SOHC.co.uk Forums > CB350/400

Really bad cold starting. Ign or fuel?

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Scottish Badger:
Oddjob, caps are new, fitted as part of the full service.....

Gareth, levels were all checked when the carbs were apart, all seemed good.

TrickyMicky:
Just thinking outside the box for a moment, it would seem that most mechanical/electrical items have been checked, how about trying a supply of fresh, new petrol. The stuff that has been sitting in the tank, (for how long?), may well be contaminated, not unknown. If you have been using this wonderful new E10 fuel, the high ethanol content is very hydroscopic, and a significant water content could make it a bitch for cold starting. I only use Shell V-Power (E5) whenever possible.  Good luck, Mike.
     

Scottish Badger:
Hi Mike, it's being fed new, fresh super-unleaded shell fuel at present (which in the north of Scotland has no ethanol), from a separate fuel feed container.

K2-K6:

--- Quote from: SteveW on February 22, 2023, 10:24:02 PM ---My 1980 650 is an absolute bitch to start when cold but once warmed up is perfect.

I found a way to get it to start first time every time no matter how cold it was and it sits there and idles perfectly.

Choke on full, kill switch off. Throttle open fully, turn over engine a couple of times.
Close throttle, kill switch on, press start button and it fires first time and idles perfectly.

Don’t know if this is the proper way to start it but works for me.

--- End quote ---

This is effectively, by closing choke and opening throttle (presumably not cv carb) forcing it to pull vacuum through opened main jet to pre rich the combustion chamber, a little like the old brit carb tickler to make it rich.

It does suggest idle circuit and or compression as initially being the places to check and verify.

This is what the squirt of e-z-start checks for, by giving it something to easily burn then it starts and subsequently pulls more vacuum on the jets to get it running.
Routine of not cranking and try with a squirt to see if you get response will confirm too little fuel, then you need to wotk out why.

McCabe-Thiele (Ted):
My 400 was exactly the same as yours before the rebuild - it was poor cold compression in my case.
Now after rebore it starts first kick from stone cold.
My cold compressions iirc were all about 120 psi from memory as I can't find a record - when checked hot they were closer to 150 psi.

A compression test will definitely give you an answer so you can rule it out if they are good.

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