Topic: 1978 Cb750k Strait pipes?

[zeus87]

What kind of jetting is needed to run strait pipes like these on a stock 78' 750k?   They have baffles...

[zeus87]

someone must have expirience with these pipes!?

[eldar]

Those look like the mac drag pipes. There is no way to say exactly how you will need to change jetting as every bike is different. You could probably start with moving up 5 on the mains and turn out the idle screws a bit. Most likely.

[754]

Those pipes  were first made in the early  70,s.. way before Mac

Usually the internal baffles are pretty  restricive, probly little power gain.

I think though, 1080 did low 11,s with these pipes..

[eldar]

Those do not look to have baffles in them. Maybe zeus can tell us. If there are no baffles, then a small amount of jetting will most likely be needed. If there is baffling, then probably not.

[754]

They look the same wether baffles are in or out, just a small bolt hold them in on the backside.

I dont think I have seen these for 77/78. a spigot would have to be installed.

[zeus87]

there are baffles, now are these pipes safe?  no long turn damage?  And also im running a k&n factory stlye filter and look to be running a little lean already.  Should i pick up 120's to jump up from 110's or should 115's be alright?  Thanks

[eldar]

The pipes themselves should be fine. You will need to usually rejet for the k&n though.

Do a search for k&n and it should pull up a lot. See every bike is different. So you may need to go 120 but you may only need to go 115. Hard to say. So I suggest reading up on those old posts to see where you think you should start.

Do you know what brand those pipes are?

[Sam Green Racing]

Best way is do a plug chop, pipes and modified filter systems can throw the mixture all over the place.
If you know you are running lean and your plugs are coming out a very light colour, go up one set on the mains.
Then check it again until you are happy.

Sam.

[firecracker]

I know a guy that had some on his '78K




For what it's worth, he got rid of them.  Too much trouble? 

[mustangcar]

i read a post here forgot the members name he drilled the main jets out with a 1/32 drill bit,says his 78k runs great,,i will be running 4into4 with dunstall  low restriction mufflers with no baffles,so thats what i did to my main jets,cross my fingers hope my750 likes the taste of more fuel its a stock motor with 14,000miles on it,better to run rich is my way of thinking,,i just had the four factory pipes chromed sure don't want them to turn blue on me so soon

[zeus87]

hey, 110's are bigger than 1/32...im drilling them with 3/64....we'll see what happens

[mustangcar]

hey, 110's are bigger than 1/32...im drilling them with 3/64....we'll see what happens

,,hmm maybe i used a wrong drill bit,can someone confirm this,is 1/32 smaller than 110?,originally mine were 110 but they are bigger now by just a bit,i place the drill bit 90 degrees up on a vise and placed the main jet on top of the bit and slowly turned with a screwdriver, a 1/32 drill bit measures.031 how does that compare to a 110 jet,oh well i have spare jets in case i screwed them up

[TwoTired]

1/32 of an inch is 0.03125 decimal and  0.7937 mm, or about a #80 size jet.
3/64 of an inch is 0.046875 decimal and 1.19063mm, or about 1 #120 size jet.

However, standard twist drills make a hole larger than their measured size.  If they didn't, they would jam in the hole.

Cheers,

[mustangcar]

 TwoTired thanks!! for clearing it up ,just bought a micrometer going to mic that drill bit,its made by Dremel got it at Home depot, its just skinned the 110 jet made a spec or two of brass dust,

[zeus87]

cool, seems to be running pretty good.  I will post some pics soon, thanks everyone!

[754]

TT,

 drills do not always drill oversize, part of that depends on material.. in some cases they will drill undersize..

[TwoTired]

TT,

 drills do not always drill oversize, part of that depends on material.. in some cases they will drill undersize..

It depends on where the drill measurement is made.  If it is measured and sized at the cutting end, it must make a hole larger than itself or it will stick in the hole.

We are talking dimensionally stable material here like metal.  Plastic and nylon distort when a force is exerted and a two flute cutting head (particularly if it is dull) will distort as it cuts, possibly making an undersize hole.

Jet material is NOT plastic or nylon in the carbs we are discussing.

You cannot have two materials occupy the same physical space.  You cannot put a 120mm wire into a 120mm hole as that is an interference fit. (Barring tricks with temperature differential to alter dimensions.)
A two flute twist drill WILL make a hole larger than it's sized dimension in a dimensionally stable material.

Are you trying to tell me that you have 3/64 drills with undersize ground tips relative to the shank dimension?  It's possible, but those are going to be expensive and not what you'll find in an Exacto set or your typical hardware store.

