SOHC.co.uk Forums > CB350/400

Really bad cold starting. Ign or fuel?

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McCabe-Thiele (Ted):
Good news that the compressions are reasonably good from cold - much easier & cheaper  to sort out the spark issue.

Pretty sure it's been mentioned here before that what looks good like a good spark can be something else under load inside an engine.

Sesman:
Just a question on compression. How low does it need to be before experiencing cold start issues? Just wondering as the manual states 170psi so I’m wondering if the measurement methodology and/or equipment accuracy might mean the compression is lower than anticipated? Spark breakdown normally manifest’s as a misfire rather than a reluctance to start? Just wondering? Hope the ignition fiddling sorts it by the way.

McCabe-Thiele (Ted):
All I know is when mine were 120  psi it took a lot of cranking but would eventually fire up - no smoke particularly but would start first touch of the button for the rest of the day - I did test my readings when it was slightly warmed up - they all rose to above the 160 mark iirc - it was a cheap gauge as well. Never rode it before the rebuild or tested it since the rebuild - in high mileage fuel injected cars I found adding a small amount of 2T oil would raise compression and make them start much quicker.

I found poor cold start could  be due to the ignition being slightly retarded on cars or advance retard not working as it should. The whole coil LT /HT  impedance/ plug resistance are just a collection of words that I understand individually but as a sentence my eyes just gloss over.

K2-K6:

--- Quote from: Sesman on February 24, 2023, 07:52:51 AM ---Just a question on compression. How low does it need to be before experiencing cold start issues? Just wondering as the manual states 170psi so I’m wondering if the measurement methodology and/or equipment accuracy might mean the compression is lower than anticipated? Spark breakdown normally manifest’s as a misfire rather than a reluctance to start? Just wondering? Hope the ignition fiddling sorts it by the way.

--- End quote ---

I'd put a notional 130 psi floor on compression as practical limitations of all round running for these and most engine that start as specified around 170 psi book spec.  One of the giveaway is "its very smooth when warm" as low compression gives that characteristic and generally without the resonance in combustion of a higher compression engine. These motor are incredibly tolerant of low pressure though, and mask it well in general running.

To build a full picture, you'd need cold straight reading as base level, then plus some oil to see if the rings seal any better, then both the same when hot to give you reasonable analysis. That's any engine you suspect of being low though.

As noted though, it's not a show stopper on these for running them,  the smaller one's would have more trouble pulling higher gear uphill though  :)

Cappodimonte:
Before you start it from cold next time open the fuel cap on the tank as it may have a blocked air breather hole. The other check is the fuel on/off tap, remove it and see if it has a collapsed internal filter or part blockage. Are you using E10 fuel ? Ethanol can strip older rubber fuel lines I have replaced all of mine with Viton tubing to prevent this along with an external fuel filter to catch any debris that the fuel tap filter misses. Good luck with your problem solving.

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