360 Tune Up

[Rock Able]

I put a 360 in my 41 Plymouth. It is mated to an AX-15 5 speed and 3.55 Dakota rear. I was expecting to get at least 18 mpg and maybe 20. So far, that is not the case. Here is what I know about the engine.

It is slightly oveerbored and has enough compression that I have to run 93 octane or it sounds like a bag of hammers when I step on it. It's great on 93. It idles smoothly and has great low end torque. I suspect a mostly stock cam profile. It has an Edelbrock Performer intake and Edelbrock 1406 carb. I put a 1/2" wood spacer plus the Edelbrock insulator on it to try to keep the fuel from boiling after shut down. I had to richen up the cruise mode jetting to avoid a cold stumble. I recently richened it up one more step to a #22 setup.


I have a Mopar mag pickup distributor on it with a MSD 6AL CD box and MSD blaster coil. The current distributor is set at 34 BTDC with the vacuum disconnected and has about 17 degrees of vacuum advance. It runs great but I would like to get decent mileage since I road trip this car.

Also, I have been trying to get the temperature to hang close to my 180 degree thermostat on a hot day. That was the reason for richening it up an extra step. Road tests indicated it was still a little lean.

Any advice you guys can give me is appreciated.

 

[RSie]

I put a 360 in my 41 Plymouth. It is mated to an AX-15 5 speed and 3.55 Dakota rear. I was expecting to get at least 18 mpg and maybe 20. So far, that is not the case. Here is what I know about the engine.

It is slightly oveerbored and has enough compression that I have to run 93 octane or it sounds like a bag of hammers when I step on it. It's great on 93. It idles smoothly and has great low end torque. I suspect a mostly stock cam profile. It has an Edelbrock Performer intake and Edelbrock 1406 carb. I put a 1/2" wood spacer plus the Edelbrock insulator on it to try to keep the fuel from boiling after shut down. I had to richen up the cruise mode jetting to avoid a cold stumble. I recently richened it up one more step to a #22 setup.


I have a Mopar mag pickup distributor on it with a MSD 6AL CD box and MSD blaster coil. The current distributor is set at 34 BTDC with the vacuum disconnected and has about 17 degrees of vacuum advance. It runs great but I would like to get decent mileage since I road trip this car.

Also, I have been trying to get the temperature to hang close to my 180 degree thermostat on a hot day. That was the reason for richening it up an extra step. Road tests indicated it was still a little lean.

Any advice you guys can give me is appreciated.

I have no idea about solving your problem.. but I'm sure there are some on here that will help you out. I just want pics of this Plymouth

 

[MOPAROFFICIAL]

I put a 360 in my 41 Plymouth. It is mated to an AX-15 5 speed and 3.55 Dakota rear. I was expecting to get at least 18 mpg and maybe 20. So far, that is not the case. Here is what I know about the engine.

It is slightly oveerbored and has enough compression that I have to run 93 octane or it sounds like a bag of hammers when I step on it. It's great on 93. It idles smoothly and has great low end torque. I suspect a mostly stock cam profile. It has an Edelbrock Performer intake and Edelbrock 1406 carb. I put a 1/2" wood spacer plus the Edelbrock insulator on it to try to keep the fuel from boiling after shut down. I had to richen up the cruise mode jetting to avoid a cold stumble. I recently richened it up one more step to a #22 setup.


I have a Mopar mag pickup distributor on it with a MSD 6AL CD box and MSD blaster coil. The current distributor is set at 34 BTDC with the vacuum disconnected and has about 17 degrees of vacuum advance. It runs great but I would like to get decent mileage since I road trip this car.

Also, I have been trying to get the temperature to hang close to my 180 degree thermostat on a hot day. That was the reason for richening it up an extra step. Road tests indicated it was still a little lean.

Any advice you guys can give me is appreciated.

