After going through a few tanks of gas, I became worried by my 20-24mpg. I thought it might just be due to the fact that if I left my petcock on, I'd overflow in one of my carbs, leaving a large puddle under the bike.
I started things off by checking float height and setting it to 22mm using a homemade credit card gauge. I blasted the jets with carb cleaner, cleaned the bowls, cleaned the float valves, and changed the bowl and drain plug gaskets. The overflow issue pretty much went away, but my mileage didn't get much better. This did however reduce a power lull from 3.5-5.0krpm (by a fair margin!).
Next thing was to sync the carbs and set the mixture, but before that, I'd have to get everything else in tip top shape:
- Set tappet clearance to 0.003" for intake and exhaust (per HondaMan's recommendation). This made a big difference on engine noise from idle to 2500rpm, plus really smoothed out the power band over that same range. Highly recommended!
- Set chain tension by loosening the lock-nut with the engine on, giving the screw a very light counterclockwise turn until no more noise, then locked the jam nut again. This made a HUGE difference on engine noise, at pretty much all rpm! Very nice. I'd like to try some of the other techniques you've mentioned to see if I can get it down even better.
- Checked my plug gap again (still 0.026"...yay!)
- Check compression while in there. Hot/dry: 108-112-112-119, and hot/wet: 124-110-130-131 (I didn't apply oil very consistently, so I think we should just ignore that). Also, during the electrical diet I unplugged my starter and never plugged it back in, so this is after 50 kicks by leg, using an automotive style compression tester.
- Set idle to 1100rpm. It can maintain idle below that just fine, but it's a bit rattly down there, and I just didn't like that!
- Checked for vacuum leak by spraying brake fluid on/near each intake tube, but found none. I was surprised by this (simply because the rubber doesn't look 100% brand new), so I tried using a propane torch (not ignited, of course) but I couldn't detect any changes in idle. I did this both while cold and hot.
- Sync'd carbs using a homemade manometer, adjusting the idle to maintain around 1100 rpm. If you need to do this ever, and don't currently own 4-cylinder a gauge type sync tool, just buy one. They're like $60 on amazon and include restrictor valves to tune out the pulsations. My homemade works alright, and i did a good job building it, but it cost me $40 (after also buying the $10 adapters that come with the kit) and added a bunch of time trying to prevent the ATF from getting sucked up too high. Mine did include a DIY lowpass filter using the foam from grey paint brushes, crammed inside the vinyl tubing, sandwiched between two inline barbed fittings, so you could have probably gotten away with $20-$30, but it's just not worth it to me. But it works, see the results in the pictures below.
- Set idle mixture screw by screwing them lightly all the way in, then turning them out by 1.5 turns.
And that's were I sit today. I'd like to go through a tank of fuel before I do anything else. Plus, I've got a squeaky/sticky front brake and a slow leaking dripping down my brake lever, so it's time to get on those.
