292 I6 tuning help needed

[RThree]

Alright guys I need some more help with my (mostly) stock '72 292 this thing gives me fits….

To main symptoms:
-Rough surging at idle until it stalls, but runs decent at higher RPM
-timing has to be advanced to 15+ deg to run with out backfiring though the carb when given throttle and have good throttle response (4+deg is default)

We have done the following:
-Verified TDC timing mark is correct on harmonic balancer
-Compression check is consistent across all cylinders (140psi + or – 10psi)
-Replaced points distributor with HEI and ran an unfused power source.
-removed intake/exhaust manifolds to check for cracks none found. Machined mating surfaces and replaced gaskets.
-Installed a new Rochester 2GC with an 2-1 bbl adaptor plate
- Set timing to default 4+tdc but idles rough and backfires though the carb with throttle.

Any ideas?

Thank you for taking the time to look this over. Please let me know what other information would be helpful.

[RustyPile]

Here is a link to a recent lengthy thread that addresses 292 problems similar to yours.. http://67-72chevytrucks.com/vboard/s...d.php?t=788162

[RThree]

Originally Posted by RustyPile

Here is a link to a recent lengthy thread that addresses 292 problems similar to yours.. http://67-72chevytrucks.com/vboard/s...d.php?t=788162

Alright I'm reading though it now, sounds like a different issue but i will go though it to see what i can glean.

[geezer#99]

If it runs good at 15 degrees, run it there.
You are plugging off the vac advance when setting the timing, aren’t you.

[RThree]

Originally Posted by geezer#99

If it runs good at 15 degrees, run it there.
You are plugging off the vac advance when setting the timing, aren’t you.

Yes the vac advance has been removed and plugged during timing.

I'm feeling more comfortable with the advance realizing the differences in today's gas. But this truck seems to have always surged during idle even with different carbs

[geezer#99]

The last 230 six I had liked 14 initial.
Bump your timing up, adjust your idle back down and tweak your mix screw.

[RThree]

Originally Posted by geezer#99

The last 230 six I had liked 14 initial.
Bump your timing up, adjust your idle back down and tweak your mix screw.

So without a timing mark to go by (the indicator on the block only goes + or - 8 Degrees) what is my best indicator that ive got it timed up properly? Just engine stability and throttle response? or is there some other indication i should be looking at?

[geezer#99]

Not much else.
Smooth low idle, no popping and easy to crank and fire.
Only 8 on the tab.
Each mark is 2 degrees.
A dial back or digital light helps.

[May70]

Originally Posted by RThree

So without a timing mark to go by (the indicator on the block only goes + or - 8 Degrees) what is my best indicator that ive got it timed up properly? Just engine stability and throttle response? or is there some other indication i should be looking at?

Maybe you know this but in reading this maybe not. Realizing that the tab only goes to +- 8 degrees you could get a adjustable timing light and set it to what ever degree of advance you want, then you would adjust the distributor until you have reached the zero mark on the tab. Just remember or write down what you set your light to.

Also the previous link to my thread.... a lot of people have put a lot of good trouble shooting tech tips specifically for a 292. I would read through it, just because you may see something you may have over looked. Ive gone through so much on this thing its crazy. Next step is machine shop and starting new for me.

Are you sure you are not 180* off on your TDC verification? How did you verify? The mark on the harmonic balancer will pass zero once and then when it passes the second time, for a full stroke. So in simple terms when the mark hits zero every other time it is not at TDC. I know im a newbie and had a lot of help going through all that. Not trying to insult you in any way.

Mine wouldn't run decent at idle till 12* so 15 doesnt sound bad. Your compression is much better than mine and I have some valve seal issues.

I also learned that ignition and fuel symptoms are all the time incorrectly diagnosed for each other. Have you gone through your ignition? Since it idles bad I would assume weights and vac advance (not working) is irrelevant in your case since neither of those should be impacting an idle at less than 700ish. UNLESS maybe a spring is broke and the weight is not retained to the center. I would check you are getting fire at all your plugs. Not sure how you would check the hei aspect, im sure there is a thread. Then I would look at fuel/carb. Plenty of that in my thread also (lol).

Good luck

[RThree]

Originally Posted by May70

Maybe you know this but in reading this maybe not. Realizing that the tab only goes to +- 8 degrees you could get a adjustable timing light and set it to what ever degree of advance you want, then you would adjust the distributor until you have reached the zero mark on the tab. Just remember or write down what you set your light to.

