Author Topic: Drilling Jets HELP! (Read 4747 times)

fletcha221 · « **on:** February 12, 2010, 08:18:07 AM »

Another member on the forum here stated that since the jets for the cb's are hard to find and expensive, they had figured out what size drill bits equate to what size jets. Before spending 50 bucks for jets from sirius consolidated, I thought I would ask about it. Has anyone done this? Any advice? I need a 90-95 jet.

kslrr · « **Reply #1 on:** February 12, 2010, 08:27:20 AM »

What bike? What carb? Jets of most sizes for Keihin and Mikuni are less than $2 @ Z1 Enterprises.

I drilled out the jets on my CB650 only as a way to approximate the size I needed and then I ordered from Z1.

fletcha221 · « **Reply #2 on:** February 12, 2010, 08:36:37 AM »

My bike is cb350f, keihin carbs. I just ordered 98's from Cycle Re cycle for $3.25 a pop. I'm just worried that this might be a little too much fuel.

mystic_1 · « **Reply #3 on:** February 12, 2010, 08:46:14 AM »

Tom Summers, the admin over at www.hondachopper.com, sells jets for $10 per set plus $6 shipping:

http://hondachopper.yuku.com/topic/28523

see also his website: http://www.lowridersbysummers.com

mystic_1

Triffecpa · « **Reply #4 on:** February 12, 2010, 08:46:57 AM »

I run CB350F and CB400F bikes on the track in the southwest. Typically I run 85 main jets. Now I'm at 5000' of elevation, but I also run open velocity stacks and basically open exhaust. So take that for what it's worth. I've run 90 and 95 jets and they turned out to be way too rich.

Drilling them is fairly simple. I googled .85mm drill bit and found a place that sells them cheaply. I bought two .80 and two .85 bits. Spent a few bucks apiece on them if I remember correctly.

Drilling them is fairly simply. Hold the bit in a pair of pliers and screw the main jet onto the bit and then pull the bit straight back out. There are going to be all kinds of people who say that you can't drill an accurate round hole with a drill bit. I won't argue with them, but the bikes run well now and in my opinion, that is the ultimate score of success.

Tracy

kslrr · « **Reply #5 on:** February 12, 2010, 08:49:35 AM »

I agree with Triffecpa, 98's are a bit large. I'm still using the stock 75 mains and 35 idles with the needle clip on the bottom with my somewhat open 4-1 pipes and pod filters. I will need to revisit these settings once I complete the fresh-air intake, but back in the '90's it ran well, able to do the TON.

Triffecpa · « **Reply #6 on:** February 12, 2010, 08:49:46 AM »

Quote from: mystic_1 on February 12, 2010, 08:46:14 AM

Tom Summers, the admin over at www.hondachopper.com, sells jets for $10 per set plus $6 shipping:

http://hondachopper.yuku.com/topic/28523

see also his website: http://www.lowridersbysummers.com

mystic_1

jets for the 750 are pretty easy to come by. The press in, O ring sealed jets that are used on the "little fours" are harder to find in the correct sizes.

Tracy

fletcha221 · « **Reply #7 on:** February 12, 2010, 08:53:41 AM »

Tracy,

Thanks for the heads up! I have 80's in there right now and those plugs are white. I run pod filters and a custom free flowing exhaust (4-2) Built off the stock exhaust headers, then to 79 sportster pipes & baffles. (Put in torque cones and baffles). What type of exhaust do you run?

Triffecpa · « **Reply #8 on:** February 12, 2010, 08:58:46 AM »

I've got two 350F's and two 400F's.

The 350's run a stock head pipe, 4-2 set up with the mufflers cut off and basically open megaphones. The second one runs an aftermarket 4-1 with an open megaphone.

The 400's run a stock 400 header with an open megaphone and the second one runs one of those cool Yosh knock off pipes from MC Ready in Japan.

On drill bits, buy a .80 and a .85 Drill the jets first with the .80 (even though they are 80 jets) and see if that makes a difference. if not, go to the .85 bit.

