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Author Topic: M91 spray from inlet  (Read 1951 times)

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Re: M91 spray from inlet
« Reply #15 on: January 05, 2025, 09:46:15 am »
Have to say, the spray coming from the inlet is a lot less. After running it a bit it's hardly noticeable.

Maybe the ignition timing did the trick ? Maybe the inlet valve seated itself better after some run time ?

Still a bit of a mystery to me, these little engines.
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Re: M91 spray from inlet
« Reply #16 on: January 05, 2025, 10:16:49 am »
I'd say it was likely a combination of both improved timing and run time seating the intake valve better!

In any case, congratulations on your efforts to make it run better!
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Re: M91 spray from inlet
« Reply #17 on: January 05, 2025, 10:30:59 am »
It's still a far way off, lots of tweaking required. But first I'll just run it a bit, get everything seated a bit, and just enjoying
the sounds and smells of it.

Also now I think of it, I'm running it on the same fuel I use for the Cison V-Twin, which required 1:25 oil content. Might be too
much oil for this engine. Using Zippo. Difficult to get Coleman fuel here in Greece....

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Re: M91 spray from inlet
« Reply #18 on: January 05, 2025, 11:05:40 am »
Bonjour Hans,

Great progress indeed, bravo !

Based on my experience with these little engines (I have made a Playlist for this topic : https://www.youtube.com/watch?v=Q91Ju0Peu2Q&list=PLCVcZL8L7sBJ-ILRmMDuBQk9Jz-aOd-AE&pp=gAQB) they are AWFULLY difficult to adjust.

I work by trial and error on only a few degrees forward or backward for every new test as the temperature of the room is never the same.

Furthermore, I noticed that sometimes the mixture air/gas has to be just a little little bit adjusted (closed) when the engine gets warm.

As gasoline, I was suggested Alkylate 2T and it is great advice :
https://www.google.com/search?client=firefox-b-d&q=essence+alkylate+2+temps
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Raphaël, Membre du Modèle Yacht Club de Paris : http://mycparis.fr/
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Re: M91 spray from inlet
« Reply #19 on: January 05, 2025, 12:41:20 pm »
All is well, but yes when warmed up and after running out of fuel, it is almost impossible to get it running again. Very
sensitive, the adjustment.

Also, when setting the needle, I close it, and then turn it back let's say 1/4 turn. But sometimes the fully closed point
seems to have moved ? I made a little dot on the adjustment screw, but it is not always on the same position when I close it
fully.  Not sure what's happening here.

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Re: M91 spray from inlet
« Reply #20 on: January 05, 2025, 01:43:17 pm »
Answering my own question here. The spring under the adjustment screw is too strong, making it hard to
feel the end point of the needle. I'm gonna look for a softer spring to put there.

Also I modified the 'coasting link' (what IS the name for this thing that keeps the exhaust open) so it can never
touch the cam gear again.

.... and I just noticed the little shaft that the cam follower is on has moved inwards a bit. Have to look at that
tomorrow. And find a better way to fix this in place.

Any of you using loctite ? I'm hesitant to use it. In case of difficulty when something needs to be taken apart again.

I'm trying to find this Alkylate. Not easy here in Greece it looks like....
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Re: M91 spray from inlet
« Reply #21 on: January 05, 2025, 02:51:44 pm »
Bonjour Hans,

Personally, I have never dared to put any thread locker on my little engines, considering the small sizes of the different screws, nuts and bolts and their possible fragility when unscrewing.

I wish you will find this alkylate type of gasoline, it is really great, before I used to put 4 drops of 2 stoke oil in the tanks with SP98. I can see it on Amazon :
https://www.amazon.fr/s?k=essence+alkylate+2+temps&crid=31S8BUW01EW35&sprefix=essence+alkylate%2Caps%2C105&ref=nb_sb_ss_ts-doa-p_2_16

In all the cases, never more than 4% oil in your mixture otherwise the engine spits it out !

I have also tried "Essence F" but have not been convinced.
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Re: M91 spray from inlet
« Reply #22 on: January 05, 2025, 03:39:39 pm »
> It's new. Shouldn't it be lapped and ready to go?

Yes, it should be, but don't bet on it. QC varies in model engine land.


The problem could be something simple like a bit of trapped swarf or a burr.  If not....

Because the valves are quite tiny, they need fine abrasives.  Things I've used that work at varying levels of efficiency are toothpaste, tin oxide, and bathroom cleanser.  I've had good results with SoftScrub.

Apply just a small amount, you don't want the abrasive to get onto the valve stem and into the valve guide. Check frequently, rinse and repeat until you get a clean seat.


Re: Running too fast

Use softer springs on the governor weights.  With the OEM springs, using a single spring will help a bit.
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Re: M91 spray from inlet
« Reply #23 on: January 06, 2025, 12:06:11 am »
Yes I did notice quality is not consistent. But then again, we get this magnificent jewel for this price that we
can actually afford. And with it comes the ability to improve on its flaws by speaking to fellow enthusiasts.

Compression has increased noticeably during run in, and with it the spray has decreased significantly. So the diagnosis was correct, but
I'll leave the head on for now. Can always do it later. Indeed I had gathered the normal lapping cream would be too rough.

My project for today is to fix the migrated pin where the cam follower bearing is on. It has moved inwards and now the exhaust pushrod
is sticky. I'm thinking getting the steel pin out of the brass part, clamp it in the vise to damage the end a bit, then drive it back
in with a bit of epoxy.

The springs, I'm on it. Scouring the house for some old equipment that might contain some....
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Re: M91 spray from inlet
« Reply #24 on: January 06, 2025, 04:51:44 am »
Quote from: Raphael on January 05, 2025, 02:51:44 pm
Bonjour Hans,

Personally, I have never dared to put any thread locker on my little engines, considering the small sizes of the different screws, nuts and bolts and their possible fragility when unscrewing.

I wish you will find this alkylate type of gasoline, it is really great, before I used to put 4 drops of 2 stoke oil in the tanks with SP98. I can see it on Amazon :
https://www.amazon.fr/s?k=essence+alkylate+2+temps&crid=31S8BUW01EW35&sprefix=essence+alkylate%2Caps%2C105&ref=nb_sb_ss_ts-doa-p_2_16

My thoughts also on the loctite.

I see 55 different brands. Which are you using ? The Stihl brand seems ok for price & shipping to Greece (10 euro / ltr) but it doesn't mention
the word Alkylate....
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