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

So I spent a lot of time researching how to tune the 360 better. Went out and purchased a hand vacuum pump and already have a dial-back timing light. My research led me to this post on the IFSJA forum, which I ended up as my primary distributor tuning instruction.

First my specs:
AMC 360 with MTA-1 cam
TFI dizzy upgrade
MSD 6
RN12YC plugs gapped at .45 (new last weekend)
Initial timing @ 10*BTDC
Centifugal advance cam on the 16* limiter (changed from 21 as per the post above), left stock springs alone.
Manifold vacuum advance at rear of MC2100 carb and comes in ~7.5"hg caps ~14"hg
MC2100 carb with 1.08 venturi, idle mixture tuned to get ~15"hg manifold vacuum (a bit low but probably acceptable given the mild cam)
First tank is 90 octane non-ethanol fuel, second tank is 93 octane with 10% ethanol.

After using the noted post to tune my distributor, and another few instructions to tune my carb, this engine ran beautifully. Smooth idle, good throttle response, was able to cruise the highway at 65 with nary a ping...awesome hot or cold.

Then I drove 2 hours on the interstate to a wheeling spot in the mountains where I filled up my second tankful of gas (actually got 13.5 MPG on the trip up!). Spent all day there wheeling with the club, the CJ did great, no issues. On the way home though the engine would start that annoying ping at around 55-60 MPH and especially under load like going uphill or accelerating. I rechecked initial timing when I got home and it was still rock steady at 10*. So, why after running the engine all day long did it start to ping again? Everything seems to be exactly where I set it up initially. The plugs are in the correct heat range for the engine, good octane gas, new carb gaskets that dont' leak, hose clamps around the manifold vacuum hoses. The only thing I can think of is the engine is not staying cool enough...runs ~208* ish on my dash temp gauge while on the highway. I was also thinking of backing initial down 2* to 8*BTDC.

Anyway...thoughts from the gallery?

 

Usually you look at ignition advance or mixture when you have a ping. What do the plugs look like?

I wouldn't be happy with the cooling unless it stayed right at the set point under all conditions. If you are running more than a few degrees above the thermostat set point, it means that the cooling system capacity has been exceeded, and you are depending on the higher temperature of the coolant to regulate the temperature. As I'm sure you understand, a lean mixture can also make your engine run hot. Is the radiator new? Do you have a fan shroud? Are you running an EGR valve?

 

Plugs are brand new last weekend. The prior plugs are the Champion "14" series and looked good when pulled after about 2K miles with nice light gray ash.
I do have an EGR, drawing through the CTO to ported vacuum. Appears to be operating correctly, but I haven't pulled it to see if there is much carbon build up underneath.
I have the "Contour" fans installed, obviously already in the shroud. Seems to run hot only at extended highway speeds...maybe too restrictive at that speed and load?
The radiator was cleaned and rodded at a radiator shop before I put the engine back in, it flows well.
I'm not lean right now, in fact, it's probably a tad rich. I tuned the idle mixture screws in until it was about to stall, then back them out while using a vacuum gauge until I got a steady vacuum at about 14"hg to 15"hg...backing it out any more loses vacuum so I think I found the spot.
Engine is about 5K miles old.

Would higher engine heat contribute to spark knock at load?

 

"Usually you look at ignition advance or mixture when you have a ping."

Agreed. I had an AMC 360 in a jeep truck. After a total rebuild it knocked badly, and I could never cure it. Drove me nuts trying. Played with timing, advance, springs, all that. If I got rid of the ping, it ran lousy.

I blamed this on two things - it was designed to be run on high octane gas, back when "regular" was 94 octane. And the machine shop further raised the compression ratio by skimming the heads. Possibly the replacement cam was a contributor. Plus in the era it was built (1970's) the emissions system was a Rube Goldberg mess.

Confirming my feeling was that at one point I tried filling it up with "racing" or "aviation" gas and then it ran great. But that was too expensive and largely unavailable.

In the end, on a several hour trip away from home, it blew out a piston.

