Author Topic: New carbs? (Read 2854 times)

8 Track · « **on:** December 15, 2005, 02:30:22 PM »

Hello everyone. I need some advice... I have carb issues with my '76 cb750 that a rebuild may fix. The bike has an indicated 32,000 miles on it. But I have an opportunity to buy a new set of original carbs for $200. Is this a wise purchase? I'm looking to gain new slides, seals in the "lids", and of course, then I don't need the rebuild kits. Only problem is they're for a '70-'74 and I understand those were jetted richer. Any help is greatly appreciated.

Twostrokecrazy

Bob Wessner · « **Reply #1 on:** December 15, 2005, 02:42:50 PM »

Are the current slides damaged? Ordinarily, just a rebuild kit and a good cleaning should do the trick and is a good deal cheaper than new carbs. Is there something about the overall condition of the current carbs that is leading you to think you need to replace them?

Jonesy · « **Reply #2 on:** December 15, 2005, 02:45:09 PM »

The carb casting numbers were the same up through '76. The '76 carbs had the jet needle clips on the 4th notch down from the top and 105 mains. The idle jets were the same (#40). So, if the carbs you're looking at are new, you'd need to check the needle clip position and install 105's. The will likely have 110's or even 120's, but if you buy them check the jets before ordering more. In some books the K3's even are listed with 105's, so you might be in good shape.

The little seals around the throttle slide shafts are not available separately (according to the parts books) and if you buy new carburetor tops, they do not come with them. I wonder why this is? I've never seen slides worn to the point where the carbs won't work properly, but then again, I haven't looked at lots of carbs, either...

8 Track · « **Reply #3 on:** December 15, 2005, 03:03:20 PM »

Thanks for the input; what's been said is so true. My slides appear good, but 30 years of sliding around and they may be sloppier than I think. Like I said, it's a chance to get all new, start from ground zero, new parts. The problem I encounter currently is at slow speeds (25ish), 1/8 throttle or so, it's really choppy, stumbly and generally ill mannered. I have checked EVERYTHING that could affect it. But while I'm tooling along nice and slow, if I turn off the gas and the mains are no longer in the gas, it runs great. Turn on the gas, roll it on and she runs great everywhere else in the rpm range. I think my needles and mains have worn and the needles have a straight area in the taper. I've also heard mixed reviews on the keyster kits. Any input on those rebuild kits?

Twostrokecrazy

bowhunter · « **Reply #4 on:** December 15, 2005, 03:18:11 PM »

Up to 1/4 throttle is controlled by the slow jet, mainly. Sounds like your's are getting clogged. Try running some Seafoam gas additive/carb cleaner though them for a couple of tankfulls of gas. You may not have to do anything else.
Worked for me.

Bowhunter

8 Track · « **Reply #5 on:** December 15, 2005, 03:27:33 PM »

I've been through them three times total, and the primary jets are clear. Checked them with light. While I'm putzin along in the stumble zone, choking makes it worse and turning off the gas (for a mile or so) makes it better. I honestly think it's a rich condition. Reading the plugs verifies my suspisions. To make it more aggravating, it's right there in the throttle range that's used to gracefully pull away from a stop. One more thing- it runs perfectly before it warms up completely.

Bob Wessner · « **Reply #6 on:** December 15, 2005, 03:32:34 PM »

Quote

choking makes it worse and turning off the gas (for a mile or so) makes it better

It almost sounds like stuck float/float valve or float height problem.

8 Track · « **Reply #7 on:** December 15, 2005, 03:39:14 PM »

Bob, I thought that too, so I checked. Nope. In fact, even with the floats set correctly, I poured each bowl out and measured. They were deads nuts the same. Stock is 26 mm, I played around with different heights and I think I'm at 28 mm right now. A mm here and a mm there didn't seem to affect the situation.

damn_yankee · « **Reply #8 on:** December 15, 2005, 04:14:48 PM »

Do all the pipes get hot at idle (1000 rpm) ?
If not your slow jets are clogged.
I cleaned mine with some fine lamp cord wire.
A bike with only 32k miles should not need new carbs....unless someone screwed around with them?

8 Track · « **Reply #9 on:** December 15, 2005, 04:34:49 PM »

Yeah, damn yankee, I agree with you. It says 32000, but it's sooo clean! I posted a pic in the gallery. Looks like I can get the mains and needles I suspect are bad for $100. Honestly, the carbs look fine. I swear those primarys are clean, but since I've been told twice to check them, I'm gonna check them. All pipes get hot and I check them with an infared thermometer. Like clockwork, the insides run hotter, but close to each other and the outsides run the same and cooler. We're talking about 180 and 140. Chrome doesn't give a very accurate reading, but one gets the idea.

damn_yankee · « **Reply #10 on:** December 15, 2005, 04:38:47 PM »

My check is to use a water spray bottle on the exhausts. It should steam off on contact. If not the slows are clogged.

8 Track · « **Reply #11 on:** December 15, 2005, 04:42:45 PM »

I do the more daring spit test. Get your finger good and wet and brush the pipe. I learned that test from my grandmother when she tested her iron. If it doesn't burn you, it's hot! Yeah, you know, the steam that's produced doesn't allow your finger to touch the pipe.

Jonesy · « **Reply #12 on:** December 15, 2005, 05:42:55 PM »

I know you said you checked a lot of this stuff, but bear with me as part of this is me "thinking out loud"..

