Merc 90 won't get on plane

[Chris115]

I am new to the forum and boating for that matter. Having a problem with my 2003 merc 90 4stroke. It idles fine and runs well at low rpm but when I throttle up to get on plane it idles way up to 4000 rpm and more without gaining much speed. It just plows through with the bow ridding high, and some vibrating at around 10 knots barely gaining speed. Eventually it planes out and I can reach top speed around 35 mph and sounds fine. It feels like the power just isn't there in the mid range. I changed all the fluids and filters and plugs. Oil was nasty, the plugs had all similar wear and found real bad chunks of shellacked gas in the fuel filter. Also the bulb and exterior gas line was lined with this shellacked gas so we changed this out for new parts and added an in line filter. Took apart all the crabs and gave a good cleaning to the jets and ports which were not clean but not to severelly dirty either. Ran fresh gas directly to the motor and it's still behaving the same, maybe just a little improved. Anybody got ideas or a direction I should head in.

Thanks

 

[usskydvr]

Re: Merc 90 won't get on plane

You may have a prop issue
1) Current prop diameter and pitch
2) Wide open throttle RPM and speed with an average load
3) Boat make model, year, length and weight

 

[Chris115]

Re: Merc 90 won't get on plane

Thanks for the reply, but the first thing I did was pull the prop,thinking it may be a spun hub, but the hub was not the problem. As far an improper prop it's the same one I have always used but the boat and motor are behaving much differently now. Not being as knowledgeable as I would like, i think I'll do a compression test and spark plug test next but I might be throwing darts in the

 

[Chris115]

Re: Merc 90 won't get on plane

Sorry I didn't really answer your question Usskydvr, it's a 2003 trophy 1703 17 foot cc about 2000lb with motor. I usually cruse at 4000 rpm doing roughly 26 mph with a wot at 5400 rpm and about 36 mph I'm not sure what the prop stats are but I believe it's the original set that came with the boat which is sold as a package. I bought it last year off the original owner who said he had regular maintenance done and didn't use the boat all that much, but people say all kinds a things when the re selling you a boat.

 

[usskydvr]

Re: Merc 90 won't get on plane

Not to beat the issue, but how did you check for a spun hub?

 

[Chris115]

Re: Merc 90 won't get on plane

I actually replaced it, I removed the prop and inspected the groves on the spindle, they looked fine, then knocked out the plastic hub that sits in the prop. I had to bang it out pretty good it was seized on there. There were no scores on the hub or inside casing of the prop and no sighns of spinning or wear. Sorry if I don't call everything by it's proper name I'm just trying to learn as much as I can before submitting to a mechanic I don't know.

 

[kevinclark93]

Re: Merc 90 won't get on plane

This past weekend I had a similar issue with my boat. I have a Mercury 150 and it took for ever to get on plane. I checked for spark and my #6 has very very weak to no spark. I have traced the problem back to a bad leg on one of my switch boxes. The new switch box is in the mail and I hope it fixes the problem. All that to say I would check the spark.

 

[Chris115]

Re: Merc 90 won't get on plane

sounds like a good idea, i'm going to get a spark plug tester and maybe even a compression gauge this afternoon and try to get the boat back on land again tomorrow to test things. thanks for the advice, i'll post what i find after i check the spark.

 

[CharlieB]

Re: Merc 90 won't get on plane

How old are the spark plugs and what do they look like?

A spark plug can idle fine and run with light loads fine, until the load increases that the plug begins to short instead of firing, then the motor misses, vibrates, and hangs until the plug eventually burns just clean enough to get up to speed.

If your inspection reveals carbon deposits that will not clean off with carb cleaner and a rag (assuming you have the surface gap style), then it is time to replace the plugs.

 

[Chris115]

Re: Merc 90 won't get on plane

plugs are brand new, i'm not real great with engines and feel like it is an ignition problem, possibly the switch box(cdi) or stator. i feel like a spark plug test may be a waste of time for the reasons you have just stated and want to try and test the switch boxes, but am not quiet sure how to do this. is it dangerous to remove spark plug wires will the engine is under load? i thought this might isolate which plugs are possibly not igniting enough and then could trace this to a switch box. does this make any sense.

 

[usskydvr]

Re: Merc 90 won't get on plane

Keep in mind that this is an ECM engine, very complex controls. Look at your Private Messages

 

[Chris115]

Re: Merc 90 won't get on plane

finally did my tests, compression was between 150 and 130 so i'm just below a 15% difference with high numbers(done after engine was warn) and during spark test all wires jumped a 7/16 inch gap with blueish color. was hoping something would be wrong to send me in a particular direction but all i have is more questions. today i will drain as much fuel as possible and try and run it again but this is just wishful thinking. I want to test further ignition components, such as the tps, stator and cdi but am not sure if the recommended tester and harness are neccessary.