Help understanding vacuum test

[banderberg]

Engine is a 1990 3.0L, serial number 0D389001.

In the water, in gear at idle of around 680 RPM I get around 15 hg. The needle bounces around a little bit.

I have gone back and forth between idle speed and mixture screw and this is as good as I can get it.

The needle is over a section on the gauge that says late timing. The engine is set to 8 deg BTDC.

Any suggestions? See my video here

New video by Barry Anderberg

photos.app.goo.gl

 

[Rick Stephens]

A little low. Mine was down there as well. Not too bad though at <700 RPM. Have you done a compression test to find baseline?

 

[banderberg]

A little low. Mine was down there as well. Not too bad though at <700 RPM. Have you done a compression test to find baseline?

I did one with the engine cold and had ~160 psi on all four cylinders

 

[dubs283]

15 to me indicates a sound engine. As long as there is no variation of more than 1 inch and it returns to 15 after a throttle open/closed fairly quickly I'd say you're set.

Of course the ultimate idle vacuum is approx 17 with little to no variation but I would have no hesitation to run full wot with your engine at 15.

One thing to try is to back idle timing down to 6 or so, nite any changes and adjust accordingly, just make sure to monitor total advance, iirc on older Gen 3.0 would be roughly 18-20 (?) plus initial

 

[Rick Stephens]

I was going to suggest you ignore it. The tiny bit of bounce is next to nothing, and being at 15 may just be gauge inaccuracy or a teeny bit of wear on your valve guides. You would not have high compression if the standard and truly low vacuum causes were present - worn rings, tight valves. Only other thing is a vac leak and that seems unlikely as smooth as you're running.

 

[achris]

In the water, in gear at idle of around 680 RPM I get around 15 hg.

This puts a slight load on the engine, which will drop your number slightly. If say you're good to go...

 

[dubs283]

Didn't even notice the in gear reference, agree 100% with chris

 

[banderberg]

15 to me indicates a sound engine. As long as there is no variation of more than 1 inch and it returns to 15 after a throttle open/closed fairly quickly I'd say you're set.

Of course the ultimate idle vacuum is approx 17 with little to no variation but I would have no hesitation to run full wot with your engine at 15.

One thing to try is to back idle timing down to 6 or so, nite any changes and adjust accordingly, just make sure to monitor total advance, iirc on older Gen 3.0 would be roughly 18-20 (?) plus initial

I don't fully understand how to check total advance. I believe I need to be at a high enough RPM for the advance weights to be active and use a timing light to check. So that would require someone else driving the boat pretty fast while I check. Does that sound right?

 

[Scott Danforth]

First, the propane method is better for setting idle screws

Your vacuum reading is normal your compression gauge reads high

 

[banderberg]

First, the propane method is better for setting idle screws

Your vacuum reading is normal your compression gauge reads high

160 is high its supposed to be what like 140? Is that bad or is the gauge wrong? It's an inexpensive one off Amazon

 

[Scott Danforth]

160 is high its supposed to be what like 140? Is that bad or is the gauge wrong? It's an inexpensive one off Amazon

150 psi is what the engine reads new with a good gauge

 

[Rick Stephens]

I don't fully understand how to check total advance. I believe I need to be at a high enough RPM for the advance weights to be active and use a timing light to check. So that would require someone else driving the boat pretty fast while I check. Does that sound right?

You are going to have electronic ignition. The controller for that determines advance, there are no mechanical weights for advance.

The proper way to check is to use a electronic advance timing light, set it to the maximum advance from that motors curve, point the light at the timing mark and gently rev your motor to 2800-3000 RPM for a couple of seconds. Should see the mark come into position as the motor comes up to speed.

 

[achris]

I don't fully understand how to check total advance. I believe I need to be at a high enough RPM for the advance weights to be active and use a timing light to check. So that would require someone else driving the boat pretty fast while I check. Does that sound right?

Nope. You can check the total advance in neutral, but it still needs to be in the water. Flushers can't supply enough water...

Not sure which ignition system you have. Could be points or 'distributor-less'... Here's how to tell the difference. Any doubts, post a photo..



Correct advance for each system is as follows.
Points: Initial timing 8°BTDC, total advance @4100rpm 24°BTDC.
Distributorless: Initial timing 8°BTDC, total advance @4600rpm 26°BTDC

 

[banderberg]

You are going to have electronic ignition. The controller for that determines advance, there are no mechanical weights for advance.

The proper way to check is to use a electronic advance timing light, set it to the maximum advance from that motors curve, point the light at the timing mark and gently rev your motor to 2800-3000 RPM for a couple of seconds. Should see the mark come into position as the motor comes up to speed.

I have a points ignition, it has advance weights

 

[banderberg]

Nope. You can check the total advance in neutral, but it still needs to be in the water. Flushers can't supply enough water...

Not sure which ignition system you have. Could be points or 'distributor-less'... Here's how to tell the difference. Any doubts, post a photo..

View attachment 366264

Correct advance for each system is as follows.
Points: Initial timing 8°BTDC, total advance @4100rpm 24°BTDC.
Distributorless: Initial timing 8°BTDC, total advance @4600rpm 26°BTDC

So basically I'm just eyeballing the notch since the marker on the engine only goes up to ten I think

 

[Scott Danforth]

So basically I'm just eyeballing the notch since the marker on the engine only goes up to ten I think

That is how you ruin your motor. But a proper timing light or Pend $4 on the right size timing tape

 

[banderberg]

That is how you ruin your motor. But a proper timing light or Pend $4 on the right size timing tape

I have a timing light with advance. When I use it for idle timing I see the notch at the 8 deg. BTDC, so if I use it for advance where does the notch line up?

 

[Scott Danforth]

I have a timing light with advance. When I use it for idle timing I see the notch at the 8 deg. BTDC, so if I use it for advance where does the notch line up?

At the 0 point if your timing light is set to 8BTDC

 

[achris]

An advance timing light enables you to dial in an advance and it will fire the light that many degrees late. So if you dial in 8° BTDC and you point it at the marks, if you've set the timing correctly, the marks will be at 0°. If you dial in 24°BTDC and take the engine to 4100rpm, you should see the marks near 0 also.

Chris....

 

[Eric 3.7LX Alpha 1]

Adjust the timing light knob so the timing mark on the balancer is at zero, then read where the knob is pointing at which will be your timing.