Spark plugs indicate lean mix on carbureted V6

Jack wrote in news:uXh_k.8787$yr3.5373 @nlpi068.nbdc.sbc.com:

Are there deposits of discernible thickness on the insulators?

What has your water temp been? What has the timing been doing?

What you may be seeing is the alcohol content of the fuel cleaning the plugs as well.

You may try dropping a heat range or two on the plugs and see what you get. From your driveability report it sounds like your engine and the carb are pretty close out of the box. If the engine temps look OK and it isn't knocking I would try the plugs and see if you can cool them down.

No, none. Very thin light grey deposits on the bendable electrode though.

Dashboard coolant temp gauge shows normal temp at all speeds and loads.

I just checked timing. It was at 8 BTC. Spec is 7 BTC. I adjusted to spec.

Wouldn't all engines in my area, using the same gas, have plugs with snow white insulators?

Same stock plugs that I have always used. Champion RN12YC

I'll try it.

Thanks, Jack

Not being able to affect the mix or basically tune the carb is troubling isn't it. When I see that, it usually implies a massive vacuum leak is happening some place.

I also have seen strange things like that when the carb base plate got put in upside down or with a torn gasket. Some base plates have notches in them that direct the vacuum into the carb and when these go in wrong or if the gasket goes in wrong, the carb's internal vacuum is messed up which can make the carb not tunable.

Some carbs also like to just come loose at the base after a new install and need the nuts re-torqued.

I think I would take a can of WD40 or carb cleaner and carefully spray around the running carb's base and the intake manifold's gasket area to see if I got an engine burp or miss. This would indicate a vacuum leak.

I would next pump the brakes like crazy when sitting at an idle to see if that affected the engine. This would be to rule out a leaky power brake unit.

Did your other carb just stop reacting to the inputs to the mixture solenoid? Maybe the carb wasn't the trouble? Maybe the computer went into limp mode because of something else gone bad like a leaky intake manifold gasket or something?

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

The burned white insulators may be an indication of an overheating problem. Lean mixture of course can cause heads and plugs to run hot, but there are other causes of overheating in that area. Also, you might try a cooler plug.

All the symptoms seem to point in that direction, don't they?

I recently changed all the vacuum hoses, though, and did a thorough test of each individual line and found no leaks, at least none that could be characterized as 'massive'. Plus, vacuum reading at warm idle, at 700 rpm (spec) is steady at 19-20 inches Hg and there are absolutely no driveability issues. I couldn't ask for better performance.

The thick *mounting gasket* for this carb is impossible to install incorrectly. It's the same no matter which way you turn it. There is a 1 inch aluminum *spacer* that has and 'UP' arrow on it and it is pointing up, if that is what you mean by 'base plate'.

All mounting bolts are tight and I used the purple Permatex high-tack gasket sealant when mounting the carb. The intake manifold was remounted by me about 30-40,000 miles ago when I re-did the valves. This white spark plug condition did not arise until after I installed the new non-feedback carb. It is basically the exact same body as the feedback variety but with with minor modifications. The Holley 2280 is the carb that Chrysler used for it's 318 engine from the early seventies until carbs disappeared. The 3.9L engine in the 87 Dakota uses the 6280, which is the feedback version. The mixture control solenoid is no longer available and they wear out.

Chrysler refers to these carbs as "lean burn carburetors", so there may be something in the design that causes a lean mix, But the only real difference between this "lean burn" variety and the feedback variety that came stock on this vehicle is the extra main jet controlled by the mixture control solenoid, and the auxiliary idle air bleeds that are also controlled by the mixture control solenoid.

I tested the brake booster by applying vacuum and observing vacuum gauge. No leaks evident, but I will try your suggestion as it requires almost no pain :--)

The the mixture solenoid went bad. Small, molded rubber parts wore out and residual magnetism caused it to get stuck in one position.

