Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: 350Simon on August 07, 2022, 12:48:48 PM
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When I bought my 350f the dealer said 'always run these old bikes in the reserve position'....... in other words there's a problem with your fuel tap but I don't want to tell you haha
Anyway it runs great so no issues there, yesterday out of interest I thought I'd put the tap in the on position and see what happened. Literally 15 seconds later fuel is pissing out all 4 overflows onto the drive. Turn it to reserve, fuel stops flooding out the carbs.
Started engine with tap in reserve, lovely idle and good response. Turn tap to on position, bike starts spluttering and flooding carbs again.
Could it be that the tap is broken? It seems very odd to me as you'd expect if it was a float / seat issue it would also flood in the reserve position. Anyone had this before?
Cheers
Simon
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That's weird as if the tap internals are as they should be you would have a greater fuel pressure in reserve - lower in normal ON position.
Out of interest what happens if you are on reserve and open the filler cap.
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That's weird as if the tap internals are as they should be you would have a greater fuel pressure in reserve - lower in normal ON position.
Out of interest what happens if you are on reserve and open the filler cap.
Yes that's exactly what I thought. Pressure should be greater on reserve setting than 'on'.
An internal restrictor perhaps broken down? Someone has rebuilt it back to front maybe? Is that even possible?
Not tried with the cap off yet I will try that shortly!
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Wonder if the rubber diaphram/gusset thingy inside is split or in back to front as you suggest (if thats indeed possible) weird it behaving like that!
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If the tap is switched off whilst the engine is running to empty the carb bowls you can sometimes get flooding when you open the tap again if the valves stick a little. Before my rebuild I found rocking the bike whilst on the centre stand would fix the flooding immediately.
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Strange, the pressure when there is no flow will be the same as it's the height of the fuel in the tank to the needle valves in the carbs that determines the pressure. When there is flow there will be a change dependent on restrictions in the path. But the needle valves should be strong enough to hold back pressure from a tank filled to the brim. (but in practice they often drip a tiny bit, so always turn it off)
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The floats and float valves will do their job irrespective of what position the fuel tap is in, the fuel can only pass down through the same delivery pipe. Is there a chance that the tap is actually leaking in that position, and gravity is allowing the fuel to run down the outside of the pipe, running over the outside of the carbs, and following the easiest path downwards which would be via the overflow pipes. Make sure everything is bone dry, kneel down next to the bike, then using a torch,(NOT A MATCH!!), turn on the fuel and see if the outside of the delivery pipe becomes wet and shiny. Have fun, we love this hobby don't we? Late PS:- I meant to say that it really does sound as though the tap has been wrongly assembled. There's an exploded parts diagram on the CMSNL website, might possibly help.
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I think I would be looking for a new tap myself by now!
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The floats and float valves will do their job irrespective of what position the fuel tap is in, the fuel can only pass down through the same delivery pipe. Is there a chance that the tap is actually leaking in that position, and gravity is allowing the fuel to run down the outside of the pipe, running over the outside of the carbs, and following the easiest path downwards which would be via the overflow pipes. Make sure everything is bone dry, kneel down next to the bike, then using a torch,(NOT A MATCH!!), turn on the fuel and see if the outside of the delivery pipe becomes wet and shiny. Have fun, we love this hobby don't we? Late PS:- I meant to say that it really does sound as though the tap has been wrongly assembled. There's an exploded parts diagram on the CMSNL website, might possibly help.
This seems like the most plausible answer to this conundrum. I shall try tonight when I get home and see what happens, a leak at the tap crossed my mind but I just imagined it would drip down onto the crank cases and so dismissed the idea but as you say the fuel may well be flowing down the outside of the feed pipe!
Ta
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I think I would be looking for a new tap myself by now!
I don't really want a shiny new tap as my bike is nicely aged apart from the exhaust which will tarnish fairly soon anyway. If anything I'll try and rebuild the old tap I think.
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If it's the same as a 400 I used the DS repair kit - I re-used the one O ring rather than using the two thinner ones.
Can't see how it will fix the carbs from flooding changing the tap?
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One of the biggest problems are the washers under the two screws holding tap to tank as they are a thick fibre and i have only ever been able to find them as genuine, expensive, Honda. Generic are too thin and split, i have had sucsess with alloy bodied Dowty washers but they are not easy to get now as they have changed the body to stainless.
It is worth changing the 4 hole rubber disc as they do wear and leak
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DS has some in stock these days. I reckon he's bought someone out in the states as he's got stuff he didn't have before.