Another Compression Test Question

[MDJEEPER]

Your metering pins look like they're in the perfect position. I can't think they're responsible for... your Jeep sputters & won't run if you romp on it after it's warmed up? Na - I can't think that.

Quick check - get it to that point (hot, ready to romp on it) with the air cleaner off (do it in the driveway so you don't pick up a bunch of dust), and have someone else romp on it while you hold your hand partway over the carburetor throat to close it a little - your hand will be acting like a choke.

See if you can find a hand position (more air, less air) that makes the problem go away. If you CAN'T, then your motor is definitely not running lean at temperature. It could still be running rich, though - are you getting black smoke out the tailpipe? If you are, it's too rich. If not, you're not.

If you're not getting black smoke, and if you can't pretend to be a choke & improve the way it runs when it's hot, then the only thing I can think is that the problem is in the ignition... meaning possibly a poor ground or a bad connection someplace that gets worse when it's hot... or... dang. The distributor isn't slipping out of timing, is it? Is the vacuum line running to the distributor still going through the temperature-controlled switch on the intake manifold?

When it runs rough, can you improve the way it runs (somebody else romping on the pedal) by unplugging that vacuum line from the carburetor and sucking on it (I'm serious here)?

Did we check for a leaky power-brake booster?

The EGR doesn't hold vacuum, if I recall, but it does make the motor stumble when you manually move the diaphragm? What happens if you do that with the motor fully hot?

-- Tim --

Thanks! I will try holding my hand over the choke plate tonight and see if that makes any difference. Hopefully, a backfire (since it sounds sometimes like it's backfiring) won't come blasting out!


To answer your questions...

There is no black smoke coming out the tail pipe. In the past, it seemed like it smelled rich, but I think that may have been a catalytic converter issue.

The vacuum advance on the distributor is directly connected to the ported vacuum on the carb. The CTO valve is one of those "minor" things that a PO eliminated some time in the past. At idle, I can suck on the hose with a vacuum pump and make the timing advance, so it seems to be working.

Yes, pushing on the EGR valve will cause the engine to lose RPM, so I believe it is in the closed position. However, I will check it hot.

I am not sure how I would check if the ignition is falling out of timing. It has been steady at 8 degrees whenever I check it at idle.

I do not have power brakes, so no brake booster.

 

[MDJEEPER]

Here's another thought...What is the idle RPM supposed to be after doing the nutter bypass? Is it possible that my RPM is too high or too low, therefore causing the timing setting to be off?

I have already tried advancing the timing a little (say 9 or 10 degrees) but the problem did not go away, so I set it back to 8.

 

[TDHofstetter]

To doublecheck your timing... check it cold (vacuum hose disconnected) and watch it advance as you speed the engine up. Get a feel for how far it advances as you do that (you're using the centrifugal advance when the hose is disconnected).

Next... check it again hot, still with the hose disconnected. Again, try to give it some throttle & watch it advance again. It SHOULD act exactly the same way cold & hot.

Last... while it's idling & the timing light is still running... plug the hose back in. The timing should NOT CHANGE when you do that if it's at an idle. If it changes, you may not be on the right vacuum port - the port you want shouldn't give you any vacuum at idle, only at higher speed.

 

[MDJEEPER]

Another crazy question...Do the mixture screws on the BBD only impact idle or can they also effect higher RPMs?bb

 

[TDHofstetter]

They PRIMARILY affect idle... but they have some small effect on higher speeds. It really is minor, though.

At idle, the carburetor's mix comes entirely from the idle circuit, controlled by the idle mix screws. Under medium power, the idle mix stays in the circuit but the jets make up the major portion of the mix. Under heavy power draw, the power valve adds more to it because the harder a motor works the richer a fuel/air mix it needs to deliver that power. So... under heavy power, the carburetor is delivering fuel through three different circuits, with the idle circuit being the most minor and the power valve being the most major.

