Question, what octane level are people running in their stock Y`s?

Mine has a dieseling effect when you power brake and try to rev a little but when going down the road and flooring it it diesels for a split second almost cant even notice it and then it gone.

Im wondering if the 87 octane regular I am running is just not up to par.

Either that or I need to back timing off some but I never had a too advance engine act like this one is so I am wary if this is the cause.

Only other option I could think of is maybe carbon build up from running the engine at idle over the years till I got to this point.

Anyways just curious what other people are running octane wise.  Might need to upgrade to middle grade on this old engine as that is 89 octane around here.


1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi

I have wondered about this myself.  Most stock Y Blocks are not high compression engines.  My 1955 292 has been bored .060, squared, decked and align honed with flat top pistons and no valve reliefs. It's running stock 1957 G heads that have had the surfaces cleaned up with no excessive milling and composite FelPro head gaskets. I also have a 1957 distributor in this engine with 10 degrees advance on the crank and 38 degrees total timing.  I use 93 octane pump gasoline.  This engine runs great and does not run on or knock.   Should I use high test?

Mine has a dieseling effect when you power brake and try to rev a little but when going down the road and flooring it it diesels for a split second almost cant even notice it and then it gone.

That's interesting as the '56 LOM dual advance chamber (retard feature) was supposed to correct that.

I assume you still have mechanical points and base timing is set @ OEM or has it been bumped? Fuel quality also varies from pump to pump and area to area. How do your plugs read? Your first step may be to use premium fuel (if there is still such a thing). The advance feature within the DIST may also be worn and out of spec.

Make sure secondary IGN is in good shape. You could once put a carburetor car on a drip to break up carbon.

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When you say stock, are you talking about an engine that has never been apart or had any significant work since it left the factory.  Or is it an engine that has been rebuilt, trying to keep it as close to stock specs as possible?  There can be significant differences, depending on who did the rebuild and the attention to detail.



Lawrenceville, GA

Rusty, I have experimented some with tuning and carburetion and found a stock Y Block will tolerate a fair amount of ignition advance over and above the stock specs due to the lower compression, this was true by 1964 in the truck engines at least, with their 8 to 1 ratio. Fuel quality is not critical by any stretch. The factory spec was 6° BTDC base timing at that time, and further allowed another 5° for high altitude or to optomize performance and economy. So 11° BTDC was the initial timing spec for a stock truck curve. A car being generally lighter would likely tolerate even more if you play around with the curve and springs with a post '56 distributor. Some people run close to 40° on a stock Y Block, regardless of the number give it what it wants and don't worry about the number, if it isn't knocking or detonating. Keep in mind too, engine knock isn't necessarily audible!

Running lots of ignition advance, lean fuel mixtures, and electronic ignition may benefit from a slightly cooler heat range spark plug to minimize engine knock due to hot spots. I would believe you've got something else going on other than fuel octane. Keep an eye on the plug ceramic for evidence of detonation though, excessive advance can burn holes in pistons and other fun stuff. A small amount of spark knock on occasion during part throttle acceleration is not a cause for concern. Tuning is an art as well as a science!

A worn distributor will cause trouble with this for example, excessive distributor cam wobble will require detuning the engine to prevent knock. Excessive carbon buildup will do it. I found using both a wideband O2 sensor for carburetor adjustment and an ignition scope really upped my tuning game, it's remarkable how far out of tune something can get and still run reasonably well.

Florida_Phil (1/12/2019)

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I have wondered about this myself.  Most stock Y Blocks are not high compression engines.  My 1955 292 has been bored .060, squared, decked and align honed with flat top pistons and no valve reliefs. It's running stock 1957 G heads that have had the surfaces cleaned up with no excessive milling and composite FelPro head gaskets. I also have a 1957 distributor in this engine with 10 degrees advance on the crank and 38 degrees total timing.  I use 93 octane pump gasoline.  This engine runs great and does not run on or knock.   Should I use high test?

Same here, I typically run the regular grade fuel but we got that stupid ethanol blend around here, once I move I can get 93 octane in straight gas which I will go with.  But here I can get 87 Octane regular, 89 Octane premium, or 93 Octane super.  Ive been putting 87 octane regular in here cause of the low compression and that's what I run in my 302 in my '82 truck that has the same compression ratio along with in my 351W in my '78 Mercury.  But this is acting a bit different which could be due to points and the lack of mechanical advance.

The ignition system I replaced it completely about 3 or 4 years ago but I haven't drove the car just ran it from time to time while I did other work on the car.  The plugs, plug wires, point, condenser, cap, rotor are all new basically with limited use.  The dist how ever is original to the car and from all the service records I have in my grandfathers notebook that was in the ash tray there is no mention of replacing the dist as far back as 1963 so I assume this is original from 1956 and has never been pulled.

KULTULZ (1/12/2019)

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Mine has a dieseling effect when you power brake and try to rev a little but when going down the road and flooring it it diesels for a split second almost cant even notice it and then it gone.

That's interesting as the '56 LOM dual advance chamber (retard feature) was supposed to correct that.

I assume you still have mechanical points and base timing is set @ OEM or has it been bumped? Fuel quality also varies from pump to pump and area to area. How do your plugs read? Your first step may be to use premium fuel (if there is still such a thing). The advance feature within the DIST may also be worn and out of spec.

Make sure secondary IGN is in good shape. You could once put a carburetor car on a drip to break up carbon.

Timing is bumped up to 12* initial which at idle in neutral it is 16* with the vacuum lines attached.  I saw the timing go retard when I hit the throttle fairly hard from an idle when I was checking the dist operation.

