One step forward ...

[Myumi-2]

Im restoring a 1972 CB750 after it spent twenty years in a shed.  It has been my daily rider for the past couple of months and Ive been slowly repairing/improving different parts of the bike after work each day.

I knew that its performance was below par and I was experiencing some astoundingly poor fuel economy 160kms (100 miles) per tank, around town but it is a 10 minute ride to work at 60km/hr (40miles/hr).  The plugs also told me it was running rich.  The problems I would describe as inconsistent power in the bottom 1/8th to 1/4 of the throttle (i.e. very jittery turning a corner) and lacked power (woudl wash out) at the top end (i.e. accelerating up hill toward 100km/hr or 70miles/hr).  

The bike has the 657B carbs, with Lynx maxlow dual pods and a 4-1 exhaust.  I do not have an original airbox, which would have helped. The stock main jet is a 105 however, it was fitted with a 120 main jet. The original clip position was #2 where #1 is the bottom position and #5 the top.  I did my research mostly via http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm and http://www.salocal.com/sohc/tech/carb/carb.htm

Then removed the carbs and adjusted the needle clip up. Outcome was improved, but not ideal in the first 1/8, terrible mid range and unnoticeable change at the top end.  The next night I pulled the carbs out again, moved it up another notch.  Outcome was a much better first 1/8 an acceptable mid range and flooding at the top end of the throttle however, the throttle was sticking open.  I assumed that I'd streched the spring return.   So I pulled them out again to discover that the large ring that screws onto the top of the carbs was crossed threaded and therefore was not allowing a return.  I only discovered this after a visit to the Honda dealer to discover the carb spring is no longer available and then found a suitbale replacement at the hardware store.

By this time Ive spent every night for a week pulling the carbs out of my bike and it is the weekend.  So I pulled the carbs out again, moved the clip up to the top position.  Whilst I had it stripped down I decided to give it a tune.  checked the valves, replaced the oil filter, plugs, timing etc.  

Then whilst replacing the sump plug I gave it one more tighten for good luck and head a "clink".  I backed it off, had a look, could not see anthing wrong.  replaced everything and filled it up with oil and almost immediatly there is oil running out across the floor.  

On closer inspection I now have a hair line crack in the sump that runs from the bottom of the thread up about 15mm (an inch).  

So I just put more oil in and took it for a ride.

Now I have a nice smooth first 1/8th, good power through the middle.  I can still get it to flood at the top end if Im pulling heavily up a hill and drop the throttle.  I seem to have also acquired a ping an deceleration.  But I'll live with this for now (I'll re-check the valves).  Now I know the carbs inside out I think Ill re-jet with a 110 main and go through this process again.  I wait a week or two then check the plugs before making this decision.

So tonight I'm pulling the sump off my bike and will make a temporary fix with some epoxy until I can source a new/used sump.  I might even have to epoxy the sump plug in, just to make sure I get a proper seal?  

Finally, Ill just say it is a very demeaning experience to park your bike at work and put a kitty litter tray underneath it to catch some brand new 30w 50.  I don't think I could do that two days in a row.  It is perhaps equally diappointing to work on your bike every night for a week only to end up one step back.

Ahhh, the joy
Tim

[Re-run]

Well you are not off ot the best start but the bright side is most of it is not your fault!
Anyways, lets start with the oil pan. Those threads crack easy. It is pretty cheap aluminum. Ebay or a cycle junkyard if you have one is what you should do. Do NOT epoxy the bolt in as when you change oil, you need to remove this bolt. You CAN jb weld the outside of the crack and that will help some. See these bolts use crush washers to seal. After a couple times of use, they need to be replaced otherwise a person tightens too much and, well you know already.

The carbs are a pain since the previous owner had no clue what he was doing. I would strongly suggest going back to the airbox, also an ebay thing, and back to the 105 jets. If you do not want to do that yet, then you have some work to do.

I would suggest you pull the carbs and fully clean everything, think " clean enough to eat off of."
Pull the jet out too and clean them. Very important this step is! If you pull the slides out and adjust anything there, us a smooth round object to make sure the slides have the same opening distance when closed. Called bench syncing. Do this on the cutout side, side that curves up. Should be on the engine side if I remember correctly.
Lets start at the idle mix.
Put the clip in the middle position. Set the air screw to 1.5 turns from lightly seated(I think).
Clean your plugs and see if she starts. It always takes a little here to find the sweet spot for starting so you might want to use the kicker so you do not drain the battery.
Once the engine is warmed up good, pull the plugs and clean them again quickly. Put them back in and start and idle for a few minutes. You should aim a fan at the engine. After a few minutes, check the plugs. If they are black are very dark, turn the air screw out. now the amount youturn out depends on how black they are but a half turn should be ok. Once they are all a medium tan, move to the mains.

Do the same thing here except get on some open road and floor it and keep it floored and go through the gears. Keep the rpms up and hit the kill switch and pull the clutch right away and stop. Pull plugs and see how dark they are. If pretty dark, you will probably be best off getting a smaller jet but you can drop the needle or raise the clip, same thing.

This is all a start.