Not 100% sure of the engine’s vintage as it is in a 1978 T3 California frame, and is new to me.
I’ve changed the fluids, petrol and spark plugs as it would not run at purchase. New plugs immediately allowed it to start. I am unsure how long it had sat before I bought it. I have set the timing to the marks with a strobe and the advance works. It has Lucas electronic ignition. I have checked the valve clearances on a cold engine. 0.20 mm inlet and 0.25 mm exhaust. Compression test on right hand cylinder at maximum was 175 and left 180psi. Hot engine and with full throttle.
It now starts immediately with choke from cold and when hot off the key. Carburettors are PHM 40 Dellorto. I have balanced the cable uptake to the slides and checked the balance with vacuum gauges.
Problem is I cannot get a good throttle response from idle except from between shutting off the choke and the engine getting up to operating temperature. At that warm stage before the engine gets hot the throttle response is perfect, but as it gets hotter the throttle pick up is not as sharp from idle. I’ve set the fuel idle mixture screws and followed the protocol of slight tweeking on each side of the idle mixture screws for the sweet spot, but a happy throttle response when hot eludes me.
I’m presuming this is a low running mixture issue rather than a degrading coil issue as the engine is happy to rev once the main jets take over.
Is there someone with more experience of these carburettors that could give me some pointers please?
It sounds like someone has swapped the carbs, I’m sure the standard size on these is 36mm. It might be worthwhile calling Eurocarb for a bit of advice on jetting. Maybe it needs smaller pilot jets? Can you get it picking up better by playing with the mixture screws when it is hot?
I have only tried to tune the idle mixture screws once it’s up to temperature. A check suggests both Mk1V and V models should have these 40mm pumper carbs - but I’m obviously missing something.
Have you thoroughly cleaned the jets and emulsion tubes?
Old fashioned petrol goes gummy in tight spaces and needs dissolving out.
e5 is worse if left for more than 6 months,
e10 is truly awful for deposits and corrosion if left for more then 3 or 4 months.
Good point. I’ve been avoiding that so far as I’m not familiar with these carbs. The last Le Mans I had was in 1981 and it never needed the carburettors stripped down.
I’ve had a further look and now suspect that due to the bike being a mongrel (T3 chassis with later square finned engine) that the choke cable may have inadequate inner cable length to fully shut off the chokes. The adjusters are all at a fully compressed state and it is hard to know if the plungers are actually seating properly.
This could tie in with my disappointment over the lack of difference in tune by adjusting the fuel idle mixture screws.
looks like I’m going to have to try to pare back the outer choke cable sleeve to ensure there’s more useable length of the inner cable, find flip top versions that do away with a cable, or, blank off the chokes somehow.
Any ideas on the blanking off option?
You have the plunger controlled starter jets?
If so the easiest and reversible way of blanking is to disconnect the cable and reassemble the plunger with sprung and cap.
If the plungers are dropping down fully, you should be able to feel a little bit of slack in the cable outers as you try and lift them out of the socket on top of the carb. Pull the rubber covers back first.
If you change to the flip lever type they come in two different lengths, make sure you get the correct ones for your carbs.
Thanks, that makes sense.
That’s the issue, as there doesn’t appear to be any slack or free adjustment available at either of the adjusters on the carb tops or at the adjuster on the cable itself.
This needs sorted as otherwise I’m suspecting the choke is remaining slightly on which is negating any useful adjustment at the mixture screws.
If you stick with the cable arrangement, you need a pair of lower cables to go from the splitter under the tank to the carbs and make the inner cables with a bit more free length. Or alternatively, try and trim a bit of the outer cable back to give you more free length.
The standard cables should allow the plungers to drop and give 3-5mm slack,
The only way i can see that slack being taken up is if the cable-splitter is not assembled correctly or is tied to the frame too far from the carbs- worth checking that
Unless the PO has had their own cables made up?
If the carbs have accelerator pumps can you disconnect the choke cables to test the theory? My experience is that if you give the throttle a couple of twists, cold starting will be ok with the bit of fuel that puts into the intakes.
That’s my plan. I’ll see if I can ease off the a silver cap prior to the splitter and snip off a small 6mm section of the outer cable sheath.
I suspect the previous owner either had a custom cable made up (and the measurement for free play was off), or the original from a Le Mans isn’t right due to the California bars.
Thanks for an idea of the necessary free play. I’ll ease off an end cap and snip off a good 6 mm as all the adjusters are fully in at the moment, including a seemingly extra one 9 inches from the lever at the bar.
Good point too. 40 years ago I had an early series Le Mans with similar carbs that was used for the Battle of the Twins series. But it had ticklers to flood the bowls to help cold starting. A couple of twists of throttle, and on a warm day and it sometimes started if you pre-warmed the sump with a Primus stove. Otherwise it involved a tow from a car -which was always a pain as my house was in the dip between two hills. I only had that bike for two summers as the starting proceedure from cold meant short lunches out, carrying a tow rope and forever asking favours🤣
Just spent an hour at it grinding off 10 mm from the outer choke cable to give some free play at the plungers.
Although I haven’t had time to fine tune the mixture screws there’s now a noticeable difference when they are adjusted which was absent with the choke cable previously.
I now have enough slack to properly adjust everything.
Thank you all for your inputs!
If you need it Venhill do cables and stuff!