Carb rebuild questions

[Andy4k6]

I am in the process of rebuilding the carbs on my 95 speedster (twin 657x fitted with dual carbs which I think are 38's)

First of all, I am using the SBT rebuild kits, I have reused the original spring and arm for the needle valve assy as it seemed better than the SBT replacement, has anyone used the SBT kits at all and do you have an advise to offer on them?

My other question is regarding the needle valve assembly adjustment, when placing a straight edge on the arm should it be perfectly level and applying pressure onto the needle valve? When level mine is not lifting the valve at all but is not pressing the valve either it's half way between hitting the lifting arm and pressing the needle valve which I am assuming is incorrect?

I have checked the pop off and with my bike pump it says it's about 20 psi which according to the manual should be no more than 21psi however this was done with the cover and diaphragm off the carb.

Many thanks

Andy

 

[Dr Honda]

As per our earlier discussion...

The aftermarket kits are bad. You can get away with using the gaskets, O-rings, and valves. (the clear plastic) Sometimes, even the O-rings are questionable. I've seen a bunch of kits where they were simply the wrong size.

As far as the Diaphragms... they are useless. The "Normal" problem with them is... the bellows on the edge is too tall, and the rubber is stiff. In turn... they will keep pressure on the needle. This problem will normally show up as a "Leaky" needle, and making re-starting hard. The other way it shows up, is that the needle doesn't want to open all the way... and then it will feel like it's running out of fuel. (so, problems at both ends)

The issues with aftermarket needle and seats is that they aren't machined right, and makes tuning hard, to impossible.

And... the aftermarket springs are always wrong.



One thing to remember... we test the carb with a "Pop-off" test, putting pressure behind the needle, and seeing where it opens. Well... I personally think it's a bad test, (misleading) since that's not the way the carb actually works. The carb uses vacuum to suck the diaphragm down... and pulls the arm, and lifts the needle. Unfortunately... there's no way to do it that way on the bench. And the Pop-off test was developed using OEM parts. So... as soon as you use a part that has softer/harder rubber (tip of the needle) or the seat is a different height (poor machining)... or a part with different tension... then the test parameters will change.

With that said... the other hard parts may be able to be used... but it would take someone experimenting with them, to build new pop-off parameters. (maybe) But the diaphragms are just made wrong.


To your last comment of the pop-off being done with the cover off... that's the way it's always done. You have to be able to get to the needle, so you can submerge it in oil, to get a clean pop. But, I bet if you put the cover on, with the aftermarket diaphragm, it wouldn't hold pressure.


Anyway... to step back to what you PM'd me about... assuming that the gaskets and O-rings are OK... I would make sure to have OEM needle/seat, springs, and diaphragms.


That's my story... and I'm sticking to it.

 

[Andy4k6]

Thanks for the detailed responce.

I guess I shall use the sbt bits and try and find 4 genuine diaphragms to complete the carbs.

I have no idea how manufacturers get away with manufacturing parts that do not work but at least I have the rest of the kit at a cheap price I suppose.

Many thanks for your time

Andy

 

[Coastiejoe]

You will be re-doing the job..

The aftermarket simply are not the same.

Even when you do get the OEM kits, I suggest NOT changing the springs as long as your pop-off is correct at this time. The replacement springs seem to cause issues..

 

[Andy4k6]

I have kept the same spring and arm in the one that I have rebuilt so far and pop off seemed perfect at approximately 20 psi.

Andy

 

[1983]

Listen to Joe, if you don't use a complete kit with all parts with OEM Mikuni then you will be back in the carbs soon. Some things you can skimp on not going OEM with, the carbs arent one of them.



Rob