Yanmar low RPM under load

[Mike Banks]

You have checked everything else--now check your throttle linkages, governor and your camshaft and pump timing and the pump calibration.

I think your engine is probably OK mechanically.

Check your pump timing with a protractor. I have seen instances when some smart-arse at the factory, once the engine was properly assembled and tested, deliberately etch the timing marks so that the engine will run but run retarded the next time the timing is reset to the incorrectly positioned marks.

[Wotname]

Originally Posted by jmurray109

installed rebuild fuel pump and still no change and out of ideals. Could be just a tired motor and time for rebuild.

Originally Posted by Q Xopa

Dont give up.
The positives are that you have eliminated many things it ISNT.

So now keep on working down the list of possibles.

It really can only be 2 things.

1. Engine is not making enough power, or

2. The prop you are trying to drive is too big.

Try the easiest, or most likely things first.

You are chasing Engine power at the moment. You said the middle cyl didnt seem to be firing right. Is that better now with your rebuilt pump?

You can check if one cyl has less power than the others by cracking indivdual injector supply unions. If a cyl is not making power the engine will not decrease RPMs or tone when you crack that injector. Be carefull doing this as it is a high pressure squirt of diesel that comes out. You dont want it injected in your blood stream or eye etc. It can easily pierce skin. If you are not confident get a mechanic.

Of course it could also be something that is common to all cyls - exhaust elbow, fuel filter, air filter etc.

If the engine all checks out you might need to delve deeper into the prop. I dont think you positively eliminated that yet.

Good luck.

Originally Posted by Compass790

I'd still suspect air leak somewhere or fuel supply issue. If your compression ok it's not just tired.

Yes, this is not the time to give up.

As Q posted, there are only two possibilities - engine not making power or prop too big or fouled (and a dirty bottom).

Now is the time to recap:

After the symptoms still the same? That is, low RPM at WOT in gear but full rpm in neutral.

Did it ever make full power (about 2,600 to 2,800 rpm)? If yes and if nothing has changed with prop or bottom, then we can rule that out but if no (or unknown), this remains an active suspect.

Engine only needs compression, enough fuel and air to make full power.

If the head has been serviced and the compression figure are correct, then the engine isn't getting air or enough fuel.

We will assume it is getting enough air after you checked the air cleaner earlier

You has stated the injectors are new so we can eliminate them (for the moment).

You have had the injector pump serviced so we should eliminate that also.

So that leaves not enough fuel getting to the injector pump or not enough fuel being delivered to each cylinder at the right time (i.e. injector timing).

I would start again -

1. Confirm the actual fault i.e. low RPM in gear at WOT but full rpm in neutral.

2. Confirm prop was suitable and nothing has altered (broken tip or such like) and bottom is still clean.

3. Chase potential fuel issues by supplying a clean source of gravity feed fuel directly to the injector pump (bypass fuel delivery pump and fuel filters).

4. If no change, confirm all three cylinders are working by decompressing each one in turn (or cracking the fuel feed pipe as described by another poster above) and noting any differences. Do this at WOT in gear. If one cylinder is behaving differently, chase fuel issues to that cylinder.

5. If all three cylinders are identical with the above test, the timing asn't changed when the injector pump was refitted (it uses shims IIRC).

6. Check the fuel rack / governor linkage hasn't changed when the injector pump was refitted.

The reason for 5 & 6 is that it is possible that the orginial problem was fixed but a new problem was introduced giving the same symptoms. This happens more often than I care to remember .

Report back please .

BTW, Mike Banks is saying the same thing essentially but he is far more concise

[Compass790]

Originally Posted by jmurray109

installed rebuild fuel pump and still no change and out of ideals. Could be just a tired motor and time for rebuild.

Did you tell injection pump rebuilders it wasn't firing on 2nd cylinder when you took it in? Just wondering if that's why they said it had a problem on the 2nd plunger part on the pump. Wouldn't be the first time a pump had been "rebuilt" unneccessarily.
Still you are at their mercy I guess.
Wotname gave you a good checklist.