90 HP 2 stroke timing HELP!

[ollyjscott]

Had great response with other questions about my motor on this forum so hoping that you folks can assist with this one..

My serial # plate is missing from my motor so I'm not sure exactly what year it is (any other way of finding this out??) I think its somewhere between a 1987 and 1990 motor. Live in Europe so not sure if timing advance/retard is different from US versions..(? can't see why it would be).

Looks like the painted stopper screws have been messed with at some point and the motor was running very rough/stuttering at low and mid throttle when tested a few weeks ago. Runs very well above 4000rpm and WOT. I want to take the chance to set up the linkage base settings as per service manual and get it set up as per factory spec. TROUBLE IS.... I have bought a 1987 and 1990 manual and in the procedure for checking timing at full retard they are listing different specs for ATDC degrees! Extracts from manual attacehed in images to this post...see for yourselves..

Full advance timing is the same for both, so no worries there. I don't see why the full retard timing on these two models should differ by a whole 5 degrees! The motor is the same in bore and stroke, the compression ratio is the same, the hp output is the same...

Is full retard timing that critical to get right?

Help would be most appreciated!!

[ollyjscott]

Also worth noting is that BOTH manuals have the SAME dimensions given when first settign up the length of the stopper bolts, so the arm for the magneto control lever should have the same position when at full retard/advance, which should result in the same numbers on the flywheel when checking timing..?

[rodbolt17]

once you make sure spark is ok,compression is ok and the carbs are clean.
no ATDC timing isn't that critical. as long as it idles well.
pick up timing,not so critical as long as it accelerates well.
WOT timing, absolutely critical, miss it to far above TDC and you can detonate a piston.

before I start setting timing I index the flywheel to insure the pointer is correctly showing TDC.

[ollyjscott]

Hi there - thanks for your reply, and also for your advice on my other threads. Appreciate the quick response and the knowledge here.

Compression checked on all 3 cylinders - 120 psi even across all cyl which seems good to me, if a touch down on manual spec. It's an old engine after all.

Fuel pump replaced due to a punctured diaphragm which no doubt contributed to poor running and a very fouled plug on cyl # 3

Carb kit is still in transit so I'm waiting on that in the post (live in Norway and spares availability is something I'd rather not talk about..)

My main aim in checking the timing is to eliminate this from the rough running causes. I would like to check the full retard timing at idle and then get the boat in the water and check full advance at WOT.

Have a gauge to help me get accurate TDC and will check timing plate alignment at TDC on cyl # 1 and adjust if needed before proceeding as per manual.

Essentially, all is in place to start checks, I'm just stumped by the manuals - two vastly different degree settings for full retard. As you say most critical is WOT full advance and that is the same for all 90hp Yams for all years so no issues there.

Any ideas why there is this difference and how I can find out the model year of my motor?

[JohnOrrett]

Here is an extract from another Yamaha manual, which covers several models, including Europe. Presumably yours in a 90 AET or AETO.

[ollyjscott]

Thanks for the reply and the extract. I've got this manual too but I think it covers newer models as in the pages preceding it shows setting timing also involves making sure an indicator or lever on the CDI box cover is aligned with a sticker or something like that. That's the newer Yamaha 90A engine. My model does not have that extra lever not indicator to the left of the magneto control lever. Interesting that it shows yet another different setting for full retard. Maybe I should just take an average of the three and hope for the best!..

[99yam40]

I would put the timing light to it and see what it is set at before going through a bunch of thinking and searching. See what it is set at and then decide if any changes need to be made

[ollyjscott]

Exactly my thinking. I'll be running it up tomorrow to check the timing at idle /full retard. I checked the link and stopper bolt lengths vs what the manual(s) I have state and they are way off. So far off I don't even know how it ran before. But as you say I'll get a timing light on it and work from there.
Checked the TDC mark vs cyl #1 position and it's correct.

Think I've also found the second serial number stamp on my motor below the starter motor on the engine block.

I'll let you folks know what he result is.

[ollyjscott]

So for the sake of keeping a record here and potentially assisting others in the future here are the results of my timing check:

Timing at idle 800rpm was found to be 2 degrees ATDC so confirmed my suspicions that it was out of adjustement. Adjusted to what I believe it should be 5 degrees ATDC and engine idle and low rpm running improved.

Took her out on a calm day and hung over the back end and motor while the flywheel was spinning at 5000 rpm WOT / 37knots (interesting experience) and discovered full advance timing was around 27 degrees BTDC so too advanced. A couple of runs later and some adjustement and it was set to 22 degrees BTDC as per manual. Motor runs sweet which is helped by new fuel pump at low to mid rpms.

So on and old motor that looks like it's has had some work done I'd recommend you check the timing. My motor was previously running fine according to my ears at WOT by it was actually too advanced on the timing which in the long run is not a good thing. Just make sure you have a calm day and a steady handed wheel man while you're checking timing when running flat out!

Cheers for the input and advice so far people

[TownsendsFJR1300]

Thanks for the updated and informative information!!

[99yam40]

Hopefully you rechecked idle timing again after adjusting everything to make sure nothing got changed

[Nightfisher]

90 AETO (2 stroke 3 cyl) timing adjustment -- service manual pages

I know this is an old post, but have the same issue.

I just got an older 90 AETO 2 stroke and the timing adjustment is way out of whack.

I am not a mechanic but do own a timing light. I wonder if anyone knows where I can access the service manual pages for this process without paying for a service manual?

[boscoe99]

https://www.vansoutboardparts.com/se...x-75x-90x/file

[Nightfisher]

Thank you

https://www.vansoutboardparts.com/se...x-75x-90x/file

Much appreciated Bosco!!! Great forum.

[Nightfisher]

I thought I was relatively handy with engine stuff but I guess it is over my head. My engine does not even have the throttle sensor control link adjustment that is described in the picture.

Does anyone know a reasonably priced mechanic that can do the timing on this engine in the Dallas Fort Worth area?