If I could somehow tickle my Makuni

I have an early 850 with a single 34mm Makuni. My bike would start with one kick if I could get some gas to my carb. It is an Interstate so plenty of flow pressure and I run my fuel line similar to the one Old Britt sells. I choke it but it still takes too many kicks to get the gas built up in the carb. For some reason it doesn't even try to start with the choke on but after some kicks with the choke on and then I remove the choke and that is the first sign of life. It just seems like the fuel will not flow into the carb but after it starts, it runs fine. I have made sure that my tank vent is clear and the problem is only after the petcocks have been turned off for the day and then reopened for the next outing. Any thoughts would be appreciated.

If these symptoms only seem to occur after the petcocks have been off for a while, it sounds as if there is either very limited flow thought the petcocks and/or the Mikuni float chamber venting is restricted or the float needle is sticking.
Try disconnecting the petcocks from delivery pipes. Then open the taps and catch the petrol in a cup or something, observing the flow rate. Petrol flow is fairly gentle but judge for yourself if it seems to be sufficient. The seals inside the petcocks tend to either swell or break up after a long time and this either causes leaks or severe flow restriction. See if the symptoms are eased by having a full tank of fuel. If so, replace the petcocks.
The routing of the fuel tank pipes has a slight effect on the ability of the fuel to find its way into the carb float. Is it improved, for example. if both petcocks are opened? An air lock usually sits in the reserve fuel pipe. This should not matter unless the needle valve, for example, is a bit tacky and needs that little extra head of fuel to open it. Opening both taps would provide marginally more fuel pressure.
Check that the Mikuni float vent is clear. Failing all these, take the float chamber off and clean possible gums and distilates from the needle float area.
Have you had the Mikuni a long time or has this fault only just developed? What pilot jet size are you using? Presumably compressions and valve clearances are ok. Does it ever backfire of kick back? What ignition system is fitted?
My VM36 Mikuni always needs choke to fire from cold but once fired I have to flick the choke off immediately, otherwise it would be over-rich. It seems to be unable to fire cold without choke. The choke circuit is crude and non-progressive but does give the desired mixture to excite cold cylinders. I usually crank through 3 or 4 times slowly with choke on just to introduce some vapours into the bores. Then swing it with gusto and it fires first time. Then I flick the choke off and keep throttle tickled until it stabilises.

I keep both petcocks open and do have good flow because if I disconnect the line going into the carb, gas flows and then it seems to get gas to the carb after the disconnect reconnect sequence as if there had been an air lock. It runs fine after it starts first time and starts easily after it has been running. I will see where the carb vent is and check that. Does the carb need to be disassembled to clean the carb vent? The bike does not kick back and my ignition is all stock. Engine number is 303012.

Different choke jets should be available, possibly you need a richer one?

Check that the choke plunger mechanism is seating correctly when the choke is on. And check the float height.

Also, that type of plunger choke works independently of the main carb bore, and the throttle needs to be kept closed, or very nearly closed for starting, if the throttle is opened then the intake air will pass directly through the main carb bore, and not through the carb choke enrichment circuit, so the carb will not supply the correct rich starting mixture.

Hi Yellow_Cad. I have not had need to strip my Mikuni, apart from float bowl to change pilot and main jets, but I would think it a lot easier to have it on the bench to check the breather and float system. Check the float height and cleanliness of needle valve seating. I cannot quite figure out why disconnecting and reconnecting the fuel supply pipes should clear what, I agree, sounds like an air lock in the supply pipes. If the fault was inside the carb then tapping or shaking the carb (should be possible on rubber mount) should have an effect on the 'air lock'.

scim77 said:

I cannot quite figure out why disconnecting and reconnecting the fuel supply pipes should clear what, I agree, sounds like an air lock in the supply pipes.

Click to expand...

If we think about this logically, then there is likely to be a certain amount of fuel already in the float bowl and supply pipe before the tap is switched on, unless the bowl and line has been completely emptied?

In which case, if a small amount of fuel enters the float bowl, the float will rise and the needle will shut off the flow before the line is purged of air, and maybe give the impression of an air lock?


Once the engine starts, then the Mikuni float/needle will open, and the fuel should then start to flow to the carb?

L.A.B. I can follow your reasoning but it still does not explain why the engine will not fire up unless the fuel pipe removal procedure is carried out. It seems that, whatever level of fuel is remaining in the float bowl, no more fuel will flow into the carb to top it up unless the pipes are removed.

Well, the next time I started the bike I took the fuel filter out of the line and used the sequence scim77 gave me and it acted completely normal starting with the choke on and so forth. The next time after that starting from cold wasn't that good but wasn't as bad as previously. I read a post somewhere that advocated screwing the idle screw in before starting and then backing it up as it warms up. I have been doing this but I think the only thing that does it to keep from feathering the throttle after it starts. After it is running, it takes a few times to get it adjusted back to where it should be. Does this sound like a good or bad thing to have in the starting sequence?

Not good.

You shouldn't have to disturb the idle adjustment, it is not part of the cold starting enricher circuit which should be entirely responsible for proper starting.

If I were unable to tickle my carbs the thing would never run.