Author Topic: That's why they press in the slow jets.... (Read 8713 times)

Cvillechopper · « **on:** August 16, 2005, 04:39:47 PM »

OK, I'm not new to the motorcycle maintenance scene, but sometimes I feel like it. Today I figured out why the guys (or gals for the pc folk) used press-in jets on the cb750K 8 . So idiots like me don't end up drilling them out about 4 times their size while trying to clean them. Yeh, hurts to hear, doesn't it. Soaked them for a few hours then got an old spring and ground down the end into a point. I figured the holes on either end of the jet is larger than the spring (straight portion) so I couldn't do any damage. Well, after the 3rd one I started to wonder why it was taking so much effort to get the gunk out but figured they just might be that bad since they've been sitting for a few years. On the forth I noticed the small brass shavings. I've installed them as is and got the bike running, but she's got gas to spare for sure. I don't think any amount of adjustment will bring her anywhere close to clean. Any suggestions as to where I can order slow jets for a 78 cb750 K? I've seen 69 through 76 and 79 up, but not 77 or 78. Here's hoping I didn't just set my project back another month or 2 looking for jets.
(hangs head in shame)

eldar · « **Reply #1 on:** August 16, 2005, 04:49:49 PM »

I have not seen ANYTHING for slows for the 78k. You might be able to make some using brass or maybe copper tube. Otherwise maybe someone has something else to offer. The only other Idea that just popped into my head is that you might be able to put threads into the carb bodies to use screw in jets.

Terry in Australia · « **Reply #2 on:** August 16, 2005, 11:23:49 PM »

Well those holes are tiny! I had one blocked on my 77 F2, it was so bad I had to replace it, I couldn't unblock it. How many do you need, I might have some spares? Cheers, Terry.

Cvillechopper · « **Reply #3 on:** August 17, 2005, 03:18:51 AM »

All four are shot. Let me know if you have some and how much you want for them. I would appreciate it greatly.
Thanks
James

Joe Danger · « **Reply #4 on:** August 17, 2005, 05:30:58 AM »

I seem to recall a post on the old site that recommended a particular jet that motorcyclecarbs.com sells.

The trick was, you had to turn it on a very small lathe, or use a soft vise and file, to take off the threads. I have #42s in mine, I'd suggest any slow jet you can find that has a reasonably similar shape and a .42mm aperture. (Or whatever your particular desire is.)

It won't be fun, but the other option is a new rack of older carbs. There's a bit of munging to get the rubbers on, but we are hardy and inventive souls here.

Cvillechopper · « **Reply #5 on:** August 17, 2005, 05:57:46 AM »

I thought about new carbs right off, but have been told that the 77/78 carbs were better, just harder to find parts for. Anyone know if this is true or just someone buying into the Honda sales schpiel from those days?

Also, does the emulsion tube separate from the jet or is it one piece? I could probably make something to work if I can separate the 2.

I'll try that website first. Maybe I'll get lucky.

bryanj · « **Reply #6 on:** August 17, 2005, 07:03:07 AM »

If you find anybody selling those jets post it on hear cos i been trying for years

carpy · « **Reply #7 on:** August 17, 2005, 08:21:38 AM »

Hey Gang
Have you tried SUDCO?
They are about 45 mins from me and are a main importer of Kehien carbs, they have over 15 sandcasts there and more 750's I have ever seen in one place, more than i have owned and I have owned over 100 CB750's in 5 years.
I think they have a website but need to check it out and see, as i just drop in there.
carpy

dusterdude · « **Reply #8 on:** August 17, 2005, 09:12:04 AM »

carpy!!!!,you`re alive!!!!!

Cvillechopper · « **Reply #9 on:** August 17, 2005, 09:57:06 AM »

Carpy, I found the website for the place you're talking about and e-mailed them to see if they can order teh jets. Of course they're not listed with the rest of the Keihin jets, but here's hoping. I also just thought about something while looking at a chart of the throttle range effectiveness of different adjustments to your carbs. It seems that the slow jet and the pilot screw effect the same range and from what I've seen/ read the pilot screw is downstream from the slow jet... If this is the case, would it be possible to use the pilot screw to lean it out enough? I'll try just about anything once...

