Low Compression?

12347 Views 33 Replies 8 Participants Last post by Rcfish90, May 28, 2016

MathieuOuellette

Discussion Starter · Jun 8, 2012

So Im riding my pocket quad this morning, everything was running smooth, have a new clutch and just put in a moded flywheel, when it suddenly chokes out. I check the gas tank, still more than halfway full, so I give it a crank... and another, and another. I let it cool down for about an hour and gave it another try. Still no dice. I then notice that the engine compression doesent feel right, when I pull the starter it does not give it's usual resistance.

I sat down and thought about it for a few minutes and concluded that either my piston rings need changing or my reed block needs to be checked. So I take the cylinder out, carefully inspect the piston, piston rings, wrist pin, etc. As far as I can tell the piston rings are fine, and the piston itself has very little wear on it, aside from a few scratches ontop from when I was doing my Squish adjustment. I then removed the carb and reed block. the reed block is fine.

Does anyone know what else could be the cause of this loss of compression?

1 - 20 of 34 Posts

MathieuOuellette

Discussion Starter · #2 · Jun 8, 2012

So Im riding my pocket quad this morning, everything was running smooth, have a new clutch and just put in a moded flywheel, when it suddenly chokes out. I check the gas tank, still more than halfway full, so I give it a crank... and another, and another. I let it cool down for about an hour and gave it another try. Still no dice. I then notice that the engine compression doesent feel right, when I pull the starter it does not give it's usual resistance.

I sat down and thought about it for a few minutes and concluded that either my piston rings need changing or my reed block needs to be checked. So I take the cylinder out, carefully inspect the piston, piston rings, wrist pin, etc. As far as I can tell the piston rings are fine, and the piston itself has very little wear on it, aside from a few scratches ontop from when I was doing my Squish adjustment. I then removed the carb and reed block. the reed block is fine.

Does anyone know what else could be the cause of this loss of compression?

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CAM2

· #3 · Jun 8, 2012

You should of performed a compression test on it before disassembling it....The reeds sound like they could be the culprit...Make sure theyre sealing tightly

drgnprelude

· #4 · Jun 8, 2012

is this a double post i saw do a compression test to make sure and its prob the reeds from CAM right or was that another place or forum

MathieuOuellette

Discussion Starter · #5 · Jun 9, 2012

yeah my bad I didint realize there was a help section to the forum, delete away.

MathieuOuellette

Discussion Starter · #6 · Jun 9, 2012 (Edited)

I'll probably reassemble tonight and do a compression test, how much psi should a pretty much stock cag be running and any idea if there is a way to gauge how tight the reeds need to sealed?

StrayDawg63

· #7 · Jun 9, 2012

Should be well over 145psi. Yes you can check the reeds by holding the reeds to a light loking at the motor side if you see light no good if you dont a ok...

MathieuOuellette

Discussion Starter · #8 · Jun 9, 2012 (Edited)

I threw everything back together this morning and I am only getting about 65-70 PSI. there are no visible cracks in the crank housing. I tried starting the motor with no success aside from one loud popping noise, but the compression did not improve.

I am now puzzled. should I keep troubleshooting to find a problem or should I invest in a new head kit and reeds?

MathieuOuellette

Discussion Starter · #9 · Jun 10, 2012

Ordered big bore head kit and carbon reed. hope this does the trick.

MathieuOuellette

Discussion Starter · #10 · Jun 10, 2012 (Edited)

I found the culprit! some how a piece of broken clutch spring made its way into the head kit, notched the side of the piston and ended up mached into the cylinder head. Derp, Derp and Derp again.

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MathieuOuellette

Discussion Starter · #11 · Jun 10, 2012

http://imageshack.us/photo/my-images/151/img2012061000206.jpg/

CAM2

· #12 · Jun 10, 2012

MathieuOuellette said:
Ordered big bore head kit and carbon reed. hope this does the trick.

If ya didnt order one from someone reputable like boosted306 then its likely you will see very little performance and half the actual gains if you dont take the time to blueprint,,deburr and polish the components to work like smooth silk with way more than expected power and last a little bit more than they actually do...In my thread I bought a 5 port headkit and just slapped it together as someone unknowledgeable would do and it didnt last too long.......Ill be posting pics of the damage done in a very short time ;-later in my thread...