I'd suggest you perhaps consider diagnosing this from the aspect of fuel octane/compression-ratio.

 

I'm sure you know this - idle mixture only affects the mixture at idle. Above 1000 RPM or so, the idle circuit shuts off, and the main jets take over. So adjusting the idle mixture is pretty much irrelevant to your problem.

If you have a 2100 that's meant for a 304, it could be a little too lean. The plugs should be brown-tan - I suspect that the 'gray' could be called 'white' and is an indication of overly lean. But maybe not. You can measure the air-fuel ratio with an aftermarket O2 sensor. These require a bung welded into the exhaust pipe. Eventually, when you go to TBI, you can incorporate the wide-band O2 sensor for the TBI's mixture control, and it will make tuning much easier than using a factory-style narrow band sensor.

Jets used to be available for the Motorcraft carbs - I have purchased them in the past (by my source - Parker Carburetion - is out of business now). They look like Holley jets, but unfortunately they are different. I would go a couple of numbers larger and see how that affects the operating temperature. Also, if I had not replaced the radiator, I would consider it.

 

The rebuilt engine is stock compression...so I think that's around 8.5:1 or something. I also regularly run higher octane fuel than 87, and non-ethanol if I can find it.

My first impression after tuning is that this engine ran beautifully and was tuned spot on...no ping at highway speeds or under load, great throttle, great idle warm or cold. The issue only popped up again after running it all day long and then on the way home it started again. In the mean time, nothing changed from the tune.

The jets are either 47 or 48, I forget which. I'm also pretty sure that the 1.08 venturi was run on both 304 and 360...or were you referring to the jet size vs V8 size?

 

Jet size vs. engine size. The venturi size only changes the VE.

The float setting also controls mixture to some degree, but not as much as the jet number. So I would pull the top off the carb and make sure that the bowl is full, and the float is set right. I would also do a pressure/volume check on the fuel pump. Unlikely that's the issue, but it would be good to eliminate that.

I would not use the jet number as an absolute measure. Different years, applications and configurations will have different jet sizes. It's only relative to the performance on the engine. If you are too lean now, you have to increase the jet size.

Are you running a single-stage power valve? Should not have any effect on cruise, but it will affect the mixture under load.

 

Okay, I get it now on the jets, makes sense.

Single stage PV at 7.5-8.5 I think...hard to remember now.

I'll have to run it a bit longer to get a rich/lean from a spark plug read, they barely have 200 miles on them. The carb was also rebuilt when the engine was installed and the float was set right then. I'll verify that too. Never have done a fuel pressure check, something new to learn, I'll check the TSM for that, although the pump is new...and it ran great off road climbing some pretty steep hills and idling over rock.

I keep going back to the fact that the stars aligned and everything was in harmony until the return trip...which is why I keep thinking too much engine heat.

But I'll double check for vacuum leaks, correct timing, spark health, etc. before getting into anything else at this point.

 

Non-ethanol there, but ethanol in return trip...I'd do the simple thing first and refill with non ethanol fuel again and duplicate the trip to your original destination.

 

What does the manuf of the cam recomend for total advance since the MTA-1 probably has a 5* advance already ground in? What does your 360 require for total advance (no vacuum adv) at WOT? When you are pulling heavy, such as long steady inclines, your manifold vacuum drops off drastically. You say you have the dist adv keyed into 16* (32* crank) +/- 5* cam, and initial 10* then your total adv (no vacuum) is 47* +/-. What does FSM for 360 say it should be? Are you in the ball park?

