engine hood choking engine?!

[redfury]

1972 'rude 100hp. Long story short, bought the motor used- it didn't run right in the lake...ended up being a loose wire on the points system. Ran great in the water once it was firing on all the cylinders, but then bogged down. Water pump was replaced and got a good stream from the pee hole, so some of the worry of overheating was gone, but still wanted to err on the side of caution. Took the engine cover off to keep an eye on the engine and the bogging down was gone. Seemingly the engine cover is choking the air supply to the engine. It runs fine if the cover isn't completely snapped down with all 3 clamps, but if you seal it up, it bogs down again. Where in the cowl is the air supposed to enter, because it isn't readily obvious by looking at it.

 

[F_R]

Re: engine hood choking engine?!

Are you sure the cover latches aren't contacting the spark plug wires and shorting them out?

Possibility #2: If there is an exhaust leak under the hood it will smother the engine due to lack of oxygen in exhaust fumes.

I don't remember where the air gets in that particular model, but it does. Sometimes it comes in under the grab handle for tilting. But wherever, don't worry about that. Focus on other possibilities.

 

[redfury]

Re: engine hood choking engine?!

Well, it runs fine for a few minutes before it starts bogging down, which is why the conclusion on airflow instead of something just plain wrong ( such as sparkplug wires grounding out.

 

[Beeched]

Re: engine hood choking engine?!

I was or am fighting the same problem with my "65" 40 HP. On mine there is a cover that is about 3" diameter right under the carb.. It is right at the front of the motor. The manual calls it a silencer. At any rate I took the 3 screws out that holds the cover on. It has some type of fiber type filter in it. I replaced it with a cut out piece of foam air filter for a lawn mower, (not sure if it was the right thing to do), the original material was totally soaked with gas/oil mixture. I used the foam filter because of it's resistance to chemical and its air flow application as air cleaner. Ran the motor in a tank and it did run better with the cover on. I had to set a fan by the tank to keep the exhaust from fouling the intake.

This may or may not help, but is a suggestion.

"Keep on Fishin"

 

[redfury]

Re: engine hood choking engine?!

Hrmmm, I'll have to check on that. This motor has the "silencer" carb cover also, tho' we haven't had it off yet.

 

[iwombat]

Re: engine hood choking engine?!

Sounds like a classic exhaust leak to me. There's nothing like an air filter in your setup anywhere.

 

[kmk_7110]

Re: engine hood choking engine?!

Well, it runs fine for a few minutes before it starts bogging down, which is why the conclusion on airflow instead of something just plain wrong ( such as sparkplug wires grounding out.

Just because of that makes me think of exhaust getting in the hood as mentioned before.

 

[F_R]

Re: engine hood choking engine?!

OK, I looked it up. The air intake is under the carburetors. A large cover with a silencer pad. Check it out if you really think that is the problem. There is a first time for everything, I suppose.

 

[redfury]

Re: engine hood choking engine?!

If I'm getting exhaust into the cowl, how's it getting there? What's fouled up?

I can only assume that the exhaust would be leaking into the extent that it eventually chokes out the motor by introducing more exhaust than fresh air?

 

[hoeser]

Re: engine hood choking engine?!

Probably a bad exhaust gasket if you are leaking exhaust up there... it should be a noticeable leak, you may want to get yourself a spray bottle full of soapy water and spray around the gasket area and see if you get any bubbles when its running.

 

[reeldutch]

Re: engine hood choking engine?!

could be your powerhead to adapter plate gasket leaking.

when you run it with the cover on and it dies on you
take the cover off quick and do a sniff test.
it might smell like 2stoke exaust under your cover.
than you know for shure.

that would be the first thing that comes to mind.

 

[wilde1j]

Re: engine hood choking engine?!

BTW, running OB's with the silencer removed is generally a very bad idea. Causes a too lean mixture, which will fry the cylinders.

 

[redfury]

Re: engine hood choking engine?!

No plans to run this engine other than the way it was designed. The sniff test is a good idea though. We are going to get the boat out on the water on monday to check the timing, etc. I'll do the sniff test the second it bogs down.

 

[redfury]

Re: engine hood choking engine?!

Well, taking suggestions from here into account, and a need to pull the lower unit off anyways.... We found 2 things.

One, we found out that the lower unit gasket for the exhaust was damaged, Aaannnd, we found out where the combustion air comes into the cowl.

For those that can't find it, the air intake is under the carbs in the lower part of the engine housing. It sucks air from the transom area and is buffered by the air intake for the carbs being located by the flywheel. That was a relief. The other good news we got was that after fixing the gasket leak, the engine no longer bogs down, so we were getting choked out by exhaust. About 20 seconds to get to top speed from a dead stop! 35mph, and I'm sure we are not properly propped, but we need to get a tach on it before we look into that. For now, it is running smoothly, fast and doesn't sound like it's over revving or under revving at all, so we are pretty close to spot on with the prop that came with the boat...

So, thanks for the suggestions, the engine is now viable, even though we'll still need to do some fine tuning. For a while there, we thought we may have gotten a dud motor.