I haven't pulled the plugs to read them but the whole ignition system was replaced minus the dist about 4 years ago.  But mind you the car hasn't been driven in those 4 years just idling and a few revs here and there just to maintain the battery as I worked on other aspects of the car.

Im wondering if its just bad weak gas that is truly not 87 octane or if its carbon buildup or if its something else.  I did turn my idle mixture screws out half a turn at a time and the knocking dieseling feel got better.  at 1 turn out from where it was on both screws the engine now doesn't stumble but I can hear a knock for a moment on the initial hit of the throttle.  but loading the engine up its running a little rough but no sound of a knock like it does when you first hit the throttle with the brake applied.

charliemccraney (1/12/2019)

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When you say stock, are you talking about an engine that has never been apart or had any significant work since it left the factory.  Or is it an engine that has been rebuilt, trying to keep it as close to stock specs as possible?  There can be significant differences, depending on who did the rebuild and the attention to detail.

Correct, stock as in the engine has never been apart as far as I know.  Only work that's been done on it was pulling the intake to put a newer 2V intake and late model 2V carb on it which I  pulled off and reinstalled the original 4V intake that came off the carb and a reman holley 4000 along with a NOS dual reservoir vacuum advance.

Im kind of wondering if it was carbon build up I don't think its advance because the engine spins over too easily with the starter to be too far advanced.  I currently have timing set at 12* initial which is 16* at full idle with the venturi and manifold vacuum lines attached.  Reving the engine I saw it advance smoothly with a rev and when I hit it quite hard to nearly full throttle I saw the TDC mark quickly fall back behind the pointer for a split moment before it went back to around 30* advance at around 2,000 rpm I guestimate.



That is what I am concerned with is if it might be my old dist.  I did turn my idle mixture screws out half a turn equally and putting car in drive and stepping on the gas it got better, wasn't shaking as bad and wasn't dieseling as bad.  Went another half turn out on both screws and its gotten better it actually tries to pull.  I can feel transmission slipping in second gear so that is probably my problem on the transmission falling out of gear, a band that is worn out, needs adjustment, or a leaky servo not applying enough line pressure.  But the car pulls now and doesn't shake dieseling but it is still knocking for a split moment when I first hit the throttle.  if I load the engine up like checking for a miss it doesn't knock it just knocks on the initial hit of the throttle.

I thought about going another half turn out on the idle screws but I don't know how far out they are now and don't want to have them fall out.  Like wise I don't know if another half turn out will remove the initial knock that is there.  I did get the stumble out and the roughness out.  Guess I had the carb set ideally and not slightly rich like is wanted so with the air filter on it made a change.  I know I put my hand over the choke housing to check for a lean condition when I initially set the carb idle mixture and it didn't race up like it was running lean it stayed idled the same with no change indicating an ideal mixture.

I guess what I will need to do is look at getting this dist rebuilt and then go from there.  I got just under 3/4 of a tank not sure how quickly I can burn that out to get some 93 octane in there to see if that is the cause that I have now.


1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi

It sounds more like a lean tip-in condition. You might consider adjusting the ACCEL PUMP ADJ or possibly going to a larger primary jet.

As for the LOM DIST, the re-builder would have to have a DIST MACH with a manometer, have access to needed parts and know and care what he is doing. The system is unlike a DUAL ADV SYSTEM.

You could plot the DIST ADVANCE CURVE with a timing tape, light and TACH to see if the advance curve is near correct before sending it off. The slight hesitation in advance @ WOT indicates the retard function is working.

As for carbon build-up (which raises CR and interrupts fuel mixture flow) is a given in an engine that old, especially if it ever ran a fat fuel curve or weak IGN.

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I will need to look at the carb manuals I have, I don't know if the accelerator pump can be adjusted or not on this Holley 4000 I never looked.

I know on my truck I went one notch up it removed some of the stumble but not all.  I got most of the stumble out but thought maybe its not squirting enough fuel that could be why I can off set it with fattening up the idle circuit.

Maybe best to see my old carb that came off here see where its accelerator pump is set at and check this reman holley 4000 and see if its in the same place.


1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi

I just looked, my old Holley 4000 is set on the bottom most hole which is from what I am reading the summer setting.

The Holley 4000 I paid a core on to get a spare to use for now it is also set on the summer setting in the bottom hole.

Im tempted to take and move the accelerator pump rod up to the winter setting and resetting my idle mixture back where I had it at and see how it does then.  Could very well be a lack of fuel cause I know when I was trying to drive the car I was fluttering the gas pedal to get the car to move off the line like it just wouldn't take any gas like it was running out of fuel.

Might very well be the accelerator pump isn't giving me enough fuel squirt to make it run right.  Might still need to leave the idle mixture where they are at but I rather revert the changes I made and see what changes just the accelerator pump made and then go from there.


1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi

For a stock street engine that does a lot of stop & go or just off idle acceleration, the mixture setting is important. Most driving is done there.

It's called the idle circuit but the throttle plate exposes the transition slot "just so" when the RPM and mixture is set just right. It's probably in play up to around 40 miles an hour. It needs to be set just slightly rich from a point that is just about as lean as you can get it consistent with a smooth idle. If you start cranking on the screws it will be way off. They are "precision cut", about an 1/8 turn on a single screw changes the engine AFR by a whole point.

Lean out the idle mixture and fatten up the pump shot. It sounds counterintuitive but that's how it works. Too much pump shot can cause a bog or hesitation too.