Max RPM Problem

[johnrmclean]

Always something. My 1994 Baja with a 454 Mercruiser, Bravo 1 , and 21 pitch stainless prop. sat for about 3 years. Over the last month I have been debugging one thing after another. On Saturday I finally thought I had it all worked out , got it fueled with 50 gallons, and launched however I was unable to get it to rev over 3000 RPM. It ran good up to 3000 which is maybe 1/2 throttle but then even giving it full throttle it would not increase revs over 3000. After many tries the engine would always max out at exactly at 3000 RPM. Then, after cutting back the throttle to maintain 3000, it seemed like it was starting to bogging a bit or having a slight miss or fuel starvation. So now I'm looking for probably causes. My first thought is water in the fuel filter or system but was thinking that would create missing but not limiting RPMs to exactly 3000. Any thoughts from those more experienced that me?? Thanks in advance!

 

[johnrmclean]

BTW .. I just noticed after reviewing the method for mounting the prop it should be torqued to 55 PSI.. I know I did not tighten it that much. Could that be the cause??

 

[achris]

Carb, EFI or MPI? How is the bottom of the boat? Covered in 3 years worth of growth? Was the tank empty (and clean) or did you pour 50 gals straight into more old fuel?
Best if you post up the engine serial number to help us indentify the engine.
And a bit more information on exactly what has been done to 'debug' this engine/boat....

Chris........

 

[achris]

BTW .. I just noticed after reviewing the method for mounting the prop it should be torqued to 55 PSI.. I know I did not tighten it that much. Could that be the cause??

No. And it's 55 lb-ft, not psi....

 

[johnrmclean]

No. And it's 55 lb-ft, not psi....

Duh ..yep 55 ft lts ...

 

[johnrmclean]

Carb, EFI or MPI? How is the bottom of the boat? Covered in 3 years worth of growth? Was the tank empty (and clean) or did you pour 50 gals straight into more old fuel?
Best if you post up the engine serial number to help us indentify the engine.
And a bit more information on exactly what has been done to 'debug' this engine/boat....

Chris........

Exterior of the boat, a1994 Baja 272 runabout was and is very clean. The hull is waxed and clean. It was running fine and the mechanicals were all in good and clean working order when put away and winterized. It was stored outside with about 25 gallons of stabilized gas in it for 3 years in Michigan. I could boat for a couple years due to work and last year had to rebuild rotting seat frames and redo vinyl before it could be put back in so that ate up last season. Unfortunatly The new gas went right into the old gas. Its a 1994 boat with a 454 Mag 350 hp but the engine was replaced in 2003 when a bearing was spun. The new motor was a roller cam short block but used the same Mag heads and Rochester carb. It has a Mercathode which I assume is the Thunderbolt ignition system. I have a couple written serial numbers in my records for engines . One says 1347-9863AS and one that says OF305171 but I can't find those numbers in Merc service records anywhere and not sure which is for which. I'll try bildge diving to try and find it on the engine. Oil is clean and new. Replaced the cap and rotor, new plugs, cleaned and replaced all the grounds I could find, 2 new batteries, new seal water pump, pre lubed the upper half of the engine and cylinder walls with Marvel oil and rotated the crank to pre lub bearings etc before the first start up. Starts on the first or second crank. Hoping to get the fuel filter replaced and check to see if the rear throttle plates are open and linkage are working correctly at full throttle today. Thanks for your time and consideration.!

 

[wahlejim]

Do you have a fuel/water separator? If not, install one. If you do, change the filter, empty the contents and check for crud. Try running on a small gas tank of fresh gas to see if issue persists. If the issue still persists, I would put money on crud in the carb after sitting 3 years.

A lot of times when people store these boats, they don't think to run the stabilizer through the system before storing. They just dump it into the tank and call it good. If that was the case, the fuel in the lines and the carb will varnish and you have some cleaning to do.

 

[achris]

0F305171 - 1995 Gen V, 454 Mag, Weber Carb, TB-V, power steering, 2 pce Exhaust, Bravo drive.

Mercathode is a corrosion protection system. Thunderbolt V is the ignition system.

 

[johnrmclean]

Do you have a fuel/water separator? If not, install one. If you do, change the filter, empty the contents and check for crud. Try running on a small gas tank of fresh gas to see if issue persists. If the issue still persists, I would put money on crud in the carb after sitting 3 years.

A lot of times when people store these boats, they don't think to run the stabilizer through the system before storing. They just dump it into the tank and call it good. If that was the case, the fuel in the lines and the carb will varnish and you have some cleaning to do.

just replaced the fuel/water separator this pm. No water or crud in the filter but it was a bear to get off. The rear lower butterflies appear to be opening all the way when not running and I go to full throttle on the controls but the upper rear butterflies have no tension on them but stay closed. I'm not sure how they work. Firing order is checked and correct. Can't see how the timing would have got changed. Does that bring us down to the carb or ?????? possibly fuel pimp???

 

[NHGuy]

The upper butterflies don't open til they are pulled open by engine vacuum. They should be able to be pushed open with a finger.

 

[Bondo]

Does that bring us down to the carb or ?????? possibly fuel pimp???

Ayuh,..... Have ya run it on a can of fresh clean gasoline connected right to the fuel filter,..??

That 25 gallons of ole gas mighta given ya 75 gallons of bad gas,....
Could take quite a few fuel filters before it clears up,.... If it ain't gotten to the fuel pump, 'n carb yet,....

