OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Chainsaws and Concrete Cutters Petrol Chainsaws and Concrete Cutters Zenoah Chainsaw Won't Start. Help!

Have Zenoah G3800 c/saw, 4 yrs old, has not done a lot of work. Started fine a week ago, and was fully cleaned up following use. Will not now start.Good compression and spark. When saw is tilted on its side for fuel filling, fuel leaks from fuel delivery line tank grommet. Fuel line seems to be OK, but grommet is not a tight fit into tank. Sucking through fuel delivery line/filter assembly can be done, with a little resistance.
As carby is a Walbro diaphragm type, what amount of suction effort should be required to provide required fuel flow?
Could it be that the carby diaphragm has failed?
I am at a loss as to explain what has occurred, as it has happened so suddenly.
All helpful suggestions welcome.
(Relevant fuel delivery line part numbers are 22,23,24 on manufacturer's parts list.

Thanks.
mustang

I suggest you begin by establishing whether the problem is due to fuel. Remove the spark plug and pull the starter cord briskly several times. Then put a small spoonful of 2 stroke mix into the cylinder through the spark plug hole, and reinstall the plug. Do not choke or prime the engine, but pull the starter cord briskly several times with the ignition on. If the engine then starts, runs for just a couple of seconds, then stops, you will have established that the problem is lack of fuel delivery. Come back and tell us what happens.

I have established that fuel starvation is the problem. I bought a carby repair kit for the Walbro WT 749A carby and pulled it down for cleanup and repair. Found some grit in filters and jets. Blew all idle jets, mixture control passages and similar clear with carby cleaner and comp air, but main jet is blocked solid and no amount of carby cleaner injection or comprssed air application has cleared it. This jet is a brass unit pressed (not screwed) into the venturi opening. I have tried pressing it out with a screwdriver blade, but so far no go. Further, I have not yet been able to find a replacement jet block either as an individual part or as part of any repair kit. The number stamped into the brass jet block is 34, presumably a bore reference? Am I now up for a new replacement carby? If replacement jet blocks are available, I would drill out the block, remove with an "easy-out", pop in a new one for a fix. At the moment I am stuck.
Any advice would be most welcome.
Thanks in advance.
mustang

mustang, I suggest you focus on cleaning the pressed in jet, without damaging it. That job on carburetors is conventionally done by getting one of these sets of stainless steel oxy torch tip cleaners:


Find the largest rod that will go through the jet, and see if it will push through. If not, start a bit smaller. When you get the plain part on the beginning through the jet, push it through further and work the beaded part back and forth through the jet. Then squirt some carb cleaner through, followed by some compressed air to blow the carb cleaner out. Do not expect any other solvent to work: use carb cleaner to remove that gum.

If you just can't get any of the tip cleaners through, the last available measure is this one (shown on a Honda mower idle jet):


The tool is called a pin vice, and you use it to hold a jet drill. They can be bought in various sizes, down to 0.3 mm. The worthwhile ones are made from High Speed Steel, and they are used by touching them lightly against the center of the jet, holding the vice steady with an index finger on the octagonal part, and twirling the vice clockwise with the other fingers. Of course it is just as happy to drill through the jet, or the carburetor body, or a nearby piece of cast iron, as through the soft gum, so considerable delicacy is needed to use this approach. I normally use them to drill out jets as part of carburetor development, but I have been known to use the tiniest one on a Honda idle jet when necessary.

grumpy,
Thanks for your suggestions. As the no. 34 is stamped on the base of the jet block, I am presuming that this indicates a 0.34 mm dia jet. By eye, it would appear somewhere in that region. So I have ordered a pin vise and jet drill set from ebay, as many seem to contain a 0.30 dia drill. All the oxy tip cleaner sets that I could find seemed to have approx. 0.45 mm as their smallest rod.
So, we shall see how we go. I have noted that some hobbyists drill out the offending jets, fill with solder, and bore out the solder with a jet drill of correct size. I hope it doesn't come to that. I will let you know how things turn out.
Thanks and regards,
mustang

Thanks for the update. I don't think it likely it has a 0.34 mm jet, because that is not a standard drill size, and is way too small for a main jet, even of a whipper snipper. However if you get a set of jet drills, they will certainly extend across the size you need. The only jet I've found to date that is smaller than 0.45 mm is the idle jet in the small GX & GXV Honda engines - main jets are normally 0.5 mm or so.

