T3 - Odd Vacuum Gauge Readings

After spending 12 months and more cash than I care to remember I’ve finally got my T3 ready for the MOT. One problem - it isn’t running all that well. I’ve got it to tick over OK with vacuum gausge readings of -5 ins Hg on both gauges. When I open the throttle to @ 3k revs, the left hand cylinder shows a steady reading of -12 ins Hg whilst the left stays low at 3-5 ins.According to a web site I found this indicates an air leak on the intake side. I’ve checked the air filters (K&N) and the intake gaskets (new ). The only thing I can think of is the intake manifold sleeve.After a quick test ride the LH side was running slightly rich, whilst the right hand slightly weak which (I think) supports the leaking manifold theory.I’ve rebuilt the carbs with all new gaskets and replaced the air screws and idle adjustment screws. All the cables and adjusters are new.Has anyone else had this problem or can shed any light before I start taking it to bits again ?

Just a thought but slightly sticking choke on L/H carb throwing the balance out at idle, compensation made by adjusting but becoming apparent at 3k?Can’t say that I pay much attention to the actual readings on my Carbtune or Synchromate just the relationship between both sides at idle, pulling off idle and at about 4k. Sometimes it’s a compromise but I try and get a smooth matched ramp up to and including 4k (engine cruising speed).
guzzijack2014-07-17 16:56:04

It may tickover ok but are the carbs balanced? Take the filters off and put a straw or cocktail stick etc under the slides with the engine off. Open the throttle and both should start moving at the same time. If not you will need to adjust the cable lengths to balance them. Either on the carb tops or at the bar ( or both ) depending on your setup.
Where are you ?

Love & kisses

Balance at tickover is done by adjusting the idle screws. Then balance the throttle cables by opening the throttle a smidgeon and adjusting the cable adjusters on the top of the carbs. Reading level is pretty irrelevant, it’s getting them the same on each carb that’s the thing at both tickover and when the cables lift the slides.

Have you fitted new cables?
This can cause problems due to overlength outers, odd lengths etc.
Just another thing to check
Best of luck

Thankyou all for your helpful comments. I have removed both carbs and rechecked everything including the inlet sleeves, needle length, jets, cables, adjusters etc. Both carbs are now back on and set up but getting a consistent tickover is proving difficult.

Both cylinders have been tested for compression at 150psi so nothing wrong there.
I’m going to re-set the tappets and check the ignition timing (again) in the morning followed by another carb setting session.

Both slides are worn and fairly loose in the carb body. Is this the real problem do you think ?

I would double check that your enrichening pistons have good rubber seals at the bottom ( you did replace them didn’t you?). Also are they moving freely and, very importantly, are the springs pushing them downwards properly to form a seal when in the “off” position.
Next, when the bike is warmed through, set the slow running mixtures by screwing in the small screws one at a time until you hear the engine “stumble”, then screw outwards until the revs stabilise and first reach a max level but no more. Then set the slow running up via the throttle stop screws using a pair of vacuum gauges or an eBay ball-bearing based gadget so each cylinder is producing equal power. Keep blipping the throttle so the stop spikes do not dig into the slide surfaces. Fiinally set throttle cable length by very gently opening up the throttle and watching the gauges or gadget to see which cylinder is powering up first and set the cable length so they both power-up equally.
All the best, and good luck!

while you are checking bits swop carbs left to right and see if the low reading stays on the same carb if it does its a carb problem if its still same cylinder its engine related

^ What petethebee said, you have to do gauge measurements twice, one for throttle stops at idle, second for slides hanging on the cables. Adjust throttle stop screws for when on the stops (well obviously).

Adjust the cable adjusters on top of each carb to get the lifted slides vac readings the same. Oh and, still need to ensure there is some slack in the cables to make sure slides will close properly. There’s also a bit of an art in terms of routing the cables so it all works smoothly and is not altered by turning the bars.


PS: forgot to add that once throttle stops gave been set to equal Hg, and then slightly open throttle ditto, what should happen then is that when you slowly open the twist grip from closed, the gauge needles should move simultaneously.

Here’s one that I prepared earlier so I did

Cut and paste this…


OK then type in Vacuumate into YouTube and that will get it !

Love and kisses

xtheone&onlymin2014-07-20 07:25:18

Linkee no workee

Hi Min,
Tried the link without the . and I couldn’t get the link to work with the end bit via YouTube either.
Was there another deliberate error ?

In the meantime, to all, yes I’ve replaced everything including the choke and totally cleaned out the carbs 'cos they were a right mess. New everything apart from carb bodies, needles, main jets and slides, mainly because these bits were either OK or irreplaceable.
Been the pub and had a day off 'cos I’m fed up and feeling sorry for myself, even if Wigan did batter Hull and Widnes did the decent thing against the Wires. Normal service will be resumed tomorrow.

Oh-kay, we’ve told you you need to balance them twice, on the stops AND on the cables? Then should be good to go?

OK, got this sorted. The inlet flanges were warped and no fibre washers in place - I rebuilt everything as I found it and there were no washers at all. I got the flanges as flat as I could and re-fitted them with the correct seals and washers. The needles are OK but the slides are worn (though not totally u/s).The pair are now in synch at tickover and 4k revs.Thanks for your help etc.Stuart


Didn’t know about that did we? That would explain a lot Mike H2014-07-23 18:55:10