Rebuild K760 Carb or not

[JOEPRO]

I bought a good used K760 petrol 2 stroke saw a few years back.

First year or so it was 100% ran like a champ lots of power.

Then it began to cut out during use. It would start and rev up fine but randomly cut out when running. This got more and more frequent until I stripped it down and ordered a carb rebuild kit.

The inevitable 2 year interlude later and I tool the carb off tonight. To discover that it is a new genuine Tama carb



Is there any point in stripping this down? It is a few years old so maybe something is wrong with it.

I did also buy a full service kit, air & fuel filter and spark plug.

 

[JOEPRO]

I think I might have an idea now on one possible cause elsewhere given the carb is mew genuine.

This saw would have had the fuel sensor to check for yellow 2 stroke oil on the system. (You have to use genuine H oil which I have)

But the wires have already been cut off the sensor.



I think it's best to remove the sensor / valve now on the fuel line and install one piece new fuel hose from the carb to tank filter.

I'll have to mail order this hose but I will report back for what it's worth.

 

[Migmac]

Strip it down and rebuild it. Easy enough to do

 

[frank horton]

plus if u loose the screws etc...a new one from china can be anything from £15-25.....
I get a few in that even the ultra sonic wont clean.....
just order new...tske sround 2-4 weeks even here.......

 

[JOEPRO]

Strip it down and rebuild it. Easy enough to do

plus if u loose the screws etc...a new one from china can be anything from £15-25.....
I get a few in that even the ultra sonic wont clean.....
just order new...tske sround 2-4 weeks even here.......

I had the gasket repair kit on hand anyway so I stripped the carb down there and its in my ultrasonic cleaner now for a good hour or so. (It's clean anyway but incase there is gunk somewhere in a bore)

I'll also order new fuel hose and remove that fuel sensor which isn't needed anyway. Other saws I have seen on you tube just have a hose there instead.

All other rubber intake parts are good, no cracking. So I can't really see what else would be wrong with it. I'll get it back together next week and if its still not running right I'll take it to a repair place and let them take care of it.

 

[armalites]

When I got my ultrasonic cleaner I bought a 20L which seemed like a good idea at the time. I soon realised it was a lot of water/chemical that was going to get manky pretty quick.

I now just fill it with water and put stuff like chainsaw carb bodies in a jam jar of petrol and put that in the tank. Works really well and far less mess.

I've had chainsaw carbs that I've had to run through the cleaner multiple times before they would run 100% right, it's frustrating but rewarding when it all works out.

 

[JOEPRO]

I did rebuild this carb. It was easy to do. I also got new fuel hoses and removed the oil guard sensor. There is also a new decompression valve on it as the old one wasn't engaging correctly.

It's back together now but I realised the ignition coil is also unique to the oil guard system. So I ordered a new non oil guard coil for this saw which will be here next week.

It's had a full service kit installed too.



Fingers crossed with the new Coil and fresh fuel it will run again without cutting out like it did before. I do wonder was this oil guard related which is why its good to have that eliminated now as I'll always know to use 2 stroke fuel in the saw so its useless really and just something else to give trouble.

(As a reminder the oil guard system is looking for a special yellow dye in the oil to prevent someone running it on raw fuel or another colour of oil)

If it still messes around then next step is a complete new carburettor from ebay. The carb on the saw looked like recent new and was genuine Zama which is the main reason I wanted to use a repair kit on it.

 

[JOEPRO]

FWIW

I got this saw back together today. The complete cutting out issue it had before is resolved.

I have "deleted" the oil guard system now on this saw and I suspect that may have been the previous issue.

Whilst it is running it is revving up and down a little on its own so there is something up with the carb. So I ordered a new aftermarket one within UK for £21. Will fit that when it turns up and hopefully that is it all sorted.

The tip / button on the new decompression valve also immediately melted off! I'll order another one and hope its not also garbage.

 

[JOEPRO]

Update, I did order and fit a new carb on this saw. EBay cheapie carb. (I did not try tuning it if that matters)

Edited post:

Not starting at all with the new China carb.

Sod it, I just ordered a new Genuine Husky / Zama carb for it on ebay. Guy is selling them for £55 Inc postage.

