spudulike

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

Good news, thanks for the update, always good after many putting forward possible solutions.

And I second this. How many actually blow all that chip out of the side cover of their MS200/201/150s? How many have stripped it down and not pushed that inner cover fully home as it should? How many notice the brake guard flapping around in the breeze because the flat spring has failed. How many operate a top handle without a chain catcher? I see many machines in from many operators of different sizes. I mentioned getting in two MS260s from different operators, both not running but both with failed chain brake springs which were not mentioned at hand over. I regularly see the MS201s in and the chain brakes don't operate on the very edge of the guard nearest the oil cap. This part of the guard not actuating the brake is the first sign of the chain brake having wear in its components or being stuffed full of chip - the actuation sound is also a clear indicator on the latter as a new unit is "crisp" and rings, a bunged one is much duller in note. As I mentioned earlier, in commercial manufacturing, machines have Health Checks, Preventative Maintenance and regular servicing. It would seem from what I see, chainsaws are maintained at the point of failure i.e. not running, impact damage or at the point it is totally unusable! I am not looking for work but have been around long enough to know the dangers of bleeding out and say that those elements mentioned above get serviced whether a customer asks for it or not!!

Common reasons for flooding are pulling the saw over too many times with the choke on without it starting or issues with the metering part of the carb that could be metering arm set to high, needle worn, metering spring weak, metering diaphragm deformed and the old favourite - filling the tank right up to the top and leaving the saw in a roasting van in the middle of summer.

Try fitting a new fuel filter if it hasn't been replaced. Having the carb apart and checking the gauze strainer if the fuel filter doesn't fix it. The tappets often need adjustment but sounds like they have been done - the pull start being difficult to pull over is a sign they aren't adjusted correctly. I have had one unit where the valve timing was one gear tooth out. The machine would never run as it should if this is the case. This would only occur if someone had stripped it down and got reassembly wrong!

I have recently replaced two chain brake springs on two MS260s. Funny thing is ( not so funny) is that both came in because both machines had packed up. I can deduct from this that both machines were being used with no working chain brake until they clapped out...both with impulse lines off if of interest. I regularly have 200s and 201s in with their chain brake mechanisms stuffed full of chip to the point it is obvious they are becoming ineffectual. These are all from pro commercial outfits....perhaps it is time saws had preventative maintenance or MOTs as the only time I see 90% of saws is when they don't run or are smashed. BTW, I am not looking for more work as am semi retired now but reporting back on how it is out there!! I agree much of the issues are with weekend warriors knowing nothing but the lack of maintenance in this industry is at odds with my past in the manufacturing industry.

OK, mapp gas torch....is that better...you know exactly what I mean...move on....and it works as a fix, proven many times through the generations!

Sounds low on compression, relatively common. A new piston and a bit of work on the bore would probably bring it back. The poor man's compression check sort of works but prefer a proper test.

Knowing the danger points of potential injury and how to avoid them is the largest part of being safe. Do you wear all the kit and not bother to switch the engine of when walking or know a trip with a running machine could cause injury and switch it off whilst walking ...two different outlooks!