5.7L Mercruiser test drive

[FreeBeeTony]

I am still considering purchasing a '84 24' Grady White Offshore. It has a rebuilt (in late '01) 5.7L Mercruiser.<br />I plan on taking it out for a test run this w/e.<br />What should I look for specifically here? The owner said he thinks the carb needs to be rebuilt because it doesn't seem to be reaching the RPM's that it should. I didn't ask him what the WOT was.<br /><br />Any hints would be appreciated.

 

[skicat]

Re: 5.7L Mercruiser test drive

WOT for the 5.7 is 4400-4800<br />An easy thing to check is for any water in the oil.

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

I guess checking compression would be a good idea.......but it doesn't look to easy to get to...will the WOT give me an idea if all cylinders are firing?

 

[Limited-Time]

Re: 5.7L Mercruiser test drive

FBT,<br />If you’re not hitting on all 8 cylinders you won't likely reach the correct WOT RPM. You can check around to get an idea of that hull/power plant performance expectations, top end, planing (sp?) speed, cursing speed and RPM. That should give ya a good base line before your test cruse.

 

[ziggy]

Re: 5.7L Mercruiser test drive

FBT, i've seen yer posts on yer il6. ya have done much work to it. bet it's real easy to get to and work on too. if i ever consider a new boat, i'm sure gonna be looking at how much fun it's gonna be to work on. yer statement, "i guess checking compression would be a good idea, but it doesn't look to easy to get to" leads me to think it won't be as much fun to work on as yer IL6. if ya can't get a compression tester ontoit, i'd assume that changeing sparkplugs will be fun too... what are ya gonna have to remove to go get them. the engine? i'm over emphasizing there, but i think ya understand what i mean. <br />as far as the carb needs to be rebuilt. hum, well isn't that what everyone says to get their boat sold if it don't run right. might not be the carb. as ya can see, i'm a pesimist. just my two cents. have a good time on the sea trial.....

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

Where would I get these performance numbers?

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

Well.......took the boat out today, was disappointed.......only reached ~3400RPM and WOT. Didn't seem like it was just fuel, made some noise....didn't sound like pinging which would indicate timing.<br /><br />Is it possible to only be running on 7 cylinders and still idle smooth? I thought maybe it might be ignition. Think it's a rebuilt '84. What type of ignition does this have? I saw a coil.....what about this thounderBolt ignition? How do you know if it has that?<br /><br />I looked into the carb.....with the engine off I asked my son to open the throttle......the primaries opened but not the secondaries......are the mechanical or vacum?<br /><br />I didn't buy it because I didn't know what was causing the loss of ~1000RPM's.<br /><br />The temp ran ~150*, oil pressure was ~50#.<br /><br />Any opinions?

 

[Don S]

Re: 5.7L Mercruiser test drive

Well.......took the boat out today, was disappointed.......only reached ~3400RPM and WOT.

could be overpropped, could be a problem.<br /><br />

Is it possible to only be running on 7 cylinders and still idle smooth?

I would notice it, but had a guy bring in a V8 boat that wouldn't turn up, but said it ran smooth and sounded good. I started and it sounded like a 6 cylinder. Did a compression test and it had 2 cylinders with 0 compression....... Ran smooth he said....hmmmmmmmmm<br /><br />

.the primaries opened but not the secondaries......are the mechanical or vacum?

What kind of carb??????<br /><br />My Opinion, have him fix the rpm problem and do another test ride before buying.

 

[Mahoney]

Re: 5.7L Mercruiser test drive

Yes, I was running my 5.0 on 7 cycliders due to a defective plug, and id idled and ran smooth although could not get more than 4000 RPM, replaced with good plug and it ran up to 4500-4600 RPM.<br /><br />Pull the plugs after a run, if one is black and wet, its a good indication there is a problem with that cylider not firing.

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

Hi Don....<br /><br />Don't think it was over propped.......<br /><br />Guy was asking 12K, I originally offered him 8K, he said he would take 10K......this was all before the test drive......I'm now thinking of offering him 6K.........he needs a quick sale!<br /><br />What about the ignition.....I had an uncle (real old school), he used to short out each plug wire with an awl to see if the plug was firing......can I do this with this ignition? How about that ThounderBolt? Does this sound like it has it?

 

[Don S]

Re: 5.7L Mercruiser test drive

I wouldn't go poking holes in the insulators unless you own it. You can do the same thing by removing the plug wire from the distributor cap one at a time and see if the engine changes. if it doesn't some thing is wrong with that cylinder. Could be cap, wire, plug, or even damage to the engine. A compression check would verify.<br />The Thunderbolt IV ignition was around for years before the Thunderbolt V series came out. Same cap and rotor assembly, just different electronics.

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

does this sound like the thounderbolt ign?<br /><br />as far as the carb.i'm not quite sure what type it was.....how about those secodaries? how do they open? could this be the problem?

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

any ideas on what the chances are that it's not the engine? Meaning fuel or ignition? I am thinking of offewring him the 6K and hoping for the best..........would be a good deal......

 

[Don S]

Re: 5.7L Mercruiser test drive

It could be anything tony, and guessing what it could be isn't going to help. The chances are that it's just a minor problem. I would at least want to do a compression check on it before jumping in. If the compression is good, the rest if fairly minor. <br />Thunderbolt ignitions has been used by Merc for years, and in that year boat that is what Merc put on it. Rather it's been changed, hard to say.<br />Pictures of the distributor and carb would help to identify.

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

thanks Don........problem is the guy needs to move fast and doing a compression test is next to impossible........guessing the carb is a Rochester?.....the distributor seems kinda ordinary with a seperate coil. hope that helps some.<br /><br />What is different about the thounderbolt ignition? not just a coil and points? i'm not familiar with it at all.<br /><br />thanks again.........think i'm going to make a decision tonight about the offer.

 

[Don S]

Re: 5.7L Mercruiser test drive

It's Thunderbolt, not thounderbolt Tony, and it's not a points type distributor.<br />It's electronic and has a module that is mounted on a plate attached to the port exhaust riser.<br />(Or the back of the distributor on some newer models)<br />Here are a few links on the TB4 ignition.<br /><br /> http://www.marinemechanic.com/site/page253.html <br /><br /> http://www.marinemechanic.com/site/page252.html <br /><br /> http://www.marinemechanic.com/site/page251.html <br /><br /> http://www.marinemechanic.com/site/page250.html

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

here is a pic of the engine...looks like something mounted on the port riser.<br /><br />see how difficult it would be to to a compression test?<br /><br />

 

[tommays]

Re: 5.7L Mercruiser test drive

tony <br /><br />how much of that stuff in the way can be removed if you need to <br /><br />or are you screwed when you need to service the motor<br /><br /><br />tommays

 

[FreeBeeTony]

Re: 5.7L Mercruiser test drive

looks pretty tight!!!

 

[Don S]

Re: 5.7L Mercruiser test drive

I would say that engine was a complete repower, not just a rebuild. The 350Mag didn't even come out till 86 and the name is still used today. What's the serial number of the engine? It's on that plastic cover over the carb.<br />GW's were always noted for saying to hell with the mechanics, lets make more deck space. They also made some sorry engine covers (See funnels in tony's pic) that put all the water right on top of the engine.<br />And yes, that is the ignition module on the port riser.