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Author: Subject: Bike Carbs (again!!!)
myke pocock

posted on 14/10/19 at 06:39 PM Reply With Quote
Bike Carbs (again!!!)

I run my Skoda Estelle trials car on 4 Honda Fireblade Keihin bike carbs. They are fed through a Facet pump and then through a pressure regulating Filter King. On occasions when I am tackling a particularly steep part of a competitive section the engine appears to die momentarily. The Facet is set to 3psi. One theory is that as the bike carbs presumably are designed to work across the bike (I am not a bike person so am guessing the engine in the bike is across as opposed to inline) the pick ups from the fuel chambers are designed for that and not in line as I now have them. Does that sound as if it could cause an issue if I am right. OR am I running at too low a fuel delivery pressure?

[Edited on 14/10/19 by myke pocock]

[Edited on 14/10/19 by myke pocock]

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posted on 14/10/19 at 09:08 PM Reply With Quote
It will always be said that a bike fuel pump would be better. Whether that is the proper cause of your problems are not in this case is debatable. I wouldn’t of thought it make any difference which way round they are. I get it guess it’s possible you get a moment tree fuel surge which floods the engine?
If anything three psi is too high

[Edited on 14/10/19 by ReMan]
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David Jenkins

posted on 15/10/19 at 07:37 AM Reply With Quote
I had problems using bike carbs with a setup like yours - although my car isn't a high-power job. Changing to a bike pump made all the difference, with all issues going away.

Bike pumps are low-pressure, high volume (normally), but I guess that you'll need a pump from a bike that used carbs, rather than a fuel injected one (but don't quote me, as I'm not sure).

The older I get, the better I was...

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posted on 15/10/19 at 07:39 AM Reply With Quote
we used the same carbs and pump set up on a Luego - it never worked properly.

Changed to a bike pump (off a scrapped fireblade to match) never had an issue again.

I believe its to do with low pressure, high flow. the bike pump 'ticks' as it comes on to get the pressure right, then only kicks in when needed. We always found the facet would flood the carbs, whatever we did.

Worth a try. this looks cheap enough that if it didnt work, you've lost very little: Link

[Edited on 15/10/19 by Norfolkluegojnr]

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posted on 15/10/19 at 03:45 PM Reply With Quote
The problem is quite likely the operating angle of the carbs on a steep incline.

Having your longitudinal installation pointing up or down at a steep angle is not quite equivalent to a bike leaning sideways as the the fuel will be held level by the cornering forces in the bikes case. In your case the fuel level on one side of the float chamber will be high enough to keep the floats raised up, but the level in the middle where the fuel is drawn into the venturi will be lower.

Not sure there is a simple solution to this. You could convert the carbs to a floatless return system, but that would involve a fair bit of work and probably one return pump per carb.

It may also be possible to modify the float pivot so the float can tilt to some extent, but new floats are far from cheap and these carbs are getting harder to find so it could be a costly experiment if it doesn't work or causes flooding etc under other conditions.

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