Honda CB750 Sandcast
The Restoration Shop => Fuel (gas - petrol) Tanks, Side Covers and Air Box => Topic started by: markb on February 26, 2014, 09:09:03 am
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For the #97 carbs I'm planning on using 28 tops, 5mm cables and short springs and slides. I scored on a set of carbs that has everything I need except it was missing one of the springs. Anyone have one they could sell me? I think the part number is 16185-300-004. There is a company in town who can make some for me but the minimum order is $300 so pretty much the same price for 1 or 25. I just hate to spend $300 when I only need one. So the other option would be if anyone needs/wants some I could have extras made.
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Is that the spring CMS sells for € 2,50 ?
No. 35
http://www.cmsnl.com/honda-cb750k0-four-usa_model14346/partslist/E25.html#results (http://www.cmsnl.com/honda-cb750k0-four-usa_model14346/partslist/E25.html#results)
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#35 is a throttle stop screw spring. i think Mark's looking for a thr.return spring; actually fits inside carb.l
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That's right Steve. It's like #36 only shorter (approximately 114mm long).
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keep asking around, Mark. somebody should have a single 28 spring, i would think. i used to have a couple extra, but i'm pretty sure i gave them away. i'll look through my stuff, but i can't promise i've got one.
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A 28 spring! That's a good name for them. Thanks.
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I'm still looking for a 28 spring. Anyone?
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If I decide to get some of these springs made does anyone else need some?
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Mark, i looked thru the few parts i have and i have none. sorry.
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Mark,
You can count me in for several sets. Just guessing, you can probably have 1000 made for the same price as 50.
Keith
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Is anyone else interested in some? Once I know the quantity I'll let everyone know the price and get some ordered.
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Mark,
Count me in for several sets.
Thanks,
Duane
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sure, ccunt me in for a couple sets.
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Looks like I've got a enough to get the price under $8 each. Anyone else want any. Might be able to shave a little more off the price. Before I order I want to bring an actual spring for the manufacturer to look at. Sor far I've just given them the exact dimensions and they said they could do it but I want them to confirm they can duplicate them before I place the order. I'm going to try to get there this week and it would take 30 days.
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I was about to place the order but after a couple of discussions with members I want to make sure I have the correct dimensions. I've got one complete set and one set of three 28 springs. The set of three has a free length of 112-115mm (they've been in the carbs sitting on a bench for probably 40 years). The other set measures 125-130mm. (By the way, they all had 15 coils and the wire diamter was .9mm). Some later ones I have measure 140-145mm and have a wire diameter of 1mm and 22 coils. One of the guys thinks the 28 springs were 20mm shorter than the later springs. Some of all of these may have relaxed and are therefore shorter.
Does anyone know the original length of the 28 springs? Or the difference in the length of the 28 and later springs? Does anyone know the length of the later springs or have some they could measure? Right now I'm thinking that somewhere around 125mm is close to the original 28 length but if they were a bit on the long side it probably wouldn't hurt. Any input is appreciated.
Mark
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Mark, Honda Parts Bulletin dated March 23, 1970 states "original" springs are 128 mm long; 1.15 pounds tension. "Modified" springs 145 mm long; 1.54 pounds tension. I cannot find any documentation about the number of coils.
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Bravo Steve! Excellent detective work! Didn't think to check the bulletins. We were also missing the tension rate just measuring old springs. Very important info right there.
Based on measuring 40+ year old springs, the "28" springs are definitely relaxing or taking a "set". The K0's I measured appear to be holding their length pretty well.
Obviously, Honda changed them for a good reason. I'm guessing the "28" springs were relaxing in a fairly short timeframe.
Also makes me wonder if they changed the material since the K0's seem to be holding up pretty well for this long.
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Plus, excluding any additional friction implications, the throttle "pull" went from 4.6 pounds to 6.16 pounds. Bet you could feel that after a few miles.
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Found the long springs. Was fitted for say the 5 minutes it took me to decide they were too strong for my weak arms. They now measure 141mm 143mm and twice 144mm!! took one original spring out of carburettor and it was 111mm long! 0.9 thick 18.5 outer diameter same as new ones and had 13 1/2 turn to the 19 3/4 of new ones of 1mm thickness and still 18.5mm outer diameter. With the higher carburettor slides and thicker cables never again had any troubles of cables nibles "jumping" out of the bottom. Of cause the thing has not been used since fall 78.
