Seems like jet kits have a love-hate following. Just about every time we see a posted question about rejetting we get/give the same suggestions:

49% say to just change the slow jet.
49% say to install a jet kit changing/fine tuning more than just the slow jet.
2% say some other scenario.


I've done it both ways on my own bikes. I know both methods work. BUT.................I'd like to see a test done to see the results of each method. I know DJ advertises about 10-12 HP gain from their jet kits + AC (shown in their fact sheets that come with the kits and on their website). But I know of no similar information showing results for simply changing a slow jet.

To keep it fair we need one stock bike to start with and a dyno. We run it stock to get a benchmark performance standard. Then change the AC. Up the slow jet and retest. Take the carb back to stock and then install the jet kit. Test again. Then compare and publish the results.

I'm not attempting to define a "winner" or suggest that any one method would be better or the right answer in every situation. But testing would give everyone an indication of what you will get, or give up, running one method over another through definitive results vice personal opinion.

 

A friend and I had near identical bikes, mine a 1995 Roadking and his a 1996 EG Standard. I had the Dynojet kit on mine. On his we changed the slow jet, installed a 1988 1200 Sportster needle, cut two coils off of the slide spring, and drilled the vaccum port. Both had SE air cleaners, and free flowing exhaust.

I don't know what difference a Dyno would have shown, but seat of the pants you couldn't tell one bit of difference in the performance of the bikes, and we rode them together for several years.

Only difference I could find was $90.00 for mine, about 20.00 for his.

Regards,

Bill

 

You just need to be careful and use the correct jets, as explained in the below FAQ...



FAQ's:

(Q What happens if I don't match the correct carburetor jets with my intake and exhaust flow requirements?

(A It could create an unnatural flow and lead to a possible timing paradox, the results of which could create a chain reaction that would unravel the very fabric of the space-time continuum and destroy the entire universe! Granted that's a worst case scenario. The destruction might in fact be very localized, limited to merely our own galaxy.

 

OMG OldFenderGuy bwahaa [sm=funnypostabove.gif]

 

I heard something about that space time continuum but thought that was only if you had put cams in too.

[sm=roll.gif]

 

Every motor is different. A gain found by setting a carb up a certain way on one bike will not be the same if set up the same on another bike. Also, a 45 jet in a DJ kit is the same as a 45 jet purchased alone. The main difference in the DJ kit is the use of a clip on the needle to adjust height when the same effect can be had using shims. I guess I don't understand the point of your proposal. Adjustments are made to the carb to optimize the air/fuel mixture entering the motor. There is only right and wrong.

 

Two main schools of thought on carb tuning. One group advocates using a jet "kit" (Dynojet, Dynojet Thunderslide, etc). This group believes that your bike yields more power gains and is better tuned. The other group feels that they can achieve comparable results using an adhoc method; changing single jets and maybe shimming the stock needle. Some of the latter feel they can actually better the results of a jet kit.

I know they all work. I just want to know how well they all work. My proposal is to take one bike, stock from the factory, and conduct a legitimate evaluation of both methods in a controlled experiment. Test horsepower, torque, and air/fuel ratio for:

- Stock
- AC/Exhaust + 45 Slow jet
- AC/Exhaust + 45 Slow jet + shimmed needle
- AC/Exhaust + 45 Slow jet + shimmed needle + Yost Power Tube
- AC/Exhaust + 45 Slow jet + ……for as most commonly suggested/used part combinations
- AC/Exhaust + Dynojet Kit
- AC/Exhaust + Dynojet Thunderslide kit

Basically do the same thing Progressive does for insurance quotes but apply the concept to carb tuning. The result would be some hard data which would compare the actual results of jet kits versus individual part combinations. In the end, this would confirm and/or refute claims made by supporters of both methods.

The idea was spurred by reading thread after thread where someone asks about tuning their CV carb for new pipes and/or higher flowing air cleaners. Replies are often very polarized and opinionated; based on personal experience and word of mouth but not a lot of hard, scientific evidence. The result is many suggestions are made for the "right" way to do it. Or you'll get one of those, "Don't waste your money on...." replies. I think everyone is genuinely trying to help but ends up discouraging the guy who asked the question away from certain, potentially beneficial tuning solutions in favor of the one the responder used on his bike, heard his buddy talk about, or has the most experience with.

