Mantis Tiller

[Wburton]

Hiya,
Got a "good" Mantis that's driving me nuts!! I bought it new and it's always been 'good' but temperamental. Usually a slight carb tweak and off I go. I'm good on small engines (2&4 stroke) but I'm stumped.
This one is a 10 yr old 2 stroke. Starts on 2 pulls and runs great- for about 2 minutes then quits. Starts right back up with a pull. Throttles up and down fine. I rebuilt the carb. No help and no change. I swapped it with another Mantis Zama carb. No change. All new fuel lines, vent, plug, cleaned air filter, cleaned muffler. No change. Bought a whole new OEM carb. No change. Compression is great. Visual inspection thru the muffler port and the piston and rings look fine. I only burn non-eth 93octane, 50/1 mix fresh.
So today I heard the vent line might not be working right. Ran 2 hours of tests with a stop watch. I can crack the gas cap open and leave it on fast idle (via the throttle cable adjustment screw) and run a half tank of gas. But when I screw the cap tight it will 'stall' in about 4 minutes. I swapped the brand new vent line with 2 others I have on hand. All are spotless and clean. I get the same results.
I've got tractors, gas and diesel, 6 chainsaws, a '70s cast iron Ariens Rocket 7 tiller, Shindaiwa blowers, weadeaters, push mowers, 4 riding mowers and a big JD 0-turn. I maintain and do all the repairs on all of them!
But- this tiny little thing is killing me

 

[Bird]

Sounds like the same machine and age that I have. I recently had to replace the primer bulb. The old one was cracked and leaking air. My first thought on yours was a dirty, partially plugged, fuel filter in the tank. But now it does sound more like a gas cap that isn't venting as it should. Good luck with it.

 

[Wburton]

Yeah, new primer bulb along with the new carb, fuel lines and filter and vent, air filter, spark plug, muffler screen clean, proper fresh fuel/mix...
Mantis says my compression should be 120-140 psi. I've got around 102-105. They say it won't continue to run below 120. It's a possibility but I don't think so. The dude that checked it says anything above 95 is ok. Over 100 is good.
After 2 hrs of running it with the gas cap open and closed, it is evident it's not venting properly. Now I just have to figure out why.
Thanks!

 

[Sawyer Rob]

So vent the cap... You have a drill, don't you?? lol

SR

 

[Wburton]

Funny you would mention that. I actually have a cap off a scrap mantis and drilled a small hole in it just a bit ago. It worked but I'm getting gas out of it when it's bouncing around. I think I need to add some sort of "deflector" inside the cap.

 

[Toyboy]

Does yours have the little vent line filter that goes up under the engine shroud like mine? If so, that may be clogged.
A356000031 Fuel Tank Vent $6.22

eReplacementParts.com

 

[Wburton]

Does yours have the little vent line filter that goes up under the engine shroud like mine? If so, that may be clogged.

Yep. Brand new. Actually tried 3 new ones just to make sure one wasn't defective

 

[Toyboy]

Once you put a hole in gas cap your screwed. Gas will keep coming out. sounds like your missing something on the venting but I don't know what.

 

[Toyboy]

Here's a thought.
Go to your Husqvarna dealer and get a duckbill valve HOP 530026119. Measure the OD and then enlarge the hole in your gas cap just enough to firmly seat the valve by pulling through from the bottom. You want it tight enough to have a good interference fit. That will stop the gas from coming out ans give positive venting to gas tank.
I would use a touch of Permatex Aviation sealer on the OD of valve when installing it to make sure it stays in place.
Just make sure cap is degreased .

You shouldn't have to do this, the Mantis vent should be working properly, but this will save you gas cap plus giving you venting.

 

[Wburton]

Interesting idea.
I'm still bumfuzzeled as to why it just started this. I bought it new.
Oh well...

 

[Wburton]

Fortunately, I picked up a scorched mantis at a yard sale for 20 bucks. The owner's son ran it with straight gas. Use it for parts so I have a spare cap.

 

[Toyboy]

The only other thing that makes sense to me, is the vent hose is too long and/or submerged in the gas and not allowing it to breathe. Otherwise, I'm perplexed also.

 

[Wburton]

Nope. Checked it and confirmed the proper length with Mantis tech. Also matches the old one.
A question on the Husqvarna vent; Do you know if the vent keeps dirt from entering the tank? Is it capped similar to the standard tank vent? I haven't researched it yet.

 

[bcp]

Part of my diagnostic process would be to run fuel direct to the carb, using some other small engine fuel tank, or even a siphon from a gas can.

Bruce

 

[Toyboy]

Nope. Checked it and confirmed the proper length with Mantis tech. Also matches the old one.
A question on the Husqvarna vent; Do you know if the vent keeps dirt from entering the tank? Is it capped similar to the standard tank vent? I haven't researched it yet.

No, it would be open on the top. You could put a piece of foam like air cleaner or some other porous material over it.

Here's the valve
Husqvarna Craftsman Poulan Chainsaw Replacement Duckbill Check Valve # 5326119 | eBay

You can get one from your Husky dealer for less then $2 I can here anyway.

 

[Wburton]

Looks just like the inside piece of the stock vent. I can pop one out of an extra I have.

 

[Wburton]

Back again: It occurred to me- I have another problem that your thread might help me with... Have a Shindaiwa EB630 blower with an invisible crack in the bottom of the gas tank. Tried everything to patch it (vs a new one around 25 bucks). JBWeld worked for awhile. Do you think the Permatex Aviation would work? Cheaper than a new tank. Anytime i'm working with plastics i'm careful to make sure they're degreased. Can't get to the crack from the inside.....
And! still farting around with the Mantis. Runs like a scalded dog wide open for ten minutes now but-poof- quits. Just like before but longer.

 

[Bird]

The plastic fuel tank on my 1995 B7100 Kubota got a crack in the bottom of the tank when it was about 3 years old. The dealer did no have a new tank, and it was pretty expensive if I ordered one, so I patched it with JBWeld. It lasted just about a year before it started to leak again just when I had made a deal to trade it in on a new B2710.

 

[Toyboy]

Back again: It occurred to me- I have another problem that your thread might help me with... Have a Shindaiwa EB630 blower with an invisible crack in the bottom of the gas tank. Tried everything to patch it (vs a new one around 25 bucks). JBWeld worked for awhile. Do you think the Permatex Aviation would work? Cheaper than a new tank. Anytime i'm working with plastics i'm careful to make sure they're degreased. Can't get to the crack from the inside.....
And! still farting around with the Mantis. Runs like a scalded dog wide open for ten minutes now but-poof- quits. Just like before but longer.

I've done this to repair plastic outboard gas tanks, and maybe it'll work on yours if you want to give it a try.

After washing tank with warm soapy water I also spray some degreaser on area to be repaired. In my case I use brake cleaner.

Get/borrow a small soldering iron (like used for circuit boards) and when it's hot, trace the crack with it. On the red plastic tanks it will melt and re-fuse the plastic. Once you see the plastic starting to melt keep it moving. You can't stay on one spot for more then a second or two.

Once you think you've got it, put 5 to 10# of air to the tank and put a little soapy water on repair. If it bubbles, dry it and re-touch the spot. If you don't see bubbles your good to go.

I always try to trim a few shavings from the inside edge of filler, and will melt them into the crack as I go.

I've repaired several outboard tanks and put 20# of air in them, and they will hold it until I release the pressure.

Check YouTube, there's probably some examples there.

 

[sixdogs]

It's not the fuel, is it? Mine ran better with non-ethanol gas.