Vaccum lines at air plenum; Tachometer

[tony-muscle]

Here I go again with a few questions about my 1971 Mach 1 with a 351C and added RAM-air:

1- I am trying to understand the two vacuum lines that are connected under my air filter plenum. They seem to connect to a small square adapter that sticks on the inside of the plenum. See two pictures below. The issue is that the nipples where the vacuum lines connect don't seem to have an opening. I wonder if they were painted over. One of the lines go to the flap in the snorkel of the air filter plenum. I assume this is used to control the temperature of the air inlet by allowing the air to either go directly or being heated through the exhaust. I don't see this flap moving.

2- My tachometer seems to mark up to about 2,800 RPM. At that point if fluctuates and doesn't move up even though the engine is revving. I have only checked the RPM reading at idle with my timing gun ant it matchse, but I don't know if the reading is good after that. Why will it stop at 2,800 RPM?

THanks.

 

[72HCODE]

that connection is for the snorkel.

works like this. that double vacuum connector has a Bi-metal switch inside. when the intake manifold gets hot enough it opens the switch and that flows vacuum from one side to the other.

so one connection goes to full manifold vacuum. the other goes to the snorkel, when the engine is cold, the snorkel door is closed and it allows warm air from the heat riser over the exhaust manifold to rise into the air cleaner and down the intake. this warm air helps heat up the engine to proper operating temperature.

when the engine reaches temperature and the intake manifold transfers heat to the air cleaner base the switch opens and vacuum goes to the snorkel vacuum motor it then opens which shuts air from the heat riser and allows cooler air from the top of the engine bay to feed into the motor intake. it is kind of a warm/cold air intake system.

now if you Don't have the snorkel then you don't have to hook up that heat sensor to anything, you can just leave it disconnected.

Ram air works like an Aux port on the V4 air cleaners, so it only comes on when Vac drops to 0HG" otherwise the flappers are held closed all the time, and the engine is feeding from the internal snorkel. the idea is when you stomp on it then the ram air flappers open and allow cold air right into the engine. once you reach cruise, the ram air shuts down and you feed air from the snorkel as either warm air when the engine is cold or cooler air when the engine is warmed up.

here is a vac diagram

issue:

#2 do you have an aftermarket ignition system like MSD?

 

[tony-muscle]

Thanks for such a detail explanation. I did not see that flap close when i started the engine yesterday. Do you happen to know at what temp does it trigger? Also, if you install headers, does the hot air riser stays?

issue:

#2 do you have an aftermarket ignition system like MSD?

There is a Pertronix II module.

 

[72HCODE]

It takes a while for the switch to activate the engine has to reach operating temperature.

Basically go for a drive for 10-15 minutes then pop the hood and look at the snorkel.

The flap should open.

Now the switch can be bad or you have a vacuum leak at the vacuum motor.

If you have headers you loose the heat riser shield and there is no where for the riser tube to attach to.

Thanks for such a detail explanation. I did not see that flap close when i started the engine yesterday. Do you happen to know at what temp does it trigger? Also, if you install headers, does the hot air riser stays?

issue:

#2 do you have an aftermarket ignition system like MSD?

There is a Pertronix II module.

 

[tony-muscle]

Now the switch can be bad or you have a vacuum leak at the vacuum motor.

Is 12 mmHg too low for vacuum? Thats the highest i was able to achieve at idle after adjusting mixture.

 

[Bill73Ragtop]

Typical vacuum should be 16 - 18. If you have a radical cam, that could affect vacuum.

As for the tach, it sounds like an internal electrical issue with the tach itself.

 

[tony-muscle]

Typical vacuum should be 16 - 18. If you have a radical cam, that could affect vacuum.

These are the specs of my cam. It is not a radical cam, but not stock. Do you think I may have a vacuum leak somewhere?

As for the tach, it sounds like an internal electrical issue with the tach itself.

Does that mean a new tach? or I should try opening it to see if I find something obvious inside.

 

[turtle5353]

If your gonna pull your tach send it out to Rocketman. He does a great job with them. And hes a member on here.

http://www.rccinnovations.com/index.php?show=menu-tach-all

 

[tony-muscle]

issue:

#2 do you have an aftermarket ignition system like MSD?

I have the Pertronix II ignition module. After your question I did some internet searching and read some people having issues with bouncy tach needles. My needle actually becomes bouncy when it approaches 2,750 rpm. So, it may be that I need what they call a tach filter.