Funny on how a hot day the air can get thin and adding fuel only makes for a slower burn and more heat. When it gets really hot out usually you want to lean it out a little bit and if it gives you trouble still, it's probably needing a little timing taken out. Now consider your two problems are related, bag of hammers timing setting and that running hot
Jmo

 

[rumblefish360]

What is your overdrive ratio? Tire size? Do you know the total amount the distributor is pulling? 52*’s?
What mileage are you getting now?

 

[Rock Able]

I have no idea about solving your problem.. but I'm sure there are some on here that will help you out. I just want pics of this Plymouth

Hot Rods - Finished my 41 Plymouth

 

[Rock Able]

What is your overdrive ratio? Tire size? Do you know the total amount the distributor is pulling? 52*’s?
What mileage are you getting now?

Overdrive is .79.
Tires are 225/70/15.
34 +17 = 51 degrees.
15-16 mpg

 

[RSie]

Hot Rods - Finished my 41 Plymouth

Wow! Sweet ride!

 

[rumblefish360]

Wow, that equals a 2.80 final drive ratio and your tires aren’t super tall so cruise rpm has to be on the low-ish rpm side?

18 isn’t to bad. You should be able to squeak out another 1 or maybe 2.

Personally I think your down to fiddling around to see what it likes best on the carb & distributor. 34 + 17 = 51 but I ask if it was measured and verified. Just Incase.

The best I ever did was 21 on my 360. Backed by a 904 & 2.76 gears and a 235/60/15 tire, or 26 inch tire. This was with better gas than I get anywhere at home. Seems when I’d hit the road, out of NY, the fuel quality increased and it shown that n the odometer.

The best mileage on could get with the OE spark box was about 17/18. Adding the MSD 6A added another 1-1.5 mpg. At home, 19-1/2 was as good as it ever got. Absolutely the best.

My compliments to your ride. Very nice, very nice.

 

[Rock Able]

I tried going back to the #1 base setup for my carb but it stumbles on part throttle acceleration, so I went back to 4% richer to eliminate the stumble. Once I burn this tank of premium, I'm going to put some 89 octane in, back down the timing 2 degrees and see what happens with the plunging. It's supposed to be in the 90's here this weekend, so I can do some more testing after I change fans.

 

[yellow rose]

I tried going back to the #1 base setup for my carb but it stumbles on part throttle acceleration, so I went back to 4% richer to eliminate the stumble. Once I burn this tank of premium, I'm going to put some 89 octane in, back down the timing 2 degrees and see what happens with the plunging. It's supposed to be in the 90's here this weekend, so I can do some more testing after I change fans.

I would NOT fatten it up at cruise to cover a stumble. You need a bigger accelerator pump shot.

All you are doing is making it rich at cruise for no reason.

 

[MopaR&D]

I had to richen up the cruise mode jetting to avoid a cold stumble.

The only thing on the carb you're supposed to mess with when it's cold is the choke... any other tuning you do when it's fully warmed up. As long as it isn't so bad that it stalls out, the way it runs when cold shouldn't really matter. My carb (750 Street Demon) has somewhat sluggish response and sometimes hesitates until the engine is fully warmed up then it feels almost as responsive as EFI. Before the SD I had a 625 cfm Carter AFB on my old 360, when I used my wideband to tune that for a leaner cruise (I took it from 13.5:1 to 15.5-16:1 at cruise) it ran kinda crappy when cold.

Lean the primary circuit back out and move the accelerator pump linkage to the hole closer to the carb body, there should be 3 positions.

 

[Rock Able]

Ok. I will try that. I already had the accelerator pump linkage in the short side hole.
Thanks.

 

[rumblefish360]

Ok. I will try that. I already had the accelerator pump linkage in the short side hole.
Thanks.

Short-side hole? Is that the hole lowest to the carb body? If so, you may need to ether and or do both below.

1: Re-bend the linkage rod to adjust the plungers height so the proper stroke to the plunger is correct.

2: Change out the pump shot cluster. Order a set. They come 3 to a set. 2 larger than stock, 1 smaller.