TwoTired · « **Reply #10 on:** August 17, 2005, 12:37:47 PM »

Quote from: cvillechopper on August 17, 2005, 09:57:06 AM

It seems that the slow jet and the pilot screw effect the same range and from what I've seen/ read the pilot screw is downstream from the slow jet... If this is the case, would it be possible to use the pilot screw to lean it out enough?

It would seem like it. Possible stumbling block. Are the slow jet orifices now of uniform size? If not, each idle mixture screw will have its own unique setting and may be quite sensitive.

I think if you yank out the pilot jet, you will find it does indeed have an emulsion tube attached as part of it. You will also probably find that the emulsion tube holes are plugged too, which may be contributing to your rich slow mixture.

A thought. I haven't tried it but.. Couldn't the body of the carbs be tapped to accept a screw in jet? Like the N424-22-XXX found at :
http://www.sudco.com/keihinjets.html

You might need to grind down a tap to make it a bottoming type.

Cheers,

eldar · « **Reply #11 on:** August 17, 2005, 12:43:57 PM »

Thats what I was thinking. Maybe tap the carbs to accept screw-in slow jets. I think that might be easier than getting slows and machining the threads off.

Cvillechopper · « **Reply #12 on:** August 17, 2005, 12:58:58 PM »

I just thought of something else. Would making a plug for the bottom end of the jet and getting a jewlers bit to drill the proper size hole work or is there a reason the diaphram was half way up the tube?

eldar · « **Reply #13 on:** August 17, 2005, 01:07:04 PM »

I think what might happen is that either the pressure of air in the emultion tubes would cause the gas to not be drawn up into the jet or the jet would not meter correctly and fuel would load up in the tube then get sucked into the engine causing backfires and fouling.

TwoTired · « **Reply #14 on:** August 17, 2005, 01:11:37 PM »

Quote from: cvillechopper on August 17, 2005, 12:58:58 PM

I just thought of something else. Would making a plug for the bottom end of the jet and getting a jewlers bit to drill the proper size hole work or is there a reason the diaphram was half way up the tube?

Well, yes. Has to do with laminar flow of fluids and retension of fluid in the emulsion tube. I can explain if you think it will help. But, it's wordy and technical and bores some people.

BTW, it's an orifice, not a diaghram. Technospeak, you know.

Anyway, I would plug and redrill only if there was no other way. I believe some aspect of performance will be lost. FYI twist drills leave jagged, irregular holes, whereas reamers leave smooth round ones.
Cheers,

Cvillechopper · « **Reply #15 on:** August 17, 2005, 02:54:27 PM »

Well, I tried the "tighten down the mixture screw and pray" method, but still looks like a diesel rig. There's a place that says they have the slow jets for the CBX style carbs but they needed my measurements. I'm thinking they don't know about the press-in thing. I sent the dimensions in great detail including the taper length and steps and mentioned once again about them being press-in so hopefully they'll have something that works. Anyone know about the CBX jets and compatability?

Also, just in case you were wondering, the measurements are as follows:
The large part of the jet is 14mm long x 5mm od
The seat for the press-in part is 4mm long tapered from 4.5mm to 4.25mm to 3mm in stages
the emulsion tube is 9mm long x 3mm od.
Total length 28mm
Inside diameter is approximately 1.75mm other than the orifice (see, I do learn)

TwoTired · « **Reply #16 on:** August 17, 2005, 03:08:51 PM »

CBX carbs are CV types and the fuel jets are all screw in types. But, I don't remember if the slows had emulsion tubes on them.

From your dimensions, it sure seems like the one from the Sudco site might work.
N424-22-XXX
For some Honda PE Series carbs. Sizes #35-#80.

Why don't you call them to see if other dimensions match up?

Cheers,

Terry in Australia · « **Reply #17 on:** August 17, 2005, 04:19:08 PM »

Quote from: cvillechopper on August 17, 2005, 03:18:51 AM

All four are shot. Let me know if you have some and how much you want for them. I would appreciate it greatly.
Thanks
James

I'll take a look tonight and let you know James, the fee will be a porno mag or a bike mag (no dirtbikes and no gay mags though, I do have some standards! Cheers, Terry.

Cvillechopper · « **Reply #18 on:** August 17, 2005, 04:57:08 PM »

I'd say at least one of each, maybe my whole stash plus a new one....