The BBD (factory setup) is a little different, in that the solenoid controls the range between jets & power valve. It doesn't have a profound effect, just enough to maintain (when it's running right) the leanest possible mix that still keeps the motor running under power. To do that, the computer monitors the exhaust for oxygen content (O2 sensor) and adjusts the mix to keep exhaust-borne oxygen at a very specific level within a very narrow range. When the solenoid is disabled, the exhaust can contain any level of oxygen over a broader range with no effect on fuel mix. When the plunger is at the full-rich position, the mix always stays somewhat rich (usually not enough to notice from the driver's seat) at the cost of fuel economy. When the plunger is at the full-lean position, the mix stays somewhat lean, but again usually not enough to notice.

 

[MDJEEPER]

They PRIMARILY affect idle... but they have some small effect on higher speeds. It really is minor, though.

At idle, the carburetor's mix comes entirely from the idle circuit, controlled by the idle mix screws. Under medium power, the idle mix stays in the circuit but the jets make up the major portion of the mix. Under heavy power draw, the power valve adds more to it because the harder a motor works the richer a fuel/air mix it needs to deliver that power. So... under heavy power, the carburetor is delivering fuel through three different circuits, with the idle circuit being the most minor and the power valve being the most major.

The BBD (factory setup) is a little different, in that the solenoid controls the range between jets & power valve. It doesn't have a profound effect, just enough to maintain (when it's running right) the leanest possible mix that still keeps the motor running under power. To do that, the computer monitors the exhaust for oxygen content (O2 sensor) and adjusts the mix to keep exhaust-borne oxygen at a very specific level within a very narrow range. When the solenoid is disabled, the exhaust can contain any level of oxygen over a broader range with no effect on fuel mix. When the plunger is at the full-rich position, the mix always stays somewhat rich (usually not enough to notice from the driver's seat) at the cost of fuel economy. When the plunger is at the full-lean position, the mix stays somewhat lean, but again usually not enough to notice.

OK. So, the BBD is designed like most other carbs then when it comes to the idle circuit.

By "plunger" do you mean the vacuum piston? More specifically, how on earth to you measure the gap accurately to set it to .035??

I've tried different types of feelers, but a) I haven't found anything but a spark plug gap measurer that's even small enough to fit down in there, and b) it is not at all obvious to me where the gap is that's being measured.

Right now, I have the set screw turned almost all the way in to fully rich (since the assumption has been a lean condition), but like you said, it really doesn't seem to make too much of a difference from the driver's seat.

I'm working on your other tests right now...but haven't found anything yet.

 

[MDJEEPER]

I have completed all of the suggested tests and cannot find anything that makes the situation better. However, I can say that in the garage I can see it backfire when I romp on throttle.

To answer the questions specifically...

Your metering pins look like they're in the perfect position. I can't think they're responsible for... your Jeep sputters & won't run if you romp on it after it's warmed up? Na - I can't think that.

Quick check - get it to that point (hot, ready to romp on it) with the air cleaner off (do it in the driveway so you don't pick up a bunch of dust), and have someone else romp on it while you hold your hand partway over the carburetor throat to close it a little - your hand will be acting like a choke.

See if you can find a hand position (more air, less air) that makes the problem go away. If you CAN'T, then your motor is definitely not running lean at temperature. It could still be running rich, though - are you getting black smoke out the tailpipe? If you are, it's too rich. If not, you're not.

I could not find a choke position that would make the engine not backfire. However, it was a very fine line between closing the choke so far as to kill the engine and have it open far enough so that the engine would run.

There is also no black smoke or other obvious indication that it's running rich.

If you're not getting black smoke, and if you can't pretend to be a choke & improve the way it runs when it's hot, then the only thing I can think is that the problem is in the ignition... meaning possibly a poor ground or a bad connection someplace that gets worse when it's hot... or... dang. The distributor isn't slipping out of timing, is it? Is the vacuum line running to the distributor still going through the temperature-controlled switch on the intake manifold?

When it runs rough, can you improve the way it runs (somebody else romping on the pedal) by unplugging that vacuum line from the carburetor and sucking on it (I'm serious here)?

I checked the idle timing, both hot and cold, and it remained steady at 8 degrees. I also used a vacuum pump to check the vacuum advance and it is working.

I did not try to suck on the vacuum advance to see if the problem stopped. However, I did try romping on the engine with both the vacuum advance hooked to ported vacuum and also completely disconnected and got the same results -- lots of backfiring.

Did we check for a leaky power-brake booster?

