boese1978

Morning,
68 350 olds stroker with Aluminum heads, holley 670 street avenger. Turning down my street after a short drive car just petered out almost to dying. i feathered the gas and made it home running roughly/wanting to die. Got home and parked it. over the next couple days I would start it up, fires right off, runs smoothly...for a short time then the idle drops away ( when the car is running it sounds normal making me think it is not internal issues ie: valves or??). I can keep it running by feathering the gas. If I let off the gas it starts to stumble and die. If I feather the gas to any RPM and try to hold it steady, it will start to lose rpms. stumble and die without my feathering the gas. As mentioned it will idle fine for a short period, approx until when the choke has opened about 1/2. I watched the choke from the drivers seat and it is working as it should. I checked the few vacuum lines I have and see no leaks or loose fittings. I checked the plugs and wires for arcing /looseness, found none. Fresh tune by the way, only a couple hundred miles on all. Visual inspection of the points show no burning and checking with a dwell meter shows it steady at 30*. Timing has not changed is not jumping around or anything. The contacts inside the dist cap are dark and towards the edge of the brass, seems pretty dark and "burnt" for only a couple hundred miles but why would it run good at start up then fade if the contacts were an issue. Looking for some tips and trouble shooting advice so go ahead and advise me! I'm leaning towards a carb issue..........Thanks in advance!

72455

Sounds like a fuel delivery issue...check your filter and fuel pump.

boese1978

Replacing the cap today, struggle to get the hooks to swing into place...will not buy an MSD cap again.....would a bad or going bad coil be able to work sometimes and not another?

boese1978

Fuel pressure steady at about 8psi, new pump, no leaks..........

joe_padavano

Put a vacuum gauge on it. This sounds like a leak. How does it run above idle?

boese1978

From initial start up to approx 1/2 choke it runs fine then the issues start so no, it will not run above idle without feathering the pedal. I have a vacuum gauge, use manifold vacuum, correct? Tips? what to look for? ( I have the chart showing what the various readings / needle movements equate to, just looking for suggestions....)
Thanks!

joe_padavano

Yes, just use straight manifold vacuum. If it's low (like 15 inches or less), consider plugging all the vacuum ports on the engine and see if that makes a difference. If not, verify that the carb bolts to the intake are tight. The fact that it runs when the choke is on reinforces my suspicion that this is a vacuum leak issue. When the choke is open and the engine is running poorly, what happens if you move the choke plate towards closed with your hand?

boese1978

Good info Joe, I'll have to get a helper to see what happens if I start closing the choke while running. I will report back once I get time to take another look.

boese1978

Hooked up the vacuum meter. From start up to “when the problems” start I am getting a steady 19-20. When I let the car struggle and feather the gas to keep it running I get a reading of about 15-16. I then unhooked and plugged, at the manifold, the dist advance, the power brake assist and the transmission modulator. This made no difference. I checked carb bolts, all snug. If I close the choke as joe mentioned above I can keep it running longer but as soon as you let the choke start opening , sputter and die- always restarts well. Tried directing an open butane tank stream around the carb and manifold base. No obvious increase in rpm’s but difficult to tell while having to feather the gas and monitor the choke. And, on closer inspection the brass terminals in the cap are not all that bad. Carb issue???
Other thoughts / trouble shooting tips please....

oldcutlass

Try turning the carb a/f mixture screws out 1/2 turn.

boese1978

Easy enough to try but why would the a/f ratio have changed “just like that” at the beginning of this issue ? It started while rounding the corner down my street out of the blue.

droldsmorland

Check these items in order

1. Fuel related:
Started rounding a corner = likely schizt in the carb bowl.
When was the last carb rebuild?
Is the fuel filter good? Pull it, cut it open, inspect.

2. Vac related:
Hold your hand over the air horn when it wants to die. RPM pick up? If yes you have a lean condition or vac leak.
Do the propane, carb cleaner, brake cleaner, WD40 test at the carb base gasket to see if the RPMs pick up.
Block off ALL vacuum accessories. Then plug back in one at a time. PCV, Vac Can, Booster, Trans modulator etc.
Is the 19-20" of vacuum steady while the chokes on?

