Can someone check my logic? Jet size and relationship to pinging ... (edited: now Advance Curve) (1 Viewer)

[fyton2v]

At freeway speeds, a lean condition contributes to pre-detonation, correct? I can't seem to eliminate some light pinging. It happens in two circumstances. Going up a hill under heavy load with WOT (4th gear only). And 4th gear 65mph+ when rolling on the throttle. Cruising seems fine, but a bit more throttle can cause some pinging for moment. 91 octane does improve the situation, but it doesn't go away completely. Maybe my total advance needs some help, but I'm not sure that's it and I want to rule out some other stuff.

I'm looking at my primary and secondary jets and they're currently 136 and 177 on stock '78 Aisin carb. I think those numbers are correct, they're a bit worn and hard to read with an endoscope. Those seem smaller than what many people are running. Before I go looking for larger jets, am I on the right track? I'm at sea level.

Thanks.

 

[1911]

At freeway speeds, a lean condition contributes to detonation, correct? I can't seem to eliminate some light pinging. It happens in two circumstances. Going up a hill under heavy load with WOT (4th gear only). And 4th gear 65mph+ when rolling on the throttle. Cruising seems fine, but a bit more throttle can cause some pinging for moment. 91 octane does improve the situation, but it doesn't go away completely. Maybe my total advance needs some help, but I'm not sure that's it and I want to rule out some other stuff.

I'm looking at my primary and secondary jets and they're currently 136 and 177 on stock '78 Aisan carb. I think those numbers are correct, they're a bit worn and hard to read with a endoscope. Those seem smaller than what many people are running. Before I go looking for larger jets, am I on the right track? I'm at sea level.

Thanks.

Retard your timing a tiny bit. Way easier than changing jets and 100% more likely to help.

 

[fyton2v]

I worry a bit about retarding timing because my base timing is only 7* before TDC. Retarding from there taxes performance in a way that I'm hoping to avoid. Perhaps foolishly, I'm searching for a compromise somewhere in the 7+ range.

I going to try disabling the vacuum advance and see if that has an effect. I'd like to see if it's influenced by vacuum. Guessing it is. If that resolves, perhaps I can try slightly stronger springs on the dizzy's mechanical advance to lower the overall advance at that particular RPM. I don't have a tach, but I'm guessing when my truck is doing 65mph I'm probably above 2500 RPM. That's another thing I'll need to figure out.

Anywho, am I in a safe range with those jets? I was just thinking that they may be too small ... but it looks like they are the stock jets for the '78 carb.

 

[mattressking]

Jets are a little small, you shouldn’t have pinging with 7* at sea level.

Try 138/200 or go a little higher. Is your vacuum at least 18in at idle?

 

[fyton2v]

When the engine is warm vacuum is 19+.

 

[fyton2v]

Ok now. I headed over to the local Harbor Freight and bought a fancy digital timing light, so I can more easily figure out the advance curve on this retard/advance distributor. With vacuum disconnected, I'm getting too much advance... I think. These figures aren't precise, but they're pretty close.

1000 rpm = 9*
1500 rpm = 15*
2000 rpm = 22*
2500 rpm = 25* ??
3000 rpm = 28* ??

Adding vacuum from carb to the retard port of the dizzy reduces all that by -6* starting above 1000 rpms. At 1500 rpms it's 9* advanced instead of 15*. Adding vacuum to the advancer port seems to add about +16*. So, for those counting along, at 2000 RPM in 4th gear (advance only kicks in in high gear ... because smog police) it's likely advanced 38*. Just a guess. This was all done in neutral, so the engine didn't need to work hard. It seems, however, that I'm a bit off the charts above 2000 and this is likely my problem. Not sure what my rpms are at 65 mph, but it's definitely above 2000.

Am I missing anything? Back several months I cleaned up my dizzy and added a 4mm bushing to the stop pin. It doesn't seem like it's helping. One thing I wonder is if it's assembled incorrectly. The cam plate has 2 notches where the stop pin does its thing. One notch is longer than the other. The stop pin is currently in the longer notch ... which I guess could be advancing timing further. Is it backwards?

Not my pitcure, but the is the notch I'm referring to. The other side of the plate has a smaller notch (in my situation).