I've been doing that solder-and-drill trick on brass jets for quite a lot of years, it is the easiest way to tune old carburetors for which jets are not easily or cheaply available.

grumpy,
I must admit that this carby has me a little puzzled. I have attached some images for your advice and comment. I appreciate that the exploded view covera the whole WT series, as my carby is much simpler and has no manual priming gear. The carby image shows the main block and the choke and throttle shaft assemblies only. For interest I have inserted a copper wire through the power needle seat as far as it will go. (Wire diameter is 0.67 mm.) You will see that it passes across the main jet fuel pick up hole to the right of the main jet check valve/block?. It appears to enter a side opening in the base of the main jet block, which seems to be solidly blocked.
Compressed air and carby cleaner fluid will not pass through the main jet when fed from either the power needle passage or the base of the block which can be seen in the image. The base of the main jet block bears the number 34, and has a small hole at its centre. An even smaller diam. hole is centered on this one.
Looking at the Walbro manual images which illustrate operation at idle and full throttle, a check valve is shown preventing main jet fuel flow at idle. As a check vale is usually operated by pressure difference across the valve, can you tell where such a unit would be sited in this carby - maybe the valve is stuck shut? As the fuel appears to enter the main jet block from the side and not the bottom, can you suggest a means of cleaning it while it is pressed in place in the carby block?
Thanks in advance,
mustang

The specific-model manuals are OK for tuneups, but not for carburetor overhaul, or for helping you understand fully how the carburetor works, or for trouble-shooting. Start by downloading the Service Manual, which covers all Walbro diaphragm carburetors:

http://wem.walbro.com/distributors/servicemanuals/ServiceManual.pdf

(An 8.8 MB manual must be better than a 1.3 MB manual - all those extra pages must tell you something useful. In this case, the real manual tells you how everything works in nauseating detail.) It also tells you how to remove the main nozzle and its check valve, but it seems fairly unlikely that is a problem area. Difficulties with the diaphragm carburetors seldom seem to stem from dirt that far inside the carburetor, perhaps because the dirt would be more likely to affect the high speed mixture adjustment. One of the most common problems after an overhaul, is either too much or too little fuel being supplied due to misadjustment of the metering lever: this is a critical adjustment.

You didn't ever explain whether the engine will start and run briefly if you put fuel in the spark plug hole, or the carburetor air intake. Also, you didn't explain whether the spark plug gets wet or stays dry whey you try to start it.

hello grumpy,
Thanks for your reply. I already have a downloaded copy of the manual to which you refer. If you have a copy- consider this-on p.15 of the manual at top image, showing the test air tube location - placing an air tube over the opening in my carby at this check valve location will not allow air to pass. I think that my check valve is of the long tube nozzle type described at the last sentence of para 3 on p.16. I have attempted to press it out from the venturi side as per manual with a flat bladed screwdriver, but all that is happening is that the soft brass nozzle is becoming misshapen - it's certainly not moving. Its shape will not allow it to be pressed out into the venturi area. I might have to drill it out and get another.
Unfortunately, I have seen neither hide nor hair of replacement main jet blocks/check valves on the internet. Do you know where I might find jet blocks?
There was grit on the internal filter screens and all three progressive idle jets were either partially or totally blocked.
Thanks, as usual
mustang

You placed your air tube in the same place as shown on the top of Page 15? By my reading, that might not work on a WT. Look at this diagram from Page 2 of the WT manual:





It seems to me if you blow in the hole surrounded by the green circle, you won't achieve much because it is blocked off below the main jet. If you blow in the hole surrounded by the yellow circle, and you have the high speed mixture screw backed out several turns, it should pass air. If it doesn't, it may be time to take physical action regarding a possible blockage.

Note that if the diagram is correct, you also can't drive the main jet out from the green circle hole, because the plug is pressed in against a shoulder, and the bore the main jet is pressed into is smaller.

To amplify what I've said above, there is a small block of metal which incorporates the non-return valve, and has the main jet pressed into it. This block is pressed into the carburetor body from below (i.e. from the metering chamber) until it reaches a stop, which ensures that it projects the right distance into the venturi. From the workshop manual it looks as if this may be one of the cases where to remove that block, you are supposed to drill a hole in it from the metering chamber, then screw in the self-tapper-like thread on the end of the little slide hammer puller that is included in Walbro's toolkit. You extract the original block, then you drive or press in a new block. However I don't see why you would want to do that until you had tried blowing air or carb cleaner into the yellow-circle drilling, to see whether it comes out through the main jet. If it does, all is good. If it doesn't, after some head-scratching it might be time to resort to the slide-hammer puller, provided you can get one, plus you can get another jet block cheaper than you can get another WT carburetor from a junked chainsaw. However I'm guessing that if you blast some carb cleaner into the yellow circle passage, it will come out through the main jet and all will be well.