3. Ignition related:
A fresh tune-up means you have recently changed some parts. New doesn't equal good.
If you still have the old parts you could try swapping them one at a time, if the above steps do not find a vac leak. I have had bad condensers and rotors give similar symptoms. But start with #1 above and work down to here.
USA made Corvette spec points, cond, cap, and rotor is the way to go. Corvette Central, Ecklers, Mid America, Long Island. Not sure where Fusick & Year One source points. You want the vette spec points for small block high out put engines, L79/L84/LT-1.

coppercutlass

A few things. if its only doing it when hot. an engine needs fuel air and spark. all of those are affected by heat. , fuel can be the only one affected by motion by a few things . heat can kill voltage to the coil. we has a similar issue on a mustang at work. once the coil hit 172 degrees the engine would not start or want to idle ( temp gun works to rule that out) that covers spark to an extent , Heat will make materials expand any vacuum leaks or bad seals around the intake will manifest themselves more when hot. fuel..... well that can be a little tricky. i have literally had a carb flood over around a turn due to too high of a float but a sticky needle and seat will have the same symptoms which can literally happen just like that. It sounds like your car is well taken care of and driven so i wouldnt suspect a clogged fuel line but something minor. Another odd thing i had was a bad coil that would smoke caps. It would have no running issues but would carbon track the cap so bad once i shut it off it wouldn't run until i cleaned the terminals in the cap but when it ran it would run perfectly fine even with the carbon tracked terminals it just wouldnt start. I have also had an intake leak where the gasket was almost blown out past the intake flange it would run normal when cold but horrible hot. . Just my 2 cents.

Dynoking

Excellent thought! If the A/F ratio was set and car ran good, then all of a sudden the A/F is off and car runs bad. Should you just adjust the idle mixture screws? We know the screws didn't tighten by them selves and lean out the A/F ratio. So what went bad? The usual problem is dirt, gum, or varnish in the internal fuel passages of the carburetor. You should try to en-richen the A/F ratio by turning each screw out a 1/4 turn at a time while watching you vacuum gauge, trying to achieve the highest reading. If this helps you know you are on the right track. If your idle problem is corrected you may want to leave well enough alone. If it helps but doesn't correct your issue 100% or you are a purest you may have to/want to remove, disassemble, and clean the carburetor.
This issue may be caused by the ethanol additive used in todays gasoline. Ethanol acts as a solvent loosening rust that maybe be present in the fuel tank and fuel supply lines. The small particles may also flake off inside your 50 year old carburetor casting. I have also found that the ethanol gas becomes unstable after a couple of weeks, and turns into real crap after three weeks. Experience has taught me to add fuel stabilizer at each fill up and a fuel system cleaner such as Techron or Lucas be added every 3000 miles or six months.

Cutlass Fan

A plugged fuel filter would affect higher RPM more than idle and the OP said he only has problems at idle.

Cutlass Fan

You could try removing the A/F screws (count the turns) and shooting in a blast of carb cleaner. If the jets are dirty this may help. That would only be a quick fix though and the carb should be properly cleaned.

VORTECPRO

yes

boese1978

Still no luck ( although I have not spent a great deal of time looking) I bypassed the in line fuel filter, no change, I removed the mixture screws and gave a shot of carb cleaner into each, no change. Some have suggested adjusting various things on the carb, while running, not gonna happen with me trying to feather the gas, work the choke etc in order to keep it running. FYI, Holley 670 Avenger, new 4 years ago.

Quote-A few things. if its only doing it when hot. an engine needs fuel air and spark. all of those are affected by heat.
The car will not run long enough to get to operating temp- I'm hesitant to "make" it run for too long knowing something is not right.

Quote-Check these items in order

1. Fuel related:
Started rounding a corner = likely schizt in the carb bowl. FYI carb is about 4 years old, new when installed
When was the last carb rebuild?
Is the fuel filter good? Pull it, cut it open, inspect. Bypassed the filter, no change

2. Vac related:
Hold your hand over the air horn when it wants to die. RPM pick up? If yes you have a lean condition or vac leak.I can manually hold the choke partially closed and feather the gas to keep it running, the rpm's do go up a little bit but it is hard to tell because of the feathering.
Do the propane, carb cleaner, brake cleaner, WD40 test at the carb base gasket to see if the RPMs pick up. I've bypassed and plugged the vacuum accessories / carb ports ( advance, brake booster, modulator, pcv) same issue. I've tried propane and the carb cleaner to look for leaks. Nothing obvious but it is hard to tell because of the steps I need to take to keep it running.
Block off ALL vacuum accessories. Then plug back in one at a time. PCV, Vac Can, Booster, Trans modulator etc.
Is the 19-20" of vacuum steady while the chokes on? 19 vacuum until the choke starts opening.

3. Ignition related:
A fresh tune-up means you have recently changed some parts. New doesn't equal good. Why ignition related if the car runs perfect until the choke starts opening?
If you still have the old parts you could try swapping them one at a time, if the above steps do not find a vac leak. I have had bad condensers and rotors give similar symptoms. But start with #1 above and work down to here.
USA made Corvette spec points, cond, cap, and rotor is the way to go. Corvette Central, Ecklers, Mid America, Long Island. Not sure where Fusick & Year One source points. You want the vette spec points for small block high out put engines, L79/L84/LT-1.

