1971 johnson 50 bogs at WOT

[Jconnell43]

Have had boat out twice in last week, top speed used to be about 22 1/2 MPH.
15 Ft fiberglass tri-hull
Sorry no tach.
2 days ago it was running fine but topped out at 20 MPH, thought that I needed to adjust trim plate to right as it was pulling to right.
Adjusted trim plate boat runs straight.
Now I can't get it past 15 MPH it bogs down at about 2/3 throttle, fastest speed is about 2/3 throttle.
Have tried adjusting full advance stop to several positions, no change.
Questions
1,Doesn't this sound like a classic case of clogged fixed hi speed jet on at least one carb?
2.Have read Joe Reeves WOT procedure, need an informal answer, when I'm advancing the throttle, engine not running, at what point should the timing arm hit the stop, 3/4 or only at WOT?

Thaks to all!

 

[jbuote]

Really has nothing to do with what position the throttle is at necessarily.. Though it should reach full advance before throttle control is at full..
​Our 71 50's should be at full timing advance (timing arm hits the stop) at about 3500 RPM, and full advance is 19? +/- 1?
​From there, it's only opening the throttle butterflies further the more you go to WOT.. Should be open and flat when full throttle. Be sure it's not trying to go past full open, and closing again after center if that makes any sense.. lol

​Yeah.. Sounds like maybe a clogged hi speed jet as one possibility..
​Sure you're not misfiring or losing spark on 1 cyl at higher rpm?
Did a link & sync on it yes? lol

For what it's worth..

 

[Jconnell43]

Have done link & sync, think I have it down pretty good.
Idles, starts fine, pulled plugs, both clean, changed anyway, no change.
How to check for hi RPM on 1 cylinder?

 

[jbuote]

One really easy way, is if you have an inductive timing light, hook it up and watch the flashing when it bogs..
​If you see dark spots, that cylinder is not firing all the time..
​Then do the same on the other cyl..
If you constantly have the flash on both, then it's not a loss of spark at higher rpm..

​Another is a cylinder drop test. Not sure how I'd do that on the lake at speed though.. Would need help..

​I'm sure others will have more (and probably better) ideas...
​Like you, I'm new with this.. Had mine for only a little over a year now... lol

​sounds like I'd revisit the carbs and the hi-speed passages..

 

[Jconnell43]

Have timing liht
Can I check this in a tub on muffs with the prop removed?

 

[jbuote]

I wouldn't..
​You do not want to rev past idle (fast idle for cold start) when not on the lake with proper load on engine..
​Very real risk of runaway and blowing the engine..

​All "At Speed" or "High RPM" tests should be done on the lake with proper engine load..

 

[F_R]

Read this: http://forums.iboats.com/forum/engi...-75hp-stinger-bogging-down-after-1-4-throttle

So you messed with the full advance stop? Now you gotta re-time it, if you did.

 

[jbuote]

^^^^ This!!!

I was going to link you to that same thing, as it's a better thought than what I came up with, but F_R beat me to it.. LOL
​Definitely pay attention to that first...

 

[Jconnell43]

Had flywheel off this summer and cleaned and lubed advance mechanism, it's very smooth.

Will keep researching advance specs.

I'm at 50/50 on clogged jets or advance probs.

 

[jbuote]

Have tried adjusting full advance stop to several positions, no change.

Well, this says you'll have to adjust your WOT timing back to spec..
​Either the Joe Reeves method, or on the lake..
​If on lake in gear, you want to set the advance so it hits the stop at 19? +/- 1?..
​If Joe Reeves method, then you want to set to 15? +/- 1?

That should be done first..
​Then if still bogging, check the high speed jets in the carbs..

​My 2 cents.. lol

 

[jbuote]

And just an fyi.. .
Based on my factory service manual for the 71 50hp's,
​When adjusting the full advance stop screw.. Stop the engine, then 1 full turn of the screw will provide approx. 1? of adjustment.

​Once that advance stop is set to 19?, you should never have to change it again..., Ever... lol

 

[Jconnell43]

Did take apart carbs, one hi speed jet was clogged, ran boat, got to all time hi speed of 23 MPH, then motor quit.
Tried everything at home in tub with muffs, nothing, did timing light, no spark at either plug.
Just installed new power pack from CDI this morning, started on 1st try, ran for 5 minutes, will test on water on the very few remaining days here in Colorado.
Thanks for all the input!

 

[oldboat1]

Don't know if trihulls are any more prone to water logging than other old hulls, but mine carried a bunch of water and sogged foam under the floor. Lot of weight.

If you have the ignition issues handled, check the tank for water or other contaminants, check the hull for water. If memory serves, I was hitting mid to high 20s in mine (same power) -- think that was after I dried out the hull.

 

[Jconnell43]

Not sure if my 1972 15ft Scott Engineering has any foam in it, I have a steep driveway and I get little to no water coming out after use.
Would the foam be attached to the hull or directly attached to the underside of the inside floor.
Gas is fresh and I empty it out end of every season.

 

[oldboat1]

Not sure if my 1972 15ft Scott Engineering has any foam in it, I have a steep driveway and I get little to no water coming out after use.
Would the foam be attached to the hull or directly attached to the underside of the inside floor.

Usually both, I think. Mine was a Chrysler (same vintage), and I found the issue when I noticed some water dripping from a crack in the gelcoat at the keel. I drilled some holes low on the transom, and found water and wood rot. (can fill holes with epoxy or MarineTex, or something similar).

I'm not suggesting you have issues, though.