Gerbutt

Hello all Does anyone know if you can get the hd air filter and cover for a 390xp in the uk? And, is it worth getting?

id say it is your rails, mates saw was doing it when disking up. same chain on new bar and it was fine.

Thats the thing, it doesn't skip them, they just need the raker setting to them. If you have one side of, the chain down to a quarter and the other side brand new, it'll still cut straight so long as the rakers are set. But everyone has different views on this, I'm just going by what I've tried and seen. Sorry to derail the thread a bit!

Yeah when I did my courses years ago I was told that all teeth had to be the same size, so if you hit some metal with a new chain and badly hit five teeth, you had to take all the others down to the same size once sharpened. I don’t know why they teach that cos if you get a new chain and take one side down to half and set the rakers then it’ll still cut straight. Of course all that goes out the window if you don’t keep your rails level and flat ?

It is reality though bud ?

Remember your rakers are the important part, it doesn’t matter if every tooth is a different length so long as the rakers are set [emoji106]

Splendid, I shall have a gander! Thank you.

What’s ope?

Well I've got it back together again. I ended up leaving the piston alone, smoothed out the lip on the intake, widened the intake and exhaust just a tiny bit, it didn't look like there was much room to do any more, enlarged the exhaust outlet and matched the gasket and exhaust. Its defo got more compression and runs a treats so all's good. Will just see how long it lasts now

Interesting read above dude, thanks for that. The key is built into the flywheel so i couldn't measure it. I only took a smidge off, probably a 1/4 or less. Id read that folk take a 1/3 off but didn't want to risk going to far. Infact, when fitting the flywheel again i couldn't really tell if there was anymore play when moving it forward it was such a little amount. I advanced the timing just because I'd read that's what people seem to do, and I was interested to see if it made a difference. The saw seems a bit more responsive, and it's probably just my imagination but it also seemed to start better - not that it really had a problem starting before. I'd just ported and raised the comp so it was just part of that. All seems good though, will have to wait until i've something large to cut up to see how it performs. At least it works hey

That was an interesting read. How did it turn out Matt? I’ve decided to put some new rings in mine so whilst at it I thought I’d check the squish to see if it could be improved. It measured in at around 0.9mm, so quite large. I took the cylinder off, cleaned band, which wasn’t really dirty and replaced without gasket. It then measured in at 0.69mm which from what I can gather is still too high. I looked at the bottom of the cylinder and it was real rough, you could see milling makes and when held up to the light with a straight edge it was all over the place. This is probably not the way to do it but I got my 8 inch diamond plates and very carefully flattened the bottom until is was flat and marks had gone. Put the cylinder back on and now the squish measures 0.49. I don’t really want to run without a gasket so made one from my thin paper and it now measures 0.62mm. Is that too high or should I flatten the bottom a tad more. The other thing I notice is there is a real lip on the bottom of the intake port. If I was to flatten this, and with lowering the cylinder it will have dropped the port a bit. Should I take that measurement off the bottom of the piston and maybe raise the exhaust the same? Or do you think it won’t make much difference?

Tee hee thank goodness [emoji28] Whilst you’re here sir, should the max rpm be around 13,000?

Oh no, it’s that a bad thing?

Nice! Here’s the 390 vid

I can remember watching the vids of it cutting if it’s the same one, sounded like a right beast! After a bit more reading I think they are usually at 13,000rpm. Strange thing with mine is I’ve just ported it and I’ve not touched the H jet but it’s at 11,900. All I can think is when I did the duel port muffler a couple of years ago and I turned the H jet up a tad, I must have been scared and gone way to far - but it’s always seemed to cut fairly well. I think next week I’ll go down the yard with the tach and have a play and then stop the saw when flat out in a cut a check the plug. At least after all this tampering it’s still going [emoji3]

Yeah it’s easy enough with diamond files, and I just did it a little bit. It does feel more nippy, and I’m sure it started a tad better too [emoji106]

Hiya Spud, what sort of top rpm would you expect from a 390xp? Just checked mine and it’s at 11900, does that seem a little low? The plug colour looks right and it’s nice and responsive. It has the blue coil, would that be making the readings hard to find? Thanks.

Well I’ve just done it and it does seem more responsive [emoji106]

Hiya spaceman, yes that makes sense, thank you for explaining [emoji106] I may have a dabble at doing it on my 390xp. I hear folk saying 1/3 of the key. I wouldn’t want to go to far so I may do a 1/4 and see what happens first!

Hiya I’ve been pondering about timing advance and I understand what happens when you do it, it sparks a little earlier. What I don’t understand is why this is an advantage and how come it seems to give more power? I’ve seen vids where a 200/201 had been done and it looked quite a lot better, faster in the cut and holding rpm better. Why is this?

Thank you Steve [emoji106]

Thanks for the advice there Spud. I don’t think I fully understand what you are saying, but would you say that the little triangle on the cylinder transfer that I marked in yellow is the part that needs blending? From what I can see everything else matches perfect, according to the gasket. I’ve just made a new gasket so it’d be great to grind that part before putting it back together.

I think .020 is the same as 0.5, I will aim a little higher than that because my micrometer is a cheap one. I’ve just got the gasket paper through and that’s meant to be certain thicknesses so will check them on that. I’ve only done little changes on this saw, just a bit wider on exhaust and less so on intake, not raised or lowered or touched the transfers apart from smoothing out lips. I would like to match the transfers to the case but finding it hard to get an answer whether I’ve marked the right part in the picture above. I’m sure it is right because there’s nothing else that doesn’t match, but want to be sure first. I do t expect much from the changes, certainly nothing like the change that Spud made to my 346xp, that things a little beast!! Thanks for your wise words of warning bud [emoji106]

Thanks for your input bud, that was nice and easy to understand and makes total sense [emoji106] It may be a tad larger than 3mm, it was just a guess looking at it.

I’ve read about matching the transfers to the case using the gasket as a template. Is the bit marked in the photo the bit that needs blending in?