Remember when setting float heights, the greater the float height dimension when measuring them, the less fuel there will be in the float bowl. Ie, float height measuring (because when doing it, the carb is upside down as such) so it's not actually the float height or the fuel level which are the same things. That unfortunately is what Chris and I argued about (he's forgiven, so hopefully I am too ). If you want to check the actual fuel height in the float bowls, you can do it on the bike, although I have never had to do it. You can probably also do it with the carbs off the bike with them held in a vice at the correct angle. If you look at the underside of the float bowls you will see the word 'Mikuni'.......if you imagine a line across the bottom of the letters and measure down, you should find it probably coincides with a parallel casting line. If you connect a clear tube to the float bowl drain and hold it upright, it will give you the fuel level in the bowl. As far as I am aware, that level in the tube should be near that casting line or, 15.5 to 16.5mm below the bottom of the 'Mikuni' word.
Further to Al’s post above I found info on another site stating that with the floats set at 14mm the level in the clear pipe should be level with centre of the circular feature (with pin) on the side of the carb, this confirms the fuel level is correct, It’s possible to set the floats correctly but still have an incorrect fuel level for various reasons, leaking needle valve for example, so it’s worth confirming th level with the clear pipe, photo of my M900 and as you can see it’s nice and level with the circular feature (actually it’s difficult to confirm from the photo due to the angle it was taken at but trust me it was level).
That pin is roughly in the same location as the casting line I mentioned (and roughly the same dimension I mentioned). I reckon the OP's problem might be low-ish fuel level in the float bowl (IE 14mm float measurement) and leaking float needle valves.......if the fuel level was higher, it might then be peeing out of the overflow pipes. I have had two genuine new float assemblies and needle valves which leaked like mad, causing similar problems. I didn't find that out until I reduced the float height measurement to 12mm (more fuel in the bowls), and petrol ran out of the overflows.
so guys, taking into acount the carbs have been rebuilt with new parts including valves etc, you think that it is float heigh setting that may be causing my banging issues and not starting?
It might be.......it took several times of carbs off to get mine right so that it would stop a similar issue. How did you check the float heights? Assuming the float assemblies are all in good condition and the O-rings aren't damaged...... The correct way to do it (IMO) is to take the float bowl off, then turn it round so that two screws only hold the float assembly in the carb body. If you don't do that, the whole assembly can move in the O-ring while you are setting the height. Once that is done, turn the carb body upside down and gradually turn it so the gasket face is almost vertical, while watching the assembly just make contact with the little sprung loaded pin in the needle. The pin should not be pushed in by the assembly. You then measure the rectangular float so that the 'top' corner is the dimension you need above the gasket face. With regard to the dimension, IMO it is arbitrary @ 14mm. My bike needed 12.5mm; in fact 12.00mm would have been better. Although my bike was a 750, I picked up a pair of carbs from a 900 which was a write off, but running OK until it got bent. When I removed the float bowls, I checked the float heights. The horizontal cyl one was set at 12mm, the vertical cyl at 9.5mm. I made a measuring gauge using two credit cards as the frame, with a sliding cut down card in the middle, but I can't find a photo of it. If I have time I will dig it out and take another photo. The photo below will give you an idea, though (and how to hold the carb). PS. I haven't gone back through this thread......Is a Dynojet kit fitted?That might have some bearing on the issues.
I didn't make that one.....I think that is a Cycleworks photo. Post a pic of yours if you have it handy......I can't find mine at the moment.
Ill take another look at the float level measurements again I used a digital caliper and a piece of .005 plastic card lay accross the floats. with the floats gently resting onto but not pushing the valve i took the measurement from the gasket faces. One was at 14.3 and one at 11.8. I checked re settled and masured 3 times before adjusting then did the same again having bent the tab to set them. This is what is confusing me. The popping banging crap was happening before this process started and yes she has a stage 2 dynojet 144y mains with the needle set at 4 from the top. If the next effort fails, then I may pop the carbs back to stock, see what happens.
Have you got a footpump with a pressure gauge? If so, set the carbs in a vice at roughly the right angle they would be on the bike. Fit a hose so you can fill the carbs with fuel from something like a squeezy bottle. Give it an extra squeeze when you think they are full and see if fuel pisses out of either the overflows or if it runs out of the carb body........I expect it will do one or the other.......it will depend upon the fuel height/float height measurement. When they have stopped dripping, fit the footpump to the fuel line hose and give it a squeeze while watching the gauge and the carbs......the float needle valves should 'let go' at about 12 psi and fuel will pour out. If the 'let go' pressure is significantly lower, then you have a float assembly/needle valve problem. PS. For Stage 2 Dynojet you need straight through zorsts and a partially open airbox.....just try it with the trumpets off first. Pilot screws turned out by 3.5 to 4.5 turns (don't expect them to be the same).
