Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readings

[insttech1]

OK so Volvo's engine manual (downloaded--thanks DonS!) utilizes manifold vacuum readings at idle as the primary metric (measuring stick) to analyze engine performance and as a diagnostic tool. And from what I see, it doesn't even have a really great explanation for setting timing w/ a light....but I may be wrong there. It basically says start it, put the shunt in, and point that light at the now-illegible, poorly marked teeth, and guestimate between the 4-degree-increment teeth, until timing is correct.

I'm asking the question below maybe to help others with some small method of timing "check" without a light for all of the "my engine dies" and "hesitation" posts that I myself was experiencing when I first came here for advice.

First assumption: Idle mix screws have been meticulously adjusted for maximum vacuum at idle, in a freshly rebuilt carb.

So, for instance, if all is "normal" with any given engine, manifold idle vacuum is at least steady within 1", and it seems to be operating correctly (going up to reasonable WOT, having decent engine idle, etc....then you should be seeing 17 to 21" of vacuum from the manifold, after warm-up, and at idle RPM.

However, if you are only seeing 14 to 17" of vacuum, then the manual says you have "late timing".
And anything beyond that, per the volvo manual, should show serious fluctuations in a vacuum gauge, so you know there are other, more serious issues.

If one does not have a timing light, or timing shunt, then can one deduce that tweaking the timing adjustment on the distributor (GENTLY!) until the vacuum rises until it is greater than 18", that this is an acceptable confirmation that the timing is within correct spec's?

I know it's kind've an inverted educated guess, and we're talking about minor (my case very minor--I have a solid 16.5" of idle vacuum) differences, but would changing the timing until I get another 1.5 to 2" of vacuum be detrimental?

This one will open up debate, I'm sure, but let's hear it.

Thanks!

 

[Howard Sterndrive]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

If one does not have a timing light, or timing shunt, then can one deduce that tweaking the timing adjustment on the distributor (GENTLY!) until the vacuum rises until it is greater than 18", that this is an acceptable confirmation that the timing is within correct spec's?!

not a chance....
get a timing light

 

[insttech1]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

I've no problem with getting all of the proper equipment, as I have access to all of it, even the shunt.

So that's why this is theoretical....you didn't say "why" no to use vacuum as a reference.

Thanks for the input.

 

[bruceb58]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

Because its not the proper way to time an engine. You use a timing light and the timing settings are in the service manual. I have a Volvo manual and its extremely clear how to do it including the shunting procedure. Curious what section of the manual you are looking in. It obviously isn't the electrical and ignition manual.

 

[Howard Sterndrive]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

try it both ways and you'll find out how accurate this theoretical method is. Maybe you can write a blog if it works. Timing lights are $12.99 at Harbor Freight so how many hours would you expect us to spend working it all out and discussing it?

 

[Tail_Gunner]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

You do understand that the engine does not have a vacum advance system, timing should be controlled by predeterminded curves in the dist..its electroncially predeterminded and the results are the vacum readings...


......Does this thing have points and a shunt.....Points use vacum advacne....must be a old motor

 

[wellcraft19]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

Maybe a better way to say "why" is that the vacuum really does not matter. The (use of the) increase in vacuum is only a "driver" used on many engines to advance the timing (on mine it is it 100% electronically controlled, so no vacuum), a "cheap" and reliable method to deliver the rpm mechanically to a device, instead of having something much more complex as a mechanical rpm "reporter" (simplified, but you get the idea, get a timing light is the recommendation).

 

[Maclin]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

I have an old old vacuum gauge, and it has green, yellow and red ranges and says things like late timing, etc. I think it really means valve timing events, but not sure of that either. In any event it is not meant as the primary ignition timing tool. Set the timing via the light. Then adjust carb idle and idle mix to get best vacuum. If vacuum is still low then there could be cam timing problems, or the mark on the dampener is not in the right place, or not being read right, things like that.

