CARB BREATHER: THE HOLE STORY
Cutting a hole in the side panel for a carburettor to breathe isn’t easy; here are a few tips to help you get it right.
On a tuned Lambretta the chances are it’ll be running a bigger carburettor, and more often than not this will mean cutting a hole in the side panel to help it breathe more easily… or just to get it to fit! The problem arises in where to cut the hole to get it central to the mouth of the carburettor. If it’s off-centre then not only will it look odd, but is also pointless as it’s not doing the job it was intended for. Before cutting the hole there are certain things that needed to be looked at to make sure you get the position as accurate as possible.
Tools
Before you mark out the side panel there are several things you need to check which can alter the measurement. If it is a new project then it can be difficult as you may not have the engine already built. If so then to get it exactly right the engine does need to be fitted, not complete but with the top end and carburettor in place.
First, make sure the shock absorber you use when taking the measurement is the one that is used when the machine is built. There are plenty of shock absorbers on the market and all vary in length slightly. If you measure where the side panel hole goes then change to a different absorber that is perhaps 10mm longer then it will have moved the position of the carburettor. If you are using offset mounting cones then these must also be used when taking the measurement. Then there is the manifold itself, if you change to a different or upgraded one then this too may sit at a different angle.
Presuming that you have everything in place ready for taking the measurement is fine but there is one other thing you must do. When the side panel is fitted, make sure the rubber trim is correctly sitting home in its groove. Also, there are different types of trim which vary in thickness which will mean a difference in exactly how the panel sits. Again, whichever trim you use in the measurement must be the one used in the final build. Finally, make sure the panel clip is fitted and locked on the panel. This will draw the panel into its final position. Only when all of this has been done can an accurate measurement be taken.
Where to start marking out
With the panel now fitted and the clip locking it in place you can start to take a measurement. The problem now is you are looking at the side panel but can’t see the carburettor and guessing where the middle is won’t work. From the other side of the frame, you can look in across the main tube and will be able to see near enough the position the carburettor mouth sits in relation to the panel. Take a marker pen and make a mark near enough where the centre of the mouth will be against the panel.
Having taken the panel off from the inside, drill a hole around 10mm diameter straight through where you made the mark. Refit the panel and look through the hole to see how accurate the position is. You can even put the drill piece through the hole and look from the other side of frame see how central it is. If the hole drilled is central then fine but if it is slightly off then don’t worry. As the hole is only 10mm wide then you can open it up to one side using a grinding attachment until it is central.
It may be you have made the hole around 15mm to do so but this is still way smaller than the final one so it doesn’t matter. By now you should have a hole that is central to the carburettor mouth ready to move on to the next stage of making it full size.
Making a template
Now you need to mark out the outer circumference of your circle ready for final cutting. The best way to do this is to make
a quick template out of paper and position it on the panel’s outer surface. It is easier to do it this way as there is a gentle curve on the panel and marking round something such as an aerosol can lid for instance means you may not get an exact circle. This can cause the hole to look slightly oval afterwards. The diameter of the hole to be cut depends entirely on the width of the mouth of the carburettor you are using. When measuring the mouth you can allow it to be 2mm bigger if you are going to fit a rubber trim around the edge which will narrow the hole slightly when fitted.
Mark a cross on a piece of paper and from the centre put another mark half the distance of the mouth measurement you took. Using a compass this will allow you to scribe a complete circle which will be the template for your hole. Place that centrally over the hole in the side panel and scribe around its edge. This will now give you the exact hole you need to cut out.
Cutting the hole out
The easiest way to cut the hole out is to drill a set of smaller holes just around the inside edge of the outer circumference. This can then be easily cut with some small tin snips to remove it. Using a half round file it only takes a few minutes to file off the edges and leave you with near enough the perfect shape. You can now put the panel back in position and this should leave you with the perfect hole in the panel centrally in position to the carburettor mouth. To finish it off fit a small flap wheel into the drill and smooth off using a circular motion. Now that panel is finished and ready for painting.
Easy way out
There are a couple of options that help if your metal work and measuring skills are not the best. Fitting a small rubber trim will compensate if you don’t have the perfect circle after it cutting out. This will still give a good finish and hide any small edges that didn’t clean up when you cut the hole out. You can fit a gauze or grille over the hole by using perforated steel plate or wire mesh. The advantage of doing this is, if you change to a different shock absorber or manifold at a later date then you cannot see that the hole is now slightly out of position. At the same time, it will still allow the carburettor to breathe adequately enough. If you are going to use this method then cut the grille to a slightly bigger shape and fit from the inside. It’s best to tack weld it in position to make sure it sits flat before blending it in with filler prior to painting. The grille can be painted a different colour to the panel to give it a professional finish.
Remember to take your time when doing the procedure of measuring and cutting the hole in the side panel. If you get it drastically wrong then the panel will be scrap and you will have to start all over again with a new one. Getting it right is not difficult so long as you take the right steps and the end result is very rewarding.