MathieuOuellette

Discussion Starter · #13 · Jun 11, 2012

I am likely going to remachine and port my stock piston/cylinder to use as spare parts,

you say If I didint get from a reputable source it will require deburring and polishing, I got the stage 1 kit from big bore motorsports, should I remachine the kit before installing aswell or is it fine to just plug and play it? I know you reffered Big Bore Motorsports to me when I asked about the headkit

.

EPR

· #14 · Jun 11, 2012

All this stuff comes from China, no one inspects the parts, there not all the same. I had a new kit w/ a warp cylinder, took me months & months before I sent it to Sam, and he fixed it and put a 3rd Stage Billet Kit and started it then sent it back to me. You need to know how much space is between the piston at TDC, or Squish to know how much compression you have. Have to raise exh and lower intake ports, and time the ports to make it right.

MathieuOuellette

Discussion Starter · #15 · Jun 11, 2012

yeah I always knew the quality of china parts was terrible. We'll see whats what when I receive the kit. Who is this Sam you speak of? I might do the machining myself (father owns a precision sheet metal engineering shop and I have access to a wide variety of fabrication tools), but if he's good at doing cylinders I might just as well send him mine if I see the need.

EPR

· #16 · Jun 12, 2012

If you have the tools to work with it will be easy. You still need to mod all the ports and debur & sand & polish, and find away to cut your Squish down. At stock B/B head or kit could be 1.5mm, on my Red Head engine it's .45mm, that's a big difference. I only get the kits the have a compression piece so you can change from 12 to 17:1. Makes it much easier to make more power.
Then you need a bigger carb and better exhaust an so on.

MathieuOuellette

Discussion Starter · #17 · Jun 13, 2012 (Edited)

SO... I got my head kit in the mail, and I have some concerns with it. firstly the spark plug hole, is no where near the same size as the stock head, and will not accommodate a BM6A NGK spark plug. Secondly, the quality of the aluminum is really bad, I flick the cooling fins on it and it sounds like a xylophone. The Cylinder is ported, but will need some work, as for the piston, shouldent it be modified aswell? looks the same except new as my stock one.

as far as the exhaust goes, I have a Jet Pro pipe, but try fitting that **** in a mini quad, the manifold attachment doesn't clear the cylinder head.

edit: any good tips on how to tighten the piston rings? my tool was designed for much bigger pistons and thus will not close up enough for this size cylinder.

Thanks!

EPR

· #18 · Jun 13, 2012

Its sounds like a typical B/B kit. You need a 10mm plug, all Kits have that small plug. Line the ring grooves w/ the pins in the piston grooves, put alittle oil in the cylinder and around the rings push it into the cyld, and pinch the rings w/ your fingers or ice cream sticks and push it up in the cyld, there is a taper on the bottom of the cylinder for that. I put the piston on the rod w/ No rings and the cylinder over and see where the piston is a TDC. If you leave the base gasket out and yamabond of glue it down get alot more power.

MathieuOuellette

Discussion Starter · #19 · Jun 13, 2012 (Edited)

I used some after run oil to get the piston in with the rings, and decided to measure squish before bonding it down. I'm getting about .72 without gasket. wont it rise when I put bond between the cylinder and case? could I grind a few mm off the bottom of the cylinder to negate this effect?

What kind of spark plug would you recommend? I was looking at the NGK CR7HSA but not sure if it will fit properly... and perhaps Champion offers a better plug of this size?

edit: Substitute for yamabond? Cant seem to find any in my neck of the woods.
this is what I'm using:
http://www.rona.ca/en/gasket-maker?select_store=05160

EPR

· #20 · Jun 14, 2012

Yamabond is at a Yamaha Bike Dealer, Hondabond works too. I use that copper RTV but not on headgaskets & cylds. The stuff I use is real thin and rubbery and dries very fast. I spread it on the cases very thin with my fore finger and drop the cyld down, put the bolts in just touching the base area. An hour later, finger tight, the next day torque them down. An 8 plug is colder than a 7.

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