Well I had a challenge yesterday with the carburetors.
Last week, when I replaced the clutch cable, I had to remove the tank and to do that, of course I had to undo the fuel hoses to the petcocks. Those fuel hoses were beefy aftermarkets and they had hardened quite a bit over the years. When I went to re-attach the right one to the petcock, I struggled quite a bit to get it on, and in the process, I was concerned some old rubber pieces might have gotten into the fuel hose. Nothing bad happened during my test drive though, so I figured all was good.
Then, one day during the week, I left for work (at 5:30 AM, nice and quiet on the LA roads) and after about a mile, the engine started to hesitate badly. I only made it back home with the choke pulled out. I figured I was right after all, and debris from the hose had gotten into the carburetor.
Luckily, the new Yamaha fuel hose I had ordered with Partzilla had finally arrived, so yesterday I set to finally taking off the carburetors, figuring I would test the slide diaphragms, given them a good cleaning and replace some of the parts from the rebuild kits I had gotten from MikesXS a while back. I studied the carb how-to here on the site.
So I did find the two small pieces of old rubber in the right carburetor. The slide diaphragms were in A-ok shape, fortunately. With the bowls off, I figured the only pieces I would replace were the ' valves' - the widgets that close off the fuel suplly when ordered to do so by the floats. Since the bike has only 5,300 miles on it, I am assuming that the needles, springs, jets and everything else that wears, should all be as good as new still. There was no dirt or varnish in the float bowls and I decided not to take off the side covers and leave well enoug alone.
One of the bowl gaskets I replaced, the other one was still in good shape so I left that one in place. The new gasket was quite a bit thinner than the old one but I figured that would not make any noticeable difference. I re-assembled the whole lot, with the new fuel hoses, which went ok. I did get very dirty hands so I decided that, next time, I would make sure to wear some of those latex/nitrile whatever gloves.
I forgot to mention that a previous owner / their mechanic had at some point put 1 inch sections of rubber hose into the breather pipes on the cylinder head, thereby restricting the unburnt air/fuel mixture flow back into the air boxes and back into the engine. I knew these were not stock, as one was longer than the other and they were sort of raggedy-cut. I imagine this was done as a 'tuning' measure, back in the day? The most interesting part about this discovery though, was that one of the hoses had gone soft and had swelled up so much over time (from those unburnt fumes no doubt) that I was barely able to blow any air through it! I had noticed the bike would smell a bit when sitting at a traffic light for example, like an old Beetle without pollution control, and now I was hoping that by removing the blockage, the engine would ' breathe' better and the smell might get less.
So on to the test ride. Idle was greatly improved, super smooth. It was good before, but now clearly better. However, on the road I had a big problem: anything over 3,500 RPMs the bike would hesitate very badly, and refuse to rev much higher. It ran awful.
Oh boy, what have I done?
So this morning, I took everything apart again (went a LOT quicker this second time), and I decided to do two things: (1) swap the old valves I had saved back in, and (2) replace the one old gasket with the new one I still had. I had read the float bowls had to be at the same height, and the slightly thicker old gasket would interfere with that. Although I could not imagine the half millimiter difference would have that much of an impact, I figured that, and the new valves, were the only things I had changed, so that was all I could come up with to try and remedy this newly created problem. I measured the float height with the new valves (24 mm), and then with the old valves re-installed (23mm). Both within spec for these 38s (24 +/- 1 mm).
Everything back together, my hopes were not high, but onto the test ride and boy, what an improvement! The hesitation was completely gone, and the enginge pulled a lot stronger through the entire rev range than before, when it would hit just a bit of a flat spot, around 4,000 RPMs. Now it pulls evenly and strongly, all the way to 7,000. It's a lot faster now, really noticeable.
So moral of the story, for me, is that either the half millimeter difference between the old and new gasket, OR the new valves were not up to the task. Or perhaps I had somehow goofed up on one of the many other small steps in putting things back together, I don't know, but I am not touching the carbs again until I really, really have to.