Youers Erling.
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Mark, Honda Parts Bulletin dated March 23, 1970 states "original" springs are 128 mm long; 1.15 pounds tension. "Modified" springs 145 mm long; 1.54 pounds tension. I cannot find any documentation about the number of coils.
Thanks Steve! That is excellent information. It coincides with my guess of 125mm. I'm assuming the 1.15 lbs. is when the spring is fully compressed. That info will be very helpful for the spring winder. I noticed my coil count is a little higher than others. In fact my count ranged from 15 to 15 1/2 but that included the closed ends. If I don't count them then I am very close to 13 1/2 active coils. Can anyone else confirm this? Thank you all for your help. I'm very close to placing the order and feel that these repros will be very close to original. By the way, the price should come in about $6 each.
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the other thought i have, after i read Keith's email.... is the tension in pounds ? or is in in kg ? the Service Bulletin does not denote what "1.15" measurment scale. i wonder if there is somethign in the Honda Service Manual addressing spring length, seems i remember there is not..... ?
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Hmmm. Another good point. I would guess the units are kg. That said if we have the wire diameter, spring diameter, free length and number of coils we should be about as close as we can get. I will give all the information I have to the manufacturer included a cap and slide just to make sure everything fits up. If anyone else has anything to add it will probably a few days yet before I place the order. Thanks again.
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My guess and it is a guess would be pounds because:
4 x 1.15kg = 4.60kg = 10.14lb
4 x 1.54kg = 6.16kg = 13.58lb
Either one sounds like an extremely heavy throttle pull to me.
Going to do a little research.
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I've been thinking about this too. Not meaning to get too anal about this but the force would be compression not tension. That number 1.15 that Steve cited may be the spring rate which would be in units of force per length (example lbs./in. or kg/cm) so one would have to measure how much the spring is compressed to determine the force. Either way it does seem that kg would be quite high but it seems odd that Honda would mix units. Steve, what is the number of the Service Bulletin? Again, as I said earlier, if all the dimensions match, the spring rate or force should come out in the wash.
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Don't sweat the anal part. We need to get it right. The rate or force is also effected by the modulus of elasticity of a given material. These are relatively small springs, so the material selection will probably have minimal impact other than holding their size.
Yes, the spring is in compression but when you are twisting the throttle, you are experiencing the resulting tension from compressing the springs and the numbers I was citing doesn't take into account any friction in the cables, slides, etc. Nor does it take into account any leverage benefit that exists from the spool diameter on the throttle pipe. You may be correct about the number mixing but the kg force is very high. And the bulletins were written by American Honda. I have seen them mix and match before. I plan to take some "used" springs to work today to measure their force and see if there is any correlation.
I think the bulletin is #15. Unfortunately, my copy is the Rev 1/31/75 which does not include that info.
Steve, sending PM for you guessed it!
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I plan to take some "used" springs to work today to measure their force and see if there is any correlation.
I think that is a great idea. I agree that kg seems way too high.
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I plan to take some "used" springs to work today to measure their force and see if there is any correlation.
I think that is a great idea. I agree that kg seems way too high.
I am not 100% convinced but in my opinion must be 1.15N (newton), International System (SI) not kg or lb.
HOOKE LAW
F = k * x
F is the force that the spring exerts when the spring is either stretched or compressed by a distance x , unit N (Newton).
x < 0, means compression, x=(a-b) , unit m (meter).
k is the so-called "spring constant." K = - F/x , unit N/m.
so,
F= 1.15N
b= 0.128 m (extended measure)
a= ____m (compressed measure applying 1.15 N)
k= -F/(a-b)
1.15 N = 0.117267 kgf = 0.258530 lbf
The power applyied by hands to accelerate must be about 4x1.15N = 4.60 N = 0.4690 kgf = 1.03412 lbf , plus friction.
Seems plausible?
Marcello
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I measured 1 of each, a "28" and a K0, spring compressed to 1/2 of their free length respectively. Results as follows:
28 - at 58mm = 1.20 lbf
K0 - at 73mm = 1.90 lbf
The "28" spring came in pretty close to the numbers Steve posted while the K0 came in on the heavy side.
