Jeep CJ7 258 Six Rough Idle

[Jeepdog]

I have a 1983 CJ7 with the 258. I have done a home garage restoration. I kept it stock and tried to keep everything oem. The previous owner had installed a rebuilt long block. It had 20000 miles on it when I got it. I basically put everything new on it when I put it back together. It has a non computer controlled nos Carter BBD I found on Ebay. It has a MSD 6al, stock distributor with team rush upgrade. Timing is set at 8°. I have rebuilt the carb twice and adjusted it every which way I can. It idles a little rough. I haven't found any vacuum leaks. I think it should be smoother. I thought it was running rich but now not so sure. The first picture is of my plugs before the last time I adjusted the carb. They are black especially the middle ones. I set the float a little lower hoping to lean it out some. Now my plugs are clean except for a black stripe under the electrode like pictured. I changed them after the adjustment. That's the no 1 plug after around 100 miles. 

Any suggestions would be appreciated

[Moses Ludel]

Jeepdog...You photos are very helpful.  Assuming that the MSD ignition is firing optimally, the plugs are showing every indication of a rich condition. The BBD without computer feedback could be jetted improperly (combined jets and metering rods) for a 258 cubic inch engine. 

Checking spark plugs for a rich mix requires testing the main jets on the carburetor's high speed circuit.  You do this by running the engine at or near wide open throttle then quickly shutting the engine off.  This can be done in a lower gear since the test is strictly rpm and throttle position related.  Hold the throttle open steadily for a bit without over-revving the engine.  Inspect the spark plugs under that condition.  To eliminate confusion, the plugs should be relatively clean to start this test.

The Carter BBD was popular on Mopar V8 engines.  Considering the cubic inch displacement of these V8s, the carburetor jetting can vary widely.  The base mounting pattern is the same as a 258 application.  If there is a tag or stamping I.D. number on the carburetor, let's verify the original application.  Otherwise, you'll need to disassemble the carburetor again and verify the jet sizes and metering rods. 

If jets are right, confirm that the carburetor is assembled correctly on the metering circuit.  Note check ball(s) placement and make sure any check balls are in their correct locations.  If this carburetor was rebuilt professionally or DIY, go over it carefully and verify all the linkage adjustments, including the choke pull-off.  I have a lot of carburetor data if we can determine the BBD's original application.

I snagged this schematic drawing (courtesy of Carbs Unlimited) of a typical Carter BBD carburetor without a feedback system.  Note the check balls at positions #39 and #28 in this illustration.  Check balls and their size are critical:

Here is my rebuild of a factory Jeep 258 feedback carburetor.  You can get instructions for your non-feedback carburetor from either a shop manual or a carburetor rebuild kit for your specific application carburetor:

https://4wdmechanix.com/moses-ludels-4wd-mechanix-magazine-rebuilding-the-two-barrel-bbd-feedback-carburetor/

Your photos also reveal the way an inline six distributes fuel.  This is a graphic example of the outer cylinders running leaner than the inner cylinders.  Assuming that the compression is normal at all six cylinders, this lean/rich condition is typical of inline sixes with a center mounted carburetor and uneven length intake runners.  Check the engine's compression just to be sure it's okay.

Let's work through this...

Moses

 

[Jeepdog]

Here is pictures of the tag and one of the jets. You will know better but from what little Info I found the jet was correct. I have a couple parts carbs. I played around with different metering rods from them but it didn't seem to change much. I did verify the check ball size and location last time I had it off. The small ball was in the venturi and large was in the bowl area. What do you think about the plug in the second pic of my first post. Clean with a black stripe. They all look like that now after 100 miles after the float adjustment. I will have to do a compression check this weekend.

[Moses Ludel]

Jeepdog...Regarding the spark plug with a clean outer porcelain and rich mix near the ground strap, if each plug looks like this, it could be the MSD ignition burning the fuel mixture in this manner.  If each plug looks like this, the pattern would have to be firing/spark related.  (It's highly unlikely that every plug would line up or "index" at the same position in its combustion chamber, which rules out intake flow anomalies.)  Due to the lower float level, you still have a rich air/fuel mixture just with less available fuel.  The combustion mixture still appears rich.

