Author Topic: Pilot Jets  (Read 789 times)

Online Laverda Dave

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Pilot Jets
« on: May 21, 2021, 02:47:58 PM »
Apologies if this has been asked before but I did try a search and found nothing so thought I'd ask the experts for some advice........
So, the Kawasaki Z400J had been sitting unused and full of fuel in the carbs for 39 years. I stripped and cleaned the carbs and bought new Keyster kits although I cleaned and used the old jets in the end. The carbs were cleaned four times in the utrasonic cleaner using ali and zinc cleaning solution.
Back on the rebuilt motor and it refuses to run unless on choke, turn off the choke and it dies. It also hangs on the throttle when revved. This points to a classic pilot circuit blockage or an air leak (or both). I have now replaced the inlet rubbers as they were a bit hard. The new parts are pattern carb to cyl head rubbers and new carb to airbox rubbers so they should be good and have no air leaks but still the problems persist. 
It looks like the carbs will have to come off again for another inspection and clean :(. When i inspected the pilot jets I could not see right through them although the holes on the sides were clear. I passed a carb cleaning wire through the jet but it stops dead halfway up (just beyond the holes in the side of the jet). This applied to all four pilot jets so I thought there is a deliberate factory set restriction at this point? Is my thinking correct or should the drilling in the jet be clear from top to bottom and what I though was a restriction for fuel metering ids actually crusty old petrol deposits that needs clearing out?
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline AndyD

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Re: Pilot Jets
« Reply #1 on: May 21, 2021, 06:27:08 PM »
Dave,
Sounds like blocked hole on the jets.
I've just finished cleaning a set of CJ360T carbs in the same way with ultrasonic etc.
All jets and passageways looked clear but I had the same thing with the pilots and after careful poking with a strand of very thin copper wire and repeated blasts with carb cleaner the holes opened up.
At first the jets didn't seem to have a hole right through at all - luckily I had a set of aftermarket replacements (bought kit just for the O rings and gaskets) so could look at the shiny new one for confirmation and the hole clearly went through.
Good luck
Cheers,
Andy

Online K2-K6

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Re: Pilot Jets
« Reply #2 on: May 21, 2021, 07:35:04 PM »
It does look like the idle fuelling is impaired from symptoms.

Small jets can be so reluctant to clean out fully and drive you nuts.

Sometimes the only thing that's got the hardened and dried fuel debris out for me is to leave them simmering in and old saucepan of water for 30 mins or so. Running a chemy lab, all the apparatus was "steamed" out to get anything to dissolve and release as routine practice as it'll generally remove most things.

Online Moorey

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Re: Pilot Jets
« Reply #3 on: May 21, 2021, 09:23:37 PM »
 No doubt some will cry don't do this but if all else  fails get a set of micro drill bits and very gently clear the jet by using a smaller bit in your fingers than the jet size. Once there is a passage through then you should be able to clear it. Kehine jet sizes are in mm a #100 jet is 1.00mm in diameter a #20 jet is 0.2,mm etc, etc Practice on something else first not your carb or jet as these break quite easily and if you break one in the jet or carb  its game over.
  Only do this as a last resort.
     .

Online Laverda Dave

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Re: Pilot Jets
« Reply #4 on: May 22, 2021, 09:36:52 PM »
With utter amazement the carbs came off really easily for a change. I swapped the old jets for the new jets from the Keyster kit blasted the idle circuit with carb cleaner and left it for an hour to soak. Following a blast through with compressed air and reassembly back on to the bike (again so easy this time) and it started and idled without choke. A quick check with my old Gunsons colour tune to get the mixtures spot on followed by a carb sync and the bike idles really well, picks up and drops back down to a smooth idle.
Thanks again everyone for the great advice as always 😊👍.
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

 

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