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Discussion Starter #902 (Edited)
This bike never stops kicking me in the nuts.

while changing out the jets this happened:



Yup that is the seat for the jet on the #4 carb broken off. This is apparently a common issue as I went through every rack of carbs last night and the #4 in all of them was unuseable. The worst ones looked like this:



I then remembered I carried a set back from NY with me this summer that I was going to build and sell:


yup, you are reading that float bowl right that is a set of carbs off the exact same model and year I am currently working on. So it forced me to do some research. Here is why I was banging my head against the wall with the carbs:

The carbs I bought and installed on the bike are 657B model carbs for a k bike (around 1972 vintage or so I am told). The original carbs on the bike are 064As. This new set are 064As as well. I had to go to 4 different resources plus the forums to figure this out but here is the issue. Essentially the carb bodies are all the same for most of the 1971-1976 carbs, however the numbers denote differences in hard parts installed on each body. The 657B carb and the 064A have completely different slide needles, emulsion tubes, jets, float heights, needle heights, and air bleed screws. There are also some differences regarding location of return springs and such but they aren't really relevant for this conversation.

Remember a few pages back when I said it felt like the needles may be the wrong shape as well? well I guess I should have listened to my gut instead of reasoning that this was a problem with the jet. Don't get me wrong the jets were wrong too, 064As use 105 and 657Bs use 120. The reason for the differences are both tightening emissions standards and different cams used in the bikes from the K2 models (which are the last of the lest strict emissions US models) . The 064A air bleeds are drilled to give a different air bleed profile, the needles are #27301 (657B carbs use a #27201 needle), and should be in the 3rd notch (2nd or middle notch on the 657Bs). The slow jets are supposed to be #35 in the 657b carbs but someone swapped them out for #40 slow jets which is what the 064A carbs are supposed to run. In fact I went through every slow jet I had and every one was a 40 so this is a common mod.

By the way there is a lot of mis-information on the internet, but also a ton in the actual factory manuals and parts books. Some of the older manuals I had didn't even have references for 064A carbs and the aftermarket manuals (clymer and haynes) have "generic" one size fits all setting for the different carb numbers. No wonder so many people have trouble setting up the carbs on their honda 750s.

So it looks like I have to do the world fastest carb rebuild and setup as the buyer is coming a long distance on sat and come hell or high water this bike is going to be ready. At least the set I am starting with looks to have been stored without much fuel in them, doesn't have any pitting or corrosion, and is set to the factory settings. I should be able to have them done by tomorrow night. fingers crossed.
 

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Discussion Starter #903
by the way, since we are discussing needle shape and how it affects fueling I thought I would post this from the SOHC4 forums:

 

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That's actually very interesting! Who would have thought that very similar looking bikes from a fairly short time period would have such variation in carb and cam specs, all improvised around stupid EPA restrictions.
The EPA sure knows how to fuck up a good time.
(Said the jeep enthusiast who also owns a diesel volkswagen)
 

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Nice bit of research which I too enjoyed reading. Hope nothing else bites you on the bum with this one. You seem to have gone through a lot to get to here.

Best of luck old chap!
 

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Discussion Starter #906
for their whole production run there are like 14 or 15 different carb variations spread across 3 different carb bodies (Sandcast and K0 carbs, K1-K6 and F0-F1, and 77-78 Keyhole carbs). All these variations address fuel metering in some way and there is no one source for the information.

this chart, which I posted earlier:
Carburetor Specifications U
doesn't even mention some of the variations of carbs it covers (064A carbs are not on there, neither are some of the K0). It's treated like gospel.

I had an interesting thing happen yesterday that made me think about this. While going through my old racks the physical size of jets the same orifice size were different between versions. The 120 jets that were in the 657B carbs I had were physically larger than 120 jets I had in another set of carbs. same orifice, different physical dimension. weird.
 

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I've come across similar spurious jets with the carburettor hinckley triumphs. The only thing I can put it down to is someone being in there before and reaming out the jet.

I also remember reading this the other day:

20161020_182730.jpg 20161020_182745.jpg

If, at the time, the carb manufacturer was playing around with flow rates for their jet design then they could appear a lot different between two 120's from different carbs. Worth taking a jeweller's eyepiece to for a comparison but I think slicing them in half to see would be taking it too far...
 

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One of the CB750's i owned years ago had the same thing on the #4 carb where the emulsion tube threads in. Broke clean off. PO had used JB weld to fix it. It worked....for a short while. I replaced the rack of carbs in the end though.
 

