Balanced throttles on twin diesels

[boatmike]

Think I know the answer to this but I will ask anyway. 2 Volvo TMD41a diesels. Both in good nick. Have just cleaned out the exhaust elbows and removed the dreaded damper thingies. Good flow of water now and minimal steam/smoke evident. Oil pressure great, temperatures OK but one engine makes 3800RPM at WOT the other only 3600RPM neither appear to be labouring and bottom of boat relatively clean. One idles at 650RPM the other at 720RPM. Also the Turbo boost on both is the same at 3600RPM but the one that will go to 3800 rises further when opened up that extra 200RPM (as you would expect) The cables are adjusted OK and both throttle levers on the fuel pump hit the stops. I see no reason not to simply adjust the stops to let the slower engines throttle open further and adjust the slow running stops to get idle in balance. Anyone disagree?

 

[PeterBoater]

Are your tachometers accurate? You can buy a cheap hand-held optical rev counter on the internet; aim it at a dab of white paint on the crankshaft pulley, mine came with little reflective strips to stick on the pulley wheel. At WOT, does the boat run in a straight line with the wheel centred? Are the gearbox ratios the same, are the prop dimensions the same?

 

[ChromeDome]

The first point is to make sure your figures are correct.

If the instruments are off it makes no sense to use them as a baseline for correction.

These counts rev on anything moving (so be careful what you point it to)
Amazon.co.uk

 

[Montemar]

My rev counters differ by 100 rpm

 

[boatmike]

I have very accurate digital rev counters but to eliminate any errors I swapped them over and the readings are identical

 

[DavidJ]

Buy one of these. I’d sell you mine but they are only a tenner on eBay

 

[ChromeDome]

Buy one of these. I’d sell you mine but they are only a tenner on eBay

View attachment 164052

Link in #3 at £6

 

[boatmike]

Link in #3 at £6

I don't think you guys are listening! I swapped the rev counters over and the readings are the same which proves without doubt that one engine achieves 200 RPM more than the other. I would welcome input from anyone who can answer the original question but this is drifting away into an argument about the accuracy of the rev counters which has been proven to be a non issue........
To be clear, both engines have adjustable throttle stops on idle and WOT. I was hoping someone would either say it is normal to adjust these so that both engines agree or conversely give me a reason why I shouldn't

 

[ChromeDome]

Well, my #3 was before you stated your #5.

Hard to listen to something that hasn't been said.

 

[kashurst]

check/replace the airfilters then check and probably replace and flexible fuel pipes, feed and return. I had a pair of KAMD300s do something similar. In the end after trying everything else I replaced all the flexible pipes on the slower engine, problem solved. Never sure why but fuel pipe is cheap.

 

[boatmike]

check/replace the airfilters then check and probably replace and flexible fuel pipes, feed and return. I had a pair of KAMD300s do something similar. In the end after trying everything else I replaced all the flexible pipes on the slower engine, problem solved. Never sure why but fuel pipe is cheap.

Good thinking, Will check fuel pipes and fuel filters to ensure no restriction in fuel supply but the air filters are new(ish) so I don't think its that

 

[oldgit]

Assume this is shaft ?
Does this occur at both Low and Hi idle and/or when under load.
Having checked all the basic stuff, is this new phenomenon, could be one of the prop blades has sustained damage or become fouled.
One of the engines having to work "harder" than the other.
Is one of the shafts binding in the "P" bracket ?
Simple to check, just turn the shafts by hand at the reverse gear flexible joint and note if any difference in resistance.
At WOT my engines do sometimes not achieve the same revs depending on sea conditions/load etc but as it amounts to a hundred or so RPM at 3400, simply not that bothered.
Did once have boat where the boat always revs matched ...possibly !

 

[Seastoke]

I don't think you guys are listening! I swapped the rev counters over and the readings are the same which proves without doubt that one engine achieves 200 RPM more than the other. I would welcome input from anyone who can answer the original question but this is drifting away into an argument about the accuracy of the rev counters which has been proven to be a non issue........
To be clear, both engines have adjustable throttle stops on idle and WOT. I was hoping someone would either say it is normal to adjust these so that both engines agree or conversely give me a reason why I shouldn't

You are lucky to get replies , with that attitude .

 

[scottie]

Is power steering fitted as usual on one engine only also uneven battery load can affect revs
other possible reason used to be differences in reduction ratios and or prop sizes but all these don’t suddenly happen

 

[boatmike]

Assume this is shaft ?
Does this occur at both Low and Hi idle and/or when under load.
Having checked all the basic stuff, is this new phenomenon, could be one of the prop blades has sustained damage or become fouled.
One of the engines having to work "harder" than the other.
Is one of the shafts binding in the "P" bracket ?
Simple to check, just turn the shafts by hand at the reverse gear flexible joint and note if any difference in resistance.
At WOT my engines do sometimes not achieve the same revs depending on sea conditions/load etc but as it amounts to a hundred or so RPM at 3400, simply not that bothered.
Did once have boat where the boat always revs matched ...possibly !

Yes its shaft
All figures quoted are under full load.
The props are new fitted only recently and both shafts spin freely by hand
There is no power steering
The key fact for me is that both engines behave identically up to 3600RPM with identical turbo boost
I do think I need to check the fuel flow but I don't think I would get the same boost from both if one had fuel starvation
I am still inclined to adjust the WOT throttle stop on the slow engine to allow higher revs.
Can't think of anything else.

 

[lynallbel]

Do not forget the rev counter signal comes off the alternator which is belt driven, so even a slightly worn belt will change the figures.

Plus as the pump is a Bosch unit, adjusting the throttle stop will increase max rpm at a standstill, but not whilst under load as the internal governor will take care of that bit.

 

[scottie]

Yes its shaft
All figures quoted are under full load.
The props are new fitted only recently and both shafts spin freely by hand
There is no power steering
The key fact for me is that both engines behave identically up to 3600RPM with identical turbo boost
I do think I need to check the fuel flow but I don't think I would get the same boost from both if one had fuel starvation
I am still inclined to adjust the WOT throttle stop on the slow engine to allow higher revs.
Can't think of anything else.

I would check prop details if newly fitted just to eliminate the possibility of error

 

[Momac]

Whats to lose by adjusting the cable?

 

[burgundyben]

Where does the tacho signal come from?

I've seen a boat where one engine had a bigger alterbator pulley, so the rev counter was under reading.

 

[scottie]

Probably remove the doubt in your mind at least