Two stroke problems
This is a typical enquiry we get from time to time. We will help in any way we can. With years of two stroke tuning for customers as well as my own bikes I am able to draw on more than 30 years of knowledge.
Phil wrote
I came across your business when looking on the web for someone who can competently rebore two stroke cylinders. Many can but I’ve seen too many horror stories of where it goes wrong! Reading through your site gives me the confidence that I’m approaching the right company to carry out work. We’re looking at a rebore on a Suzuki TC200 “twin cylinder” from 1968. The cylinders have at some time prior to my acquisition and renovation had a rebore as the pistons are 1 stove size and marked as 0.5. I did measurements with pistons and rings prior to a rebuild and tolerances were in spec but strongly suggestive of wear. Now running and back on the road I’ve top-end noise and a serious loss of compression on any incline so am fairly certain of a need for a rebore and over size piston and rings. I will have a full set of NOS 3rd oversize piston and rings “1.5” soon, have all the technical details and tolerances in an original manual and wonder if you could rebore the cylinders to suit and what the cost might be. The cylinders are sleeved aluminium and I’ve gone for 3rd oversize to allow for any serious corrective machining as I’ve no history of the use/abuse that the bike has had prior to my acquisition. I hope you’re able to help. Cheers.
Terry replied.
Hi Phil we will have absolutely no problem doing them for you and we also do not leave razor sharp port edges as all other engine reconditoners so. Present turn around is up to one week. To bore and hone and radius ports is 50 per barrel. We will need a piston ideally. Kind regards.
-Deliver to Phil was one week late but would have been sooner as I needed some new honing stones-
Phil said.
Many thanks, they arrived this morning. No doubt the bank holiday weekend slowed delivery of the parcel. I’ll let you know how the engine runs/sounds once I have it built.
Terry replied.
Brilliant to hear that they arrived as we never had an incident yet of anything going astray. Remember to run them in on your fantastic bike. I just bought a 1965 registered Yamaha YL1 that I cant wait to get restored. Any feedback is much appreciated. Cheers.
Phil replied
Help needed! I got around to reassembling the engine, double checked everything – rings right way up, piston right way around, new small end bearings, pins, ect.- and then kicked it over (ignition off) and all ok. That was it for 2/3 months but getting around to firing it up properly I find myself getting nowhere and just flooding the engine – so fuel getting through and sparks all present and correct, I checked I’ve tested compression and kicking it over I’ve got what I think is bugger all on each cylinder – at best about 30 PSI after dozen kicks. The pressure builds up on successive kicks to a max of 30 PSI. Surely there should be more as the Clymer manual quotes 123 – 131 PSI at 1000 RPM . Am I right in expecting have more compression prior to the engine firing? Its only the top end that has been touched and all was running fine on the old barrels/pistons; albeit noisy/slappy, hence the rebore, any thoughts/help much appreciated. Cheers.
Terry wrote.
Hi Phil it is important that you do the pressure test with the petrol on and the throttle full open. If you do not get above 110 I doubt it will fire the fuel. The next thing to do for the second test is leave the fuel off and put about a teaspoon full of heavy oil such as hear oil into the cylinder you want to pressure test and see what reading you get . If it is still too low you will have to put the spark plugs back in and get a spray bottle with fairy liquid and water mixed. Spray the bottom of each barrel and turn the engine over slowly to complete about one revolution then the next thing is to spray the cylinder to barrel joint. What we are doing is trying to visually find a leak. If there are no leaks then I would suspect the bottom end seals. These are common tests that I do to my racing 2 stroke.
Phil said
Terry many thanks for such a prompt and detailed reply. It’s always food to get advice form an expert. Forgot to say I’d fitted new gaskets and everything torqued down as per specification but who knows. I’ll do the pressure test with some heavy oil in the cylinders first and then double check for gasket leaks After that it’s going to be a rebuild of the bottom end seals although where I’ll get these from I’m not sure as many spares for the TC200 are effectively obsolete given it’s relative rarity. Hey ho, such are the pleasures of owning a classic motorcycle. Cheers.