 

[pb4ugo]

Vacuum disconnected and the line plugged? Those advance 's are what I would expect. Actually I would have expected 30*+, but it's a tractor motor. Total timing is typically considered base timing plus centrifugal advance all in at a rpm where timing doesn't advance any more. That's usually around 3000 rpm. So, limiting the centrifugal advance or retarding base timing is the same thing. Search the net for an rpm calculator to figure out what your rpms should be at a certain speed. Measure the actual size of your tires. A 4 speed in 4th is 1:1 ratio. Enter the speed you want and the calculator will predict the rpms.

 

[fyton2v]

Yes, both vacuum hoses were plugged with high performance golf tees.

I went ahead and reversed the notches just to see what would happen. The side with the larger notch had a 13 stamped on it and the other side had an 11. The stopper pin is now in the size 11 notch. I also reversed the springs so that the spring with the longer loop was on the side without the stopper pin. They were reversed before. This spring orientation is what is shown in the FSM picture, but no reference is made to why or which you should do.

New curve numbers look like this for mechanical advance:
1000 rpm = 10*
1500 rpm = 17*
2000 rpm = 23*
2500 rpm = 24*

About 24* seems to be the new limit. I read somewhere in ancient mud history that 24* was supposed to be about the factory dizzy limit. Who knows?

More importantly, the truck performs about the same but no longer pings under any condition. Even up a decent grade, WOT doing 65mph is good, for now. And according to the cluster thermostat, the truck is running cooler. I think I’m going to leave it like this for a bit and do 87 octane my next fill up and see what happens. I took my fancy new timing gun on the test run and 65 mph is about 2900 RPM.

I guess there is a reason behind two different advance notches. It would be interesting to know which one is used out of the factory.

 

[firemanj92]

nice detective work!

 

[zerotreedelta]

Seems close to the numbers for mechanical advance in the FSM, depending on flavor. Factory "base" was 7*, with 15-16* of mechanical advance adding up to 22* or so. If you're at a base of 10*, ending up at 24* seems reasonable. Bumping your base may have warranted reducing total advance, but seems like there was also more afoot.

(I haven't played with the mechanical dizzy a ton, but did dig quite a bit at timing curves for the Holley controlled Hyperspark).

 

[fyton2v]

That 2 degree difference in the "governor advance angle" seems to correspond to the notch sizes. Do we think the larger figure is a high altitude option?

 

[peesalot]

The pinging is because all the systems are advancing/fueling and the motor has not physically caught up. I second the retard the timing suggestion.

 

[fyton2v]

yeah, so retarding base timing will totally fix this, but it just causes another problem. I'll admit that I have a lot to learn here, but through bunch of experimentation on fuel grades and advance tweaking ... retarding timing later than 7 degrees leads to pretty poor motor performance. This truck has a vacuum advance AND retard dizzy. The retard function works in the lower gears. In 4th, a switch on the transmission tells the VSV to reroute vacuum to the advancer port. Going to something like 5* BTDC makes this a slow truck.



Slow enough where you know something just ain't normal. Flipping the limiter pin notch for the dizzy's centrifugal advance doohickey has solved the problem, I think. I'm almost certain. I have a good feeling that I can run 87 octane with no pinging while climbing a tree in 4th gear. That was the goal.

I have no idea how the notches ended up reversed. Perhaps the PO did it to do some high altitude wheeling, or perhaps I did it and just forgot. I knew squat about motors before I started working on this truck. The first few years I was just monkey with a hammer. I'm pretty happy right now because I've been chasing this particular problem ever since I got her road worthy.

If anyone in the Guru Club reads this and thinks I've made a terrible mistake, please say so. I'd be forever grateful.

Thanks all!

 

[brian]

Yes, both vacuum hoses were plugged with high performance golf tees.

I went ahead and reversed the notches just to see what would happen. The side with the larger notch had a 13 stamped on it and the other side had an 11. The stopper pin is now in the size 11 notch. I also reversed the springs so that the spring with the longer loop was on the side without the stopper pin. They were reversed before. This spring orientation is what is shown in the FSM picture, but no reference is made to why or which you should do.

New curve numbers look like this for mechanical advance:
1000 rpm = 10*
1500 rpm = 17*
2000 rpm = 23*
2500 rpm = 24*

About 24* seems to be the new limit. I read somewhere in ancient mud history that 24* was supposed to be about the factory dizzy limit. Who knows?

More importantly, the truck performs about the same but no longer pings under any condition. Even up a decent grade, WOT doing 65mph is good, for now. And according to the cluster thermostat, the truck is running cooler. I think I’m going to leave it like this for a bit and do 87 octane my next fill up and see what happens. I took my fancy new timing gun on the test run and 65 mph is about 2900 RPM.

