Missing AQ170 Power

Hi like a number of thread beginnings I have read I am also new to forums. I like nostalgia and have been restoring a 6Mt glass boat that has an AQ170 engine running a 270 2.15:1 leg with a 15/15 prop. The engine has been overhauled and had new rings, all new valves carbi kits, electrics ect and actually runs very nicely the plugs are a good colour but unfortunately I have come to the conclusion it is lacking in power firstly as I have only ever been able to achieve a max of 24 knots at 4500 rpm with this prop and the prop guy's tell me it should achieve 30 knots minimum with this set up. I have also done some extensive inspection of the hull but it is also fine.
What leads me to consider lack of power is when the boat is performing at its best the carbi butterfly's are only half open and it makes no difference to the operation of the engine if they are fully opened or anywhere in between. I have considered the usual suspects being either incorrect combustion air flow, electrics or fuel and I am fairly confident it is not combustion air or electrics so that leaves fuel it has 3 x Solex P44 carbi's of unknown origan so as a first I would like to know what size the main jets should be or if anyone can suggest another area to look it would be much appreciated.
Thanks
Denis

Re: Missing AQ170 Power

Denis Gibson said:

Hi like a number of thread beginnings I have read I am also new to forums. I like nostalgia and have been restoring a 6Mt glass boat that has an AQ170 engine running a 270 2.15:1 leg with a 15/15 prop. The engine has been overhauled and had new rings, all new valves carbi kits, electrics ect and actually runs very nicely the plugs are a good colour but unfortunately I have come to the conclusion it is lacking in power firstly as I have only ever been able to achieve a max of 24 knots at 4500 rpm with this prop and the prop guy's tell me it should achieve 30 knots minimum with this set up. I have also done some extensive inspection of the hull but it is also fine.
What leads me to consider lack of power is when the boat is performing at its best the carbi butterfly's are only half open and it makes no difference to the operation of the engine if they are fully opened or anywhere in between. I have considered the usual suspects being either incorrect combustion air flow, electrics or fuel and I am fairly confident it is not combustion air or electrics so that leaves fuel it has 3 x Solex P44 carbi's of unknown origan so as a first I would like to know what size the main jets should be or if anyone can suggest another area to look it would be much appreciated.
Thanks
Denis

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The AQ170 should run with a 1.89:1 ratio. The 2.15:1 is for 4-cyl engines. But that makes it even mores strange, that you can only achieve 4500 rpm. A lower ratio should make your engine explode in rpms. The AQ 170 should run 5000 rpm max, under normal circumstances.

I suspect an ignition issue.

Have you checked the ignition timing?

With the dist. cap off, can you turn the rotor a bit, and does it jump back, when you release it. This will indicate if the advance mechanism works.

Are you still running points/condensor, what condition are these in?

Re: Missing AQ170 Power

HorizonblueDK said:

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The AQ170 should run with a 1.89:1 ratio. The 2.15:1 is for 4-cyl engines. But that makes it even mores strange, that you can only achieve 4500 rpm. A lower ratio should make your engine explode in rpms. The AQ 170 should run 5000 rpm max, under normal circumstances.

I suspect an ignition issue.

Have you checked the ignition timing?

With the dist. cap off, can you turn the rotor a bit, and does it jump back, when you release it. This will indicate if the advance mechanism works.

Are you still running points/condensor, what condition are these in?

Click to expand...

Thanks for the reply, I din't mention but I have both an 1.89:1 and1.65:1 and have tried both some time ago with a number of differant props but with similar results lack of power.
The engine has only done 74 hours since rebuild when it had all new ignition parts and the advance is operating ok, it still has the genuine Bosch disy with points but I think I may start again at the timing and give it a twist at speed and maybe partially block one of the carbs to see what result I get.
I was chasing the main jet size first as the carbs are an unknow and could well be from a B20.
Cheers

Re: Missing AQ170 Power

Dennis, I agree with Horizonblue, the ratio for this engine should be the 1.89:1. The two other ratios will simply not work, and you'll be chasing your tail trying to prop the 2.15:1 or the 1.61:1.

Be sure that you re-shim above the vertical shaft bearing race when you switch lower units. The OEM workshop manual will give you the how-to.

If carburetion is not the low WOT RPM problem, check the progressive ignition advance. See if it meets the OEM specs at the higher rpm range. Not enough ignition advance will hold back on power. Too much will cause problems and damage, so be careful.

Re: Missing AQ170 Power

Page 48 of the manual says that the main jets are the same for the 4 cyl and the 6 cyl engines. I know that doesn't help a lot but at least it rules out the issue of main jet sizes being incorrect for the engine. If you want to check them out, they can be easily removed and compared.

Re: Missing AQ170 Power

billbayliner said:

Dennis, I agree with Horizonblue, the ratio for this engine should be the 1.89:1. The two other ratios will simply not work, and you'll be chasing your tail trying to prop the 2.15:1 or the 1.61:1.

Be sure that you re-shim above the vertical shaft bearing race when you switch lower units. The OEM workshop manual will give you the how-to.

If carburetion is not the low WOT RPM problem, check the progressive ignition advance. See if it meets the OEM specs at the higher rpm range. Not enough ignition advance will hold back on power. Too much will cause problems and damage, so be careful.

Click to expand...

A very important procedure when replacing the lower unit. Off course, if he has a couple of complete outdrives to switch between, it doesn't matter.

Re: Missing AQ170 Power

HorizonblueDK said:

A very important procedure when replacing the lower unit. Off course, if he has a couple of complete outdrives to switch between, it doesn't matter.

Click to expand...

Thanks for your assistance, I actually have complete legs so it is relatively easy to change them over. It does sound like timing and although I had a manual it didn't give timing at 2000 rpm and now I have the info I thought I would try and track down someone that still had a distributor calibration machine but it appears they are now all gone. I stripped down the distributor just to make sure it was to specs so I will mark up the balancer and see what the timing is at 2000rpm.

As there appears to a couple of different Bosch disy's for the Aq170 I wondered if someone could advise what mechanical advance could be expected from Bosch part number 0231311002 as the manual I downloaded says the engine should be fitted with either a 0231152060 or a 0231121008?
Cheers
Denis

Well, I think I have eliminated mechanical advance as the culprit but that depends on what the maximum advance should be. I set up the distributor as per spec's but not the dwell as I have no meter, I adjusted the valve clearances, set the timing at 700 RPM at 12 BTDC ran the engine to 2000 RPM that advanced to 30 BTDC, (the book say's 32 -34), I then ran the engine up to 2500 RPM and it advanced to 35 BTDC so I took it up to 3000 RPM but 35 BTDC looks like its maximum. I wonder if this is to specification?