The background on this project is that I bought this Daytona from the original owner's son with 5000 miles on the odometer. It was ridden occasionally until the late 80s when it was parked in a warehouse where it stayed until I bought it. The son thought that it had been parked due to his fathers age and not because there was anything wrong with it, but could not confirm. Based on that information, I decided to clean it up and fix only the parts that obviously needed fixing before trying to start it up. I rebuilt the carbs, replaced the head/exhaust/cover gaskets, and replaced all the rubber parts (intake manifolds, tires, air box pipes, etc.) as they were crumbling. I did not pull the cylinders or split the cases at this point, but did a leak down to verify the engine was still sealed. When I got it all put back together, it started up fine, would idle stably and rev out on the center stand. When I tried to take a lap around the block, I found that it wouldn't take throttle under load...i.e. I could putt around the block, but not under any significant power...the engine would stumble and fall on its face if you applied much throttle. Sooo...I decided to do a complete tear down on the engine. The pictures below are what I found. Substantial blow by is evident on the pistons, which is strange because it passed compression testing. I measured the ring gap and they were symmetrical...top ring on both cylinders was at 0.0350" and second ring on both was 0.030"...WAY above spec. Interested in your assessment on what needs to be replaced. I'm assuming new pistons and bore are in order...should I rebuild the crank too? Is it safe to assume that whatever caused accelerated wear in the cylinder also wore the bearings? Is there a way to check the crank/rod bearing health? Any thoughts or recommendations are welcome.
Also...the pistons look to be different. As far as I know, no one has been inside the engine...would it have come from the factory with different markings between the pistons?
Chris
You can check the crank by rocking the small end, side to side, and see how much it moves. If it is more than 2mm the lower rod bearing is worn out. You could probably use a good ruler to check that and with the crank still in the motor. The piston with the numbers on it is a standard sized oem yamaha piston, I don't know about the other one. Are the pistons the same size? It looks like they are just at the end of their life span. Your carbs may still have a problem if it doesn't that throttle. The crank seals may be problem with that after being parked for 40 years.
(https://i.ibb.co/tLvTFLH/crankcheck.jpg) (https://ibb.co/J3Gwg3c)
As said above the piston on the left in the photo of the two crowns is an OEM factory piston. The other is uncertain. Normally Yamaha OEM pistons will have numbers stamped in the top denoting the size. This is true for stock originals and oversize ones. Do they both say "ART" and "2V0" on the underside inside the skirt? (Actually I think the 2V0 pistons may have been cast with "360" on the underside if I recall correctly.)
Regarding the carbs and your stumble and lack of power, I would make sure you completely disassemble them and thoroughly clean all passages and jets. Be sure to remove the needle jets (emulsion tubes) from their bores and clean them. Also check to make sure the slides are in the correctly (cut-outs facing the rear) and that the enrichener hose is connected between the two carb bodies.