Ts410, sorry another

[Mike the builder]

Hi. Ill try to be brief. We’ve done the carb kit, filters and replaced the coil and plug twice. Sparks good so we check the compression using the decompression port and it’s at 75psi. I understand it’s supposed to be maybe 150. I’ve taken the cylinder off but the piston, rings and cylinder look sweet. Nice and smooth, no scoring, rings are good. Does look like it’s done much as there’s only minimal carbon. Ideas please ?

[openspaceman]

4 hours ago, Mike the builder said:

piston, rings and cylinder look sweet

but did you measure anything, like the ring gap at various parts of the bore. Air has to be escaping somewhere for that low pressure figure.

[ChrisNewport]

is the plug getting wet? Take plug out, spray some carb cleaner or easy start in the hole, replace the plug and pull it over, if it fires but doesnt run its a fuel problem. Buy a new chinese carb off ebay.

 

75psi is low but i imagine the tester is  at fault if the p and c look good.

Edited October 9, 2021 by ChrisNewport

[spudulike]

How are you measuring the compression and with what gauge? Some gauges are meant for cars and a small engine won't register the correct compression measurement.

75psi, if correct and measured on 5-7 pulls is terrible. It should be 150+ and 75 on the first pull.

Stone cutters usually are not looked after, the stone dust is very abrasive and it is easy to abrade the ring, piston and cylinder to the point you don't have much compression. 

If you rest the cutter on the ground and lift it by the starter handle, does it fall to earth in a second or two or can you lift it by the handle and have a few seconds of suspension each time the piston reaches top dead centre (compression)?

No disrespect but a cylinder and piston "looking fine" by a novice may not be what an experienced engineer may think. It is possible the ring end gap is too large, the bore nikasil plating has worn off or the piston and ring have been abraded badly....as I said, stone dust is extremely abrasive and these machines are usually abused.

[Mike the builder]

Thx, great advice. I’ve a gauge to measure compression but surely 75psi is 75psi regardless of whether it’s a car or bike or anything. 
yes I’m a novice and I’m going to try picking it up by the cord to try your yr advice. I will have to reassemble though. I’ve ordered chinese cylinder and piston to try out.

I didn’t think the plug was wet enough at all and I guessed that was because there wasn’t enough compression to pull fuel through the carb, if that’s how it works. 

[AngrySquirrel]

Another way to check if rings ok or not if not sure gauge is right? is to drop a little oil down bore leave a short while so runs top of piston then give it a pull see if registers higher?

[Mike the builder]

Magic idea, I’m trying that as well. I’ll report back once I’ve done these suggestions, wish me luck reassembling 

[adw]

TS 410 was not Stihls finest hour, however as has been said these machines run in a very hostile environment, once concrete dust enters the cylinder it mixes wthe fuel and becomes lapping paste, a sure sign is if you look at the inlet port from the inside you will see a dull area either side, this is where the plating has worn away and the alloy is showing through, the rings will be worn to excess, a new pot and piston is then required, you also need to be looking at the big end and main bearings, it's pointless assembling a new cylinder before checking these points, look at the paper filter and the housing behind it, any signs of dust beyond the filter is bad news, never blow out paper filters as this opens the pours and will allow dust through.

 

[Darkslider]

13 hours ago, Mike the builder said:

Thx, great advice. I’ve a gauge to measure compression but surely 75psi is 75psi regardless of whether it’s a car or bike or anything. 
 

Spud wasn't disputing that 75psi is low, but saying that a vehicular compression tester designed for being pressurised with thousands of CC in a short cranking cycle might not be as accurate with a 40cc cylinder being pulled over by hand. I've got a motorbike/small engine compression test kit I use, I've had a few in the past that under read by miles which could lead you down the wrong path, hence the questioning of the starter cord behaviour.

[Mike the builder]

Thx for advice, as you’ve guessed I’m a novice. I’ve some photos for fun. I found an Aluminium tube to use in tester instead of the rubber hose and I now understand the ration difference between small and large cylinder affects the volume of tester so I see the issue surrounding different pressure testers. That being said it’s what I have. With ali tube I’m now reading 105psi with oil added to the cylinder, I’ve taken it apart again and cleaned cyclinder, piston and rings, retested and still getting 105psi. I think I might of been barking up the wrong tree. 
pictures of tester, piston and inside of bore. Can even see reflection of my finger nail in one of them. 
time to order a chinese carb ?
( done carb kit already )