Poor Vacuum on left cylinder Le Mans 2

Technical Big Block Tonti and Loop Frame models
1

Trying to balance the carbs on my Le Mans 2 but the left cylinder has no vacuum when I open the throttle. Both cylinders similar at tick over.
I have swapped the manometer over to check it isn’t a fault with the instrument.
Stripped down the carb and connected the float height but still no vacuum
Any ideas?

Peter

2

Compare compression in each cylinder , then you can rule out piston rings, valve guides and valves, check tappet clearance first.

Open throttle wide when doing compression check.

Will it run on one cylinder with the other plug removed?

Check the cable operation in the bad cylinder.

3

Thanks for your advice Andrew. Compression gauge on order!
I did some googling and started to worry that something bad has happened to that cylinder. It was running very hot ( maybe due to weak mixture because the carb float was very low)
It does run on that cylinder only but maybe no power?
Will keep you informed of progress.
Thanks again!

Peter

4

Compression gauge arrived. Both cylinders have similar compression (approximately 175psi after a few spins on the starter)

Vacuum similar in both cylinders at tick over. As soon as I open the throttle the vacuum on the left cylinder falls and the vacuum on the right cylinder increases (as expected)

I haven’t had the bike that long and have been slowly getting into shape. If the left carb has the wrong main jet or needle could this do it? Seems unlikely that is the progression system?

5

Possibly a leak on the left hand side intake piping? Between head and carb. Can you spray some e.g Easystart on the intake side connections on the left cylinder? If the tickover is affected then that would seem to indicate a slight leak. That would also give you a weak mixture in that cylinder.

6

Thanks for your ideas Bob. I will give it a try

It is pretty suspicious that the vacuum is fine when the carb is running through the idle jet and as soon as I touch the throttle to open the main jet the vacuum disappears?!?!

7

Try adjusting the throttle cables, sounds like one is slack :+1:

8

Phil. Thanks for your thoughts. Both throttle cables have about 1.5mm slack with throttle closed (per the manual)

I am attempting to balance the carbs with a vacuum gauge (get both throttle cable tensions/ pull the same) but the left carb vacuum falls away as soon as I open the throttle

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9

OMG! After further googling it appears that the vacuum is supposed to drop as soon as you open the throttle. Logical really, open the throttle, let air into the inlet system, less vacuum in the inlet manifold

I now have a more confusing problem….. Why does the vacuum increase on the right cylinder when I open the throttle? There is no air filter, nothing to stop air getting in, the throttle slide is opening.

Well confused!!!

10

I was thinking exactly this earlier, had it on my LM mk3 - try adjusting the carbs as follows

  1. Wind the tickover adjuster screws out until they are not holding the slides off bottom.
  2. Adjust the throttle cables until you get even vacuum across both sides as the engine is revved and held at a steady speed - they are sychronised when the needles move in unison. You need to maintain some slack in the throttle cables at the end.
  3. Adjust the tickover screws to get the engine ticking over evenly (likely balanced on the vacuum guages too)
  4. Adjust the idle air screw on each cylinder to get the fastest idle, do this slowly a eighth of a turn at a time as it takes a few seconds to react. One and a half turns out is a good place to start, and adjust in or out for fastest idle.
  5. Readjust the tickover screws to get even tickover again.

Hope this helps

Russell

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11

Thanks Russel. I am pretty happy with the tickover.
I will strip the right carb down and try and find any faults I will then try your set up method