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Author Topic: Loss of RPM  (Read 1197 times)

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Jeff Tancock

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Loss of RPM
« on: May 14, 2019, 07:06:13 AM »

Looking for some thoughts and group wisdom regarding my engine's poor behavior.
I have a 25XP. Owned over 25 years.
It has always run the same. It's always been hard to start. Full throttle in gear has always been 2600RPM pushing boat at approx. 6.5 knots depending on condition of the bottom.
Full throttle in neutral has been 3200 RPM.
Took her out for a spin recently. Still hard to start, full throttle in neutral is 2800 and in gear 2100. boat speed 5.4 knots. Had this problem at end of last year but it might be a bit worse. Not sure.
Something is definitely wrong.
Hauled last year so shouldn't be the problem. Fuel pump ticks away as usual and I have been told (or read) that these old pumps either work or not...not in between.
No big smoke to indicate injectors.
Recently replaced both water pumps.
Suggestions??????
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Jeff Tancock
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Jon W

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Re: Loss of RPM
« Reply #1 on: May 14, 2019, 07:35:49 AM »

Have you checked the throttle cable clamp(?) at the engine didnít slide out of place?
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Jon W.
s/v Della Jean
Hull #493, 1987 MK 1, M25XP, Manson Supreme 35
San Diego, Ca

Jeff Tancock

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Re: Loss of RPM
« Reply #2 on: May 14, 2019, 07:44:54 AM »

Yes I did check for that. It has slipped out before.
Thanks for the suggestion though.
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Jeff Tancock
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Ron Hill

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Re: Loss of RPM
« Reply #3 on: May 14, 2019, 02:39:40 PM »

Jeff : Have you ever replaced your injectors?  Have you checked your engine RPM instrument readings with a digital laser tach? 

On the hard starting have you changed the glow plug wiring to the solenoid upgrade?

A few thoughts
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Jeff Tancock

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Re: Loss of RPM
« Reply #4 on: May 14, 2019, 02:49:26 PM »

Thanks for your thoughts Ron.
I've done the wiring upgrade. It turns over with lots of power but doesn't fire up. Seems like a fuel issue???
I have not done anything to the injectors....ever. Does it sound like an injector issue to you?
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Jeff Tancock
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Roland Gendreau

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Re: Loss of RPM
« Reply #5 on: May 14, 2019, 06:12:12 PM »

Could be multiple issues.

Suggest you verify that the glow plugs are functioning properly.
Have the fuel filters been serviced?
Any chance something is blocking the air filter intake?
Are you sure the throttle is opening 100%?
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Roland Gendreau
1992 MK 1.5
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Bristol, RI

Jeff Tancock

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Re: Loss of RPM
« Reply #6 on: May 15, 2019, 07:20:09 AM »

Glow plugs? I can't be sure they are working properly but the volt meter does dip significantly when engaged.
Both fuel filters changed and I checked the filter in the facet fuel pump.
Took off the air filter It's clear and made no difference.
I think the throttle opens until it hits the nut acting as a governor. I've looked at that but will double check that today.
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Jeff Tancock
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Ron Hill

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Re: Loss of RPM
« Reply #7 on: May 15, 2019, 03:18:53 PM »

Jeff : Hard to believe that you are still using 31 year old injectors!  I'd spring for some new ones.
Have you checked your engine instrument tach against a laser digital tach??
Have you change your glow plug wiring so you now use a solenoid?  Look in the critical updates!!  You could (maybe) use a new glow plug or two??  Pull them and inspect them!!

A few thoughts for answers!!
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Roland Gendreau

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Re: Loss of RPM
« Reply #8 on: May 16, 2019, 02:53:33 PM »

My NAPA fuel pump failed but continued to exhibit the tick, tick, tick.  It wasn't till I disconnected the output hose from the pump to use it to pump out my fuel tank that I found out it wasn't pumping at all.   

So don't assume the pump is working just because it is making the noise.



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Roland Gendreau
1992 MK 1.5
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Bristol, RI

Jeff Tancock

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Re: Loss of RPM
« Reply #9 on: May 17, 2019, 08:15:26 AM »

All good thoughts
I just did a 4 hour run at full throttle and it runs well, just doesn't have the top end. The last 20% of throttle doesn't change anything.
I am thinking that I will change the fuel pump first after hearing that the ticking doesn't mean the pump is healthy. (Thanks Roland).
Even if that takes care of it I will pull the injectors and have a look at the glow plugs as they are both high on my suspect list.
I've read up on pulling the injectors but know nothing about pulling glow plugs.
Thanks for all your thoughts.
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Jeff Tancock
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Ron Hill

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Re: Loss of RPM
« Reply #10 on: May 17, 2019, 01:13:59 PM »

Jeff : To remove a glow plug : remove wiring, insert socket over G plug and back it out!!

BTW, because the fuel tank is higher than the engine (just like Roland attested to!!) the fuel will gravity flow and keep the engine running when the lift (fuel) pump is not operating!!

A few thoughts
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Roland Gendreau

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Re: Loss of RPM
« Reply #11 on: May 17, 2019, 01:48:34 PM »

When my fuel pump malfunctioned, the engine went from full power to stalled.  After a few minutes, I restarted it but could not run the engine at much more than idle even with the level of fuel in the tank more than half full.    Although the tank is higher than the engine, I believe the malfunctioning pump and maybe the Racor were obstacles to gravity fuel flow.

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Roland Gendreau
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Jeff Tancock

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Re: Loss of RPM
« Reply #12 on: May 18, 2019, 10:05:57 AM »

Before I buy the new pump does anyone have any recomendations or remorse after buying either the NAPA 610-1051, the Pro13SV or the FAC 40105 CUBE model?
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Jeff Tancock
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Jon W

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Re: Loss of RPM
« Reply #13 on: May 18, 2019, 11:47:43 AM »

FWIW if you're in California - Yacht Supply Depot is listed in the Tech WIKI as a low price supplier, but they don't ship to California. I just ordered a Gold Flo series cylindrical pump 476087E for $79.95 from Aircraft Spruce who does ship to California. Others online are selling this same pump for between $87-$122. I haven't received it yet but it's on its way.

Sounds like you're looking at the "cube" series not the cylindrical. A quick look and the NAPA price is $66, these two suppliers are ~$37. From a price point of view it's worth taking a look before buying from NAPA or someone else.
« Last Edit: May 18, 2019, 11:50:42 AM by Jon W »
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Jon W.
s/v Della Jean
Hull #493, 1987 MK 1, M25XP, Manson Supreme 35
San Diego, Ca

Ron Hill

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Re: Loss of RPM
« Reply #14 on: May 18, 2019, 01:43:14 PM »

Guys : I ran my M25XP engine with out a lift pump for over a year.  I had wired the electric fuel pump to the blower instead of the ON pole of the ignition switch.  We traveled from the Potomac to Key West and back like that - we did not travel like that with the engine in idle!!
I would periodically turn ON the blower and note if the RPM increased.  It usually didn't - if it did I changed out the Racor filter and then the RPM remained the SAME!!

Jeff : If you have the Racor filter before the fuel pump you do not need the cylindrical fuel pump (with its filter in the cylinder). Buy the cube!!

A few thoughts
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