Low RPM at WOT on Mercruiser 260

[cantaris]

Hi all and thanks in advance for any help and advice. I have a 1985 Bayliner 2550 Ciera Designer's Edition that I put a 350 engine in coupled to a mercruiser 260 outdrive. The engine has a Pertronix ignitor in it, Pertronix Flame thrower coil. Carter 4 BBL carb(9781s 1875). Closed cooling system. Compression on all cylinders is 125-160. Which according to Mercruiser service bulletin is good. I bought a 16in dia 16 pitch prop from the local prop shop based on their recommendation. The problem that I am having is I can only get 3500 RPM at WOT doing 26 MPH(2 adults, 2 children, 50gallons of fuel, 20 gals of fresh water, empty holding tank). From what I have read on other forums is that that is about what cruise RPM should be. Things that I have done:
1 Read the Adults Only post for low RPM at WOT
2 Changed fuel water separator
3 Rebuilt Carb
4 verified no air bubbles in boat side fuel lines, added 60 gals fresh fuel to 25 gals of old fuel
5 Adjusted Timing to about 30 degrees advance at 3000RPM in neutral?? using timing light with advance feature but I am not certain if I am doing this correctly.
It will not go beyond 3500 RPM as if there is a mechanical stop. It is slow to get on plane. If I drop below the 26mph the boat drops off plane. Temperature climbs to about 220-230 but the new gauge is suspect. When I drop down off plane temp stays at 200 but infrared temp says temp at probe is about 160 ish. The boat is in the marina so I am limited to checking stuff in the boat. Thanks again for any help.

 

[alldodge]

Re: Low RPM at WOT on Mercruiser 260

Will the prop shop let you try a 14 or less pitch prop?

 

[Don S]

Re: Low RPM at WOT on Mercruiser 260

The prop is not your problem, no way, no how. You are about 1000 rpm off and an inch of pitch is going to change 100 to 150 rpm change.
There isn't a 350 out there running a 16" prop, let alone a 14". 19 is about minimum in reallity.

Do you use the trim at all or just run it all the way down?

 

[alldodge]

Re: Low RPM at WOT on Mercruiser 260

The prop is not your problem, no way, no how. You are about 1000 rpm off and an inch of pitch is going to change 100 to 150 rpm change.
There isn't a 350 out there running a 16" prop, let alone a 14". 19 is about minimum in reallity.

Do you use the trim at all or just run it all the way down?

You talking diameter of pitch.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

The original prop that came with the 260 out drive was a 14.5 x 19 but was only giving me 3000 wot. The prop shop sold me a 16 in dia 16 pitch and said that was what was needed to push that heavy boat and that I would be happy with it. There is only about 3/8 in clearance between tip and cavitation plate. Don I did have it trimmed all the way down to get up on plane but if trim it I loose speed. Now another part of the puzzle. This boat started out with a volvo penta 350 in it that was an insurance write off due to water damage. I glasses in new transom and stringer and cut new hole for mercruiser package as that was what I had in my garage for an earlier scrapped project. I was debating putting the 14.5 x 19 back on since I have messed around with Timing since that first run and see if it makes a difference. The prop shop told me he had a 15 x15 he trade for the 16 x16. but he didnt think it was a prop issue.

 

[Don S]

Re: Low RPM at WOT on Mercruiser 260

Have to ask, when you cut out the new transom, did you have the X dimension in the right place?

Compression on all cylinders is 125-160. Which according to Mercruiser service bulletin is good.

That is boarder line at best. What are all the numbers?

Again, you DO NOT have a problem that can be fixed with props. Just too many rpm to make up.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

I have a 24 searay sun dancer with a 350 and alpha drive. I have even 165 on all cylinders . But it is a new engine. The boat is 6500 ish loaded and with a 17p 15.5 diameter stainless prop I run 41 mph @ 5000. It comes out of the hole really good and will old a plane down to 17 mph in rough water. I would guess your engine may be weak , or the exhaust is plugged

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Have to ask, when you cut out the new transom, did you have the X dimension in the right place?

I used the mercruiser paper template and did use the x dimension. But there is nothing that can be done about it if I did not get it exactly. I am pretty sure it is within 1/2 an inch .



