Low RPM at WOT on Mercruiser 260

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Too much prop overloading a motor will definitely create a lot of fuel flow, way more than working the motor at 4600 - 5200.

16 inch dia is a LOT of prop.

Lord how I miss living in Everett!

 

[NHGuy]

Re: Low RPM at WOT on Mercruiser 260

I think Dogdave is on to something. He has a similar boat and same motor. You should expect the performance he gets with a 17" pitch prop. Soooo, either your boat is too heavy, or lacking power. You know how to run your trim right? Start off with it all the way down. Don't raise it til you are up on plane; then raise it a little at a time til you get some slip or porpoising, then trim it back down enough to get stable.

If you find that the throttle IS opening all the way then maybe you should weigh the boat, or get a book on your carb and make sure it's perfect. A flawed carb could be the problem, but so could a waterlogged hull. There are other possibilities but those are your best bets.
Keep at it, you are probably getting close to the answer!

Edit: I don't think the drive you have will handle a 16" diameter prop, 16" pitch sure. But as I recall they only take 14.75" or so diameter.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

Cantatis, I did not see you out there I would have pulled in to see that everything was ok, because I am that kinda guy. We are headed back out this weekend . Sounds like our boats are in the same marina. As for carburation I have a q jet. My knowledge is mostly from the car world,, but a 750 carter or edelbrok is big for a stock 350. I would check the manual to see if you have a stock carb. If not tht may be a place to look. For jetting, make a run on the primaries of the carb, shut it down and take a plug reading. Do the same on he secondary's and you will find out your jetting. Economy, running back and forth to hat island and shrimping off mukelteo and then north to penn cove, I averaged 7 gph with a fair amount of idling . According to the boat logs the set up I running produces 7.8 gph on a cruise to Victoria @ 3200 rpm.

 

[CharlieB]

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 .........................

How old is this unit?

Have you had the Distributor cap off?

With the cap off, remove the rotor and look at the advance mechanism, springs, weights, the pivot points look good? Lightly lube, just one drop of oil or a very light weight grease. Look at the slots and limit pins, turn the upper shaft by hand, will it turn freely pulling against the springs to the end of the slot against to stop pins? If not then this could be part of the problem, the ignition is not advancing fully and limiting power.

Q-jet carbs work very well, the secondary air gate adjusts air flow to the demand of the engine. The same 800 cfm will work on a number of small blocks as well as a few big blocks with minor adjustments. As long as fuel flow is good and the throttle cable pulls wide open, and of course the carb is clean, shouldn't be a problem.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

Good call Charlie, I did not think of stuck advance in the petronix. I have a thunderbolt in mine .

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Ok so the outdrive is a mercruiser 260 "I" drive it is what some people call pre Alpha. I believe when I bought it a few years back the PO said it was an 1980ish. The Carburetor is a Carter 9781s 1875 which is the predecessor to the Edelbrock 1409 or 1411. From what I gather the 7 after the 9 in the part number signafies that it is a 700cfm which is what the 1411 is. The primary and secondary jets are both 120s. The Edelbrock specs for the 1411 say that primary should be 113 and secondaries should be 107.
I do need to take off the plate that is in the distributor to look at advance weights. seems that I did see a light coating of rust when looking through the holes in the plate.
Hey Dogdave I am at H38 at marina if you feel like mosying over. I will be there Saturday afternoon

 

[Sunny2007]

Re: Low RPM at WOT on Mercruiser 260

Well i used to have an 86 Bayliner Capri and bought it used.The guy told me it had a governor on it.That's why it only ran at 3500 rpm wot.I found the pickup tube,ball and spring check valve at the fuel tank was partially plugged with varnish from lack of stabalizer treatment.The boat ran like a top with no miss or sputter.
After cleaning the check valve it hit 4300 rpm wot.

You never know

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Well i used to have an 86 Bayliner Capri and bought it used.The guy told me it had a governor on it.That's why it only ran at 3500 rpm wot.I found the pickup tube,ball and spring check valve at the fuel tank was partially plugged with varnish from lack of stabalizer treatment.The boat ran like a top with no miss or sputter.
After cleaning the check valve it hit 4300 rpm wot.

You never know

Thanks Sunny2007 I will look into that. Someone else mentioned it here. I am thinking that the nipple that is at the tank is welded to the fuel tank with no way to inspect pickup for debris etc. I have a list of things I will check tomorrow after work.

 

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Do NOT change the jets in the carb. It is way too early to make any change to what may still be factory design.