This links shows the setup, http://www.camaros.net/techref/electrical/tachfilter/index.htm

And this other link tells you what size resistor and capacitor are needed: http://www.camaros.org/forum/index.php?action=printpage;topic=8132.0

From the link above, I will need "a 270 ohm, 1/2 watt resister, a 1 ufd electrolytic capacitor"

Has any of you done this or had this issue? or this is only for Chevy

 

[Bill73Ragtop]

I've been running a Pertronix for over a decade and the tach works perfectly. With winter approaching, it may be well worth having Rocketman check it out and refurbish anything it may require.

I'll let others with more technical knowledge comment on the cam profile, but I ran a similar hydraulic Boss grind back in the 80's - 90's in a 71 M-code and the best I could do was 13 - 14 inches of vacuum. Had a lope at idle, but once you passed 3,000 rpm it got real interesting.

 

[tony-muscle]

I've been running a Pertronix for over a decade and the tach works perfectly. With winter approaching, it may be well worth having Rocketman check it out and refurbish anything it may require.

I'll let others with more technical knowledge comment on the cam profile, but I ran a similar hydraulic Boss grind back in the 80's - 90's in a 71 M-code and the best I could do was 13 - 14 inches of vacuum. Had a lope at idle, but once you passed 3,000 rpm it got real interesting.

Yeah, mine also has a lope at idle but have been afraid to rev it too much due to the tach not working and still getting familiar with it. It may be that 12 vacuum is what I should expect. The vacuum lines look in a generally good shape.

Bill, with that cam did you set idle a little higher than the 800rpm (600 with trans engaged)? Also, what idle timing advance did you use?


Let's try to expand the questions in this thread. When I start the car in the morning there is a wet black path behind the exhaust that dirties the garage. I assume is the water condensation being pushed out with some of the exhaust "soot". If this is normal, how do you guys deal with it? or do I have to get used to clean it every time I start the car? Also, the exhaust has a "stronger" smell than newer cars. I sort of remember this smell from the older days so I assume is normal.

 

[72HCODE]

i have the same wet black soot on my garage floor. when the engine is cold it will be spitting out some unburnt fuel and lots of water vapor.

as far as the tach, something sounds messed up with the gauge. if you had a MSD ignition and didn't run the tach adapter that causes a similar issue with a very low tach reading. but with a petronixs II the tach should work as factory intended. i would send the gauge out for service/repair.

on the vac, its possible to mess around and gain some vaccum but it might not be worth the effort if the car runs good i would leave it alone.

since the engine has been reworked all the factory guidelines go out the window. even with a mild cam there are combinations of heads and pistons that cause low vac readings and low compression.

yup the exhaust is going to smell and the smell changes depending on operating and atmospheric temperature, modern cars have oxygen sensors and catalytic convertors, EGR and of course fuel injection with computer control so the mixture is optimal and measured constantly and anything that gets into the exhaust is burned at very high temperature and some of it recycled back into the engine.

 

[tony-muscle]

i have the same wet black soot on my garage floor. when the engine is cold it will be spitting out some unburnt fuel and lots of water vapor.

as far as the tach, something sounds messed up with the gauge. if you had a MSD ignition and didn't run the tach adapter that causes a similar issue with a very low tach reading. but with a petronixs II the tach should work as factory intended. i would send the gauge out for service/repair.

on the vac, its possible to mess around and gain some vaccum but it might not be worth the effort if the car runs good i would leave it alone.

since the engine has been reworked all the factory guidelines go out the window. even with a mild cam there are combinations of heads and pistons that cause low vac readings and low compression.

yup the exhaust is going to smell and the smell changes depending on operating and atmospheric temperature, modern cars have oxygen sensors and catalytic convertors, EGR and of course fuel injection with computer control so the mixture is optimal and measured constantly and anything that gets into the exhaust is burned at very high temperature and some of it recycled back into the engine.

BTW, I just read your posts in this thread, http://www.7173mustangs.com/thread-carburetor-hesitation, and they were extremely helpful. Now I have some homework to do in regards to my idle hesitation issue. Thanks....

 

[Jeff73Mach1]

I'm going to be the bad guy here.

You should be able to get a much healthier vacuum reading with that camshaft.

Vacuum is affected by ignition timing, vacuum leaks(obviously), valve condition, exhaust restrictions etc. More important than the number is how the gauge reacts to opening the throttle quickly and whether the needle is steady or bounces or swings back and forth more slowly-it can tell you a lot.

