not enough RPM's!

[mikbaek0]

I'm having a little trouble with a boat I just bought in Florida. It has a 1997 Volvo Penta 5.7 GS with a SX drive and is supposed to run with a 15x17 prop.<br />When I got the boat, the prop was damaged, so I got another one in a hurry, but it was way to big.(15x21). I put it on anyway, just to get my first testdrive in my new boat, and it did around 4000 RPM. Since the prop was to big, I was certain that the RPM's would come up with a smaller prop - THEY DIDN'T! With a 15x17 it only did 4200 RPM's. Then I had the carb. rebuild because there had been some water in the fuel, so idle was un-stabil. Now idle is stabil and engine runs smooth, but it still won't do more than 4200 RPM's. Does anyone have an idea?<br />Thanks - Mikkel.

 

[tommays]

Re: not enough RPM's!

mikbaek0<br /><br />the results your are geting make no sense with the 15*21 pulling 4000 rpm a 15*17 should turn at least 4800 with out doing anything to the motor<br /><br />how big is the boat this motor is on as a 15*17 sounds like a small prop for a 5.7 <br /><br />tommays

 

[mikbaek0]

Re: not enough RPM's!

tommays.<br />The boat is a glastron gs249 (25 ft)and weighs around 4500 pounds.<br />I know it doesn't make any sense - and I agree, it should be doing around 4600-4800 without problems.

 

[bayman]

Re: not enough RPM's!

For the new version of this boat on Glastron's site it says: " With a top end of 50 mph and a cruising speed in the 30s"<br /><br />No info on RPM's though.<br /><br />Details:<br /> http://www.glastron.com/pages/BoatTest_GS249.html

 

[waterone1@aol.com]

Re: not enough RPM's!

What was the top speed you were getting with the<br />"big"prop on it ? What top speed are you getting now ? Are you sure the dash tach is accurate ?

 

[mikbaek0]

Re: not enough RPM's!

The top speed was just about the same with both props - 35-37 mph (gps).<br />I've allready been to Glastron's site to see what i could find, and I wonder if those test results were made with the bigger engine (315HP)?. Anyway I do think that 35 is a little low though. I had my mechanic with me on my last test drive to see if he could find anything, and he says that the secondary valve only opens 1/4 of full throttle. It's a 4V Holley carb.<br />When he tried to open it fully by hand, it produced a lot more gas, but no more RPM's.<br />Someone said that the engine oil has to be very thin due to the hydraulic lifters - ??????

 

[Don S]

Re: not enough RPM's!

Is the bottom clean? If it's covered with stuff, it will sure kill the rpm in a hurry.<br />When was the last time it was tuned up. May very well be just the need of a new set of plugs, cap and rotor and all is fine.<br />Have you done a compression test on it. <br />

Someone said that the engine oil has to be very thin due to the hydraulic lifters

Don't listen to that person anymore regarding engines. Obviously doesn't have a clue about them.

 

[waterone1@aol.com]

Re: not enough RPM's!

Hmmm...this just isn't making sense. Everything Don said needs to be looked at, and lets face it, a tune up never hurts, especially when you buy a used boat. Bayman is also correct with the expected performance,(50 would be ideal smooth lake, but you should be able to get at least 44-46 out of it). <br />Since you are new to this boat, did you "play with" the outdrive trim/ tilt while at full throttle ? Did it make much difference from all the way down to being up a few "notches" ?<br />Going back to the one thing that has some of us stumped.....The two "new" props you tried....are they brand new? Have you verified that they are the pitch that you were told they were ?<br />As a side note, the secondary's of the carb will not open fully untill you are close to the max rpm, which I think would be 4600-4800.

 

[mikbaek0]

Re: not enough RPM's!

Don.<br />The bottom has just been polished so that shouldn't be the problem. When the carb. was rebuild, we changed the plugs aswell. The engine only has 285 hours on it and besides from the missing rpm's, it runs very smooth. We cleaned the cap and rotor just to be sure, but we still need to do the compression test. I just don't think there's anything wrong with the compression since the motor runs the way it does. It has a very good pull, and accelerates great with good response to the throttle. It comes pretty fast up to the 4200 rpm's and then stops accelerating. There's no mis-fire or anything, it just won't go any higher. As for the props, -waterone , they are brand new with the size stamped into them, so they should be correct. I tried both the trim and the tabs and they have a lot of effect. I usually start with the tabs up and the trim down. Then when the boat picks up speed, I raise the trim until the boat starts bouncing, and then go a little back down again. I usually only use the trim-tabs for balancing the boat once planing.<br />Thanks for the note on the oil, have to admit that I've never heard that before, but then again, I've never worked with gas-engines before - only diesel's. Thanks so far........<br />-Mikkel.

