71 50hp won't idle with butterflies closed

[dezeldave]

Greetings all!

So after reading and researching and trying to figure out my Evinrude, I’m at a loss. I’m working with a 71 Evinrude 50hp with electro-hydro shift. I did a complete teardown and rebuild of the motor and lower unit. Everything was cleaned, painted and reassembled according to the factory service manual. Motor looks great and after some testing finally got almost all of the bugs worked out.

Before I pose my question, here is where I stand right now:

1. Both cylinders are at 135 on compression.
2. Both ignition coils have great blue spark at 7/16 gap with a spark tester.
3. Both carbs were totally rebuild and cleaned out. Low speed settings were done in accordance to the service manual and Joe Reeves method. Currently sitting at around ¾ turn out.
4. Max advance timing was set using the Reeves out of water method and is sitting at 15 degrees. 4 degrees short of the factory spec of 19 BTDC, but according to the Reeves method. I have not been able to get a timing check on the water at higher throttle setting yet.
5. Engine will run and idle smoothly, has full range of power, shifts nice as well.

This leads me to my question:

Being an avid tinkerer and also an aircraft mechanic, I can’t leave well enough alone. Right now the motor runs seemingly fine but it’s not within the specs it should be. I have the idle timing set at 3 degrees BTDC, right on where the factory manual states. However, to get there, I have the idle stop screw (that directly links to the timer base arm) set all the way in. I’ve done the link and sync about 10 times, using the service manual and the bits and pieces off the reeves way. I’ve swapped timer bases to check that. Also swapped flywheels to check that. None of that changed any of the timing readings. I’ve verified the timing pointer is dead on at TDC as well.

My issue is I’m seeing this motor is supposed to idle with the carb butterflies completely closed. Meaning that the idle speed is controlled by ignition timing alone. I cannot accomplish that. I have the cam scribe mark lined up perfect with the carb throttle arm but I have to have the butterflies cracked open for it to idle. No matter where I move the timer base (with the carb throttle disconnected) I cannot achieve an idle with timing alone.

I followed the service manual to a T and found 3 degrees BTDC for the idle timing, then reset the nylon yoke so that the carb cam scribe line is perfectly in alignment with the throttle arm right as the cam starts to pickup the throttle link.

Could not get an idle at these settings. I had to advance the nylon yoke further to hold the carb butterflies open a tad.

Maybe I’m just picky but I always like to fully understand why something works and more importantly why it works or doesn’t the way it was designed.

As I’ve said, motor runs just fine but just not exactly the way it was designed. I’m in need of some assistance to help understand if I’m thinking correctly about this motor idling with the butterflies completely closed. (which really baffles me as to how the engine gets any air) And if so, what am I doing wrong that is not enabling me to accomplish that. I’m also looking at why I cannot get to 3 degrees BTDC on the idle unless I have the stop screw maxed out.

I did notice on disassembly that someone put a 4 degree advance timing key on the flywheel. Its basically and offset key to mechanically change the timing. I took that out and put a regular key in it’s place. I’m wondering if I should put that back in.

I’ve read other posts about guys having similar problems but never saw where they reached a conclusion on what was going on.

Looking to the experts here for advice.

Thanks

 

[racerone]

No such thing as an " offset " key for those motors.------Put the proper new ( factory ) key in the flywheel !-------When throttle plates are closed are there not holes in the throttle plates to let a calibrated amount of air in ??----As you must know a 2 stroke MUST have fuel , air and OIL going into it while idling.

 

[dezeldave]

Racer, appreciate the response. I know there is not an offset key for these motors, but the previous owner fit an automotive offset key in there. And after a close look at the butterflies, I do see the bottom is notched to allow air in. And I'm well versed in 2 stroke engines. I have a 1960 40hp johnson that I restored and rebuilt and it runs great.

And i have put a new OEM key in it and got rid of that offset key.

I have air, fuel, and oil in the engine. Just can't figure out why i need the extra butterfly opening to hold an idle.

 

[racerone]

Some trouble shooting needs to be done.---With the extra information posted I am sure you can figure it out.

 

[dezeldave]

Agreed, I do need to get into it further. I was really looking to see if anyone else had ever had a similar situation occur.

 

[Faztbullet]

You say you rebuilt motor...did you replace pistons?

 

[dezeldave]

Only the top cylinder piston was replaced. Both cylinders were honed to clean them up. Both pistons got new rings. Piston to cylinder clearance was checked and is within specs.

 

[Faztbullet]

Hard to believe it was in specs as these motors have sever side thrust and wear the cylinders around the ports in a taper..when these are worn you have the symptoms you are posting.

 

[dezeldave]

That is a very interesting point there. I checked the cylinders with a bore gauge but only checked about 1 to 1.5 inches down. Mainly looking to see if they were out of round and if it was still within spec for standard pistons. I didn’t check for taper around the ports, wasn’t sure how to do that without the gauge going into a port opening.

What effect does the taper cylinder cause when it’s that low down in the cylinder? I’d have to assume it’s related to how the piston and port work to draw fuel in and get exhaust out. Or possibly blowby not allowing enough fuel into the cylinder.

I might be back knuckle deep in that engine over the winter!

 

[Faztbullet]

Result is lower crankcase pressure due to flowback or as some people call blow by. This is because the skirts of piston (widest part of piston) have excessive clearance past ports.This will cause a weak carb signal.

 

[dezeldave]

I thank you fine sir for the tech advice. That’s an avenue worth pursuing. This was the sort of discussion I had hoped for when I first posted. I figured someone out there had to have seen similar results and had some ideas on where to look.

Thanks again!