SOHC.co.uk Forums > CB750
Progress on the 750
McCabe-Thiele (Ted):
Brilliant news - I guess even experienced rebuilders are relieved when it starts as it should.
Johnny4428:
Indeed Ted! It’s certainly not a given that everything is going to be right first time. I’m not a mechanic, just an enthusiast. I was well impressed with the way this bike burst into life compared with the 550 after rebuild. Even now unless you are running every day it needs a bit of coaxing to start and warm up but great on a descent run. Must be down to carbs. I am going to raise the needles one notch in the 550 just for interest before spring time run. But defo looking forward to getting out there on the 750. I wanted one when I was 17, so I’ve waited a while. Just a small note on oil pressure. I tried and better tried to raise oil pressure without starting be blipping the starter button and also kicking over in bursts of 50, also putting bike on side stand and squirting a dose of oil into oil gallery port to rear of cylinders, but to no avail. After reading much on the forum across the pond, I was given the encouragement to try and start the engine and shutting it down if there was still no oil pressure. I also poured 0.5 litre oil into rocker cover. Maybe all my prior purging helped?
K2-K6:
The initial oil feed is affected by air locking in the oil pump head ordinarily with these engine, especially if it's been evacuated from sitting or stripping the engine.
All oil has to pass through the filter housing to get to the galleries, and onward distribution. If the route out from the pump head is effectively sealed from oil used to assemble (or complete drain down from sitting unused for long periods) then the pump is reluctant to fully prime and push the oil against an ongoing fairly well airlocked distribution.
If you remove the oil filter (complete housing) and crank with the spark plugs removed, then you'll see the oil come out of the feed tube into the filter location (no air pressure to work against lets the pump head prime quickly) as soon as you see that, then replace filter housing and then it won't take long for the fully primed pump head to complete the rest.
K2-K6:
"Even now unless you are running every day it needs a bit of coaxing to start and warm up but great on a descent run. Must be down to carbs. I am going to raise the needles one notch in the 550 just for interest before spring time run."
If you want to try an easy bias (without dismantling the carbs) to assess it a small amount richer, then use the idle airscrew and set them all 1/4 further inward (so if 1 1/2 out, go to 1 1/4) which will just bring up the low speed running mixture faster and may give you the effect you need to stop it feeling so "cold blooded" initially.
The idle jets on these bikes generally contribute about 1/3 of total jetting demand, and so influence the overall fuel strategy quite a fair way up in throttle opening.
Using the above adjustment won't alter the overall jetting, but it will bring the mixture up on a steeper response curve (vs vacuum curve) and change the way it runs.
Virtually free time wise to try it too.
K2-K6:
Just a note, if the "airscrews" are on the filter side of carburettor body they work as above description.
If they are on engine side of carburettor body they generally modulate fuel rather than air, and so work the opposite rotation to get the same mixture effect.
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