G80 compression.

crawsue · Post by **crawsue** » Thu Nov 01, 2007 8:49 am

Doing my best to absorb all this great information,BUT (!) I don't understand Don's remark about not getting a reading with the rocker box OFF.Surely with the box off the valves must be closed as tight as the springs (or gunge in the seats) will allow,as there are no push rods or rockers in place to hold them open? Or am I missing something?

crawsue · Post by **crawsue** » Thu Nov 01, 2007 12:27 pm

Well another wet day here in Glasgow meant rereading all the advice and the message was loud and clear,don't do things in half measures.An assisted "go" with the compression tester could only register 50 psi max ,so off came the head.Good news first of all,the piston and cylinder bore seem to be good,and squirting some 3 in1 oil onto the piston seems to show that it is not leaking away.Some signs of "blow through" on the head and head gasket adjacent to the exhaust port,but the major problem seems to be the valve seats and faces which are badly pitted.I will have them checked by an expert but there seems to be enough "meat" on both the valve and seats to re-face and grind.The valve guides are good.I had been offered a 2nd hand spring to replace the broken one,but is the consensus to renew valve springs? Thanks for all the input,amazing how much fun you get with old 'bikes---ride 'em all summer, pull them to bits all winter and spend the rest of you time waiting on Don getting out of bed in California!!!

paul knapp · Post by **paul knapp** » Thu Nov 01, 2007 1:52 pm

Crawsue, to perform a compression check, the throttle must be held wide open to allow the cylinder to fill with its full volume of air at atmospheric pressure as the piston sweeps down, ready for the piston to compress it to get a compression reading.
If the rockers are removed, this won't happen because the bl***y inlet valve won't be open to let the bl***y air in

It actually drizzled rain for most of today also..10 mm by my rain gauge...good stuff

Pablo

..Bl***y spellin' mistake!!Edited by - paul knapp on 01 Nov 2007 1:55:38 PM

crawsue · Post by **crawsue** » Thu Nov 01, 2007 5:33 pm

My inept reply regarding the readings with the rocker box off/on will show all correspondents what a "bodger" they are dealing with here!Off course the readings will be affected by the fact the inlet valve ain't letting air in to compress,what a NUMPTY I am. No matter,though,it primed me to remove the head,as ITMA had implored from minute one,and half an hour at my local repairer has shown that the 'head is slightly warped -by about 5/6 thou-adjacent to the exhaust port,so that was probably my biggest compresion problem.But here is the next dilema---both valves and their seats show extensive signs of "pitting"---really sharp "jaggy" holes,almost as though they had been aggresively sand or shot blasted,could this be as a result of unleaded fuel? Like nearly all of these machines ,it is not fitted with an air filter,but this is the West of Scotland,40in rain per year.I have read replies to "newbies" for the past couple of years who always bring up the unleaded question,and have still to hear of any long term problems.The dilema is--for aprox 1750 miles per year ,do I have the head made "unleaded friendly" or merely re-grind in new valves and start using an additive? Questions,questions!

itma · Post by **itma** » Thu Nov 01, 2007 7:34 pm

I bet you wished you had never asked now?

I doubt myself if this is the result of using unleaded, more likely through other reasons, weak valve springs, tight tappets, even lack of use, having it parked with the valves part open for instance.

Unleaded conversion involves only the exhaust valve seat.
For myself , considering the other problems you have , I would box it up and send it to the Cylinder Head Shop, not cheap, but it will be 100% when you get it back.

http://www.cylinderheadshop.co.uk/Edited by - itma on 01 Nov 2007 7:35:19 PM

crawsue · Post by **crawsue** » Thu Nov 01, 2007 8:14 pm

Cheers ,Ken,I have sent them an e-mail for a quote.It seems that Glasgow has a dearth of old fashioned "engine shops" especially ones that have any experience with air -cooled stuff.My local 'bike shop,where the owner/engineer has over 50 years of motorcycle experience,unfortuneately doesn't have a tool big enough for these big old valve seats,and any local garage just throws any stuff that needs fixing in the skip and replaces with new!I'm glad I took your advice (belatedly !) and had the 'head off ,as it would have niggled at me having read other replies inferring standing on kickstarts etc. etc.!!

itma · Post by **itma** » Thu Nov 01, 2007 8:34 pm

I have confidence in the guy

you may need to sit down when the reply comes, but he knows his stuff and has the kit .
He made a nice job of sorting out a loose exh guide, on a head which is not the easiest to work on.

PS I see from the very bottom of the site that he only answers emails on SaturdaysEdited by - itma on 01 Nov 2007 9:36:55 PM

Don Madden · Post by **Don Madden** » Thu Nov 01, 2007 9:43 pm

Can I go back to bed now?
Cheers, Don.

crawsue · Post by **crawsue** » Thu Nov 01, 2007 10:02 pm

Thought you were just getting up? !!!

crawsue · Post by **crawsue** » Sat Nov 03, 2007 7:43 pm

I think I have found a local "shop" that will re-cut my valve seats,letting them see the head on Monday. My local "man" has trued-up my 'head on a plate ,there was a 5 thou "kink" just above the exhaust port.Having "emeried" (?) the 'head true is this going to give me problems with piston/valve clearance? Should I take a bit off the "lip" of the barrel? What is the feeling on valve springs? Does an old engine with this level of compression benefit from fitting new ones? Questions,questions !!!