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PostPosted: Tue May 15, 2018 8:53 am 
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Tom Rae wrote:
The jetting will also be wrong as the foam filter will be less restrictive than the standard set up, have you checked the mixture? I guess it will be too rich throughout the rev range?
Tom


Tom - please can you explain a little more? I was assuming that a less restrictive foam filter = more air = leaner mixture?

I am ready to be corrected :oops:

cheers, Robin

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PostPosted: Tue May 15, 2018 10:22 am 
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MartinClan wrote:
Tom Rae wrote:
The jetting will also be wrong as the foam filter will be less restrictive than the standard set up, have you checked the mixture? I guess it will be too rich throughout the rev range?
Tom


Tom - please can you explain a little more? I was assuming that a less restrictive foam filter = more air = leaner mixture?

I am ready to be corrected :oops:

cheers, Robin


Robin, you are indeed correct however the symptoms you describe suggest to me that the return is the issue and could be causing over richness low down the rev range but the max revs could be restricted by a lean mixture? Just a guess but you need to check the mixture throughout the range. The float should be 23.5 - 24.5 mm from the face of the carb to the lowest point of the float measured at the point when it just contacts the needle valve.

Tom

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PostPosted: Tue May 15, 2018 11:58 am 
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Robin
Just a thought, if fuel is dribbling out of the rear of the carb, are the carbs leaning back?
Have you checked the float level? You could try setting 1mm lower than spec.
Tony

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PostPosted: Tue May 15, 2018 12:54 pm 
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Martin,
The PMC method of making an excess fuel return to the tank is to use a metal tee in the supply line before it splits for the 2 carbs. Block one arm of the tee with solder then drill a small hole for the return to make sure that fuel only returns when the chambers are full. I doubt a drop from 8mm to 6mm would be enough to prevent some of the primary flow "escaping".

On the bike, the choke operated with a small lever on the carb. Eurocarb can supply higher than gravity float jets and needles for the PHBH.

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PostPosted: Tue May 15, 2018 1:01 pm 
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MikeM wrote:
Martin,
The PMC method of making an excess fuel return to the tank is to use a metal tee in the supply line before it splits for the 2 carbs. Block one arm of the tee with solder then drill a small hole for the return to make sure that fuel only returns when the chambers are full. I doubt a drop from 8mm to 6mm would be enough to prevent some of the primary flow "escaping".

On the bike, the choke operated with a small lever on the carb. Eurocarb can supply higher than gravity float jets and needles for the PHBH.


Mike, if the supply lines are higher than the float chambers at some point they will be effectively gravity feeding in which case the size of the return is less important? Mine is the same as the supply at 8mm and seems to work fine.

Tom

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PostPosted: Tue May 15, 2018 1:40 pm 
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Tom,
Gravity plus whatever psi is in the main supply line, after the regulator?

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PostPosted: Tue May 15, 2018 3:18 pm 
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Thanks all. I shall take some, all (delete as appropriate) of the comments into account :)

It's interesting to get so many replies. I guess it must be a commonly encountered problem...

Cheers, Robin

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PostPosted: Tue May 15, 2018 4:41 pm 
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Robin,
It's surprising how different people have experienced "idiosyncrasies" of twin carb setups, but the value of sharing those experiences and the "old school remedies" which sorted some, is invaluable. Who would have thought whether a solid foam float might actually be a "sink"? A jam jar of fuel is "the solution" - those that stay on the surface are indeed "floats" , those which hover half way, or worse, sit on the bottom of the jar are not!

I presently have a problem with Bolide, 10 years on, with fuel pressure suddenly dropping to the point where a tickover and on a light throttle, the engine won't rev above 2,000 rpm under load (so suddenly you have a 30 mph max car). Replace the pump with a new one, calibrate the regulator and the car runs normally for 1100 miles, able to rev out in 4th until suddenly, you have a 2,000 rpm, 30 mph car again. I have this to sort presently, but it does remind me of an early series 1 Lotus Elan that did exactly the same - sometimes. The cure was to remove the waxed sweetie paper that had somehow got in to the tank and when the fuel level was low, got sucked on to the delivery pipe. Refill tank and sweetie paper floats off.........

The value of this Forum is that everyone is keen to share. As I've said before, take ALL advices, think and choose the ones that seem most appropriate to you.

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PostPosted: Sat May 19, 2018 12:38 pm 
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well.... The problem seems to be that the float and / or needle valve is sticking. Unfortunately I cannot see why. I have stripped the carb and examined it visually and it seems fine. Put it back on the car and as soon as I run the fuel pump petrol drips from the carb. If I tap the carb with the handle of a scewdriver it sorts itself out. Which is probably why the problem only occours on startup. After the engine has been running and vibrating for a few minutes it sorts itsef out.

Any clues anyone? I would rather be driving the car than tinkering with it in this good weather! I have to admit I am tempted to buy a new carb. Relatively cheap at £100 odd quid.

Cheers Robin

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PostPosted: Sat May 19, 2018 12:42 pm 
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MartinClan wrote:
well.... The problem seems to be that the float and / or needle valve is sticking. Unfortunately I cannot see why. I have stripped the carb and examined it visually and it seems fine. Put it back on the car and as soon as I run the fuel pump petrol drips from the carb. If I tap the carb with the handle of a scewdriver it sorts itself out. Which is probably why the problem only occours on startup. After the engine has been running and vibrating for a few minutes it sorts itsef out.

Any clues anyone? I would rather be driving the car than tinkering with it in this good weather! I have to admit I am tempted to buy a new carb. Relatively cheap at £100 odd quid.

Cheers Robin


Robin, if you have no return then the float valve may be overloaded, even the uprated one?

Tom

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