Dynojet vs. Factory Pro - Any Real Differences?

[threejagsteve]

Some people use the Dynojet kit, others use Factory Pro.

I've heard what amounts to trash-talking, but I've never seen any concrete reasons to choose one over the other.

From their websites, the kits look very similar and prices are comparable.

Input, anyone?

Also, I've run across something called the "Slide Hole Mod" - anybody know what that does, and is it generally effective or only suitable for heavily built motors?

[andyb]

Use the factory pro kit.

If nothing else you'll find it easier to translate the jet sizes to OEM (because they already are).


However, you'd probably do just as well skipping the jetkit completely.  Shimmed stock needles, $10 in mains, $10 in pilots, you're done.

[the fan]

On deans 93's I had great luck with the factory kit. not so much on the bike with the DJ kit.

The factory kit was good to go right off the bat. The DJ kit required a lot of fiddling and never did run really well.

[SlowOldGuy]

On deans 93's I had great luck with the factory kit. not so much on the bike with the DJ kit.

The factory kit was good to go right off the bat. The DJ kit required a lot of fiddling and never did run really well.

I guess you're just not that good then.     Of course, it was Dean's bikes, so maybe it wasn't all you.

I've got DJ kits in '85 and '93 FJs and both run well.  The key is to not get greedy and install the biggest jets.  I've gone through quite a range of needle settings and jet sizes on my '85 over the years just for experimentation sake and it's been extremely tolerant of whatever settings I've tried.  Usually the only noticeable effect is a decrease in mileage when I go to a richer setting.

Either kit should do just fine, but I'd agree with Andy that shimmed stock needles are usually adequate.

DavidR.

[threejagsteve]

However, you'd probably do just as well skipping the jetkit completely.  Shimmed stock needles, $10 in mains, $10 in pilots, you're done.

Yes, that's what I'd do if I knew what jet sizes would be right.

I seem to recall it being passed around as a "rule of thumb" ~30 years ago that you'd be in the ballpark if you went up 2 jet sizes for each mod, i.e., + 2 sizes if you ditched the airbox for K&Ns, + 2 sizes if you added a pipe, + 2 sizes if you advanced the rotor, etc. I never really knew if it had any truth to it, but it certainly was repeated as if it were gospel.

I had all three of those mods on my KZ650 back in the early '80s, and all I did was drill the main jets a little bigger, fatten the idle screws, and raise the needles with washers.  Voila, an improvement (and the motor actually ran decently!), with none of that tedious jet-switching, testing, or plug-reading!

'Course those 2-valve motors were rather more forgiving, but I never bothered to find out if a more disciplined approach might've yielded a better result...


So nobody's ever heard of the "Slide Hole Mod"?

[SlowOldGuy]

I seem to recall it being passed around as a "rule of thumb" ~30 years ago that you'd be in the ballpark if you went up 2 jet sizes for each mod, i.e., + 2 sizes if you ditched the airbox for K&Ns, + 2 sizes if you added a pipe, + 2 sizes if you advanced the rotor, etc. I never really knew if it had any truth to it, but it certainly was repeated as if it were gospel.

So nobody's ever heard of the "Slide Hole Mod"?

That would result in a main jet size of 125 for all those mods.  I believe that would be way too large for all but flat out racing with the engine sitting sideways and getting very little airflow in a Legend car.

You could safely run all those mods with the stock main jet "IF" (there's always an IF) you never spent a significant amount of time at Full Throttle.  It would be REQUIRED to shim the needles, but the mains are only working for that last few degrees of full throttle.

Are you referring to drilling the vacuum hole in the slides as the "Slide Hole Mod?"  My '85 has that mod.

DavidR.

[andyb]

A 125 would be pretty close though.  A 122 would be ideal, assuming you're riding in cool air and not in the mountains.

[threejagsteve]

Well, now we're getting somewhere!

As I said, I was never sure whether to believe that old "rule of thumb"... it just seemed too cut-and-dried, one-size-fits-all to be much more than a guesstimate.

Of course climate conditions should be considered... but around here, it's literally possible (on occasion) to be riding at sea level, 50*F, and at 4000 ft., 100*F, in the same day! 

Having read what David and Andy had to say (thanks, guys!) and factoring that into my next planned round of upgrades (pods, advancing the ignition, and coil and headlight relays), I'm now thinking that 117.5s would likely be plenty. Raise the needles, of course, and figure on some plug reading to nail the amount down. Pilot jets a little bigger as well, I guess... And no big bucks for a whole handful of extra jets I don't need!

And BTW, this is the copied-and-pasted reply to my query about the "Slide Hole Mod" that I got from 6 Sigma Engineering, the outfit that sells the kit that offers it:
"That is enlarging, or adding holes to the carb slide. This allows a little more air through the carb from 1/4-3/4 throttle. It is beneficial for the acceleration. On your model however, no advantages were found by doing this."

[SlowOldGuy]

And BTW, this is the copied-and-pasted reply to my query about the "Slide Hole Mod" that I got from 6 Sigma Engineering, the outfit that sells the kit that offers it:
"That is enlarging, or adding holes to the carb slide. This allows a little more air through the carb from 1/4-3/4 throttle. It is beneficial for the acceleration. On your model however, no advantages were found by doing this."

Why would I need to allow more air through my slide in the 1/4 to 3/a throttle range?  The carb supplies all the needed air in that domain along with the correct amount of fuel for it.  If I was to add a hole and add more air, then the carb would need to add more fuel and the end result would just be a higher RPM.  Sounds like snake oil to me.

I would predict that "On ANY model, however, no advantages were found by doing this."
It just sounds like a bad idea.

DavidR.

[threejagsteve]

Why would I need to allow more air through my slide in the 1/4 to 3/a throttle range?  The carb supplies all the needed air in that domain along with the correct amount of fuel for it.  If I was to add a hole and add more air, then the carb would need to add more fuel and the end result would just be a higher RPM.  Sounds like snake oil to me.

I would predict that "On ANY model, however, no advantages were found by doing this."
It just sounds like a bad idea.

DavidR.

Oh, I agree! 

I just figured I ought to post that reply since I'd raised the question to begin with... it intrigued me since I'd never heard of it or seen it mentioned anywhere else.
And now I know why! 

[SlowOldGuy]

They could be talking about the slide vacuum hole.  This is the hole that allows the vacuum produced by the velocity of the airflow through the carb throat to act on the diaphragm and pull the slide assembly up.  The early DJ kits included a drill to enlarge this hole (not by much) which was supposed to "improve throttle response."  Some people thought it did, others thought it just ruined their slides.  Of course if it did ruin them, they could just JB Weld the holes and redrill smaller.

It's interesting to note that about the time DJ quit including the slide drill, their new kits began including stronger slide return springs.  Kevin Cameron alludes to this in his Sprotbike Performance Handbook.  Kevin's theory was that it was probably undesireable for the slide to be overly reactive.  If the slide raises too quickly (and too far) in response to a throttle increase, the needle could allow too much additional fuel into the intake which could create a stumble.  Seems it is considered best for the slide to rise slowly and avoid rich overshoots for smooth throttle response.

I've experimented with stock slide springs and stiffer DJ springs and I'll be damned if I can tell any difference in response.  Just another example of how tolerant the FJ carbs are if you get the settings close. 

DavidR.