Need New Piston

[edzy]

i took the bike in for a sevice today....

the guy said it has no compressioon
so i need a new piston and rings.

should i get just the standard one or hi-comp?

will hi-comp give me much more power?


if i put the hi-comp in would i need to do anything else? like HD valve springs or anything?

 

[Sidney]

IF the bike starts there has to be compression. does it run fine?

why don't you service it yourself?

 

[edzy]

well it has very very very low comp.
it starts after like the 10th kick.
but sounds horrible and goes bad and slow.

i didnt have the tools to do the valve clearences because i though that was the problem.

 

[Sidney]

oh ok, just get a big bore kit for it

 

[Cactus Jack]

First off , check that you have adequate rocker to valve stem clearance on both valves at TDC on the power stroke ... if both rocker arms can be wiggled and movement felt then your rings and/or bore may be worn and you have way too much ring end gap ... IF so , it should be puffing a bit of blue or white smoke ... if there's no smoke then one of your valves could be bent or otherwise damaged and not seating or the head gasket has a bad leak (which you'd hear) .....

At any rate you need to pull the top end down to check ..... if it's the bore worn out of parallel then it needs a re bore and hone ....... NEVER buy a new piston or rings until you've had the bore checked for wear as it only needs to be out a few thou' for it to be no good ...... and in that case fitting a new piston and rings won't solve the compression loss ..... it's better to buy a complete cylinder , piston and ring kit or get yours bored and honed to suit an O/S piston .... after you've got the new piston since the machinist needs it to be able to get the bore clearance right ..... a sloppy fitting piston (especially a cast one) will give poor ring seal , will be noisy and can shatter ......

 

[edzy]

yeah , im going to pull it all down over the w.e and buy the piston and ring kit of dhz.

would a hi-comp give much more power?

 

[Cactus Jack]

the guy said it has no compressioon
so i need a new piston and rings.

should i get just the standard one or hi-comp?

will hi-comp give me much more power?


if i put the hi-comp in would i need to do anything else? like HD valve springs or anything?

Higher compression will give you more power everywhere as long as you don't go overboard and detonate the engine or kick start it the wrong way and bust the kick starter gears ....... in which case higher compression can make your bike go much slower with far less power than it has ever had ........ ZERO HP and torque readings on the dyno in fact .......

 

[Cactus Jack]

yeah , im going to pull it all down over the w.e and buy the piston and ring kit of dhz.

would a hi-comp give much more power?

Before you go tearing it down .... CHECK the valve lash clearances ...... I just fixed my sisters kids bike .... a GT90 ........ His father reckoned it was stuffed ...

I actually typed the info below out to post the other day but saved it so now here it is .........

My lil' nephews GT90 had the same problem .. his step Dad (a typical 2 stroker) had tinkered with the carb screws etc because it wouldn't start ...... even with the electric starter . He reckoned it must be electrical or piston and rings stuffed . My sister who bought him a near new 2006 CRF 80 as a replacement after giving up on the GT asked me to check the "stuffed" cheap crap chinese 4 stroke bike out . (He's too small and unable to ride the CRF80 due to it being manual) Straight away I said tight valves . On checking , sure enough they were super tight to the point where the bike had no compression .. I reset the gap by feel since they had no feeler gauges ..... the bike starts instantly but splutters and won't idle ... turned the idle up to keep it from stalling then did the in and out air/fuel screw tuning to find the sweet spot until it was plonking nice , slow and solid and starting super easily on the first gentle kick with no throttle ...... bike spluttered when revved ....... I instantly said "WATER" ..... air filter was dirty so I removed it to clean and re-oil it via the Cass38a dishwashing liquid , vege oil method ......

Water pours out if it ..... "OH we went riding in the wet at Pacific Park but after paying the bike wouldn't start" ........ Durrhhh !!!!!! I washed it and put it back on still slightly damp but not oiled to test if it WAS the filter ....... bike revved cleanly without any spluttering ......... lil' nephew extremely happy ....... sister says bike is running better than it ever did from new ...... brother-in-law looking very sheepish .......

 

[edzy]

oh ok
i can push the kicker down with my hand as many times as i want. its really easy to push down.

could this still be the valves?

 

[Cactus Jack]

Yes it can be , that's exactly what was wrong with the GT90 . The valves being held off the seat makes the cylinder lose all its compression and in this case it was mainly the inlet valve . So the piston was sucking air and fuel in ...... then blowing most of it back out thru the carby on the compression stroke leaving the plug wet . The plug can only ignite gassed up compressed fuel/air charge so the low compression makes it seem like either the ignition or plug aren't sparking .....

Adjust the valves and bingo ... it starts after a few kicks when the cylinder clears ....

 

[edzy]

ok
im going to go get it to TDC and then take some pics of the valves so you guys can judge if they are alright.

there is alot of oil in the end of the exaust , burnt oil , like a moist black powder...

still could be the valves?

 

[edzy]

ok guys
officially fixed....
i did the valve clearences myself today
the guy in the shop must have not even looked at the valves. they were so tight...not even one little bit of movement.

now she purrs and idles great

 

[edzy]

ok guys
officially fixed....
i did the valve clearences myself today
the guy in the shop must have not even looked at the valves. they were so tight...not even one little bit of movement.

now she purrs and idles great

 

[Cactus Jack]

EXCELLENSIO !!!!!! Yes , the soggy black soot is from lousy combustion efficiency due to the lowered compression ...... the fuel separates from the air so the bike appears to run rich ...... also the cylinder gets flooded with fuel from all the relentless kick , kick , kick ..... the charge gets pulled in ..... the fuel drops out ... then the air minus fuel gets pushed back out past the inlet valve ......

