MP DISTRIBUTOR TIMING CURVE 340

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Bewy said:
What carb?
The engine is no longer near stock, so stock timing curves go out the window. That is why the engine is going to need 20+* for best idle.
If you want to dial in the timing curve with this modified engine, it has to be done on a dyno; anything else just becomes a good guess, which might be good enough.
I have run plenty of carbs with the t/slots closed off....& never had a problem. Many TQs & AVS carbs had bypass air holes in the base which can be blocked/reduced in size to correct the T slot position. The problem with T slots is when there is too much exposed at idle.
PCV can be run from front or rear of carb, whichever is more convenient. It is all manifold vacuum. On Pontiac V8s, the PCV does not connect to the carb. It goes into a junction in the intake, about 2" in front of the carb. The junction accesses both planes [ dual plane intake ].
Click to expand...
Carb is a 6139. I guess I'll revisit these questions when the actual 340 is on the stand. I just figured that I could get some insight on the current curve, and TQ characteristics using the "mule" 318 as a test pilot. At least the 340 should run good enough to break in with this combo.

Thanks for the input everyone. :)
 
MopaR&D said:
The Summit billet distributor I have has the same style of internals. I'm trying to wrap my head around how changing the amount of mechanical advance changes the initial spring tension...?
Click to expand...
It doesn't change with mallory style guts.
Here's the directions with curves. Every .010 in gap setting is approximately 1* advance at the crank.
 

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Moving right along...............

The 340 is on the stand and I was messing around with initial timing. It seems that this engine likes around 25* at idle (advanced timing until the rpm stopped going up, then backed it off a couple of degrees.

I tried manifold vacuum on the VC as @Bewy suggested. It added almost 20 degrees of advance, so I dropped my initial down to 5*. The mechanical is limited to 14* centrifugal for a total of 39* at 3000 RPM (No load).

So if I use the MV, do I set my initial at 5*, then figure out how to let my mechanical advance to add another 29* by 3000 rpm giving me 34* at said rpm while under load.......Then, once I am cruising (light load), the VC adds another 20* for a total of 54 at cruise (actually would be a little less as I will not be cruising at 3000 rpm.)

Am I getting this concept?
 
CFD244 said:
Moving right along...............

The 340 is on the stand and I was messing around with initial timing. It seems that this engine likes around 25* at idle (advanced timing until the rpm stopped going up, then backed it off a couple of degrees.

I tried manifold vacuum on the VC as @Bewy suggested. It added almost 20 degrees of advance, so I dropped my initial down to 5*. The mechanical is limited to 14* centrifugal for a total of 39* at 3000 RPM (No load).

So if I use the MV, do I set my initial at 5*, then figure out how to let my mechanical advance to add another 29* by 3000 rpm giving me 34* at said rpm while under load.......Then, once I am cruising (light load), the VC adds another 20* for a total of 54 at cruise (actually would be a little less as I will not be cruising at 3000 rpm.)

Am I getting this concept?
Click to expand...
Tried manifold vacuum on the valve cover, and added 20 degrees of advance ? Huh ?
Can you explain ? Any pics of the carb hooked up ?
 
Johnny Dart said:
Got any pics of what you are working on ?
Click to expand...
1971 340.....XE262 cam with OE Intake and exhaust manifolds and OEM Thermoquad.

Experimenting with ported vs non ported vacuum advance.

20231208_160311.jpg


20231208_160401.jpg
 
Would like to see a pic of how you got manifold vacuum to the VC ?

Are you using the timed port on the Thermoquad ?
 
Johnny Dart said:
Would like to see a pic of how you got manifold vacuum to the VC ?

Are you using the timed port on the Thermoquad ?
Click to expand...
There are 2 ports on the rear of the TQ. The one closest to the pull off supplies metered vacuum to the pull off/dashpot and the one beside that is manifold vacuum, and where the vc gets connected to. The timed port is at the front of the carb.,and in this case, gets plugged.

I am using a 6139.....Fuel metered 1972 model.
 
CFD244 said:
There are 2 ports on the rear of the TQ. The one closest to the pull off supplies metered vacuum to the pull off/dashpot and the one beside that is manifold vacuum, and where the vc gets connected to. The timed port is at the front of the carb.,and in this case, gets plugged.

I am using a 6139.....Fuel metered 1972 model.
Click to expand...
So do yourself a favor.
Hook the ported port on your carb, to the vacuum advance like it was designed. Set your timing between 12-14 initial 34-36 all in. And watch it purr like a kitten. And your cam is close to stock, so there should be no issues with idling.
 
CFD,
Post #30. Congrats!!! That is EXACTLY how you find the correct amount of idle timing.
Now for the next step:
- you can use any combination of MVA + init to get the 25* [ eg, 5 +20, 15 + 10, etc ]
- do not go less than 25*, but a few degrees more will not hurt.
- if the centri curve in the dist is ok, you can probably use the factory init [ static ] timing & not touch the dist.
- make sure you use an adj VA unit. Check that timing is steady at idle.
 