 

Walt is reading my mind, I think you are over advanced at cruise. I think 10* initial is good, 16* in the dizzy is actually 32* at the crank (a bit high) you have 42* total with no vac advance which is a bit high, 36* total would be the number to shoot for with vac advance can disconnected and pluged. On top of your total timing of 42* you have the added advance of the vac advance can at cruise part throttle (At WOT there is no vac), which is usually around 16* but some are as high as 20something, add that to your total timing number of 42 and you are over advanced at 58* cruise "if" your vac can is only giving 16* of advance. You want to keep your total with vac advance part throttle around 52* or less. You need to know how much your vac can is adding, you can do that by putting your hand pump vac gauge on the vac can and see how much it will pull. If it is over 16* you will need to limit it. I think once you know how much the vac can is adding you will need to limit the amount of timing in the dizzy from 16*(32 at crank) down to 12* (24* at crank), this will get your total (no vac) down to the acceptable 36* instead of 42, then with 16* of the vac can at cruise part throttle you will have 52* total at cruise which is right in the ball park.. I think you are just a bit over advanced at cruise and getting detonation at times. If you lower your initial of 10* to lower your total, your engine will feel like it has lost some power. This is why I suggest limiting to timing in the dist from 16* to 12* instead of lowering your initial.

 

this topic is very confusing to many people because you have three separate timing settings working together as one unit. Your initial set at the crank BTDC, the centri built into the dist, and the timing added by the vac can. I can break this down to make it simpler if you don't understand but it will be a very long post.

 

No need to expand on it for me, I've read enough to understand it from other posts around the Internet. Feel free to write up an explanation if you think others will benefit since this will be archived forever.

I think you guys are right though, I was thinking the same thing on the advance as i was running the numbers it seems like I'm still getting a bit too much advance all in. I originally up the vac can to start at about 7.5"hg which makes max at about 14"...I can back that starting number down a bit and the max will follow. The only other way to limit advance on the dizzy is to install heavier springs to try and keep the centrifugal advance from coming in too quickly, right now it's one light and one heavy which I assume to be original. The limiter on this dizzy is already on the lowest setting for the centrifugal at 16* (i.e. 32* crank), it used to be on 21* (if the stamped numbers are indeed distributor degrees).

Honestly I've been too lazy to fully map the timing curves specific to my engine and was trying to tune based upon "best practices" that I've found online.

To answer one of your questions Walt, I have found no reference on the MTA-1 recommended advance, but you are correct that the cam has 5* built in.

I'll start with vac advance down a bit then test it out and make incremental changes from there one at a time until I get this. I think its really close based upon how well it worked the first time around so it shouldn't take too much. I'll also get a helper to work the RPM's so I can see how quickly the centrifugal comes in and where it maxes out. I'd rather not mess with the springs unless I have to. Worse case is I have to back out a couple degrees of initial also.

 

You really need to just map it, if for no other reason than to make sure it makes sense and give you a starting point.

Even if you do idle/1000/2000/3000 rpm, vac both connected and not, should be enough to generally tell what's going on. No need to do every 250 rpm or anything like that unless you really want to get down to it.

 

I'll probably end up doing that...at least I'll have a better sense of where the advance is coming and going.

 

Bringing the vac can down will only lower your total cruise timing, and not your WOT timing. the vac can is not active at WOT, limiting the can will help you though at cruise, but to limit the dist further to bring your WOT down to around 36-38* you will need to make a limiting plate that will not let the cent weights fly out as far. If you look on the net, their is a ton of info on this. I had to limit my points dist to 20*, 16* initial and 10* from the vac can, because I changed dists, I now have hei and love it.

 

The 5* built into the cam has not effect on you setting your timing, the factory takes that into account. Cam timing and ignition timing are diff

 

It does sound like you have too much timing at cruise, but gray spark plugs are also indicating a lean condition. 208* is too hot also. The lean and hot conditions can also lead to spark knock or detonation. Now, the lean may be causing the hot. Try pulling some timing out like others have mentioned, and go up a few sizes on the jets.

 

This is true.

With the initial 10* crank and the dist mechanical 16 (32* crank) for a 42* then what the vacuum can adds (example=8*) which you have to double at the crank of 16* puts you at 58*. If the vacuum can is non-adjustable (not all are) then it will need to be replaced.

 

Why do we think he's getting 32 degrees of centrifugal advance at the crank? Not saying you are wrong but I've just never heard of a street/strip type distributor giving that much centrifugal advance without grind out slots, etc.