Computer works fine. It's not in limp mode at all. Absolutely no driveability issues with this carb. I got hold of the Chrysler breakout box that reads the fault codes and tests the computer and solenoids for this pre-OBD system. The problem was definitely with the mixture control solenoid, no question about it. I monitored the signal from the computer with a duty cycle meter and the signal was there even when the solenid was stick and not responding to it. It would get stuck open or closed and the the engine would stall when coming to a stop. The computer could not see that it was stuck and continued to send the operating signal based on input from the O2 sensor. I have been at this for years and I have no doubt what caused the feedback carb to malfunction. It was definitely the mixture control solenoid.

I guess I will have have to bite the bullet and yank the new carb, break it down, check it out and reinstall it .

Isn't the intake different on the lean burn carb also? Or maybe the EGR or even the cam is different? On my Jeep there are 5 flavors of EGR that can be used according to other emission components. I had one lean burn once and remember something odd about it, beside it not being able to get out of it's own way.

Mike

Mike Romain wrote in news:493af7ae$0$26770 $ snipped-for-privacy@unlimited.newshosting.com:

I would be the only one here to believe it was fine and dandy all along. the new carb was ok, unless you have another indicator other than the plugs it is lean, I doubt it is. white plugs unless bubbled are common with todays gas. get a airfuel ratio check if you must, but I bet it was ok before you started messing with it. KB

He could rev the engine and dump a little sand down the carburetor. He needs to get it to burn a little oil to get that nice tan glow he wants in his spark plugs:^)

-jim

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I have heard that, but live where there is minimal alcohol in the fuel so haven't seen it.

Mike

Could be. I may be wrong but I seem to recall that Chrysler called it a lean burn "system", so it may have included more than just the carb. At any rate, the replacement carb body is the essentially the same as the feedback variety with minor porting differences. For instance, the port designated on the replacement carb for "spark vacuum line" is the one that I use for the EGR valve. It is stronger than the port designated for EGR. The designated EGR port was not strong enough to fully open the EGR valve used on this truck. The spark vacuum on this truck uses manifold vacuum, so that port was free.

At this point, I am tempted to remove the main jets to see if I can force a rich mix before I yank the the whole carb, but that would not explain the lean mix at idle. Like you pointed out, it seems like a massive vacuum leak, but other than the white plugs there is no evidence of one. At any rate, if I can force a rich mix in the main system by removing the jets, I can tune it with the jets.

After reading up on the lean burn system, I am beginning to suspect the wire from the carb that signals the computer whether the throttle is opened or closed. If the computer never senses that the throttle is opened, the spark advance would never kick in. As I recall, this wire is housed in the same plug as the wires from the old mixture control solenoid. I will have to check to see if I got this wire screwed up when I changed carbs.

But I doubt that I would be getting over 21 mpg on the freeway without spark advance. Well, my day is spoken for tomorrow :--)

I would be fine with that, Kevin, but on the last California smog test, it failed the CO portion on both 15mph and 25mph. After fiddling, it

*barely* passed the HC test at 15mph. Unfortunately, I would have to do some serious memory recall to remember what I fiddled with.

Plus, there is the part about seeing the plugs in another engine in the area with toasty tan plugs.

But I will keep in mind what you posted about white plugs not being out of the ordinary. The only things that I am concerned about are 1) not killing the engine, 2) passing the next smog test and 3) getting good mileage and performance. Plug color is way down the list, as you rightly point out that it should be.

Jack wrote in news:ING_k.5676$8 snipped-for-privacy@flpi147.ffdc.sbc.com:

I am guessing, but without the feedback carb your going to have trouble passing a smog anyway. If you can find a non conputer dist. it might pay to get it and play to see what can be done. there usually is a smog setting and a good running setting. find out each and change only when smogging it. KB Ps a dyno tune might pay here.

I was also thinking on the distributor. The system AMC Jeep used on their last generation of carb engines that could almost be called a lean burn system with a feedback carb needs the distributor signals modified when the carb is changed out to a non feedback type or we can even just manually set the feedback system's stepper motor's air bypass pins and manually tune the carb to nicely pass emissions.

