Jeep CJ 304 V-8 Carburetion Issue

[Rinky Dink]

In the on-going saga that is the Rinky Dink resuscitation, I thought I was done, or nearly so. After a decade or two of silence the mighty 304 coughed and sputtered to life. With a bit of fiddling it seemed to run pretty good though it was certainly running rich. Towed it down to the muffler shop and with a few hours and $650 swapping hands it now possessed a California Compliant Cat exhaust. Towed it  back home and started it up to the muted whisper of the Flowmaster muffler. As it sat there idling it seemed to progressively run worse. My son happened to stand in front of the exhaust outlet and commented on how hot it was. Sure enough, the exhaust was hot enough to seriously burn you. Shut it down and looked at the cat. It had already started to discolor. Obviously the carb was dumping a ton of fuel into the engine. The sooted up plugs confirmed this. Now this got my head scratching. New carb, plugs, wires, cap, rotor, distributor, fuel pump, filter, rubber, tank. Timed correctly @ 5 BTDC.

Last night it occurred to me that the fuel pump was over pressurizing the carb and pushing fuel into the engine. I figured the the fuel return line must be plugged. When I had replaced the fuel tank I had flushed the fuel inlet line but not the return line. This is the 1st carbureted vehicle I have had with a fuel return, just like an injection system. At the time, I thought it was only a vent. Wrongo! Home for lunch I did a quick disconnect at the filter and tried blowing air through the line. Nothing happened. I disconnected the return line at the tank and tried blowing again. Still nothing. Tonight I'll try the air compressor.  Hope this is the cure. 

[Moses Ludel]

Rinky Dink...Many find the Jeep third pipe return line interesting.  I also believe you could have an issue here...

The third or return pipe on either the fuel filter or a mechanical fuel pump came into being during the early carburetor emissions years.  A classic trouble issue with carburetors was fuel vapor lock, and with the emissions non-vented fuel bowls (a requirement in the late 'sixties/early-'seventies), carburetors were even more prone to vapor lock.  

A simple solution was the three-pipe fuel pump or AMC's approach with a three pipe fuel filter.  Instead of holding high pressure against the float needle, the excess fuel returns to the fuel tank.  The float needle is less likely to unseat with a full fuel bowl.  High pressure can raise the fuel level or pressurize the carburetor like you believe may be happening.

The third pipe returns excess fuel volume to the fuel tank.  The mechanical pump is designed to deliver way more fuel volume than needed, more than enough to keep the carburetor bowl full.  This continual return flow to the tank prevents fuel stagnation in the line to the carburetor.  Stagnating fuel heats up from the engine, even more with an emissions engine.  This causes vapor lock within the fuel line.  Many years ago, in the Jeep L-head or F-head era of no return line to the fuel tank, putting clothes pins on the fuel line to the carburetor would draw off some of the heat, a homespun remedy to reduce risk of vapor lock!

Note: If the fuel bowl has a vent pipe and hose to the emissions EVAP canister, make sure this hose and the vent are not restricted.  Make sure the EVAP canister is functional.  A restriction here can pressurize the fuel bowl and force fuel through the carburetor's circuits.  Of course, a defective needle and seat or a high float level can be an issue.

The return pipe's recirculating fuel keeps the fuel line constantly cooled by the movement of the excess fuel.  The needle still unseats readily at the carburetor, so there is a ready flow of fuel into the bowl.  It's only the excess that returns to the tank, and if that line is restricted, it would boost pressure within the carburetor, which could cause a rich mixture.

As for the 2150 Motorcraft carburetor, this is a very reliable carburetor and arguably much better than a 2100 series Holley two-barrel.   I like the 2100/2150 Autolite/Motorcraft.  It's relatively easy to rebuild and bench "blueprint" if you need to do so.  The settings hold for an endless period.  It is an excellent carburetor for start-up and fuel mileage.  You'll have more of an issue with the 304 intake manifold's inefficiency (two-barrel plenum) than the carburetor's functions.

