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9020

SUNSTAR 20 losing revs/power

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9020

Greetings on a reverent Memorial Day weekend. Got out to mow today and after about 20 minutes the engine lost revs and power. No smoke at all, black or blue. Did seem a bit hot as the oil dipstick handle was definitely dicey to touch. Oil is good, no mice, all the cooling fins clear. This I know because just yesterday I had all the shrouds off and cleaned out EVERYTHING that was caked and dirty. It did do this one time before about 2 weeks ago. Any suggestions or tips would be greatly appreciated!

Thanks!

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Richard Wright

You didn't say whether this is an earlier Sunstar with a Magnum or a later one with the Command engine, but I'd be suspecting a fuel issue, possibly a clogged filter. Pretty common after being put up all winter. 

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9020
58 minutes ago, CarlH said:

Might also check the hydro oil cooler fins.

Cleaned those too, the whole engine is quite clean, even the shrouds are degreased. I'm wondering if it is something electrical once it heats up.

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SimpleOrange

Start easy when cold hard when hot check the valve clearances as the engine warms up tolerances get tighter and the valves do not fully close. Results in poor engine performance.

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SimpleOrange

An easy way to check for a leaky exhaust valve, a piece of paper with the thickness of card stock folded to create a hinge. Works best on larger tractors and automotive that have a tailpipe.

Place the paper against the pipe with the hinged part on the bottom of the pipe, while the engine is running a bad leaky valve will cause the paper to make a flapping sound as the paper is drawn against the end of the pipe while a good seating valve constantly pushes the paper away as the exhaust gas's escape.

A minor leaky valve gives intermittent flaps while a bad one goes flap, flap, flap, flap, flap, flap.

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9020
7 hours ago, SimpleOrange said:

An easy way to check for a leaky exhaust valve, a piece of paper with the thickness of card stock folded to create a hinge. Works best on larger tractors and automotive that have a tailpipe.

Place the paper against the pipe with the hinged part on the bottom of the pipe, while the engine is running a bad leaky valve will cause the paper to make a flapping sound as the paper is drawn against the end of the pipe while a good seating valve constantly pushes the paper away as the exhaust gas's escape.

A minor leaky valve gives intermittent flaps while a bad one goes flap, flap, flap, flap, flap, flap.

I'll give that a whirl and see what it tells me. Thanks!

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9020
8 hours ago, fishnwiz said:

Doesn't sound like electrical problem to me unless you have a hard start issue when hot.

It'll start again, but just not easily and won't rev.

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SimpleOrange
1 hour ago, 9020 said:

I'll give that a whirl and see what it tells me. Thanks!

If you feel foolish playing with paper while a neighbour looks over your shoulder and would like to look more professional, use a vacuum gauge.

Leaky exhaust valves shows up as a tic on the gauge needle.

 

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9020

Amidst many a frosty beverage yesterday I discovered that my compression is low, 90 psi both sides, and that did not change with 15w40 squirted in so I'm thinking my rings are decent. I tried the card deal on the exhaust and not sure what I was seeing. Will try again today. My old vac gauge is dysfunctional so I'm gonna have to get a new one before going further...

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9020
On 5/26/2018 at 11:14 PM, SimpleOrange said:

An easy way to check for a leaky exhaust valve, a piece of paper with the thickness of card stock folded to create a hinge. Works best on larger tractors and automotive that have a tailpipe

A minor leaky valve gives intermittent flaps while a bad one goes flap, flap, flap, flap, flap, flap.

Just tried it again and now I think I see what you're saying. It was flappin' like, well, it was flappin'!sm06

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9020

Wait a minute, now I'm learning about this "compression release mechanism"? I know all my Olds 455's don't have that, but they do have "positive valve rotators"!dOd

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SimpleOrange
1 hour ago, 9020 said:

Just tried it again and now I think I see what you're saying. It was flappin' like, well, it was flappin'!sm06

Awe, I think your just funning me.

Just after the intake valve open with the piston on its sudden downward stroke a massive vacuum is created inside the cylinder.

Atomised fuel from the carburettor fills the void, but with a leaky exhaust valve surrounding atmospheric pressure also tries to enter the cylinder.

With a leaky exhaust valve your engine is being starved of fuel, adjust the valve lash or regrind the valve face to fix the vacuum leak.

Pay attention to the animation figuring in a leaky exhaust valve.

 

 

 

Edited by SimpleOrange

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9020

What do you think of the compression readings (90 both sides, didn't change with adding 15w40 oil) and are they that low because of that "compression release mechanism" that I just learned of? I did the compression test simply turning the engine over with the starter.

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MikeES

M20 does not have an internal compression release.  90psi is Ok for the Magnum, not the best but at the bottom end of OK. 

When you do a compression check, have the engine warm and the throttle wide open and the choke open.  Just run the starter until the gauge stops moving up.   Restricted exhaust can affect compression test and it can  be a culprit of power  loss.

Open it up and you will probably find lots of carbon build up.

Edited by MikeES
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9020
2 hours ago, MikeES said:

M20 does not have an internal compression release.  90psi is Ok for the Magnum, not the best but at the bottom end of OK. 

When you do a compression check, have the engine warm and the throttle wide open and the choke open.  Just run the starter until the gauge stops moving up.   Restricted exhaust can affect compression test and it can  be a culprit of power  loss.

Open it up and you will probably find lots of carbon build up.

Thanks for the info, I got a feeling I'm gonna pop the heads off next week and check it out. Before I do I'm gonna test it one more time just to be sure.

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9020

Got the nuts and bolts soaking with PB Blaster, putting it on twice a day until I get to it and hopefully make it easier. Last thing I want to do is break one!wah

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