 

[achris]

Thunderbolt V on a 454 Mag has a knock sensor. 75 gallons of rotten fuel will make that engine knock like nobody's business. Wouldn't mind betting the ignition system is retarding the spark in an attempt to save the engine. Late spark means less power. That could account for your problem.

Solution: Pull the carb and fuel pump and clean them out. Next pump 75 gallons of rotten petrol out of the tank and dispose of it. Pull the tank out and have it steam cleaned. Pull all the fuel lines out and toss them in the garbage, and fit all new.

Not sure if you are willing to do that, but that's what needs to be done. It's what I would do.

Chris.......

 

[johnrmclean]

The upper butterflies don't open til they are pulled open by engine vacuum. They should be able to be pushed open with a finger.

ok that sounds about right ..they move really easy

 

[johnrmclean]

Ayuh,..... Have ya run it on a can of fresh clean gasoline connected right to the fuel filter,..??

That 25 gallons of ole gas mighta given ya 75 gallons of bad gas,....
Could take quite a few fuel filters before it clears up,.... If it ain't gotten to the fuel pump, 'n carb yet,....

No not yet but there was no visible water or crud in the filter I took off today.

 

[johnrmclean]

Thunderbolt V on a 454 Mag has a knock sensor. 75 gallons of rotten fuel will make that engine knock like nobody's business. Wouldn't mind betting the ignition system is retarding the spark in an attempt to save the engine. Late spark means less power. That could account for your problem.

Solution: Pull the carb and fuel pump and clean them out. Next pump 75 gallons of rotten petrol out of the tank and dispose of it. Pull the tank out and have it steam cleaned. Pull all the fuel lines out and toss them in the garbage, and fit all new.

Not sure if you are willing to do that, but that's what needs to be done. It's what I would do.

Chris.......

Gotta do what I gotta do to get it running right. The fuel filter I took off today had no visible water or crude in it, looked pretty clean and I removed the fuel line to the cab which also looked clean. So far the engine runs pretty good with no knocking .. it just only revs up to 3000 at about 1/2 throttle and above so would it still be trying to retard the spark? I can see fuel being pumped into the carb in the front primaries and the rear butterflies are opening full at full throttle, without the engine running. however I do not think I noticed fuel being pumped in to those rears.(need to check tomorrow) Is it possible the secondaries are being starved, possibly by a bad float or sticking needle/ seat assembly? I could see a case for old fuel sitting in carb or fuel pump gumming things up. Not saying it isn't but if it was really a water problem I would expect the engine to be running rough or missing a lot.

 

[achris]

Very old fuel has varnish deposits and loses it's volitility. There are no accelerator discharge jets in the secondaries, so not seeing a squirt of fuel in the secondaries is correct. You still need to pull the carb, and fuel pump, and strip and clean them.

 

[johnrmclean]

Very old fuel has varnish deposits and loses it's volitility. There are no accelerator discharge jets in the secondaries, so not seeing a squirt of fuel in the secondaries is correct. You still need to pull the carb, and fuel pump, and strip and clean them.

So today a freind and I sea trialed the boat for about an hour but we did not find or cure the problem. We changed the fuel filter today and temporarily removed the fuel line check valve to insure free fuel flow from the tank. Then we went to sea. The boat ran strong up to 1/2 throttle which again is just at 3000 RPM . We have pretty much ruled out water in the fuel as an unlikely cause as the engine runs to smooth. Opening to full throttle did not increase RPMs. On occasion it seemed to create a slight stumble, miss or pop. Below 3000 it ran good. I checked before starting the engine and the secondary butterflies do open at full throttle but while at sea and 3000 RPM the upper secondary butterflies, while very loose and free, did not open. I used a small wire to open the butterflies manually but nothing happened. (not sure if the lower butterflies were open but if they are mechanical and open when engine is off at full throttle, they must also open at speed and full throttle) Next I sprayed raw fuel into the rear secondaries several times but nothing happened, no change in engine and still no increase in RPMs. Finally we turned our attention to he possibility of timing. At idle the engine appeared to be timing at or about 8 to 10 degrees BTDC. We again went to sea and experimented with altering the timing at speed but nothing we did could improve engine performance or get us over the 3000 RPM hurdle. I can and probably should/will pull the carb and fuel pump however, if there was no increase pumping raw fuel into the carb does that still make a dirty pump or carb a probable cause of the problem? Thoughts??

 

[johnrmclean]

BTW . I hate that Cadet /Seaman Apprentice rating .. at the rate I'm progressing I think I should be in line soon for at least U Boat commander!

 

[johnrmclean]

So this may seem basic but I don''t understand .......if there are no accelerator jets in the secondaries how does it get its fuel and accelerate the boat and RPMs?

 

[achris]

You need to read up on carburretor basics. But in brief, the engine is running fast enough, and the intake vacuum is such that as the secondary throttle plates open the fuel is drawn directly from the secondary venturis without the same 'weakening' of the mixture as occurs with the primaries. That weakening is caused by the difference in densities of air and fuel. The air can accelerate faster than the fuel as the primaries are opened, so raw fuel (from the acc. pump) must be added to the air stream to compensate for the slower fuel speed (in the primary venturis) before it can 'catch up'.....

This is what wiki has to say.

Liquid petrol, being denser than air, is slower than air to react to a force applied to it. When the throttle is rapidly opened, airflow through the carburettor increases immediately, faster than the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which makes the engine misfire (or "stumble")?an effect opposite to that which was demanded by opening the throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of petrol through a jet into the carburetor throat. This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means.

HTH,

Chris......