Topic: CB 450

[Haoleman]

Well, My son brought out the 73 CB 450 today.  Checked the compression (142 lbs on both cyl) Wouldn't run so pulled the carbs.  One float has gas in it and the other carb had a stuck CV diaphram.  I'm looking for carb kits and find kits for 69 to 71 but that's it.  I believe they all share the same Keihin carbs.  Could someone please enlighten me on that?  Anyone have an extra carb float around and know where I can get kits for these carbs?

Thanks,
Terry

[tbpmusic]

Honda lists the minimum compression on a 450 as 164 psi......

There is no "diaphragm" in a 450 carb.

You're wasting time and money messing about with the carbs, you have more serious issues to deal with.

[bucky katt]

try doing a "wet" compression test and see if the compression comes up significantly after that. just squirt a little oil in through the sparkplug holes

[eurban]

Ya'll might want to ease up a bit on the gloomy predictions for this engine.  Here in SOHC4 world, the subject of compression readings has been coming up a good bit. . . . .Compression readings are significantly influenced by the testing apparatus used and the procedure (throttle wide open/carbs removed, all spark plugs removed, good battery, good starter, good seal of tester to head) used. The testers internal volume (how long is the hose on your tester??) adds volume to the combustion chamber during a test.  This increased volume effectively lowers compression and can give low gauge readings, particularly if this volume is relatively large like on a automotive style gauge with a long hose for connecting to the head.  On a car with a 3000cc engine this added volume is insignificant, while on a 450cc motorcycle its effect can be very significant.  Who knows what type of tester Honda used for their numbers but comparing what you are getting on a particular gauge to what Honda published is problematic, particularly if you are using a typical automotive style gauge.  Consistent readings between cylinders is usually a good sign as it would be highly unlikely for multiple cylinders to wear evenly.  Sure, he may a have a compression problem but indentical compression readings in both cylinders would tend to indicate otherwise.  This subject came up just a day or two ago in the SOHC4 bikes section and Two Tired has written up a nice discussion in the Tricks and Tips Section. . . .

[tbpmusic]

Ya'll might want to ease up a bit on the gloomy predictions for this engine.  Here in SOHC4 world, the subject of compression readings has been coming up a good bit. . . . .Compression readings are significantly influenced by the testing apparatus used and the procedure (throttle wide open/carbs removed, all spark plugs removed, good battery, good starter, good seal of tester to head) used. The testers internal volume (how long is the hose on your tester??) adds volume to the combustion chamber during a test.  This increased volume effectively lowers compression and can give low gauge readings, particularly if this volume is relatively large like on a automotive style gauge with a long hose for connecting to the head.  On a car with a 3000cc engine this added volume is insignificant, while on a 450cc motorcycle its effect can be very significant.  Who knows what type of tester Honda used for their numbers but comparing what you are getting on a particular gauge to what Honda published is problematic, particularly if you are using a typical automotive style gauge.  Consistent readings between cylinders is usually a good sign as it would be highly unlikely for multiple cylinders to wear evenly.  Sure, he may a have a compression problem but indentical compression readings in both cylinders would tend to indicate otherwise.  This subject came up just a day or two ago in the SOHC4 bikes section and Two Tired has written up a nice discussion in the Tricks and Tips Section. . . .

OK, fine - Haoleman, go ahead and rebuild the carbs.
Let us know how that works out for you........and post a photo of that "diaphragm", I'd be interested to see that.

I've read all that compression gauge stuff- very interesting, and the logic is certainly seductive.
But this is a 450, and a bike I know more than a little about - if those compression values (142) are correct, there's little hope that it will run well......you might get it to run, but not well.
My last 450 rebuild (valve job, bore job, new pistons/rings) measured 182 psi before it was ever started, using a regular ol' auto gauge. The third 450 I've done this year, all with equivalent results.
It's nice that both sides are the same, I guess, but that probably just means it's equally worn out on both sides - again assuming those values are correct.

Try the compression test again, warm engine, throttle wide open, NO CHOKE.
And like Bucky suggested, try again after adding a little oil in the cylinder - if it jumps dramatically, you have bad rings most likely.

