Mercruiser +Alpha one, high RPM's and now a popping sound

[gsknuffke]

I have a 1989 Chaparral 198 XL C with I/O Mercruiser, 4 Cyl pretty well maintained & inside a garage when not on the water. Beginning of this boating season, changes Antifreeze and oil. Lower unit good from servicing 1.5 years before. It usually pulls skier out of water at about 3500-3800 RPM. This year I saw the RPMs were higher than normal with/without load (500-1000 RPM more). Also the RPM bounces around more (maybe my imagination). I researched in the pages of this wonderful forum and there was the discussion of props that can strip out and cause symptoms like a slipping clutch in a car. That seemed to explain it, so I replaced the prop. Went out this weekend- No change. So we had fun with high RPM's then coming in at the end of the day (2 mile trek), I heard this pop, like a bulk door flying open and my wife even looked for that. I thought it came from back. 2-3 minutes later it happened again, and I felt the boat slow abruptly for sec then continued on. I nursed it in the rest of the way without problems. Then, on Sunday back out again same thing higher than normal RPMs then at the end of the day on the way back, popping again, with abrupt short hesitation in boat. I see in the pages of this great site similar examples where it was the engine, this doesnt seem to be the engine, sounds like the drive to me. Scares the crap out of me.
Thoughts are appreciated

 

[alldodge]

You went from 3500-3800 to 4000-4800 rpm. In essence your running the motor at WOT (max rpms). If your tach is correct then your probably running out of fuel and that is a backfire from running lean. Might need to check fuel pump output and pressure.

If you tach is not accurate you might be over reving the motor, and bad things can happen.

In any case, over rev, lean condition can all cause damage. It's also not a good idea to run at WOT for extended periods of time

 

[gsknuffke]

Thanks AllDodge. I agree that I don't ever want to over-rev. Thing is.....it doesn't sound like it is over-reving. Sounds normal or below normal (I have US Govt calibrated ears :lol: ). The popping sounded like it was coming from the stern drive. I'll take it back off and open the engine compartment and listen more directly to where the popping comes from.
Thanks

 

[Bondo]

Ayuh,..... Welcome Aboard,..... The changin' rpms, I'm gonna blame on the tach, not the boat,....

Risin' rpms ain't from a bad prop hub or coupler, they just go out, 'n ya can't go much faster than idle, if that fast,....

'bout the only cause I know of that's like a slippin' clutch is a prop fulla weeds,...
A quick burst of reverse will clear 'em, 'n get ya on yer way,....

The poppin' is most likely a lack of fuel,....
Check the contents of the filter for water, 'n crud,...

 

[gsknuffke]

Thanks Bondo. Yep, it isn't the prop. How do you check the Tach? Hook up an auxiliary one (which may be a pain??) Checking the filter is EZ Enough.
Thanks

 

[NHGuy]

You can remove & reinstall the tach wires. If the ends are corroded give em a brushing with a brass or steel brush. Disconnect the battery negative first. One end is at the tach, the other is at the coil. Also there is a switch on the back of the tach that controls 4 ,6 or 8 cylinders. It wouldn't hurt to turn that through it's stops a time or two.

 

[Rick Stephens]

A shop tach is a good way to check. If you have a friend with one or a shop nearby that'll let you hook up to their hose, you can verify your onboard tach pretty quick.

If I was hearing things in my outdrive I would probably pop the top off the drive and take a quick peek, just to make sure gears aren't shredding. It goes back on without even needing to buy a gasket, has an o-ring. 3/8ths 12 point socket is all you need.

Rick

 

[gsknuffke]

Thanks to everybody. I did clean up the wires on the tach and reviewed the lead to the coil.....all looked in good shape. I took it out on the same routine. RPM's still riding high. Also, I opened the engine compartment. On the way back I heard the pop (only once this time) but it did not come from the engine. It again sounded more mechanical and it wasn't a carb back-fire.

I checked the lower unit oil....level is fine and viscosity looks good.

I called my boat shop and briefly talked about the issue. He also thought engine at first until I mentioned my tests and my thought it was mechanical. He thought Prop as well until I told him it was a new prop. He then mentioned a 'connector' between the engine and drive. Just like the rubber/plastic inside the prop he said it too is rubber and with age & under load can heat up and pop. Then when the engine cools, it will re-form and work the next day until it heats up again. He cant even schedule to see it for 2-3 weeks. I looked up this part on the internet. I think it might be called the "coupler" ?

Does his thinking make sense ?

Oh....I haven't and still need to hook up an aux tach.

 

[nola mike]

I've never heard of an incomplete coupler failure, which doesn't mean anything... But that explanation doesn't make sense to me either. I'm also not sure how the outdrive gears would make that sound. In both cases, if you had enough slip to make that noise, you probably don't have many wot runs left. A wonky tach could potentially cause a misfire. Through the exhaust that could sound like it's coming from the drive

 

[alldodge]

He then mentioned a 'connector' between the engine and drive. Just like the rubber/plastic inside the prop he said it too is rubber and with age & under load can heat up and pop. Then when the engine cools, it will re-form and work the next day until it heats up again.

Man like nola said, that's a good one ound:

I would strongly recommend do not take your boat to this guy.

If your hearing a pop with a slight hesitation (loss in power) then you may be running lean or there is a slight break in connection causing a miss