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post #1 of 15 Old 10-17-2019, 06:17 PM Thread Starter
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Small Leak Coming from Crankcase

Hello everyone,

I just got my 1974 8251 project up and running. After putting it all together I got it to run pretty good but after some time I noticed a small bit of fuel film running down from the upper crankcase gasket area. It's only one spot that's leaking/bubbling and there are no signs of lean running such as a hard start or surging. I was wondering if this is normal for a rebuilt engine. Gaskets are all new and mating surfaces where cleaned.

Thanks,
LB90/Seth

Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #2 of 15 Old 10-17-2019, 08:48 PM
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Re: Small Leak Coming from Crankcase

There is a seal on the top of the crankshaft I suspect is leaking. Roger
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post #3 of 15 Old 10-18-2019, 12:15 PM Thread Starter
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Re: Small Leak Coming from Crankcase

I suspected that too but the area on the top of the crankcase where the magneto plate attaches is where the bubbling and leaking are coming from. Seals should still be holding good since they are also new. I think it's just the gasket breaking in since the engine is running really good and starting easy both got and cold. Just never encountered this before.

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Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #4 of 15 Old 10-18-2019, 05:05 PM
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Re: Small Leak Coming from Crankcase

Well it doesnít sound like itís bad enough now to cause starting and running problems, but it may get that way, from my experience gasket leaks usually get worse over time, not better. The mag plate-to-crankcase gasket comes in two versions, a thin one and a thick one. I think if I were you I would install a new thick gasket. While you have it apart I would inspect the sealing surfaces and if you notice any imperfections (using a good straight edge across the top) you could probably have a machine shop to shave off .005 inch to true the surfaces up a bit. It shouldnít be bubbling any fuel/mix there at all. Any leak in a crankcase of a two stroke will effect performance to some degree. If you do hire a machine shop to true up the surfaces I would also give them the crank and the specs, so that they can get the proper clearances with the gasket installed and plate torqued.
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post #5 of 15 Old 10-18-2019, 05:31 PM Thread Starter
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Re: Small Leak Coming from Crankcase

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Originally Posted by Lawnboy77 View Post
Well it doesnít sound like itís bad enough now to cause starting and running problems, but it may get that way, from my experience gasket leaks usually get worse over time, not better. The mag plate-to-crankcase gasket comes in two versions, a thin one and a thick one. I think if I were you I would install a new thick gasket. While you have it apart I would inspect the sealing surfaces and if you notice any imperfections (using a good straight edge across the top) you could probably have a machine shop to shave off .005 inch to true the surfaces up a bit. It shouldnít be bubbling any fuel/mix there at all. Any leak in a crankcase of a two stroke will effect performance to some degree. If you do hire a machine shop to true up the surfaces I would also give them the crank and the specs, so that they can get the proper clearances with the gasket installed and plate torqued.
That's what I suspected. It's bubbling again and not improving. I got it out today and the mower was very hard starting. When I did get it started. It ran rough and smokey all over again. Which gasket do I get? P/N 602590 or 602763? They look the same.

Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #6 of 15 Old 10-18-2019, 05:36 PM
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Re: Small Leak Coming from Crankcase

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Originally Posted by LawnBoy90 View Post
That's what I suspected. It's bubbling again and not improving. I got it out today and the mower was very hard starting. When I did get it started. It ran rough and smokey all over again. Which gasket do I get? P/N 602590 or 602763? They look the same.
Iím not sure on the individual part numbers. I always have ordered the complete gasket set, which comes with both gaskets and I just use the thick one if I need the extra clearance. Iíll do some research, I may have one in my spares. I have a habit of not throwing anything away. Lol if I do find it itís yours.
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post #7 of 15 Old 10-18-2019, 06:00 PM Thread Starter
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Re: Small Leak Coming from Crankcase

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Originally Posted by Lawnboy77 View Post
Iím not sure on the individual part numbers. I always have ordered the complete gasket set, which comes with both gaskets and I just use the thick one if I need the extra clearance. Iíll do some research, I may have one in my spares. I have a habit of not throwing anything away. Lol if I do find it itís yours.
Thank you kindly in advance. I really appreciate it. Do you think adding a little gasket sealant would also help as well? I really want to make sure this engine is sealed good.

Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #8 of 15 Old 10-18-2019, 06:03 PM
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Re: Small Leak Coming from Crankcase

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Thank you kindly in advance. I really appreciate it. Do you think adding a little gasket sealant would also help as well? I really want to make sure this engine is sealed good.
Yeah I donít think it would hurt to use a thin smear of gasket sealant. It might be about all you really need to fix the issue.
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post #9 of 15 Old 10-18-2019, 06:38 PM Thread Starter
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Re: Small Leak Coming from Crankcase

Oh good. I was thinking of getting some to use for when the time comes for the repair. Haven't tried it before but I figured it wouldn't hurt to try it out

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Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #10 of 15 Old 10-19-2019, 12:48 AM
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Re: Small Leak Coming from Crankcase

I did a little digging and I do have 2 of those gaskets p/n 602763. You can pm me with your address and Iíll get one out to you pronto.
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Last edited by Lawnboy77; 10-19-2019 at 12:55 AM.
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post #11 of 15 Old 10-19-2019, 11:09 AM Thread Starter
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Re: Small Leak Coming from Crankcase

Quote:
Originally Posted by Lawnboy77 View Post
I did a little digging and I do have 2 of those gaskets p/n 602763. You can pm me with your address and Iíll get one out to you pronto.
Thanks for taking the time to find them! PM sent!

Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #12 of 15 Old 10-21-2019, 01:43 AM
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Re: Small Leak Coming from Crankcase

It is hard to find but there is a gasket sealant that is resistant to gasoline. I bought some from a local Caterpillar dealer years ago. I tend to put sealant on almost every gasket I install except for carbs.
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post #13 of 15 Old 10-29-2019, 08:28 PM Thread Starter
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Re: Small Leak Coming from Crankcase

Well an update. I got the gasket from Lawnboy77 today and installed it. A little Loctite gasket sealant and a new seal later I got it to run. Dialed it out around two and one quarter turns out and it seems to have all of its power.

One thing I learned is with these self propelled units they run a little richer at idle compared to the push models. You actually want them to with more of a burble when the drive is engaged since your putting the engine under a load.

Here is a short video of it going. It started pretty easy for 47įF weather.


Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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post #14 of 15 Old 10-30-2019, 05:45 PM
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Re: Small Leak Coming from Crankcase

Great job Seth! Runs really good. That shaft drive D-400 is in really great shape.
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post #15 of 15 Old 10-31-2019, 09:18 PM Thread Starter
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Re: Small Leak Coming from Crankcase

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Great job Seth! Runs really good. That shaft drive D-400 is in really great shape.
Thank you! It runs nicely enough I think. It's got a newer deck from a 1978 model since the old one was busted. This deck is in almost mint condition and it's made of aluminum which I believe is better. Used an upgraded drive system as well and it's pretty fast now. Now when the weather breaks I'll test it in some leaves see how it runs then

LB90/Seth

Mowers:
1970 - 5239 (D-408)
1973 - 3058 (D-433) (Rebuilt)
1974 - 8251 (D-480)
1975 - 5273 (D-601) (Rebuilt)
1982 - 7071 (F-101) (Custom)
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