saw not drawing fuel from tank - where to look next?

[sfinnie]

Hello all, a follow up to a previous post *.

I have a McCulloch 3500, quite an old saw. Belongs to a friend and I'm trying to get it working. It will run temporarily if I prime with fuel directly into the carb; however it stalls once that fuel is used up. So far I've done the following:

1. Removed carb, sprayed through with carb cleaner & blown out with compressed air. Replaced diaphragms, needle, gaskets then re-assembled.
2. Replaced fuel line and filter
3. Confirmed there's a strong spark and decent compression.

I'm stumped on where to go next. It'll start 2nd or 3rd pull when primed, but just doesn't seem to want to lift fuel from the tank. The only thing I've noticed that might(?) be questionable is the saw body onto which the carb assembly sits. It's split vertically - I'm assuming it's where the crank case itself is split. There's a very slight difference in the height of the two halves. Much smaller than 1mm but noticeable to the touch. I've replaced the gasket that sits on top of it with a cork one, on the basis it should accommodate the slight difference.

Other than that I've drawn a blank. Any suggestions appreciated.

Thanks,
--
* After much digging, it appears the 3500 never had a purge valve. I know that'll likely make it more difficult to start, but it suggests it should run without one.

 

[Bob Hedgecutter]

Welcome along.
These wee Mac's are not something I am very familiar with, but as far as I know at least some of the 3500 (if not most) models had purge valves and it may be that the lines are incorrectly installed to it, connected in the wrong order.
This gap of less that 1mm that was repaired with a cork gasket? If I understand correctly is between the two case halves- which is not a good thing.
The new cork gasket may not be sealing correctly and has increased the height of the combustion chamber relevant to the piston top at top dead centre- thus dropping compression. Cork gasket is very possibly blocking the vacuum channel that pulses the carb diaphragm as well.
Photos will help no end trying to spot or confirm problems.

Have a look at about the 6 minute mark of this for fuel line and purge valve connections.

 

[sfinnie]

Thanks Bob. Took some pics tonight, this is probably clearest (ignore the gunge, needs cleaned up). You can see the gaps at top- & bottom left of the intake rectangle. They're ~1mm wide and there's a very small height difference between the two halves at the bottom. I'd guess it's a few thou: noticeable to the touch but not big enough to measure with a ruler.

The cork gasket is 1mm thick and sits around the inlet (i.e. doesn't interfere with the centre rectangle). I'm reasonably sure the saw didn't have a purge valve; the hole that I thought held the valve is actually access for the carb adjustment screws. And the carb only has one fuel port (per previous message). I wouldn't be averse to swapping in a two-port carb and fitting a purge valve; difficulty is knowing which carb to go for. It would have to be compatible with current (mounting screws, inlet/outlet diameter & position, probably other things too I'd imagine?)

Anyway. Thoughts on the pic below appreciated, as well as any suggestions on what to do.

Thanks very much.

 

[Bob Hedgecutter]

My thoughts are- what is that wee cork like circle- centre bottom of the photograph-pinky width to the fight of the case joint as viewed? If that is an impulse port- there is your problem- it should not be blocked.
Not sure how to handle that step in the cases- but cork and silicone is not the answer.
Original gasket coated each side with a proper liquid gasket maker like permatex, yamabond, Hondabond, Dirko etc would be my try and fix.

 

[sfinnie]

thanks Bob. Unfortunately, the wee cork circle is just debris - there's no port underneath it. The "gunge" is actually clear liquid gasket, not silicon (forget the brand name). The original gasket was torn so not serviceable: I do have gasket paper though, so can cut a new one. I'll give that a try with liquid gasket on both sides.

Other than that, I'm stumped. I assume that trying to address the gap would mean splitting the crank case. Never done that before, assuming it's not a trivial job.

Feeling a bit conflicted tbh. The saw seems fundamentally fine: good compression, oil pump works well, runs if primed manually. It's just this fuel supply issue, and it's become a bit of an obsession! Need to take a step back and decide if it's really worth the time and effort. Hmmm.

 

[Bob Hedgecutter]

Be good to get it solved though and figure it out. Might be within the carb- blockages or stuck needle valve not allowing fuel in.
As I said, I am not familiar with the model enough to tell you if the case step is a common thing, or which carb they use.
Finding a known good saw and swapping the carb would prove something- even finding a similar model wrecked and trying that carb might work.

 

[sfinnie]

Thanks. The carb is a Tillotson HU-57A. I've had it out, cleaned and blown through with compressed air. Replaced the needle vale as well as diaphragms. I'll keep an eye out for a swap. Nothing showing up on the usual sites (ebay etc.). Ideas for other places to look would be good if you have them (I'm in the UK). Meantime, I'll keep fettling, obsessing and hoping...!

 

[sfinnie]

afternoon all. Quick update to close things off. I found a second hand carb on ebay, fitted that, and the saw is now working. Hurrah! Don't know what was wrong with the other one, but I can live with that. Returned saw to its owner today, and he's delighted. So a happy ending. Thanks again for your help Bob.