TR

NitroHunter · « **Reply #9 on:** February 12, 2010, 09:36:17 AM »

For what it's worth, here's something to keep in mind about drilling jets.
Not as critical with carbs as injection but the theory still applies.
Some years ago (on our dragster) we commonly drilled jets oversize as needed, only to find that some caused a leaner condition than when we started.
Go UP in jet size and burn a piston - WTF?

Finally boxed them up and had Enderly flow everything we had, it wasn't cheap but we needed to know. On return we found almost every one of them were re-marked, as the actual flow didn't match the stamped numbers on the jet body. Many of the modified ones flowed quite a bit less fuel than our self-drilled hole diameter would indicate.

We were told that a magnified view will show that the flukes of the drill bit leaves little scratches down the hole, and maybe some of the entrance taper is cut away, and fuel doesn't always react positively to this.
All that being said, I still think drilling is a cost effective way to get you in the ball park with bike carbs, but once close, myself I'd purchase new ones.

fletcha221 · « **Reply #10 on:** February 12, 2010, 10:25:28 AM »

Nitro,

Thats really strange! I Wonder if it would make that much of a difference in a little street bike.

Tracy,

I really have to wonder if something else is holding up fuel flow. The carbs were apart, I cleaned everything but the tubes. They were off a running bike.....but i've come to find that "Running" sometimes means sputter coughing and backfiring to a slow and steady death.

Basically, the bike starts easily, idles nice, and rev's smoothly all the way up. However, when in gear, she really hesitates at the high end, as (I believe ) the bike is starving for fuel. When I put my new exhaust on, (which is more constricting), the plugs went white. So, it's really starting to lean out.

Any Ideas? Here is the exhaust

http://cb350fcafe.blogspot.com/2010/02/full-exhaust.html

NitroHunter · « **Reply #11 on:** February 12, 2010, 10:39:54 AM »

Check the screen filter in the petcock?

fletcha221 · « **Reply #12 on:** February 12, 2010, 10:44:04 AM »

Nitro,

Brand new petcock rebuild, tank was professionally cleaned and lined. Is there a possibility that my fuel tank has too much of a vaccum? I replaced the rubber gasket on the cap....did i screw it up?

Triffecpa · « **Reply #13 on:** February 12, 2010, 11:00:14 AM »

First thing that I would do is to disconnect the gas line from the petcock. Install another length of hose and turn on the petcock and run the gas directly into a clean white bucket. run the tank dry and make sure that the flow doesn't decrease as vacuum builds in the tank. I've also had partially clogged tank vents on these bikes that will do that as the run for awhile. You can drill a hole in the base of the gas cap (after it's flipped open) to provide additional venting.

Second thing that the clean white plastic bucket does is to see if there is any crud coming thru the fuel system and into the carbs.

Also make certain that you don't have a partial miss on one cylinder. The engine will rev fine, but once you put a load on it, and the cylinder starts missing, the engine won't pull cleanly.

What kind of ignition do you have on the bike? When you check plugs, are you pulling all four of them? Do you have stock coils and plug wires?

Tracy

NitroHunter · « **Reply #14 on:** February 12, 2010, 11:06:38 AM »

Tracy,
Those all sound like good places to start.

Fletch,
Maybe try riding it once - under controlled conditions - with the gas cap cracked open? That will show you if you've got a venting problem.

Of course you can try drilling the main jets...

It should get you close if you creep up on it. Lean is mean - err on the rich side. When you get to sputtering, then order new ones a size or two smaller.

Again, just my .02

fletcha221 · « **Reply #15 on:** February 12, 2010, 11:13:03 AM »

Tracy,

I will have to try that tank trick when I pull it this weekend. I can hear the vent pulling air through when I turn the bike off....so there is some substantial vaccum.

The bike pulls great until the very top end. I Cleaned and oiled the spark advancer, put in a Dyna S ignition, and replaced the spark plug caps. The coils and wires are stock.

