What the f..........is going on here?

[traffic903]

Some of the terminology may play a part in the confusion, different names/terms used for the same part, ie. check valve, check nozzle, governor valve have all been used to describe the same part, maybe has to do with what is commonly used in a local area. Problems like this discussed out in the open helps many members that have less experience gain some more even if it becomes boring for those that already have been there and done that.

Well said!

 

[Wonkydonkey]

I couldnt agree more, that’s why I like it here, even when you get a derail, you still learn sumting, and just some times it makes you think outside of the box, so to speak

And on a last note, I really hate chucking things in the bin, just cos you can buy a new one, if we all did that, we wouldn’t learn anything.

Cheers

 

[Basher]

We all had to learn this stuff somehow, sometimes through trial and error, sometimes discussion and asking for help, doubt any of us was born with a headfull of mechanical knowledge. I grew up in a remote rural area where the only saw dealer was 12 miles away and there was no two stroke repair shops at all, a couple gas service stations had a mechanic at each but they knew nothing really about two stroke equipment, maybe replace a sparkplug and turn the screws on the carb, unfortunately not always correctly.
Chainsaws were piling up around me if anything went wrong mechanically and at the age of 9 I started pulling them apart with my one and only slotted screwdriver and a pair of Peterson Vice Grips. More tools were acquired as more saws got fixed and more showed up to be fixed. First few years were trial and error repairs, think I learned more those first few years about how two strokes worked than in the years since as they are still the same basic design with many improvements. Within a year of starting to work on saws I got my first hand me down one, a Pioneer 600 , best I can tell that was 1962.

 

[Chainsaw Jim]

The check valve isn't the problem.

 

[Wonkydonkey]

The check valve isn't the problem.

Just asking for learning purposes,,,,what is it then,

Cheers

 

[Chainsaw Jim]

Just asking for learning purposes,,,,what is it then,

Cheers

Lack of quality with the muffler bearing.

 

[PogoInTheWoods]

My money is on something in the metering side, or the pump diaphragm just caves at a point corresponding to around 13k, ( only when hot, of course). All of those components could have been swapped out several times by now to either isolate or eliminate possible sources of the problem.

Since fixing the problem isn't the objective here, but rather simply identifying it, the only way to a conclusion is still by the process of elimination. It's either going to be the carb body itself, or with the replaceable components attached to it. OP needs to approach it more like cutting down a difficult tree..., just keep trimming and cutting away everything that doesn't look like a stump..., or in this case, a cheap aftermarket carb.

 

[traffic903]

Lack of quality with the muffler bearing.

Possible! I thought it was more likely a warped bell housing.

 

[traffic903]

All of those components could have been swapped out several times by now

They have been swapped out several times, but just not with oem stuff (remember what I said about 2 week wait minimum AND insane pricing). I will order an oem kit today (if the price is reasonable) and it will hopefully arrive by mid to late April. If my oem used carbs arrive before that then i will use one of them to start the swapping process sooner. I'm not messing with my good 660 because I need it to cut the bigger stuff I am attacking today and next week.

I know 24 hours in computer land is a long time and so it can SEEM like your suggestions aren't being followed through. But in reality life is always getting in the way of the things I could be be doing. And I'm in no hurry. I'm not after the quickest fix, I'm after ideas to try, so that when I look at this next I have some other brains helping.

 

[davidwyby]

No saw pro here, but I like a good mystery problem solve. I’d have good and bad carbs apart next to each other comparing.

 

[CR888]

Just food for thought I'd also be checking the function of the choke making sure it firmly stays open under operation. This includes linkage, master control, shaft & butterfly. A choke has the ability to flood a saw under operation and vibration at high rpm can make them do funky things if not secured well. Just somthing to consider.

 

[Stihlms]

Or he can send me the saw, I'll throw a 396 bbc under the hood, send it back to him and he'll be smiling from ear-to-ear. Lol

 

[Terry Syd]

This is a curly one. The check valve in the nozzle is to keep air from being sucked back through the nozzle during idle. If you had problems at idle, then a bad check valve could be the problem. Since it idles fine, then I would rule out the check valve.

If the tank was becoming pressurized by getting warm, that could produce more fuel pressure on the metering valve. However, since this is happening at high RPM where the metering valve is already off the metering orifice, then an issue with the metering valve shouldn't be affecting the mixture - unless it was somehow getting stuck OPEN.

With pressure in the tank, the wetside would be filling with fuel and the diaphragm SHOULD be closing off the flow. Perhaps the diaphragm is getting hung up, or perhaps the metering lever is getting hung up, or the metering valve is slipping so far out of the orifice that it gets hung up.

Basically, the diaphragm should be moving to close off the metering valve when there is excess fuel flow going into the wetside - that doesn't seem to be happening.

 

[traffic903]

I think I have found something.......

That line coming from the nozzle orifice is either a gouge or a crack. It is also visible inside the nozzle orifice and runs vertically. Could it be possible that fuel is being pulled through this crack/along this gouge at high rpm when pressure is at its lowest??

 

[Terry Syd]

Possibly. You might see if you can smear some GB Weld into the crack, clean it up, let it sit and then put the nozzle back in. If it fixes it, you found the problem.

The crack could be acting like a second 'unregulated' high speed jet.

 

[traffic903]

Well I did some more forensic work on this carb today. And I also looked over the other one that does the same thing. Internally they are very different. The one from Huztl looks exactly like the genuine Walbro internally, except for a couple of differences. Those being a 0.54mm main jet (instead of the OEM 0.64mm) and a different high speed screw. There is definitely a gouge or even a crack in the main nozzle orifice which goes into the high speed screw seat. I can't imagine how it got there, but it was obviously there from manufacture. I tried to "seal" it up today, but failed. I'm writing this carb off!

The second carb, which has "Walbro" cast into it, is very different internally to the others. It does not have a replaceable jet. It has a sizeable hole in it which leads directly to the nozzle, via the high speed screw. I couldn't figure out at first why it was having basically the same problem as the other until I pulled out the metering spring. I should note here that this was the first time I had pulled this carb apart, as I had tossed it aside initially and focused on the original one. I could see the spring was different to the Huztl carb and the OEM Walbro also (which I reluctantly pulled apart as well). So I came up with a super basic (and probably flawed) test.

I got my digital scales out. I put the Huztl spring on the scales, standing upright, and then pushed down with my finger to the point that the spring was just fully compressed. The scales read 50 grams of weight. I then tested the OEM spring - 52 grams. Then I tested the China Walbro and the spring read 23 grams! It also didn't look "right". The other springs are a heavier gauge shiny steel, but this one was a dull, galvanised colour and much flimsier.

I swapped the Huztl spring into the China Walbro and installed it on the saw. Fired up, ran perfect. It has a tendency to go rich with a quick throttle blip, but otherwise runs good. I then took it off, because I wanted to mod my "other" Farmertec carb which has been running well on the saw, but at 2 turns out on the high screw. I pulled it apart and it had the same 0.54mm high speed jet. So I drilled it to 0.65mm and put it back together. Back on the saw and it runs really nice. H screw is now at 1.5 turns out.

So I think I have sorted this out. 2 different carbs with an almost identical problem, but from 2 totally different causes!

 

[davidwyby]

Well done.

 

[a. palmer jr.]

I was gonna say to try a stronger metering lever spring but that's strictly a guess..