The quest for emissions continues, misfiring issue

[Dr. Marneaus]

Hi folks!


K, I'm planning to swing the waggy over to a no-pass no pay place tomorrow to get it tested again, after a few small changes, and I want to make sure i get the truck good and hot this time.

Engine is a 1987, with 1987 ignition. Truck is a 73. No smog equipment.

I retarded my timing slightly, but made sure it's right at 12* runs nice and quiet and smooth, mostly. When idling it misfires. Not in any rhythm, not 100% consistantly. If I hold my hand down by the exhaust its mostly smooth, but ever 3 or 5 or 7 seconds i can feel it puff and hear it puff. Sometimes it'll be 1 little puff, then it'll go 5 seconds and go PUFFPUFFPUFF, then it'll go several more seconds and puff once or twice. It's not rhythmic. It does, however, seem more apparent after having retarded the timing from 15*.

Hooked up a vacuum gauge, at idle in park (650-700rpm) when warm its pulling 19" at the manifold. It'll dip for a split second when it misses. It's hard to hear it miss from the engine bay, its most noticable at the exhaust.

I watched the vacuum gauge to set my idle mixture. Turned each screw in until it dropped, then backed it out 1/4th turn. holds steady at 19"

I have installed a new coil, new cap, new rotor and new plugs. Old plugs didnt look terrible, but looked a little carbon fouled, indicating a rich mixture.

So, those are my adjustments, and those are my issues. Obviously I'm running rich still, i had high CO and high HC when I tried to smog it. I figured I might as well do some tune up work, couldnt hurt.

What could be causing the misfire? It has no effect on drivability. The truck is driving like a dream right now, plenty of power, doesnt stumble or anything, no noise other than the normal stuff. I dont think this is a HUGE issue, but obviously it's pumping unburned gas out my tail pipe, and I need to know what to look into to fix it. Could this be carb related?

What else would be causing the rich condition? What do I need to look into on the Carb? It's an edelbrock performer. Is it possible that it just needs a rebuild and could be leaking? I'm not super familiar with carbs or tuning, this portion of the job has been my issue ever since I put in the newish engine. With little experience I was able to remove and install an engine, and kinda sorta rebuild a transmission, but the finer points of tuning are not in my skill set.

Just need some advice on the miss and the rich condition.

[billyj7175]

This has nothing to do with the overly rich mixture, but mine used to do that puff....puff-puff thing before I converted to HEI...after that, it never idled so smooth. A cheaper option is the TFI mod.

However, I'd start tuning on the carb first (http://www.edelbrock.com/automotive_...rs_manual.pdf), then worry about the ignition/timing.

[Dr. Marneaus]

Okay...

I got it good and hot, and after the changes I've made I got the emissions down slightly. I am now officially less than 2x the legal limit for CO, but my HC at idle went up, presumably from my now noticeable misfire.

I took her home and started to fiddle a bit. I remembered I was running manifold vacuum to the distributor for advance, so I switched it over to timed vacuum, because IIRC, that was to help emissions at idle by retarding the timing. Well, it slowed everything down a bit and the misfire went away, at least I couldn't feel it or hear it from the exhaust. Also, running the timed vac advance, I was able to way lean out my idle mixture screws. Perhaps this will help with the idle HC.

I startec checking for vacuum leaks, and noticed a decent one alone the edge of my intake manifold. I was able to spray starting fluid alone the edge between the manifold and the head, and it bogged the engine down. I checked and one of the intake screws was loose enough that i could put a good half turn on it with a wrench. I tightened that up, and the leak appeared to nearly disappear. Could this be a cause for the rich condition or incomplete burn? I'd think it would have the opposite effect, unless it was causing a specific cylinder to continually misfire?

Other than that though, I'm not sure where to turn. Is there any way to check different parts of my carb, or would the whole thing just need to come off and spend an afternoon on the bench being rebuilt?

[Rich88]

I startec checking for vacuum leaks, and noticed a decent one alone the edge of my intake manifold. I was able to spray starting fluid alone the edge between the manifold and the head, and it bogged the engine down. I checked and one of the intake screws was loose enough that i could put a good half turn on it with a wrench. I tightened that up, and the leak appeared to nearly disappear. Could this be a cause for the rich condition or incomplete burn?

No, that would be the cause for a lean condition for the cylinder near the leak.

[The Colonel]

Are you running dual exhaust or single?
The reason I ask- if you are running dual you could try sniffing the other bank to see if it's on both banks or just one.

[Dr. Marneaus]

No, that would be the cause for a lean condition for the cylinder near the leak.

Right, i didnt think that would be the issue. Maybe it could contribute to a misfire, but it wouldnt be the main cause for my truck to be running super rich.

Are you running dual exhaust or single?
The reason I ask- if you are running dual you could try sniffing the other bank to see if it's on both banks or just one.

Running single only.


I guess the first step would be making sure the carb is good to go?

[babywag]

Might want to do a compression test as well.
A burned valve or bad seat could cause issues as well.

High CO = incomplete combustion (carb)
High HC = raw fuel/poor ignition (timing,ignition parts,bad CAT,lean mixture,low compression)

When's the last time you changed your oil?
Is this an idle test or on a dyno?

Tuning to pass smog is a whole different animal than tuning to run good.

[Dr. Marneaus]

Might want to do a compression test as well.
A burned valve or bad seat could cause issues as well.

High CO = incomplete combustion (carb)
High HC = raw fuel/poor ignition (timing,ignition parts,bad CAT,lean mixture,low compression)

When's the last time you changed your oil?
Is this an idle test or on a dyno?

Tuning to pass smog is a whole different animal than tuning to run good.

It's idle and high RPM. They basically hold your engine at 2000 rpm in neutral for a few minutes while it sniffs, then hold it at idle for a minute or two while it sniffs. It's never under load.