The EGR doesn't hold vacuum, if I recall, but it does make the motor stumble when you manually move the diaphragm? What happens if you do that with the motor fully hot?

-- Tim --

I did check the EGR valve, both cold and hot, and manually moving the diaphram does make the engine stumble. I also sprayed all around it and inside of it with carb cleaner and did not find a vacuum leak.

To doublecheck your timing... check it cold (vacuum hose disconnected) and watch it advance as you speed the engine up. Get a feel for how far it advances as you do that (you're using the centrifugal advance when the hose is disconnected).

Next... check it again hot, still with the hose disconnected. Again, try to give it some throttle & watch it advance again. It SHOULD act exactly the same way cold & hot.

Last... while it's idling & the timing light is still running... plug the hose back in. The timing should NOT CHANGE when you do that if it's at an idle. If it changes, you may not be on the right vacuum port - the port you want shouldn't give you any vacuum at idle, only at higher speed.

The centrifugal advance behaved the same way, both cold and hot. As I cracked open the throttle the timing steadly advanced.

The vacuum advance is definitely connected to the ported vacuum on the carburetor.

What else can I try??

I sure wish there was a way I could teleport you guys here!!

 

[Old Dog]

back firing is usually not carb related it's usually ing. related. Is it back firing through the carb or the exaust?

 

[MDJEEPER]

back firing is usually not carb related it's usually ing. related. Is it back firing through the carb or the exaust?

As best I can tell, it's back firing through the carb. There was no actual flame, but with the air cleaner off I could hear the loud pop and actually feel the burst of air coming out of the carb.

Here's a really dumb question...Reading back through this thread, it looks like this problem got really bad after I took the spark plugs out to do a compression test. I don't remember dropping a plug...BUT who knows. Could a damaged or mis-gapped plug cause a backfire??

I would say, maybe yes...but wouldn't the problem exist any time the engine is running and not just hot?

 

[Old Dog]

As best I can tell, it's back firing through the carb. There was no actual flame, but with the air cleaner off I could hear the loud pop and actually feel the burst of air coming out of the carb.

Here's a really dumb question...Reading back through this thread, it looks like this problem got really bad after I took the spark plugs out to do a compression test. I don't remember dropping a plug...BUT who knows. Could a damaged or mis-gapped plug cause a backfire??

I would say, maybe yes...but wouldn't the problem exist any time the engine is running and not just hot?

The only hot thing is still confussing me, but if I were you I would check the plugs, the firing order and the wires to be sure you didn't cross any wires. If all is well there, something in the timing is making it fire early to backfire through the carb.

 

[MDJEEPER]

The only hot thing is still confussing me, but if I were you I would check the plugs, the firing order and the wires to be sure you didn't cross any wires. If all is well there, something in the timing is making it fire early to backfire through the carb.

Me too!

I guess I should have added that it's this backfiring thing that has appeared more recently. The no guts part has existed for as long as I've had this project.

I'm going to check the wires and the plugs right now. Meanwhile, what in the timing could cause a backfire...and how would I find it considering all of my previous timing tests have shown the timing to be solid??

And again why the only hot thing!!

 

[Old Dog]

The only other idea I have is to check you cap & rotor the be sure it's not cracked or has any carbon traking. Also check the bushings in the mech. advance to be sure they are not wore out. I had to change my bushings not long ago, they were egg shaped and it was starting to run bad at higher rpm's.
Maybe Tim will have some other ideas.

 

[MDJEEPER]

The only other idea I have is to check you cap & rotor the be sure it's not cracked or has any carbon traking. Also check the bushings in the mech. advance to be sure they are not wore out. I had to change my bushings not long ago, they were egg shaped and it was starting to run bad at higher rpm's.
Maybe Tim will have some other ideas.

Cap and rotor are pretty new, but will check for anything obviously amiss. I assume it's pretty easy to spot carbon tracking....looking for dark streaks right?

How do I check the bushings?

 

[MDJEEPER]

Me too!

I guess I should have added that it's this backfiring thing that has appeared more recently. The no guts part has existed for as long as I've had this project.

I'm going to check the wires and the plugs right now. Meanwhile, what in the timing could cause a backfire...and how would I find it considering all of my previous timing tests have shown the timing to be solid??

And again why the only hot thing!!

