Risor/elbows/manifolds system/s question.

[Drewmac]

Boat/merc 5.7 runs absolutely perfect at cruising low speeds with no overheating and starts fine. Compress perfect, carb cleaned, all new electrical, impellor, pump, and then some. However, when I give the motor a handful of throttle it cuts out, coughs, but eventually comes barely on plane. At this point, the motor then starts to overheat quickly. This boat used to do 45mph, but now when barely planed, only 22-25.

Hopefully enough of the the basics to understand my question which is....will clogged (see title) cause the boat to have these symptoms ie no holeshot or top end speed and high speed overheat.

I've read enough to know that a high speed overheat is a symptom, but what about the speed? It seems logical to me that it would as water and exhaust flow is not optimum and basically "backing or blocking" the system up, but just not sure. The boat was in saltwater and these systems have never been changed to my knowledge. All the mechanic's just want to keep saying carb and/or timing, but those have been done. I can't help to think they just want to do all that stuff again even though I keep stating it has been done. However, I also don't won't to spend a grand or so on the ris/elb/mani's if that is not it either.

Any suggestions? Thank you!

 

[81 Checkmate]

Re: Risor/elbows/manifolds system/s question.

First thing....did you replace the impellor or the shop...reason i ask is, if it was burnt up some of the peices could be lodge in the system reducing water flow.

2nd... when it's running out of the water on muffs does it have good water flow out the releive holes on the drive?

3rd... The guys are going to ask for engine serial numbers to know what we are working on.

 

[alldodge]

Re: Risor/elbows/manifolds system/s question.

Boat/merc 5.7 runs absolutely perfect at cruising low speeds with no overheating

Compress perfect, carb cleaned, all new electrical, impellor, pump,

Who cleaned the carb?
Have the fuel lines been checked for restrictions?
Has the fuel filter been changed?
How about fuel pump, how is the flow and what is the pressure?
Does the "new electrical" mean cap, rotor, wires, (points)?

However, when I give the motor a handful of throttle it cuts out, coughs, but eventually comes barely on plane. At this point, the motor then starts to overheat quickly. This boat used to do 45mph, but now when barely planed, only 22-25.

What is the rpm change between before and now?

Hopefully enough of the the basics to understand my question which is....will clogged (see title) cause the boat to have these symptoms ie no holeshot or top end speed and high speed overheat.

Clogged exhaust system parts will cause over heating brought on by the inability of sufficient water being able to flow through the engine to cool it.

I've read enough to know that a high speed overheat is a symptom, but what about the speed? It seems logical to me that it would as water and exhaust flow is not optimum and basically "backing or blocking" the system up, but just not sure. The boat was in saltwater and these systems have never been changed to my knowledge. All the mechanic's just want to keep saying carb and/or timing, but those have been done. I can't help to think they just want to do all that stuff again even though I keep stating it has been done. However, I also don't won't to spend a grand or so on the ris/elb/mani's if that is not it either.

Slat water exhaust system last 5 to 10 years and this depeneds on how well the owner flushed the system after each use. If it sat in the salt water 5 years would be good.

Edit: HOW about a serial number

 

[Drewmac]

Re: Risor/elbows/manifolds system/s question.

Thanks. I do and don't understand why it matters who did the work for my particular question, as I would think this should apply generally to any/most similar inboard motor's, but since you asked, it has been both marine mechanics and a marine mechanic turned Nascar mechanic. This guy was awesome, but he left my area before we could get everything worked out. He did all the obvious areas first...carb, fuel pump, filter (many times) and yes electrical cap, rotor, wires, plug, (no points) and timed all perfect. He also replaced the impeller.

That year the boat was used, but the symptoms mentioned were still there. Winterized, and got back out once again late next season and used, but obviously still had same problem, but developed leak in boot. Took in to local marina, as my "nascar" marine guy was no longer close to help. The local marina re-did all the lower end, plus cleaned the carb. Low and behold still same symptom, but was late in season, so just winterized and put up. Haven't had out now in two seasons.

Again simply and please just assume everything other than risr's/elbows/mani's are new and/or working properly...will or can a faulty/clogged/full or partially blocked system cause a boat not to gain full speed? Thanks for replies, but starting to sound exactly what I have run in to...noboby can answer this question.

Guess all I'm looking for is a yes or no shouldn't affect speed or yes possibly could. Then I would feel more comfortable looking/spending more money elsewhere.


One thing that has changed when this boat ran fine (which was many years ago, family boat, now mine, and too sentimental to let go, and want my boy to enjoy it, but no good unless we can at least ski again) is the temperature. I know it has a new thermostat, but now the boat runs (at cruising) at 125. Before it was always 150. Surely that couldn't be anything to affect that much of performance loss..yeah, however that has always been a question mark in head.

I'm starting to ramble a bit, I know, but just really ready to move this project on.

 

[Bikeluver43]

Re: Risor/elbows/manifolds system/s question.

Well a clogged cat/exhaust on a vehicle will cause lack of power while driving, so with everything else being in good working condition then I'd say yes a clogged exhaust on your boat could cause this.

An engine is simply an air pump, air in, air out. If timing, fuel, air, compression are all good and you aren't getting up on plane then it begins to overheat - it sounds like exhaust is not being allowed out causing a lot of backpressure. Secondly, as mentioned before, the water jackets in the exhaust may be bad as well not allowing proper circulation.

-I'm not a boat mechanic, cars / trucks are really my thing, but this sounds like its your issue.

 

[Eric 3.7LX Alpha 1]

Re: Risor/elbows/manifolds system/s question.

Have you ever checked the firing order to ensure the plug wires are on the correct spark plugs? It wouldn't be the first time...last one on here checked and double checked... then after checking again two wires were crossed and the engine was running on 6 not 8.

 

[alldodge]

Re: Risor/elbows/manifolds system/s question.

Sorry if we appear to be coming on a bit strong, but think of it this way: Your telling folks about an issue that many have seen parts of before. We may not necessarly have seen your exact case but many (not all) have seen these types of cases which turn out to be what has happened so many times before. This is the reason we try to find out everything about the problem before we come accross with a best guess. So the entire "exhaust system is new and without problems" and if you want a guess I'm sure we can try that, but keep in mind it is just a guess.

So my guess is that you have a problem with your water circulation system and this is why you are overheating.

Since you say your compression, fuel, timing and electrical system is perfect (spark, air, fuel, bang), then my guess is you have a bad coupler or spun prop hub.

I would really like to know more but this is my WAG.