Ported Vaccuum?

[Odmark]

Since we are on the subject, does anyone know why my dist does that? Instantly going to 40° when att full vacuum.

When i hook it up to my ported slot i think i have it set up about 16 initial and 36 total. And i can adjust the total timing at the vacuum canister.

But at full manifols vacuum its just 40 initial and won't move at all when adjusting.:huh:

It is because when you hook the vacuum advance up to a vacuum source below the throttle blades it sees vacuum as soon as the engine is running.

Therefore whatever advance the vacuum can will allow is immediately pulled in and increases your initial timing.

Ignore my post, i realise now how stupid that was since i just read about it in 72HCodes post :whistling:

I think my real question is how do i tune it for manifold vacuum? Since it wont respond on adjusting at the vacuum canister.

Cant ride around with 40 initial :angel:

 

[TommyK]

Since we are on the subject, does anyone know why my dist does that? Instantly going to 40° when att full vacuum.

When i hook it up to my ported slot i think i have it set up about 16 initial and 36 total. And i can adjust the total timing at the vacuum canister.

But at full manifols vacuum its just 40 initial and won't move at all when adjusting.:huh:

It is because when you hook the vacuum advance up to a vacuum source below the throttle blades it sees vacuum as soon as the engine is running.

Therefore whatever advance the vacuum can will allow is immediately pulled in and increases your initial timing.

Ignore my post, i realise now how stupid that was since i just read about it in 72HCodes post :whistling:

I think my real question is how do i tune it for manifold vacuum? Since it won't respond on adjusting at the vacuum canister.

Cant ride around with 40 initial :angel:

You need to reduce your initial timing and/or limit the vacuum advance. Some vacuum advance units are adjustable. On the adjustable ones you insert a 3/32"? allen key through the nipple where the vacuum line goes and you can change how much vacuum is added. I had to make a stop for the arm inside the distributor to get mine where it needed to be.

 

[Odmark]

It is because when you hook the vacuum advance up to a vacuum source below the throttle blades it sees vacuum as soon as the engine is running.

Therefore whatever advance the vacuum can will allow is immediately pulled in and increases your initial timing.

Ignore my post, i realise now how stupid that was since i just read about it in 72HCodes post :whistling:

I think my real question is how do i tune it for manifold vacuum? Since it won't respond on adjusting at the vacuum canister.

Cant ride around with 40 initial :angel:

You need to reduce your initial timing and/or limit the vacuum advance. Some vacuum advance units are adjustable. On the adjustable ones you insert a 3/32"? allen key through the nipple where the vacuum line goes and you can change how much vacuum is added. I had to make a stop for the arm inside the distributor to get mine where it needed to be.

I have an adjustable one, thats what i mean by adjusting at the vacuum canister. When i have it hooked up at the ported nipple i can adjust the total timing but when att full vacuum it does not respond at all.:huh:

 

[Stanglover]

Ignore my post, i realise now how stupid that was since i just read about it in 72HCodes post :whistling:

I think my real question is how do i tune it for manifold vacuum? Since it won't respond on adjusting at the vacuum canister.

Cant ride around with 40 initial :angel:

You need to reduce your initial timing and/or limit the vacuum advance. Some vacuum advance units are adjustable. On the adjustable ones you insert a 3/32"? allen key through the nipple where the vacuum line goes and you can change how much vacuum is added. I had to make a stop for the arm inside the distributor to get mine where it needed to be.

I have an adjustable one, thats what i mean by adjusting at the vacuum canister. When i have it hooked up at the ported nipple i can adjust the total timing but when att full vacuum it does not respond at all.:huh:

To recap on what I have done so far on my engine, '71 351C 4V manual trans, which is not yet perfect, but getting there.

The stock factory reman. distributor is set up with an L10 slot, 20* ( which I had to weld and recut myself) with 1 stronger spring on the left side and a Mr. Gasket spring on the right (can't offhand remember the number). This gives me a fairly good curve, but not yet perfect. All in at about 3000rpm. The initial is 14* on timed ported vacuum on the Holley 670 4160 carb. The vacuum can is a new replacement, likely Chinese!! as it has a metric adjuster screw, 3mm I think. The engine has approx 10:1 pistons and a slightly better than stock cam, but even on 91 Shell gas, best I can get, no ethanol, I get some minor pinging with much more advance. I only have about 4* vacuum advance on the can, for a total of 40*. The screw is turned out anti-clockwise 12 turns from all in clockwise. This seems way off the recommended 3-4 turns, but it seems to work for my motor. The next step when I get the car on the road again after winter, will be to cut the mechanical down a tad more with a sleeve on the slot pin and increase the initial to 16*, play with springs again and then use a fuel/air meter to get the carb right as well. It all works together. This is from what I have learned here and figured out for myself, so not necessarily what others will agree with.

Hope that isn't too confusing and good luck with it.

 

[basstrix]

I think my real question is how do i tune it for manifold vacuum? Since it won't respond on adjusting at the vacuum canister.