We do however need to hook the ignition module directly to the distributor to do this. This eliminates the computer's input to the timing advance and makes a radical power difference. Things like the knock sensor are disabled also, so the timing and octane levels of the fuel need to be set up right.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 'New' frame in the works for '08. Some Canadian Bush Trip and Build Photos:

Here is what I have concluded from input from posters here and from available info on the net and in various publications, including the shop manual for the 87 Dakota and the book entitled "Holley Carburetors and Manifolds" by Mike Urich & Bill Fisher, and from my own tests and observations.

1. For more than one reason, a white plug insulator does not necessarily indicate combustion temperatures over 2200F degrees or stoichiometric mix far outside the ideal of 14.7:1.

1. There are no vacuum leaks in the system.

2. I was finally able to induce a rich mixture by obstructing the air bleeds, so the carb is tunable.

1. Coolant temperature, as indicated by the dashboard temperature gauge and the hand-on-radiator-hose test is normal.

2. Three inputs control the spark advance feature of the computer: coolant temperature, engine rpm and available manifold vacuum. I hooked up a timing light and observed the difference in the spark advance at cold start, partial warmup and full warmup for a given rpm. The spark advance increased as the coolant temperature rose. I also observed the spark advance increase with rpms for any given coolant temperature. Spark advance also responded to vacuum input to the vacuum transducer on the computer. Based on this observation, I am concluding that the spark advance feature of the computer is not non-functional. Based on performance and mileage I get I am concluding that it functions pretty well.

1. The idle contact wire does not affect spark advance, only air fuel mixture capabilities, which this carb does not have so it doesn't matter if it is properly routed (it was).

I will use the present condition as a baseline. The floats are about

3/32 to 1/8 inch high and the jets are slightly oversized from stock. Before making any further mods, I will keep track of mileage and take the truck for a dry run at a smog check place.

Thanks to all for the input, especially the reassurance that white insulators are not necessarily indicative of a faulty condition. This was also stated in the Holley carburetor book in a couple of places.

I expect this showed up as some shade of gray on the spark plugs? (black would be the shade indicating extremely rich)

Holley carbs often have a vacuum operated power piston. Lean condition usually show up as surging with a vacuum power piston type carb. If it has a mechanical linked power circuit then it is harder to tell. Usually if the fuel metering is mechanical linked it is adjustable.

-jim

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It was a very, very dark shade of grey on the plug insulator, almost moist-looking. I had plugged the main air bleeds with small sewing needles and the larger idle air bleeds with a large paper clip. I had originally done this with the engine warm, first the mains, and then a high speed run and a quick shut down and pullover. I checked a plug and it was still white. I then plugged the idles and ran the same test and the plug was white. But after a complete overnight cool-down, the engine ran rough at startup. After a couple of blocks of slow driving I pulled over and pulled a plug. That's, when I saw the typical evidence of a rich mix. I want to try this again with the engine warmed up just to make sure that it is only a cold condition situation. Perhaps after warmup there is not so much of a need for full air bleed openings to deliver a good air/fuel ratio.

Indeed, the power piston is vacuum operated on this model. But there is no surging. In fact, there are no driveability issues whatsoever. The

*only* indication that the fuel mix might be lean are the white insulators on the plugs. I have great freeway mileage, 21.4 mpg on a recent 200 mile trip with a heavy lumber rack and a fully loaded 2x2x4 foot tool box. This is with the original 3-speed automatic, no overdrive, that came on the first Dakotas. There are no flat spots and no hesitation whatsoever.

As others in the thread have pointed out, alcohol in the gas may have a tendency to clean the plugs. This prompted me to find out what the percentage of alcohol is in the gasoline in my area. It is 6% ethanol. So I am not going to worry so much about plug color for the present. I will reset the float to specs, leave in the larger jets and see what kind of reading I get on my next smog test. Based on that I will either replace the stock jets or put in a still larger size.