SOHC.co.uk Forums > CB750

Pressure gage tripping after 15min ride

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totty:
It's not the oil breaking down as it still did it with fresh. I don't think it's overheating at those temps, I also don't think it can be oil dilution with fuel, so if the mixtures out that may be a separate problem. It sounds like worn bearing or pump, or an inaccurate gauge.
There's a chart that I don't have to hand which shows oil viscosities against ambient temperature,  I'd try the thickest that's ok for the current temperature range and see if it made a difference.

When are you checking the plugs? Remember it could be really lean at one throttle opening and really rich at another, it could also be darkening them with a bit of burnt oil.

mgy66:
Yep,  i have ordered draper oil dainage tank and will look at oil chart. Hope it is not the bearings as that can be misery ahead. Is it easy  access to replace inacurate gauge, i.e. does it requires opening the engine?

Good point on the dark plugs. The colour is rather mat than shiny so thinking more toward rich mixture than burnt oil. Meanwhile, you are right, difficult to say where on the curve throttle is mixture out. All i can say is that i use the bike in london so rarely more than 30mph, loads of stops and goes and half an hour commute. I usually check the plugs after a commute.

What about carbs levels, can it affect the mixture on the lower part of the throttle curve? I have also noticed idle can increase to 3000 rpm without touching the throttle, forcing me to adjust idle screw down at red lights and then up when it revs go down to avoid stalling at the next red light.

Painful... ::)

totty:
Wondering idle can be an air leak at the inlet or carbs, this will also affect the mixture so it's worth looking at before looking at float heights.
I don't know the 750, but on the 550 the pressure switch is easily accessible, it screws into the top of the pump.

mgy66:
Ok, so this may be of interest for oil visco. I will probably go for 15w40 which i think is easily to source.

http://www.ratwell.com/technical/OilSelection.html

Here attached two of the 4 plugs...

Re air entries, i have new inlet silicons from the airbox and a new air filter. Can it come from carbs top gaskets where the needle are being pulled up?

Thanks

totty:
Spray a bit of something flammable (wd40) will do on the carbs and inlets a small area at a time listening for a change to idle. If it only wonders when warmed up them it'll need to be warmed up for the test. If the idle changes you've found a leak.
I'm not sure if a leak at the carb tops would be that noticeable, the problem is generally on the engine side of the carbs.

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