Okay...I need to eat a little crow here...I went back and checked my notes. The first time I jotted down a sputter/bog/backfire was back in May. However, I don't have anything again until much more recently. It has been within the past 6 weeks that it started happening all the time when the engine is hot.

However, I am still going to check the plugs and wires.

...and the no guts thing still goes back to the beginning...

 

[Old Dog]

When you take the cap and rotor off you should see the weights in there, just take them out and check to see if the hole that fits over the pin os is fitting right or if it's wore out (egg shaped) here's a thread I did awhile back.http://www.jeep-cj.com/forums/f2/distributor-advance-2528/

You can make your own timing tape by measuring the circumferance of your balancer, divide that by 360 to find how far a degree would be. than make marks on a piece of masking tape and put it on the balancer so you can watch it with a timing lite. I'm not sure what a AMC 258 i6 / 4.2l should be for total timing maybe someone will chime in. I do know a chevy 350 wants 36-38 for total with no vac. hoked up.

And yes traks would show up as black lines.

 

[MDJEEPER]

Here are some pictures of the dizzy, cap, and rotor. I found nothing in the cap, and just a little discoloration on the rotor, mainly on the tip.

This afternoon I got a new timing light with an RPM counter that actually works! After hooking it up I found that the timing was still right on at 8 degrees, but that my RPM was up about 900.

So, I reduced the RPM down to about 700 and the engine just really started running pretty rough...even at idle. Readjusting the mixture screws helped a little, but not very much.

Also, there is a pretty good whistle coming out of the top of the carb which goes away as you gently raise the RPMs.

And, at this idle level, when it got hot rather than backfiring, the engine either bogs really badly or just plain dies the moment you tromp on the throttle. You can only get the RPMs up if you gently open the throttle.

Does this shine any light on the mystery??

 

[Old Dog]

I have no idea, but I was wondering if you have ever read this post that has to do with the nutter...http://www.jeep-cj.com/forums/f2/problems-931/ it almost sounds like what you are going through.

 

[MDJEEPER]

I have no idea, but I was wondering if you have ever read this post that has to do with the nutter...http://www.jeep-cj.com/forums/f2/problems-931/ it almost sounds like what you are going through.

It does! My only concern is, looking back through my notes, this problem first appeared BEFORE I did the nutter bypass. Of course, the reason I had to do the nutter bypass is because the PO stripped most of the stuff (including the O2 sensor) out, so I really don't know what the computer was doing.

BUT it has only been more recently that it has started doing it consistently.

Oh, the other thing is that I have not yet reconnected the vacuum advance just to eliminate it from the equation.

The thing tonight also has me confused...with the idle turned down it stumbles and/or dies so fast when the throttle is cracked that I have to wonder if we're back to the carb again.

This is just turning out to be an amazingly hard problem to solve. Hopefully, we'll hit upon some kind of bright idea!

 

[TDHofstetter]

Let's look REALLY HARD at the firing order... and if the distributor cap or plug wires aren't new let's get 'em replaced. In fact... if the firing order is definitely right, I'd be tempted to replace the cap even if it's new - sometimes a factory cap gives problems. I've got an Astro van that simply will NOT run unless it's got a genuine GM distributor cap on it; aftermarket caps all give it a terrible miss.

Backfiring through the carburetor will be one of three things, and nothing else:

(1) a plug is firing while that cylinder's intake valve is still open (ignition timing), or
(2) a plug is firing while that cylinder's intake valve is still open (valve timing or sticky/warped valve), or
(3) hot carbon deposits inside the cylinder are hot enough to ignite the inrushing fuel-air mixture.

Nothing else (within reason) can cause that to happen. Carburetion can't do it (a lean or rich carburetor doesn't cause backfiring through the carb, although it CAN cause backfiring through the exhaust, as still-unburned gases ignite in the exhaust system).

Firing order is my biggest suspect right now, although the hot-engine thing kinda' points toward either a sticky valve or hot carbon deposits, too.

Let's trace each plug wire from distributor to plug. The plug wires SHOULD be 1-5-3-6-2-4.

 

[priceg]

A lean fuel mixture can backfire thru the carb. If it severe enough, it will destroy the diaphram in the power valve, which will cause a lean mixture at low manifold vacuum.