Finish going over the mechanical advance , the ignition has to be top notch in order for the motor to perform.

The fuel supply system, tank fitting, hoses, filter, all must be clean and clear in order to flow sufficient fuel for the motor to make full power. ANY restriction in fuel flow will seriously limit power and may cause the cylinders to run lean overheat and melt the pistons. These motors work their butts off, far harder than any automobile does. Fuel flow is absolutely critical.

All built in tanks have a check valve at the fuel fitting to prevent fuel leaks in case the boat sinks, This is the fitting that can get trash in it and restrict fuel flow. Check it, remove it, look, blow air, etc., it should flow one way and seal off the other.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Thanks CharlieB. I bought the caruretor on EBAY and may have been set up on a 454. So may have been jetted for that. I went to the boat yesterday and looked at advance weights and put a couple drops of light oil on everything. It looked fine with no apparent binding. Changed thermostat but temp still runs high(200ish on new gauge and tempprobe). I believe Advance is working. Was able to rev to 4400 RPM in neutral. Didn't go beyond that but I know the throttle still had a little to go. So I believe 4600-4800 is achieveable. I did however discover (due to antifreeze on manifold) air bubbles on intake manifold at the exhaust crossover that warms carb. So now I am wondering if it is a vacuum leak at manifold to head. cylinders 3,5,4, and 6 were alot lower on compression check. If it is sucking air that would lean out the fuel going to those and could cause burnt valves maybe?. I will have to pull manifold to replace gaskets. Should I also pull Heads?

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

Check the center 6 bolts on the intake to see if they are loose. You could get a vaccuum leak like you mentioned but coolant I don't think so. Not knowing how your engine is plumbed for cooling . The center section of the stock intake heats the carb with exhaust. As for the temp, look at the sea water pump and the heat exchanger . Mine ran hot last weekend, brought it home , pulled the lower unit to find that the local shop that rebuilt the drive did not clean the old gasket off the mounting surface for the water pump and it was pulling exhaust gasses into the sea water area of the drive.... Pretty happy I am. If you we're going to pull the heads, I would get the numbers off the old heads and buy a set of rebuilt heads so you could just swap them out. Pretty big job with the boat on the water.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Thanks Dogdave. the coolant came from me changing thermostat and not getting it quite tight enough and spewing over the top of manifold. that was how I was able to see bubbles. I may have to pull boat anyway and pull the engine back out to change power steering pump and figure out where the oil is coming from in the bilge. BTW I think I tried to talk to you on Saturday if you were filling your Sundancer with water from a Neon yellow hose, but you couldnt hear me.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

Oh man pulling the engine for a power steering pump that's a bummer. At least you can have the engine on a stand to do the heads. That was not us on the dock, we pulled the boat around 2 pm . Good luck with the project

 

[500dollar744ti]

Re: Low RPM at WOT on Mercruiser 260

Thanks CharlieB. I bought the caruretor on EBAY and may have been set up on a 454. So may have been jetted for that. I went to the boat yesterday and looked at advance weights and put a couple drops of light oil on everything. It looked fine with no apparent binding.

You can't put a 700-750cfm carb jetted for a 454 on a 350 and just expect it to work. People always think bigger is better, I see that crap at the drag strip all the time. People slap a big carb on a small block and blow black smoke all the way down the track while getting beat by a honda civic.

You need to get your engine fueled properly or you will be beating a dead horse. Ideally your air/fuel ratio should be 12.5-13.0 at WOT for max power on a naturally aspirated v8. With your current carburetor, I estimate 10.5-11 AFR, you must be drinking fuel and not going very fast with that setup.

You can't achieve WOT rpm at the wrong fuel mixture.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

You can't put a 700-750cfm carb jetted for a 454 on a 350 and just expect it to work. People always think bigger is better, I see that crap at the drag strip all the time. People slap a big carb on a small block and blow black smoke all the way down the track while getting beat by a honda civic.

You need to get your engine fueled properly or you will be beating a dead horse. Ideally your air/fuel ratio should be 12.5-13.0 at WOT for max power on a naturally aspirated v8. With your current carburetor, I estimate 10.5-11 AFR, you must be drinking fuel and not going very fast with that setup.

You can't achieve WOT rpm at the wrong fuel mixture.