Cant ride around with 40 initial :angel:

It may be that the level of manifold vacuum is greater than ported. If you have a vacuum gauge, you can answer that quickly. The premise is that you haven't adjusted the canister enough to back off the max advance at the (suspected) higher vacuum level.

 

[72HCODE]

high timing at idle is not a real issue because there is no load. the determination for a real problem will be if you hear the engine ping/detonate(which you can if you have a problem)

the second the motor starts to load down the vac drops, reving the motor in park or neutral in the driveway and watching the timing go to 56 degrees as example tell you nothing.

it is what the car does when actually driven that tell you if you have too much timing.

the first thing you do with timing is disable the vacuum advance.

check mechanical advance and see how the car is running.

if you are pinging on the mechanical then you have a few options

1) lower timing

2) add more fuel

3) adjust the springs on the mechanical to change the advance delay.

4) reduce mechanical range(install a rubber vacuum hose piece on the stopper.

once mechanical is sorted then you bring in vacuum advance.

on the vacuum advance you have adjust ability with an a replacement diaphragm.

additionally there are Vacuum delay devices that can be installed inline to the advance that basically start to simulate ported type of vacuum.

seeing the motor rev'd to 4000rpms in the driveway with timing going to 40-50 is again ok as there is NO load on the engine.

you would want to disconnect the vacuum advance and repeat the same test on mechanical only to get a real indication of your timing end point. the issue is heavy load and high timing, like stomping on it 0-100mph and you miss a shift go to red line on heavy load and grenade the engine. at WOT(wide open throttle) the vaccum is going to be low so vacuum advance will turn off and you will be on pure mechanical advance that is where you would like to see the timing at 30-38 degrees.

once sorted out with mechanical you start adjusting the vaccum advance until the engine starts to ping then back off 2 turns for a good safety margin.

as for what to listen for, it literally sounds like you took marbles and threw them into the muffler, you get this TATATATA noise. you will then start to get lean misfire you stomp on it and the engine cuts off then suddenly goes. that is the motor going lean, if you kept going next would be massive carb backfire usually blowing off the air cleaner and then you start to damage pistons.

the opposite end of the spectrum is going super rich then when you stomp on it the engine goes, cuts off then goes, and either you blow massive Black smoke out the tailpipe or massive backfire out the tailpipe.

so rich tailpipe backfire, lean carb backfire.

carb backfires are really scary especially when you are standing over the carb making an adjustment and you blip the throttle and lose eyebrows and go deaf.

 

[Stanglover]

72HCODE, you should write a book on timing!! I learn from you every time you post on it.

Geoff.

 

[Twins]

72HCODE, you should write a book on timing!! I learn from you every time you post on it.

Geoff.

I can vouch for his skills. 72HCODE squared away the timing on my car himself.

 

[4Vforever]

high timing at idle is not a real issue because there is no load. the determination for a real problem will be if you hear the engine ping/detonate(which you can if you have a problem)

the second the motor starts to load down the vac drops, reving the motor in park or neutral in the driveway and watching the timing go to 56 degrees as example tell you nothing.

it is what the car does when actually driven that tell you if you have too much timing.

the first thing you do with timing is disable the vacuum advance.

check mechanical advance and see how the car is running.

if you are pinging on the mechanical then you have a few options

1) lower timing

2) add more fuel

3) adjust the springs on the mechanical to change the advance delay.

4) reduce mechanical range(install a rubber vacuum hose piece on the stopper.

once mechanical is sorted then you bring in vacuum advance.

on the vacuum advance you have adjust ability with an a replacement diaphragm.

additionally there are Vacuum delay devices that can be installed inline to the advance that basically start to simulate ported type of vacuum.

seeing the motor rev'd to 4000rpms in the driveway with timing going to 40-50 is again ok as there is NO load on the engine.

you would want to disconnect the vacuum advance and repeat the same test on mechanical only to get a real indication of your timing end point. the issue is heavy load and high timing, like stomping on it 0-100mph and you miss a shift go to red line on heavy load and grenade the engine. at WOT(wide open throttle) the vaccum is going to be low so vacuum advance will turn off and you will be on pure mechanical advance that is where you would like to see the timing at 30-38 degrees.

once sorted out with mechanical you start adjusting the vaccum advance until the engine starts to ping then back off 2 turns for a good safety margin.

as for what to listen for, it literally sounds like you took marbles and threw them into the muffler, you get this TATATATA noise. you will then start to get lean misfire you stomp on it and the engine cuts off then suddenly goes. that is the motor going lean, if you kept going next would be massive carb backfire usually blowing off the air cleaner and then you start to damage pistons.

the opposite end of the spectrum is going super rich then when you stomp on it the engine goes, cuts off then goes, and either you blow massive Black smoke out the tailpipe or massive backfire out the tailpipe.

so rich tailpipe backfire, lean carb backfire.

carb backfires are really scary especially when you are standing over the carb making an adjustment and you blip the throttle and lose eyebrows and go deaf.

Great write up and very easy for even the novice to understand. These write ups should really help those out to understand the workings and be able to fix and adjust correctly their timing woes themselves. Once again great write up, well done.