Mantis cultivator no power under load

[fespo]

I bought a Mantis cultivator at a yard sale for $50.00 with extra attachments, thats like new. I drained the old gas and it started right up. But under a load it has no power so I took the carb off and cleaned it out. It was not bad but did smell like old gas. i tried again and same thing, so I took everything apart to get the clutch and it like new. I was going to order a carb rebuild kit but a aftermarket was only $9.00 shipped to my door. I tried it but the same thing. I also removed the muffler and the spark screen, The screen was like new, no carbon at all, the piston is clean and shinny, no scoring at all.... Im lost on this one Help Fespo

 

[jharkin]

Does it have the two stroke or the honda four stroke engine?

Most obvious thing to check is if the carb is tuned properly. An overly rich or overly lean high speed needle will both cause loss of power at WOT. See if you can find out what the factory starting settings for both low and hi sped needles and retune from there.

 

[fespo]

Does it have the two stroke or the honda four stroke engine?

Most obvious thing to check is if the carb is tuned properly. An overly rich or overly lean high speed needle will both cause loss of power at WOT. See if you can find out what the factory starting settings for both low and hi sped needles and retune from there.

It's the 2 stroke

 

[jharkin]

Ok, being the two stroke then you don't have to worry about a valve issue.

I'd start with retuning the carb (let us know if you need some tips on that) and if that doesn't work I'd suspect the magneto timing is off...

 

[semipro]

I'd bet on a carb problem. Though the rebuild kits are cheap I've not had a lot of success rebuilding them though I've rebuilt many carbs. Check the price on a replacement carb. It may be relatively inexpensive and worth the investment.

 

[fespo]

Ok, being the two stroke then you don't have to worry about a valve issue.

I'd start with retuning the carb (let us know if you need some tips on that) and if that doesn't work I'd suspect the magneto timing is off...

I did install a brand new aftermarket carb. Run the same as with the old carb. Starts fine, run at low idle no problem, full throttle runs great UNTIL you set it on the dirt and try to use it.; Just bogs down

 

[semipro]

I did install a brand new aftermarket carb. Run the same as with the old carb. Starts fine, run at low idle no problem, full throttle runs great UNTIL you set it on the dirt and try to use it.; Just bogs down

Maybe a fuel supply to the carb problem or a vented gas cap clog then.

 

[fespo]

Maybe a fuel supply to the carb problem or a vented gas cap clog then.

I will check the fuel filter and cap. I was busy today splitting wood,

 

[jharkin]

I did install a brand new aftermarket carb. Run the same as with the old carb. Starts fine, run at low idle no problem, full throttle runs great UNTIL you set it on the dirt and try to use it.; Just bogs down

Yes, but when you installed the aftermarket carb, did you adjust the needles for max power? The carbs come from the factory typically with the needles set at a default setting that is VERY rich, especailly for generic carbs that may be used on a variety of engines in a variety of tools in a variety of climates.

The two stroke engine on your mantis should have a diaphragm carb like the ubiquitous Walbro - looks something like this. The two spring loaded screws you see at the bottom of the photo just below the air purge bulb are the mixture screws. they are probably labeled "L" and "H" for low speed and high speed jet, but if in doubt, the low speed is always the screw closest to the engine.



Adjustment goes something like this. Having a small engine tach is helpful but not mandatory. Adjustments are made in very small increments - 1/16 of a turn at a time. Turning the screws in (clockwise) adjust it leaner, turning them out adjusts it richer.

#1 - Set the needles to the factory default starting position. If the manual doesn't specify one a safe starting point is 1 and 1/2 turns open from fully closed.

#2 - Start the engine and let it idle a few minutes to warm up.

#3 If the engine is very rough at idle you can turn the low speed needle in a bit until it smooths out.

#4. Now do an initial set of the high speed. Hold the tool at WOT. Turn the hi needle in in small increments until RPM peaks (by ear or with a tach). Once you find the peak back it off just a hair like 1/16 turn so you just barely hear it richen up.

#5 Now go back to idle. Let it idle 15 seconds or so and then snap it to WOT. It should instantly rev up with no hesitation. If it bogs a little and then speeds up turn the low needle in a little and test again. Keep repeating this test until you can snap the throttle open and it advances to full with no hesitation. If you snap the throttle open and the engine stalls you have gone too lean, back the low needle open 1/8 turn and start again.

#6 Once the transition is good go back to WOT and repeat the high needle test.

Once its adjusted optimally if the idle speed is too fast you can slow it down a bit with the throttle stop screw... generally these small two strokes are set to idle around 3000-4000rpm and the clutches are adjusted to engage a bit above 6000.

That's all there is too it. There is some interaction between the jets (at WOT the engine runs off both hi and low jets, at idle it only runs off low) so you might have to go back and forth a little, but once you get the hang of it its easier than it sounds.

Once you have tuned it optimally like this (no load) if it bogs down when you start tilling and load the engine try richening up the high needle a little.


Another useful reference to have for diagnosing issues is the Walbro carb service manual:
https://www.lawnmowerpros.com/pdfs/walbro/walbroservicemanual.pdf

 

[fespo]

Thanks I will try it and let you know. I was playing with the jets a little but maybe just to much at once. Thanks again


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[sportbikerider78]

All good advice.
Did you take a look at the spark plug? Whats the color?

2 strokes love new spark plugs if they have been run rich. Quick test is to pull it and hit it with brake cleaner and some scotch brite.

 

[Doc C]

Step 1 adjust the carb following the instructions you were given above.

Step 2 verify the spark plug is the correct plug and the gap is fairly close. Also look for carbon running down the ceramic on the plug. Your looking for compression getting past the metal ring on the spark plug and will be pretty obvious on the ceramic. And I'm not talking about carbon tracking. That's a different issue. Sometimes a spark plug develops a leak at the base of the ceramic and at full power the compression is essentially escaping and you will see what looks like carbon build up where the ceramic meets the metal. Exact symptoms are very little power at top end. For some reason I see it alot on Husqvarna chain saws.

Step 3 If neither of these have fixed it then remove your carb and inspect for a Crack at the mounting point of the carb.

I am small engine mechanic and have seen these issues alot of times.

USUALLY it's a carb adjustment issues. OCCASIONALLY it's a compression leak or cracked causing vacuum leak.

 

[johneh]

Have you checked the compression ?
An engine may run fine with out a load on it
but add a load and it will crap out if the is not
enough compression Just my nickles worth

 

[fespo]

Thanks everyone, I have been so busy at work and other jobs that I do that I have no had time to try anything yet. But I will update once I try everything out. Thanks agsin

 

[fespo]

Two problems where found. The main one was the gas line had a VERY SMALL crack that I did not see when I remove the carb and replaced the first or second time. Once that was replace today I had to do was adjust the carb. Works like new now!! Once I had everything set right, you could smell that new engine smell. I don't this thing ever ran more them a few minutes. Thanks everyone for ALL your HELP. Frank

 

[Doc C]

Two problems where found. The main one was the gas line had a VERY SMALL crack that I did not see when I remove the carb and replaced the first or second time. Once that was replace today I had to do was adjust the carb. Works like new now!! Once I had everything set right, you could smell that new engine smell. I don't this thing ever ran more them a few minutes. Thanks everyone for ALL your HELP. Frank

Since this ethanol stuff came out i made it a habit to replace all hoses anytime I rebuild a carb. Even on own stuff. It's hard to find non ethanol some time.