With a hesitation issue, I'd start by working with my ignition timing to achieve absolute maximum vacuum at idle then back it off 2 degrees.

If you don't see better vacuum after that, I'd consider a compression test and if that is suggestive of problems-then a leak down test. You may need a valve job, you may need rings, or you may just have a combination of minor tuning issues that are adding up to create your problems.

 

[tony-muscle]

I'm going to be the bad guy here.

You should be able to get a much healthier vacuum reading with that camshaft.

Vacuum is affected by ignition timing, vacuum leaks(obviously), valve condition, exhaust restrictions etc. More important than the number is how the gauge reacts to opening the throttle quickly and whether the needle is steady or bounces or swings back and forth more slowly-it can tell you a lot.

With a hesitation issue, I'd start by working with my ignition timing to achieve absolute maximum vacuum at idle then back it off 2 degrees.

If you don't see better vacuum after that, I'd consider a compression test and if that is suggestive of problems-then a leak down test. You may need a valve job, you may need rings, or you may just have a combination of minor tuning issues that are adding up to create your problems.

Not a bad guy, reality is what it is. I know I am running at 10 degrees of timing. The engine was much more happy at more advance, but I did not measure vacuum at that time. I will try the vacuum method to see if it improves in addition to a quick vacuum troubleshooting. If not I can proceed to a compression test. Do cheap ones like the Innova from Amazon are a decent option (http://www.amazon.com/INNOVA-3612-Compression-Tester-Piece/dp/B000EVU89I/ref=sr_1_1?ie=UTF8&qid=1410892915&sr=8-1&keywords=compression+tester)?

 

[Jeff73Mach1]

That looks exactly like the one I have been using for the last 10 years or so. It works wonderfully. But I rarely use it. As cheap as they are, they still clutter up your tool box and many places loan them out like Auto Zone.

When setting your timing, remember to check total timing also. Have the engine held at 3000 RPMs to check the advance and see where it is topping out at. Different engines will like different total advance, but anything over about 34 degrees is really high strung. When using a timing light, I generally start by setting total timing to 32 degrees and then look to see where base timing resides. If your dampner isn't marked they make timing tape that you can add to it temporarily to be able to see the marks at that level of advance.

http://www.autozone.com/autozone/accessories/MSD-Timing-tape-for-harmonic-balancer/_/N-255s?itemIdentifier=139829_0_0_

 

[Bill73Ragtop]

I've been running a Pertronix for over a decade and the tach works perfectly. With winter approaching, it may be well worth having Rocketman check it out and refurbish anything it may require.

I'll let others with more technical knowledge comment on the cam profile, but I ran a similar hydraulic Boss grind back in the 80's - 90's in a 71 M-code and the best I could do was 13 - 14 inches of vacuum. Had a lope at idle, but once you passed 3,000 rpm it got real interesting.

Yeah, mine also has a lope at idle but have been afraid to rev it too much due to the tach not working and still getting familiar with it. It may be that 12 vacuum is what I should expect. The vacuum lines look in a generally good shape.

Bill, with that cam did you set idle a little higher than the 800rpm (600 with trans engaged)? Also, what idle timing advance did you use?




Sorry: I have not checked this thread in a dew days. I was running a 4-speed so my idle was set at 650 - 700 rpm. Initial timing was 16 deg / total was 36 (at 2600 rpm). Looks like you are getting some good information on the responses. Good luck.

I've been running a Pertronix for over a decade and the tach works perfectly. With winter approaching, it may be well worth having Rocketman check it out and refurbish anything it may require.

I'll let others with more technical knowledge comment on the cam profile, but I ran a similar hydraulic Boss grind back in the 80's - 90's in a 71 M-code and the best I could do was 13 - 14 inches of vacuum. Had a lope at idle, but once you passed 3,000 rpm it got real interesting.

Yeah, mine also has a lope at idle but have been afraid to rev it too much due to the tach not working and still getting familiar with it. It may be that 12 vacuum is what I should expect. The vacuum lines look in a generally good shape.

Bill, with that cam did you set idle a little higher than the 800rpm (600 with trans engaged)? Also, what idle timing advance did you use?




Sorry: I have not checked this thread in a few days. I was running a 4-speed so my idle was set at 650 - 700 rpm. Initial timing was 16 deg / total was 36 (at 2600 rpm). Looks like you are getting some good information on the responses. Good luck.