ooheck Posted September 23, 2013 Report Share Posted September 23, 2013 I have two TY175's of similar vintage, one runs perfectly with crisp throttle response an plenty of low down power. The other only has power at high revs and feels like it has an elastic band between the twist grip and carb. The barrel is not marked and they have similar compression. Same fly wheel I have replaced; all the ignition system, new points, condenser, coil, plug. Timing set the same on both bikes, and I've done it 5 times to make sure. reeds to fiber and they have the same opening on the petals (9mm). (came with petals from an 50/80 with 7mm opening). When you suck through the reed block they fully seal shut. all the carb is now back as standard (came with larger carb fitted either 24 or 26mm) Exhaust decoked with no restrictions. This bike has slightly lower gearing than the good one, so can eliminate that. The only thing I haven't compared is the porting in the barrel, but this looks standard as far as I can tell and the piston. The piston has large ports but I can't decide if this will have much effect as there seems to be large and small port pistons for these. Looking at the piston in the barrel the intake transfer port in the piston never closes, but I presume it relies on the reeds closing to stop blow back through the carb. I've completely ran out of ideas other than changing the piston to one with small ports, but am not convinced thats the problem. Any ideas?? Quote Link to comment Share on other sites More sharing options...
thats_a_five Posted September 23, 2013 Report Share Posted September 23, 2013 Crank seals leaking? Air filter clogged? Rubber band hidden inside throttle cable? 1 Quote Link to comment Share on other sites More sharing options...
bestrcpilot Posted September 23, 2013 Report Share Posted September 23, 2013 Ok it could be a lot of things going on. Try the carb and air box off the good one on the bad one. Check the intake rubber for cracks or spilts. If all is good and the exhaust is clear then into the motor. Try changing the heads. The compression ratio on these bikes changes a lot from head to head. While the heads are off check your bored size you may have a 125 jug. A 175 has a stock bore of 66mm. What is the piston to bore Clearance? What is the ring gap? Is there a stuck ring? Is the piston Scored? is the bore good, still round? Quote Link to comment Share on other sites More sharing options...
ooheck Posted September 23, 2013 Author Report Share Posted September 23, 2013 Forgot to mention, have already done crankcase seals and replaced air filter as this was missing when I bought the bike. Swapped carb and intake makes no difference. Its definitely a 175 (66.5mm) with a 0.5 piston. I have a spare 125 top end as well Rings aren't stuck, barrels not scored, Cranking them both over slowly by hand hand has very similar compression Will have a look at swapping heads, but can't remember noticing any difference Where/ how do you measure piston clearance? Quote Link to comment Share on other sites More sharing options...
cvgmmartin Posted September 23, 2013 Report Share Posted September 23, 2013 I assume you swapped the reeds when you switched carbs, so now it would seem the only difference could be either the exhaust system or the barrel and piston porting, although you would have thought a plugged exhaust would affect the high end more than the low end which doesn't seem to be the case. Try switching the exhaust first since it easier, after that it looks like you are left comparing the porting. Can anyone comment whether a weak ignition system could cause poor low speed power yet not appear to affect high speeds? Quote Link to comment Share on other sites More sharing options...
bestrcpilot Posted September 23, 2013 Report Share Posted September 23, 2013 I check the piston with a Micrometer and the cylinder with a bore gauge. And I check the cylinder from the top to bottem. As I have found that the bore at the top and bottem can be good and just below the exhaust port can be a lot out of round. A little work with a die grinder can make a huge inprovement to the way the motor runs. Start with the tranfer ports where the cylinder meets the case. Match the cylinder to the cases and make sure the gasket does not cover any part of the port. The Bridge between the transfer port is thined out and knife edge as much as you can get it. This is at the bottem of the cylinder base. Then clean up the exhaust port and get rite of any casting flash and make the port as close to a mirror finish as you can get it. Then do the intake port the same way. There are lots of vidios on utube for Porting and polishing a two stroke. I run my Ty 175 without a base gasket just a light film of RTV in order to bump the Compression a little and to lower the exhaust port timming a little. As I said earler the stock heads can be all over the place as far as compression and Squish are Concern. I find that getting the squish set to .050 works for my ty. There also videos that expain what the squish is and how to check it and how to set it. Quote Link to comment Share on other sites More sharing options...
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