Another Compression Test Question

[MDJEEPER]

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.

I checked the firing order last night and everything appears to be in order. Attached are a couple pictures. Do you see anything a miss?

The wires are basically brand new.

I guess I could be mis-interpreting the backfire (intake vs. exhaust) but I could certainly feel air being blown back up through the top of the carb.

Going way back to the beginning of this thread, I mentioned I had a couple cylinders that actually were slightly above specs on the compression test. Could this also be a sign of carbon deposits?

I am also wondering now how the rough engine idle piece fits with all the rest of this which I discovered last night when I put the idle into specs...

 

[MDJEEPER]

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.

If the problem is a lean mixture, wouldn't putting my hand partially over the air horn cause the symptom to go away...or at least get better?

 

[priceg]

If it is a lean pop, using your hands as a choke will help. If the engine runs fine when cold, then as it gets to temperature, starts running poorly and popping through the carb, it is lean.

 

[MDJEEPER]

If it is a lean pop, using your hands as a choke will help. If the engine runs fine when cold, then as it gets to temperature, starts running poorly and popping through the carb, it is lean.

Well, it was running "finer" when cold (at least at idle) when I had the idle RPM set at 900. Now that the idle RPM is at 700, it's definitely a litte more rough.

But, you are correct that I can still get to WOT when it's cold without the backfiring/sputtering/dying...or at least I could yesterday. I haven't tried it this morning yet.

 

[TDHofstetter]

If it is a lean pop, using your hands as a choke will help. If the engine runs fine when cold, then as it gets to temperature, starts running poorly and popping through the carb, it is lean.

I don't agree at all - if the engine runs fine when cold, then the fuel/air mixture is RICH enough for that to happen. If that same mixture is run while it's warmed up, it'll run very poorly... because it's running far too RICH - just as if the choke was still closed when it's warmed up. I don't think we can deduce that it's running too lean when it's warm, especially since using hands to simulate the choke didn't improve the situation.

I'm not saying a too-lean mixture won't make it run poorly - it will. I'm also not saying it's NOT too lean... but I don't believe it is, in this case.

Now... we can also experimentally make it run LEANER by introducing a vacuum leak in any of the unported vacuum lines. If that improves things, it's too rich.

 

[MDJEEPER]

I don't agree at all - if the engine runs fine when cold, then the fuel/air mixture is RICH enough for that to happen. If that same mixture is run while it's warmed up, it'll run very poorly... because it's running far too RICH - just as if the choke was still closed when it's warmed up. I don't think we can deduce that it's running too lean when it's warm, especially since using hands to simulate the choke didn't improve the situation.

I'm not saying a too-lean mixture won't make it run poorly - it will. I'm also not saying it's NOT too lean... but I don't believe it is, in this case.

Now... we can also experimentally make it run LEANER by introducing a vacuum leak in any of the unported vacuum lines. If that improves things, it's too rich.

I had that thought as well. I can say that it seems to run worse at idle when you open a manifold vacuum port. I experienced that yesterday everytime I connected my vacuum guage. However, I didn't change the idle mixture screws, so they are presently set to reach highest vacuum with the vacuum port plugged.

What I didn't do what actually try to crack the throttle with a vacuum port open to see how it runs at higher RPMs. So, I'm not sure if I can say anything about whether it is running too rich off the idle circuit??

 

[priceg]

I don't agree at all - if the engine runs fine when cold, then the fuel/air mixture is RICH enough for that to happen. If that same mixture is run while it's warmed up, it'll run very poorly... because it's running far too RICH - just as if the choke was still closed when it's warmed up. I don't think we can deduce that it's running too lean when it's warm, especially since using hands to simulate the choke didn't improve the situation.

I'm not saying a too-lean mixture won't make it run poorly - it will. I'm also not saying it's NOT too lean... but I don't believe it is, in this case.

Now... we can also experimentally make it run LEANER by introducing a vacuum leak in any of the unported vacuum lines. If that improves things, it's too rich.

The same mixture is not run as it warms up if the choke is pulling off correctly.You only have a few choices for a for this condition, ignition, timing, valve train or a lean condition. I will bet a Yankee dime that one of the basics is being overlooked. So much has been done to this vehicle without an accurate diagnosis. Lots of guessing and parts being thrown at it. Its a wonder it runs at all. A diagnostic charge at a reputable shop seems reasonable at this point. Just my opinion.

 

[BajaEdition]

The same mixture is not run as it warms up if the choke is pulling off correctly.You only have a few choices for a for this condition, ignition, timing, valve train or a lean condition. I will bet a Yankee dime that one of the basics is being overlooked. So much has been done to this vehicle without an accurate diagnosis. Lots of guessing and parts being thrown at it. Its a wonder it runs at all. A diagnostic charge at a reputable shop seems reasonable at this point. Just my opinion.

I tend to agree
it seems that a basic test, then a test with a bit of oil squirted in are what we tend to depend on as non professionals without compound test equipment. these two test can tell us a lot, as the numbers tell us the conditions of the rings and valves. but that is where the limit of the test is without further test equipment.
Once the findings have been reviewed and we do as we can for those issues, if we want to now more we need better test. Or opening up the engine, which at that point we need to be prepared to do what it takes to fix it.

