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Discussion Starter · #1 ·
I recall reading the advice that when servicing an older Onan engine, one should split open the intake manifold and reseal it. Mine is definitely two pieces, but the halves are pinned together and the pins have been peened over like a rivet. I would have to drill out the pins and then use some small fasteners or new rivets to put this back together. Now, I have no way of knowing if my intake is even leaking to begin with, so I don't even know if this is a worthwhile endeavor. Advice?

I've got the intake off because I wanted to check things over before the snow flies, as I observed a marked increase in my gas consumption when mowing this summer.

Paul
 

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Discussion Starter · #3 ·
mikebramel said:
If its good dont mess with it.

You can do an absolute test with some propane
That was my thought too. I'll go looking for other possible problems. Any thoughts on my initial problem: poor gas consumption? I've had this tractor for 7 years and while it was never easy on gas, this year it was worse than "normal".

Paul
 

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Assuming you have electronic ignition. (?)
I`d check valve clearance while the intake is removed.
Might as well pull a cyl head and inspect for carbon deposits.

Of course, you might be hogging fuel when mowing due to a fault with the deck. Bearings ? Bad idler pulley ? Belts ?
Does everything spin nice and smooooth ? :wave:
 

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Look at the two previous posts. A leaky intake manifold causes a vacuum leak. It will turn the engine lean, meaning it will use less gas. Less gas because some of the engine vacuum will be lost sucked through the cracked intake manifold rather than the carburetor and fuel circuits
 

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Discussion Starter · #7 ·
99flhr said:
Assuming you have electronic ignition. (?)
I`d check valve clearance while the intake is removed.
Might as well pull a cyl head and inspect for carbon deposits.

Of course, you might be hogging fuel when mowing due to a fault with the deck. Bearings ? Bad idler pulley ? Belts ?
Does everything spin nice and smooooth ? :wave:
Yep. Electronic ignition, P218.

I'll take a look under the heads. I had a loose intake valve seat on one side that I had repaired last year. If I yank off the heads, am I going to have to replace the head gaskets?

The mower deck is ok.

WVshooter said:
Are you smelling much gas in the exhaust or is it smoking at all? I'd look for junk in the carb or a mixture screw not set right.
I cleaned the carb this summer and its still really clean looking. I do smell gas in the exhaust. I also noticed that my oil consumption has gone up a little, at least I feel like it has.
 

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i took my onan manifold off, and split it. some of the pins broke, some pulled out. iam planning on using metric # 3 screws for my R/C jato to put the onan manifold back together,
. have not done it yet. but iam sure i can do it. and i will only use permatex #2 to seal the manifold.
 

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Hylomar is the best I've found for a sealer; If needing to use something instead of a gasket the permatex black when used correctly is good.

I use latex gloves and spread it thinly with my finger. do not assemble wet, let it skin over slightly, loosely assemble, let it cure, then tighten.

I don't think the product is bad, but a lot of techniques certainly are!
 

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In cases such as this, I think that you have to have a plan.

To me, that plan consists of making sure the engine is properly serviced.

- remove all the tins to look for any air blockage that might cause higher operating temps

- remove the heads, decarbonize the combustion chambers, clean the underside of the valves, inspect the valves and seats, set the valve lash and reinstall the heads. Torque according to the manual.

- remove the carb, check for junk in the float bowl, make sure the float level is set properly, make sure the needle and seat are working properly, make sure the float has no fuel inside of it, blow out all passages with compressed air, make sure there are no air leaks between the block and the manifold and the manifold and the carb

- clean and regap the spark plugs. If they look dicey, replace them. Same with the ignition wires, a commonly overlooked item.

- conduct a proper leak down test and also a compression test.


You want to KNOW whether the basic engine is sound or not. The compression should be fairly even between the two cylinders and definitely higher than 100 PSI. Sure, the engine still has life at 90 PSI but let's be serious here. There's some wear issues that have brought the compression down to that point and that means the HP is definitely much less than 16 or 18 and that causes the engine to work harder and use more fuel.

The leak down test will tell you whether air is getting past the valves or the rings or both. The results will guide you as to a course of action.
 
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