 

[Rock Able]

Short-side hole? Is that the hole lowest to the carb body? If so, you may need to ether and or do both below.

1: Re-bend the linkage rod to adjust the plungers height so the proper stroke to the plunger is correct.

2: Change out the pump shot cluster. Order a set. They come 3 to a set. 2 larger than stock, 1 smaller.

It is the hole nearest the carburetor body.

 

[yellow rose]

It is the hole nearest the carburetor body.

They also make pump nozzles with bigger holes in them. A bigger nozzle gives more fuel for less duration where a smaller nozzle gives less fuel for a longer duration.

The linkage controls how much the pump is depressed. You need to work with both, the linkage and the nozzle size.

 

[Mattax]

I put a 360 in my 41 Plymouth. It is mated to an AX-15 5 speed and 3.55 Dakota rear. I was expecting to get at least 18 mpg and maybe 20. So far, that is not the case. Here is what I know about the engine.

I can't say if that's realistic, but its a great goal However in my experience as long as you are tuning and testing mpg will be low. So don't sweat it too much right now.

I had to richen up the cruise mode jetting to avoid a cold stumble.

I agree with MopaR&D. A cold engine, a warm engine, and a fully heat soaked engine are three different things. Make adjustments to carb with warmed and fully heat soaked for tuning. Choke for cold. Choke setting are the hardest thing to nail on a hot rod.

Road tests indicated it was still a little lean.

Indicated how? At higher speed cruise, too lean will show as engine reving and dying.

It is slightly oveerbored and has enough compression that I have to run 93 octane or it sounds like a bag of hammers when I step on it. It's great on 93.

OK. Sounds like timing is about on the edge, at least for some conditions.

I put a 1/2" wood spacer plus the Edelbrock insulator on it to try to keep the fuel from boiling after shut down.

You may be still on winter fuel. June 1 all stations in the US will be back to summer fuel.

The current distributor is set at 34 BTDC with the vacuum disconnected and has about 17 degrees of vacuum advance.

These numbers look OK but need to get the rpm and vacuum to really know.
If the mechanical advance is too quick, then it will ping at part throttle loads when used with a vacuum advance.
34* by 2400 rpm, plus vacuum advance of 17* is very different than 34* at 3400 rpm with another 17* added by vacuum.
Stick a golf tee the vac hose. Then check the timing at the slowest rpm and every 250 or 500 rpm until 3000 or it stops advancing.
Result will look something like this when graphed.

Go to section 3 here for Mopar Performance's baseline for timing.
The Mopar Performance baseline timing recommendation assumes a timing curve like the one posted above.
That particular distributor idled around 12* BTC when set at 31* at 3000 rpm. On a stock 340 or 360 that would be OK. On a hot rodded engine, 16 to 20* might be needed depending on the cam and compression. That's one reason why I wasn't able to get a timing below 900 rpm. The engine I had it on needs around 17* initial.

 

[MopaR&D]

Mattax brings up a good point, what RPM is your 34* BTDC at? If that's at idle then no wonder you have pinging issues. If the cam is close to stock as you say you need more like 12* idle advance and the 34* shouldn't be until at least 2500 RPM. Also is your vacuum advance canister hooked to manifold or ported vacuum?

 

[Rock Able]

Thanks for the challenge, guys. I had already checked this distributor and since it was new, I ASSumed it was ok. After mapping the centrifugal advance curve, I plugged in the vacuum line to find that the diaphragm was open. No vacuum advance. So, I'm starting over literally. Fortunately, I had another distributor and mapped it. I also returned the Edelbrock 1406 to base #1 tune.

The 2nd distributor has the following map.

1. Idle (750 rpm) 7 degrees
2. 1000 rpm - 12 degrees
3. 1500 rpm - 24 degrees
4. 2000 rpm - 26 degrees
5. 2500 rpm - 28 degrees
6. 3000 rpm - 31 degrees
7. 3300 rpm - 34 degrees

The vacuum canister on this one has 15 degrees of advance. I get 17-18" of vacuum at 750 rpm and 22" at 2000 rpm. Vac advance is hooked to manifold vacuum.

