Piston Rings

[eamer1989]

Hey all, i know this is a weird question i think my race bike maybe down on abit of compression i rebuilt top end and gone to fire up and nothings firing up, anyhow manged to use a shit compression tester as i dont have a normal adaptor for the head of the VFR and its coming only with about 75psi but then again i dont like the compression tester as its not a secure fit into the head, i only tested one of the cylinders also as i didnt have time was in a rush tonight. should be around the 150 mark i recon, its a 50k motor but was running before hand sweetly

my timing was spot on and also my clearences i checked for bent valves and replaced an exhaust valve which was slightly bent before putting heads back on

looking like ill have to take heads back off and check again.

may try abit of oil down the bores tommorw and then try fire up. left me pissed off really

if they are nessoryary are there any bottom end overhaul kits say oversized rings ?

Thanks Dave

[Neosophist]

I'm a little confused (but i've had a few beers) so i'll re-read this tomorrow.

If your cylinders are worn enough to cause low compression you'll need it bored out to the next side up and fit bigger pistons & rings. You can't just fit bigger rings to your standard piston.

You really need to check all 4 cylinders.. especially if you think the gauge is duff, a reading roughly equal over all 4 cylinders should indicate that its good.

Dont' forget that these motors are often a royal pain in the backside to start after the carbs have been emptied etc.

Double check all your timings are right and that your getting a spark with carbs that have got petrol in them before you start tearing it down again.

It's often something simple / overlooked that is usually the cause.. espeically if it was running seemeingly fine before it was messed with....

[venom51]

Low compression can be a simple as carbon build up on the valves and valve seat. Popping off the valve cover and playing wack-a mole while feeding air pressure to the cylinder can clean the valve seats enough to get a better seal. Compression tester will onky tell you so much. A leakdown test would give a you a better idea of what's going on.

[eamer1989]

Y so today I checked compression again with same tester as u can't find a good one from anyone and I can get readings from all cylinders over 100 psi but that tester will never give a true Reading anyhow there's enough compression to fire.

I've just cleaned and rebuilt carbs again tonite, unblocked te 4 little brass holes which u block from ram air also

Any other suggestions ?

[venom51]

Since I have no idea what a stocker makes for compression and I know what my high comp pistons and head work makes can someone give me an average.

[Cammo]

anyhow there's enough compression to fire.

Have you checked for fuel and spark??

Try the basics first.

[eamer1989]

yes sorry didnt i put that down i do have spark and i have fuel in the float chambers, i even tryed abit of brake cleaner to get the old girl to fire, first time i tryed i got abit of popping and flames out exhaust just the usual ofcorse.

i spoke with greame via email and just got a new set of plugs from him, im hoping that will be the issue now, gonna build and put the rads etc back on tommorw and hope for the best if not i have a seruoios issue somewhere


compression side of things im sure bikes are slightly higher than normal petrol cars, saying that the new high comp cars run something like 220 psi which is kinda nutts but i think the bike should be like nearer the 150 mark but dont quote me on this lol

[venom51]

Mine makes 210 with a couple drops of oil, 185 dry.

[eamer1989]

well i rebuilt top end, still standerd bottom end which i didnt touch at all, once its fired up that first time it should be okey after running abit and getting hot, maybe i should of overhauled the bottom end but it was running fine before so no reason it shouldnt now really

will just have to see after new set of plugs in the girl. not sure when they will arrive either so. i do have half an engine spare so if worse comes to worse ill have to spend some more time working.

Grrr damn V4's, they work a treat when running but cause problems when been sat for 3 months, atucaly it was fired up before i stripped for about 30mins of idleing for the first time in a year i belive lol, didnt take to much to start either

will get there in the end just lots of faith