Engine Misfire & Stumble

[toolboxchev]

So I have been pretty lucky with my older rebuilt engine so far, until today.

Going down the road under mild acceleration I get a ever so slight misfire. I have been having a problem with a lifter under cold starts for a while now but it will quite up after running for 10 seconds and after a couple of minutes it is virtually non existant.

Fast forward, under half throttle it is stumbling and a few more backfires I am thinking the HEI ignition module is going out again. I go through one about every 2 years due to the older style of charging system which can give out voltage surges.

Replaced it and I have the same problem, now I thinking I am losing fuel pressure or in need of a fuel pump. Have not checked pressure yet.

What is a good next direction to look at?

Carb is about 9 years old, Eddy, stock mechanical fuel pump (not leaking yet) Large can style fuel filter, I checked between fuel filter and carb by cracking the line open, fuel came spraying out.

HEI was just gone through last fall, new adjustable vac advance, cleaned weights, and made sure springs were functioning ok. Everything was ok until today until that little backfire. Ughh I start a new job tomorrow and this is my driver.

Stumped for now, looking for some hints, Any pointers are appreciated.

[geezer#99]

Maybe your cam is going flat.
Pop the covers off and check the rockers for equal movement.

Or maybe it’s just flooding a bit. Eddy carbs don’t like more than 5 psi.

Or maybe you need more timing.

[toolboxchev]

Originally Posted by geezer#99

Maybe your cam is going flat.
Pop the covers off and check the rockers for equal movement.

Or maybe it’s just flooding a bit. Eddy carbs don’t like more than 5 psi.

Or maybe you need more timing.

Checked timing, 14 btdc. Pulled vac advance off untimed port, left disconnected. Even though I have 2 degrees of slop in the timing chain, I am pretty sure I have not skipped a tooth.

Cleaned out the distributor, cap contacts, weights, springs, lightly lubed weight pads.

Cleaned out primary metering rods on Edelbrock, Pcv valve.

Took it for a drive afterwards, Way better in terms of missing and popping around 2500 rpm, once I get past that rpm I can hammer on it and away I go like normal.

Stiil does not like 2450 to 2800 rpm, once past there It feels solid. Yet to pull those valve covers.

[cadillac_al]

Your description does sound more like carb popping than backfire. Backfires are usually nice and loud and out the tailpipe. I don't usually see multiple backfires unless the plug wires are crossed. Carb popping would have a nice rythem to it. Maybe a little more description can narrow it down more.

[toolboxchev]

Originally Posted by cadillac_al

Your description does sound more like carb popping than backfire. Backfires are usually nice and loud and out the tailpipe. I don't usually see multiple backfires unless the plug wires are crossed. Carb popping would have a nice rythem to it. Maybe a little more description can narrow it down more.

It did have somewhat of a rhythm to it, yet next time I laid on the fuel it would go away, then after accelerating ( normal ) it would come back.

Today being the first day of little to no rain here it did act up a little this morning. I laid on the throttle pretty heavy more than a few times. I has definitely kinda cleared up. On the way home from work it was even better with only one or two stumbles right after warm up and then all clear on the way home.

Weird??? I am going to work on the fuel line issues, they are the old rubber hoses. This weekend I will check fuel pressure and blow out those areas noted on the carb.

Thanks to all whom have gotten back, I was worried it would have died on me this morning going to a new position with BENEFITS the date of hire!

[Dead Parrot]

Check all the suction lines from the tank to the pump inlet. Bad line can let air in and create fuel starvation.

If that Eddy carb is one of the 140x family, pull out and clean off the power pistons and give where they move a good spray of carb cleaner. They can get fouled and stop moving freely.

Don't overlook the simple possibility that you got a bad batch of gas. Rare but it does happen. If you have dual tanks, switch to the other one.

[cadillac_al]

If you have a vehicle that runs bad on rainy days, that is a classic sign of bad spark plug wires. Are yours original wires or parts store replacements?

[mattfranklin]

Originally Posted by cadillac_al

If you have a vehicle that runs bad on rainy days, that is a classic sign of bad spark plug wires. Are yours original wires or parts store replacements?

x2

[toolboxchev]

Originally Posted by cadillac_al

If you have a vehicle that runs bad on rainy days, that is a classic sign of bad spark plug wires. Are yours original wires or parts store replacements?

They were just replaced last year, Accel Super Stock brand. Not leaning that way yet, but will check also.

Started sputting again under medium load and a misfire/backfire every now and then again today, its mostly dry out.

Still working the problem, thanks again. Fuel pump, carb, over 8 years old. Carb has never been dipped/cleaned and as a daily driver it has never sat more than 3 days. I run a fuel system cleaner between once a year to over 4 times a year.

[toolboxchev]

Update!

Just checked the fuel pressure, filter, and replaced line at carb. Filter is in place between pump and carb. Should I have more pressure under a hard rev in park?

Fuel Pressure, 5-6 at idle, drops to 4 under hard throttle in Park.

New thoughts? Checking Coil and Pick up Coil assembly, will update later.

Checked the Coil, and Pick up Coil assembly, both check within spec.