If the tag is valid for your carburetor, it was built March 9, 1984.  The number "4" on the tag is for the year, in this case 1984.  My 1984 Jeep CJ factory workshop manual shows two BBD carburetors:  8383 and 8384.  Main metering jet numbers are 120-392 for each of these carburetors.  Main jet size is 0.092".  Metering rod numbers are 75-2384 for both OEM carburetors.  I have other detailed specifications for these two CEC/feedback carburetors that are your engine's era. 

The number on your tag is lower than Jeep CJ 1984 applications but much higher than 1974 AMC tag numbers.  Why are there no Sole-Vac or feedback features?  What was the listed model application for this carburetor when they sold it?  Both four- and six-cylinder engines had computer controlled (CEC feedback) systems from 1982-84.  CJ 258s were Computer Emission Control from 1982-86 with 1987-90 YJ Wrangler 4.2L/258s also having this system. 

I went to eBay, and there is a bumper crop of carburetors for these model years without CEC features like Sole-Vac or feedback systems.  Are these hybrid carburetors?  Off-shore knockoffs of the BBD?  They have no provision for a CEC system.

Your carburetor looks like a genuine BBD.  There were 258 sixes in Eagle cars and J-trucks in 1984.  I-H used the 258 AMC sixes but went out of the light truck business after the 1980 Scout II.  I can look further for the actual application of the 8128S carburetor if that would be useful.  We could determine where that tag originated.

A few other factors to consider:  1)  Is there sufficient heat below the intake manifold while the engine is warming?  2)  Do you have a thermal air cleaner installed for the warm-up period?  2)  Are the venturi and other pieces original to this carburetor?  The main jets seem correct.  Are the metering rods right?

Moses

[Jeepdog]

It has been a while since I bought the carb. I do not remember what the listed stated it was for. I have the original Carter box. My Jeeps wiring on the engine compartment side was very hacked up when I got it. I rewired everything following diagrams online. I kept it all very simple and basic. That's one reason I purched a non feedback bbd. I am running the stock air cleaner but the tac system is not hooked up. I tried but could not get it to work properly. Only thing I have hooked up it the door that opens up under vacuum. The manifold heater is hooked up but I do not know if it operates properly. I do have the coolant hoses hooked up that runs through the intake. No egr or emissions are there. Can the MSD be tested to see if its working properly? Here are some pics of the engine. I'll have to check on those metering rods. Something else i thought i should mention is when i screw both mixture screws all the way in it does not shut the engine off. RPMs will go down and get a little rough but still run. I thought it should completely kill the engine.

 

I really appreciate you taking the time to help me out.

[Moses Ludel]

Jeepdog...Glad to help, you have a really clean Jeep and great looking engine.  The engine needs to run right.

The idle with needles turned in all the way is a distinct clue.  Either the carburetor is flowing excess fuel or the engine is getting fuel from somewhere.  Closing the needles cuts off fuel flow through the carburetor passageways.  If the idle is unaffected, there is another source of fuel sustaining the rpm—or there is a vacuum leak either at the base of the carburetor, the intake manifold gasket or at a vacuum pipe/hose to the carburetor.  

Using WD-40 or an equivalent low volatility spray, spray a light mist around the carburetor base with the engine idling.  Listen for engine speed changes that indicate a leak.  Check other vacuum hoses that might allow air to enter the intake stream.  Avoiding hot areas while spraying a light mist, spray at the intake manifold junction with the cylinder head.  If speed changes, the intake manifold gasket is leaking.  Check around the vacuum switches, too.

