Why So Slow?

Well the jeep mostly drives now. Its a 1974 with the 258. It has always felt slow relative to the 79 (which also has a 258). Today I drove it to work and had to hold WOT to maintain 50 mph on flat ground. All stock but with 31" tires and the factory optional 4.27:1 gearing. I tested compression and all cylinders were 115-130 psi. With it idling, you can hear the engine run rough everytime a plug wire is pulled-so all cylinders are working at least at idle. New distributor cap and rotor, new coil, new fuel pump and filter, new vacuum advance thing, verified timing and dwell. It starts right up and runs decently smooth. No smoke. It does pop sometimes when you're cruising and let off the gas quickly, but thats in the nature of an old catless carbed vehicle is it?

What should I check next? Would the crank pulley have spun so the timing marks might be wrong? Bad cam?

 

Mice in the air cleaner?

Exhaust obstruction?

Brake dragging?

Speedometer error?

 

Does it have air injection? If the diverter valve is not working, that will cause a backfire through the exhaust.

I would expect a '79 to have the BBD in place of the YF. That makes a difference. It's also a different transmission; IMO the T-150 has a more favorable 2nd gear ratio than the T-14.

I owned a '75 CJ-6 with a hard top and 3.73s in the late 70s. Probably had 30ish tires on 8" rims (10.00-15s). I drove all over southern California with that on the highway, keeping up and passing traffic. My J10 is a '77 258 with 3.54s and a BBD originally. The PO flat-towed his CJ-5 all over New England with it. It does fine on highway, 65-70 NP. And a considerably heavier vehicle.

Did you try a wet compression test? My J10 compression is 160/160/160/150/155/152, and my notes say Jeep specs the compression at 150 +/- 10 psi.

Plugged exhaust? Should show on a vacuum gauge.

 

I mentioned exhaust obstruction, above. Then right after that I went out and took my YJ for a drive. A lovely smell of roasted nuts... in the exhaust.

One winter the %$#* squirrels, or mice, totally disabled my '56 by filling the muffler with acorns. So I recognized the aroma.

 

Good point. That brings up my question, when the accelerator pedal is floored is the carb butterfly fully open? Is the choke fully open when the engine is warm?

 

Hi Dane, How are you ? . Long time, No type ?
OK , How about your plugs? Newer ? Correct heat range & Gap ? ? These things , out of scope, could cause your problem. I would also check that your dwell doesn't change & your Timing advances properly, corresponding to your RPM's . I have seen worn distributor shafts cause what you describe, particularly the backfire. Loading up unburned fuel .......... Go to your TSM for tune -up numbers.
Good luck to you with this .....you'll find it.

 

I'm gonna check into some of this stuff tomorrow/this weekend and address all these responses! I'm looking forward to figuring this out Thanks for reading

 

Wide open like revved way up or mashed and bogging?

 

I would note where the distributor is, then loosen and advance the timing a few degrees to see if that perks things up.
-Donny

 

Could it be that you lost a lobe on the cam? I had this happen earlier this summer to me. It seemed to run fine but down on power and some exhaust noise in the carb. Upon further checking of this and that I had a flat exhaust lobe. Easy to check, pull the valve cover and look for a rocker arm not moving as much as the others.

 

Before I get out there today I wanna ask: do you guys have anything against setting ignition timing with a vacuum gauge as opposed to using the timing light? I wonder if I should give that a shot

This is a 1974 258 if it matters

 

I watched my dad do it once years ago, it was how he learned as a kid.
It would help to verify your timing marks.

 

Alright guys, I wonder if I'm onto something. I'll answer your questions soon but I'm wondering if I found the problem? Looking at things with the timing light, the timing mark is supposed to move if I rev it, even with the vacuum advance disconnected right? Because of the centrifugal advance? The timing mark only moves while revving it if I have the vacuum advance connected.

 

Sounds like you might need a drop of oil on the weights.

 

Freeing up the weights did it! It runs good now. I set the dwell and went on a quick drive. The only problem is when revving in neutral it breaks up around 2000 rpm. I didn't notice that under load and didn't hear any pinging or anything, but I think I can fix that if I look over everything again a bit closer.

 

Yes Sir, It should advance about 4 to 5 degrees , vacuum off. With vacuum connected, it should advance incrementally with higher RPM's ( hence increased Vacuum in the intake manifold ) .

Ya Know.....now that I think of it , you could have a vacuum Leak somewhere up top.? That would also cause it to run rich & possibly backfire .

 

The backfiring was kinda weird cause it only did it that morning, that said it was mild.

Sorry this thread has been all over the place, I've been trying to gather some ideas! I think we're good now if I figure out that break up when revving in neutral-but again I need to double check everything.

Edit: While the dwell was perfect at idle, I found it goes up with RPM. Messing with the points a little was good enough to fix the break-up, but I'm still gaining like 15º of dwell as it revs. Distributor bearings/bushings I guess? I'll update again when it's truly fixed.

Edit 2: With a new distributor everything is good! Steady dwell at all rpms. They're hard to find by the way...