Help, my jet drive is having problems

[wolfgang]

I have a 1998 eveenrude 115/85hp jet drive on a 19 foot lowe jon boat. Over the last three years the top speed has decreased and the top end rpms are only at 4000. I have had the engine rebuilt after being told it was a compression issue but it did not help. Does anyone know what is going on?

Wolfgang

[RMSOSF]

First, do the basics.... check compression, spark,timing and fuel quality. Any compression reading 10 pounds or more lower than the next lowest cylinder means bad news. The next steps would be diagnoses, does the engine get to a certain speed and still have throttle, but no RPM gain? Does it bog at high speed?

[wolfgang]

The engine does reach top speed with trottle remaining. All cylinders are 185 to 189 psi. It doesn't bog but it doesn't go faster either. Someone mentioned that if the exhaust tube is rusted it might have an effect. Have you ever heard of such a thing? Thanks for the reply.

[ondarvr]

Have you checked the pump? the impeller will become dull and the liner will wear down, so it will be less efficient. Did you check the comp or did they? 185 to 189 is very high, but most gauges don't read the same, so it could be just the gauge.

[RMSOSF]

A dull, worn pump would result in too much RPM. 185 psi in compression is way too high for that engine, should be down around 125 psi. What do the spark plugs look like? are they all lightly oil covered? Do any of them have a white/silver residue on them? As far as the exhaust goes, if it is all carboned up inside, yes it will give too much back-pressure and cause a reduction in top end speed. The only way to check that is to pull the back cover off the engine and remove the gearcase so that you have a clear view of the entire exhaust system.

Now, you did say it reaches 4,000 rpm with more throttle to go. You can try doing two things at that point...... pump the primer ball and see if it picks up speed and hit the choke and see if it picks up speed. If it works with the choke, your carbs are fouled, if it works with the primer bulb, you have a restriction or air leak in your fuel system or a bad fuel pump.

[RMSOSF]

One other thought..... when was the last time you changed the oil in the pump drive?

[ondarvr]

There is no oil in this type of pump, just a grease fitting and it should have a shot of grease after every use. I wasn't sure on the RPM 4000 statement, but I have seen a reduction in RPM at times with a worn out pump and he didn't say how much more than this it would turn when he first got it. If the pump is badly worn, it may just cavitate after a certain RPM with no gain in speed. A plugged exhaust could limit the speed and RPM, but more info is needed to tell which way to look.

[RMSOSF]

His RPM should be 5500 - 5800. With an engine almost 10 years old, I am leaning towards a blocked exhaust cavity as you are. Either that or a fuel delivery problem.

[wolfgang]

My mistake, the compression ranges from 120 to 125. The engine has new rings and was completely broken down on the top end. I had the problem before the rebuild. The exhaust tube that comes out the back of the jet drive is rusty and may be coroded within. Could this be part of the problem?
The cam is fully engaged before the throttle is all the way forward. Also, the pump oil was recently changes during the rebuild.

[RMSOSF]

Can you poast a picture of the rust you see?

[ondarvr]

The exhaust comes out of a fairly large hole in the pump but changes shape up higher. When they had the power head off didn't they look down into the midsection to inspect it? You can take the pump off pretty easily, I think there are six bolts that hold the shoe on, then take off the impeller, it's not hard with normal tools. at this point you will be able tell if your problem is related to the pump. The impeller should have fairly sharp clean edges, if it's rounded over, or bent that could be your problem. If you have never serviced the impeller before, it should be toast by now if you run in sand or rocks. But you said they changed the oil in the pump, there is no oil in the pump, only grease, and if they did change it, they should have checked the impeller and liner, plus shimmed it correctly. To do all of this they would have needed to take the pump off, if they did, there's nothing really left to be plugged up, (assuming they even looked).

I have the same basic setup you have, a 1998, 90/60 Evinrude jet and I use it a lot, 4-5 days a week sometimes and it's not even close to needing a rebuild. So if you had low compression, it was for reason, if they didn't find the problem that caused it, then you may need another rebuild soon.

[redtech]

i've been looking at this postist for most of the day thinking about what could be going on. so many things come to mind i could just send you off in the wrong direction. so here i go with my to cents.
start with the basics do a sync and link
check timing and don't rebuild the motor again
as stated above check the carbs ether by tapping the choke or be running it with the airbox off and just let the end of a rag float in the throat of the carb
if rpm pick up you have a bad carb if not don't go there
another thought is check for a double fire this is done by indexing the flywheel and a timing light once you have marked the cyl on the flywheel use the timing light on each plug lead you should only see one cyl at a time
sorry just thought about that
if you install a short peice of clear line on your fuel supply and a vaccum gauge you should only see 3" or less vacuum and no air bubbles
this finishes the fuel and spark if this is good dig a little deeper
go small then big in trouble shotting it save money

[wolfgang]

I finally contacted the company that makes all the jet drives and their techs believe that a rusty exhaust tube in the pump unit will cause my problems if pinholes leak exhaust into the housing causing a buildup of back pressure and would lead to loss of rpm and speed. I will post the results when I replace that exhaust tube.