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post #1 of 15 Old 07-19-2019, 06:00 PM Thread Starter
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C10a Loosing power

I have a 1975 C10a that I put a new plug and points on last year, and it ran great up until a week or so ago. Now it is loosing power super quickly. I'd love some help with why...I'm at best a middling gravely mechanic.

I have replaced the ignition coil, plug, and plug wire, and that hasn't fixed it.

When I pulled the old plug out, it was *super* fouled. This plug was probably 6-8 months old, and I had replaced it because the old one was super fouled. The beast pumps out a lot of smoke when it starts, but when gets up to full power, I can't see anything smoky.

When it gets bogged down (unduly quickly) there's definitely black smoke coming out of the exhaust.

Thanks much!

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #2 of 15 Old 07-19-2019, 11:04 PM
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Re: C10a Loosing power

Is the air filter (super) dirty, as in clogged?
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post #3 of 15 Old 07-20-2019, 02:09 PM
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Re: C10a Loosing power

Yeah sounds like you are running too rich. Like ultane said.
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post #4 of 15 Old 07-26-2019, 02:10 PM Thread Starter
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Re: C10a Loosing power

I removed the air filter for a bit, after trying to clean it, and it still bogs down when under load.

Its possible that if I adjust the carb to not run as rich I might get different results? This behavior only started a couple weeks ago. I replaced the carb last summer.

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #5 of 15 Old 07-26-2019, 02:14 PM Thread Starter
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Re: C10a Loosing power

When I open the throttle all the way, the engine does start doing an rev/die cycle, so the idle at high speed at least may be off? I don't know if that is useful info or not.

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #6 of 15 Old 07-27-2019, 04:20 PM
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Re: C10a Loosing power

I am not a mechanic, and I did not stay at a Holiday Inn Express last week, but the rev/die cycle makes me wonder how if the governor is some how messed up.
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post #7 of 15 Old 07-28-2019, 11:17 AM Thread Starter
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Re: C10a Loosing power

Could be. But given the smoke I sometimes see on the tractor (idling yesterday I did see some whitish smoke coming of the exhaust) I think maybe I should try to replace to the head gasket. I think the carb probably needs to be adjusted, as the rev/die thing only happens at full throttle, and I suspect the frequency of plug fouling may further indicate as others have suggested that it is running to rich.

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #8 of 15 Old 07-28-2019, 12:03 PM
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C10a Loosing power

If the head gasket is blown, it seams that you should easily see evidence of hot exhaust burning the paint off whatever is near, or be able to feel the gassed leaking out, as I was able to do on my SOs C8. And Im not sure that a blown head gasket would cause it to run too rich. I wonder if the carb is out of adjustment? Surely someone more knowledgeable than I am can help you figure this out.
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post #9 of 15 Old 07-28-2019, 12:07 PM Thread Starter
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Re: C10a Loosing power

I definitely don't see that with the paint.

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #10 of 15 Old 09-04-2019, 08:30 PM Thread Starter
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Re: C10a Loosing power

I was able to spend some time with my c10a tonight.

Adjusting the carb helped enormously. No appreciable power drops when attacking a dense tangle of weeds!

Two questions remain:
1) The main jet screw bottomed out against its spring before any noticeable drop in engine speed happened. I think the tractor is still a bit too rich, though I'll observe it more in the daylight.

2) There is a third screw on the carb not referenced in the documents I found. It is on top of the carb, sitting under the arm where the governor connects to the carburetor. I've no idea what this screw does....air maybe?

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #11 of 15 Old 09-05-2019, 09:27 AM
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Re: C10a Loosing power

I would guess another case of bad gas!
The current formulation of what is joking called "automotive fuel" is deadly to our older small engines/carburetors. Here in Central New England it is almost impossible to find non-corn gas at any local filling station. I can get $9 a gallon race gas at a station 15 miles away but in the Gravelys that would get expensive.

Here's what I do to minimize the bad gas problems:
1) always run the carb dry when you shut off the machine for more than a day.
2) mix the maximum amount of Stabile in the 5 gallon can before you fuel the tractor
3) as an added precaution I add either 2-3 oz. of 2 cycle oil or 4-6 oz. of Marvel Mystery oil to
each 5 gallon can so if there is any of the corn gas left in the carb or fuel lines it will at least
not dry out to that hard dirt like stuff.
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post #12 of 15 Old 09-05-2019, 01:33 PM Thread Starter
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Re: C10a Loosing power

This particular carb has never seen ethanol fuel...I have 3 gas stations in a 15 mile radius I can get non-ethanol fuel from, and that's what I use in all my small engines.

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
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post #13 of 15 Old 09-05-2019, 03:21 PM
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Re: C10a Loosing power

I'm envious; we can't get that stuff up here.
I would still check out the carb to make sure there is no junk in the passages. Some of these are really pretty small and it doesn't take much to upset the mixture.

Oh, the top screw you mention; it may be the idle speed limit screw. Just a guess.
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post #14 of 15 Old 09-05-2019, 05:51 PM Thread Starter
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Re: C10a Loosing power

I moved an hour north about 6 years ago, and I went from not being able to find it to three stations near by. I felt very lucky!

I found the screw descriptions!! (I googled the term you suggested.)

I was way off on the purposes of each screw. So now I need to go adjust again :-D

The three screws are:
* Idle speed adjuster
* Idle air/fuel adjuster
* High speed main air/fuel adjuster

i had adjusted the idle speed and idle air/fuel adjust, but *not* the high speed. So...yeah. Gotta go fix that.

Another opportunity for learning

???? Gravely L
1965 Gravely L8
1968 Gravely CI
1975 Gravely C10-A
2015 Mahindra 3616
cegeddin is offline  
post #15 of 15 Old 09-12-2019, 12:01 PM
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Re: C10a Loosing power

You might consider cleaning the top end with head off. Then check valve lash. If you do this follow torque instructions given in manuals.
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