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Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

 
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post #1 of (permalink) Old 05-21-2012, 08:57 AM Thread Starter
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Latest Jeep project

For a long time I've suspected that the engine was running rich. We're talking about taking the Jeep to Colorado and points west this summer, and at seven or eight thousand feet I feared that it would be WAY rich.

My original plan was to swap on a Motorcraft 2150. I thought that I could cut the 1-barrel top off the intake and weld a 2-barrel plate on but, with the exhaust passages for warming the carb, that proved impractical. (Thanks to Dux4Life, I have two MCs to choose between. If anyone has an unwanted 2-barrel manifold . . . )

Instead I decided to fix up the Carter YF, seal the manifold leaks and make the best of it for now. As mentioned in another thread, I now have the Jeep high-altitude jet and metering rod for the YF, so the mountains should be friendly territory.

The exhaust manifold was warped 0.050", so I sent it to an automotive machine shop to be surfaced. The face of the intake that mates to the exhaust was also warped, but the milling machine fixed that. I put it back together and after about five hours of run time re-torqued the bolts and got a quarter to half turn on every one. The leaks aren't likely to come back for a long time.

While it was apart I also made a linkage to control the heat riser butterfly. The thermostatic spring had rusted off years ago, so the butterfly had just been flapping in the wind. Winter warm-up seemed to take forever.

The big change is an AutoMeter air-fuel ratio gauge. ($234.00 from Summit, shipping and a $20.00 coupon included) This is an amazing piece of technology. It uses a Bosch broadband oxygen sensor screwed into a bung on the exhaust pipe just below the manifold. After about thirty seconds of warmup it displays the ratio so sensitively that after going over a sharp bump, the needle bobbles for a second as gas sloshes around in the float bowl.

The good news is that the carburetor seems to be doing just what it should. At light cruise, 45 to 50 MPH, the ratio is about 15.5:1 and as high as 16:1 around 40. Full throttle it drops to 13:1 and goes lower as engine speed increases. By the consensus of what I've found on the net, that's pretty good.

The worst I've seen is that it doesn't handle throttle position changes very well. Opening the throttle produces a brief lean blip followed by a very rich one before it settles down. Closing the throttle produces similar wild swings, but at any steady position the mixture seems to be pretty good.

The dial is a poor match with the rest of the gauges, but that's of no consequence.

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Last edited by Jim_Lou; 05-21-2012 at 09:21 AM.
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post #2 of (permalink) Old 05-22-2012, 11:38 AM
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Jim, you can't have too much information on the engine as it runs.. .but honestly, I'll have to ask... What are you going to do with the information?

I'm wondering if you could have done a similar thing with an 02 sensor and an inexpenisve meter hooked to it?

Is this going to be too responsive?

Last edited by LEVE; 05-22-2012 at 11:40 AM.
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post #3 of (permalink) Old 05-22-2012, 02:23 PM Thread Starter
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The inside of the exhaust system is coated with black soot. I suspected that the engine was too rich, but also realized that it could be too lean and misfiring, also causing the soot. If I started throwing jets at it, larger or smaller, it would be difficult to tell if things were getting better, and how much farther it needed to go. With this instrument I could tell what the current state is and see what effect jet changes were having, allowing me to dial it in with just a couple of adjustments. As it turns out, it seems to be jetted pretty well, so that's the end of it. Until something changes, like the air cleaner getting dirty, the PCV valve sticking open, the needle valve leaking, a vacuum hose cracking or falling off, the altitude changing . . .

In retrospect, a sensor and a meter probably would have told me everything I needed to know, but I didn't know that then. The air/fuel meter replaces the oil temperature gauge, which I never saw read more than five degrees above the water temperature, so it's no great loss.

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post #4 of (permalink) Old 05-23-2012, 02:35 PM
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About (depending on the fuel blend) 14.7:1 is stoichiometric air–fuel mixture. This is only suitable for light load conditions as it burns hot. A richer mixture is desired for higher loads and acceleration. At 15.5:1 to 16:1, your engine is running lean.

I would have looked at the spark plugs to determine if the mixture was lean or rich, not the exhaust pipe.

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post #5 of (permalink) Old 05-24-2012, 05:40 AM Thread Starter
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That would indeed be too lean if there were a lot of it going in, but it only sees that at very light load conditions. Before the throttle approaches half open it's down to stochiometric mix and goes richer from there. According to several sources that lean mixture is just right for maximum economy at light load.

That 14.7 number popped up quite a few times in my research, but I wonder if it should be a little richer for 10% ethanol. I also saw 14:1 mentioned, but neither number was in conjunction with the fuel blend.

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