Vacuum Guage take off pipe.

[wanderinstar]

While checking round engine yesterday I noticed that the bit of tube I use to blank off the vacuum pipe had got hard and snapped off. The engine revs increased and it seemed to run quite smooth with this pipe open. When I put my finger over end of pipe revs decreased and tickover became rather lumpy. Can anyone tell me why?
Ian.

[Willie]

Yes, with the pipe open to the atmosphere you are allowing more air into
the mixture thus weakening it. A result of a weaker mixture is to increase
the revs at tickover so,if you think it sounds happier with the pipe open it seems likely that you are running slightly rich and should adjust the mixture
HOT, with the aid of the carb lifting pin, or simply turn the mixture nut up ONE
flat at a time until it reaches perfection! Weaken it too much and the engine
will tend to stall on deceleration.

[bmcecosse]

It's on LPG gas - but the idea is right - too rich, extra air makes it idle better! Can you adjust the 'mixture' Ian ?

[wanderinstar]

Yes there is an adjuster on the side of vapourizer. Will adjust it today.
Ian.

[bmcecosse]

So - does that adjust the gas pressure? How does it respond to you opening the throttle- is the gas simply drawn in at a fixed rate by the additional venturi on the front of the carb ? Since the carb is not used (or do you start up on petrol) - have you ever tried it with the piston removed - just leaving the throttle plate ?

[wanderinstar]

Yes it adjusts the gas flow on tickover.You need the carb to control gas intake into engine,. When piston drops down it stops creating vacuum behind it, which in turn stops the flow of gas.
Ian.

[bmcecosse]

Just closing the throttle will cut off the vacuum - after all the LPG can be used with other carbs that have no piston - it's there to regulate the liquid fuel which you don't have !! I suspect you don't need the piston and without it the car will be much more responsive. If you get a chance - try it without piston - just as a trial.

[bigginger]

Isn't the point that if you run out of LPG you can run it on petrol?

[wanderinstar]

Yes your right Andrew, you do have to be able to revert to petrol when LPG runs out as Gas guages are renowned for not being accurate.
However the idea appeals to me Roy. Been giving this some thought. Would it be possible to lift piston up and drill small hole in top of the part of carb that piston slides up. Then fit a pin to hold it up, connected to a cable so that pin can be withdrawn from inside car. It could then be put back in when you fill up with gas again.
Ian.

[wanderinstar]

Well anybody.
Would it be possible?
Ian.

[Kevin]

You really need to be speaking to an LPG expert I think Ian as I dont know how many users on here have LPG and the alterations you are suggesting would have no relavance to a standard car methinks.