Topic: OEM vs Aftermarket Jet testing

[brewsky]

Just for kicks I decided to compare a couple of slow jets to see if there is any significant difference between brand sizing. I know Dynojet has their own numbering for sizes that doesn't match OEM, but I thought the other common replacement sizes were supposed to be equivalent.

Not having anything to actually measure the holes with I decided to try a home made flow test.

Turns out, the common #40 push in replacement from SCI (don't know what brand it actually is) flows more than the stock #42 Keihin.

The 2 jets, while having tha same overall dimensions, do have a slightly different design.

The stock #42 Keihin jets took about 11 seconds more to pass 15 ml of fluid (water) than the smaller sized #40 SCI jets in the test.

Some photos of the test:

[brewsky]

More pics:

[brewsky]

Pics:

[scottly]

Holley jet sizes are related to flow, rather than the size of the hole, but I've never seen anything about how much flow in how much time. You have the right idea, but left out some data. How much total time did each jet tested take to flow 15ml?

[brewsky]

The oem keihin #42's took an avg of 54 seconds...
The replacement #40's from SCI took an avg of 43 seconds
That was with a head of about 27" of water pushing.
I don't have any idea how that pressure compares to the actual in the carb, but I could definitely tell the difference between a dribble and a stream between the two.

[lucky]

Are both of the jets in the photo the same size?

[brewsky]

No...one is 40 and one is 42...
BUT The 40 flows more than the 42??

[scottly]

Wow, that's a significant difference in flow rate! Well done, Sir!

[brewsky]

Thanks.....I think I'll try some different  mains......hopefully there won't be that much difference in them.

[scottly]

No wonder some folks have had such a terrible time jetting for pods and such...

[killersoundz]

The #40 idle jets I got in my rebuild kits have to be sized bigger than the stock ones. I never ran my bike with the stock ones but with these aftermarket jets, and pods, and straight pipes, the idle circuit is perfect and maybe even a little rich if I turn the mixture screws in more. So no way they are stock #40 size.

[seanbarney41]

You can easily see the difference in orifice size in the photo...

[brewsky]

You can easily see the difference in orifice size in the photo...

The pictures of the jet ends are a little misleading, as the smallest opening is buried INSIDE the jets in both cases.

The OEM Keihin design is much harder to clean, as there is a larger diameter "chamber" in between two smaller openings inside which traps debris inside.

You can soak them in cleaner, blow air or water thru them, run a wire thru and pick them up later only to find them partially blocked again.

I think this happens  not so much from particles that enter from the outside, but because built up varnish and deposits  are loosened by the cleaning processes and are too large to escape from the chamber.

It almost looks like the fat end of the Keihin has a pressed in plug to reduce it's diameter.
?

[trueblue]

Those varnish build ups in the jets are quickly and easily removed by soaking the jets in acetone for a short period.  I have an old coffee jar with about 1" of acetone in it.  When doing carbs I pull them down, drop the jets in the acetone, clean the rest of the carbs, then pull the jets out of the acetone, blow them out and refit them, it is that easy, unless there is silicone in the jets, but the acetone even softens that.

[brewsky]

Those varnish build ups in the jets are quickly and easily removed by soaking the jets in acetone for a short period.  I have an old coffee jar with about 1" of acetone in it.  When doing carbs I pull them down, drop the jets in the acetone, clean the rest of the carbs, then pull the jets out of the acetone, blow them out and refit them, it is that easy, unless there is silicone in the jets, but the acetone even softens that.

Thanks for the tip!

I usually soak them overnite in sea foam or carb cleaner sprayed in a jar, but will give the acetone a try.

Other jets you can actually see from each end whether they are clean, but not these.

[Tugboat]

Very interesting! Thanks for posting.

[brewsky]

No problem.....

FWIW, I checked a screw-in #40 Keyster slow jet and it was close to the #40 SCI push-in  at 44.5 sec.

Also, checked an unkown main marked "AB 115" vs a Keihin 115.... and they were roughly the same at 4.7 seconds.

So hopefully the major differences are limited to the push in jets.

[ekpent]

No problem.....

FWIW, I checked a screw-in #40 Keyster slow jet and it was close to the #40 SCI push-in  at 44.5 sec.

Also, checked an unkown main marked "AB 115" vs a Keihin 115.... and they were roughly the same at 4.7 seconds.

So hopefully the major differences are limited to the push in jets.

Are you able to compare a stock Honda Keihin screw in #40 to the Keyster #40 ?  I am always suspect with some of these aftermarket jets. Have eyeballed some big differences.

[aperry]

Pretty interesting stuff.  Just for sharts and giggles here is a pic of my Keihin #40 screw in compared with the #40 that came in my PartsNMore rebuild kit (don't know what brand).  The Keihin is the darker one and its hole size looks a little bigger because it is. I have a pin that fits in the Keihin but not in the other one.  I replaced these with a #42 which seems to work well so I'm not complaining!

[brewsky]

No problem.....

FWIW, I checked a screw-in #40 Keyster slow jet and it was close to the #40 SCI push-in  at 44.5 sec.

Also, checked an unkown main marked "AB 115" vs a Keihin 115.... and they were roughly the same at 4.7 seconds.

So hopefully the major differences are limited to the push in jets.

Are you able to compare a stock Honda Keihin screw in #40 to the Keyster #40 ?  I am always suspect with some of these aftermarket jets. Have eyeballed some big differences.

