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volvo penta AQ151c

R

rossh

New member
hi all,

im new to the forum and relieved to find somewhere I can get advice on this engine.... recently bought a boat with 2 x AQ151c engines and AQ250 drives

ran fine initially but after a few hours on load (after I paid for it!!!) I ran into issues.... i've summarised them below and i'd appreciate any help as none of the local mechanics in the Chichester area will work on it as its so old.... gone to the DIY clouds in the sky is how it was described to me....

Port engine
Recently had carbs overhauled with Volvo service kit. Set to factory settings (not tuned)
Runs fine unto 2k rpm
Seems to struggle under load from 2- 3k rpm where it won’t pick up any more speed.
Once fully warmed up under load (by the end of the Chichester channel) will choke/ stall when load is applied.

Stbd engine
Recently had carbs overhauled with Volvo service kit. Set to factory settings (not tuned)
Runs fine unto 2k rpm
Won’t rev above 2.2k rpm
Oil pressure and temp show slightly lower than port engine on the boat gauges

Both engines have revved higher under load before the carb overhaul although the Stbd engine has always lagged a bit and not got above 3k rpm. Boats speed was 25-27 knots, it now cant do more than 10knots.

advice and help appreciated. I don't fancy the alternative of fitting desiel engines although it may come to it!!

regards,
ross
 
You haven't said anything about the ignition system... there are a myriad things there to cause this...
1) Distributor cap
2) rotor
3) points ( dwell can change if points not properly installed)
4) plugs
5) wires
6) sticky advance weights in disti...
7) Bad fuel/water saturated seperator

.... and last but not least..
8) a botched carb overhaul ( I once bought a Bertram 25 for very short money because a PROFESSIONAL mechanic on LBI in NJ had installed the choke thermo springs backwards on a carb overhaul and the owner thought the rings were shot...engine hard to start and blew black smoke when warm)

Re: "alternative of fitting desiel engines "... this is like having a s$x change operation to cure a mild case of ED!
 
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sandkicker said:
Re: "alternative of fitting desiel engines "... this is like having a s$x change operation to cure a mild case of ED!
Click to expand...

well thats the first time i've laughed about this boat since I got it :)

the ignition has been converted to electronic. the coils and leads are new. i've got a new set of plugs coming

the reason I hadn't looked at that was because it happened quite suddenly. the boat was motoring along fine at 20+ knots when the port engine died then ran with the symptoms I described. I thought of the fuel issue first of all. the tank was almost empty so I filled it to the brim with fresh. the separators are clear of water too. I was thinking fuel pump failed as it feels like fuel starvation. starts and runs fine unto 2k rpm. in neutral, it revs freely. its just an issue under load.

ive got new plugs, coil, filters and fuel pump coming for the weekend. ive also got a timing strobe to check the advance/ retard...oh, and a workshop manual... been a while since I got my hands dirty but got no choice. ill check all the other point you mention too.
thanks for the feedback so quickly....
 
So today I’ve changed the ignition coil as it was looking corroded.
fitted new plugs and checked the timing (5deg at idle and 35 at 4000)
compression test was good with 160psi on all 4 cylenders
i noticed the water separator had been connected the wrong way round, i had a new one to fit anyway so now the fuel getting to the engine is clean. Fuel pump works fine.

Starts and runs fine off load but still not taking load above 2500rpm..... anyone got any more ideas?
 
Hi bob,

the carb was stripped and cleaned using the standard Volvo service kit so new gaskets etc. All settings were set back to factory. I was going to balance the carbs with vacuume gauges but cant find the connection point on the manifold. I’m used to balancing carbs on bikes but not sure if connection points are different on boats.

its got to be something minor but just not sure what....
 
WOT rpm is 5,500rpm. If running with points use a dwell meter to set the point gap at exactly 62 degrees the check timing is at exactly 6dbtdc. What is the pitch of your props?
 
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If the coils are getting hot do no hesitate and put a ballast resistor to coil posative. Exactly how are you adjusting the idle air needles for the carbs?
 
