'78 stock carb | Yamaha SR500 Forum

strick
Full Member

Posts: 112

'78 stock carb Jun 9, 2008 13:10:34 GMT -5

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Post by strick on Jun 9, 2008 13:10:34 GMT -5

1st - Can someone tell me which Mikuni this is?

I am nearly done with my street tracker, other than finishing touches on the paint, and one minor carb issue, I'm done.

Here is my carb issue: The float needle is done (not functional). The spring actuated pin that suspends the float does not contain enough pressure to suspend the float when holding the carb upside down with the base horizontal. The float just drops and will not supsend at all.

I ordered a needle and seat from Thumperstuff and got a needle and seat that looks NOTHING like stock. The needle is attached to the brass seat with a wire loom type of device. According to Mark at Thumperstuff it works the same = WRONG! I used a feed tank this weekend to start the bike, but the fuel is not being introduced into the cylinder. I believe the float needle does not drop down far enough to feed the carb & accelerator pump. Also, if I let the feed tank sit on the carb long enough it evenly starts coming out the overflow. Has anyone else had any experience with this 'new' needle and seat?

Also, I assumed that when I was using my feed tank to start the bike that I was to PLUG the vacuum hole (tubing with a golf tee attached slid over the vacuum port). Is that correct? I don't have much experience with vacuum controlled tank/petcock setups.

Thanks guys (mates)!

Strick[br]'78 SR500E, '04 CRF450, '83 XL600R, '80 SR250, '04 VRSCB[br]'67 Camaro 'vert (408", 505hp) and on and on!

solo2racr
Senior Member

Oh what fun, it is to ride, on a one hole cafe' bike!!

Posts: 651

'78 stock carb Jun 9, 2008 14:28:47 GMT -5

Quote

Post by solo2racr on Jun 9, 2008 14:28:47 GMT -5

Jun 9, 2008 13:10:34 GMT -5 strick said:

1st - Can someone tell me which Mikuni this is?

A picture here would be VERY helpful. Particularly of any writing beside the enrichment plunger (choke)

Jun 9, 2008 13:10:34 GMT -5 strick said:

I believe the float needle does not drop down far enough to feed the carb & accelerator pump. Also, if I let the feed tank sit on the carb long enough it evenly starts coming out the overflow.

If the "float needle does not drop down far enough to feed the carb & accelerator pump" then there is no way it would reach the level in needs to overflow. Not sure what you are trying to say here.

Jun 9, 2008 13:10:34 GMT -5 strick said:

Also, I assumed that when I was using my feed tank to start the bike that I was to PLUG the vacuum hole (tubing with a golf tee attached slid over the vacuum port). Is that correct?

Yes, it needs to be blocked off to use an remote tank setup. Your method is as good as any and I have used it myself before.

www.N-RPerformance.com

strick
Full Member

Posts: 112

'78 stock carb Jun 9, 2008 16:15:49 GMT -5

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Post by strick on Jun 9, 2008 16:15:49 GMT -5

Jun 9, 2008 14:28:47 GMT -5 solo2racr said:

[If the "float needle does not drop down far enough to feed the carb & accelerator pump" then there is no way it would reach the level in needs to overflow. Not sure what you are trying to say here.

Not true, with most carbs the gravity feed into the float bowl will continue until it reaches the overflow (brass tube inside the float bowl base) and OUT the carb.

Keep in mind that I am saying the float needle never opens up enough to feed the carb, so the fuel keeps flowing in to the bowl until the float is in its' top out position until it overflows out the brass overflow tube inside the base and out the plastic overflow tube attached to the carb.

Have you ever had some novice load their bike up in a race trailer with a non vacuum actuated tank, leave the gas on, and open the trailer at home and see gas everywhere?

Last Edit: Jun 9, 2008 16:19:30 GMT -5 by strick

Strick[br]'78 SR500E, '04 CRF450, '83 XL600R, '80 SR250, '04 VRSCB[br]'67 Camaro 'vert (408", 505hp) and on and on!

solo2racr
Senior Member

Oh what fun, it is to ride, on a one hole cafe' bike!!

Posts: 651

'78 stock carb Jun 9, 2008 18:53:34 GMT -5

Quote

Post by solo2racr on Jun 9, 2008 18:53:34 GMT -5

Jun 9, 2008 16:15:49 GMT -5 strick said:

Jun 9, 2008 14:28:47 GMT -5 solo2racr said:

[If the "float needle does not drop down far enough to feed the carb & accelerator pump" then there is no way it would reach the level in needs to overflow. Not sure what you are trying to say here.

Not true, with most carbs the gravity feed into the float bowl will continue until it reaches the overflow (brass tube inside the float bowl base) and OUT the carb.

Keep in mind that I am saying the float needle never opens up enough to feed the carb, so the fuel keeps flowing in to the bowl until the float is in its' top out position until it overflows out the brass overflow tube inside the base and out the plastic overflow tube attached to the carb.

Have you ever had some novice load their bike up in a race trailer with a non vacuum actuated tank, leave the gas on, and open the trailer at home and see gas everywhere?

I think that your wording is what I'm confused with. To me when you say "the float needle never opens up enough to feed the carb" The way I read it is that the float needle never leaves the seat. IE, the float needle is still in contact with the seat and CLOSING off the fuel flow. To me, the carb IS getting fed. Too much in fact in that you say it is overflowing. It's like you are saying one thing and then saying the opposite thing. I guess I'm just maybe reading it wrong.

All that aside, If you can post a pic of the carb, Let's see what you have and go from there.

Last Edit: Jun 9, 2008 18:55:25 GMT -5 by solo2racr

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