500 RPM LOSS? what can cause this? 2.5L Iron Duke

[Adam_c]

I was Running around 4200-4400 rpm WOT 19P prop and recently i noticed im running about 3500-3800 RPM WOT which of course means i'm not going as fast now by no means would i constantly run it WOT but its nice to have that extra little bit.

what could cause this?
I've checked my throttle linkage, I always run mid grade gas 89 or 91

I've read maybe spark plugs however they were changed at the beginning of the season but i did have a few misfiring issues early on which have been fixed.

I have 2 carbs and both of them seem to have my boat running the same way so I don't suspect a carb issue.

Just wondering if anyone else has every experienced this?

Thanks!!

 

[Tycer]

That’s a low compression engine. Run 87. Running higher octane than an engine can burn can lead to excess carbon buildup on back of valves, rings, pistons and head. Not saying that’s your problem but decarbonizing with oil and gas treatment would be something I would do.
Check your dwell. The rubbing block may be worn and points aren’t open long enough to fully collapse the coil field.

 

[Scott06]

Verify the tach is correct with ashop tach first , common that boat tach get inaccurate. Have you verified hull is clean and not water logged, and ignition system is in good tune ?

Seeing your title , the iron duke is a Pontiac engine that is comepletley different from the mercruiser 2.5/3.0 which is Chevy based that you have. Just keep that in mind when ordering parts

 

[Adam_c]

Tycer funny enough I was reading today that lower octane ignites better in low performance engines I always thought higher octane ran better. I do suspect a worn block as I am getting blowby dripping.

Scott06 tach was jumping around a bit today never notice that before I'll order a new tach and see what happens also curious if my recently installed bimini top has something to do with my 10kph drop with the resistance. It was closed but still in the up position which would cause some drag. I don't believe I'm water logged but I assume cutting the floor open would be the only way to check? As for the iron duke I thought I read somewhere that's what the old 70-80's 2.5s were called but I've never ordered part looking them up that way thankfully. Just installed new points, condensor, and coils trying to solve a previous problem but that turned out to be my condensor. Haven't changed the plugs yet but bough some new ones just to verify to see if all my previous backfiring problems and stalling may have ruined them but have taken them out and cleaned them since all that.

​​​​​​​Thanks for the advice.

 

[HT32BSX115]

I was Running around 4200-4400 rpm WOT 19P prop and recently i noticed im running about 3500-3800 RPM WOT

Howdy,


There is a list of "things" that will affect WOT RPM. To view them click #15 here: https://forums.iboats.com/forum/eng...es-outdrives/295720-adults-only-no-exceptions

Octane number has nothing to do with carbon build up, Fuel-air mixture does. Using a higher octane number fuel in an engine not needing it only reduces your bank bank account size, nothing else.

Decarbonizing is a 2-stroke "thing" and almost is NEVER needed in a properly running 4-stroke engine. If you are getting excessive carbon build up, either re-jet the carburetor, properly adjust the choke, or disconnect it altogether. (I have been running without a choke in my boat for 15 years)

Timing (one of the items in the above list) has a large effect on WOT RPM, so you might check that one first along with points gap and a couple of other ignition related items (like total mechanical advance)

Regards,


Rick

 

[Scott Danforth]

first, the 2.5 is not the Pontiac 151 cubic inch "Iron Duke"

when was the last time you did a tune up/

check the list of low WOT items that Rick posted

if you still have points ignition, file, re-gap, verify dwell and re-time

 

[Adam_c]

Thanks guys I changed the points and condenser not long ago but today I figured out it was the gap in the points had closed and I reset the gap and voila runs perfect and regained my rpm/top speed ugh boats there's always something lol

 

[Tycer]

Howdy,



Octane number has nothing to do with carbon build up, Fuel-air mixture does. Using a higher octane number fuel in an engine not needing it only reduces your bank bank account size, nothing else.

Decarbonizing is a 2-stroke "thing" and almost is NEVER needed in a properly running 4-stroke engine. If you are getting excessive carbon build up, either re-jet the carburetor, properly adjust the choke, or disconnect it altogether. (I have been running without a choke in my boat for 15 years)


Regards,


Rick

Check your PM

 

[Tycer]

Check your PM

So after a side discussion with Rick - very thoughtful response from him by the way - I can no longer make my octane claim. Maybe I could in 1987 but I cannot find any corroboration for my claim with today’s gas or even with old ethyl gas.
Thanks Rick!