Loss of revs at Wide Open Throttle

[nigelmercier]

A few weeks ago I noticed a loss of revs at WOT. Thinking there was something wrong with the propeller, I was pleased to find a plastic bag wrapped around it; however, the next day I still had the problem.

Although this is a new propeller, I'm 99% certain that it is the same specification as the old one, and 100% certain that I've had full revs with the previous one, which was indeed slightly larger than the original. The blades are clean. The propeller turns freely by hand, and when under sail.

Symptoms: the last 1/3 of the throttle does nothing when the engine is under load. In neutral, it revs up to 4000 (then I chicken out).

Here is an extract from my log book:

Engine, lack of power at WOT: Rev counter reading about 15% low, max revs under load on quay therefore 1,700 RPM (2000 RPM displayed). Both fuel filters replaced, pick-up pipes blown through (some grit found), return pipe checked for air bubbles, fuel tap blown through (small piece of black plastic removed, possibly from a 10 litre tank), air inlet cleaned, exhaust elbow removed and de-coked, exhaust pipe cleaned with cloth pull-through, each injector disconnected in turn: same loss of max revs (about 500) in each case. Final test run with direct fuel line to lift pump, no different. However, a sea-trial showed that maximum revs now displayed 2500 RPM into a head wind.

A diver at the marina has suggested that there could still be some plastic jammed on the sail-drive shaft, and that it only makes contact when the propeller is driving the boat, thus pushing the propeller towards the sail-drive. There is a 5mm gap between the propeller and the anode.

I therefore open this poll: is this a likely solution?

 

[misterg]

... the last 1/3 of the throttle does nothing when the engine is under load. In neutral, it revs up to 4000 (then I chicken out).

Do you get black smoke during that last 1/3rd of throttle under load?

"Yes" = yes that could be your problem (or your new prop, or dirty hull, or... ...)

"No" = no it's not your problem - look again at your fuel supply (filters, etc.)

Andy

 

[nigelmercier]

Do you get black smoke during that last 1/3rd of throttle under load?

No smoke.

Regarding the propeller. I replaced it with a larger one about 5 years ago, and it was fine. The new one I fitted this year was ordered with the same specifiction, but I confess I didn't check the markings. Both are made by Volvo Penta.

 

[davidej]

For G*d's sake, get you mask and shnorkel on and find out instead of running this poll.

The water is still quite warm!

 

[jonathanhsm]

Air in fuel lines?

Does it slow down and then speed up again? If so I suggest air in the fuel lines.

 

[nigelmercier]

get you mask and shnorkel on and find out...

I would do, but I'm not the next one out to the boat. I want to convince my partner to take a look.

BTW, it's not air in the fuel - we tested for that with the direct line to the lift pump.

 

[misterg]

No smoke.

It's a diesel engine, yes?

QED:

"No" = no it's not your problem - look again at your fuel supply (filters, etc.)

One other thing is to check you're getting full travel on the "throttle" linkage and that the fuel cut-off is completely open.

Oh, and is there a sneaky gauze filter at the inlet to the lift pump?

Either the engine isn't being asked to deliver the extra fuel (linkages, etc.) or it can't draw the extra fuel (filters, blockages, "the bug", etc.)

Andy

 

[wooslehunter]

If the problem ocurred at the same time as the bag, the chances are it's linked. But, maybe not. Is there a gauze filter in the lift pump. There is in mine & when it got blocked I suffered frfom a severe lack of revs. i.e none at all.

 

[Fire99]

If the engine is running consistently at lower revs and not dying etc then it's a power issue. This could be from an over-pitched prop to dirty filters or leaking boost pipe etc.

My normal course of action is to do a basic 'visual' check of the engine, linkages, filters etc to discount anything obvious and then consider anything that has been changed recently.

i.e, double check the prop size / pitch against the old one. Worth doing this before getting too involved.

 

[misterg]

It's not the prop (or turbo, etc. ) - If he isn't getting black smoke, then the engine isn't getting fuel.

Andy

 

[jonathanhsm]

air

Or it could be a dodgy lift pump or beyond - try a direct line from fuel can to injector pump with fuel can high up to provide gravity pressure. That was my problem turned out it was a dodgy banjo point AFTER the lift/jerk pump.

I would do, but I'm not the next one out to the boat. I want to convince my partner to take a look.

BTW, it's not air in the fuel - we tested for that with the direct line to the lift pump.[/QU

 

[Tranona]

Almost certainly the prop is too "big". All Bavarias are propped to get just about full revs - in your case at least 3600, probably 3800. An inch too much on pitch or diameter will reduce max revs by 250-300. Volvo dealers have a chart which will show the correct size for your boat (assuming it still has the original saildrive).

It is unlikely to be the remnants of the plastic bag as it it had got into the saildrive it would have likely wrecked the seal by now. There is nowhere it can be trapped if the spacer on the shaft is still there.

While you have it out to check (and probably replace) the prop worth considering fitting a rope cutter as that will protect the saildrive from damage from plastic bags, ropes, nets etc.

 

[ianj99]

I had a similar occurrence with my 4108. It had been running fine then after a few weeks of disuse, would not rev over 1600rpm in gear due to what looked like a fuelling issue. (ie no smoke)

I changed the engine filter head, both filters (both clean, having been changed 12months earlier), re bled it to no avail.
I found the 30year old exhaust hose had partially delaminated internally so it was replaced. This helped.

I then threw away 35galls of very old diesel (very brown but had been filtered and treated with Soltron and Fuel Set 6months earlier). It didn't smell very diesel like so I concluded that due to its age (probably over 5years) and the various additives, it had partially decomposed into goodness knows what!

After these steps the engine would rev to 2000rpm, but the final cause was a heavily barnacle infested prop.

My point: there may more than one cause.
Ian

 

[Fire99]

My point: there may more than one cause.
Ian

Absolutely, but unless you have pockets deeper than the Atlantic, check the obvious (cheap) stuff first. Before major surgery.