Also make sure the EGR valve is seating properly.  It operates from ported vacuum and should not be open at an idle, which can affect performance at an idle.

Moses

[Rinky Dink]

Moses,

Turns out the return line was plugged, well enough that 100 psi wouldn't clear it. Installed a new hard line. Still running rich enough that it is fouling plugs so I checked the fuel pressure. At idle it runs 1.5 - 3 psi, spiking to 7 on revving it so pressure should not be an issue. I pulled the coil wire from the dist and cranked it, showing nice white fat spark. Did the same with one of the leads from a fouling plug - orange and weak. Leads me to think the wires are no good. Installed a set of Accel 8mm  wires + new plugs and turned the idle screws in a 1/2 turn so they are 1 turn out. Restarted and let it idle. By the time the water temp was up close to normal it started to idle rougher, obviously missing on one or more cylinders. I should add the cat was starting to glow red so obviously getting too much fuel. WTH! Crank open the throttle and the engine revs well with the occasional  carb backfire and closed throttle over-run popping. I can't find any manifold leaks. Sprayed carb cleaner everywhere to no effect on the idle.

7 out of 8 cylinders have 150-160 cranking psi with the remaining @ 135 rising to 145 with a bit of JB squirted in the cylinder( probably either worn or stuck rings).

New (professionally rebuilt) carb, new fuel filter, new dist, cap, rotor, wires, plugs. Timed @ 5' with vac advance off & plugged, Ported vac to the dist.and it shows advancment Everything I can think of is done with the only thing remaining would be a stuck float needle. As it is new, I really didn't want to open the carb. Looks like that is next. I'll be checking the main jets also. From what I have found they should be 47-49. Do you know of anything different? Any ideas?

Tim 

 

[Moses Ludel]

The richness can be a high float level, Tim.  If you did the carburetor rebuild yourself, I'd be more confident.  I've seen many "rebuilt" carburetors with mismatched jets and metering rods or worse.  The cranking compression sounds good with the exception of the one cylinder, and I'm optimistic that your stuck rings theory might prevail here.  Could also be carbon on the valves.  A leak down test could pinpoint the compression loss at that cylinder.  You might try torquing the head bolts if you haven't done so already.

I would take the carburetor apart and check all measurements and metering factors.  Confirm the check ball sizes and other trouble possibilities.  The 2100/2150 is a relatively easy carburetor to rebuild, and if you do it yourself, it will work properly.  There is too much fuel flowing through this carburetor.

Other possibilities are an EGR valve malfunction or back-pressure/clogging at the EVAP system that boosts fuel pressure in the float bowl.

Moses

[Rinky Dink]

The carb was bought from a professional carb shop so I doubt there would be any issue with it. I talked with them today and questioned them re the setup. They did question whether there was visible fuel leakage in the throttle bores. That's kinda hard to check with a mechanical pump. They said it was highly unlikely that the float level would be wrong as they wet test every product they build. They did suggest leaning the altitude aneroid, starting with a 1/2 turn. Tried this this afternoon. Leaned the aneroid, let it warm up and it still had the miss. I threw the vac gauge back on the manifold  and advanced the dist until it showed 19" steady on the gauge and it seemed to run the best. This is about 5-6" improvement from the spec'ed timing numbers. The miss is greatly reduced and the best part is the cat no longer glows. I figure to just take it to the smog guys and hope it passes. I know them so they should be good to me.

This timing advancing makes me wonder if the dist is off by one tooth, and, would it make any difference, as it fires at 5 btdc on the #1 as per the manual and yet it idled like crap. 

I haven't touched the head bolts. There are no obvious leaks and the coolant isn't dropping. At this point I don't want to risk opening a larger can o  worms. There is always the possibility that low cylinder could be leaking over to the adjacent but I think it would show on both cylinder's numbers.

I checked the EGR with a Minivac. The diaphragm opened and choked the idle rpm so I assumed it was ok. I guess I should take the evap can apart and go through it just to be sure. 

Thanks for your insight on this project. It's getting there.

[Moses Ludel]

See comments...