Noticed timing chain has about 2 degress of slop. Performed this check by simply turning the fan. Any thought on that?

Only have 70k on that gear and chain as it was a complete top end rebuild 8 years ago.

[RedRider]

Originally Posted by toolboxchev

Update!

Just checked the fuel pressure, filter, and replaced line at carb. Filter is in place between pump and carb. Should I have more pressure under a hard rev in park?

Fuel Pressure, 5-6 at idle, drops to 4 under hard throttle in Park.

New thoughts? Checking Coil and Pick up Coil assembly, will update later.

I found this distributor on Amazon and it has worked like a champ for me. For $45 you have a brand new distributor that will rule out the coil, module, cap, rotor, etc...can't beat that deal.

[cadillac_al]

FWIW I've had good luck with Accel Super Stocks. I did read a thread somewhere recently where a guy had a sticking weight in the distributor. I run Skip White distributors in all my vehicles. I would never spend much time on an old one.

[toolboxchev]

Originally Posted by cadillac_al

FWIW I've had good luck with Accel Super Stocks. I did read a thread somewhere recently where a guy had a sticking weight in the distributor. I run Skip White distributors in all my vehicles. I would never spend much time on an old one.

Thank you, I am looking at that Distributor Weight issue again, it caught my eye 2 days ago.

[toolboxchev]

Originally Posted by toolboxchev

Thank you, I am looking at that Distributor Weight issue again, it caught my eye 2 days ago.

The distributor is a 7 year old unit with brand new module, coil, pickup assembly comes out within electrical spec. I have never had to worry about a grounding issue, ....yet.

[Hollow65]

I would up the initial timing. That’s just me though.

[cadillac_al]

Since it only happens at 2400 rpm it must be a lean spot in the carb. I always said the 1406 Eddy was jetted for a 305. It's pretty lean for economy.

[toolboxchev]

Originally Posted by cadillac_al

Since it only happens at 2400 rpm it must be a lean spot in the carb. I always said the 1406 Eddy was jetted for a 305. It's pretty lean for economy.

Thats exactly what I have been thinking. I did enrich the carb 7 months ago by adjusting the accelerator pump and adding new springs. Yesterday I pulled the top of it off and did an inspection. The floats were not adjusted properly as far as I can tell and had some junk in the bottom of the float bowls. Proceeded to a clean out of the said areas and took it for a drive, same problem.

Makes me think somewhere in there the middle of the transitions has some junk in it also. Have yet to pull the valve covers, and do a complete clean out of the carb, that thing has gone 10 years with ethanol and held up, no corrosion inside so far.

[jeffahart]

Check out this post I did a few months ago http://67-72chevytrucks.com/vboard/s...d.php?t=742187

Also, I pulled the air horn off a Quadrajet this morining and found two of the tubes had fallen out of the air horn and were laying in the float bowl. I was going to advise this could be the problem, but you already popped it apart.


j

[toolboxchev]

Ok taking bets now. I pulled plugs 1 & 5 a week prior to this problem going on. Those 2 usually soot up and need a good cleaning every 4 months or so.

So I tear into the plugs just before night fall and this is what I see. Last thing to check as I just put these in last fall.

This is #3, now who is wagering any money this little problem is going to be fixed in the morning.

[cadillac_al]

I can barely get 2 years out of regular AC spark plugs. You'll have to keep changing them regularly if you don't like modern plugs (a lot of people don't).

[toolboxchev]

Well same problem so far, just less severe. I kinda figured that I have an issue that will be a two fold solution.

[my67c20]

"under half throttle it is stumbling and a few more backfires I am thinking the HEI ignition module is going out again. I go through one about every 2 years due to the older style of charging system which can give out voltage surges."

You sure that does not have something to do with it. Seems like its time to chase down the voltage regulator delete button and clean that ignition system up some. Also check your grounds.

[toolboxchev]

Just put a new module in it noted earlier in the thread. Checked the coil and Accel spec, Accel techs say its on its way out.

Will be doing that today, Kinda a hap hazard way to diagnose in which I have been going about this. I have been applying fixes that are the least costly and not labor intensive.

I have yet to do a compression test.

[mattfranklin]

Sometimes dirty valves can cause stumble. They act like a sponge. So on acceleration they are soaking up the accelerator pump fuel and not letting it get in to help combustion, during the transient, when it needs it most.

My go-to easy fix for that is Techron Concentrate (a couple of jobs ago I did a lot of dyno testing with Techron). It cleans dirty valves, carburetors, (and if you have injectors, those, too.), and combustion chambers. Put in a bottle at the recommended dosage and drive it normally until the tank needs refilling. Do it again, if you think it needs it.

If you decide to do this, make sure it says "Concentrate" on the bottle. That's the good stuff. The cheaper ones are made to compete with cheaper competitors at their level.

That might not be it (but again it might be) , but it's a cheap easy test and then you know you've covered all of the bases.

[toolboxchev]

I am a believer in that stuff, I had used it in the past. I cleans the heck out of stuff. Chevron fuel is the best stuff out there in todays world.