spudulike

Have you been doing those crossword thingys again Or was it a new key word in a powerpoint, bit like "synergy and holistic" were back in the day!! I would agree with giving the carb a wake up call on the H&L screw, it may be an easy fix and if not....back to the hard way.

I rarely use it but Walbro do this gauge: -

Some businesses work out what parts are regularly used and then buy in "STOCK" so they can fix customers saws quickly so they are not inconvenienced. You don't obviously hold stock of very expensive items or ones you use once every three years but I can be sure a MS201 handle, a MS200 chain catcher, MS261 bearing, a Zama C1Q carb kit may be used in the next week or two Not rocket science and a good supplier allows me to get six trimmers in on Tuesday and be invoicing for a full service and new blades for all of them by Saturday....what a life It is just annoying when customers supply a load of parts that they don't actually need, that I have in stock and it makes it look like they don't trust me not to rip them off!!

The idling and revving up then dying - what happens is you start the machine on choke, this fills the metering section of the carb with fuel. As the saw idles, it empties this part of the carb of fuel and what should happen is the diaphragm pushes against the metering arm opening the needle valve and another slug of fuel enters the metering section and shuts off as the diaphragm comes away from the metering arm. What your Stihl is doing is using the first slug of fuel and then it basically runs out of fuel as no more is coming through and that is why it exhibits the same symptoms of running out of fuel. The Stihl needs the metering arm height checked and perhaps a new diaphragm in the carb. I have come across the same thing in BG86 blowers, exactly the same symptoms. Whilst you are in there, check the positioning of the metering arm spring and that the needle is moving freely up and down. Why the new fuel should cause this....no idea as I don't know the history but your issues with the Stihl are exactly as I have said! You could check the metering arm height and see if you can get it to work just by doing that if the diaphragm is relatively soft and hasn't hardened. If I get a machine like yours in, I just work through the carb and if a going through doesn't sort it, I fit a new kit. If you want to prove it, let the saw run and then stop then strip the carb, the metering section will be dry and there will be fuel in the pumping section!

So, the saws are idling OK so the idle circuit is providing enough fuel for idle but not mid to high speed running. The first checks would be fuel filter and gauze strainer - on the latter, I don't mean look at it, I mean take it out and view it against a bright light with a magnifying glass! On from that.... Holed fuel line, blocked tank breather, holed pump diaphragm flaps, hardened metering diaphragm, dirt in the metering section of the carb, metering arm set too low, dirt under the welch plug (very uncommon), faulty accelerator pump (If fitted), blocked high speed check valve, blocked air filter, faulty choke flap, faulty ignition timing ramp on the coil, arcing on the HT lead/plug......and so on. Highly unlikely that two machines would suddenly develop the same issue with many of the above causes. Have you tried opening up the high speed screw on the carb?

There do appear to be two types of crankcase that may be for the two different types of coil. If the OP can give us a serial number, I can tell what flywheel and coil he should have! I still don't get this air filter on....backfires and air filter off.....runs fine thing I think I would start it with the filter off and then put it on with the engine running....all seems a bit weird!!

Yes, ADW means the crankshaft seal or radial seal and it is pushed in to the actual bearing - it is a sod even with the factory tools and requires a real knack and isn't helped that they are dam expensive. The manifold - The yellow mark is where they typically split and it is often very hard to see. If the saw has been stripped in the past, the whole impulse doofa may be deformed where someone has located the manifold incorrectly or you could say....hasn't got a clue!! If you are purchasing parts to give to me when you run the saw in....don't as I have them all in stock and charge what you are likely to purchase them for!

Is that 100% as I think the 660 has three mounting holes but both the earlier three hole and later two hole coils fit. Just trying to totally discount the mix of parts scenario!

The stihl vents are one way and only let air in so high pressure on the tank through heat may cause the metering needle to leak if the spring is weak. Not sure if the vents on Makitas are similar, don't do enough!