So, still trying to solve this. I do have a new in the box 770 avenger, again, hesitant to just swap the carb with possible further disappointment if it doesn't correct the issue. Any other thoughts or suggestions????
Thanks!!

joe_padavano

Because the same engine heat that causes the choke to open can also cause heat-related issues with either an ignition module or a coil

boese1978

Understood, not trying to be argumentative etc but there is (IMO) not enough heat being generated in the small amount of run time I get. I can touch the intake and heads w/o getting burned, heck even the headers don't get super hot , FYI my coil is mounted on the firewall well away from the minimal heat there now. I do have another I could swap and try......what do you think about swapping the carb out......

joe_padavano

Which it why it was lowest priority on the list of things to check.

I think that adds another unnecessary variable.

Fun71

I don't know anything about that carb, but in days of old Holley carbs were notorious for the power valve blowing out and leaking internally due to a backfire. At some point a check valve was added to prevent this. Just a piece of information to be aware of.

Koda

My money is crap on the needle seat. However, for pedantry, you could under the fuel line going into the pump and the return and run rubber hose to a 5 gallon jug to get the fuel tank sock and lines out of the equation. But, I really think it is crap in the carb, and you should open up the carb and look in the bowls and needle seat and blast canned air through it (wear glasses).

boese1978

Everything from the sock, lines, tank etc is 4 years old so I think I’m good with getting the proper amount to the carb.
Been driving working etc these cars for 45 years and have yet to take a carb apart. Possible to perhaps get some cleaner in the fuel system as opposed to spraying it ? Could I give the bowl a wrap with a rubber mallet to see if that frees up anything that is sticking?
sll the input is appreciated

boese1978

Would I be able to visually see stuff you mention with the bowl off?

oldcutlass

Did you get gas on your short trip?

cherokeepeople

just what part of the country are you in.i see snow on your avatar pic,what do you have for a choke?manual or electric?these carbs need heat hence the heat tube on the snorkel of the air cleaner.you might have to tighten the choke up a little.

cherokeepeople

if your up here in iowa yes you need a choke or your walking,

OLDSter Ralph

If I remember where both of you are, he is about 75-100 miles north of you.

cherokeepeople

i'm south of worthington about 70 miles.you say first started it will idle until chokes about half way open.can you start it and while its running run the mixture screws in and see if it affects the idle?

boese1978

Yes, I did get gas, at the same spot I always do, a Qwik Trip. Note- that day I filled and the pump did not shut off so there was an over flow. I put the cap back on - fuel right to the cap, could this be an issue? is a vapor lock possible? My tank is vented to atmosphere so not likely, correct? When I get home today I'll open the cap and see if there is any difference.......

boese1978

Minnesota Cold. Electric choke ....as mentioned in another post, if you don't use the choke up here you flood your engine / get a wet plug or two and then your walking.......

boese1978

I can try but as mentioned, I only get 2-3 minutes before the stumbling starts so very difficult to tell if anything I do / try affects the problem while I try to keep it running.

boese1978

Yes, I'm in St Paul too.........

oldcutlass

While it doesn't happen as much as it used to, perhaps you bought contaminated fuel that had water in it.

boese1978

would that not affect the engine all the time though? including at start up / choke? Remember it runs well until the choke starts opening

olds 307 and 403

Try your other carb, I bet it is carb related. My 83 super lean carb runs the same. Runs great on the choke then needs the idle set to 1300 rpm to run. Probably something plugged a passage in the carb. Honestly your motor should be able to use the 770 cfm carb anyways.

droldsmorland

To split the dictionary toss an inline spark tester on #1 and see if it dies. If the spark tests out Id be pulling the carb apart.
If it runs with your hand over the air horn its the carb. Restriction or leak somewahere.

Cutlass Fan

I think that is possible. My theory being that with the choke on the richer mixture hides the bit of water in the gas and as soon as it starts to lean out the moisture has a greater affect on combustion. You could try putting some moisture absorption additive in the gas or plumb in a temporary fuel supply.

boese1978

Appreciate all the help everyone! I'm going to try one more time to try and find a vacuum leak. Regarding the inline spark test, I have a tester, what do you mean by "see if it dies"? I'm figuring you mean put the tester inline, number one, start the car and verify consistent spark, correct? Next step i'll swap the carb with the new 770. Open to other diagnostic suggestions!

Vintage- Appreciate your input, I'm convinced it is not ignition related simply because I can watch the choke start opening and then the stumbling, all correlated and hand in hand. It starts on a dime and runs fine until......

Old Cutlass and Cutlass Fan- I also filled up my snow blower gas can at the same time and it has been running fine.....