Thanks 500dollar... I am not sure what the ratios you are saying are. So what would the best course to take? When I bought the carb 4-5 years ago I wasn't thinking bigger is better. I was ignorant. I was told to buy a carter/weber/edelbrock marine carb. this one fit the bill. It wasn't until recently that I figured the second digit of PN referenced the CFM of the carb. I don't remember the original posting for the carb on eBay but it seems that it came out of a ski boat. Don't remember size of engine. Another thing I just figured out is When I posted that the primary and secondary jets were 120's, I was wrong. Carter/Edelbrock use a 120 followed by a dash and another number ie. 120-410 which is a .110 jet. so I honestly don't know what jets are in there with out taking top off and look at them. I do have a Rochester Quadrajet that is on the 350 I pulled out of the boat originally that was paired up with a VP dual prop out drive. Probably need to rebuild it. could get an adapter plate and throw it on and see if it makes a difference. But I am starting to think that I am over carbed. And yes it is guzzling fuel. As near as I can tell about 15-20 gallons and hour.

 

[500dollar744ti]

Re: Low RPM at WOT on Mercruiser 260

Try rebuilding that Q-jet you got, if it's the right match for the 350 and it's a marine carb, it's going to perform like a champ!

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

What intake manifold do you have on the engine that should need an adapter. If its a square flanged auto manifold you ma have an issue there . What cam shaft is in this engine

 

[tfret]

Re: Low RPM at WOT on Mercruiser 260

i don't know if this helps you or not. I have a 1984 5.7L with Thunderbolt 4 ignition. Timing is supposed to be set at 8DBTC. I've struggled for years with low WOT, and poor acceleration, hesitation, even some back fire. It's always had low vacuum at the intake, only around 12Hg. I've replaced the Qjet. Learned all there is to know about the Qjet. Adjusted the Qjet like crazy. Replaced cracked cylinder heads on both sides. Reinstalled intake manifold twice to ensure no vacuum leaks. One day out my friend says "sounds like a timing problem". I check the timing (in nuetral) and it's right on the money 8dbtc. So I play with it. I dial it back (retard) 3 degrees. HOLY COW! Now it runs absolutly fabulas! I have checked to see if the pulley had slipped so that the timing marks were no longer accurate, but TDC is right on so that's not it. I don't understand it. Maybe the timing chain slipped a sprocket or something. I really don't understand it all. But all my performance problems went away. Better idle, better WOT, way more power, acceleration, everything! So if all else fails, try playing around with the timing a little. You might get lucky like I did. Good Luck.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

What intake manifold do you have on the engine that should need an adapter. If its a square flanged auto manifold you ma have an issue there . What cam shaft is in this engine

So the Manifold is an Edelbrock Performer that came with the engine block I built. It came with a Holley 6120-3 double pumper. I have it in the garage. I was going to put an Edelbrock 1409 on it but after some research found that Carter was the same thing. So I purchased one on Ebay. The cam well that is a good question. It was with the engine I built up. There is a story concerning the engine and why I received it in pieces with a new block and new rebuild kit. the person was running a 19ft ski boat with the Performer intake and Holley and had straight exhaust out the back of the boat. I scrapped the boat, sold fuel tank and trailer and volvo penta outdrive and ended up with free engine which I then started piecing together all Mercruiser stuff.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

i don't know if this helps you or not. I have a 1984 5.7L with Thunderbolt 4 ignition. Timing is supposed to be set at 8DBTC. I've struggled for years with low WOT, and poor acceleration, hesitation, even some back fire. It's always had low vacuum at the intake, only around 12Hg. I've replaced the Qjet. Learned all there is to know about the Qjet. Adjusted the Qjet like crazy. Replaced cracked cylinder heads on both sides. Reinstalled intake manifold twice to ensure no vacuum leaks. One day out my friend says "sounds like a timing problem". I check the timing (in nuetral) and it's right on the money 8dbtc. So I play with it. I dial it back (retard) 3 degrees. HOLY COW! Now it runs absolutly fabulas! I have checked to see if the pulley had slipped so that the timing marks were no longer accurate, but TDC is right on so that's not it. I don't understand it. Maybe the timing chain slipped a sprocket or something. I really don't understand it all. But all my performance problems went away. Better idle, better WOT, way more power, acceleration, everything! So if all else fails, try playing around with the timing a little. You might get lucky like I did. Good Luck.

I have my timing set to 8dbtdc at about 650-700 rpm in gear. I put in 2 new timing chain sets both double roller because I thought timing had jumped when I first put engine in boat this spring. But there was no evidence that chain had slipped or jumped sprocket. This rebuild has less than 30 hours on it.