Merc RPMs Getting higher & higher

[BuzzM]

I have a 88 Sunrunner 245 with a Merc 260 ( 350)& alpha 1 turning a 17" aluminum prop. It used to run up to about 4800 rpm @ WOT. Then it started going as high as 5000 rpm. I changed the prop to a new one ( also 17" ), no change. Everytime I take it out, it seems to increase RPM. Now, it shows 6100 rpm!!!!, but it runs great, planes well for a 6000 lb cruiser, and the engine does not seem to be screaming or pistons swapping cylinders. I don't think it is cavitating or the hub is slipping because it seems to be running so well. One clue, a previous mechanic added a hot wire from the starter solenoid to the coil, saying it should have been there all along. ( that is when it started going to 5k rpm and bagan to run sluggishly). I checked the manual, and it shows NO such wire on this thunderbolt system so I removed it. That's when I saw an immediate increase in INDICATED rpm and performance. A major difference. The only problem is that I am afraid I'll blow the engine. Any ideas? I am thinking a tach problem.

 

[Dunaruna]

Re: Merc RPMs Getting higher & higher

I'm also thinking tach problem.<br /><br />If you have a Digital Volt Meter (DVM), they often have an RPM function. You can connect that to the '-' coil terminal and verify your tach readings.<br /><br />When cold, does the motor start O/K?

 

[BuzzM]

Re: Merc RPMs Getting higher & higher

As a matter of fact, no. I actually have to push the throttle handle forward about 12 times before it will sputter to life. That is even after replacing plugs, distributor cap & rotor, & professional carb rebuild. I change the fuel /water filter twice a year.<br />Once warmed up, it is fine.

 

[Dunaruna]

Re: Merc RPMs Getting higher & higher

I stuffed up, when you mentioned the extra wire that the mechanic fitted I immediately thought 'points ignition'. <br /><br />You don't have points ignition. My bad.

 

[zeezer]

Re: Merc RPMs Getting higher & higher

Has that boat always had a 17" prop on it? I got an '89 wellcraft nova 23 with the same motor/drive setup in it. It spins 4100-4300 with a 21" prop on it. Your motor is supposed to spin 4200-4600 rpms. Are you sure that it's the tach, or is that prop just too easy to spin? I'll be honest, I don't know anyone that has a 17" prop on a 24' boat with a 260 Merc. Most have 19, 20, 0r 21. Just my .02 cents

 

[BuzzM]

Re: Merc RPMs Getting higher & higher

I think it came with a 19. I had it transported during a relocation and had thought the prop was different that previous. I wrote it off to poor memory ( since established a maintenance / repair spreadsheet). I took it out yesterday twice, once with 4 adults onboard. I focused on trim technique, all the way down until on plane, then fine tuned it. It ran great. WOT RPMS was 5500 - 5600. Again, the engine sounding normal for full throttle. Every phase of the process, however, seemed to occur 500 - 800 rpms higher than before moving it to Houston. Thanks for any suggestions. I plan to pruchase a 20 prop and replace the tach.<br />With the hurricane impact, this is really a very, very small problem.

 

[Haut Medoc]

Re: Merc RPMs Getting higher & higher

Have you pulled the drive and looked at the splines on the drive shaft? Do you see any metal near the engine coupler?.....JK

 

[BuzzM]

Re: Merc RPMs Getting higher & higher

I just had the outdrive removed to have it serviced (gearcase seals, bellows, cable, & trim<br />sensors ). I asked them about the coupler & shaft condition & they said it was fine. I would hope they would have checked it out, but who knows. I tried a few more agressive shots of throttle and did not hear anything unusual and feel anything. I did check the tach connections and they seemed o.k. I have had aircraft tachs go bad, but usually they shake a while then just break and go inop. So, I am hesitating about dropping $ 100 doing the part swap dance. I appreciate this suggestion and may have it checked anyway.

 

[BuzzM]

Re: Merc RPMs Getting higher & higher

I think I found an important clue! I was taking her out in a slight chop and she " lost bite " once. She got back up without a problem. Made me remember something I read about cavitation. I looked at the prop and much of the paint is off the blades in a swirl pattern, starting at the hub and moving out. Maybe pitch isn't the problem, but the diameter is. It peaked at<br />5200 rpm this time instead of 6100. This is starting to drive me a bit nuts. Also, I think I need a basic leesen in proper starting technique. Pumping it when cold cannot be normal.

 

[Haut Medoc]

Re: Merc RPMs Getting higher & higher

Pumping it cold is very normal, try two pumps and set at 1/4 throttle. Once it kicks over you will need to 'feather' the throttle to keep it running. This is not a fuel injected engine and you need to set the choke. The choke should close after one pump, verify this by checking it with the flame arrester off. If it fails to close, you will have a cold hard start condition. If it fails to open when warm, you will have a hot hard start condition. You are under pitched, you should have a 14-14.25d x 19P or so to achieve the desired 4200-4600 W.O.T.........JK

 

[bomar76]

Re: Merc RPMs Getting higher & higher

If you are indeed cranking 6100rpm in a stock SBC, you need to be shopping for a new engine.<br />Shortly there will be posts describing how the valve poked a hole in the piston or how the connecting rod peeked out from the side of the block.<br />For it to be soley an underprop situation you would need to be running a trolling motor prop now.

 

[BuzzM]

Re: Merc RPMs Getting higher & higher

Thank you all for this info. I'll get a new prop<br />as suggested and stay out of those very high rpm ranges. I'm just burning fuel anyway, but it is good to know what she'll do when the weather kicks up and we have to head for the barn.

 

[BuzzM]

Re: Merc RPMs Getting higher & higher

Update: I removed all wires from the coil & cleaned & tightened. I removed the connector to the tach. I cleaned & plugged it back in. I changed the tach calibration screw from 8 to 6 then back to 8, in case there was a spot of crud in there.<br />Took her out & noticed the idle was back down to 650, up on the step at plane at 3000 rp.m., & WOT at 4200. I have no idea what may have changed exactly. But, it ran great.<br />I think I'll quit messing with it. I don't want to tempt whatever powers influence such mysteries.