Hi All
So I have my 48 mil Mikuni all mounted up on my fresh 113+20 S&S it has extensive head work and I'm told is running about 305 cfm on the intakes, I have the 600 cam fitted and also R&R Cycles Overflow air cleaner & backing plate. R&R make the best backing plate that i could find IMHO as the transition into the Mikuni venturi is the correct size without any restriction (as opposed to the adapter you can get from Mikuni which causes restriction back down to 42mil size).
I fitted the Mikuni as it came in the box with #25 pilot, #190 main and the 8CFY15-97 needle (stock for the 48mil).

The engine fired straight up and runs fine, except it has what I feel is lean backfire on deceleration (popping not banging), I have only done about 30 km on it so far really just as a bit of a shake down to see all is well. I pulled the plugs for a look as I said I'm thinking its lean out of the box with the stock jetting and the plugs are white and clean as a whistle, I know what I did is not a true plug chop but I was just looking for first indications. I'm not a novice when it comes to jetting as I've done two stroke and 4 stroke dirt bikes over the years and corrected the jetting on the super E that was on my old motor.

So, I'm thinking I'll go 27.5 on the pilot and go up to the next richer needle (I have the dealer Jet Kit) and leave the main for now as I'm still at Run In stage as not using that part of the throttle range yet. My plan is to get some color on the plugs working on the bottom and mid-range and work on 3/4 to WOT later.
Long winded I know but now you are all in the picture 😊.
My Question is does any have a similar set up 113 & 48 Mikuni and what jetting did you end up with?

 

I just purchased a bike with an EVO 96" S&S with the same carb and 600 cam according to the serial number on their site. I dont know the jetting yet but I have test ridden it a few times prior to pulling the wheels last night to swap on new tires. With the cam and all the obnoxiousness going on (I love it) I would never expect the engine on decel to be smooth, I expect what I call burbling. As a test when setting up webers I dial the idle mixture screws out to cover a lean pop, burble or sneeze and if I end up too fat at idle I up my idle jet then turn my mixture screws back in. The same can be done here. Main jets are addressed separately as you know.

 

I don't have an evo with a hsr48 but have a 131 ci TC with one.

Jetting is the stock needle -97 in the middle, 30 pilot, and 200 main..

IMO you won't get rid of the decel pop on a 48. You might be able to reduce it some but the bigger carbs use the same basic dimensions of hsr 42 and 45 for slide width.. There is less hangover on the slide and when you roll the throttle off, air can leak past the slide and cause a lean pop..My 131 has always had decel pop..

BTW changing to a thinner needle will likely make the pilot size go down as these carbs still suck fuel through the needle jet at idle..

 

Ok, Good info and seeing as I won't have to pull the tank off if I leave the needle for now, I might just go richer on the pilot and main and get a few more miles on it and see where I'm at. Ultimately, I want to put it on the Dyno and see exactly where I'm at, but I want to get some run-in miles on it first. plus dyno time is expensive here and i don't have a local guy anymore.

 

A little rich is going to be better than a little lean while doing the break-in to help reduce the heat. Just don't overdo it.

Griz

 

Without a meter, everything you're doing is just a guess. I would turn the air screw in an 1/8 at a time until the popping goes mostly away. That will usually get it pretty good. If you can't get it tuned out with the screw less than a turn out, try pulling the enrichener out just a little. If that cleans it up, then go up to the 27.5. The rod change won't do anything in the idle circuit.

 

Hey thanks, yep did that I'm sort of past that point, I understand the way the carb works and what circuits are active for a given throttle position, wasn't getting much joy with the air screw on the idle circuit so I have gone up to the 27.5 just can't test it at the moment as its winter here in Perth, no snow but cold and raining.

 

From tuning a Mikuni 45 on the street with a Wego3 find the low jet goes deeper into the needle crossover in the cruise area than realized, keep attacking the low jet and idle air adjustments which is the path you are on. Have a 130" with a bored Super G and 3" exhaust baffle, need to over fuel the idle afr's to remove intake pop on slight thottle after a decel, tried tuning by "13.8 -14.2 normal" afr's but wants to blow the carb off the engine, give it what it wants and not overthink it, mine idles @ 12.5 afr which keeps it cool in traffic and kills the decel throttle pop with fuel carry over.

 

I've ended up just going straight to the 30 pilot and 200 main, i haven't been able to get it out on the road again yet due to work and weather but i gave it a start on the lift and she seems to be running cooler at idle.

 

You never stated how many turns out on the air mixture screw you were initially.. 2-2.5 would be an acceptable range to identify proper size.. And understand that this circuit is only recognized up to 1/8 throttle..
The following is an excellent manual on the HSR..
https://www.mikunipower.com/Manuals/HSR42-48_Manual.pdf
Other things that need to be checked are the intake and exhaust seals..