Tuning the 360 - still has spark knock at highway speeds after engine is hot

All the references I have seen says that the notation on the centrifugal advance limiter is 1/2 the crank degrees so that if it's marked 16 that means that it is providing no more than 32 degrees as measured at the crank.

 

Before I make any changes I mapped the curve in 500 RPM increments up to 2,500 RPM. Of course this is without any load, but vacuum is adding about 15*, except at the 1,000 RPM mark where it's only adding 10*. I checking this with a dial-back timing light.



The starting RPM on vacuum is actually ~700 RPM. Since the curve is kind of flat between 1500 and 2000 I'm thinking that the larger advance spring is too strong and not allowing full centrifugal advance early enough.

I could check the 3K RPM mark if that is helpful, but I rarely get there other than running up RPM on acceleration to shift to the next gear. Cruising at highway speed is usually at that 2,500 range.

When I read the '72 TSM and the '74 TSM it appears as if total advance is in the range of 32*-39* between 2,000 RPM and 3,000 RPM, but I don't know if that includes vacuum or not which is around 7*-9* according to the book.

I'm not sure if timing is causing the pinging or if it's fuel related such as a lean v rich situation. I'll be checking the current jet sizes to compare to the TSM which recommends 48.

 

I really think that is backwards............1/2 the crank degrees if you saying it has the ability to move 16 would be 8 and not 32 degrees..........I have never seen a stock engine needing anywhere near 46 degrees with the centrifugal & vacuum hooked up. Most basic engine's even the most non-efficient combustion chambers only have 6-12 degrees initial lead and then perhaps 6-8 on the centrifugal or vacuum advance side for a total timing of maybe 20 degrees. On Race motors we lead the Ignition to burn the extra fuel........perhaps 30-32 degrees total.
Looking back at your post you mentioned that the motor was hot..........was there any change in elevation?.......excess heat & elevation will cause detonation.......depending on where your fuel lines are running you could also be experiencing over heated fuel or a version of vapor lock whereby the fuel is overheated once it gets to the Carb...........look at your lines and make sure they have shielding and are not up against the block or manifold where they can absorb heat.
If it runs good at normal operating temp...........control the heat first.......and keep in mind the fuel we buy today is terrible.

 

Interesting considering that the TSM calls for up to 39* advance around the 3K RPM range.

Anyway, my elevation is ~900 feet give or take 300. Fuel/return lines are stock location and stock sizes not up against any heat sources.

I really think that the issue is between either a lean condition or timing or a combination.

 

Distributor degrees is referenced in half crank degees, 8 distributor * is 16* at the crank, the crank turns twice to one cam rotation. You may be correct that the larger spring may be to stiff, take the motor to 3000rpms and see if your numbers go up. Don't exceed 52* with vac, and 38 without. Most total timing numbers are given with the assumption of no vac advance added. If you get around the 36* range at 3000rpms with no vac, you may be over the 52* with vac and that will cause pinging, if that turns out to be the case you would need to get a new vac can with lower vac numbers, or limit the travel of the one you have. With out a distributor sun machine, this process can take a long time. You make a adjustment, test drive, make another adjustment, took me several weeks to get mine right, so be patient.

 

Your curve looks pretty darn normal to me for stock heads, stock cam, and compression ratio. Even a bit mild.

I really don't think its detonation unless something is really awry somewhere else. Don't touch a thing until you look at the plugs. They won't lie.

 

I can adjust the vac can downward more. I need to check vacuum at those RPM's though to estimate where to dial it in.

The plugs are pretty new, only about 200 miles on them. My last set was more gray than tan colored though, they were in about 1,500 miles.

 

Moot.

Mute is different, as in deaf-mute.

Sorry to be picky - but this one bugged me.

 

Its not color you are looking for. Just pull the plugs and see if there are any with specks like black pepper on them. Those specks are basically metal from the combustion chamber being left behind on the plugs after detonation. If you don't see any specks, its not detonation. Any trace of detonation will leave those specks on the plug so its a pretty good test.

Keep in mind detonation can also be caused by too lean of a fuel mixture too, and not necessarily a fault of the timing curve. A common fuel injection tuning strategy is if a motor "likes" a certain amount of advance, or maybe more, but its getting into detonation, the fueling will get adjusted rich in that area before backing off the timing. Sometimes a little extra fuel is all it takes to run a degree or two of extra timing somewhere. If it still pings, the timing gets backed-off and the fueling returned to the "normal" desired A/F ratio. Lack of pump shot or jets that are too small can cause what you describe, although they wouldn't suddenly become an issue after one wheeling trip as you describe.

 

None of the plugs have had any spots on them...I'll have to pull the recent set to check.

BTW...my carb has 48 jets in it, which is what the TSM calls for in the '72 and '74 TSM's.

 

Tim, ...............Correction noted.

 

In my opinion that was a terrible time for engine technology, since the makers were struggling to develop smog reductions, including leaner mixtures.

I think it is possible an ethanol mix could result in effectively leaning out the fuel further, as the engine sees it, since alcohol has less BTUs. I'm not sure but it may be something to look into.

(On the other hand I think I recall that boy-racers years ago used to add alcohol on the theory it would cool the mix and make it denser, carrying more btu's, a kind of poor man's supercharging.)

My Model T Fords supposedly can run on either gas or ethanol - but you can adjust the timing and the carb mix from the driver's seat. Those were the days.

 

Have you rerun straight gasoline through it before trying to change anything?

 

I did put the non-ethanol back into it, but haven't been able to do a highway run with it.

 

If these were the plugs that were in there when it was pinging on your return trip, its not detonation or they would be peppered. Regardless of ethanol mix in the fuel, etc.

I would start looking for things that sound like detonation (i.e. an exhaust leak.)

 

Detonation comes in many forms...............severe may pepper the ground strap and pistons giving them a sandblasted effect.
His problem which seems to be at higher Rpm's and load ...... sounds more like the fuel is probably preheated in the combustion chamber and is just breaking down under pressure from the lack of better word not enough octane or lead additives..............basically the ping is the fuel igniting itself prior to the Ignition event.........
Timing may not change a thing since the fuel is just breaking down.
I would put some octane booster in a tank of fresh fuel and see if that fixes the problem..........if it does you just need to find a fuel vendor that has a constant supply of fuel you motor likes.

 

FWIW, the ethanol tank (when ping occurred) was 93 octane. I normally buy non-ethanol 90 octane around home.

 

I would put some octane booster in a tank of fresh fuel and see if that fixes the problem..........if it does you just need to find a fuel vendor that has a constant supply of fuel your motor likes.[/QUOTE]