1998 Sportster 1800 Carb Issue - Likely Diaphragm Replacement?

[lauramsportster1800]

I think I know what the answer here is but any input is welcome.
Totally new to all this but through this forum and youtube I've learned a lot in the last few weeks.
My machine starts and runs well - when its wide open. When I am at around 6000 RPM it is great. The issue is my starboard side engine.
All is well slowly increasing throttle until about 3000 RPM. It then sinks down to 2000 RPM and BLASTS up to 6000RPM.
I changed plugs, checked fuel lines (previous owner replaced grey lines, no green gunk).
I believe I have a diagnosis. Looking into the starboard carb opening (everything turned off) when I throttle up a bit, gas continuously comes out of the needle. When I compare this to the port, nothing comes out when I throttle up.
Conclusion: Either the starboard diaphragm is torn, or there is something wrong with the needle and it is flooding too much fuel into the carb.
Can anyone confirm if this sounds right? I know that the answer is likely carb rebuild I've found the correct kit on amazon and will likely be hitting Checkout soon!
Cheers

 

[TheGTIjetguy]

It seems you are dead on. When advancing the throttle the accelerator pump should dump a steady stream of fuel into the carb throat, as you saw on your starboard motor. Will probably need a carb rebuild, but you can check the adjustment of the accelerator pump linkage first to make sure the throttle is actuating the pump during acceleration.

 

[lauramsportster1800]

It seems you are dead on. When advancing the throttle the accelerator pump should dump a steady stream of fuel into the carb throat, as you saw on your starboard motor. Will probably need a carb rebuild, but you can check the adjustment of the accelerator pump linkage first to make sure the throttle is actuating the pump during acceleration.

To clarify - the starboard motor is the one experiencing the issue. Sluggish, sparks fouling etc. I think there is TOO much fuel going into the starboard and not enough going into the port. Is there a way to establish the right amount?

 

[TheGTIjetguy]

Interesting. Are your engines dual or single carb? To clarify, the engine is off, does it only dump fuel into the intake when advancing the throttle, or does it continue if you hold it steady, at half, full throttle etc.
Too much fuel can definitely foul plugs. A common failure point in carburetors is the needle valve and seat. That can allow excess fuel into the engine at idle if leaking. It could load up when reving up. You can perform a “pop off” test to see if they are holding a good seal.

 

[lauramsportster1800]

Interesting. Are your engines dual or single carb? To clarify, the engine is off, does it only dump fuel into the intake when advancing the throttle, or does it continue if you hold it steady, at half, full throttle etc.
Too much fuel can definitely foul plugs. A common failure point in carburetors is the needle valve and seat. That can allow excess fuel into the engine at idle if leaking. It could load up when reving up. You can perform a “pop off” test to see if they are holding a good seal.

Single carb. When the engine is off if I advance it to about 1/4 throttle it dumps a little squirt in. I will look into the pop-off test! I ordered a carb rebuild kit anyway since the gaskets and seals have probably never been replaced anyway and it can't hurt.
Thank you for your helpful input!!!

 

[tobytd]

Only genuine Mikuni kits ! After market kits will cause nothing but problems. Order needle and seat separate, they do not come in the kits