Performance difference between early and late T80's

I own two T80's - one of 1991 vintage with 27k miles and another of 1995 vintage with only 9k miles. The engines in both of these had fairly lacklustre performance, a top speed of about 45mph on the flat with no wind, and not very spectacular accelleration.

However I have two spare engines, one from a 1984 (B) and another from an 1986 (C) model. I do not know the mileage of these engines but I imagine 30k or more. If I fit either of these engines to one of my bikes with no other changes - i.e. same carb, same exhaust, same CDI, etc, the top speed in the exact same conditions increases to 50+ and accelleration, particularly higher up the rev range is noticeably better.

As far as I can tell, the newer engines are in very good condition - i.e. have good compression and I've checked the cam timing, camchain tension and valve clearances to be correct. So I'm wondering what Yamaha changed on later models to decrease the performance. Anyone know?

One of the older engines is a bit rattly and produces a little blue smoke on cold mornings, but ironically this is the one that gives the highest top speed!

As far as I can see there is no change in spec of engines, I must admit I have not noticed any difference in performance when I have changed engines in same bike. I suppose its just that some engines perform a bit better than others, & as you say with higher mileage may have loosened up. The only other thing that can effect performance is the exhaust system, different makes/patterns may have an effect on performance but you say you are using the same one, so that rules that out. The original equipment exhaust fitted to the T50 although looking identical to the T80 was restricted as I found when I put one on a T80 & it struggled to do 35 mph. Cutting it open I found a baffle restriction at the front end, once cut out & silencer welded up again the bike was back to its normal performance. Must admit I never take my bike over 45 mph & usually cruse at 40mph unless in a 30 zone. I find this gives around 120 mpg but if you go faster this diminishes down to nearer 100 mpg. As most of my journeys are within 30 & 40 mph limits its not a problem.

Yes it's odd isn't it. Top speed is important to me, as many of my regular journeys entail using a stretch of the A14! So I'm using the 1986 engines in my 1991 bike. I'm amazed by how much abuse these engines can tolerate.

teapot wrote:Yes it's odd isn't it. Top speed is important to me, as many of my regular journeys entail using a stretch of the A14! So I'm using the 1986 engines in my 1991 bike. I'm amazed by how much abuse these engines can tolerate.

Talk about abuse, have you seen this on Youtube https://www.youtube.com/watch?v=-9eKvHT_rGc

I have noticed this my b reg t80 is by far the fastest over my 1990 ,,91 when warm my b reg will show 60 miles a hour down a longhill of the a27 at brighton don't think its any faster up hill may be a tad faster on the flat tho 50 ish some times with the wind

My 1984 model got up to about 47-48mph today, fastest it would go. I am though a big man. Bike has almost 13,000 miles on it.

ive noticed my c reg t80 is faster then my old g reg bike to an its faster then my mates t80 must be the old girls love it more lol