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Lawnboy 77, you do have the plugs ( 611545) installed in the two round holes at the back of the rectangular F carburetor air box?
This one doesn't have those plugs (611545) in the filter box. I think those plugs you are referring to are on the older OMC F series carb boxes. Mine is a 1993 model 10420 with the metal Walbro carb. I'm glad you mentioned that though because I think you are on to something. I need to check to see if that air box/base is warped enough to cause this because I don't think I have a gasket between the air filter base and the carb. The IPC doesn't call for a gasket there and I usually just stick with whatever the parts catalogue says, but obviously if the surface is warped I would need some way to get a good seal there. I don't think there is any other way for dirt to bypass the filter, other than the two mounting holes, but the screws heads have those holes covered well enough I suppose. I should have checked that out the last time I had that carb off. There probably should be a gasket there anyway because who knows what happens when it heats up. It could be sealing okay when I look at it cold, but when hot the plastic may expand and create openings. I had mistakenly thought that it was just a bad design and the fix would be a remote filter, but that may not be necessary. Thanks guys!
 

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Yes the self-propel crank and push cranks are the same for that era of the F series engines. Not all of the Self-propel and push cranks from an F series are interchangeable though, some of the real early ones were gear drive and had a worm gear machined directly into the crank.

The flywheel is kind of the same deal, looks like there were at least 3 different ones over the 20 years or so of the F engine production. I went and looked at the parts catalogue and from about 1991 and on the F series used flywheel p/n# 683580.

The latest F series engines (mid 1996 through the end of production) had thrust bearings and washers installed at the lower end of the crank (pn# 252-76 thrust bearing, and 252-80, thrust washer) to combat the problem of excessive vertical play in the crank. It might be something to consider, and they are still available. You can look at models like the 10401 (latest version), or 10401C see those parts in the diagrams. If memory serves me correct those bearing sets (1 ea stacked) measure about .062 inch, so it may take a couple of the washers to shim you back into specs with yours having approx .250 inch of play. I have yet to incorporate these thrust bearing sets in an engine not originally equipped with them, but the part numbers are the same for both the crank and the crankcase, so it should be doable.

Below is the parts catalogue link that I use to check parts effectively and interchangeability for different model of mowers.

https://www.partstree.com/parts/lawn-boy/mowers-walk-behind-lawn-mower/
I have found things differently. The crank on my c21zpn and on my former s21zpr did not have holes drilled for the self propel pulley so those are another one to add along with the capstan drive crankshafts that will not work if you want the drive system to function.
 

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I have found things differently. The crank on my c21zpn and on my former s21zpr did not have holes drilled for the self propel pulley so those are another one to add along with the capstan drive crankshafts that will not work if you want the drive system to function.
Yes you are absolutely right! I was thinking that hole for the self-propel was present on all those cranks, but just checked a couple of my original pushers this morning and the hole was not there. I do know that you can use the self-propel crank on a pusher though, so it's a one-way interchange. I did just that on my 10420 a few years ago. Thanks for the correction Ben.

During my hunt this morning for some parts for a fellow member I ran across another F series crank that I had forgotten about. It's a blade brake version I suppose, If anyone needs one let me know. I think it's NOS as well. It was on a NOS short block that I used for another pusher and swapped out the crank to make it work for the pusher.
 

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Where can I find crankcase housings for my F engine?
Is my post in the correct forum section?
I assume you mean the halves of the engine. The cylinder and reed halves are a machined fit. Meaning you can' take the cylinder from one engine and use the reed plate off another. Personally, I'd look for another F engine machine on craigslist and rob the engine off that.
 

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I am wondering why you would need to tweak a LB engine? They are plently effecient for cutting the grass just as they are. Did you realize that the DF engine uses a tuned exhaust and a rear boost port?
I'm with you. If it is just cutting grass...well a properly jetted lawnboy or duraforce does it better that about anything else out there.
No reason to waste the time, effort, or even bother as they don't need the extra 5% hp or heat dissipation.
I would rather focus or obsess on one of the many other things I do that on...LOL

Now if you are using the engine for another purpose like so many Briggs were on go carts and mini bikes, then TWEAK TWEAK TWEAK away.
I was also, always the tuner who had his stuff running faster then the other peoples. Started with bicycles with sprocket changes and even rear wheels, then Puch mopeds with milling out exhaust port and opening up muffler, then motorcycles, then cars from 73 Buck Century with 350 2 barrel in it originally, to 67 and 68 GTOs with 400 Pontiacs.
I learned that if you opened the gap on the points to about twice as wide..It ran a lot stronger...until they burned up in about 10 days. But Standard Blue Streak lasted longer.
Learned how to tweak a Quadrajet and Carter ARF carbs. The air vane on GM is almost always to tight and jetting can be tweaked a lot after other mods.

