Stumble @ 2500 rpm

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27 Jun 2011 22:35 - 27 Jun 2011 22:35 #459839 by Patton
Replied by Patton on topic Stumble @ 2500 rpm

55fbomb wrote: ...have set the floats....


Fuel level is the critical factor, not float height.

Measured "float height" is a stab toward achieving the proper fuel level, but should wind up being whatever is required to attain the correct fuel level.

See clear tube illustration below.

Would remember to turn the pilot air screws out counter-clockwise, which adds more air to the pilot circuit mixture, and should thereby lean the existing apparently over-rich pilot circuit mixture.

Good Fortune! :)


1973 Z1
KZ900 LTD
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Last edit: 27 Jun 2011 22:35 by Patton.

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27 Jun 2011 22:39 - 27 Jun 2011 22:39 #459840 by Patton
Replied by Patton on topic Stumble @ 2500 rpm
If carbs have bottom-located pilot mixture screws, the screw turns in clockwise to lean the mixture, and turns out counter-clockwise to richen the mixture.



Good Fortune! :)

1973 Z1
KZ900 LTD
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Last edit: 27 Jun 2011 22:39 by Patton.

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27 Jun 2011 23:20 #459850 by 55fbomb
Replied by 55fbomb on topic Stumble @ 2500 rpm
Thanks Patton, i will try the clear tube thing, i kinda figured that what i had to do but wanted to be sure about it. i will give that a go, i bet the level is high. thanks again.

1974 CB750
1975 CB750
197? CB750 mutt
1977 KZ1000

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28 Jun 2011 00:03 #459863 by cavanaughracing
Replied by cavanaughracing on topic Stumble @ 2500 rpm
It might like the 17.5 pilots :) But unless you have the fuel levels correct, you'll chase your tail trying to dial in jetting. Get that correct and the rest will fall into place a lot better ;)

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01 Jul 2011 10:45 #460454 by 55fbomb
Replied by 55fbomb on topic Stumble @ 2500 rpm
Ok, i put the 17.5 pilots back in and set the mixture screws to 1 turn out and it runs much better now, a little fine tuning and i think its gonna be OK. I did uncover somthing else tho, the mixture screw on the number 2 carb doesnt seem to do anything, i can run all the other ones in the whole way and hear the engine start to sputter, and number 2 i can run in the whole way and it doesnt seem to make any difference. anyone know why this would be? i cant get any of my bowl drain screws out so the clear tube test is gonna be a problem, and i dont wanna risk stripping them out and having my bike down.

1974 CB750
1975 CB750
197? CB750 mutt
1977 KZ1000

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01 Jul 2011 12:36 - 01 Jul 2011 12:48 #460464 by Patton
Replied by Patton on topic Stumble @ 2500 rpm

55fbomb wrote: ...17.5 pilots back in and set the mixture screws to 1 turn out and it runs much better now...the mixture screw on the number 2 carb doesn't seem to do anything, i can run all the other ones in the whole way and hear the engine start to sputter, and number 2 i can run in the whole way and it doesn't seem to make any difference. anyone know why this would be? i cant get any of my bowl drain screws out so the clear tube test is gonna be a problem, and i don't wanna risk stripping them out and having my bike down.


If turning the pilot adjustment screw fails to influence rpm, would suspect an imperfect pilot circuit for whatever reason, despite the reported cleaning, oftentimes due to an obstructed passage (see earlier posted diagrams of pilot circuit), or perhaps due to a damaged pilot screw (perhaps the pointed tip is damaged or its seat - orifice - is damaged), or using an incorrect pilot screw.

Where carbs are in fact clean and in otherwise perfect condition, an excessively rich pilot circuit may result from a too high fuel level (as earlier noted).

:unsure: Has water mist been sprayed onto #2 exhaust pipe while engine is running at idle rpm to determine whether #2 cylinder has combustion?
Mist is supposed to instantly vaporize (i.e., sizzle off).

With float bowl removed, may help to remove the drain screw by allowing to soak in PBBlaster or mixture of 50-50 acetone and ATF.
And could also try applying heat to the bowl area around the drain screw.

Are the drain screw heads in good condition? Or have they been damaged during efforts to remove them?
It's essential to use a screw driver having a tip that perfectly fits the slot in the head of the drain screw.
Have in the past resorted to using a hand held impact driver, but must be careful so as not to over-do it ala Godzilla.

Likely could be especially important to check and assure correct fuel level in the #2 carb.

Good Fortune! :)

1973 Z1
KZ900 LTD
Last edit: 01 Jul 2011 12:48 by Patton.

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01 Jul 2011 12:50 #460466 by Patton
Replied by Patton on topic Stumble @ 2500 rpm
Forgive if already covered, but what is spark plug reading on #2? :unsure:

1973 Z1
KZ900 LTD

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01 Jul 2011 22:47 #460569 by 55fbomb
Replied by 55fbomb on topic Stumble @ 2500 rpm
All cylinders were around 450 degrees except #2 which was about 75 degrees hotter for what ever reason. im wondering if something may be wrong with the idle circuit like you mentioned. its still loading up a little but is much better than it was. im going out for a ride tomorrow and will pull the plugs once i get home and it cools, i have NGKs in it and they dont seem to show color very fast. last time i pulled them they were pretty dark on #1 and #2 and 3 and 4 were a nice golden brown. once i get home from my ride the float bowls will be getting soaked in seamfoam in an attempt to get at least a couple of the drain screws loose. i have been using an impact driver to try to take them out since the larger flat bit was a perfect fit in the screw head but was unsure about actually hitting the driver with a hammer. I wish i had a set of smoothbores lol. its not popping out of the carbs and no popping from the exhaust on deceleration so i think im getting close i just have to keep taking you guys advise and be persistent i guess. thanks again everyone.

1974 CB750
1975 CB750
197? CB750 mutt
1977 KZ1000

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02 Jul 2011 08:34 #460605 by Patton
Replied by Patton on topic Stumble @ 2500 rpm
Getting only one of the drain screws out would allow the same float bowl to be used separately on each carb for the clear tube test, which may be performed while carbs are removed and on the work bench.

To save some time during the process of making any needed float tab adjustments, usually only one float bowl screw is necessary to attach the float bowl before performing the clear tube test.

It may not leak at all with only one screw, but a slight fuel leakage past the gasket won't adversely affect the test results, because the volume of incoming fuel surpasses the leakage.

Good Fortune! :)

1973 Z1
KZ900 LTD

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02 Jul 2011 08:45 - 02 Jul 2011 08:46 #460606 by Patton
Replied by Patton on topic Stumble @ 2500 rpm

55fbomb wrote: ...float bowls will be getting soaked in seamfoam in an attempt to get at least a couple of the drain screws loose. i have been using an impact driver to try to take them out since the larger flat bit was a perfect fit in the screw head but was unsure about actually hitting the driver with a hammer....


If not already done, would try applying heat to the float bowl area around the drain screw, so that it's plenty hot when using the impact driver.

A propane torch should work nicely, using a pencil tip flame.

Before applying flame, the float bowl should of course be sitting alone on top of the work bench and far away from the carbs or any fuel or other flammable substance (such as a canister of carb cleaner).

Good Fortune! :)

1973 Z1
KZ900 LTD
Last edit: 02 Jul 2011 08:46 by Patton.

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