7 hours ago, Rinky Dink said:

The carb was bought from a professional carb shop so I doubt there would be any issue with it. I talked with them today and questioned them re the setup. They did question whether there was visible fuel leakage in the throttle bores. That's kinda hard to check with a mechanical pump. They said it was highly unlikely that the float level would be wrong as they wet test every product they build. They did suggest leaning the altitude aneroid, starting with a 1/2 turn. Tried this this afternoon. Leaned the aneroid, let it warm up and it still had the miss.

The shop offered a confidence inspiring, knowledgeable response.  Most are unaware of the altitude compensation device on the 2150.  This is a touchy mechanism and needs factory settings without fiddling.  Our 360 V-8 in the '87 Grand Wagoneer had the device, I set it by the book during a rebuild and never had an issue.

I threw the vac gauge back on the manifold  and advanced the dist until it showed 19" steady on the gauge and it seemed to run the best. This is about 5-6" improvement from the spec'ed timing numbers. The miss is greatly reduced and the best part is the cat no longer glows. I figure to just take it to the smog guys and hope it passes. I know them so they should be good to me.

This timing advancing makes me wonder if the dist is off by one tooth, and, would it make any difference, as it fires at 5 btdc on the #1 as per the manual and yet it idled like crap.

How far did you advance the timing to get 19 in/hg at idle?   If extreme, this would be more a matter of valve timing.  That should not be the case, you just installed new timing sprockets and chain set.  Has the damper ring crept on the crankshaft damper/pulley?  See if TDC on the damper aligns with actual TDC of #1 (or #6) piston.  

Distributor tooth location would only dictate the rotor position, not the degrees of timing advance, which is relative to the #1 piston-to-damper mark.  As long as the distributor housing rotates enough for a base timing set, the shaft/rotor position would not be an issue. 

I haven't touched the head bolts. There are no obvious leaks and the coolant isn't dropping. At this point I don't want to risk opening a larger can o  worms. There is always the possibility that low cylinder could be leaking over to the adjacent but I think it would show on both cylinder's numbers.

Good call for now...Again why I'm a strong proponent of leak down testing, which would indicate/pinpoint leakage at the rings, valves, head gasket, a casting crack and more.

I checked the EGR with a Minivac. The diaphragm opened and choked the idle rpm so I assumed it was ok. I guess I should take the evap can apart and go through it just to be sure.

EGR should be ported vacuum supply.  Checking the EVAP can make sense if you had the clogged fuel return line or other fuel supply issues.  These canisters get overwhelmed over time. 

Thanks for your insight on this project. It's getting there.

It's a process, and you're being thorough...Moses

 

[Rinky Dink]

Hello Moses,

It has been awhile and I need to post an up date on the carb issue. After talking about the problem to a mechanic that I know he directed me to a local one man carb shop. One of the things I have learned (per the Calif State smog ref), is that the MC 2150 never came on the 304. Now when I was looking for a carb I saw the bore difference, the 2100 with 1.08" and the 2150 with 1.21" so of course I went with the bigger carb. Little did I know. This little carb shop uses an exhaust sniffer to jet the carb to suit the engine. He pinched shut off the air injection thereby disabling the cat in order to see the true exhaust chemistry. What gasses came out the exhaust port is what came out the tailpipe. It turns out the rebuilt carb I bought was jetted for a 360. It was way over jetted. Like 6 or 7 main sizes too rich. $100 later I have an old jeep that easily passes smog, with emissions way below the allowed average. Starts easy, will idle all day and meters without a hiccup off road. That was money well spent.

[Moses Ludel]

Very pleased to know you resolved the rich state and now meet emissions requirements, Rinky Dink!  This is the old adage that just because a carburetor fits does not mean it works right.  The 2300 Holley, as a comparison, was used on everything from a 266 Scout V-8 to big-block Fords!  The only similarities were the series and basic design.  Autolite/Motorcraft used the same approach.  A 360 V-8 carburetor fits your manifold but did not have the right jetting for a 304.

Thanks for sharing!

Moses