Is the benefit gained from shimming a stock needle the same as the adjustable needle used in the jet kit? Can you get the same throttle response from the Thunderslide if you drill out a stock slide? What about adding a Yost power tube? Is it the same for everyone? Hard to say because like we all know and you pointed out, every motor is different. A gain found by setting a carb up a certain way on one bike will not be the same if set up the same on another bike.

I was suggesting doing an experiment like this so the results would have a legitimate basis for comparison and therefore have greater meaning and usefulness for the person who is interested in tuning his bike but isn’t sure what direction he wants to go. He could grab the spreadsheet and dyno results from this experiment and be able to assess his options and make an informed decision. If the test bike showed there was 2 horsepower difference between a Jet kit and a Shimmed/45 combo, the guy can assume the same 2 horsepower difference if he applied those methods to his own bike. He might not get the same max output that the test bike got. He might get more. But the variance between methods could be assumed to be a constant.

Perhaps the concept is way too far into the weeds and way too much information for the average rider. If you are an uber gear head and want to squeeze every ounce of power from your bike, this would be useful. I guess most just want to know if a method will work and aren’t necessarily concerned with the extra 1.5-2.5 horsepower and torque gains had using one method over another.

 

For the engine to reach it's max potential, the fuel/air mixture needs to be optimized. How you get to that optimization is personal preference.

The same needle raised the same amount by moving a clip or shimming will yield the same result.

The dynojet kit is a waste of money because it makes people, like you, believe that their product will eek out some additional horsies simply by using their product. Like I said before, a 45 jet is a 45 jet no matter where it came from and a needle raised a specific amount is the same whether you use a clip or shims.

As far as the mighty "Thunderslide" kit, there have been dyno tests that prove it offers no gains yet makes tuning the mixture even more unpredictable. When people start modifying parts that are unrelated to mixture, they begin throwing off the balance of the carb itself which only makes tuning even harder.

If you are looking for some magic solution, and don't want to invest any time into learning how your bike works, and are comfortable buying into marketing b.s., then by all means, support companies like DynoJet.

 

Uncledaddy, I'm going to assume from your reply that you would fall into the "hate" column with regards to jet kits.

You were right when you said you didn't understand my proposal. I'm not advocating jet kits. I'm not advocating a single jet and a few washers either. I am fully aware of the different methods available. I'm simply suggesting there could be some benefit from a controlled experiment to determine the effectiveness of those methods.

Go back and read my original post again. You will see I have jetted bikes with jets alone AND with a jet kit. I have a pretty good working knowledge of how my bikes work. But I would bet money that the majority of the people that solicit advice on jetting their carb in this forum, and others, do not have a solid working knolwedge of their bike. If they did, they would not be posting the question in the first place.

I'm of the school of thought that those people deserve to be informed of every option that is available to them, not just the ones you like. Further, I think we do those people a disservice by specifically putting down 'a' way of accomplishing the task required; like you did to BlackCatalyst here. If there is a legitimate reason for not recommending something, that's one thing. But you said yourself that a jet and shims are the same no matter where they come from. So you agree that a jet kit will work, you just don't like it.

Is a jet kit really a waste of money? You think it is. But not everyone is you. Oh, and its not your money. Someone else might appreciate the benefits of getting everything they need in one box, with detailed directions that will walk them through the process. They might like having multiple jets together that give them the flexibility to adjust things as their setup changes. Maybe down the road they get new cams, different exhaust, etc. They don't have to guess what the next step is. They grab their kit and its all there. To some that piece of mind might be worth it. Someone else might be sure that they will never change anything again and thus find the money spent on extras they will never use to be excessive.

But the fundamental assumption you are making is that the jet kit is expensive. How much is too much? What if someone could get one for under $75, $60, $40? When does a jetting option become too much money?

According to you, anyone who buys a jet kit is a moron and wasting their money. Harley Davidson endorses, recommends, and uses jet kits (DynoJet kits specifically) for all their Stage I upgrades. So Harley buys into marketing BS and has no idea how their bikes work? You trust Harleys judgement when they are building your bike, just not when it comes to jet kits? What about brakes, do you trust their selected method for stopping?

My proposal was a way to factually define the different options people have to jet their carbs, without emotion. Give them the information and let them make up their own mind. Just because someone might want to do it a way different than you doesn't make that way wrong or them stupid. Putting information together like I proposed would prevent, in large part, people like you, from putting rhetoric and conjecture ahead of the main reason we are all here; to help other Harley owners.

 

Stock Bike ???????? Unfamiliar term...................