The lack of vacuum advance would explain the higher temps cruising under load at 2400 rpm or so. Correct? It is supposed to get hot this weekend. So, I plan to do some more temperature testing. I have a new electric fan coming today and hope to get it installed tomorrow. Mileage tuning may have to come later.

What is the recommended amount of total centrifugal advance and the recommended total of vacuum advance? Is 34 BTDC about right?

Thanks for your help!

 

[MopaR&D]

Thanks for the challenge, guys. I had already checked this distributor and since it was new, I ASSumed it was ok. After mapping the centrifugal advance curve, I plugged in the vacuum line to find that the diaphragm was open. No vacuum advance. So, I'm starting over literally. Fortunately, I had another distributor and mapped it. I also returned the Edelbrock 1406 to base #1 tune.

The 2nd distributor has the following map.

1. Idle (750 rpm) 7 degrees
2. 1000 rpm - 12 degrees
3. 1500 rpm - 24 degrees
4. 2000 rpm - 26 degrees
5. 2500 rpm - 28 degrees
6. 3000 rpm - 31 degrees
7. 3300 rpm - 34 degrees

The vacuum canister on this one has 15 degrees of advance. I get 17-18" of vacuum at 750 rpm and 22" at 2000 rpm. Vac advance is hooked to manifold vacuum.

The lack of vacuum advance would explain the higher temps cruising under load at 2400 rpm or so. Correct? It is supposed to get hot this weekend. So, I plan to do some more temperature testing. I have a new electric fan coming today and hope to get it installed tomorrow. Mileage tuning may have to come later.

What is the recommended amount of total centrifugal advance and the recommended total of vacuum advance? Is 34 BTDC about right?

Thanks for your help!

Looks OK for the centrifugal advance, I think it would be better to limit it by about 5 degrees so you can have more initial timing. Also the vacuum advance should be hooked up to ported vacuum, not manifold. I recently helped a friend with a stock 360 in a '79 pickup that was running funny; didn't idle well and had some hesitation when accelerating along with bad gas mileage. Turned out the vacuum advance was hooked up to manifold vacuum; once I switched it to ported and reset the base idle advance the idle smoothed out and it was much more responsive and nicer to drive. Some engines can use manifold vacuum advance but from my experience stock or near-stock Mopar engines really don't like it.

 

[Mattax]

I plugged in the vacuum line to find that the diaphragm was open. No vacuum advance.

The vac cans are relatively easy to swap out. First couple times are the hardest, like most things.

The lack of vacuum advance would explain the higher temps cruising under load at 2400 rpm or so. Correct?

Yes it might.

It is supposed to get hot this weekend. So, I plan to do some more temperature testing. I have a new electric fan coming today and hope to get it installed tomorrow.

Are you running mechanical or electric fans now? In my opinion, the mechanical fans will generally get the best cooling. I think in part because the shrouding for electrics blocks too much of the airflow at speed. With mechanical, a hood seal and fan shroud help a lot at idle. I realize with your car, there just may not be room. So this may all be impractical blather.

What is the recommended amount of total centrifugal advance and the recommended total of vacuum advance? Is 34 BTDC about right?

At 2800 to 3000 rpm it would be about right for a typical iron headed LA. Click that link in my earlier post. Use the Direct Connection recommendations reprinted there. If its too small to read let me know. Next post we can look specifically at the timing curve you measured.

 

[Rock Able]

I'm running electric fan and that is all I can use.
Thanks for the link. I will read it later and experiment with the curves.This is what I needed as all my previous experience was with SBC's. The vacuum can on the unit I installed today is adjustable. The previous one was not.

I usually use manifold vacuum on SBC's. Why would ported be recommended on SB Mopars? My engine is idling fine.

 

[Mattax]

The 2nd distributor has the following map.

