Lost RPM's

[Bomber86]

I have an 86 Bomber Bass Boat, with a 110 Johnson motor. Early in the year, I was having problems keeping it running at idle, and brought in for repair. Turns out I had water in the cylinders and had head gasket, water pump, thermostats, plugs replaced. Also pulled powerhead to inspect for damage and replaced gasket.

Took the boat out for test drive and seemed to run OK, then hit a stump and spun the prop early, so didn't get much chance to fully test. Before the repair, I was getting 5500 rpm and 45 mph with a Propco 13 x 19.

Went to the marina and they only sold OEM props, so ordered 13.25 x 19, which only gave me 4700 rpm and 42 mph. Exchanged (at the direction of marina) for OEM 13.5 x 15, which gave me 5800+ rpm (didn't open up all the way) and 35 mph. Exchanged again for OEM 13.25 x 17, and only got 4700 rpm and 41 mph. Ordered another Propco 13 x 19, and still only getting 4700 rpm and 41 mph.

It seems as though the motor has lost power, as with all but the 13.5 x 15 prop, it struggles to come up on plane. Even when replaced with the same prop, it does not perform like it used to.

I'm not sure what could be causing the loss of power, but started by pulling the plugs to see how they looked. I noticed the plugs they put in were Champion VL77V that do not have a side electrode. Could it be the plugs that are causing the lost power?

I am only a novice when it comes to analyzing or repairing a motor, but with a little help, from the veterans on this board, I'm hoping to get better. As I've read through numerous posts, there are a lot of experts who have provided great information to novices like me. Any help is greatly appreciated!

 

[perkdp]

Re: Lost RPM's

Sounds to me like the throttle linkage has changed some, during the powerhead work, changing WOT with the original prop pitch. The one prop allowed the rpm above "original normal" because it wasn't pushing as much water.
The other prop reduced rpm because it was catching too much water and the throttle wasn't opening up for the rpm needed.

 

[Bomber86]

Re: Lost RPM's

Thanks for the response, Perkdp. The more I think about how the engine responds, you may be right. When I hit the gas to come out of the hole, it seems like there is a noticeable drop in power. The engine seems to run OK other than the lack of power.

I pulled the cowling and looked at the throttle linkage. The last adjustment arm that contacts the throttle plate is at it's lowest position possible. When I turn,it opens up the throttle. My question is how much should I open it up? Is it something you just have to play with until you get back to the correct rpm at WOT or is there a preset location?

 

[Silvertip]

Re: Lost RPM's

With the air box removed, look into the carb bore with the throttle wide open. The throttle plate in each bore should be perfectly horizontal. If it's not, link&sync and throttle cable adjustment is required (in that order).

 

[Bomber86]

Re: Lost RPM's

Thanks for the help Silvertip. When I remove the airbox and looked in the carb bore with the trottle wide open, the plate was pointed down. I adjusted the screw attached to the linkage until the plates were perfectly horizontal. This positioned the screw just about right in the middle.

I'm just learning the terminology associated with engine repair, so I'm not exactly what you meant by link & sync and throttle cable adjustment and hopefully what I did was correct. I will take it out on the lake Monday and report back.

Thanks again!

 

[Bomber86]

Re: Lost RPM's

First off, I want to thank perkdp and Silvertip for the advice in helping me get this issue finally resolved. I have gone through several different props trying to get this straight, and had I just turned here first, it may have been fixed quicker.

I took the boat out yesterday, and the adjustment to the throttle worked! The boat gets right up out of the hole like it used to, and seems to run well. However, the rpm's top out at 5000, and the speed at 42 mph at WOT. I can probably live with this, but still not sure why it dropped.

I noticed with all the different props I tried, they all pulled hard to the left when I trim the motor up for cruising. The old prop never did, and I just trimmed it up until the boat would porpise, then back it down. I could let go of the wheel and it would run perfectly straight. I know there is an adjustment to correct this, but not quite sure how to proceed. I didn't want to make any changes until I finally settled on a prop. Can you steer me in the right direction once again?

Thanks in advance for your help!

 

[perkdp]

Re: Lost RPM's

the link and sync has to do with the relatioship to the butterflies opening, spark advance and throttle handle positioning. I'm not familar your motor but it probably don't have a "manual" spark adavnce (linked into the throttling mechanisms) but you may have a slight out of tune between the throttling and spark advance causing a slight rpm loss. i still think you may have a cable adjustment problem. open the throttle wide open and make sure all butterflies are flat centered in the carb throat(s).

 

[ezeke]

Re: Lost RPM's

Just above the propeller there is a trim tab, pointed down. Turn the rear of the trim tab in the direction of the turn, using small increments and testing frequently.