Johnny Dart said:
So do yourself a favor.
Hook the ported port on your carb, to the vacuum advance like it was designed. Set your timing between 12-14 initial 34-36 all in. And watch it purr like a kitten. And your cam is close to stock, so there should be no issues with idling.
Click to expand...
Agree with this approach as a starting point. The motor running on your run stand is under NO LOAD. Once in the car fine tuning can be tweeked under real world conditions. Under hood temperature and drivetrain load changes will dictate how much ignition advance the motor will tolerate without detonation. Another big variable is fuel octane available at the pump in your area.
 
CFD244 said:
Moving right along...............

The 340 is on the stand and I was messing around with initial timing. It seems that this engine likes around 25* at idle (advanced timing until the rpm stopped going up, then backed it off a couple of degrees.

I tried manifold vacuum on the VC as @Bewy suggested. It added almost 20 degrees of advance, so I dropped my initial down to 5*. The mechanical is limited to 14* centrifugal for a total of 39* at 3000 RPM (No load).

So if I use the MV, do I set my initial at 5*, then figure out how to let my mechanical advance to add another 29* by 3000 rpm giving me 34* at said rpm while under load.......Then, once I am cruising (light load), the VC adds another 20* for a total of 54 at cruise (actually would be a little less as I will not be cruising at 3000 rpm.)

Am I getting this concept?
Click to expand...
I think you're on the right track!
 
Bewy said:
CFD,
Post #30. Congrats!!! That is EXACTLY how you find the correct amount of idle timing.
Now for the next step:
- you can use any combination of MVA + init to get the 25* [ eg, 5 +20, 15 + 10, etc ]
- do not go less than 25*, but a few degrees more will not hurt.
- if the centri curve in the dist is ok, you can probably use the factory init [ static ] timing & not touch the dist.
- make sure you use an adj VA unit. Check that timing is steady at idle.
Click to expand...
How does the VA adjustment factor in?

I assume that you want it to come in quick as to get to 25* as soon as it starts......That would entail the adjustment screw all the way out. (weakest spring pressure)

But, on the other hand, you would want it to leave quickly upon acceleration.....That would entail the adjustment screw further in. (heavy spring pressure)
 
Looks great Troy. From what I've read/"learned", the VA will have a # stamped on it. Double that # and that's how many crank degrees it ads. The VA adjusts WHEN, not how much timing it ads.
Not sure if that helps.
 
4spdragtop said:
Looks great Troy. From what I've read/"learned", the VA will have a # stamped on it. Double that # and that's how many crank degrees it ads. The VA adjusts WHEN, not how much timing it ads.
Not sure if that helps.
Click to expand...
Yup...It adds about 20* and is stamped with an 11. I understand the when with regards to the VA can adjustment. The tighter the spring, the more opposing force applied to the diaphragm thus slowing it down as opposed to low tension on the spring doing the opposite.

However, it would appear that you would want a quick application at start up, but you also want it to leave quickly under light acceleration to avoid detonation. Just trying to figure out where the spring tension needs to be.
 
The VA spring [ rate ] adjustment using the Allen Key [ AK ] .

One of the biggest reasons MVA gets a bad rap such as, 'It didn't work for me' is because it was used with a low vacuum cam & the AK was not adjusted. More a problem with auto trans.

Turn the AK fully CW [ softest setting ].

Engine idling, in gear if auto, make sure the timing is steady. Job done.

If you get light throttle ping, turn the AK 2 turns CCW. Recheck timing. Keep going 2 turns CCW & rechecking timing. When timing becomes unsteady, go 3 turns CW.

With cams that have really low idle vac, it is better to use more initial & less MVA to get the desired idle timing.
 
Bewy said:
The VA spring [ rate ] adjustment using the Allen Key [ AK ] .

One of the biggest reasons MVA gets a bad rap such as, 'It didn't work for me' is because it was used with a low vacuum cam & the AK was not adjusted. More a problem with auto trans.

Turn the AK fully CW [ softest setting ].

Engine idling, in gear if auto, make sure the timing is steady. Job done.

If you get light throttle ping, turn the AK 2 turns CCW. Recheck timing. Keep going 2 turns CCW & rechecking timing. When timing becomes unsteady, go 3 turns CW.

With cams that have really low idle vac, it is better to use more initial & less MVA to get the desired idle timing.
Click to expand...

I agree with this approach. Another helpful technique is to use a mity-vac (hand vacuum pump) when adjusting the VA can as it will show what inHg corresponds to the amount of advance added. Use a vacuum gauge on the engine while driving to find what the manifold vacuum is at idle and cruising speeds and tune the VA accordingly. All done with the engine in the car of course, it's pointless to try to tune vacuum advance on an engine stand with no load. In my experience it's an iterative process (trial and error) to find what your engine likes best but can really make a difference in throttle response, driveability and fuel economy.
 
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