I don't think I have any #40 Keihin's....I may have some 35's of both brands though....will check

[Jeff.Saunders]

As others mentioned, the controlling orifice is not at the tip - essentially a hour-glass like shape on the inside.  Hole size isn't indicative of how the jet performs.  The taper into and out of the controlling orifice is key to how the jet atomizes the fuel.  A few years ago I struggled with the pilot circuit on my Z1 with aftermarket jets (17.5 pilot jets) - I invested in a set of pin gauges and found the hole size for the aftermarket jet was the same as the genuine Mikuni jet - but they didn't perform the same - the aftermarket pilot jet was a little leaner.  Using a can of WD40 with the plastic spout, I squirted WD40 through the jet - and the cone of atomized fluid was different.  The smaller the jet, the more impact the brand makes.  The aftermarket pilot jets work fine, but they may not map perfectly with the OEM sizes - the key isn't to just install them and assume they are the same - but assess the plug color and how the bike is running on the pilot circuit - often a small adjustment to the idle screw will be all that's needed.

With main jets, I've not experienced any appreciable difference in larger main jets between genuine and aftermarket.

[brewsky]

As others mentioned, the controlling orifice is not at the tip - essentially a hour-glass like shape on the inside.  Hole size isn't indicative of how the jet performs.  The taper into and out of the controlling orifice is key to how the jet atomizes the fuel.  A few years ago I struggled with the pilot circuit on my Z1 with aftermarket jets (17.5 pilot jets) - I invested in a set of pin gauges and found the hole size for the aftermarket jet was the same as the genuine Mikuni jet - but they didn't perform the same - the aftermarket pilot jet was a little leaner.  Using a can of WD40 with the plastic spout, I squirted WD40 through the jet - and the cone of atomized fluid was different.  The smaller the jet, the more impact the brand makes.  The aftermarket pilot jets work fine, but they may not map perfectly with the OEM sizes - the key isn't to just install them and assume they are the same - but assess the plug color and how the bike is running on the pilot circuit - often a small adjustment to the idle screw will be all that's needed.

With main jets, I've not experienced any appreciable difference in larger main jets between genuine and aftermarket.

I appreciate your comments, Jeff.

The un-scientific comparisons I tried were caused by the obvious differences between the Keihin and after-market push in jets I got to tune my 750 for mods. Not only are the outer holes different sizes, but the bores leading up to the internal orifices different.

The results do appear to verify your conclusions and I agree that the size of the hole is not the only factor that determines the flow rate at any given pressure.

It does make me wonder, though, if the Keihin #42 is truly "a step" larger than the SCI #40 when the SCI flows so much more.

I have an onboard A/F meter to actually verify changes since I don't trust my eyes to do plug chops any more. I think all the fuel additives may be polluting the plug readings anyway.

I would much rather have OEM jets, but SCI appears to be the only vendor with the push-ins for PD carb slow jets.

I couldn't find any on your site......do you know if any OEM's are available?

Thanks again

[Jeff.Saunders]

Neither the online fiche, nor the old acetate fiche show a pilot jet in the 78 CB750K carbs...  very strange.

We don't have that jet - and other than Sirius, I don't see anyone carrying it either.  The fact the jet is missing on the Honda fiche probably explains why they are hard to find - the aftermarket companies tend to use the OEM fiche as reference points when assembling kits.\

BTW, your approach to roughly gauge flow is interesting - and with a modification might give more representative readings..  One thing different with how the jet works, the vacuum created by the engine pulls the air through the carb which sucks the fuel through the pilot circuit.  Applying a vacuum on a hose under the jet might give a better simulation of real world flow.  A light duty vacuum cleaner might work (or something similar).  I'm not sure of the vacuum readings on that model, but for most Kawasaki models with mechanical carbs (not CV) you get a vacuum reading of around 36cm/hg.

[TwoTired]

Three cheers for the inner scientist in Brewsky.  I love his testing process and his results are exactly relevant!

Some things to bear in mind:
A given viscosity fluid will flow through an orifice directly relative to the pressure differential, so long as the flow remains laminar.
Using water is certainly informative.  But gasoline's viscosity and weight will make subtle flow differences in this test set up.  However, this does not negate Brewsky's test conclusion.

It matters not whether the pressure differential across the orifice is caused by vacuum or head pressure.  It is the amount of pressure differential that is of note.  I observe that brewsky's test method uses a variable head pressure as the source vessel depletes.  Again this property is consistent between tests of different test subjects.

It does matter about the speed of orifice transition if it causes the flow to create turbulence.
The ramp angles and orifice shape determine at what speed the flow goes non-laminar, and the resulting flow chart stops being a predictable linear relationship.

To see the difference in orifice shape and ramp angles, will probably require destructive sectioning of the jets.

To test for flow rate under different fluid speeds would require applying a known but variable differential pressure across the jet orifice as well as measuring flow amount under those variable pressures.  (Yes, it is more complex.)

Cheers,

[brewsky]

Jeff,
Here's probably why its not shown on the parts fiche......the manual says NOT to remove it!

Funny you should mention a vacuum.......I've been thinking about trying to get the local shop to donate their old K&N Flow Display (the one that blows up the ping pong ball) for a home made flow bench. Vacuum hose on the inlet to the machine....and who knows??

TT,
Thanks for the compliment, ....too bad the mizzez doesn't share your sentiments, especially when I'm in the kitchen with carb parts etc.!

I'm wondering if the size/number of cross drilled air holes would affect the liquid flow thru the jet orifice? The cross holes appear to be the same on both brand jets.