If it has always popped and banged there’s a good chance it’s because you have no baffles or straight through baffles, my M900 did it when I fitted carbon silencers with straight through baffles, especially on the over run, it sounded like the Battle of Britain going on behind me, I fixed it by making end caps for the baffles and drilling holes in the sides of the baffles, it’s because air gets into the exhaust through the exits and allows unburnt gas to ignite, I doubt this is your current problem though, But it could be adding to it, I think it’s more likely the primary circuit is still blocked somewhere, but that’s a bit of a guess having not seen/heard, as Al said it’s important to clamp the rear edge of the float assemblies when setting the float heights using the bowls turned through 180 degrees, if you don’t you can easily be 1.5 mil off where you think you are, I’d do the clear pipe check ref above to confirm what your fuel level is before stripping the carbs again though, when you open the drain bolt do it very slowly and allow the level to settle in the clear tube by gently moving the pipe up and down before reading it, also check there are no air bubbles in the pipe, it’s a bit of a faf but you’ll know you’ve got the correct reading when you’ve got it to repeat a couple of times. Catch you in a bit I can feel a ride coming on!
Found the photo I was looking for.....funnily enough on this forum. Get two cards and cut them both identical so the legs are straight and will sit across the float assembly without touching it and the bottom of the legs sit on the gasket face. Take a third card and trim two pieces the same size as the legs but full height. They go between the other two cards as spacers so there is a gap between the middle of the two shaped cards. Stick the legs and spacers all together....supaglue worked on mine. Cut another piece off the third card and slide it between, if it all fits nicely then mark it in mm. The photo shows it reading 9mm float height. .
First photo on link shows clamping the assembly using the bowl https://litetek.co/Guide_Mikuni_BDST_Fuel_Level.html
Interesting they show the diamond shape float should be measured, but at least it isn't 14mm on that side! I guess there could be a difference of 3mm between the rectangular float and the diamond one.
Again folks you've shown that there are other things to try before banging my head off the garage wall. The reverse clamping of the float assembly is inspired, I will do that if i have to take the carbs apart again. As this weekend is set up to provide another wet windy affair, ill get into teh garage and exhaust my options. The bike is running an open airbox with a K&N filter with stage 2 dynojet and straight through pipes. Now your saying 3.5 - 4.5 on the fuel screws, this will be my first port of call. The Dynojet instructions I think state 3.5 full stop.
Whether stock carb guts or Dynojet kit, any pilot screw settings that are specified, they are only a starting point. Also bear in mind that the pilot circuit includes the pilot screws and the fuel level in the float bowls. In other words, if your fuel level is a bit lower than a decent working level, then the pilot screws will need more turning out. BTW.....Dynojet Stage 2 pilot screw setting for the 900SS is 4 turns out.
Thanks for your support in this guys. Today is one of those days in a good way. went to the bike and tried a start. nothing. Turned out the pilot screw to the rear cylinder to 4 turns (front was at 3 1/2) , go for start, Life! zero coughing or spluttering. Set the front screw to 4 turns and warmed the bike thoroughly. set the idle to 1100 rpm. perfect! Initially there was a lot of smoke but this i guess because she was running too rich, as she ran this disappeared and she revved freely and sharply with no hanging. I pulled the plugs to find a lovely dry powdery brown colour although to completely bottom this out i will need a good run out when the weather improves. Now onto the next "issue", for some time I have had what appears to be a fine oily spray around the back of the engine, this is not what I would call wet, but a damp sooty mist. I cannot see anything obvious from the rear of the engine. There are no tell tale marks of an oil leak, The only oil related thing in the vicinity is the emissions boxes from the crank breather and associated pipework that returns to the airbox. I popped the boxes off and checked the inside of the breather which is as dry as a bone . Started the bike and it is doing exactly what it should be. I suppose the question is, anyone got any ideas what it may be? P.s Ive ordered some new hoses etc.
I refer the gentleman to the second post in this thread Glad it’s running! Ok let’s have a think about the oil
Iv'e ordered what i will call service parts including a new airbox breather hose as iv'e found a small split in the current one, albeit the wrong end. I think i may have found the spray issue. Where the rear pipe joins the horizontal pipe, there are some small, what appear to be track lines of sooty residue. Now I know the front cylinder head has a slight oil leak requiring a replacement o ring which i think might have gone brittle and the whole reason for the bike being a garage queen for so long was down to a cracked rear cylinder head and internal oil leak a few years ago. I have a feeling that a small pool of oil may have accumulated in the exhaust pipe which is being blown out of the joint. Now she is running the next job will be to replace the o rings on the cylinder and give the exhaust pipes a thorough clean through and dry out with some solvent to de-oil the pipework. re fit with new gaskets and stainless bolts and see where we are....
