Topic: 78 cb750 intake boots for CR29

[sazaver]

Hi there,

I have been draining my sleeping hours in order to find a solid solution on this issue.

For my 836cc engine, I want to buy a set of CR29 carbs for my k8 cb750. As you may know, the intake boots for 77-78 years are NLA at honda and other suppliers. Since I am dedicated to run this overbored engine without any idling or performance issues because of the air leaks, I want to change the intake boots with brand new ones.

Buzz from DYNOMAN suggested that the mounting spigots of the carbs, can be changed with the early 36mm ID ones, rather than the 40mm ID 77-78 spigots and mount the carbs with the rotated/flipped 71-76 intake boots.

Which path should I take? Should I try to get custom boots for CR29s? Will running smaller intake boots mess up the performance on cylinders #2-#3?

OR, does anyone know a source for brand new NOS 78 boots?


thanks.

[Wobbly]

OR, does anyone know a source for brand new NOS 78 boots?

# 1 and # 3 are readily available (CMS, German NOS dealers on eBay). A#2 just sold on eBay--which does not happen often. So it would take patience to get the missing 2 and 4. The alternative are aftermarket boots available here:
http://www.vintagecb750.com/products/73/featured-products#product-48-0757

[Black 750K8]

I just bought those and they look perfect but the real test will be how they hold up.

[Wobbly]

I just bought those and they look perfect but the real test will be how they hold up.

It would be great to hear back from you how well they work. I already had bought the Honda #1 and #3 new before I saw this set. But NOS is just that: New OLD Stock, and the rubber is thus 37 years old.

[Black 750K8]

I just bought those and they look perfect but the real test will be how they hold up.

It would be great to hear back from you how well they work. I already had bought the Honda #1 and #3 new before I saw this set. But NOS is just that: New OLD Stock, and the rubber is thus 37 years old.

I have made a note and will let every one know but I think I will wait until mid summer to give it time and if good will check at the end of summer. I really don't know how long it will take to see if the compound will hold up. I did the same thing as you and have a NOS 1&3. I still have  two each of 2&4 used but in really good shape a little hard on the engine end so I'm going to try the old wintergreen trick.

[sazaver]

I guess I am going to go ahead and order from this site. I saw them before but really couldn't be sure. Now the carbs are waiting so it's ok to give a try.

Still in the hunt for a tested aftermarket set of boots though! If anyone tried one brand, please let me know.

[brandEn]

I am running CR29's on my 1978 CB750 F3. I have the F head using Early K model intake boots. The carbs are for the early K model . Hopefully this helps you out.

[sazaver]

I am running CR29's on my 1978 CB750 F3. I have the F head using Early K model intake boots. The carbs are for the early K model . Hopefully this helps you out.

Hi brandEn,

If these vintagecb750.com boots won't hold on, I'll get a set of custom made... By the way, I have acquired my jets according to your build, great info there. Really

[martin99]

I just bought those and they look perfect but the real test will be how they hold up.

It would be great to hear back from you how well they work. I already had bought the Honda #1 and #3 new before I saw this set. But NOS is just that: New OLD Stock, and the rubber is thus 37 years old.

I don't think NOS necessarily means the rubber is 37 years old. Spare parts usually continue to be manufactured long after the bike/car whatever has ceased production. NOS is more likely to mean they have been sitting on a dealer's shelf for some time, but certainly nowhere near as long as 37yrs in most cases. I bought two OEM #1 and two OEM #3 from DSS and no way did they have any significant age to them. Nice and supple, new looking and fresh packaging. Parts become NLA at the point the manufacturer stops making them.

I just flipped them over to fit 2 & 4 and have had no issues with leaks, although they are a little trickier to ensure they are seating this way. Given the time and effort the OP has put into his motor, I don't suppose he would consider this as a solution, though many on here have done the same. Your US supplier PartsnMore also has aftermarket rubbers listed.

[Black 750K8]

I just bought those and they look perfect but the real test will be how they hold up.

It would be great to hear back from you how well they work. I already had bought the Honda #1 and #3 new before I saw this set. But NOS is just that: New OLD Stock, and the rubber is thus 37 years old.

