'73 Johnson 65 Surges at Higher Speeds

[little mfg]

After replacing the exhaust manifold gaskets, etc. The boat can finally reach higher speeds consistently. I say higher speed because the lake was a bit choppy for WOT. All three cylinders seem to be firing properly now. Now a new problem has emerged. The boat will run smoothly for a minute or two at higher speed, then start to surge. I cut the motor, sit for a minute, start it, and all good for a minute or two, then the surging comes back again. It also requires choke constantly to get it to start. It idles smoothly once it finally starts. The fuel lines, hose and primer bulb are new. I've cleaned the bottom carb but haven't touched 1 or 2 yet. Any suggestions would be welcome! Thanks!

 

[oldboat1]

Gotta touch 1 and 2 carbs -- pull them off and give them the full treatment. Make sure the h.s. jets are squeaky clean.

Sounds offhand like the motor is leaning out. Not a bad idea to check for air leaks (lower shaft seal?) With the engine running, try shielding the carbs with a piece of cardboard or similar, then spray carb cleaner or WD-40 around the base of the powerhead. If engine rpm change, suspect a leak.

 

[little mfg]

Thanks for the reply Oldboat. Whereabouts is the lower shaft seal? I'm looking at the diagram now and nothing's jumping out at me.

 

[oldboat1]

would be at the base of the powerhead (removal required for replacement). Try the "non-intrusive" test first -- can also remove the l.u. and shine a light up at the top of the driveshaft. If there is some evidence of fuel high up there, replace the seal.

 

[little mfg]

I gotcha. I just replaced that gasket for the manifold job. Hmm. I will check it as you suggested. And pull and go over the carbs too. Thanks for the advice. I really appreciate it.

 

[oldboat1]

Hey -- hope you get it handled!

 

[little mfg]

I'll report back, when I have news, Admiral!

 

[jimmbo]

Surging after a minute can be a fuel starvation problem, Air leak in the fuel line, blockage in the fuel system as far back as the tank, or restricted venting of the fuel tank. As it takes a minute or two, I suspect a problem back at the tank. Did you notice if the primer bulb was keeping its shape or collapsing, as the surging began?

 

[little mfg]

Hey, Oldboat, one last thing: I'd have to perform the spray treatment around the motor base on the lake, correct? I don't think it would surge in the driveway.

 

[little mfg]

Hey Jimmbo, We pumped the primer bulb and there was no real change - kept surging. Bulb seems to be working properly. I don't see any visible leaks around fuel lines, but I haven't checking the internal lines yet. The only thing that seemed to help was letting it sit for a couple minutes then restart. Then surging began again after a couple minutes of run time at higher speed.

 

[jimmbo]

If the Gaskets between the Powerhead and Midsection are sealing, you could spray or pour a gallon of WD-40 or gasoline at the base of the powerhead and it wouldn't make any difference

 

[jimmbo]

Hey Jimmbo, We pumped the primer bulb and there was no real change - kept surging. Bulb seems to be working properly. I don't see any visible leaks around fuel lines, but I haven't checking the internal lines yet. The only thing that seemed to help was letting it sit for a couple minutes then restart. Then surging began again after a couple minutes of run time at higher speed.

Try Running with the Fuel Cap loose/off

 

[oldboat1]

As long as the motor is running at idle (i.e., the driveway), spraying around the base of the powerhead should tell the tale. While shielding the carb intake, spray some WD-40 at the crankcase joint and head cover joints as well. If no changes in rpm, move on to carb cleaning.

 

[racerone]

Replace fuel pump diaphragm.-----Test run with a " known to be good " tank and hose.

 

[little mfg]

Jimmbo, I tried running with the gas cap off with no improvement. I bought this new plastic gas tank a couple years ago for another boat. Never liked it. I'll probably buy another of a different brand.

 

[little mfg]

I'lll give it a try, oldboat. Thanks.

 

[little mfg]

Racerone, the fuel pump was recently rebuilt--only has a few hours on it. I like your idea about trying it with a proven fuel tank and hose. Hose and bulb are new, but who knows? I never like the fuel tank. Can't quite put my finger on why. The fitting doesn't see quite right to me. It's one of those with the strange pressure relief button on the cap. Thanks for the reply.

 

[oldboat1]

Might try the old connector at the engine, if you still have it. I've had issues (and loss of patience) with new ones too.

 

[little mfg]

Thanks everyone for the feedback! I discovered today that about half the bolts on the exhaust manifold (there's about 18) were not very tight after taking the boat out yesterday. I had tightened them to spec after the gasket job. I retorqued them again to spec (about 84 in. lbs). Could this have contributed to the surging at high speeds? Let me know what you think! Thanks!

 

[oldboat1]

Yes,

Did you lap the mating surfaces until equally shiny all around? Warpage, as mentioned earlier. Might have another go at it.