You have the idle step-up solenoid for when the air conditioner is running.  When adjusting the idle mix, is the engine rpm (speed stop) low enough to attain a normal idle speed?  Is the step-up solenoid preventing the idle speed adjustment from dropping to normal?  The idle rpm speed should be on the tuning/emissions decal.  If not, see the 1984 Jeep CJ FSM details below:

Another concern I have is the return flow from the fuel filter to the tank.  This must be unrestricted.  Your fuel lines from the tank to the filter, from the filter to the carburetor and from the filter back to the fuel tank must be routed correctly.  The third pipe on the filter (return to tank) serves two purposes:  1) helps prevent vapor lock by continuously moving fuel through the system and 2) importantly, to prevent too much fuel pressure from unseating the carburetor's float needle.  If these lines/hoses are not routed correctly, excess fuel pump pressure can unseat the needle, raise fuel level in the bowl, and cause a flooding or overrich condition.  Also, the air vent hose above the carburetor's float bowl must be routed correctly.  Otherwise, the bowl is pressurized, which could unseat the float needle or force fuel through the carburetor. 

Note: The emission vapor canister is part of this system and its hoses must be routed correctly.  Make sure the EVAP hoses and the canister's vacuum source are correct.  Make sure hoses to the fuel tank and filler tube are routed correctly and unrestricted.  Roll-over valve(s) at the tank (if so equipped) must function normally.  Fuel tank pressure must be normal. Be sure the EVAP system is not restricted or malfunctioning.

A fuel pump with too much volume or pressure output can unseat the needle, too.  I see a mechanical fuel pump.  If necessary, check the fuel pump output pressure and compare to the OEM recommended pressure. 

Another concern is the EGR system.  EGR and the ignition spark advance are each applied with carburetor ported vacuum, not manifold vacuum.  They are both regulated by the TVS (thermal vacuum switch), which prevents EGR function and ignition vacuum advance until the engine warms.  The TVS threads into an engine coolant port (typically at the cylinder head or thermostat housing).  Around 140 degrees F, the TVS (wax pellet valve) opens and allows the ignition (vacuum) spark advance and EGR valve to operate.

If the EGR valve is attached to manifold vacuum, you will have a constant rich condition.  Unspent/unburned hydrocarbons and other emissions in the exhaust will constantly recirculate through the intake stream.  This could soot up the spark plugs.  Since carburetor ported vacuum normally applies the EGR valve, the EGR should only operate off-idle (not during idle) then tapers off as the engine speed/throttle increases.  Just like vacuum spark advance, the EGR opens under maximum (carburetor ported) vacuum at throttle tip-in and operates through approximately the first 1/4 or so throttle opening.  Check the vacuum hose routing to the EGR valve.  This should be carburetor ported vacuum supply.

As for coolant circulation through the intake manifold passage, the simple test is whether the inlet and outlet manifold coolant hoses have nearly equal temperature.  This can be confirmed with an inexpensive ("Harbor Freight") non-contact infrared thermometer.  You can confirm actual manifold temperature with the infrared thermometer.  As a footnote, your heater would not be working if this coolant passage were clogged.  

Check out the points of concern here, beginning with the EGR, which can cause real havoc if the EGR valve is either stuck open (frozen piston) or receiving constant vacuum.  This would hold the EGR valve open at all engine speeds and throttle openings.

Let us know your findings...

Moses

 

[Jeepdog]

I have an update on some of the questons you asked. 

I was able to check my compression and it seems good. It was around 160 psi on all cylinders.

My metering rods have the number 2197 on them.

The manifold heater seems to be working. the temps were the same throughout. 

I do not have the EGR system in place. 

I do have a vapor canister and roll over check valve like the one pictured.

 

Vapor canister is tied in with the PCV. small hose to CTO switch. other 2 are from the carb vent then to the tank. 

[Moses Ludel]

Jeepdog...Go through my list of concerns (last reply), including the unobstructed return flow of fuel from the fuel filter to the tank.  (This helps prevent excessive pressure from unseating the carburetor's float needle.)  If fuel pressure is unclear, check the fuel pump pressure and volume on the line going into the fuel filter.  Instructions are in the factory service or shop manual (FSM).  

Check for vacuum leaks as I suggest, including the WD-40 tests...Underhood and chassis vacuum hose diagrams are in the FSM.  Are you using an FSM for your Jeep model? 

Moses