bob654 Posted October 27, 2022 Posted October 27, 2022 Hello again. You guys have helped me a lot in the past and I did think I was around finished with the engine, on this bike,but. Suzuki GS750 24,000. miles, original pistons. I'm using oil, using around 30ml over 50 miles. Blue smoke on start up. History. Head skimmed last year, it was warped. ground in valves. new springs Finally got her set up, and running well. Replaced seals on ; clutch push rod gear change sender unit cam chain tensioner. No more leaks. I did think about a leak down test but I will be taking the head off so is there any point? When the head was skimmed, the barrels looked great, A1. ( mistake no. 1 ) I didnt change the rings. I did change the valve stem seals. How do I proceed? I will put new valve stem seals on again. Belt and braces. ( this time). Q.1. Is it worth changing the rings only? Q.2. Should I, change the pistons, and rings, and re-hone the barrels? Finally ( for the moment ?), Will you recommend manufactures of pistons and rings please? Quote
TonyGee Posted October 27, 2022 Posted October 27, 2022 usually oil burning on start up is a sign of stem seals !!! did you use good quality ones ? i cant see the valve guides being worn at that mileage but worth checking. if you are pulling the top end off then worth checking the ring gaps. 1 Quote
wraith Posted October 27, 2022 Posted October 27, 2022 As @TonyGee says stem seals, check the plugs and see if it's one cylinder or all, but if you're losing oil when riding and not just at start up could be rings or one cylinder. Q. Does it smoke all the time? Do a compression test etc. It maybe a stuck ring, if the bores are good you could give it a quick hone and replace the rings, or just a de glaze hone and not change the rings. Or get some 850 barrels and pistons 1 Quote
Gixer1460 Posted October 27, 2022 Posted October 27, 2022 One of my daily's used to puff blue smoke on cold start-up, every day, never worried about it, it was just annoying. History of bike - was first owned by an old boy who did little miles, neighbour bought it to run up and down motorway for work and then sold it to me after about a year. At that time, it was 15 yrs old with 7k miles up from new! Leak down showed rings gone but at that mileage certainly weren't worn out so I had the rings changed and bore deglazed. It was like a new bike again and i'd guess the old boy never really loaded up the engine so rings never bedded in, and the bores glazed over so the rings would never seal. Sometimes the obvious isn't the problem - a leak down will point to the problem area and is a good diagnostic tool if available! 2 Quote
bob654 Posted October 27, 2022 Author Posted October 27, 2022 1 hour ago, wraith said: As @TonyGee says stem seals, check the plugs and see if it's one cylinder or all, but if you're losing oil when riding and not just at start up could be rings or one cylinder. Q. Does it smoke all the time? Do a compression test etc. It maybe a stuck ring, if the bores are good you could give it a quick hone and replace the rings, or just a de glaze hone and not change the rings. Or get some 850 barrels and pistons May be a smart move. Same head (2valve) and carbs? Quote
wraith Posted October 27, 2022 Posted October 27, 2022 I've put the gs850 (2v) top end on the gs750 but I used the 850 head to get the CV carbs but I can't see a problem using the 750 head and carbs, probably give you a bit more combustion. You will need to tweak the carbs for the bigger cc. 1 Quote
bob654 Posted October 28, 2022 Author Posted October 28, 2022 (edited) Dont know what to do next, does the last pick show to large a ring gap ( sorry thought pic was better,gap is around 3mm when squeezed in. The compression figures were obtained from a hot engine at "full throttle". Bore looks clean.Sending off today for bore measuring tool. The black dot on the bore is just a spec of dirt. Edited October 28, 2022 by bob654 Quote
TonyGee Posted October 28, 2022 Posted October 28, 2022 not healthy you put the ring in the bore to check the ring gap. 1 Quote
Gixer1460 Posted October 28, 2022 Posted October 28, 2022 If that is the ring gap in the bore - oh dear, i'm not surprised its drinking oil. I'd guess in bore ring gaps should be 0.1 - 0.2mm with service limit x3 that? The amount of oil on the pistons and plugs is telling! Bore doesn't look bad but it is worn enough that there is no cross hatch hone marks visible . . . . . end of life IMO. 1 Quote
bob654 Posted October 28, 2022 Author Posted October 28, 2022 I was so peed off with the compression figures I did not do the leak down ( not intentionally), I just started taking it apart. I already know all the valve specs are within tolerance and will rebed the valves and reseal the stems on rebuilding. The pistons have not been cleaned they are coated lightly with oil. HUH ! Yeah, I guessed the gaps looked big ( on the pistons) . The bores are great and pistons clean. I will now take it to the engineer to have the bores measured, its £30.00 for a tool and I dont think the shop would charge much more. Then they can hone or rebore as required, and I know its good. Worried about them not being round. Thanks. Quote
TonyGee Posted October 28, 2022 Posted October 28, 2022 its not the end of the world bob, the only down side is the expense Quote
gs7_11 Posted October 31, 2022 Posted October 31, 2022 Bob, Cruzin Image do piston sets, 0.5mm and 1.0mm oversize. USD140.00. I used them in my 1000S with no problems and have a big bore kit for my GS425 waiting to go in. Precision Engine Services in Elgin will do the rebore, was £50 a hole when I got my 1000S done. You don't want to be taking it to bits again, do you? Quote
bob654 Posted October 31, 2022 Author Posted October 31, 2022 5 hours ago, gs7_11 said: Bob, Cruzin Image do piston sets, 0.5mm and 1.0mm oversize. USD140.00. I used them in my 1000S with no problems and have a big bore kit for my GS425 waiting to go in. Precision Engine Services in Elgin will do the rebore, was £50 a hole when I got my 1000S done. You don't want to be taking it to bits again, do you? Thanks mate very helpful I will check all this out. Its not that I mind taking it apart, Its that I mind not getting it right, and spending money, equally. Quote
bob654 Posted November 2, 2022 Author Posted November 2, 2022 So just a quick update, Went to Inverness to the machine shop, cos I used them before, The barrels are oval front to back, and enlarged bottom to top. The pistons are toast too. So its a rebore then. Just going to stick to 750cc cos the head has been skimmed previously. Thanks for the advice. Any opinions on IMD Pistons? Quote
bob654 Posted November 5, 2022 Author Posted November 5, 2022 Another thought! Do I look at the crank,rods ,bearings, the seals on the ends of the crank are fine? Quote
gs7_11 Posted November 5, 2022 Posted November 5, 2022 The bottom ends and gearboxes are tough, so unless you're having any problems, there's no need to strip it if all the seals are OK. Check the rods for side play, and small end wear. Take the sump off, clean it and check for errant metal. 2 Quote
bob654 Posted May 4, 2023 Author Posted May 4, 2023 160 miles in from the rebuild, things are going good. Keeping the revs down and trying to pile on the miles, looking to do and oil and filter change soon. Just wanted to say thank you ,to all concerned, again for the help, advice, and support. Thank you. 2 Quote
DAZ Posted May 4, 2023 Posted May 4, 2023 Try to use a bit of engine braking as well ,down a long hill say to create a reverse load if you like, it'll help bed the rings in as well as varying the load and the gears,you don't want to thrash it but don't baby it to much either 1 Quote
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