84 would have had the ECM, so you might make sure it was nuttered properly.
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Unread 05-16-2013, 10:49 AM   #11
Caish
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You say its got an HEI,
So with that and the weber you have effectively done away with any ecm controls.
Remember the 258 max HP is at like 3500 rpm so its not a high rpm engine to start with.
I would want to watch the timing curve with a timing light to make sure its not going flat
on top or loosing timing for some reason.
I do not have information on the webers as I don't have one so someone else will have to help there.
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Unread 05-18-2013, 12:05 PM   #12
CJKevin1978
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OK, maybe I should seek more info regarding the HEI. What is have is one an inductive trigger instead of points, I may have been using HEI as too much of a generic term. Here is the Dis.
https://dl.dropboxusercontent.com/u/...134600_979.jpg
Not stock, I think but maybe not HEI either. It still has a coil as seen in the pic.

I have confirmed that the bypass has been done so that is not the problem.

But I also found that I have no centrifugal/mechanical advance, only vacuum. I checked it with the idle so low that it would barely keep running (and the vacuum line removed and plugged) and it did not change at all when revved. I then removed all plug wires except plug #1 and I saw the same reading as the engine turned over without starting. It is set to 8deg, but I don't know if that is of major importance at this point. I reconnected the vacuum line and confirmed that the vacuum advance functions. It goes off the scale when revved.

So I pulled the dis-cap and I am able to grab what I am going to call the trigger wheel and I can turn it a few degrees with very little effort. It move back to it original position smoothly and consistently. To me, this means that the mechanism is not stuck but apparently that is not the whole story.

Does centrifugal advance operate like a switch or is it gradual?

Thanks to all for your help to this point.
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Unread 05-18-2013, 03:28 PM   #13
james04si
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That looks like a stock motorcraft distributor with the team rush upgrade which is basically a riser and taller rotor. The later 82+? Distributors had a lower 6-8* mechanical advance than the 78-81 models which had 16-18*. If you rotate the rotor by hand it should snap back to the original position otherwise you may have a damaged/broken advance spring or two. Many people change over to the earlier distributor to gain back the advance lost after a nutter bypass.
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Unread 05-18-2013, 04:31 PM   #14
swatson454
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A distributor with an 18* curve will do you a world of good. Trust me, been there.


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Unread 05-21-2013, 07:04 AM   #15
CJKevin1978
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Quote:
Originally Posted by CJKevin1978 View Post
But I also found that I have no centrifugal/mechanical advance, only vacuum. I checked it with the idle so low that it would barely keep running (and the vacuum line removed and plugged) and it did not change at all when revved. I then removed all plug wires except plug #1 and I saw the same reading as the engine turned over without starting. It is set to 8deg, but I don't know if that is of major importance at this point. I reconnected the vacuum line and confirmed that the vacuum advance functions. It goes off the scale when revved.
Once again, I need to adjust my info. After seeing some centrifugal advance curves, I went back to the timing light and revved the engine higher. It does have some centrifugal advance but it comes in kind of late, I think. I will have to get a under the hood tach that I can depend on. Initial timing is at 8+* and even at high rpm it might reach 18*. My view of the upper end of the scale is blocked and I can only see it with a mirror with the engine off.

Seems to me that I have confirmed that I have a limited advance distrib. Should I grind on the limiter of replace with an earlier one? It sounds as if the early ones are cheap since so many people prefer HEI. Plus, if I get into this thing and screw it up enough to make the Jeep unusable, then the sun comes out, my wife and daughters will be pretty upset with me.

I am going to get a tacho so I can put together an advance curve.
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