1. Idle (750 rpm) 7 degrees
2. 1000 rpm - 12 degrees
3. 1500 rpm - 24 degrees
4. 2000 rpm - 26 degrees
5. 2500 rpm - 28 degrees
6. 3000 rpm - 31 degrees
7. 3300 rpm - 34 degrees

This looks like a typical early smog curve. I don't have a mid 70s or early 80s book. Those might be similar, I just don't know.
It's not that different than the timing for the late 60s 340s. Those started with an initial around 5* (for reduced CO) and started quickly advancing right off idle ( around 700 rpm) to get to the timing that would be more efficient for driving. With a stock engine that timing should work OK. In terms of efficiency, a little more initial would probably produce more power at and put less heat into cylinder walls and exhaust at idle. To keep the rest of the timing the same with higher initial, the inside of the advance slots are shortened .

The vacuum canister on this one has 15 degrees of advance. I get 17-18" of vacuum at 750 rpm and 22" at 2000 rpm. Vac advance is hooked to manifold vacuum.

That seem like pretty good vacuum at idle. Maybe too much to get away with using manifold vacuum on the vacuum advance. Its going to depend on the cam and build etc.

The Direct Connection guidelines for setting vacuum advance require checking the engine's manifold vacuum at your typical highway cruising rpm. Then adjusting the vacuum advance as needed to get the mechanical timing plus vacuum timing so it will be around 50 (go check) at 2600 rpm or so.
So for your current setup, it looks like having the vac advance adding about 20* would be about right. Just need to find out the vacuum 20* should be added at.

 

[MopaR&D]

I'm running electric fan and that is all I can use.
Thanks for the link. I will read it later and experiment with the curves.This is what I needed as all my previous experience was with SBC's. The vacuum can on the unit I installed today is adjustable. The previous one was not.

I usually use manifold vacuum on SBC's. Why would ported be recommended on SB Mopars? My engine is idling fine.

Honestly I wish I knew a solid answer to that, my best guess has to do with SBM's having more efficient cylinder filling and dynamic compression at idle and low RPMs. I haven't played with many Chevies but I have heard SBCs do prefer manifold vacuum advance. I just know every time I've tried to use manifold vacuum on a Mopar engine it ran worse.

 

[Mattax]

I usually use manifold vacuum on SBC's. Why would ported be recommended on SB Mopars? My engine is idling fine.

As far as I can tell it most has to do with the mechanical advance curve. Generally you'll find the factories prefered using ported. Doing so keeps the two advances related to their jobs. The mechanical advance main job is mostly about time in fractions of a second. Its job is to provide more lead time as increasing rpm brings the piston up quicker and quicker. It's not a 1:1 relation because the combustion efficiencies change as rpm increases. The efficiencies are different for every engine.

The vacuum advance's main job is to provide more time as the combustion is slowed due to lower fuel density. So leaner conditions require more time for combustion pressure to be maximized through the crank angles of maximum leverage. In terms of power efficiency engines run best at relatively rich idle mixtures. So ignoring emmissions considerations, timing at idle should be around 10-14* BTC at 550 - 650 rpm for best efficiency on these engines (stock). You can see that looking in the 67 and earlier service manuals.

This is somewhat related to the fact that most of us at one time or another came to believe that ported timing was an emissions thing and manifold was the way to hot rod things. But its not true.

One way we (seems like I'm not the only one) would fool ourselves was when we noticed that the engine at idle seemed to like more timing when we turn the distributor. The engine will run faster and faster. But there's no load on the engine when we're doing this, so it can do this and run lean. In gear - which is easy to check with an automatic but of course not with a standard - its different story.

 

[crackedback]

If you have 7* initial on it, the off idle stumble is likely rooted in the lack of initital timing. Set initial at 12-16 and see if stumble gets better. Readjust the idle speed and idle mix screws before testing it out.

More initial means less mechanical to keep total without vac advance in bounds. So the fun breaking down the distributor and limiting mech advance begins.