115TL79 Shaking/Vibration 2000-2800 RPM

[verdebimmer]

Been looking through the forum posts for a few weeks and never found a similar problem to mine. I am hoping that someone here has some insight into my problem. Boat is new to me and had the problem since I got it a couple months ago. I got it so I could take my 92 yo parents out on the lake. I figured this wasn't a big problem, since the boat ran great above 3000 rpm. I was wrong.

Motor starts well and runs with minor shaking until it reaches 2000 rpm. Doesn't stall even when idling at approx 700 rpm in gear. Running along at 1000-2000 rpm seems to surge and vibrate a little, like a cylinder sometimes misses then occasionally fires with minor shaking/vibration. Then, when accelerating it shakes pretty badly until it gets to almost 3000 rpm. The shaking is pretty severe, depth finder really shakes in its bracket! Above 3000 rpm until 5100, it runs great. Since the motor is installed on a pontoon boat, I would like to run in the exact rpm range where the problem is worst. I have checked compression (120-125 all cylinders), timing (0 degrees BTC at idle, 28 at WOT), champion plugs are new (0.030" gap). The idle timing screw is screwed all the way in, so I can't advance the idle timing any further as I think it should be 4-6 degrees BTC. I had a local marina check it out. They told me it was a carb transition issue, where it goes from low speed jets to the high speed ones. They did a link and sync, removed/cleaned/adjusted floats on carbs and checked timing and said there was nothing else to do.

I've since checked all the resistances of the coils, power packs and stator that I can do with a digital VOM. All were in spec, according to factory manual. I also switched the power-packs from port to starboard, which made no difference. After all of the above, I checked the port side plugs and noticed they are very clean compared to starboard, like they are being flushed with gas making me think they are not consistently firing. I then ran the motor with #2 (top, port side) plug wire disconnected and had no vibration, although WOT throttle rpm was only 4100 rpm.

So, I'm guessing there's a low rpm ignition issue. From what I've seen on these forums, usually there's a problem at higher rpm, not lower. Can anyone please give me some advice?

Thank you.:cold:

 

[verdebimmer]

Re: 115TL79 Shaking/Vibration 2000-2800 RPM

Does anyone have any ideas I can pursue?

 

[emdsapmgr]

Re: 115TL79 Shaking/Vibration 2000-2800 RPM

Check your spark plugs after running at idle. Wet, dark plugs are normal. A dry plug indicates it's not getting fuel. A clean plug may indicate it's being washed by an internal water leak. You can check for spark while the engine is running. Put an inductive timing light on each plugwire and note the flashes from the gun. They will tell you if you have strong spark, intermittent, etc. If you find you have no spark on one plug, you can swap the dual power packs as a test. If the non-firing plug moves to the other head after the swap, it's likely a bad pack. If there is no change after a pack swap, check the stator output.

 

[Toddboat]

Re: 115TL79 Shaking/Vibration 2000-2800 RPM

Still sounds like a carb issue to me. Take the carb out and look for tiny holes in the top of the carb throat. There should be a single one, and then a row of several of them. They should all be located on the inside of the butterfly plate.
The single hole is where fuel comes out at idle speed. The several holes in a row are low speed. The big copper tube at the bottom of the carb throat is hi speed. Now remove the float from the carb and reassemble it. Then reattach the fuel line to it and pump the bulb. Careful because fuel may spew out from various parts of the carb.
Make sure fuel is coming through all of the little holes in the top of the carb throat. If not, remove all plastic and rubber parts of the carb and soak it in a bath of carb cleaner for 2 days. Then spray it clean. This should free up the gunk that accumulated in these tiny passages while the boat was sitting without use.
Always add fuel stabilizer every time you add fuel to the tank. You could also try running seafoam through the motor to see if there is any improvement, as it is a shortcut to cleaning the carbs. I haven't had much luck with it. Nothing compares to soaking the carbs and then cleaning them. Perhaps the marina did not do a thorough job of this, or perhaps gunk from the fuel pump, fuel lines or tank got back into the carbs. If you find that the problem was in the carbs, you should replace your fuel lines and drain the tank. The fuel lines could be sloughing off tiny bits of rubber into your carbs from ethanol degradation.

 

[verdebimmer]

Re: 115TL79 Shaking/Vibration 2000-2800 RPM

Thank you very much for your response. I checked each spark plug with my timing light and an inductive tachometer. I also got one of those cheap neon bulb spark testers from Harbor Freight for $4. All those checks found no difference between the four plugs at any rpm. The plugs look pretty good to me.

The port side plugs are still a little cleaner than starboard, as you can see in the attached photo (I hope I did that right). However, they all seem OK. Do you agree?

I went to Radio Shack to see if I could get the parts for a DIY DVA attachment for my meter so I could check the stator output. The only part they had was the diode, not the resistor or capacitor needed. I have both an analog and digital voltmeter. Are there any tests I can do with those?

 

[verdebimmer]

Re: 115TL79 Shaking/Vibration 2000-2800 RPM

Hey, thanks for the response. I've done just about all the testing and swapping (from port to starboard) I can do with the ignition system and it's made no difference I can detect. So, I'm going to have to dig into these carbs. My problem is that the motor is on a pontoon boat that is moored at our dock. I'm afraid to work on it while it's in the water in case I drop something. I was wondering if you knew whether these low speed and idle jets are operable at 3000 rpm and above, or do they stop working when the high speed jet begins feeding fuel?

I took the boat out yesterday for an hour and fifteen minutes running non stop at 3200 rpm basically the whole time. It ran perfect at that rpm. It also seemed like it ran a little better at lower rpm after that long ride. So, I'm wondering if the fuel might be slowly cleaning out the passages when I'm running at the rpm where it's operating perfect. It might just be wishful thinking on my part.

Thanks again. As soon as I find a way to get the boat out of the water, I will get those carbs off and do the test you suggested.

 

[emdsapmgr]

Re: 115TL79 Shaking/Vibration 2000-2800 RPM

You may be able to rent a peak-reading voltmeter from an auto parts store to check the cranking stator output. I would think you'd be looking for minimum of 150 volts. I don't believe the 79 factory manual indicated a minimum volts check when that manual was written. Later manuals used that 150 volt figure for the V4 engines. You should look at this website: cdielectronics.com. They have some excellent troubleshooting information and may have a target voltage figure for your 79 engine. Your plugs look normal.