Twin carb and LPG

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Tomc1958
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Twin carb and LPG

Post by Tomc1958 »

Hi folks, 2.00 air cooled . I'm a novice and need to check something out.  Recent engine rebuild and new Solex/Brosol twin carbs fitted, I also have an LPG conversion.  LPG guy was having problems setting the LPG up and then discovered there was no air being sucked in through the offside carb so the LPG wasn't being drawn in.  He told me the the carb is faulty. I saw it, plenty of suction on nearside and nothing on offside.  It was running fine on petrol but I was told that's because the petrol goes in at the bottom whereas the LPG goes in just under the air filter so needs the suction.

Carbs are going back to supplier who have been very helpful although they are mystified why there was no suction. I'll have to have them back if there is no fault but will probs sell them.  Now bought a 32/36 Twin choke Weber progressive currently being fitted.

Is anyone able to explain why a carb would not draw air in?  Can the carb be faulty to that extent or is it something to do with the engine rebuild not creating suction on one side?  It has chugged a little after first start but soon settled down to running smoothly.  I took the original carbs back to the mech but wasn't there when he put the offside one back on...he called me and  told me that one had suction therefore the new one was definitely faulty.  

As said I am a novice but I like to understand things and how they work, especially when it's getting a bit expensive.

The engine was rebuilt a year ago and I've been driving all year on petrol and only recently went to get the LPG set back up so the problem has just come to light.   If it is the engine rebuild, will the single carb Weber mask the problem?  I'm surprised when the petrol was set up an air flow meter wasn't used because that would have shown the problem straight away?

I hope I've made sense and posted this in the right place.  Anyone who can help fire away any questions at me you need to ask.

Sadly Steve Shaw who did the LPG conversion 3 years ago obviously is no longer around to ask.  Steve was a great guy and a huge help to so many.

Cheers for any help,  Tom

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maxstu
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Re: Twin carb and LPG

Post by maxstu »

Simple answer, Tom. If the camper drives fine on petrol and not on LPG, then l suggest LPG delivery is not correct.

True, LPG is delivered from above. But if carb was not breathing as it should then petrol delivery would surely also be affected? Petrol need suction too. Afterall, its the pull of the pistons that draws fuel (either LPG or petrol) through the carbs.

Perhaps the choke flap was out of adjustment on the rogue carb and preventing delivery of LPG?

Anyway moving to a Weber looks like a positive step.

Feedback outcome please
MaxStu
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ajsimmo
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Re: Twin carb and LPG

Post by ajsimmo »

My suspicion is there was nothing wrong with the carbs, but they had not been set up correctly. As said, any fault would have shown up at first fitting when balancing the carbs with a flow meter. It's an essential step, and If it wasn't done, or at least done when fault finding the LPG issue, then it's no surprise that there was a big difference in airflow. It would run less than optimally on petrol, and much worse on LPG.
Also, if I remember rightly, Gasure did not utilise the idle circuit on LPG, but rather fed all the LPG in via both main venturi, leaving the idle circuit running very lean. When installing for Steve on such setups I devised a means of feeding a little extra LPG into the idle circuit via the "3rd carb" (on back of LHS carb) by taking it from a port installed just below the LHS mixer venturi into the idle air intake, and opening the air screw a little, whilst blanking off the spigot on the airbox. This gave a much smoother, slower and adjustable idle, and also better emissions.
I'd recommend trying all of that before changing to a Weber. It seems your mechanic is trying to work around a problem, rather than understand, find and fix it, and at your expense, of course. The Weber is a poor choice IMO.

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Last edited by ajsimmo on 06 Dec 2020, 08:41, edited 1 time in total.
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davidoft1
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Re: Twin carb and LPG

Post by davidoft1 »

maxstu wrote: 05 Dec 2020, 20:15 Simple answer, Tom. If the camper drives fine on petrol and not on LPG, then l suggest LPG delivery is not correct.

True, LPG is delivered from above. But if carb was not breathing as it should then petrol delivery would surely also be affected? Petrol need suction too. Afterall, its the pull of the pistons that draws fuel (either LPG or petrol) through the carbs.

Perhaps the choke flap was out of adjustment on the rogue carb and preventing delivery of LPG?

Anyway moving to a Weber looks like a positive step.

Feedback outcome please

Single Webber? No it’s not a positive step, sounds like your mechanic hasn’t set up the carbs properly, twins won’t just bolt on and work out of the box, I think there nothing wrong ( well I would pretty my guarantee) the carbs, the single is a bit of a cop out fit bit being able to sort the twins

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