The valve seats can even become coated with carbon (depending on how long it's been run for with zero clearance) so the engine puffs out black crap and sparks for a few seconds when revved after the valves are readjusted .... that can cause the valves to lose some of the re set clearance again so you should re check the lash again in a few days ....

IF your engine isn't blowing smoke ...... then there's nothing wrong with your piston , rings or bore ......

I wonder if the guy knew what it was and was going to simply adjust your valves .... then charge you for a rebore , piston , rings , pin and gaskets + labour .....

 

[prox666]

i just found a bolt on 165cc kit for 150 motor its in the usa its 200 us comes with every thing ready to bolt on is that a good price they have sum cheap cams aswell would it make a 150 go much better

 

[darb110]

turned the idle up to keep it from stalling then did the in and out air/fuel screw tuning to find the sweet spot until it was plonking nice

dam man you been round em awhile then? you know your stuff =) thats great,

like too know abit more about that one if its alright,, just get her revin and wind the screw all the way in and bring it out till its going well or what? =) mines just in abit of a rough spot atm.. didnt seem to make much differnce last time i changed it

cheers brad =)

 

[Cactus Jack]

If it's for a YX150 then yes it's a good price ...... but carefully follow the Planet Minis thread to see what they sort out to make them reliable when modified ....

Akunar also has cams for those engines so I'd say they can also get the same bore kits cheaper than you can buy them from the US ...... When buying from overseas keep the total value per shipment to under $500 AUD in total (including shipping) or you'll get copped with GST plus customs levy on top .......

 

[Cactus Jack]

dam man you been round em awhile then? you know your stuff =) thats great,

like too know abit more about that one if its alright,, just get her revin and wind the screw all the way in and bring it out till its going well or what? =) mines just in abit of a rough spot atm.. didnt seem to make much differnce last time i changed it

cheers brad =)

Depends on what carb you have ..... the Mikunis that most pit bikes have on them have basically a mini second carburetor to enrichen the low speed running for four stroke engines .... they bypass the needle jet and trickle feed extra fuel in on the manifold side of the slide via a FUEL screw underneath the rear of the carb ....... but turning the idle up to get the engine so that it can't stall then turning the fuel or air screw in until the engine drops revs .... then out until you can hear it run poorer works on all carbs .... the mixture screw just works in reverse on the different carbs ....

You simply turn the idle up until the engine runs without stalling ...... then turn the mixture screw in until the engine note changes .... then while counting the number of turns ... screw it out until the note changes again ... then go in half the amount of turns ..... lower the idle speed and repeat the procedure until you get the engine idling nice , smooth and steady .... and as slow as you possibly can get it ..... if you've done it right ..... it'll start with a light kick without the throttle needing to be touched ... Once you've got that done .... you can turn the idle back up to what you think is right for you ......

IF the bike bogs or stumbles when you hit the throttle .... then you need to try different pilot jets ....... DO NOT TOUCH THE MIXTURE SCREW or alter what you have previously tuned . I get the bike running spot on at at full throttle in top gear with the slide needle in the middle position first before messing with the pilot jet ...... I first do a throttle chop run to check plug colour ..... then go up or down in jet sizes , one increment at a time until the engine peak revs cleanly and crisply without detonating or missing a beat .... for racing I lean out 'til it starts to detonate very slightly ..... then go back up a size (at that setting , you can't run the engine for long periods or let it free rev on momentum .. you have to always keep it pulling thru the gears under load) . eg with a fully worked XR75 (80 cc)... with 22 mm Keihin carb - #38 pilot #100 main for racing ~ #38 pilot #105 main for general all day long flogging ......

What Mack and others have found is that they run a bigger pilot to basically "dribble" fuel in to overcome any bog or stumble on air screw equipped carbs so that they can snap the throttle open and shut quickly ..... (a large pilot jet works better in lower vacuum conditions to supply enough fuel) but I adjust my throttling rate to deal with it since the bike runs cleanly everywhere else regardless of temperature changes throughout the day .... too big a pilot and the engines run rat shit (sluggish down low) and hard to start when they get really hot or the air filter starts to get dirty on a long ride ........ my method (as advocated by Honda for the past 40 years) makes for an engine that maintains consistent tuning from day to day , hot to cold etc ...... plus the plug always stays clean .......

When racing you can lean things out up top and enrichen the bottom end but the engine is usually only run for short periods ..... for general riding you want it to run good everywhere without the risk of leaning out further as the engine temperature rises since the fuel mixture helps to keep the engine cool .... so finding the neutral (blue) flame is the best .... the engine can swing either side of neutral safely without the risk of detonation or over heating ....... you simply DON'T ever want to swing to the high side of the edge of lean ........ or things start going wrong .....

 

[darb110]

arhhh thanks for that =) thats great.. ill have to go out and try it on my fiddy later on in the weekend =)

should get her running well., splutters abit low end but just giving the carbie a clean and yet to take it for a spin


cheers