What's on your bench today?

[GardenKit]

I would think its down to a fuel starvation. If the tank filter is clean and the pickup is good, then I would be looking at the carb again. If the carb is kitted correctly, then maybe the metering arm is set a bit low.

 

Or there may be a bit of air ingress when the vacuums high (revs) which weakens the mixture.

 

I would bang another (known) carb on, it only takes a couple of minutes, and see what happens.

[spudulike]

Don't discount the breather, it is possible some air is getting in to the tank but you are getting a varying vacuum in it causing a varying pull through of fuel - when try running it with the cap slightly loose and try that.

 

Other than that - listen to Gardenkit:thumbup:

[spudulike]

Back to the 338XPT - managed to pull the flywheel off - bugger of a job. I was spot on - a fabricated key and timing retarded by 2-3 degrees causing poor revving.

 

Have fitted a spare flywheel I had in the workshop and it now sounds more crisp. When I was trying it, I found the oiler wasnt oiling well so flushed the tank, cleaned the filter, wacked some carb cleaner through the pump and it is now working great.

 

Just need to try it out now and tach to around 13,000rpm - al I could get out of it before was 11,500 down to the bad flywheel timing!

[Rich2484]

Had the 2a round again, fuel tank corroded itself, leaking fuel like a water fall, spare tank under passenger seat is swapped over and fine again. Cost= a few bolts as others sheard off. Mot on wed. Should pass ok then it's off to logging.

[ben lightfoot]

Interesting reading spud regarding the 338xpt as mine has been a bit of a pain recently although hopefully cured after a carb rebuild and clean. the revs were picking up but when underload would barp barp barp and die. Be interested in getting mine tweaked when i get a new one, can you do much with them?

[spudulike]

You cant change the squish due to the design of the engine, porting and muffler mod should be possible.

[ben lightfoot]

Can someone explain whats meant by squish? and how this can be altered and affects it has on performance, sorry if asking a lot

[spudulike]

Can someone explain whats meant by squish? and how this can be altered and affects it has on performance, sorry if asking a lot

 

Squish is the distance between the piston crown and the top of the cylinder, lowering this increases compression - that equls more power just as low compression means loss of power.

 

There are a few ways to adjust this - fit a custom thickness base gasket, drop the gasket completely or machine the bottom of the cylinder and turn your piston in to a pop up one.

 

Squish is exactly what it sounds like - don't over do it though as too high compression can stop combustion and there has to be a safety margin between the piston and cylinder!

[spudulike]

Busy weekend, finished off the 338XPT and then dug in to two MS200Ts, one a box of bits and the other had an intermittent bogging and poor idle. Did the normal and found a tiny hole in the pumping gasket and another cracked impulse line, the carb was set way too rich hence the black plug - now coffee brown!

 

Put the other box of bits together with a few quids worth of spares and hey presto - one strong running saw. The customer wanted a port job on the box of bits saw and managed to open up the exhaust port but the inlet is just too tight on the ring pins on the piston and the piston skirt sides so too risky to do much there. Lowered the squish and got 160psi comnpression, both saws were muffler modded and got 14,000 - 14,400 rpm on the tach - both sweet.

Edited February 19, 2012 by spudulike

[spudulike]

Getting to put the 262XP back together again, just fitted one of those Meteor pistons - they really are good quality and fitted with caber rings - begining to look good.