This has been a 3 week project that is sadly making me want to sell my trusty bike.
I have an 82 CB650 with 14,000 miles, that I recently replaced the head gasket, and cleaned the head and valves. After the replacement the bike ran great and I decided that since to the best of my knowledge the carbs had never been rebuilt, if I did that it would run even better. So I ordered 4 carb kits from carbkit capital, and with a buddy that knows a great deal about carbs we tore into them. The bike is completely stock other than the air box cover is removed and the rear most baffles are taken out.
Inside there wasn't anything too unexpected other than they really weren't that gummed up. There was a fair amount of rust and the slides were dirty, but nothing completely plugged as I expected. We put the carb kits in without any problems and the numbers on the jets matched the numbers on the replacements. When we got it all back together it idled great, and revved up no problem while in neutral. When I took it out on the road however, it popped and snorted when I rolled hard on the throttle up until 6k where it rocketed to redline, making me think it was a rich mixture problem. The top speed was down to 70 from 90 previously.
As I have read in many other posts and like them, it has progressively less acceleration the higher gear I'm in. We thought the rotten vacuum tees could have something to do with it, so I came up with a solution using some half inch fuel line with holes drilled crosswise in it. I put the carbs back on and it ran a little different but not much better. So we tore them down again, went through each one and checked to see if we missed anything, and again nothing to be seen. Then in another 5 removals of the carbs, I changed the stock parts back one at a time to see if there was something I couldn't see but was different about the jets. Again nothing made any difference. Tried the air box cover on, then with no filter and again no difference at all.
I took it for a 20 mile ride with the new kits all back in, hoping something would suddenly remedy the situation. Unfortunately that wasn't the case. When I got back I decided to look at the plugs to see if it was lean or a rich problem for sure. Thats when I found the problem, cylinder 4 was black and sooty but dry and 1-3 where a nice healthy color. So I changed the plugs and went out to see if unhooking the plug wire made a difference while cruising. Shockingly it made no difference what so ever other than in the power band I was getting from 6-10k. When I had the throttle at 1/4 or less the cylinder would run fine, but when it was in the higher rpms it was popping and misfiring.
This lead me to think it might be a compression issue, although that should have been impacted from the carb rebuild, but possibly from my valve job. When I checked compression, it was 125 across all 4 cylinders, which is low per spec, but it ran great a week ago so I figure its ok at that compression. I added oil to the cylinders and the compression went up a lot but how could the rings be worn that bad in them all of them is what I kept thinking.
Then I checked the coils, swapping 1 and 4 wires and again no difference, then again why would there be? I hadn't even unhooked the plug wires during the carb work.
Thats where I stopped, I have called everyone I know who has worked on engines or bikes and nobody has any ideas. The CV carbs make it so I can't just lean that one cylinder out and maybe remedy the problem. The only thing I can think of that I haven't tried is putting a hotter plug in and see if it runs better, but that isn't a long term remedy anyways. I even switching the internals between carb 3 and 4 to see if the problem transfered and it didn't.
Any input would be great, I think I listed everything I tried, but maybe there is something obvious I'm missing. Next time I'm sticking to if it ain't broke, don't fix it.
Thanks!