Mowing grass is not something I do anyway. I fix mowers and test them for only a few minutes. As long as they run fine, I am good with that.

It never hurts to more fully understand how things work but there are times that "in theory" and in actual laboratory facts...things might be "better" but no real noticeable or worthwhile improvement in actual real world use.

Kind of like the old Splitfire spark plugs...."Up to 3.78% increased fuel mileage" if they did get full 3.78...that's an increase...sure is, but who really cares about that little bit. Were talking about 1990 when the mileage was probably 13-15 around town if lucky! I'm not going to bother to change them or pay 4.99 for them (autolite back then was .79-1.59) for the less than .5 mile per gallon....Not even if I can rationalize the savings if I keep them in for a little over a year and break even.
Cause in the real world...3.78% ain't gonna happen and if it does it will not maintain that level for long.

I actually use two in a KX750 Twin. They ran stronger for a bout 3 days and then about the same. That bike was cold blooded and popped and spit or about 8 minutes when cold. Put Bosch Platinum in and it started and idles smooth right at cold start. Not as powerful as with other or standard champions autolite etc but so much smoother on start up.

SO run run run that little lawnboy monster.
I stopped working on those around 2006 or 2007. I liked the duraforce last ones best. Hated the earlier ones with coils under flywheel IIRC and muffler under deck that clogged and needed exhaust port de carboned when they lost power. Never liked the rotating clicky throttle gov adj either. I guess I am nit wired for lawnboys because so many people love them.
I like the power and sound, but hate working on them.
 

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Thank you

What is the least expensive way to ship an F series Lawn Boy engine with Flywheel.?

what size box and weight?
Thank you.
 

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Thank you

What is the least expensive way to ship an F series Lawn Boy engine with Flywheel.?

what size box and weight?

It feels like a long block f series weighs around 20 lbs or so. I dont know if the post office has a flat rate box deep enough for it to fit.

I would personally build a 2x4 and plywood crate just big enough to fit the engine if I were shipping one though. It's a little more time and effort but itll protect things a lot better.
 

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USPS will give you their shipping limits and sizes. Seems I remember 50 pounds but do not have the size limits. I have had better luck using flat rate and priority than other cheaper methods. I could give the prices I have now but just go and get the correct ones from where you will be shipping. It cost me $50 to ship to me 2 25 pound Sears garden tractor from New England to Wisconsin in a flat rate box. Roger
 

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About the thrust bearing and washers, I can't find a picture of them installed. Where exactly do they go? do they sit on top of the lower crank bearing? What prevents the washers from spinning?
 

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John, welcome to the forum. All my Lawn Boys have lapped reed plates, I've been doing this for the past 6 years. I made small lapping plates from cast steel for both D and F models. I consider this a standard procedure when I build a new engine or rebuild an old one. I does make a difference with better crankcase sealing and cutting down on fuel fog. (reverse flow).
I have been considering getting a piston coated by Swain Technology for a test engine. As you know the piston is the critical part in a two stroke and anything that can shield it from heat is a big plus.
Most of my work with Lawn Boys has been to identify wear points and excessive manufacturing clearances, then make corrections. I think the Lawn Boy engines are excellent, but can use some finishing touches.
About 4 years ago I started making new throttle shafts for all my D models. The original shaft fits too loose in the carburetor body and hence wears quickly and gets even looser, allowing air to enter.


Could you post pictures of lapping procedure?
 

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Here are the pictures that I had previously posted. The first two pictures show a new F crankcase with the reed removed. Notice the machining tool marks left in the surface during manufacture. All F crankcases and D reed plates have marks like this to some degree. Lapping can remove these marks and make the surface smooth and flat giving the best possible seal when the reed closes. Because of space limitations I use a 1 inch diameter round lap for F crankcases, there is just enough room to move it around with the fingers during the lapping procedure. For D reed plates a rectangular lap works well. When lapping the lap must be held flat against the surface and moved in a circular figure eight motion as large as space permits. Notice there are still some slight tool marks around the screw holes, this is not a problem since no sealing takes place there.
 

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