 

[insttech1]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

Yeah I guess I'll clarify some things:

I don't have vacuum advance. This is a modern distributor.
I'll install my shunt, get the light, and verify my timing. That's not part of the question.

This is more of a theoretical post about why Volvo uses the manifold vacuum readings as one of the first things in their engine manual to indicate a problem (timing being a tuning issue, others being mechanical faults).

And no Howard I wasn't expecting you to spend hours "working it out". It was just a theory question. Possibly in the hopes of helping others in troubleshooting steps.

Maybe it would have been better summed up as: "What happens if someone adjusts their timing to optimize their manifold vacuum".

The idea came to me after another guy asked about timing his engine without a light, and the response he got from another poster was "put a glass of water on there, and adjust timing until you get the least amount of slosh in the water".....ha!

 

[Don S]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

This is more of a theoretical post about why Volvo uses the manifold vacuum readings as one of the first things in their engine manual to indicate a problem (timing being a tuning issue, others being mechanical faults).

That chart has typically been used by Volvo, Mercruiser, OMC and every Chevy engine manual known to man for years.
It's just some basic engine troubleshooting information, It is NOT the last word on setting timing, or finding engine problems, it's just one of many tools used by mechanics to find problems.

 

[Tail_Gunner]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

:

Yeah I guess I'll clarify some things:

I don't have vacuum advance. This is a modern distributor.
I'll install my shunt, get the light, and verify my timing. That's not part of the question.

This is more of a theoretical post about why Volvo uses the manifold vacuum readings as one of the first things in their engine manual to indicate a problem (timing being a tuning issue, others being mechanical faults).

And no Howard I wasn't expecting you to spend hours "working it out". It was just a theory question. Possibly in the hopes of helping others in troubleshooting steps.

Maybe it would have been better summed up as: "What happens if someone adjusts their timing to optimize their manifold vacuum".

The idea came to me after another guy asked about timing his engine without a light, and the response he got from another poster was "put a glass of water on there, and adjust timing until you get the least amount of slosh in the water".....ha!

Your thoughts are sound marine motors could stand to be in a better state of tune. And that is a opinion... with that said they are tuned to take high loads all across across the rpm band every time they work and do it reliabily....You cant have it all....but you can tinker if you so choose. It is expensive and requires lots of experience...can it be of course...

 

[bruceb58]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

Maybe it would have been better summed up as: "What happens if someone adjusts their timing to optimize their manifold vacuum".

That's fine. Only problem is that the consequences are a timing that is too far advanced and the engine fails because of detonation.

For best performance, you want the timing adjusted until the edge of knock. The problem is all it takes is some fuel that has lost some of its octane(ethanol evaporating as an example)and you are now in the danger zone. The setting stated in the manual is a good safe setting that the manufacturer feels has some margin in it.

Modern automotive ignition systems can actually detect knock on a cylinder by cylinder basis and adjust the timing per cylinder.

 

[Tail_Gunner]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

That's fine. Only problem is that the conequences are a timing that is too far advanced and the engine fails because of detonation.

For best perfomance, you want the timing adjusted until the edge of knock. The problem is all it takes is some fuel that has lost some of its octane(ethenol evaporating as an example)and you are now in the danger zone. The setting stated in the manual is a good safe setting that the manufacturer feels has some margin in it.

Modern automotive ignition systems can actually detect knock on a cylinder by cylinder basis and adjust the timing per cylinder.

That pretty much sums up marine motors....The tech they use is mid 80's tech from cams to injection. Nothing new here at all....after all marine motors tried to introduce OS sensors just a few yrs ago with cats....again 80's tech. Modern engines (Ford) can detect AFR and lean or enrich on the fly.....No need for cats such old tech is regressive

 

[bruceb58]

Re: Theoretical Volvo 3.0 (any model) question: timing versus manifold vacuum readi

Yes but it is reliable and reliability is the name of the game in marine.

They also have never have had to worry about emmisions before until now which is why the O2 sensors that you mentioned are now being used.