Need a Mopar guru: 273 part-throttle ping driving me crazy!

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66fs said:
When you refreshed it did you notice the pistons? Dome or flat top, 4 valve notches or two?
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I didn't refresh it, previous owner (actually owner before previous owner) did, so uncertain what kind of piston tops. What came stock on a '68 Belvedere with a 273? Were there various pistons the factory used?

66fs said:
Since it has a Prestolite distributor, did someone put a 2 barrel carb and intake on a Commando motor?
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Possibly. But was the Commando 273 motor an option on a '68 Belvedere? Also, did Prestolites came only on Commandos? I did not know that.

66fs said:
You cannot hear the solid lifters in a Commando motor, pull the oil filler and look for adjustable rocker arms.
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I will do that! Thanks!

66fs said:
Way out on a limb here, nothing else makes sense.
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That's why I am going bonkers! =(
 
d55dave said:
Has someone perhaps put some light "recurve" springs in the distributor and the mechanical advance is coming in too early?
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Possibly.

d55dave said:
Perhaps there is someone near you who has a small block distributor you could run in it to try.
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Good thing to try. Will any SB distrib do? There seem so many different ones. Or is the replacement going to be specific to a 273?
 
RedFish said:
Do this, Start it from stone cold and blast off. Either direction I leave out from my place I have to go down and more importantly up hills. Anyway...
Run the dog crap out of it and I'll betcha you cant make it ping even up hill.
The ping will begin only after it gets too hot.
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Yes, correct. After running for about 5 minutes it will ping on a hill/load, i.e., when the engine is getting hot. But, I think the radiator was recored along with the motor "refresh" and everything that goes with the cooling system (new thermostat, new pump, new hoses, etc.).
 
moper said:
I think there's a few potential issues with this.
1st - the "opened crossover" more than likely means the factory apssage under the intake or possibly in the heads was plugged by carbon deposits. The '68 318, and any 273 would not have had any provision for EGR.
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Thanks for this, that's what I thought as well. Do you think blocking where the exhaust comes into the intake manifold to keep it from getting into heat xover would help?

moper said:
2nd - the pinging. I'd look at two things:
The carb jetting which is probably 2-3 steps leaner than it needs to be for the pump unleaded we buy now. Pull the top off the carb, find the main jets, and using a drill bit enlarge them very slightly. A little goes a long way. If you have pin drills, and wire gage bits, those are the best way to sneak up on the right jet size. You also want to make sure the power enrichment circuit is working. It might not be.
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I want to wait on this a little bit for now. See below.

moper said:
The timing curve is suspect. You need to know how much timing there is, what advance is putting it in, and when. You can ajust the mechanical advance with tweaking springs, slots, and weights. And you can adjust the vacuum advance but only the amount of vacuum required to move it - not the actual advance amount using that 11/32 hex key. My impression is there are lightweight springs in it (should really only be one, the other should be the factory "light" which is 5 times stiffer than the HP light springs) and the mechcanical is advancing too much. Get a timing tape and use that and a tachometer to fully map out your timing curve. Then post that.
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I think this may be the direction for me to go right now. Seems easier/less risky that "drilling" the carb jet even if ever so slightly...

Any tutorials/advice on the hows of adjusting mechanical on the Prestolite 3438225 distributor I have?

Oh, and as I stated before, I can't get an allen wrench into my distributor to adjust vacuum advance. It won't fit.

Thanks
 
Float level and jetting are on the right track. Part throttle conditions are very sensitive to air fuel ratio and engine temperature as well. You might also try blocking the heat crossover between the heads and intake manifold. Make sure your coolant mixture is not too rich as well. Too much antifreeze reduces the amount of heat that can be transferred from the cylinders. Many engines also require a relatively high temp thermostat to actually run cooler. The 195 degree t-stat slows the water down so that heat can be more efficiently dissapated in the radiator. The 273 cylinder heads are of closed chamber design and are more sensative to octane as you have found out.
 
dodge freak said:
Pin drills, and wire gage bits are the only way to open up jets. You only want to go up to the next wire size drill. A set of wire drills close the jet size is the way to go, find the drill that goes in tight and use the pin drill to get the next size drill in the jet.

Otherwise you just ruin the jets
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Yes, this is what I figured, too.

Curious: Is the jet "soft" enough to allow "drilling by hand"?
 
moparlover said:
it's probably an exhaust leak at the manifold.
i seriously doubt that 4 gallons of 100 octane added to 10 gallons of 91 put on a car with stock compression ratio was really fixing anything. just sayin
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I can tell you that there was a SIGNIFICANT reduction in the amount of pinging when I did that. However, I also changed brands of gasoline at that time (Unocal) so maybe it was the brand of gas that did it more than the octane...

moparlover said:
i'm speakin from experience about exhaust leaks. i would bet that is your ping you are noticing. maybe some lifter noise along with.

probably wasnt high end gaskets i'm betting.

get a set of percy's exhaust gaskets.

http://www.taylorvertex.com/gaskets/seal-4-good/header-flange/index.cgi/productListing?id=300779

i'd also recommend setting the valve lash. if he put stamped rockers (non adjustable) then skip that step.
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Do I have to pull the valve covers off to determine if I have stamped non-adjustable rockers or will I be able to look through the breather/cap hole?
 
d55dave said:
A float set too low will also cause a lean condition. It is easier to play with, and correct than drilling jets. Easier to go back as well if it doesn't work. Perhaps try raising the float.
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Something else to try. OK.

