Volvo Penta AQ145 Engine Problem Part 3....

[ontheplane]

For original post see.. :- http://www.ybw.com/forums/showflat.php?C...ID=#Post1071674

Thanks to everyone on here - We are nearing the end of the nightmare of this!!

One final question to all - the engine cover states the timing should be 28-30 deg BTDC at 4200 rpm...!!

I don't particularly want to try and set the timing at those sort of revs (it's an old engine) - any ideas what the timing should be on an AQ145 engine at idle?? Then I can set it and then run the engine briefly up to 4200rpm to check it's ok.

Thanks all

Grant

 

[dt1]

What type of strobe are you using,,can you dial in the amount of advance you want,,if it is it only takes a few seconds to line up the timming marks...or is the crank pulley marked...

 

[thefatlady]

My memory of these engines from 15 years ago is that static timing was 6 deg. BTDC running on unleaded petrol.

I increased mine to 12 deg BTDC and ran it on leaded and it made quite a difference.

Don't ask me any more than that - the memory won't cope.

 

[ontheplane]

Thanks.

We're obviously on Unleaded these days, but I might try it out at about 8 or 9 then and even advance it a smidgen until I get pinking then move it back a notch.

Cheers

Grant

 

[dt1]

Remmber to take the vacum advance pipe off if your setting the timing at idle,,too much advance in the engine can lead to bad starting ,,high fuel consumption,,over heating,,imho you should go by the book and not by your ear,,
Dave

 

[spannerman]

You won't be far out at 8 degrees at idle, then as suggeted run it up briefly to check.
There is no vacuum advance on petrol engines on boats, simple reason there is never any vacuum available except at idle as the engine is ALWAYS under load. It never gets to cruise like a car engine does at high speed and light throttle.

 

[dt1]

Thats interesting,, how does the engine retard for start up ,,interest only,,or do they run such a small btdc deg that the engine would never produce max bhp.. i really respect your comments on this forum,,,i come from a drag racing background,, and it seems i have a lot to learn ,,,,,about lower powered boat engines...

 

[thefatlady]

If there is no vacuum available, how does the mixture get into the cylinder? Accepted, the vacuum will be lower than on a light throttle setting.

Ontheplane, I wouldn't advance beyond the recommended setting. You won't get any advantage and, under way under load, you probably won't be able to hear pinking. You could, if you put up with extra expense and inconvenience, use a higher octane fuel and advance it a bit more.

Having said that, this engine is probably quite tolerant and I suspect that the setting is not critical.

 

[thefatlady]

Technical info here gives static and strobe timing for 90 and 95 octane fuel.

 

[spannerman]

Looks like we have started a new topic here, Engine Vacuum does it exist?
Well let me first mention that my in my early days I was a Volvo car mechanic, then I worked for Sun Electric for 8 yrs selling and training on their range of engine testers and gas analysers and diagnostic code readers, and let me tell you the average car mechanic has no comprehension of the 4 stroke cycle and the laws of physics. I learned an amazing amount at their technical training school that college didn't teach me.
Of course there is vacuum to be able to get the air into the cylinders, or to be more accurate a difference in ambient atmospheric pressure and the pressure in the cylinder, so actually its atmospheric pressure that forces the air into the engine.
If you could fit 3 vacuum take offs to an inlet manifold, 1 downstream of the throttle plate, 2 at the throttle plate in the middle of the venturi (narrowest part), and 3 upstream you would see the following:

Idle - 1 Hi Vacuum
2 Hi Vacuum
3 no vacuum (or actually ambient atmos pressure)

Half throttle- 1 Lo vac
2 Hi vac
3 Lo vac

Full throttle - 1 Lo vac
2 Lo vac
3 Lo vac

When we say Lo vac we don't mean a vacuum but a pressure lower than 14.5 psi so that the weight of the atmosphere forces air into the engine.
What we have to consider with distributor vacuum is where we take the vacuum from and its directly at the throttle plate in the closed postion so we get partial vacuum at idle, more vac in the cruise or engine over run situation ( going downhill with your foot off the gas) and lo vac or ambient manifold pressure at wide open.
The reason we get engine vacuum is that the engine has the throttle plate restricting airflow so depending on its position and the speed of the engine the vacuum will vary.
The engine doesn't retard for start up it initially uses a combination of mechanical advance and lo vacuum at idle then at about 2500 - 3000 rpm the mechanical has maxed out and vacuum takes it further.
Also you have to consider the burning process, a rich mixture burns more rapidly than a weak mixture, so when you step on the gas in your car the manifold vac goes down and vac advances disappears but the carb throws in extra fuel so it all burns nicely, when you lift off the carb delivers less fuel resulting in a weak mixture that needs longer to burn, fortunately our vac is now hi pulling in lots of advance so we still burn the mixture effectively while its in the cylinder which stops the banging and popping you would otherwise get on overun as still burning gases enter the exhaust system. And in the cruise we have a lean mixture and hi vac so combustion is OK, remember the days of checking the colour of the plugs or checking the colour of the tailpipe after a long drive to see if we were too lean or too rich.
This all relates to car engines and I used carb engines to simplify it. When we come to boat engines we never have the equivalent of cruising with a light throttle or coasting as the boat is always going 'uphill' as far as the engine is concerned so we don't get hi vac in the manifold, therefore no need for vac advance as its redundant.
Thats why marine diesels only have mechanical advance, there is no throttle plate for the engine to suck against so no vacuum.
Hope this helps, the combustion process is one of my favourite topics. I would have enjoyed being a motor mechanics lecturer in tech.
Next week we can discuss the gas ratios in combustion!

 

[thefatlady]

Thank you for that detailed explanation. I appreciate the work you put into it and I feel sure that your expertise on this subject is greater than mine.

It was just that my little knowledge of IC engines led me to think that your statement that "there is never any vacuum available except at idle" contradicts the laws of physics. Without manifold depression, there can be no induction.

You clearly enjoy your subject and have now made your position clear. Thanks.

 

[ontheplane]

WOOHOO guys...

Thanks to all who contributed...

The engine now pulls properly and gets up on the plane (just!)

I've set it at 8deg and it seems to run well - I know I could advance it a bit and run on higher octane fuel - but 95 seems the standard over here so I'll stick with that.

Thank you so much to all who contributed to my solution - I now have to try and get some money back from the people who call themselves engineers - who actually don't have the first clue about engines I think....

Grant