Terry said.
Almost all oil seals are available at bearing supplies as long as you give them a sample to measure. At least they will all be metric sizes. Cheers.
Phil said
Thanks for the advice. I’ll let you know what I find when I so the checks in the next couple of days. Cheers.
Phil said
Apologies for not getting back to you sooner with an update re the on going compression issue with the Suzuki TC200 after its rebore by your good selves. I subsequently had the crank professionally rebuilt with new bearing and seals. Having fitted, replaced all gaskets and torqueing down correctly I’ve got better compression by still not very good, With 5ml of SAE90 oil in each cylinder , air box filter fitted and throttle wide open (no fuel on) the best can get on either cylinder is 60 PSI. I appreciate I’ll not get what it should be with the engine running (120+PSI) but I’m at a loss to why it’s so low. Any further thoughts you might have will be much appreciated. Cheers.
Terry wrote
Hi Phil having raced and tuned my own bikes as well as customers racers it sounds like crankcase compression could be the issue. I have seen seals fitted the wrong way round as well as pistons. Aftermarket pistons can have the piston ring dowels in the wrong position. They all cause low PSI in the top end. Aftermarket top end gaskets can really lower compression . We rebuild and manufacture special cranks for all types of races and road machines. Low crankcase compression causes low pressure fuel transfer phases thus impairing low combustion pressure. These are some thoughts to ponder on. Cheers Terry.
Phil wrote.
After stripping and rebuiding the engine yet again the best I could get was 50/60 PSI in either cylinder, so no chance of it ever firing up even with easy start & aspen pre-mix. By chance I bought a box of bits late last year for a Suzuki T200 (exactly same engine as the TC) as a winter project – in fact it was six boxes , two frames, one and a half engines, two tanks and so on. None of it had been together since around 1985 as shown as shown of one of the news papers stuffed into one of the barrels! I’ve now rebuilt a T200 out of the best bits, fitting a set of original barrels (not rebored but clearly attacked by someone in the past with a file around the ports) and new standard pistons and rings which I found amongst the collection of bits. Getting 95/100 PSI compression I squirted in some easy start and using aspen pre-mix it started first time, Hurrah. I then put these barrels on the T200 and got 90/95 psi and with easy start ect. It started first time. Putting the +1.5 barrels and piston onto the T200 I’m back to 60/70 PSI so no starting. Looking like the problem now lies with the barrels restored by yourself and/or the piston/rings I fitted from the box of bits a set of well used +1.0 barrels and pistons (having first cleaned them up) to the T200, got 90/95 PSI and it fired up again first thing this morning. So I guess the conclusion I’m coming to is that the crankcase compression is not an issue. Having invested in getting the original TC barrels rebored by yourselves I’m reluctant to have to accept that I’m not going to be able to use them but at present am disinclined to fork out on another set of +1.5 pistons and rings on the off chance that the fault lies there. I could be chasing my tail here and getting no further forward. If you have any further thoughts on this matter I’d appreciate hearing back from you, cheers.
Terry wrote.
Lets not get too technical and look at the basics facts. If the barrel is worn by .001 thousandths of an inch this will equate to plus .003 on top of the standard ring gap clearance when using new rings. Like if the top ring gap as standard .004 it will be .008 if you fitted new rings in a .001 worn barrel. The latest literature tells me your engine will tolerate .005 of clearance on top rings, so it looks like you need some barrel work. Cheers terry.
Phil said
Terry, if you want to lean something new ask an expert! As you did the barrel work to +1.5 OS then we’ll assume that it’s just that. My +1.5 pistons are a very good fit but the ring end gaps are way out. My manual has 0.04 inch as a limit on end gap so even though the rings were got with the pistons as a set they are clearly pants. No wonder I have so little compression. I should have measured sooner but if you buy +1.5 that’s what you expect to get. I will source a new set of rings at +1.5 and see what transpires, cheers.
Terry said
Paul you can of course use 1.6 or 1.7 oversize rings and gap them to suit but it does take a keen eye and a steady hand and some food diamond needle files. Cheers.