My thoughts in asking this question was that if the piston skirt on the inlet side is badly worn, it won't seal the inlet port and you will get significant "free-porting" where the fuel vapour isn't controlled within the crankcase and cylinder but is pushed back out of the carb hence watching it without the filter and blipping the throttle and watching for puffs of vapour from the carb. How this impacts on the engine may be dictated if the air filter is fitted or not but from my experience, you just get a poor pickup and unstable idle and have never had backfiring from it but can't understand how fitting an air filter will change the running that dramatically unless it is completely blocked. Fitting an air filter will drop the revs and from ADWs thoughts, it should be less likely to hit a rev limiter or governor etc Any back firing I have ever had has been caused by ignition/flywheel issues causing a shift in ignition timing. I would check the flywheel part number on the other machine or try the complete coil and flywheel from the other machine and see if it cures the issue.

I have just read the first post....the engine runs 100% fine without an air filter and then backfires with the air filter fitted...is this correct? If it is, when you blip the throttle without the air filter fitted, do you get a puff of fuel vapour emitted from the carb?

OK, if the key is intact and the coil is good...you have a mix of parts. This TS400 has two different flywheels and three different coils. The flywheel has to match the coil and the good news is that there is only one crank so if you shift the flywheel AND coil from the good machine, the faulty one will run and you then just need to get the correct flywheel...simple!

Useful but 6 years too late for the OP!

I am surprised you have never had a backfiring engine if you have rebuilt that many but the primary cause is the flywheel key shearing putting the ignition timing in to a position where ignition occurs where it shouldn't causing a backfire - usually retarded. The other possibility is that the coil has failed or been replaced with an incorrect type - coils have smart electronics often with advance ramps and if they fail, it may give bad ignition timing. Also, I have seen two machines of different years have different coil/flywheel types and if mixed up, this can give incorrect ignition timing. It will be an ignition timing issue....you can try everything else but a backfire is always where the ignition has retarded that much that ignition happens after TDC causing a loud backfire. If this was too far advanced, the saw would kickback like a mule!!

Just fit a new needle and metering spring to sort it. If you had a pressure gauge/pop off gauge, you can test the carb but fitting these new components should sort it. Make sure the metering diaphragm isn't hard or out of shape - replace if necessary. The Makita carb is no different than any other saw, just get to know the symptoms and fix.

You are joking...I guess when you are 4'8" and 7 stone, pulling over a 50cc saw could seem a bit daunting

Check the gauze strainer. I have had countless machines in with chip in the tank and a carb strainer stuffed with it. Water in the fuel can give some interesting running issues. Metal jerry cans can be an issue with condensing water.

I did have a vague feeling I may have seen one but only one. Can't remember too much about it though.

I don't think I have seen the Triobrake system in the flesh but have seen the stihl one....I think on a 261. First thought was....Jesus, this throttle lock is stiff but then realized what it was all about.

Stihl had a chain brake system operated by the rear handle where the normal throttle lock is usually located. The saws brake would be engaged if pressure was released from the rear handle. It was a bit alien but probably a reasonable safety device!

I have had one that was sluggish from idle but revved out after that, I changed the accelerator pump and it sorted it. You only noticed it as a hesitance to pick up off idle when grabbing a fist full of throttle.

I think our Australian comrades would disagree

I know both of you personally, got no issues with either of you or either of your experience in different parts of the same industry so please move on as there is already too much aggression and bad feeling on this site. Save it for the few that have earnt it

You forgot the split inlet manifold around the impulse connector but obviously agree on the seals, both are rather interesting to fit BTW!! I am only doing customer dropped off work now so depends where you are in the "South East". I also don't just do the cylinder work as too much depends on the rebuild checks/final setup and don't want any hassle about who did what when the saw goes pop! The carb should have the manufacturer on it....it is probably a Chinese copy!!

The "Sprial" bit confused me but am now thinking it is the Mk2 spiral affair ....the carb is a Husqvarna specific so you are stuck with purchasing the OEM part for lots of £££ or €€€ if in Eire. Part No 582366401 Info here - https://s.cdnmpro.com/129506436/content/IPL-uri/550 XP Mark II.pdf