Also the previous link to my thread.... a lot of people have put a lot of good trouble shooting tech tips specifically for a 292. I would read through it, just because you may see something you may have over looked. Ive gone through so much on this thing its crazy. Next step is machine shop and starting new for me.

Are you sure you are not 180* off on your TDC verification? How did you verify? The mark on the harmonic balancer will pass zero once and then when it passes the second time, for a full stroke. So in simple terms when the mark hits zero every other time it is not at TDC. I know im a newbie and had a lot of help going through all that. Not trying to insult you in any way.

Mine wouldn't run decent at idle till 12* so 15 doesnt sound bad. Your compression is much better than mine and I have some valve seal issues.

Oh dont worry about offending me, I’m new to working on engines to this extent and have no professional training which I’m sure is obvious

I am however familiar with the compression TDC stroke, made sure when the mark came around i could feel strong compression out of the #1 spark plug hole.

And yes the linked thread is very dense with information and i will be looking it over closely when i get a better chance to scrub it for information.

Both my trucks have 292s in them, one purrs like a kitten and the other grumbles like an old man. I just feel like I’m missing something simple. I’m just up against the clock cause i really wanna take my 72 Longhorn to the C10 Intervention show in Auburn but at this rate i might just be rolling in my 70 C10.

[BigBird05]

From reading all of this, just a thought what is your fuel pressure at idle?

[RThree]

Originally Posted by BigBird05

From reading all of this, just a thought what is your fuel pressure at idle?

Hmm not a clue, how/where would I measure it?

[May70]

Originally Posted by RThree

Hmm not a clue, how/where would I measure it?

Well since your problem is even at it idle it shouldn't require much at all for that 2 barrel, for idle it just needs enough to keep the bowl full. If you take the fuel line off at the carb (assuming you have a mechanical fuel pump) just put the line in a bucket or something and see how much comes out when you turn it over a few times. I think its typically 3-4 psi for the stock mechanical pumps. If a decent amount of gas comes out with a few turns I would cross that off the list. If you have a filter in the carb like one of those little one inch paper filter check it, I had one to collapse and block it off once.

[geezer#99]

What type of piston stop did you use while confirming your timing tab?

[RThree]

Originally Posted by May70

Well since your problem is even at it idle it shouldn't require much at all for that 2 barrel, for idle it just needs enough to keep the bowl full. If you take the fuel line off at the carb (assuming you have a mechanical fuel pump) just put the line in a bucket or something and see how much comes out when you turn it over a few times. I think its typically 3-4 psi for the stock mechanical pumps. If a decent amount of gas comes out with a few turns I would cross that off the list. If you have a filter in the carb like one of those little one inch paper filter check it, I had one to collapse and block it off once.

I agree i think it probably has plenty. The last time i checked the fuel pump to see if the carb was gettin any gas (ran out of gas while sideways on a hill) fuel flowed nicely as soon as we got it on flat ground.

[RThree]

Originally Posted by geezer#99

What type of piston stop did you use while confirming your timing tab?

First verified we were on the compressions stroke by feeling the compression with my finger in the cylinder 1 spark plug hole. Then as the cylinder came to TDC I inserted a wooden dowel into cylinder 1 while i turned the crank and verified it hit its highest point while the timing mark was at 0

[RThree]

Alright, so I’m feeling more comfortable where the timing is now and i can get it to idle pretty well at higher RPM. However when i go to try and tune in the carb at low idle it is still surging and will eventually stall out if i dont give it a little gas. I have captured a video of the surging here :

https://youtu.be/q1L5AWKIrpk


Any ideas what could cause this?

['68OrangeSunshine]

I'm hearing some intermittent hissing in the audio of your You Tube clip. I would look for a manifold leak. Spray carb cleaner around the intake and carb mount. If RPM surges when the volatile spray hits it -- there's your leak.

[May70]

Do you have that pcv input to the intake manifold plugged? I would think that is not helping your situation

[RThree]

Hey guys,

Thank you for all the help and suggestions, it seems like it all boiled down to a poorly made gasket between the 1bbl to 2bbl adapter plate and the 2GC carb. The gasket was cut in such a way that it caused a vacuum leak on the back side though the slotted mounting holes.

Finally found it once a got an air cleaner mounted I could hear and located the hiss coming from it, oddly enough it didn’t respond to the typical vacuum leak spray tricks. Either way made a new gasket and shes back to purring along. I don’t know what I would do with out this forums help!