Volvo Penta AQ271C - Any experience working on them?

seanfoster

seanfoster

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Hi All,
I've been a member of this forum for quite some time but mainly focused on the PBO and live aboard forums, but wondered if someone here could help?

I have an ageing (1990) Cranchi Derby 700 powerboat with a Volvo Penta AQ271C (5.7L V8) engine.
I have had a few problems with the engine, mainly due to fuelling issues and wondered if anyone here is familiar with the this type?

I gave the engine a complete service not long ago as there were definite carb problems (strong smell of fuel and running roughly, fouled oily spark plugs etc).
My service included:
Oil and filter change,
New plugs,
Compression test (all ok and within 5% of each other and to manufacturers spec)
Checked over ignition system, removed cleaned and lubricated all leads and caps

Complete overhaul of carb (Holley 4150), Holley genuine carb rebuild kit, replaced automatic choke with manual kit (I suspected it wasn't opening as it should), I also found that the previous owner had mistakenly swapped the jets from primary to secondary metering block.
Set up the float and mixture screws to the Volvo Penta service manual.

Initially it ran really smoothly, but had a problem over the weekend where it is running rich and has fouled the plugs with carbon and eventually conked out and wouldn't start again. (It did start the next day).

The obvious thing to look at is to set the mixture properly on the carb, I have a vacuum gauge to do this but cant find a vacuum port anywhere on the inlet manifold (or anywhere else) to attach it to. Does anyone know where I can take it from? (Service manual doesn't give any clues!)

Also, there isn't an air cleaner as such, just a flame arrestor which sits on top of the carb, with two breather hoses from the rocker covers, and another pipe that runs from the fuel pump into an attachment on the flame arrestor, which sort of goes to nowhere. Does anyone know what this pipe is doing?

One thing to note is I run the boat on the river (with a max speed limit of around 5mph) so basically runs all day on tick over. I have NGK BR6FS plugs and wondering if the low rpms are causing the carbon build up? I've seen on the iBoats forum someone recommended hotter plugs to possibly remedy this, has anyone had similar problems or experiences?

Any help would be appreciated!

Thanks

Sean
 
Last edited:
Hi Sean, and welcome to the forum. I have the same engine but in Mercruiser version. Sounds like you gave it a thorough going over, the only thing I would ask is have you checked the ignition timing just so you know all is as should be. It should be correct as the electronic systems don’t go out of spec like the old points system did.
The hose from the fuel pump is to dump fuel back to the carb if the diagphragm in the pump leaks.
These engines like to be used, I run mine at idle as I cruise through the small sounds and channels where I live, but it gets a blast along the fjord on the way out and home. So I strongly suspect here lies your problem, you coulg try a hotter plug to burn the deposits off. There is no vacuum take off for a gauge but you don’t need one to tune these engines, if its a Holley then the mixture screw is most likely a fuel metering screw as opposed to air metering as on my Mercruiser carb. So in for weak and out for rich, its best to do it tied to the dock and in gear, idle is around 650 in gear. Hope this helps.
 
As spannerman says
All good info and expertise as usual
I think I would put BR4FS in the Lump if you are only using the engine on 'tickover' virtually
Common NGK product so not an expensive try
 
spannerman said:
Hi Sean, and welcome to the forum. I have the same engine but in Mercruiser version. Sounds like you gave it a thorough going over, the only thing I would ask is have you checked the ignition timing just so you know all is as should be. It should be correct as the electronic systems don’t go out of spec like the old points system did.
The hose from the fuel pump is to dump fuel back to the carb if the diagphragm in the pump leaks.
These engines like to be used, I run mine at idle as I cruise through the small sounds and channels where I live, but it gets a blast along the fjord on the way out and home. So I strongly suspect here lies your problem, you coulg try a hotter plug to burn the deposits off. There is no vacuum take off for a gauge but you don’t need one to tune these engines, if its a Holley then the mixture screw is most likely a fuel metering screw as opposed to air metering as on my Mercruiser carb. So in for weak and out for rich, its best to do it tied to the dock and in gear, idle is around 650 in gear. Hope this helps.
Click to expand...

Thanks for that, The timing is mechanical so will check the timing with a strobe, although when it was running well it sounded like a sewing machine so doubt it is out. I'm going to renew all of the leads for belt and braces, to make sure everything is as new as it can be.
Looking at the parts fische, there is a plug that can be screwed out on the inlet mainfold, so I'm assuming I could possibly take a vacuum from there?
Once again thanks for your help!
 
Kawasaki said:
As spannerman says
All good info and expertise as usual
I think I would put BR4FS in the Lump if you are only using the engine on 'tickover' virtually
Common NGK product so not an expensive try
Click to expand...

Thanks for that, I've ordered a set of BR5FS's from ebay, if they don't seem to improve the situation I'll try the 4's.
 
Can I ask what you hope to do by connecting a vacuum gauge, as all it will show is overall vacuum and if its steady or not. In Over 40 yrs of tuning engines I have only used a vacuum gauge when setting up multiple carbs and then the vacuum is measured on the upstream side of the throttle plate, or just downstream when you have individual inlet tracts as on outboard motors or motorcycles and some older sportscars. Your Holley which probably has twin venturis feeds into a common manifold so you can only measure total vacuum.
 
spannerman said:
Can I ask what you hope to do by connecting a vacuum gauge, as all it will show is overall vacuum and if its steady or not. In Over 40 yrs of tuning engines I have only used a vacuum gauge when setting up multiple carbs and then the vacuum is measured on the upstream side of the throttle plate, or just downstream when you have individual inlet tracts as on outboard motors or motorcycles and some older sportscars. Your Holley which probably has twin venturis feeds into a common manifold so you can only measure total vacuum.
Click to expand...

Thanks for your comment, the Holley website has a guide for tuning 4150 carbs (which this one is) and uses a vacuum gauge to do so.
 
Ok I just read the guide on the Holley website, I guess you are trying to find out if you have the correct powervalve so you need to read vacuum. There is usually a plug on the inlet manifold that you can screw out, probably at the rear of the carb, There is one plug near the front but that can be for a water takeoff for a calorifier, it depends on the manifold you have, not sure if Volvo use the same manifold as Mercruiser. But you will need to make an adapter to connect a vacuum line.
 
Hi
I would still be thinking choke issues. Had loads of these GM V engines. I’d simply just manually lock off the choke in off position and pump the throttle for cold start. Most of them have this done by now.
I’d also say (despite is being unlikely to be your main issue) that the flame arrestor is defo a filter and needs cleaned at least annually. Especially if you are running the way you are. Running at low rpm on these can be an issue. They barely get up to temperature and have poor combustion with not getting quite up to temp. You then get petrol washing down the bores past the rings and into the sump. Thins the oil, vapour is thrown out through those breathers you ask about and up to the carb. Causes all sorts of issues and of course a pretty volatile environment.
Long story short...they don’t like low rpm for ages, have known choke issues. Black smoke means too much fuel or too little air (sit flame arrestor in soapy water over night and be amazed at the dirt out it) and just perhaps make sure your risers and manifolds are good and not allowing water to pass in through exhausts valves. Another common issue. Other than that they are amazing things. Last forever
 
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