Seems rather skewed to me...and i always thought that "Free" revving the engine was bad....but what do I know.

Oil was changed a few weeks ago. Less than a few hundred miles on it.

So, considering i may have a vacuum leak, is it possible that i'm running rich because of the carb and lean on 1 cylinder causing high HC at idle? It's only high HC at idle, then high CO at idle and high CO at speed.

[Jlamb]

It is unlikely to be lean on just one cylinder, the only reasonable way for this to occur is to have an intake leak on just that cylinder's intake runner.

I know you posted a little while ago, but can you put up your most recent results? and the limits. Are you saying HCs are failing at idle and CO off idle?

If CO is rich off idle, the only way a carburated car to be rich off idle is because of internal carb issues. Jetting and power valves are most likely.

[Dr. Marneaus]

It is unlikely to be lean on just one cylinder, the only reasonable way for this to occur is to have an intake leak on just that cylinder's intake runner.

I know you posted a little while ago, but can you put up your most recent results? and the limits. Are you saying HCs are failing at idle and CO off idle?

If CO is rich off idle, the only way a carburated car to be rich off idle is because of internal carb issues. Jetting and power valves are most likely.

Currently Idle HC was 1100 PPM, legal limit is 740 PPM (original test was 930 PPM, so that got worse).

Idle CO is 6.5something% legal limit is 3.5%

High RPM CO is 6.83%, legal limit is 3.5%


Running an edelbrock performer, 1403.

[Jlamb]

Any chance you have the base idle adjusted too lean creating lean miss to cause the high HC at idle? What are your CO readings at idle? If it is really low then that is what explains high HC.

I am just getting into Edelbrocks, I have the 1404 and learned a few things with them. If you are not aware there are many different adjustments unlike most other carburetors. Great carb IMO. You can make air fuel changes at idle, wide open throttle, part throttle cruise and part throttle power mode.
The fact that you are at 6% CO off idle suggest to me that you have too large of jets but you may be able to play with the metering rods and springs.
The metering rods that are located under two little plates between the primary and secondarys on each side of the carb. Unscrew the 1/4" bolt and remove the cover. Take some pliers and remove the rod. Then you will see a spring. This spring regulates the position of the metering rod and the spring kit can come from Carquest or Advance for about $11.00 (part number for spring kit is on link) But like I said I think you have to step down on your jets, to know what you have currently you will have to get to them, find the jet size and go from there. The owners manual link has a chart that in theory tell you what direction to go and what parts (jet sizes and metering rod sizes) to use based on your condition.

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

I will probably end up getting this kit for mine, looks to be a good kit so that I can play around with different jets, metering rods, and springs.

http://store.edelbrock.com/1486calibrationkitfor1403and1404performercarbs.aspx

[Dr. Marneaus]

Any chance you have the base idle adjusted too lean creating lean miss to cause the high HC at idle? What are your CO readings at idle? If it is really low then that is what explains high HC.

I am just getting into Edelbrocks, I have the 1404 and learned a few things with them. If you are not aware there are many different adjustments unlike most other carburetors. Great carb IMO. You can make air fuel changes at idle, wide open throttle, part throttle cruise and part throttle power mode.
The fact that you are at 6% CO off idle suggest to me that you have too large of jets but you may be able to play with the metering rods and springs.
The metering rods that are located under two little plates between the primary and secondarys on each side of the carb. Unscrew the 1/4" bolt and remove the cover. Take some pliers and remove the rod. Then you will see a spring. This spring regulates the position of the metering rod and the spring kit can come from Carquest or Advance for about $11.00 (part number for spring kit is on link) But like I said I think you have to step down on your jets, to know what you have currently you will have to get to them, find the jet size and go from there. The owners manual link has a chart that in theory tell you what direction to go and what parts (jet sizes and metering rod sizes) to use based on your condition.

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

I will probably end up getting this kit for mine, looks to be a good kit so that I can play around with different jets, metering rods, and springs.

http://store.edelbrock.com/1486calib...rmercarbs.aspx

I have a calibration kit full of rods and jets and springs

Last year i had it at a shop who supposedly switched out the rods and jets (i have a pair of dirty jets and dirty rods in the kit) BUT it appears as though they replaced the jets and rods with the same ones that were already in it... (the dirty jets and rods have the same part number as the ones that are missing from the kit) I'm not sure, i really should just dig into it i suppose. They supposedly adjusted it 1 step lean in both cruise and power mode, but I'm not sure. They said it passed, but i doubt they properly tuned it.

My idle was adjusted for best vacuum. Ever since switching the dizzy to timed vacuum advance (last night after the test) the miss at idle went away, at least i couldnt feel/hear it like before. I'm assuming the timing was too advanced at idle by running manifold vacuum.

the idle CO was 6.something%, i'm getting high CO no matter what, idle and high RPM.

I suppose the best thing to do would be dive into the carb and see what jets i have? The rods i belive shouldnt affect idle co, right?

[Jlamb]

If you have a miss, find that first. Before you chase the CO.

I would not put a lot of weight that the carb was adjusted properly with a vacuum gauge.

[Dr. Marneaus]

If you have a miss, find that first. Before you chase the CO.

I would not put a lot of weight that the carb was adjusted properly with a vacuum gauge.

The miss has been solved, by switching to timed vacuum.

I dont know how else to adjust the idle mixture than by ear or with a vacuum gauge. Turn the screws in equally until the idle drops....or the vacuum drops or shakes, then turn them back out a little bit until the idle smooths out or the vacuum stops increasing.

[Jlamb]

It would be with a 4 or 5 gas analyzer.

The screw in and stumble then back out 2 turn will get you in the ball park, but it will always be a guess on the tailpipe numbers.