This is a very small sampling size and I would assess the accuracy of these measurements at +/-10%.
Also, these were both used springs that have been sitting in carb sets for who knows how many years.
The compressed installed heighth of the 28 is ~55.00mm. Pretty close, 3mm more than my measured heighth.
The compressed installed heighth of the K0 is ~60.75mm. Hum, 12.25mm more than my measured heighth.
I had not measured the installed heighth prior to reinstalling the springs tonight. You can definitely feel the difference in force.
Marcello, Based on this test, I think it is lbf. The "28" is significantly stronger than 0.258530 lbf.
I am speculating that American Honda, who published the info, used metric calipers and an "American" spring tester that would have been most available at the time.
And if they were using a "standard" valve spring tester, it would have probably been more accurate at much higher force, maybe 90-150lbf. Just guessing??? :-\
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Sorry for being anal again, I just want to make sure I understand. I'm assuming lbf is lbs per foot (the spring rate or constant). So did you measure 1.20 lbs at the compressed height of 58mm or calculate 1.20 lbs per foot based on your measurement at 58mm?
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Dear Mark;
1.20 lbf seems plausible (kmb69 measure).
Assuming gravitational force g=9.806 m/s²
1lb=4.448N=lbf 1kg=9.806N=1kgf
lb is used for mass and lbf for force, but the module is the same.
Marcello
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Mark, Confirming it is pounds force. 1.2 pounds actual at 58mm. Keith
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Dare I ask how near original Sand Cast are you gone a do your model?
Now this discussion about short throttle springs. As the original setup could not close properly!
Don't for sure know exactly what change did the difference. Thicker cables or the higher slides some 2/100 mm smaller in diameter
I could use the weak springs for years afterwards without any problems.
Though changed to double cables when that was possible.
Some in this Forum insist on 28 carbtops. Are they modified for 6mm cables?
Do someone still want to use the shorter slides?
Apparently more than me had a problem since Honda gave away new parts which indeed cured the problem.
And I for one never came about to complain.
I shall not tell anyone what to do, but apparently 6mm cables and the 46mm high slides would do, and that still with weak springs!
Youers Erling.
P.S. Some 10 years ago appeared here in Danmark a Motorcycle History Book (Danish).
In the chapter about our Honda, the author claimed all 4 slides could stick wide open simultaneously!!
I had never a problem with more than one at a time, and do not remember if it was the same every time. It took some time to rectify of cause. But it did not happen that often.
Only the one time outside the camping in Milano (Italy), to start the 1000 mile ride home, all luggage had to come off again, but girlfriend could hold the tank though.
Erling.
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I am trying to make it as close to original as possble. I do have shorts slides and new 5mm cables. Depending on certain factors such as the age of the cables, the routing, etc. there can be problems. The original parts were used until March 1970 and well into VINs 17000 before Honda changed them. Obviously they had too because of the potential safety issues. I will start and ride this bike because I need to know that everthing works. However, because of the value of the finished product it will be ridden very little. I have doubts that my odometer will ever see 100 miles (160 km) so I am not going to worry. Maybe I'll even have a set of later carbs to put on it if I want to ride it more. If/when I sell it the new owner can change them or not depending on his preference.
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I've been out of town the past few days so I didn't have a chance to get the springs ordered. I am going to try to do it this week. I think I have enough information to have them reproduced exactly. Thanks for everyone's input.
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I also need 4 of the short spring if anyone is going to make some or have some for sale. Mail to: jan@tveten.biz.
Jan T.
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I ordered them. I'll put you down for a set and let you know when I get them.
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I received the springs and they look great. They have been shipped to everyone who wanted them. Thanks to all of you who helped nail down the specs and to those of you who helped me order enough to make buying them reasonable. Just so there's a record, here were the final specs in case anyone else needs to reproduce them in the future:
Material: stainless steel spring wire
Wire diameter, .9mm (.035”)
Free length, 128mm
Outside diameter, 18.5mm
Active coils, 15 ½
Total coils, 13 ½
Closed ends
Load @ 55mm = 1.15 lb.
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Mark,
Many thanks for making this happen for all of us!
Best,
Duane
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I'll have three sets
Luis