Cured the low end stumble finally!

[sparkfly88]

For the longest time I've been messing with the fuel system trying to get rid of this low end load stumble..

Taking off from a stop it'll stumble a bit, and while cruising.

Synced and colortuned to a little richer from Blue as normal... And running NGK bp6es. The NGKs have like 4k miles on them.

Well last night I decided I'm gonna try something. I colortuned it to the cobalt blue, and changed the spark plugs to autolite platinum ap63.


Wow! What a difference. Smooth idle, smooth cruising, and no stumble at any rpm.

Woo!

[motoman]

For the longest time I've been messing with the fuel system trying to get rid of this low end load stumble..

Taking off from a stop it'll stumble a bit, and while cruising.

Synced and colortuned to a little richer from Blue as normal... And running NGK bp6es. The NGKs have like 4k miles on them.

Well last night I decided I'm gonna try something. I colortuned it to the cobalt blue, and changed the spark plugs to autolite platinum ap63.


Wow! What a difference. Smooth idle, smooth cruising, and no stumble at any rpm.

Woo!

Well, since you performed the cardnial sin of changingtwo things at once, guess we'll never know which resolved the issue. But, good to hear it's resolved either way.

[CaptonZap]

Hey Brant, why don't you convince him he ought to re-install the NGK's, and see if that returns the stumble?

CZ

[jjz28]

Hey Brant, why don't you convince him he ought to re-install the NGK's, and see if that returns the stumble?

CZ

Exactly, that is what I would do. It would be nice to know if the plugs or the tune was the culprit.

[sparkfly88]

Well, the stumble is back....rarely....but noticeable

poop

[TADracer]

Well the heat range of the BP6ES and the AP63 are exactly the same (stock) so the only thing the platinum plug could be contributing is an improved spark kernel (and that is debatable). Give a hotter plug a try (BP5ES) as my machine sometimes likes a hotter plug.

I think the cause of any stumbling lies elsewhere.....valve adjustment, ignition timing, vacuum advance mechanism, less than squeaky clean pilot circuit passages, etc. Remember, setting the idle mixture to cobalt on a color tune only affects the motor at idle. As soon as you crack the throttle you are basically off of the idle circuit and onto the pilot circuit (though the idle circuit feeds from the pilot circuit).

[sparkfly88]

Well, ive already redone the advance system, valves are adjusted..ignition is timed, carbs are spotless many times over...

My next plan of attack is the pinholes in the diaphragms and new coils.

[IanDMacDonald]

Holy Moley! You've been knowingly riding around with pinholes in your carb diaphragms, and tried to diagnose other causes of stumble? Always one issue at a time as someone said above (I think Don).

[sparkfly88]

Holy Moley! You've been knowingly riding around with pinholes in your carb diaphragms, and tried to diagnose other causes of stumble? Always one issue at a time as someone said above (I think Don).

I've been broke for the longest time if you go back through my threads....

This whole bike build has been on ECONOMY

[DGXSER]

As a brethren if not a founding father of the CheapAzz tribe of XS owners, I have used RTV to seal over pinholes in the diaphragms before. Its not a long term fix, but it will buy you several months or more of service from the existing diaphragms.

And yes, that could easily be the cause of your stumbles. When you change throttle position is the biggest change in vacuum, and that is when the slides are trying to open and will flutter from the leaks in the diaphragms.

[sparkfly88]

Holy Moley! You've been knowingly riding around with pinholes in your carb diaphragms, and tried to diagnose other causes of stumble? Always one issue at a time as someone said above (I think Don).

And the stumble has been on the backburner while I took care of all of the other issues, it runs, and gets me where im going ( when it decides to ) I'm just now beginning to focus on the little stuff

[sparkfly88]

As a brethren of not a founding father of the CheapAzz tribe of XS owners, I have used RTV to seal over pinholes in the diaphragms before. Its not a long term fix, but it will buy you several months or more of service from the existing diaphragms.

And yes, that could easily be the cause of your stumbles. When you change throttle position is the biggest change in vacuum, and that is when the slides are trying to open and will flutter from the leaks in the diaphragms.

I tried using that rubber sealant in a can...it didnt work too well.
I get the stumble at cruising or when just barely getting on the throttle...like just a HAIR above a closed throttle

[IanDMacDonald]

As this is the first I've heard of the pinholes, the first fix on the otther forums was Plati-Dip, but did not hold-up over time. We now use Gorilla Glue in several layers, as it allows flex unlike the Plasti-Dip. One other alternative you could try is the gas sealer found at all hardware stores, auto parts stores, etc.

[IanDMacDonald]

Rubber Sealant? As-in Flex-All or Rustoleum variety? Yeah, they probs would not hold upto gas well. Some thought rubber cement would work; it did not.

[DGXSER]

I used the black RTV used for gaskets on engines. It held up for close to a year IIRC. I eventually spent the money a new slide with diaphragm.