I don't think NOS necessarily means the rubber is 37 years old. Spare parts usually continue to be manufactured long after the bike/car whatever has ceased production. NOS is more likely to mean they have been sitting on a dealer's shelf for some time, but certainly nowhere near as long as 37yrs in most cases. I bought two OEM #1 and two OEM #3 from DSS and no way did they have any significant age to them. Nice and supple, new looking and fresh packaging. Parts become NLA at the point the manufacturer stops making them.

I just flipped them over to fit 2 & 4 and have had no issues with leaks, although they are a little trickier to ensure they are seating this way. Given the time and effort the OP has put into his motor, I don't suppose he would consider this as a solution, though many on here have done the same. Your US supplier PartsnMore also has aftermarket rubbers listed.

I did the same and that is what is on my bike right now no leaks. When I saw the new one I just had to give them a try.
 I can tell you one thing the new ones are exactly the same in all but one. I checked the lip where they fit on the carb side and they are flush so there is no interference with air flow. The one difference is the number marking it is very small and hard to see but in the correct spot. I will try to put them on this week end. Right now the only concern is the material they are made of. Will it hold up to E-gas?? I have one in a jar of fuel five days now.  Also they will not sell just one.

[sazaver]

Vintagecb750.com boots have arrived and I have to say they don't really match to eachother. Their bent is different and their length on the outer side carb boots are different. I am not really satisfied with these!

[Wobbly]

I don't think NOS necessarily means the rubber is 37 years old. Spare parts usually continue to be manufactured long after the bike/car whatever has ceased production. NOS is more likely to mean they have been sitting on a dealer's shelf for some time, but certainly nowhere near as long as 37yrs in most cases. I bought two OEM #1 and two OEM #3 from DSS and no way did they have any significant age to them. Nice and supple, new looking and fresh packaging. Parts become NLA at the point the manufacturer stops making them.

These parts have no wear and tear nor can thy possibly break. I am certain, that Honda did not reproduce them after shutting the K8 production down--in which case they are at least 36 years old. I bought # 1 and # 3 as well recently from CMS. Not being exposed to the elements, they certainly look new as they don't age as quickly. But rubber still ages. My old ones didn't have any visible cracking either. However, I bet that if we would pick up a set recently produced, they would be much softer.

[Jerry Rxman Griffin aka MuthaF'er]

So where are you finding a problem? The 78 K head has the same size spigots as ALL bikes before it except the 77/78 F's. Someone correct me if I'm wrong on this. Is it that the 77/78 F has larger intake spigots than all the other 750's so only those require different boots?!

My CR31's fit the earlier boots just fine too.

[MRieck]

So where are you finding a problem? The 78 K head has the same size spigots as ALL bikes before it except the 77/78 F's. Someone correct me if I'm wrong on this. Is it that the 77/78 F has larger intake spigots than all the other 750's so only those require different boots?!

My CR31's fit the earlier boots just fine too.

Correct on all counts

[martin99]

So where are you finding a problem? The 78 K head has the same size spigots as ALL bikes before it except the 77/78 F's. Someone correct me if I'm wrong on this. Is it that the 77/78 F has larger intake spigots than all the other 750's so only those require different boots?!

My CR31's fit the earlier boots just fine too.

78K (405) and 77F (410) have the same size spigots on the head. I have 405 rubbers on my 77F2. The difference is in the length, can't remember which way round, but I think I'm right in saying the 405s are shorter than the 410s. No problem if using pod filters as I do, but with a stock airbox on my F2 there would be too large a gap between carbs and airbox for the airbox to carb rubbers to span. So the other way round, if you used 410 rubbers on a 78K the gap between airbox and carbs would be too tight.

AFAIK, PD carbs have a larger diameter spigot than the earlier bikes, meaning earlier (300) rubbers will fit the head, but not the carbs. So if the CR29s have the same dimensions as PDs (do they?) then the choice of rubbers is limited to 405 if using the airbox, or 410 which may necessitate moving the airbox back somehow. If using pods, you can use either.
That's my take on it anyway. I'm sure others will let me know if I'm wrong.