 

[waterone1@aol.com]

Re: not enough RPM's!

Ok.....How about ignition timing. Have you checked the base timing and followed the advance curve with an advance timing light ?

 

[waterone1@aol.com]

Re: not enough RPM's!

OK, I know I'm "reaching" here.....(with the ignition off)try putting the control in the full throttle position, at the carburator, remove the throttle cable and see if it is pushing the carb wide open. Yes I know, very simple, very basic....but this boat is new to you, and who knows what was done to it.

 

[mikbaek0]

Re: not enough RPM's!

My mechanic checked the timing, it advances about 20' as it should, and the cable pushes the carb. wide open so it's not that either.........

 

[Stan's Customs]

Re: not enough RPM's!

Hey mkbaekO..<br /><br />Still souds like a prop problem from here. A prop change should make a difference especially that big a change...Something's rotten in Denmark (pardon the pun) if it doesn't.<br /><br />There is another possibility ignition wise...see what kind of distributor module does it has. Pertronix makes one that can go in 2 different ways...both run smooth until yo get into the higher RPM ranges, then generally start acting like they are bumping a rev limiter...Can't gain any more RPM.<br /><br />Yours runs very smooth at the max rpm it will reach correct? Kinda of a long shot if that is the case. It was a real B#$@h to diagnose the one we had. We rebuilt the carb, plugs , wires changed, changed the fuel/water seperator, emptied and cleaned the fuel tank...finally determined the after market module could limit the rpm.<br /><br />You don't have a rev limiter do you??<br /><br />Good luck ..Stan

 

[Don S]

Re: not enough RPM's!

Check the fuel system for air leaks, between the tank and fuel pump. If it's sucking air it will cut down on the amount of gas getting to the carb.<br />If you notice any of the wide (1/2") hose clamps on the fittings, replace them with the 1/4" wide mini clamps. I've seen the wide ones allow air leaks many times, and with the electric fuel pump, that's all it takes to cause problems.<br />Could also be a leaky/plugged fuel pickup tube in the tank or a partially plugged antisiphon valve.<br />Was the 15x17 prop a new one or a reman. It may have been missmarked.

 

[Boomyal]

Re: not enough RPM's!

Originally posted by mikbaek0:<br /> My mechanic checked the timing, it advances about 20' as it should, and the cable pushes the carb. wide open so it's not that either.........

Is that 20 deg total? If it is, it should be somewhere up in the mid 30 degree total advance. That would definitely be a problem.

 

[mikbaek0]

Re: not enough RPM's!

Hey Boomyal..<br />Now that's something I'll have to double check right away, as I am allmost 100% sure he said around 20 degrees total advance!!!!!<br />I'll be sure to let you all know..<br />-Mikkel.

 

[deputydawg]

Re: not enough RPM's!

Is your tach correct? Make sure your RPM readings are correct.

 

[ziggy]

Re: not enough RPM's!

"I tried both the trim and the tabs and they have a lot of effect. I usually start with the tabs up and the trim down. Then when the boat picks up speed, I raise the trim until the boat starts bouncing, and then go a little back down again. I usually only use the trim-tabs for balancing the boat once planing." <br /><br />I'm know nothing about mik's motor or about the size of his large boat, i'm used to small boats. but i got a question about what his rpms are, trim down, and then trim up. all boats i've delt with ya start out, trim down, after plane, ya trim it up. rpms increase, at the same throttle position. at wot, the rpm's increase to a point where you got the drive so high it starts to ventalate, no load. what happens when ya get to this point mik. 4200 rpms? as for the tabs, perhaps i don't understand them either, have no expereince with tabs. but i thought ya started out with the tabs down, thus getting plane faster with less bow rise. after plane i though ya brought them up to a neutral position, thought this added planeing surface and perhaps speed increase. tabs up??? why. ya say that ya use the tabs for balanceing the boat, why do you need to do that too? is your load offcenter?? maybe the trim has nothing to do with your problem mik. all i do know is that the boat i got i can keep the rpms down, just by trimmng full down, about 4000 wot, trim it up and i can get 4500, much above that and i start to ventalate and get unhooked and overrev. what happens when you over trim up mik. still only 4200? if so i'd think back to the motor problems discussed above. just my 2cents, jim

 

[Don S]

Re: not enough RPM's!

Here are the book values for the timing advance.<br /><br />RPM--advance<br />600....8°<br />1200..12°<br />1600..16° <br />2000..19°<br />2400..23°<br />2800..27°<br />3200..30°<br />3600..31°<br />4000..31°<br />4600..31°

 

[Stan's Customs]

Re: not enough RPM's!

Good information Don...<br /><br />Not to change the topic..but a quick question..Would these specs be similar for a 1988 5.7 Cobra?? I have no information on this. <br /><br />Thanks.... Stan