[754]

Drill tolerance is not what reamer tolerance is.

Depending on point wear and condition of margins, drills do drag in the holes sometimes.

The trick is to comntrol it, to go the way you want it to for a specific result..

JMO but drilling jets with un micced fractional drills is asking for grief..

[goon 1492]

TT,

 drills do not always drill oversize, part of that depends on material.. in some cases they will drill undersize..

It depends on where the drill measurement is made.  If it is measured and sized at the cutting end, it must make a hole larger than itself or it will stick in the hole.

We are talking dimensionally stable material here like metal.  Plastic and nylon distort when a force is exerted and a two flute cutting head (particularly if it is dull) will distort as it cuts, possibly making an undersize hole.

Jet material is NOT plastic or nylon in the carbs we are discussing.

You cannot have two materials occupy the same physical space.  You cannot put a 120mm wire into a 120mm hole as that is an interference fit. (Barring tricks with temperature differential to alter dimensions.)
A two flute twist drill WILL make a hole larger than it's sized dimension in a dimensionally stable material.

Are you trying to tell me that you have 3/64 drills with undersize ground tips relative to the shank dimension?  It's possible, but those are going to be expensive and not what you'll find in an Exacto set or your typical hardware store.

well said TT!
Like tt said you can't put a .5 rod in a .5 hole, the rod would have to be at least .495  and it would slide in nicely and drill bits all get larger the closer you get th the shank, its the tip that is the money shot, and thats why if the drill bit is needed for a certain spec say .036 then it should not be resharpened because the size will end up at .038 or more depending on how much was ground off the tip.

[mustangcar]

what luck it is a 3/64 drill bit i used,,the drill kit made by dremel part#628 at homedepot,has 7 drill bits ,not the case or the drills have the measurements stamped on them only the paper label on the package,3/64 is the second from the left 1/32 is the smallest bit in the package ,,my concern is that i also shimmed the needles,may turn out to be too much for a stock 78k

[TwoTired]

Drill tolerance is not what reamer tolerance is.

If you were monitoring the thread, you would have noticed the discussion was about using drill bits on the jets, not reaming.

JMO but drilling jets with un micced fractional drills is asking for grief..

I rather agree on that point.  IMO if you are going to alter a jet size you should be using jet reamers and also have jet gauges, so as to make precision holes, not the far from round, scuffed ones made with two flute hardware store twist drills.  Such tools cost $60-$120.  Cafe-ing your bike on-the-cheap, doesn't often include such purchases.

Cheers,

[Roach Carver]

I have never understood why people are so averse to buying jets. Its nice because the nuber is right there in plain sight. no guessing.

[mustangcar]

gee i spent like close to 2 grand so far on this completed teardown, new chrome/paint,and i'm being a cheap ass makes no sense i know,,,just experimenting i'll get some new jets down the road ,,hey but then again i can truely claim it has the original factory brass jets thanks again

[seaweb11]

I have never understood why people are so averse to buying jets. Its nice because the nuber is right there in plain sight. no guessing.

x2

[754]

TT I am not a monitor. but I can read..

 The point I was making is not all 3/32 drill bits are the same diameter, because they are drills.. and drills are not produced to tight tolerances.

 I am not sure about your experience with drills, tolerances and charpening them, but I have drilled a lot of holes and sharpend a few drill bits..

But hey, what would I know...


Perhaps TT can help us out here and tell us what you should do to an off the shelf jobber drill bit before drilling brass..

[Sam Green Racing]

All this talk about accurate drilling reminds me of an episode many years back in a engineering journal.
At the time, not sure if it is any different today but, the smallest unit of measurement was in angstroms.
An American precision engineering company claimed it could make a 1" bolt threaded the full length complete with nuts and washers the thickness of a human hair.
Rolls Royce challenged them to prove it.
The American company complied with the request and sent the finished bolt to Rolls Royce.
Rolls Royce sent them the bolt back. It had been tapped and had another bolt screwed through it.
UK 1   USA 0 

Sam.

[754]

aH you sure that happened, or is it an urban myth..

[zeus87]

well, its running really good...but still a little lean (pipes could be darker, and cylinder #3 is blueing the pipe) anyway, where can i get jets?  i want to order them but i hear they need to be very specific...well im looking for 120's does anyone know if i can get them from say dennis kirk?  Thanks!

[Sam Green Racing]

aH you sure that happened, or is it an urban myth..

Well do you believe it Frank

Sam.

[eldar]

Zeus, #3 IS probably right. With the lean nature of these years, pipes blued a bit more.