Wacky Carbs


Premium Member
Premium Member
Hi all,
Over the winter I rebuilt the carbs in my 2003 Sportster LE 951. I purchased the boat early 2023, and the boat ran pretty bad. Rough idle, stalled when giving it gas, and would run at high rpm great for about 20 minutes, die, and not start back up until you changed the plugs. Plugs would come out dripping wet with oil/fuel.

When I rebuilt the carbs, I surprisingly discovered they were open before, and they were very clean inside. The settings were quite off from spec... Low was 1 turn on each side, high was 1 turn, 1 1/2 on the other side... I also discovered that the wrong pop off spring was used, as it had a silver spring that was popping off around 30psi. I cleaned every nook and cranny, inspected and cleaned the jets, replaced the needle valves and seats, the metering assembly, and put the proper black spring in, all genuine Mikuni parts. I tested the carbs afterwards and got a perfect pop off pressure of 20psi. I set all the carb adjustments back to factory specifications: 1 1/2 low, 0 high, and used the recommended idle set of 1/2turn after touching plate. (Trying to remember without the shop manual in front of me at the time of writing this)

I also discovered the oil injection pump was set way too rich, so I properly adjusted that back to specs.

Took it out on the water and idles perfectly smooth, and idle to 1/4 is crisp, smooth, and responsive. 1/4 to 1/2 has a slight hesitation but cleans up pretty quick. My problem is anything over 1/2. When giving it gas, the motor just bogs out. You can go from 1/2 to WOT and nothing happens, just the motor sounds like its running out of breath. Now, if you quicky pump the throttle up and down while going above 1/2, the motor runs great and will eventually get to WOT running great until you let off the throttle again.

1/4 throttle plug chop looks mocha brown, 3/4-WOT plug chop plugs have a reddish tint(fuel additive from the winter storage??)
I tried to richen the low speed adjusters and did not seem to have any effect. I turned the high speed adjusters out about 1/4 and now the boat had a hard time running around 1/4 throttle...

I have done so much research and time learning about these carbs and boats I am beginning to get burned out. What do I need to do here? I'm assuming this sounds like a very lean condition over 1/2 throttle, but why does pumping the throttle make it run fantastic? From what I have read on here, the accelerator pump is useless after 1/4 throttle, and I can see it squirting fuel into the carbs when giving it gas. I don't remember the exact compression test results I got, but I do remember that they were well within specs. I am not expecting someone to diagnose this over the web, but if someone could point me in which direction I should be taking from here I would greatly appreciate the help!
There is no "Black" spring for the 951 carbs, there is only one silver spring that is specific to 951 engines.
There is no "Black" spring for the 951 carbs, there is only one silver spring that is specific to 951 engines.
Interesting... From what I kept reading online everyone said the 951 needs to have a black spring, and that the BN46i rebuild kit comes with the wrong spring. (Not arguing with you... just stating why I went that route)

From what the shop manual states, my pop off should be between 19-23 PSI. With the silver spring that was in there, my pop off was around 30 PSI. I swapped the black 80g springs in and my pop off was right at 20PSI.

I see from other forum posts from you that the 951 should have the 95g dull silver spring, which is what I believe was in the carb when I first took it apart... Why was the pop off so high at 30PSI then?

I will say, I originally did NOT replace the spring, as a lot of people say to leave it as is. I tested 30PSI before I touched anything, and once I rebuilt it all, including new needle and valve seat, and the pop off was still way too high, that was when I swapped in the 80g that I kept seeing people recommend, and I got it right into specification.

I used this chart once verifying the needle and valve seat size (2.0) :
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Don't get hung up on pop-off as it is a test and not how the engine runs.

Make sure you have the correct needle and seat, spring and new arms with pivots and pop-off should be within spec.

No, the 80 gram spring that is not in the kits is for the 787 engines, not for the 951/947.

That chart is not for the 951 "special spring". The 951 Platinum spring is 16.4mm long. The standard Platinum is a 65 gram at 14.95mm long. The standard Dull Silver is 95 gram and 15.85mm long. DO NOT interchange these.
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