Terry in Australia · « **Reply #19 on:** August 17, 2005, 08:00:20 PM »

Quote from: cvillechopper on August 17, 2005, 04:57:08 PM

I'd say at least one of each, maybe my whole stash plus a new one....

Well for all that I'll just send ya my carbs! Ha ha, Cheers, Terry.

Cvillechopper · « **Reply #20 on:** August 18, 2005, 06:06:22 PM »

Terry, send me an e-mail with your addy and I'll send you some mags just for the offer. As for the jets... I got WAY lucky today. Went to the local dealer/ shop for a set of points and saw that they had a plastic bin with some jets in it behind the counter. Went straight home and pulled my wrecked jets and took one in to see if what they had might match up. I've now got a set of #38 screw-in jets the exact same length and width. The amazing thing is that they acted just like self-taping screws into the carb body and she idles fairly clean although I've got a little top-end noise that I'll have to look into. Tomorrow after work I'll see if I can get her dialed in. Here's hoping that everything comes together and I can get a test drive in on Saturday.

cvillechopper@earthlink.net

Cvillechopper · « **Reply #21 on:** August 19, 2005, 04:23:33 PM »

Alright. The jets I got from the dealer work great. Idle is fairly smooth. Now when I give her a quick pop of the throttle I get an immediate response, but then a backfire or pop depending on her mood. Timing is dead on and the valve clearances have been checked twice. I did notice while the rocker cover was off that when I have the crank and TDC 1.4, the cam is just a hair off from perfectly lined up with the notches up top. Looks like about .5 mm at most.
I've got 110 mains in and 38 slows with the bowl height at about 14.5. Not being all that great with carbs(evidenced by my honing the slows to 210s

) I have spent about 2 hours with the girl running at op temp and fiddling with the screw settings to no avail. I really can't distinguish slight rpm diffs and my gauge (hooked up for tuning, not mounted) doesn't really notice 50rpm changes. I was wondering if anyone could point me in the right direction. I've read all posts on tuning and haven't been able to make them work for me yet.
BTW, thanks all for the help so far and any help coming.
James

TwoTired · « **Reply #22 on:** August 19, 2005, 04:56:11 PM »

Is the only complaint the backfire/pop at throttle let off?

Is your exhaust system sealed up well?

Are the plug deposits even across the four plugs and a nice color?

Are you running a fan on the motor to keep it cool while tuning?

Do the manometers show even draw for all carbs?

What's your air filter set up?

Cvillechopper · « **Reply #23 on:** August 19, 2005, 05:21:38 PM »

Wow, didn't even think about the temp.

Only complaint is the backfire/pop at let off, yep. No smoke, fairly level idle.
I didn't have a fan on while working on her. I'll make sure to get one for tomorrow.

I know the exhaust has a little leakage around a weld or 2 that I can seal up. Wasn't aware that the pin-hole leak would make that much or a diff.

Don't have a manometer but will pick one up if the local shop has one tomorrow prior to starting work on the girl.

So far, I've only run her withour anything on the intake. Was planning on putting stacks on and figured that made little difference. That not the case?

With the bike supposedly running great when she was parked before I got her I didn't think the synch would be necessary. Chalk that one up to a youngster getting anxious and hasty.

I'll update once I get my head outta my ass and do things the right way. Thanks for asking me the simple questions that I should have known to answer myself

That's why I love this place.

James

Terry in Australia · « **Reply #24 on:** August 19, 2005, 06:47:13 PM »

G'Day James, without the airbox in place it'll run lean, as the airbox adds a bit of "choke". You'll need to experiment with your needle height (up for rich, down for lean) or just keep working on bigger main jets, whichever's easist. Those carbs were set lean in the factory to help with emission laws in the US, that's why they fitted accellerator pumps to get rid of the flat spots when you crack open the throttle. Without the airbox, they'll run really lean.

Check all your rubber mainfolds for air leaks, just spray some WD40 around the carbs while it's running, that'll tell you if they're leaking. Did you want a copy of the K8/F3 manual on CD? I'm sending one to Terry Quail on Monday, so it's no biggie to copy another one? Cheers, Terry.

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Author Topic: That's why they press in the slow jets.... (Read 8713 times)

eldar

eldar

eldar