750T carb problems

today after rebuilding the carbs, I set about balancing, the rh cylinder flooded, cleaned the plug turned off the fuel and started her up alls fine but when I turn the fuel on the engine stumbles and starts to die turn off the fuel and slowly it picks up again I have checked the float heights all ok needle jets look ok? So,stripped bowls off tonight but there’s not enough of the float pivot pin on the rh side to  pull it out any ideas 1 how to get the float pivot pin out  and 2 the stumbling fault, I have managed to set the idle, mixture and cable sync but it’s running all over the place oh, no air leaks. Help please

Float punctured …float brackets distorted…you may be able to push the pin out using a suitable size tool…why not take the float bowl off, …push the float up gently…switch the fuel on, then let the float come down gently and see when it starts to flow fuel…good luck…

Also possible, worn or dirty or damaged needle jet, float snagging on sides of float chamber due to swelling. As Kate says I would do a test that the needle jet works OK by dropping the float bowl then push float up and turn fuel on the make sure needle jet actually closes it off.


Interesting about the pivot pin, usually there’s one end sticking out you can get hold of with pliers, has somebody replaced it with a bit of wire or something?

no the pins there but I have had it in and out a few times so the exposed end has as you might say erroded away, its ok I managed to get it out in the end and ordering 2 needle jets and 2 new pins, the float is ok not swollen or punctured, but I will check its not fowling the bowl thanks for that tip.

well checked the needle valve is closing ok all’s fine there, floats not binding and float height ok, now what? think I may starting swopping bits maybe try swopping over the carbs if possible.

Sure float isn’t leaking?

def not punctured, had it in water straight after removing it, floats well and no noise when shaken, tonight rechecked the float height spot on, re checked valve clearances im sure its the carb just seems to flood the rh cylinder when the fuels switched on, on starting up and running, as i said before turn off the fuel and after a short time 3-5 seconds the rh cyl picks up, turn it back on and again in 3-5 seconds it starts to falter, during the running fine limbo she revs cleanly no smoke and idles fine, inbetween turning off and on i managed to balance the carbs on idle and balance the slides, that was fun only took 2 hours, it doesnt overflow when enginge is not running, i even rechecked the air filter pod was clear, so i have lowered the float height to 22.5 mm and will try tomorrow and see if it makes any difference. any ideas welcome
Choke piston rubber???

A good way to check the float needle seal is to turn the carb upside down (helps to have it off he bike at this point) whilst gently blowing into the fuel line. Should feel the shut off quite easily. You could compare the action of the two carbs this way.

Other than that I’d swap all the bits between the carbs and see if the problem swaps sides. Sounds like there must be some crap on the float needle jet on the problem side.

Maybe it’s a case of misinterpretation, when you say ‘flooded’, I assume you mean petrol is running out of the carb onto the floor. (?)

Otherwise may just be way too rich for some reason, choke piston would be a good culprit. May not be closing properly 'cause it’s sticking.

maybe I should explain it more clearly, its flooding the rh cylinder and not leaking externally, I have had the carb off the engine turned on the fuel and there is no overflow from the main jet or any other jets whilst the is no depression in the venturi, the only thing that would cause it to enrich the cylinder to the extent must be the choke valve leaking by the rubber seal, if you remove the spark plug and spin her over petrol mist comes out the plug hole for 4 or 5 revolutions of the engine. please let it be the choke valve ordered one today. its driving me mad this one.

Ah right thanks, that does sound more likely then. Be aware there’s also air passages for the starter jet and the idle jet, to mix air with the fuel from those two jets. Squirt something down them and make sure it comes out the other end. If the idle jet air intake is blocked it would just be sucking up neat petrol.


Well swopped over all the internals tonight, not happy days still the same, has me beat. Will ad in wants for a phbh30 bd

Have I over looked the obvious a weak spark? gonna try shopping the ignighters over maybe, just clutching at straw not and good guzzi mechanics In the club near Rochester kent, I don’t give up easy but this is now doing my noddle in.

The trouble is we’re trying to diagnose from a distance.


Try a PM to Keithguzzi on this site. Lives in Chatham and well used to playing with Big and Small Blocks. Otherwise known as “Motobradley”.


cheers ray. today I was talking to a local lambretta specialist as they use these carbs when tuning, he suggested the obvious and why I haven’t done it who knows, swop over the whole carbs and see if moves to the other cylinder, seems like a job for tonight, if it does off with its bits and boil on gas mark 6, lol or look for a RH carb body.

Well changed the cabs over and the problem has now moved to the left hand side, stripped the carb, cleaned all orafices with carb cleaned and blew them all out with compressed air, the only one that doesn’t seem to go anywhere is the lower right hole  Out of the 3 shown in the picture at the top left hand  it’s either blocked or a blind hole any ideas?

That’s where the choke jet/atomiser goes. There should be a small hole that goes into the cavity above, and that’s where the choke plunger sits.


Well she now “sort of runs” on both cylinders hasty put together and the right one fitted it was so cold out side the carbs kept freezing  so some erratic running but on 2. happy days, that’s thanks to a club member sending me a pair of carbs, thanks Don, cheque will be in the post, On inspection there are two orifices one by the choke jet and another by the main jet emulsion tube, both on the carb body in the float chamber, on my carbs these were open, on the carbs sent to me they are plugged with lead, I can only assume over the 13 years of standing they had desolved away or the ultra sound had shook them to powder, well now a proper rebuilt on both the replacements and set them up proper, will keep all posted, I’m so happy I could wee :smiley:

Re the blanking plugs going AWOL…