I will install that when it arrives and fill it with a full tank of fresh fuel. If that doesn't do the trick then all it can possibly be at that stage is a tune up required but at least I'll know its all good otherwise and can bring it somewhere for a tune up.

 

[Red'n'Black]

Crank seals would be my thought, if everything else has been done.

I have a Husqvarna 141 with erratic idle, when I get a minute I'll see about replacing them. It's 20 years old and they're probably still the originals.

On 2 stroke bikes you could often get the old seals out with a wood screw and a pair of pliers without disturbing anything else - are chainsaw engines the same, or do you have to split the cases?

 

[Migmac]

Crank seals would be my thought, if everything else has been done.

I have a Husqvarna 141 with erratic idle, when I get a minute I'll see about replacing them. It's 20 years old and they're probably still the originals.

On 2 stroke bikes you could often get the old seals out with a wood screw and a pair of pliers without disturbing anything else - are chainsaw engines the same, or do you have to split the cases?

On the Husky saws, a screw and pliers works too

 

[JOEPRO]

OK so mixed results! The new genuine carb arrived today and the engine now starts up and runs.

But it runs poorly. It does not want to maintain an idle. If it is tuned up high idle it keeps running with blade just turning. But as soon as carb low speed lowered it dies and will not idle. Also seems like alot of air is moving here:



Looking into the bore with a camera it looks mostly very good some very light marks here and there.

Compression test however reveals 100psi. Which is less than the min spec of 120psi. If I add oil into the cylinder the compression goes up to 125psi. So it must have bad rings or bore. Bugger.

I guess I'm into an engine rebuild now using whatever aftermarket cylinder, piston & seal kit that gets the best reviews. Any suggestions please?

 

[JOEPRO]

I went ahead and tore it down

Looks like it's had a China cylinder already

 

[JOEPRO]

Re the air leak, the exhaust gasket was leaking:



I bought one of these kits which includes all I need and is an exact match to the cylinder I pulled off



The China cylinder on it did me several years with my use level and who knows before I bought it as I bought it good used many years ago.

Whilst genuine would of course be ideal at £175 odd I could buy another good used saw. Will report back how I get on when it's up and running again, fingers crossed this is it this time.

 

[Migmac]

A bit late but I’ve used rock wood aftermarket pistons and cylinders. About the same price as your kit, seem
to do ok on my saws.

 

[Nomad]

I bought a chinese kit similar to your last year and it got non running commercial strimmer working very nicley for about £40

A shame it coughed and died last week so I need to look at it, will report with finding if I remember

Hope you get it fixed, I also have a dead "Merry Tiller" rotarvator high on my to-do list, will look at the website you used and see if they carry spares.

 

[JOEPRO]

A bit late but I’ve used rock wood aftermarket pistons and cylinders. About the same price as your kit, seem
to do ok on my saws.

Thanks for that, I have ordered a Rock wood one too. I note the listing says it is a Nikasil coated.

When both kits turn up I will insect them and install whichever appears to have the best internal cylinder finish and return the surplus one.

 

[JOEPRO]

I stand corrected also on the cylinder and piston I removed from my saw. On closer inspection after cleaning Mahle markings are present. To me this would indicate it must have been the original cylinder.

The cylinder walls are lightly scored. Enough to have compromised the Nikasil coating.

Hopefully a half decent aftermarket cylinder will give some useful life. Or it will melt down after a couple of days! I guess I'll find out soon enough.

 

[JOEPRO]

The first Cylinder kit arrived today. On inspection it looked good quality to me so I went ahead and installed it.

This is the most bizarre fault next- I guess I'll take it apart, again and rewire the kill switch

 

[JOEPRO]

This is the most bizarre thing. I fitted a new ignition coil wire and tested the kill switch. Still doing the same thing. Have to put pressure on the top for it to run.

Maybe intake rubber tubes is what I am thinking now. The ones on it are supple and I do not see any cracks when squeezing etc but clearly something is still wrong with it. So I'll order yet more parts and try for about the 8th time to fix this saw.

I did go ahead and buy another Stihl con saw at this stage so I can get some forking work done instead of working on the darn saw. As soon as the husky is running OK I will be selling it & keep the Stihl.

If anyone has any ideas on the current issue I am all ears.