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Yep, they're 1/8NPT thread.

I'm going to Tamworth tomorrow, was going to try and get something there.
All the gauges I have are quite a bit higher in scale than what I'm after too and I have a box full of gauges ranging from 2" diameter to 5" and up to several hundred psi but no low pressure ones.

Actually, i could pinch the boost gauge out of the Landy, it peaks at 15psi on the scale and the Landy is pushing 17psi ATM :laugh:

I'm just about to press the discharge nozzle out of a carby too
 
Check out an acetylene line pressure gauge. The max pressure for usage is around 150 kpa or 20+ psi. I think I've got an old one I might be able to use.

EDIT: Thanks for reminding me, I've got a boost gauge sitting in a box in the shed. I think it goes to 30 psi, but it might just be usable.
 
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Have a couple of acetylene gauges here, I'll go out to shed and have a look, cheers.

BTW, the fuel discharge nozzle pushes out easy peasy.

Now I can clean up this venturi and transitions.
 
tdi-rick said:
No, a couple of the Dolmar's from the early/mid eighties used a Tillo carby that when you 'choked' it, a needle valve opened that allowed more fuel through the LS circuit, there was no choke plate/butterfly.
Not sure why they died out, too complicated/more to go wrong for a saw ? although I've never heard bad about them.


Different animal to the modern primer bulb on the homeowner stuff which acts more like a purger.
Click to expand...

Here's a thread that I didn't realise had been updated with some injector piccies http://www.arboristsite.com/chainsaw/167997.htm

Not my piccies, I pinched them from that thread.

needle.jpg


needlehole.jpg


venturihole.jpg


Not sure if you twist or pull, but it opens that valve and lets fuel through to a 0.25mm/0.010" orifice instead of using a choke plate for starting.
 
Similar to an 'enrichening device' on a Mikuni.

EDIT: They probably went out of favour because they only provide a set amount of fuel during start up. The ones on Mikunis were OK for temperate climates, but inadequate for really cold mornings. I used to block off the air bleed for the system with a bit of epoxy so that they could really dump fuel. You then had to cycle them on and off as the engine warmed up.

Probably the same problem on saws. A fix would be to drill out the metering hole to dump more fuel and then cycle the actuator back and forth as the engine came up to temperature.
 
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Well, that was a surprise. The POP on this Zama carb is over 25 psi. I can't get it to pop off within the range of this gauge. I'm gonna have to find another gauge to test with.

Kinda funny, I kept expecting it to pop off around 10-12 psi like the karters carbs did. I had it up around 20 psi and tapped the metering lever to see if it would move - got a spray of petrol/WD40 mix in my face for the trouble.

EDIT: When I was talking to the rep from Zama a couple of years ago, he mentioned that they set up the POP so that a saw would idle in any position (I have doubts about that statement). If that is the case, then there may not be a lot of leeway in setting POP. I think I will just take a loop off the end of my spare spring and see how it affects the power, then another loop, etc.
 
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Terry Syd said:
Well, that was a surprise. The POP on this Zama carb is over 25 psi. I can't get it to pop off within the range of this gauge. I'm gonna have to find another gauge to test with.

Kinda funny, I kept expecting it to pop off around 10-12 psi like the karters carbs did. I had it up around 20 psi and tapped the metering lever to see if it would move - got a spray of petrol/WD40 mix in my face for the trouble.

EDIT: When I was talking to the rep from Zama a couple of years ago, he mentioned that they set up the POP so that a saw would idle in any position (I have doubts about that statement). If that is the case, then there may not be a lot of leeway in setting POP. I think I will just take a loop off the end of my spare spring and see how it affects the power, then another loop, etc.
Click to expand...

So what is the recommended popoff psi for that carb Terry?
 
They don't publish any recommended pop off pressures. Zama puts springs in them that they measure at the assembled length in the carb. The springs are measured in how many grams it takes to load the spring to that length.

I corresponded with them back in 2009 about the carbs and springs. The spring numbers and gram ratings are all over the place, it is not a sequential listing. I think they make them up as they need them and then the new spring gets the next number. I've seen some numbers down to around 16 grams and others up to 35 grams (maybe even a 45 gram??)

The carb I'm using is off a 570 and it actually had the same spring in it as the stock 445/450.

The spring has a stiff rate in the centre and 4 collapsed loops at each end so that it can be shortened to reduce pre-load. I nipped off one loop using a nail clippers. I was stuffing about trying to figure out how to do it when I remembered someone mentioning the nail clippers in a thread.

I can't tell you much at this stage other than the saw idles and revs. I will have to wait until I get it in some big wood to see if there is any change.

I'd much rather be using a mounted tach to identify and document any small changes. Then from the data base of changes, it might lead to some tuning principles.
 
The missus will have your jewels if she knows you used the nail clippers for anything chainsaw related Terry!

But then, I don't know about yours, but my toes nail present more of a problem than little spring I guess!
 
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Yeah, I'm always sneaking off with something to use in the shed. If I ever get caught I'll use the excuse that I don't have enough tools - "perhaps I should go buy some more, sweetheart?" I don't know if she'll buy it, but I could always use more tools.
 
keep up the great work Rick and Terry, I know bugger all about carbs.....except for they make you fat!! very interesting stuff. Re pop-off pressure Ive seen 7-8psi recommended for alky running saws.....how this equates to petrol I have no idea???
 
Take a look at the link I made to the EC forum. There are threads there about setting a carb up for alky. Essentially, you have to re-drill the low speed circuit to flow more fuel, re-set the POP, adjust the lever height, etc.

We don't have to worry about the alky problems on a work saw, but the fine tuning of work saw carb should be (IMHO) part of our repertoire on this forum.
 
You're thinking Karts. Yeah, it was wet - so was my face!

EDIT: I note that EC has POP gauges going up to 30 psi. I expect that saw pressures are going to be in a different range than karts. Different conditions, different pressures - makes sense to me.
 
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Rick, you also have to remember the leverage ratio of the metering lever. The Zama and the Tillotson are probably different. Also the ratio of the diaphragm to the metering orifice.

We are probably going to have to establish some principles of tuning that will apply when certain conditions appear.
 

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