When you talk about turning off the gas, the idle jets are the first to go without fuel, being higher in the float bowl than the main jets. If the bike runs better without the idle circuit, the jets might be buggered a bit so they are bigger than what they should be (the PO on my bike decided to drill them out really big, for some reason...), the idle air passage might be blocked or the screw is in too far, the floats are not working/ incorrectly set, or the upper end of the jet needle might be slightly worn, allowing more fuel to slip past.

From my experience, the keyster kits are OK; the gaskets are good and the main/idle jets seem to be a good match. Some folks have had potential issues with the float valves, but mine have been fine. However, the taper of the jet needle provided in the kits is different than the OEM needle. It's skinnier on the top of the taper, causing the bike to run rich at lower speeds and eventually fouling the plugs. The OEM needle jet kits are still available from Honda. It is listed as: Needle Jet Set (Needle Jet, Jet Needle (KeiHin #27201) and E-clip): 16012-300-014 and runs $16.28 each. I like this better since you are replacing ALL the parts prone to wear. Floats are still available from NOS suppliers like CMS or David Silver Spares.

A thought, if you pull the carbs apart again, look at the number on the jet needle. You should see the funky "K" (keihin) logo and the number 27201. If it reads different, someone might have put aftermarket parts in, which could be your problem...

TwoTired · « **Reply #13 on:** December 15, 2005, 06:07:09 PM »

Very nice, detailed post Jonsey. Well done.

Jonesy · « **Reply #14 on:** December 15, 2005, 06:31:03 PM »

Thanks, TwoTired!

8 Track · « **Reply #15 on:** December 15, 2005, 06:59:51 PM »

Thanks, Jonesy. I understand your thinking out loud comment. I troubleshoot machines alot of times and have to ask alot of redundant questions. And you're so right about the "jet heights". That was mixed up in my head. You confirmed my needle/ jet suspision. I know one can't look at primarys and tell if they've been drilled, but the hole being as small as they are, they really looked good. I will however, consider replacing them to be on the safe side. Anybody could ride this bike and say it's fine. But I remember when they were new and they were smooth as silk. I've decided not to buy the new carbs, but to run down to the honda dealer and plunk down some green for some new jets and needles. *sigh* How many days until riding season?

Bob Wessner · « **Reply #16 on:** December 15, 2005, 07:38:10 PM »

Quote

How many days until riding season?

It's looking a long way off in these parts, more damn snow today and it isn't even winter, officially, yet.

TwoTired · « **Reply #17 on:** December 15, 2005, 10:38:19 PM »

Quote from: twostrokecrazy on December 15, 2005, 06:59:51 PM

I know one can't look at primarys and tell if they've been drilled, but the hole being as small as they are, they really looked good. I will however, consider replacing them to be on the safe side.

You can take a coarse measurement of your current jets by using a shank of a numbered drill bit.
The number stamped on the jet is in millimeters.
#100 is 1.00 mm (0.0393701 inch), number 61 drill bit
#105 is 1.05 mm (0.0413386 inch), number 60 drill bit
# 42 is 0.42 mm (0.0165354 inch), number 78 drill bit
etc.

Use here to convert to inches. http://www.onlineconversion.com/length_common.htm
Most numbered drill indexes will have the inch equivalent printed on them.
Xacto makes a set for the smaller bit sizes #61 down to #80.

Bits of the exact size should not fit as that would be interference. Slightly smaller ones should have the clearance needed fit through the orifice.

The jets are of a soft material. So, don't force anything steel into them as that can change the orifice dimensions.

The bit method isn't prefect. But, it can give you an idea if modifications have been made.

Cheers,

8 Track · « **Reply #18 on:** December 16, 2005, 02:19:37 AM »

Thanks for the info, twotired. Bob, I don't know about damn snow... The snow gives me a chance to play outside with my gravely and the 4 foot snow blade!

Jonesy · « **Reply #19 on:** December 17, 2005, 06:43:08 AM »

Quote from: Bob Wessner on December 15, 2005, 07:38:10 PM

Quote
How many days until riding season?

It's looking a long way off in these parts, more damn snow today and it isn't even winter, officially, yet.

Hey Bob-

Where are the Christmas lights?? Now that your bike is running and put away for the season, you've got no excuse to slack off on your other house projects!! Get Cracking!! SCROOGE!!

(Just Kidding! Merry Christmas!

Bob Wessner · « **Reply #20 on:** December 17, 2005, 07:12:08 AM »

Quote

Where are the Christmas lights??

The only concession to Christmas lights and exterior decorations are small candle jobs in the windows (hard to make out) and a wreath on the door. My guiding thoughts on the matter are the more you put up, the more you have to take down.

Inside is another matter, but that's my wife's territory.

CAPTRON · « **Reply #21 on:** December 17, 2005, 12:23:10 PM »

Hey What snow??

I just went outside to check and I do not see any snow.

Maybe the temp is still a might too warm here... 78% ... boy I just hate to rub it in.....now if I can just remember to duck when they say huricane I will be OK.

From Paradise, Ron

8 Track · « **Reply #22 on:** December 17, 2005, 12:45:23 PM »

Hey Captron, have you ever enjoyed the look from the cagers when you go for a winter day ride? They just don't understand the phrase "layer up"! Boy, it was COLD today!

CAPTRON · « **Reply #23 on:** December 17, 2005, 03:33:28 PM »

I get those looks anyway.

The whole reason I live here and put up with the huricanes....although that is wearing then.... is that we really do not get to cold to ride down here,,,wet maybe.... but that's another story.

Ron

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Author Topic: New carbs? (Read 2854 times)

bowhunter

damn_yankee

damn_yankee

CAPTRON

CAPTRON