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70HP bogs on initial acceleration, fine after quick choke

Skiboy10

New member
This has been bugging me for years. Looking for ideas.

I've got a 1978 Johnson 70HP that has been puzzling me since I bought it in 2016. On cold start (or warm start after being left alone for 20-30 min) and under initial acceleration the engine often bogs and will stall UNLESS I tap the choke. Then it takes off and runs great. Once it is running great, bringing it back to an idle and taking off again is no problem. This pretty much just happens under load, not at fast idle or on the ears.

Things I've done that haven't made a difference:
  • cleaned carbs (local marina did this when I initially bought the boat because it pretty much didn't run)
  • re-sync'd carbs (though I'm about to go do it again from start as I'm learning more about the procedure)
  • new fuel pump
  • power reeds

One thing I've been tinkering with is having someone else drive and when it bogs, I advance the roller pin (slightly). It takes off. So I added a turn of spacing to the roller pin thinking that might help - nope.

The other thing throwing me off is the willingness to go after a quick choke. And I mean quick.

Other than that very annoying quirk (I can get around but wife and kids struggle with), the engine runs unbelievable.

Any ideas?
 
Having to use the choke clearly indicates a lack of fuel from carburetors.------What elevation are you running at.-----What numbers are on your idle jets and high speed jets ?----These jets are called " orifice plugs " in the parts books.
 
Having to use the choke clearly indicates a lack of fuel from carburetors.------What elevation are you running at.-----What numbers are on your idle jets and high speed jets ?----These jets are called " orifice plugs " in the parts books.

I'd have to pull them to find out the exact numbers. But we operate the boat at about 1825 feet.

But the part I don't understand is that the need to choke is very brief (a fraction of a second) and only once. Once warm, you can do anything.
 
Could be intermittent sticking of the timer base. If the carbs open too soon (before full timing advance), it will stumble and/or stall.
 
Did you do a link and sync? Some of those like a little timing advance before the carbs start to open.

Yeah, at least I thought I did. I was working out of one of the descriptions in an aftermarket manual. Need to see if I can find the official OMC method on YouTube or something. I will do that later this week and report back.
 
Update - I went back and did the sync n sync from scratch (exception of verifying TDC). Set it to 725 RPMs in fwd gear with no resistance, verified advance was TDC in that position (book spec for 1978). Set the cam roller as pictured below. Gave it a nice test run and it bogged again. After I choked it it ran great (here's a video). https://photos.app.goo.gl/sxw3KyCBiMvxKdRL9

Used it a few more hours for low speed aquaplaning. This is the speed where the boat is right on the edge of breaking plane. After doing that a few times then switching skiers and going again, it bogged (choke required).

I'm so confused.

IMG_0029.jpg
 
I am not there to inspect or touch this motor.----What are the compression test results on this motor ?----Post the actual numbers.---Should be around the 140 PSI.-----What are the numbers on the high speed orifice plugs?.-----What are the numbers on idle jets ?-----It should not take years to figure this out.?
 
I am not there to inspect or touch this motor.----What are the compression test results on this motor ?----Post the actual numbers.---Should be around the 140 PSI.-----What are the numbers on the high speed orifice plugs?.-----What are the numbers on idle jets ?-----It should not take years to figure this out.?

Compression was 115 dead on across all 3. When I get time I will pull the plugs. Last 2 weeks were out because of black fly season. Can't sit still outdoors without being swarmed.
 
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