I guess there is a reason behind two different advance notches. It would be interesting to know which one is used out of the factory.

the shaft needs to come out of the body to flip that, correct?.......ive been dealing the same scenero

 

[Malleus]

yeah, so retarding base timing will totally fix this, but it just causes another problem. I'll admit that I have a lot to learn here, but through bunch of experimentation on fuel grades and advance tweaking ... retarding timing later than 7 degrees leads to pretty poor motor performance. This truck has a vacuum advance AND retard dizzy. The retard function works in the lower gears. In 4th, a switch on the transmission tells the VSV to reroute vacuum to the advancer port. Going to something like 5* BTDC makes this a slow truck.



Slow enough where you know something just ain't normal. Flipping the limiter pin notch for the dizzy's centrifugal advance doohickey has solved the problem, I think. I'm almost certain. I have a good feeling that I can run 87 octane with no pinging while climbing a tree in 4th gear. That was the goal.

I have no idea how the notches ended up reversed. Perhaps the PO did it to do some high altitude wheeling, or perhaps I did it and just forgot. I knew squat about motors before I started working on this truck. The first few years I was just monkey with a hammer. I'm pretty happy right now because I've been chasing this particular problem ever since I got her road worthy.

If anyone in the Guru Club reads this and thinks I've made a terrible mistake, please say so. I'd be forever grateful.

Thanks all!

I this a California carb? You are using California gasoline, so no one outside of California is going to have the same results; unless you can find someone who doesn't have this problem and both of you use race gas (or avgas), I don't think you'll find a baseline to use to get away from your transient pinging.

The FSM doesn't allow for the crap gas California is now using, so you're kind of blazing new territory.

 

[fyton2v]

I'm at about sea level and using CA almost-gas. Because life isn't hard enough, this is a federal truck that will never be smog-exempt in this state. As Malleus said, in a bygone era with regular gas it may have been a non-issue.

Anyway, It's a pretty easy mod to the distributor. It's easier to do if you're working on a bench, but technically the dizzy doesn't need to be removed from the motor. I combed through a lot of pics of OEM Land Cruiser distributors and they don't all appear to have this swappable notch feature, so others may find different internals on older trucks.

Pull the cap and rotor. Remove the breaker assembly so you can get at the springs underneath. At the top of the shaft is a plastic cap. Remove the cap and remove the screw underneath. That screw secures what I guess is called the cam assembly to the shaft. Remove the springs and rotate the cam 180 degrees. The longer notch permits more centrifugal advance before the stop pin comes into play, the shorter notch does the opposite. If you're in there, ensure your stop pin has an intact bushing. Everyone seems to know that you can replace disintegrated stop bushings with a 4mm bushing that can be had at most hardware stores. The pin is a #8 screw size. That bushing also limits your max centrifugal advance.

Anyway, for my truck in my situation, the extra centrifugal advance plus vacuum advance in 4th gear must be taking me up to somewhere around 42*+ of total advance at near 3000 RPM. That just seems to be too much advance at my altitude and not great gas. Now I think my total advance at that RPM is around 38*.

 

[pb4ugo]

Idk, what the 2f or its distributor advance should be. I think you are on the right track. Zerotreedelta's reference of the fsm manual setting is a good starting point, then you advance your base setting +- to your liking. Pinging is bad. Probably the person you need to get a hold of is JimC at tlc performance to find out where the limits of centrifugal advance should be. I think he recurves distributors.

Maybe you know the following. Total advance is your base timing setting plus centrifugal advance. It is rpm dependant. Vacuum advance compensates for load and throttle input. It adjusts the input of your foot. It helps in fuel mileage. In a high load situation the throttle plates are close to wide open and vacuum in the engine is low, a vacuum leak is created thru the carb, air and fuel is being sucked in thru the throttle plates, vacuum advance input will be low. On level ground or going downhill you need minimal throttle input, if any, and the throttle plates are near closed so vacuum will be high and the vac advance will be near to maxed out because of the lack of load. Vac advance is only used in certain applications, a lot of engines use centrifugal advance only because they are under load most or all the time, like boats and service engines.

Do you know the history of the engine? Has the blk ever been decked or the bottom of the head been milled? Either of these things will increase the compression ratios and require higher octane fuel or possibly the timing to be retarded to eliminate pinging.