 

[TDHofstetter]

The same mixture is not run as it warms up if the choke is pulling off correctly.You only have a few choices for a for this condition, ignition, timing, valve train or a lean condition. I will bet a Yankee dime that one of the basics is being overlooked. So much has been done to this vehicle without an accurate diagnosis. Lots of guessing and parts being thrown at it. Its a wonder it runs at all. A diagnostic charge at a reputable shop seems reasonable at this point. Just my opinion.

It's pretty obvious that the same mix won't be used for a warm engine as for a cold one - I was using the choke mix as an illustration of my point that an over-RICH condition (as would be caused by a choked mix) can also make the motor run extremely poorly.

 

[MDJEEPER]

I am still trying to understand my project and its lack of power at high speed. So, I did a compression test this morning and got the following:

Cyl 1: AMC 150
Cyl 2: 140
Cyl 3: 142
Cyl 4: 160
Cyl 5: 158
Cyl 6: 162

Frankly, I am not sure what to make of these numbers. By the manual, the acceptable range is between 120-AMC 150 , with no more than 30 psi max diff between any of the cylinders.

What do you think??

I tend to agree
it seems that a basic test, then a test with a bit of oil squirted in are what we tend to depend on as non professionals without compound test equipment. these two test can tell us a lot, as the numbers tell us the conditions of the rings and valves. but that is where the limit of the test is without further test equipment.
Once the findings have been reviewed and we do as we can for those issues, if we want to now more we need better test. Or opening up the engine, which at that point we need to be prepared to do what it takes to fix it.

What is your opinion of the original compression specs?

 

[priceg]

It's pretty obvious that the same mix won't be used for a warm engine as for a cold one - I was using the choke mix as an illustration of my point that an over-RICH condition (as would be caused by a choked mix) can also make the motor run extremely poorly.

I understand your illustration.

 

[MDJEEPER]

Going back to Priceg's post, it looks like there are four systems that could be responsible for all of my problems (well maybe not all...but you know what I mean!):

1. Ignition

2. Timing

3. Valve Train

4. Carburation

Throughout these past several months, and with everyone's assistance, I have performed various tests on each of these systems. However, we have not yet been able to fully rule-out any of them. Is there a way we could take one system at a time (starting with whatever is most likely) and somehow find a way to test it until it can be completely ruled-out...moving onto the others as necessary? Is that even possible since they impact each other?

 

[MDJEEPER]

Hey everyone, I just wanted to provide an update regarding this problem. After weeks of systematically testing one component at a time, we finally figured out that I was having a major ignition problem. Specifically, it seems that the new coil I had installed back in April was bad.

BUT here's the kicker...the coil PASSED all of the various tests specified in the FSM and yet was still bad. However, now that the coil has been swapped she is once again running like a champ!

I want to thank everyone who provided ideas and contributed to this thread. Thank you!! I also have to give a specific shout out to TDHofstetter who was massively helpful and had some terrific testing procedures not covered in any FSM!

 

[Old Dog]

GREAT NEWS.

 

[bdunnam]

I have always heard not to use seafoam in a large gas engine because it supposedly will burn a hole in your pistons

 

[TDHofstetter]

I have always heard not to use seafoam in a large gas engine because it supposedly will burn a hole in your pistons

Got a new one for ya - don't use SeaFoam in large gas engines because you have much better things to spend your money on.

 

[SrBeaver]

Great thread and great guys on here, glad you figured it out. Back to the original problem, 31's + 2.73 = no power. When your tire size goes up you need to regear accordingly. Do you plan on sticking with 31's or possibly go bigger? I would run either 4.10 or 4.27 gear set for 31-33 in tires if it were mine. I run 4.88's with 35's and a T-18 and she runs out just like it was stock gears and tires.

 

[MDJEEPER]

Great thread and great guys on here, glad you figured it out. Back to the original problem, 31's + 2.73 = no power. When your tire size goes up you need to regear accordingly. Do you plan on sticking with 31's or possibly go bigger? I would run either 4.10 or 4.27 gear set for 31-33 in tires if it were mine. I run 4.88's with 35's and a T-18 and she runs out just like it was stock gears and tires.

No, I don't have any plans to go bigger than the 31s. In fact, when they wear out, I may drop down to a more street/trail combo tire. This project of mine mainly cruises the street and lights trails. It's not really set up to do serious rock crawling, mud bogging, or other rough stuff that would require something like 35s. However, I completely agree that regearing would be right thing to do to help out the engine with the big tires.

...Of course some on this forum would also recommend an engine with a few more horses!!

 

[WednesdaysChild]

I appreciate this thread as I am experiencing the spitt/sputter/backfire with my jeep. I have replaced my rusty tank, sending unit, fuel pump, filters, fuel lines, carb rebuild and many months of trying to figure this out. After reading your posts and all the helpful info posted by the other members hear I realized that my symptoms also only occur on the 2nd half of my test drive once it is warmed up. I will pick up a new coil and see what that does for me. Thanks everyone for all the input here.