That is boarder line at best. What are all the numbers?
The numbers were as follows:1= 150psi, 3= 125psi, 5= 125psi, 7= 150, 2= 145psi, 4= 135psi, 6= 135psi, 8= 160psi
I just rebuilt this engine last fall.

Again, you DO NOT have a problem that can be fixed with props. Just too many rpm to make up.

If the PO of the 260 outdrive ever changed the gears would that affect the RPM. Just speculating.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Would too much fuel cause these symptoms? The reason I ask is based on the Carter/ Weber/ Edelbrock info I have been able to find. I believe this carb is a 700- 750cfm. Also when I rebuilt it a couple weeks ago, I noticed that the primary and secondary jets are 120s. But on the a fore mentioned sites marine carb, Edelbrock 1411 specs say that primary should be 113 and secondary should be 107.

 

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Stop changing things.

First thing, you made a radical change with that prop. Increasing diameter and reducing pitch. Before making any other changes find out the drive ratio of the original Volvo drive, and the drive ratio of the Merc drive. The engines are very near the same HP so the drive ratio determines the prop size and pitch. If the same then the original prop may be necessary.

Next, check fuel flow. Being careful not to spill fuel, disconnect the fuel line to the carb, point into a suitable container, crank the motor and observe the fuel flow. WOT demands a LOT of fuel, if the stream is weak then check fuel filter, tank fitting for restrictions.

Edit; prop, increased Diameter and REDUCED pitch.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Thanks CharlieB for the reply. First I have no idea what original VP gear ratio was. I bought this boat for $130 including title transfer from local dealer. It was stored at the local navy marina. It took on water from a bad seacock for toilet. Then towed to dealer and was written off by insurance. Dealer took VP dual prop outdrive off and sold it. I was left with a 350 engine which I pulled and put my merc set up in after rebuilding transom and stringers. I have been piecing together this mercruiser setup over the last couple years. The out drive I have is a 260 and says 1.5R SN is 5029603 it came with a 14 x19 prop PN 48 78120 A40 19P. The first time I took it out, I could only get 3000RPM and 20 mph. Called Prop Shop told them what I had and they said I needed a 16x16 to push that heavy boat. I will double check my fuel flow but am running a new double diaphragm mechanical pump so should have good flow. I did check boat side fuel with clear fuel line and a vacuum gauge and everything looked fine. I did have a timing issue after trying to start it the other day and went out did hit almost 3800 and saw 28mph on gps but had a horrible pinging/ knocking. Back at dock with advance timing light I set timing to 30 degrees at 3000rpm in neutral. But am only getting 3500rpm and 25mph at WOT.

 

[alldodge]

Re: Low RPM at WOT on Mercruiser 260

Some things don't add up and if I'm way off just go tell me to go back to sleep --- You mentioned this is a heavy boat, my friend has a 2550 and gets on plane very easy with a 5.0L and a bravo 2 drive and weights around 6000. His cruise speed for him is at 27 to 2800 RPM and jumps out of the water compared to my 10000 pound boat. Your getting 3000 and barely getting on plane. In your original post you mentioned it had water damage, so my question. How much do you think your boat weights and if it weights more than 6000 is there a possibility that the hull has soaked up some water which is still there?

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

So the water was only in engine compartment and only as high as the middle of the harmonic dampener. I rebuilt the engine compartment, so I do not believe weight is an issue. I figure it weighs 6000-6500 fully loaded. I had about 25-35 gallons of old fuel in it and added 60 gallons of fresh. new fuel water separator, new fuel lines and vent lines and vent ports. I believe Part of my problem lies in the timing, part in carburetor, and part in ignition. I am just having trouble trying to isolate. I know I can get higher RPM and speed because i saw 26-29 on gps and fish finder and about 3600-3800RPM but it was pinging horribly and I had to back off. I then set timing with advance timing light set to 30 degrees and revved it to 3000 RPM while in neutral and adjusted distributor to 0 deg. The pinging went away but 3500 is all I could get and about 25 at WOT. Maybe its time to find one of the mobile mechanics here in the Everett area and have them take a look at it. Does it make a difference that my out drive is an older "I" drive?