Couldn’t tell you the model but its the same as in the OEM manual. Twin carbs, all looks exactly as per Volvo manual.
only change from standard is the addition of electronic ignition which was fitted a couple of years ago...
 
You may be over propped what is the pitch of your props? If you have never got it over 3,000 rpm on both engines do some research on the lower gearboxes and prop size/pitch. You want to reach at least 5000rpm on those two little power plants.
 
So i had a good run out today. Both engines started well. I still have the same symptoms under load though...
the props are the same as came with the boat when it ran fine Upto 27knt so its not that. When the issue occurred, it happened suddenly as if something broke or was blocked (port engine stopped taking load and stalled but restarted ok)
the port engine now revs to 3000 rpm. At this point a heavy intermittent vibration can be heard. Possibly the engine not running correctly. Every now and then it would clear and revs climbed to 3500 a few times. I opened and closed the throttle a few times to see if it would clear, it didn’t. Boat speed crept up to 12knt but didn’t reach the plane at all. I’m at a bit of a loss as to what to check next. Ideas?
 
boat speed is completely irrelavant when doing a diagnostic. Until you get the rpm up over 5000rpm WOT your working those engines to death.
 
kimcrwbr1 said:
boat speed is completely irrelavant when doing a diagnostic. Until you get the rpm up over 5000rpm WOT your working those engines to death.
Click to expand...
Ok so given the revs are 3000 what would be the next area to look at?
i talk about speed as an indication of overall performance to show the extent of the issue. Any ideas?
 
googling for answers has flagged up one suggestion I hadn't looked into....
it was suggested on an old site (2006) that the timing belt can slip if it gets wet and cause these symptoms... I always thought that would wipe the valves out and show up in the compression test but it hasn't.... does this sound plausible? my raw water strainer has a slight leak... I think I need to remove all the cooling heat exchanger to check this so anyone got any thoughts before I do?
 
That’s not quite right. The issue with the port engine began suddenly and before the carb rebuild. The rebuild was done by a Volvo dealer but they didn’t want to work on it anymore and none of the other mechanics in area will work on old petrol engines either so I’m stuck doing it myself. I guess I will need to rebuild the carbs again just to rule it out. Before I do, going back to one of my earlier questions, where can I connect the vacuum gauges so I can balance the carbs correctly?
and would a slipped cam belt cause the same loss of power I’m seeing?
 
rossh said:
That’s not quite right. The issue with the port engine began suddenly and before the carb rebuild. The rebuild was done by a Volvo dealer but they didn’t want to work on it anymore and none of the other mechanics in area will work on old petrol engines either so I’m stuck doing it myself. I guess I will need to rebuild the carbs again just to rule it out. Before I do, going back to one of my earlier questions, where can I connect the vacuum gauges so I can balance the carbs correctly?
and would a slipped cam belt cause the same loss of power I’m seeing?
Click to expand...

just following up on this note, Ive examined the cam belt without dismantling the engine cooling. I aligned the timing mark on the crank with TDC, as I can only see the back of the cam pulley, I assumed the timing mark is opposite the centre of the hole in the camshaft pulley as per manual drawings. this being the case, the belt is 1 or maybe 2 teeth out of alignment with the static timing mark on the bracket mounted on the cam cover... is that enough of an issue to stop the engine revving under load?
I haven't checked the other engine as access is much harder to that one...
 
A slipped cambelt would produce rough running and loss of power. Luckily the 151 is a non-interference engine so even a snapped cambelt does not result in damage to the valve gear.
 
ust thought I would add something. I have an old 25ft timber cabin cruiser with a single shaft drive AQ151A. I cruise along happily at a hull speed of around 10knots depending on how much I drag along with me on my hull and keel (a clean makes a big difference) at 2000 - 2500 rpm (as in cruising or in a hurry!). Fast enough for me - more throttle doesn't make much difference. Its all about wind and waves and the sound of the motor. Engine sounds happy at these rpms and quiet too. Can't imagine getting up over 3000rpm although I have heard that these motors are tough and used in "speedboats" they would be under a fair bit of strain maybe? Go faster than 15knots where I live and I worry about missing the crab and crayfish pots markers that seem to be everywhere :)​
 
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