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Discussion Starter #909
One of the CB750's i owned years ago had the same thing on the #4 carb where the emulsion tube threads in. Broke clean off. PO had used JB weld to fix it. It worked....for a short while. I replaced the rack of carbs in the end though.
I assume that it is from carb #4 being the highest carb and therefore where all the fumes end up before they make their way out on the way to varnish. I really don't know why though but I went through quite a few sets and #4 was often the only unusable one. they were all damaged in just about the same way - emulision tube threads eroded away.
 

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That's actually very interesting! Who would have thought that very similar looking bikes from a fairly short time period would have such variation in carb and cam specs, all improvised around stupid EPA restrictions.
The EPA sure knows how to fuck up a good time.
(Said the jeep enthusiast who also owns a diesel volkswagen)
In those years Honda was quite capable of complicating things even without the help of EPA. Some parts books were thicker than a sequoia to accommodate all the vin breaks. Most parts guys from that era died from alcohol poisoning or they were committed.
 

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From the bowels of the toolbox.... this is from the little blue book Honda Motorcycle Service Specifications Manual revised 5/78. Not sure if this helps or confuses the issue.


Jetting.jpg
 

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Can you turn to the page with the F specs on it?
For some unknown reason, I wondered about the float level specs, so I compared them to the spec sheet that comes with the factory float gauge. The attached pages were published by A.H. and they were usually trustworthy. The Kowa Seiki gauge sheet says 26.0 mm for 750, 750K, 750F and 750FP (P was the one with flashing red lights). I'm assuming the gauge was purchased prior to 77, cause there was a different set up for 77/78. Christ I'm getting old.
750F.jpg 750F 2.jpg 750F 3.jpg
 

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Discussion Starter #920
So when we last left our hero he had 2 days to turn around a carb from a broken emulsion tube holder in carb #4. Fortunatly I had a set of the correct 064A carbs on the shelf so I set to work.

I post this pic because....

When we talk about old carbs we always mention "varnish" and I don't know if some of the youngins working on old bikes have ever seen real fuel varnish. Well here it is. See that shiny lacquer like tecture on the side of the float? see how it looks like clear coat? that's varnish. It actually looks like wood varnish. normally that float would be a flat black rubber texture. That gray stuff on the bottom of the float (toward the top of the pic), that is old fuel turning into varnish mixed with the remnants of the additives in the fuel left behind. IT had the consistency of oatmeal.

Anyway, I managed to get the carbs turned around in a day and a half, mostly staying up late and working on it. I had 3 &4 done by Thursday night and most of the rack assembled by friday. On Saturday morning I was reassembling carb #1, putting it on the bike and tuning it. I got up at 5am to do that and the bike was singing by 9:30.

One of the reasons I think this set was so much more workable was this unique quirk to the 75-76 F series carbs:

yes, honda put a mini filter on the top of the float needle seat so the float needle can't jam with debris and hang open. neat right?

Anyway for reference this was the old plug under the 120 jetting:


and the same plug after I put in the 105 jets and all the factory specs and went on a 30 minute ride:


With the correct jets, emlusion tubes, needle shape, etc...all the carb problems seemed to clear right up. The bike was strong everywhere, except when dead cold because it's carbed afterall. I rode it for a good while looking for any issues I could find. It still needed some dialing in, the cables had the idle set at 1500 when it should have been closer to 1100-1200, and the clutch cable should be replaced in the future, but otherwise it was a punchy fast bike and behaving just like it should.

My only disappointment is it took me so long to figure out what the problem really was that I didn't get more than maybe 45 minutes of riding the bike as it should perform. It was really delivering toward the end there.

So it should come as no surprise to those reading that TheCheif86 bought the bike. Actually, his brother bought the bike. Both rode it before they had a chance to commit and both commented how much they loved the way it rode. It did have some minor tasks left that need cleaning up but, biggest being the rear brake, but otherwise it was ready to be ridden while the new owner worked on it which is what he wanted. Everybody was happy: I got my asking price, the new owner felt like he got a deal, and Eric (cheif) got to ride my ducati and is now apparently obsessed with them.

here is everybody looking happy (from left to right: Eric (theCheif86), The new owner, and Me with my gay ass scarf).


goodbye old friend, maybe we will meet again:


To be continued with my philosophical ramblings on various topics, and hopefully a discussion of the next project.
 
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