 

[Eric 3.7LX Alpha 1]

Re: Low RPM at WOT on Mercruiser 260

Did you ever check to see if your throttle cable is opening the carb to WOT by shifting to WOT then remove the cable off the carb, and see if the carb has more travel before it bottoms out?

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Did you ever check to see if your throttle cable is opening the carb to WOT by shifting to WOT then remove the cable off the carb, and see if the carb has more travel before it bottoms out?

I have not removed cable and checked for additional throttle movement. It does look like it is opening all the way. I will check this afternoon when I go down to the marina. Thanks

 

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Sometimes the fuel tank dip tube and/or the check valve in the tank fitting get crud in them and restrict fuel flow.

Easiest test is to disconnect the fuel hose to the carb, crank the motor and see how much fuel is delivered.

The engines demand a LOT of fuel. Reving in Neutral doesn't take much fuel, when under full load, it's about 26 gallons per hour flow. Any restriction will seriously limit RPM while underway, the motor will pull up to the limited fuel flow. Clue, if you can pull the throttle/control arm back from WOT, even just a little, and the motor speed remains the same, then there is a fuel restriction. The motor is running at the limit of the fuel.

I seriously suspect that 16 X 16 may be too much prop.

Double check your tune-up to ensure the motor is running correctly.

Check the Prop forum. There are some guys in there that have amazing ability to pert near dial in prop requirements for your boat/motor combination. They will want to know exact make model etc of the boat, motor, and drive (with drive ratio), weight, if possible.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Thanks CharlieB. I went to boat today and set timing to 8DBTDC at 650-700RPM while in gear. I went out and was able to get 29-30 mph on fish finder paddle wheel and about 24-25 on gps at about 3600-3700 RPM. It did seem to run better but I am still lacking on RPM . Talked to a local mechanic the deals with Mercruisers and he believes it is a prop issue. he feels I should try a 15in dia x 17 pitch. I am starting to question the accuracy of the rpm gauge. It is a sunpro which is made by Snapon but it is an auto motive gauge. I might borrow one of our digital tachs from work and have my wife drive while I try an get an accurate reading at WOT and compare.

 

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Generally changing prop PITCH and inch shoulder increase/decrease rpm about 200

Diameter changes can make a much greater difference as it is a much larger change in loaded area. That jump from 14.5 to 16 was a huge change while the 3 inch reduction in pitch wasn't.

A good prop shop should work with you and try selected props until you find one that allows the motor to work within the minimum 4400 - 4600 range.

A reputable tach is needed just to be sure, even if only to verify the boat tach accuracy.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

Like I posted earlier I have a 240 searay with a 260 and alpha drive with 1.5 ratio, I run a 15.5 x17 stainless vengence prop. I can get 5100rpm. Timing is 32 degrees all in at 3200. I hope this helps. We were out yesterday also off Everett , what a great day!!!

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Like I posted earlier I have a 240 searay with a 260 and alpha drive with 1.5 ratio, I run a 15.5 x17 stainless vengence prop. I can get 5100rpm. Timing is 32 degrees all in at 3200. I hope this helps. We were out yesterday also off Everett , what a great day!!!

I may have seen you as I was out yesterday between the yellow marker bouy and towards hat island. There was a boat that went towards hat island and I was laying on back swim step changing the prop and was worried about the wake knocking me in the water.
So I changed the prop back to the 14 x 19 while in the slip and went out to see what it would do since I have played around with different things. I could not even get up on plane or barely. was pushing about 20mph and 3000RPM at WOT. It did move me faster at idle in the no wake zone and I thought it was going to work. Boy was I wrong. Hence me laying on swim step( Oh look mommy there is a big walrus on the back of that boat) and changing back to the 16 x16 prop. So that got me up on plane but still only getting 25-26 mph at 3600-3700RPM. Is it possible to have too much carburetor. I am pretty sure the carb is a 700-750cfm. Cause I seem to have sucked about 60 gallons of fuel faster than I would have thought for the few 20-30 minute runs I have made. I mean yesterday I had 1/2 a tank was out for 20-30 minutes and am now at 1/4. Seems a little excessive.