I pulled all 4 plugs and they are identical in color. The bike runs perfectly and really sounds healthy. I've heard a lot of 350's that just sound like a F'ed up sewing machine. My carbs are sync'd and the throttle response is good, and exceleration is smooth until the very top end.....she starts hesitating, and bucking (slightly).

supersports400 · « **Reply #16 on:** February 12, 2010, 11:35:30 AM »

If a liquid flows through a jet, the surface smoothness of the jet is very important for the amount of fluid that goes through per second. The rougher the surface, the worse the flow. Don't drill the jets yourself, it's not worth it. If interested, read something about fluid dynamics, this link is interesting : http://www.scienceclarified.com/Ex-Ga/Fluid-Dynamics.html. Bottom line is that when the surface is not smooth, drag and shear will destroy the flow pattern, and thus creating an resistance. Also the difference between laminar flow and turbulent flow is of importance.

Jensen

(Ask questions, I try to will answer them)

fmctm1sw · « **Reply #17 on:** February 12, 2010, 03:14:23 PM »

I found that I actually had to size down on my main jets. I'm running 72s in one of them. Seems consistent with what HM has said about factory jetting at sea level.

Nikkisixx · « **Reply #18 on:** February 12, 2010, 03:39:44 PM »

I don't know aboput drilling with a drill bit, but I do use reamers specifically made to work on jets. The flutes are straight and they do not leave scratches that a drill bit would. I follow up this process with a size guage. Keep in mind that a set of good reamers from Germany are way more than a box of jets. It only makes economic sense if you have a bunch of different carbs to deal with.

fletcha221 · « **Reply #19 on:** February 12, 2010, 06:08:04 PM »

Ok guys, I pulled the tubes and they are SPOTLESS. These carbs are as clean as they come. I've found that (at least with this bike) The most obvious answer is usually the right one. I just need bigger jets. Tracy, from what you said, it sounds like you are running straight exhaust.....which i was before as well. I saw the same top end hesitation. I'm assuming I am making a "cleaner" and complete exhaust stroke, so essentially the bike is performing better. As soon as I put the exhaust on, she ran lean.

GUYS, Does this make sense?

fletcha221 · « **Reply #20 on:** February 13, 2010, 04:04:08 PM »

SO, Is it strange that If I put a more restrictive exhaust on, the bike will run leaner? Anyone?

scottly · « **Reply #21 on:** February 13, 2010, 04:17:27 PM »

If the problem started when you changed the pipes, then I would think that sure implies a cause and effect situation, strange or not. Did you make any other changes at the same time? Maybe the new pipes are actually less restrictive....

Duke McDukiedook · « **Reply #22 on:** February 13, 2010, 06:16:49 PM »

Quote from: jensen on February 12, 2010, 11:35:30 AM

If a liquid flows through a jet, the surface smoothness of the jet is very important for the amount of fluid that goes through per second. The rougher the surface, the worse the flow. Don't drill the jets yourself, it's not worth it. If interested, read something about fluid dynamics, this link is interesting : http://www.scienceclarified.com/Ex-Ga/Fluid-Dynamics.html. Bottom line is that when the surface is not smooth, drag and shear will destroy the flow pattern, and thus creating an resistance. Also the difference between laminar flow and turbulent flow is of importance.

Jensen

(Ask questions, I try to will answer them)

+1, don't drill unless they don't supply the size anymore. Just buy the jest if they still make them, that is the best solution unless you have the equipment to smooth them out and flare or chamfer the orifices as needed.
.

BLUE71TURBO · « **Reply #23 on:** February 13, 2010, 06:42:19 PM »

For everyone here, you do not want to " DRILL " carburetor jets, you need to " REAM " them with the proper tools.
Here is what a set looks like......http://www.socalautoparts.com/images/product/43-5715-0.jpg

Take NITROHUNTERS advice, after you figure out about what size you need, install new sized jets as they flow way different than reamed jets because of the way they are tappered.

News:

Author Topic: Drilling Jets HELP! (Read 4747 times)