grumpy,
Thanks for your post. FINALLY, I can get cleaner fluid to pass from the yellow-ringed opening through the main jet into the venturi. I will alternate between using the fluid and compressed air to ensure that the jet passage is as clean as possible prior to carby reassembly. As the main jet diameter is probably quite small, (maybe order 0.5 mm)attempting to blow air by mouth results in a fair amount of back pressure, which makes it a little difficult to judge if the jet passage is fully clear/open. Experience required? The same holds true for the use of suction to judge to if the check valve is operating correctly.
It should now merely be a case of the correct reassembly/adjustment of the carby and we're away?
I will bear in mind your caution regarding the setup of the inlet metering needle, lever and seat. (Incidentally, the carby kit I purchased for the WT 749A seems to have all the correct gaskets but the inlet metering needle is obviously for another carby as it is much larger than the one I took out. The original needle seems OK so I shall have to reuse it and hope for the best.
I will let you know how things progress.
Thanks again.
mustang

That is progress, at least. The focus now is on setting up the carburetor "by the book", especially the metering chamber. From my limited experience so far, as long as you have a matched set of parts, the main issue is the metering lever setting. However if you are planning to use a needle and a lever that do not match each other, I don't know how you would decide what the setting should be. It might be safer to find out why the needle is wrong, and what needle actually goes with the lever and diaphragm you are going to use.

I have reassembled the carby, as far as I can tell, "by the book". I am waiting on another part for the fuel delivery line which has been on back order for some time now, so I cannot try it all out yet. (It's always a bugger to get parts when the company that manufactured your original machine has been bought out. Husqvarna apparently bought out the small engine division of Komatsu, who made Zenoah machines.)
I reused the old needle as the one kit-supplied was obviously for a carby different from mine. Otherwise, I used only kit-supplied rebuild parts throughout.
I will advise the end outcome.
mustang

Hello to All,
Well, I finally got the carb and saw back together, installed a new fuel line grommet (those things shrink like crazy with use and leak), and, after a bit of fiddling with the L and H jets - away she went! Well, I thought, that's good, and then I noticed that no bar oil was coming through. I don't know yet whether the feed line is blocked or the oil pump has had it. Oil is also slowly leaking from the tank or lines somewhere - as the saw always leaves an oily mark when placed down, even when not running.
I've heard that emptying the oil tank and filling with an oil solvent, running the engine without bar and chain and checking the oil outlet for flow might work to clear the system- rather than initially pulling the machine down to get at the pump. Any suggestions?
Thanks.

There are various things that can lead to the bar oil pump working intermittently or not at all. The ones I've had most trouble with are a split in the oil pickup hose inside the tank, so it sucks air, and a blocked oil pickup filter inside the tank. However if it has ever been run without the pickup filter there's a good chance the pump is trashed, from trying to pump crud.

Another problem I've had is that the oil system discharge port near the rear sprocket, was totally gunked up by old sawdust and dirt. Some oil continued to get through, but it just dispersed into the sawdust and ended up on the ground, not the chain.

I don't recommend pumping solvent with the engine running. It would be likely to score or seize the pump. If you need to clean out the pump after you've made sure it has no problems on the pickup or discharge side, you might consider pumping some diesel fuel through it, mixed with say 10 percent engine oil, by pulling the starter repeatedly with the spark plug removed. Diesel fuel has a small percentage of a special oil in it to lubricate the high pressure injection pump (never run a diesel on kerosene or heating oil - they make decent fuels except that the lack of oil will quickly ruin your injection pump, which is extremely expensive. I think a lot of people have learned that the hard way).

G'Day, grumpy,
Thanks for your post - I shall just have to start carefully poking about to ascertain the problem with the pump.
Will let you know.

The owner manual, Zenoah G3800 Owner's Manual, has a few useful bits about replacing the oil pump suction filter and cleaning the discharge nozzle.

G'Day grumpy,
Well, more pulling down reveals that the filter comp. unit on the oil pump inlet was a loose fit in the oil tank - hence the leakage of oil. It does not seem to be a very tight fit even when pushed home - no evidence of an O-ring. (See Item 48 on Fig 1 in the G3800 Parts List). All lines are clear, but the pump is buggered. The crankshaft-driuen worm wheel is OK, as is the pump worm gear, but the pump screw section does not rotate with the worm. Must have sheared off or something. I need a new pump.
I have a question - the oil tank breather assembly does not at all resemble the system shown in the G3800 Parts List,(as Items 17, 18 of Fig 2). What is fitted is a short vertical plastic tube rising from a nipple fitting on the tank front wall and is terminated in a self-tapping screw. There is no visible opening anywhere on the tube or the screw - so how can this breathe?
Back to work.
Regards.

I don't really understand your description - are you talking about the chain-oil tank air vent? Perhaps the self-tapper is not a tight fit in the tube?

Komatsu is a very reputable engineering company - I suspect that when it left their factory, it was a fairly well-designed product. That implies either it still is now, or a previous tenant has "improved" it.