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Author Topic: Keihin Jets  (Read 276 times)

Offline desertR

Keihin Jets
« on: September 10, 2020, 11:50:16 PM »
I have an OEM keihin carb (late 400ex) on my quad, and I'm wondering if the FMF jet kit has reasonable jets in it. They are not listed as Keihin jets, but Dynojet jets, and many of you have opinions on that. Lets hear it.
desertR
« Last Edit: September 10, 2020, 11:55:16 PM by desertR »

Online The norm

Re: Keihin Jets
« Reply #1 on: September 11, 2020, 12:00:31 AM »
I have no knowledge of or any experience with dyno jets. I always buy keihin jets for my keihin carbs. I go to either a bike shop or jetsRus for carb parts.
Loren
1986 Trx250r-- Hybrid Engineering 391(Good bye super 310)
1986 Trx250r-- Hybrid Engineering  ported stock 86 cylinder
1986 Trx250r-- craigslist engine with unknown ported 89 cylinder.
1985 Atc250r-- Needs a lot of work (Super 310's new home)

Offline broken1

Re: Keihin Jets
« Reply #2 on: September 11, 2020, 01:16:22 AM »
I got some dynojet jets from a local shop because they were out of the keihin's I wanted in that size. The guy told me they seem to run slightly smaller by about half a step below what was stamped on them but the one I used worked just fine & I wouldn't hesitate to use them again.
« Last Edit: September 11, 2020, 01:18:34 AM by broken1 »
overkill is an often underrated achievement

Offline havinnoj

Re: Keihin Jets
« Reply #3 on: October 08, 2020, 12:47:35 PM »
I don't think there is an issue with aftermarket jets per se, but rather the issue arises when mixing different brands as sometimes there can be slight tolerance differences.  That can cause slight headaches when changing jet sizes (which also means changing between brands).  I believe issues are pretty rare, but personally my jet collection is Keihin so I just stick with that.

Offline Jerry Hall

Re: Keihin Jets
« Reply #4 on: October 10, 2020, 01:38:27 AM »
I only use genuine Keihin or Mikuni jets. I could fill a 55 gallon drum with all of the seized pistons, pistons with exhaust side of the piston burned through the rings, and pistons with a hole  in the top that was caused by after market jets.  Some times you get lucky and get a jet that flows more than it marked size.  Most of the time the aftermarket jets flow about the same as the genuine jets, but the consistency of the aftermarket jets are less than the genuine jets. 

I think that  anyone that has done many years of dyno work will have made a similar observation. 

Because two jets have the same diameter hole, does not they will flow the same.  The length of the hole has some influence on the flow rate, but the area where the fuel inters the jet has a huge influence on how much fuel can flow through a precisely machined hole. 

Many years ago I was working on a Yamaha 1000 cc 4 cylinder alcohol engine that I installed a Hilborn fuel injection system.  In the course of preparing to developing this engine on the dyno I would need a selection of pills.  Pills are jets that bypass fuel in the different stages that shape the fuel curve.  I ordered a set of jets that were flowed and calibrated and paid a lot of money for them.  When  the calibrated jets arrived, I inspected them with a magnifying glass, I was disappointed with the machine work.  The entry or approach to the holes in the jets did not look the same on all of the jets.  Some jets had a sharp edge where the hole intersected the cone shaped entry.  Some jets had this sharp edge or a portion of the sharp edge removed.  A few jets had small scratches and or nicks in the cone shaped entry to the machined hole.

After talking to Hilborn, the guy explained that what I saw on the entry to the holes was done intentionally to make the jet flow a certain amount.  Some holes did not flow enough so the entry had to be improved by removing some of the sharp edge or making the entry cone  a lesser angle.  Very small nicks or blemishes were intentionally inflicted on the intersection of the cone to the machined hole on some jets to reduce the fuel flow through the jets.

My recommendation is pay a few more dollars for a genuine jet because, saving a few dollars on a jet may cost you hundreds of dollars more on a new piston and or a new cylinder. 

Offline havinnoj

Re: Keihin Jets
« Reply #5 on: October 13, 2020, 12:36:05 PM »
...the guy explained that what I saw on the entry to the holes was done intentionally to make the jet flow a certain amount.  Some holes did not flow enough so the entry had to be improved by removing some of the sharp edge or making the entry cone  a lesser angle.  Very small nicks or blemishes were intentionally inflicted on the intersection of the cone to the machined hole on some jets to reduce the fuel flow through the jets.

Interesting story Jerry, not that I've had an issue with Keihin brand jets but now I'm curious to read up on their mfgr process and whether they test their flow rates for each jet.

Offline Jerry Hall

Re: Keihin Jets
« Reply #6 on: October 13, 2020, 03:46:38 PM »
I do not know what type of quality control process Keihin uses during their manufacturing process.  I do not believe manufacturers can flow every jet when they are priced at around $5.00 each.  Flowed and calibrated jet would probably cost $15.00 to $20.00 each. 

Keihin jets as well as the jets from most carburetor manufacturers are coded or labeled as the hole diameter of the jet.  A 150 Keihin main jet has an approximate hole diameter of 1.50 mm and 152 main jet has an approximate hole diameter of 1.525 and so on

A Mikuni rep. once told me that they take a sample at the beginning, middle and end of a production run of a particular jet size.

Lets say they are manufacturing 350 main jets. A sample taken at the beginning of the run flowed 354.  A sample taken at the middle of the run flowed 348.  The sample taken at the end of the run flowed 345.  All of these jets would be labeled 350.

If a sample at the beginning of the run flowed 354.  The sample at the middle of the run flowed 345.  The sample at the end of the run flowed 343.  The jets made between the beginning and the middle sample would be labeled 350 and the jets madel between the middle and end sample would be labeled 340.  As you can easily observe, all jets labeled the same size are NOT EQUAL.

Mikuni hex jet sizes are a relative flow number.  Their jets are made in increments that produce approximately a 3% to 3.5% change in fuel flow.  Mikuni makes their main jets below 200 in increments of 5.  If you go from a 150 Main jet to a 155 main jet you have increased the fuel approximately 3.3%%.  If you go from a 300 Main jet to a 310 main jet you have increased the fuel approximately 3.3%.    If you go from a 600 Main jet to a 620 main jet you have increased the fuel approximately 3.3%. 

I believe the motorcycle carburetor industry uses the 3% fuel flow increments because this is the approximate fuel flow adjustment necessary for a change in elevation of approximately 1000 feet when temperature and humidity are unchanged.










Offline 2Nubs

Re: Keihin Jets
« Reply #7 on: October 13, 2020, 08:28:27 PM »
 Amazing to see the time some have put into this platform.

Thank you, Mr. Hall. I thoroughly enjoy reading your work.

86 330

 

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