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Discussion Starter · #1 ·
Does anyone have experience with the Yost powertube, the dyno jet kit, or the thunderslide jet kit? Are you satisfied with the choice? I understand the difference between the thunderslide, is the slide included in the kit for throtal response. Is it worth the extra cost? Is the Yost powertube a rejetting kit? Thanks in advance for the input. J.T.
 

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I've installed many Dynojet kits and Thunderslide kits over the years. Never had any experience with the Yost setup. Also keep in mind that there is a Dynojet kit from company named Dynojet. It's a good kit. There are also lots of kits from other outfits (i.e. Custom Chrome, etc.) and I wouldn't use them myself. Lots of people complain about these kits running rich and losing gas mileage. I've never had this experience but am also almost at sea level. I always suspected that problems occurred because of altitude and lack of tuning after install. As to being worth the cost?? That can be debated forever. A lot depends on what you expect from the modification. You are not going to get a lot of increased HP and torque. They will solve the cold starting problem and slightly improve performance while leaving gas mileage acceptable. If you're anal about getting every last bit of performance out of your bike, they are not for you. My personal preference nowdays is just to install a needle from the 1988 XLH sportster (HD parts # 27094-88), remove the cap on the mixture screw to adjust it and ride. This gives you a good streetable bike that is not cold natured and gets good gas mileage. The info below which came from an article in American Rider or Iron explains the setup.

Taken in part from a tech article by Joe Minton in July 02 issue of American Rider.

Carbureted twin Cam and Sportster engines can have their air/fuel mixtures corrected with one part - a stock Harley part - the needle from a 1988 or '89 1200 Sportster (HD part #27094-88). There's no need for an elaborate kit or to make extensive modifications to the stock Keihin CV carb.

Even with the Screamin' Eagle or other hi-flow air cleaner and a set of free flowing mufflers, the stock main and slow jets are correct. You do not need to change the main jet when you fit a set of say, Cycle Shack slip-ons and a free-flowing air cleaner. Just install the new needle, remove the aluminum plug over the mixture screw and adjust it outward for best idle.

With these two simple modifications, your Twin Cam, Evo or Sportster motor becomes the civilized and responsive powerplant it can and should be.

Why is the solution so simple, you ask? Because the only area in the throttle range where stock Harley engines are lean is in the first quarter of throttle slide movement. That is where government controls require such lean mixtures. The straight-diameter part of the needle, together with the needle jet it fits into, controls mixture in this throttle range. When the needle's straight part is smaller in diameter - which the early Sportster needle is - and the mixture gets richer, the engine runs better.

The stock main and slow jets are OK for two reasons. The mixture screw
controls the slow jet's fuel delivery into the engine at idle and very low speed, and it is a simple adjustment. The main jet doesn't figure into emissions testing. Harley, like most manufacturers, fits a slightly large main jet to make sure the mixtures do not become too lean at full throttle with free-flowing air cleaners and exhausts. Therefore, when you fit that combination of free-flowing air cleaner and mufflers, the main jet becomes correct (AT OR NEAR SEA LEVEL). The slow jet is the right size in the first place. It simply needs to have its delivery volume increased with the mixture screw.

END OF ARTICLE
 

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Discussion Starter · #3 ·
Thanks for the reply Ed. So, the needle from the 88-89 1200 sporster you are refering is the main jet needle? I was kind of confused. Will this relieve delayed throttle response? This is because the size of the jet from the sporster is bigger?
So, installing the dynojet kit or thunderslide kit, do little more than increase gas milage, better throttle response with improved cold starting, but at a cost of about 100-200 dollars. Not well spent? Thanks again. J.T.
 

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Thanks for the reply Ed. So, the needle from the 88-89 1200 sporster you are refering is the main jet needle? I was kind of confused. Will this relieve delayed throttle response? This is because the size of the jet from the sporster is bigger?
So, installing the dynojet kit or thunderslide kit, do little more than increase gas milage, better throttle response with improved cold starting, but at a cost of about 100-200 dollars. Not well spent? Thanks again. J.T.

J.T.

The needle is just referred to as a jet needle. It actually functions as a kind of intermediate jet. The carb still contains a slow jet and a main jet but these should be OK. Changing the needle will help with delayed throttle response although the thunderslide kit is real good for solving this problem if it's that bad. The needle from the sportster has more of a taper at the bottom which allows better fuel flow in the transition from slow jet to needle or intermediate range. The needle only cost $9. That plus time spent installing it and removing the mixture plug is all it costs you. If you're not happy then, consider a kit. Another option is tweaking the carb following the insts at nightrider.com.
 

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For most street bikes nothing wrong with a pair of jets with a $6 out lay of cash. You get the fuel the motor needs to run right and the preformance is well worth the savings.
 
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