Assuming I will have to replace the gaskets after opening up the carb to get to the floats, do I have to buy a rebuild kit just for the gaskets? Or are those specific gaskets available (Holley 2280)?
 
mguner said:
Float level and jetting are on the right track. Part throttle conditions are very sensitive to air fuel ratio and engine temperature as well. You might also try blocking the heat crossover between the heads and intake manifold.
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OK!
mguner said:
Make sure your coolant mixture is not too rich as well. Too much antifreeze reduces the amount of heat that can be transferred from the cylinders. Many engines also require a relatively high temp thermostat to actually run cooler. The 195 degree t-stat slows the water down so that heat can be more efficiently dissapated in the radiator.
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How can I determine this? Also, if it was too rich, what is the proper antifreeze mixture/ratio? And how do I determine what temp thermostat I have? I guess I have to pull it out?

Thanks!
 
Usually a 50/50 mix is good on coolant unless you get down to 20 below or more. Just check it with an antifreeze tester. If it tests to -40 or so you are probably on the overkill side. Mopar had issues with what you are on some 80s model Carter BBD series carbs and hooked up an adjustable metering rod linkage to over ride the vacuum so that they wouldn't get pulled down at part throttle and cause your problem. Usually the T-stat is marked somewhere.
 
Cast iron, dual point, ball bearing vacuum advance Prestolite distributors only came on Commando / Charger 4 barrel 273's. The "Correct" 68 273 would have open chambered heads like the 318, slightly taller flat top pistons with 4 valve notches, a standard Chrysler distributor, and hydraulic lifter cam setup. What you have, may or may not be, a stock 273 2 barrel from a 1968 Belvedere. Closed chambered heads are typically less prone to detonation, but in a 273 closed chambers will give more compression. Which is going in the wrong direction for you.
 
Hi,

mguner said:
Mopar had issues with what you are on some 80s model Carter BBD series carbs and hooked up an adjustable metering rod linkage to over ride the vacuum so that they wouldn't get pulled down at part throttle and cause your problem.
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As noted, car pings with vacuum advance disconnected/plugged. =(
 
66fs said:
Cast iron, dual point, ball bearing vacuum advance Prestolite distributors only came on Commando / Charger 4 barrel 273's
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I have to check but I don't remember if this Prestolite 3438225 dizzy is a dual-point...?

66fs said:
a standard Chrysler distributor
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What is(are) the model number(s) for a "standard Chrysler distributor" I can look for so I can test to determine if the mechanical advance is happening too soon/too much?
 
Distributor # 3438225 is a 1970 318 2 barrell auto trans. unit, single point.
 
d55dave said:
mguner is not referring to vacuum advance. He is referring to a power enrichment circuit in the carburetor which may be leaning out, due to high vacuum at part throttle.
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#-o:pale:
 
I had a 66 cuda 273 commando that I overhauled back in the early 80's.As i remember the intake bolted straight down like magnum intakes and without mods would not use other small block intakes,unless they changed by 68?If its the original intake is the egr passages plugged up?If so that will lean out the mixture and make it ping...If thats okay then i would guess that the carb which i believe you posted is not the original..It probbally needs to be getted up as its just flat out running to lean..The oem carbs are calibrated to the amount of exhaust gas they design to be put into the intake mixture by the EGR valve or jets in the floor of the intake..If they are plugged it is going to lean it out,keep that in mind..m880
 
m880 said:
I had a 66 cuda 273 commando that I overhauled back in the early 80's.As i remember the intake bolted straight down like magnum intakes and without mods would not use other small block intakes,unless they changed by 68?If its the original intake is the egr passages plugged up?If so that will lean out the mixture and make it ping...If thats okay then i would guess that the carb which i believe you posted is not the original..It probbally needs to be getted up as its just flat out running to lean..The oem carbs are calibrated to the amount of exhaust gas they design to be put into the intake mixture by the EGR valve or jets in the floor of the intake..If they are plugged it is going to lean it out,keep that in mind..m880
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65 273's are closer to what you are describing. In 66 the bolt size and angle was changed to the common small block configuration. tmm
 
that might have solid lifters, if so they do tick at times. Exellent point about an exhaust leak

Why not run straight 100 for a bit and see if the "pinging" does in fact go all away, if not, it may well be not pinging
 
Part throttle "knocking" could be a rod. Just to be gloomy...
 
This won't be of any help, but about 30 years ago, I had a 68 Cuda with a 318 that had the exact same issue..and it drove me completely insane. I changed out carbs and distributors..intake manifolds...name it! Finally determined it must be a head gasket, changed them....and then the next week I changed out the heads as well because the gaskets didn't fix the problem. I flipped the car to a friend of mine who ended up dropping a 340 6bbl in it.
Granted, I was only 22 and didn't have near the knowledge and experience that I do today, but I often think back to that Cuda and the month of my life it consumed...and the fact that if BEAT ME!!!
 
toolmanmike said:
Distributor # 3438225 is a 1970 318 2 barrell auto trans. unit, single point.
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Thanks Mike, I can't find my numbers book and was going on the Prestolite info. Now it begins to make sense. The specs for that distributor are 28 degrees at full centrifugal advance. Plus vacuum advance of 21.5 degrees at 15 inches of vacuum. The 1970 318 is supposed to be timed at TDC. Start there, I'd also be suspect of the aftermarket Holley carb jetting. I'm assuming Champion N14Y plugs or equivalent. 1968 is the changeover from closed chambered heads to open chambered heads. Who knows, maybe the higher pistons with the leftover closed chambered heads made for a higher compression motor? I know my brothers 67 Barracuda 273 2 barrel short block would ping on 89 octane fuel with closed chambered heads and the big Fel-Pro head gaskets.
 
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