Have your stock TB bored out and be done.
Fuel moto offers this for $169
I see one F/S in the touring parts section
for $150 now done by fuelmoto.

 

Thought of that - and it's an excellent option. However I am having trouble finding somebody experienced at boring Harley throttle bodies here in the UK.

I hadn't thought of checking the F/S section though.....will have a look at that now!

 

Also on the boring, if you want to change the injector size wouldn't that be extra? One of the nice features with the SE unit is that it comes with the 4.9 injectors.

 

Yes it would but on the op build his stock injectors will support his build and riding style.

 

Not to start a huge tuning debate but I would consider upgrading the stock injectors especially if going through the steps of adding a larger bore TB.

The stock 3.9 injectors are able to support a plain stock 103 due to economy / EPA choices put down by HD. This smaller TB / injector combo used to be used on the 96" motors. It was carried over to the non-touring 103" motors more for epa / economy reasons than thoughts of supporting performance. The touring bikes all received 50mm TB's w/ 4.9 injectors since their extra weight / loading required a bit more power and fuel.
Lastly, all 2007-up bikes received "extra tall" overall drive ratio belt pulleys (32/66) to further maximize economy and reduce pollution/noise. This all of course hurts every aspect of performance.

Stock and stage1 HD tune maps are very lean. timing is very sedate. With my typical stage1, and a reasonably performance-oriented tune more in the 12.8-13.8 AFR ranges, I can datalog and see my injector duty cycles exceed 90% at heavier throttle / upper rpms. If I put the stock or stage1 HD tune back on, my injectors never get over 60%. but that is because there is much less gas being used due to lean VEs and AFRs.

Typically, TB's and injectors are sized according to HP requirements. the 46mm w/ 3.9 injectors theoretically supports up to 100hp, but realistically it is more like 80-90 tops reasonably speaking. I'd prefer to have a TB / injector setup that gives me a bit of overhead cushion so I'm not over driving the injectors or impeding the airflow.

Now it is 2016 and HD has outfitted all softail / dynas with the 50mm / 4.9's. If the 46mm / 3.9 setup was still deemed sufficient HD would not have changed it because that is very costly to do. Anyone shooting to go at or over 100hp on their 103 should consider moving to the 4.9's and 50mm TB (and also upgrade those belt pulley ratios!)

www.maxbore.com can hog out the 46mm to 51mm for $125 shipped. set of SE 4.9's is about $120.

Can you get away with mild cam, light riding style and keep the stock 46mm / 3.9? sure. It will still work and bike will still operate. Will it be optimal on the performance end of things? No.

58mm TB's can be used but it will make for a very twitchy ride at light throttle. A dyno tuning shop can dial this twitchiness out but it must be someone very good at tuning. It would not be my first choice but some guys end up with it as part of the SE stage 4 race kit.

JMHO, FWIW, YMMV and all

 

I agree with the porting your stock to a 50 and getting the 4.9 injectors. Or you could just get the 4.9 injectors and stay at 46. I currently running a 54mm and 4.9 on a 110. Going to the 117 probably next winter. Staying with the 54 but going to 5.3 injectors. Reasoning is good low speed manners and better low end torque. I have noticed thru a lot of research low end torque is better with a tad smaller tb. You will not have the big end others will but your low end will be sufficient.

 

Velocity doesn't matter, it's injected not carbureated. Bigger tb's can make it twitchy at very low tp's. I have a 57 on a 98" and you get used to it. Being a cable bike a tuner can't adjust the sensitivity. A bigger tb will not give up anything down low and any gains will be from approx 3500 rpm up. I would depending on expected hp up the injector size but if they are to big the issue becomes tuning them at the low end. In your case shouldn't be an issue. I would take the HPI over the SE if $ isn't an issue. Size for size the HPI will work better than the SE. Don't know how far you are from this guy but he is very, very sharp. http://www.powerglides.co.uk/machine_shop.php

 

Note; the 4.9gm injector is an upgrade for the 3.9gm on cable throttle bodies, it was never offered as an OE injector. All bikes equipped with the 50mm TBW throttle body have 4.31gm injectors. Also note the injectors do not interchange between the cable and TBW throttle bodies as the injectors are different length and have difference connectors.

With that said with your combination as some have already noted I would also recommend the SE50mm throttle body, this unit works really well and includes 4.9 injectors. We use these often, cant beat the value at $449.00 and it works with most air cleaners

 

Thanks Jamie,

I had heard that some of the SE 50mm TBs had really poor castings with flashing etc left over. But as you say, the price is right and anything like that can probably be fixed with judicious use of a Dremel tool prior to installation.

I'll probably go for one of those and maybe consider a bigger HPI unit if I ever decide to go with the 110" kit in the future.

 

We have used the SE50mm in literally hundreds of applications, never ran into any issues with the castings. They work well up to about 130Hp