opinion on 120" combination

[dave brode]

I am studying the G carbs. The XXX guy had bypass surgery, I wish him well, but not much going on there.

I had a 48 on a '99 FXDL with a +.200" tall cylinder deal. Fit under tank, but was close.

GMR makes going points. The 48 mik has gone way up in price, adapters and tuning parts are pricey. Maybe I should have bought a 51 CV that I passed up some months ago. Then again, I ready that some parts for them are hard to find nowadays. 

Thanks!
Dave

[dave brode]

How often does a G need a thunderjet?

[jrgreene1968]

Are the s&s carbs hard to tune?
I had read long time back somewhere, that they tend to run rich  and have to have thunder jets. Etc. I never really studied up on them much, I just bought a mik 45 because they are so easy to tune

[harpwrench]

When I put an S&S V111 in my bike about 5-ish years ago, John Sachs bored a super G and made the main air bleed external. It has been a great/reliable carb, easy to dial in, and runs hard. He told me that a t-jet wasn't necessary for this anyway. You might ask John what he thinks, he'd know.

[turboprop]

How often does a G need a thunderjet?

There is a common misconception that the need for a Thunderjet is somehow linked to engine displacement. Not true. Wether it's a Supper or G carb is also somewhat irrelevant. The Super carbs have three circuits that prove fuel, with the main jet doing most of the work. The issue is simply bandwidth. Tunning the main jet for optimum AFR at high RPM will produce a very rich at the lower RPM in which the main jet operates. Adding a Thunderjet effectively splits the operating range into two, with the main covering the lower portion and the T-Jet covering the upper portion. Some carbs are even set up with two thunder jets. Again, this is not to provide more fuel, but simply more control of the fuel delivery at various sections of the RPM range. The additional T-Jets require their air bleeds to be drilled to a size that brings them in at the correct vacuum draw. A larger air bleed brings the T-Jet in at a higher RPM. The air bleed for a T-Jet is at the base  of the jet holder and has to be drilled to size. That being said, it is almost impossible for the typical rider to set one of these up on their own. This really requires a dyno with an AFR sniffer to re-align the main jet and its air bleed  as well as the jet and air bleed for the T-Jet.

To give you an idea of what is involved, my Smack-A-Hoe TC124 was originally setup with a bored and retimed Super G with one T-Jet. After many pulls on the dyne it was decided that a second T-Jet was in order. Again, not to add more overall fuel but to flatten the AFR in the mid to upper RPM areas. We spent almost a week on the dyno and went through quite a few bases for the T-Jet (the part that is drilled for the air bleed) until we got it right. We also changed carb bodies to try different locations for the T-Jet (Closer to the butterfly provides more draw, closer to the Ac provides less draw). There are also different size nozzles for the T-Jet. My carb runs combination of one standard and one large nozzle. The nozzles alone required about six pulls to dial in.

Bottom line, T-Jets are not really for the do-it-yourselfer and are better left to a professional tuner. Done right, they provide a much flatter AFR line that produces great power and improved fuel economy regardless of the size of the engine.

[jrgreene1968]

Thanks TurboProp, I did not know that 

[dave brode]

Thanks turboprop,

I don't doubt a word of that. It seems to me that the placement of the t-jet, as far as distance from the venture would be very critical. Now I'm more skeered LOL.

--

Maybe I was lucky, but the 48 that I had on the 118" TC with Wood TW-9Bs and Boarzilla worked really well. Great throttle response, needed only a main jet change. Decent on gas. Repeat: I hated the rattle, and it spewed gas on decal on the dyno.

Dave
   

[jsachs1]

RE: T-Jet.
Some do and some don't. I tend to install them several thousands closer to the butterfly, and more of an angle to the right looking at the front of the carb.
Takes dyno time to see if your combo is better or worse with it. 
John

[prodrag1320]

most don't even know how a t jet works,let alone tuning one.we ran 1 to 2 in each carb on our pro gas bikes bikes in the day,CID motors from 114" to 167"

[speedzter]

Maybe I was lucky, but the 48 that I had on the 118" TC with Wood TW-9Bs and Boarzilla worked really well. Great throttle response, needed only a main jet change. Decent on gas. Repeat: I hated the rattle, and it spewed gas on decal on the dyno.

Dave

Not lucky, they always work well !!

Also, the HSR48 is no taller than the 45/42, I don't know where that came from ?.
http://www.mikuni.com/carburator/hsr_group_l.jpg
A manifold is cheap from S&S  - 160-0001A , no adapter needed, and no need for a jet kit, maybe a few single jets, and a needle.
I run one on my custom 120 in a Dyna.
You can barely hear the slide rattle, and only really at idle.
A modded "G" will be more expensive.

[dave brode]

Thank you Mr. Sachs, and speedzter,


I didn't mean for this to be a 48 vs G thing, but it's ok.   speedster, you might remember the 48 on S&S manifold thread.

Dave