Gave the old gold bagger a complete tuneup this last weekend. Adjusted the cam chain and valves, cleaned the points, did the timing, removed and cleaned the carbs and re-jetted them for the new pipes (not really new, but new to this bike), etc, etc. It was really nice weather last weekend so I went on a good long ride with TurboGrimace on Saturday and then another on Sunday. Anyway, a couple issues cropped up (don't they always). For starters, oil pressure. I've got a gauge and the idiot light. Cold pressures are about 35-40 at idle and 75+ at cruising. Hot oil pressure is around 18-22 at idle and a good solid 60+ at cruising. Now, ordinarily I'd be fine with 18 pounds of pressure at idle, but it makes the idiot light on the gauge flicker. Is that too low? Idle is set at about 1100, BTW. It's a '71 engine, never rebuilt and atleast 30,000 miles on it (probably more, it's been through a couple speedos). Should I be concerned about anything? Is there a bypass spring in the pump I can shim to get the pressure a little higher?
Secondly, the bike will occasionally (maybe once every 10 minutes) and randomly spit back through the carbs. Just once, and sporadically. Seems to generally be when I cruising at a constant speed. The settings feel pretty good on the bike. It doesn't bog, and it doesn't buck and sputter. Transitions from idle to WOT are relatively smooth, etc. What's up with this backfire? And I've never had one, even one jetted noticeably lean, spit back so loudly through the carbs. It never pops through the pipes, just a really loud cough once in a while from one of the carbs.
Well, here's the skinny on the oil pressure part:
First the running numbers are good. The idle numbers are on the margin, but there's a couple of things that you might need to know:
1. Honda stated in their owner's manual until the K5 that "occasional oil light flicker when the engine is hot at idle is normal". This was due on the earlier bikes to a low-hysteresis oil pressure switch: it opens (light OFF) at 45 PSI, then closes when the pressure hits 25 PSI. After the K5, they added more hysteresis, with it going OFF at 50 PSI and back ON at 15 PSI. Normal hot run PSI with 10w40 is 55-60 at 4000 RPM, with 20w50 it will be 65-75. Idle is around 20 hot.
2. Use of at least 15w base oil (like 15w40 or 15w50) is highly desirable. 10w base oils are pretty thin for these engines. In commuting work, I use 15w50 (wintertime, 10w50), summer either 20w50 or synth 15w50. (No, this is not an oil thread...
).
3. If the engine has been sitting a long time, the seal on the oil pump shaft may have cracked, dropping the pressure about 10 PSI overall. This seal is not listed for the CB750, but the same one is used on the GL1000 Gold Wing, and is available from Honda for that bike (in that oil pump - it looked like they started with the 750's oil pump parts in those engines!). While you're in the pump, replace the O-rings that seal the pump pan: on some K1 engines, Honda forgot to put the dowels in those pump pans, and the O-rings suffer leakage as a result. So, if yours is missing a dowel or two, get some. It can lose another 10 PSI when hot, from that issue. There are also 46x2mm O-rings around the inside of the cover.
Spitback: check for tight intake valve lash, if it's been sitting. One cylinder at .001" or less intake will do that. Also, it might have a bit of rust on the valve stem if that valve sat open a long time, so try some Marvel Mystery Oil in the gas for a couple of tanks (sorry, it will smoke a bit then) to dissolve that rust away and let the valve fully close. Or, take the carbs off, turn the engine until each valve opens, reach in with the dull side of an X-Acto knife blade, and scrape the upper edge of that crust away at the valve stem...
Other spitback: are all air screws set the same? One set lean will do this, too.