 

[fjc-man]

Ok now. I headed over to the local Harbor Freight and bought a fancy digital timing light, so I can more easily figure out the advance curve on this retard/advance distributor. With vacuum disconnected, I'm getting too much advance... I think. These figures aren't precise, but they're pretty close.

1000 rpm = 9*
1500 rpm = 15*
2000 rpm = 22*
2500 rpm = 25* ??
3000 rpm = 28* ??

Adding vacuum from carb to the retard port of the dizzy reduces all that by -6* starting above 1000 rpms. At 1500 rpms it's 9* advanced instead of 15*. Adding vacuum to the advancer port seems to add about +16*. So, for those counting along, at 2000 RPM in 4th gear (advance only kicks in in high gear ... because smog police) it's likely advanced 38*. Just a guess. This was all done in neutral, so the engine didn't need to work hard. It seems, however, that I'm a bit off the charts above 2000 and this is likely my problem. Not sure what my rpms are at 65 mph, but it's definitely above 2000.

Am I missing anything? Back several months I cleaned up my dizzy and added a 4mm bushing to the stop pin. It doesn't seem like it's helping. One thing I wonder is if it's assembled incorrectly. The cam plate has 2 notches where the stop pin does its thing. One notch is longer than the other. The stop pin is currently in the longer notch ... which I guess could be advancing timing further. Is it backwards?

Not my pitcure, but the is the notch I'm referring to. The other side of the plate has a smaller notch (in my situation).

View attachment 3276955

I believe the stop pin bushing repair / replacement is meant for 1980 and up “big cap” distributors. You’d have to ask JimC for a clarification. The centrifugal advance is allowed too much travel too quickly as well, leading to a total advance in the forties which is too much without this “bushing”.

 

[fyton2v]

I believe the stop pin bushing repair / replacement is meant for 1980 and up “big cap” distributors. You’d have to ask JimC for a clarification. The centrifugal advance is allowed too much travel too quickly as well, leading to a total advance in the forties which is too much without this “bushing”.

Pretty sure some of the slightly order medium cap distributors also suffered from the same stop bushing disintegration issue. Not 100% certain ... but almost. The pin is exactly the right size in any case.

Also, I can confirm that my pinging issue is resolved with a base timing of 7* BTDC. Hooray for 87 octane. I'm going to try advancing it until it pings and see where I land. Regarding the two different notches ... I've been thinking on them and I'm pretty sure my dizzy was rejiggered using the wrong notch and would have come from the factory configured with the smaller one. Not 100% certain about this either, but I think I'm correct. The only clue I have to go on is a note from the FSM. The FSM refers to assembling the dizzy using the notch marked with a 15. Mine doesn't have a 15. It has an 11 and a 13 (it was using 13). The only OEM units I can find online with a 15 also have a 17. I assume these refer to mm gaps. One notch is always 2mm larger than the other. And I guess you're supposed to use the smaller notch. That makes sense since you could use the other notch at higher altitudes. Anyhow, I think it's probably pretty rare for someone to be using the 2mm larger notch ... because why would you do that when you could just advance base timing? I'm pretty happy it's resolved.

 

[brian]

Pretty sure some of the slightly order medium cap distributors also suffered from the same stop bushing disintegration issue. Not 100% certain ... but almost. The pin is exactly the right size in any case.

Also, I can confirm that my pinging issue is resolved with a base timing of 7* BTDC. Hooray for 87 octane. I'm going to try advancing it until it pings and see where I land. Regarding the two different notches ... I've been thinking on them and I'm pretty sure my dizzy was rejiggered using the wrong notch and would have come from the factory configured with the smaller one. Not 100% certain about this either, but I think I'm correct. The only clue I have to go on is a note from the FSM. The FSM refers to assembling the dizzy using the notch marked with a 15. Mine doesn't have a 15. It has an 11 and a 13 (it was using 13). The only OEM units I can find online with a 15 also have a 17. I assume these refer to mm gaps. One notch is always 2mm larger than the other. And I guess you're supposed to use the smaller notch. That makes sense since you could use the other notch at higher altitudes. Anyhow, I think it's probably pretty rare for someone to be using the 2mm larger notch ... because why would you do that when you could just advance base timing? I'm pretty happy it's resolved.

on a 76 dissy, 10x8 assembled on the 10, new 3f dissy 11.5x11.5........im running the new 3f, and have been excepting the same issue you just corrected. i ended up swapping in the 10x8 on the 10 side...have not had a chance to really put it the test yet.