[Haoleman]

Thanks for all the info.  On the CV carb. It's not a diaphram, it was the piston.  Now that I have the carbs off of it, I will once again do a compression test.  Bike hasn't been run for awhile and with only 10,000 miles on it, it doesn't seem likely that it will need a ring job.  I will pull the head and cylinder as part of my redo to check things.  Rings could be gummy from sitting, right?  Nobody has yet to answer my question about a carb kit though.  Is the  73 CB 450 K6 carb the same as a 71?  Any suggestions as to where to find carb kits for mine? 

[eurban]

Quote from tbpmusic
"I've read all that compression gauge stuff- very interesting, and the logic is certainly seductive.
But this is a 450, and a bike I know more than a little about - if those compression values (142) are correct, there's little hope that it will run well......you might get it to run, but not well.
My last 450 rebuild (valve job, bore job, new pistons/rings) measured 182 psi before it was ever started, using a regular ol' auto gauge. The third 450 I've done this year, all with equivalent results.
It's nice that both sides are the same, I guess, but that probably just means it's equally worn out on both sides - again assuming those values are correct."
end quote


I'll say it again; wear almost never occurs in a consistent manner from cylinder to cylinder.  The most important thing you are looking for in a compression test is consistency between cylinders which is what he found.  Numbers from gauge to gauge will vary dramatically.  I personally have 3 different brands/ type of compression testers and on my motorcycle engines, they vary by as much as 20%.  He is not getting strikingly low numbers and unless his gauge is stuck he is getting essentially no deviancy. . . .Even if we make the assumption that he is not getting decent compression, engines that have been sitting for long periods of time will often show low compression.  This "damage" is often corrected after bringing the bike back into service and riding it for a while.  Compression values of 142 should be enough to start the bike . . . Oh, and when people are instructed to add oil to the chamber to see whether or not compression values rise, they should consider how much oil they put in.  Oil will help leaky rings to seal but it will also reduce the size of the combustion chamber by its volume.  For instance on a 450 twin adding 5ccs of oil to a cylinder will reduce the combustion chambers volume by about 20% and have a corresponding affect on compression ratio and cylinder PSI readings. 

[tbpmusic]

Thanks for all the info.  On the CV carb. It's not a diaphram, it was the piston.  Now that I have the carbs off of it, I will once again do a compression test.  Bike hasn't been run for awhile and with only 10,000 miles on it, it doesn't seem likely that it will need a ring job.  I will pull the head and cylinder as part of my redo to check things.  Rings could be gummy from sitting, right?  Nobody has yet to answer my question about a carb kit though.  Is the  73 CB 450 K6 carb the same as a 71?  Any suggestions as to where to find carb kits for mine?  

The carbs differ only in the mixture screw type and main jet size. An early K model probably has 130 mains, the later ones most likely 140/145, changed at K5.
Sirius (a Canadian EBay vendor) has kits for most of the different K models. Check your main jet size to be sure you order the right kits.

Now it's possible that when the bike is started and run for a while, any gummed up rings or something like that will free up. But it's sort of implicit that if you take the readings with a hot engine, the bike has been started and run, yeah??

Don't tear it down unless you're sure it doesn't recover. It's a lot of work to do if you don't have to, and the whole engine needs to be pulled out.

It will try to start and run - it's a Honda, after all. And a 450 engine is very durable, tough as nails, you almost have to put a couple of bullets in one to keep it from trying. They're not nearly as fussy as one of the SOHC fours.
But the difference between a 450 with good compression and one with poor compression is almost startling.

[restforyoursouls]

If you decide to rebuild the carbs on this bike, I just ordered a pair of carb kits for my '72 cb450 from Out West Motorcycles (www.outwestmotorcycles.com). 

They are out of Oregon so shipping may be less expensive than buying the kits from a company out of Canada.

I havent taken my carbs yet but plan to either today or over the weekend.

[mlinder]

Wait, how the hell is that possible? 164psi would mean a CR of 11.16 to 1.

Last I checked, the 450's were 9 to 1 CR, which, in a perfect world, would give about 132 psi. Perhaps the amount of area displaced by oil, I dunno.

[tbpmusic]

Wait, how the hell is that possible? 164psi would mean a CR of 11.16 to 1.

Last I checked, the 450's were 9 to 1 CR, which, in a perfect world, would give about 132 psi. Perhaps the amount of area displaced by oil, I dunno.

9.0:1 is correct - what oil??

Yeah, the compression is high, but it's a real-life number, Mark.
My 450 is about 185 psi.
350's run about 170 psi, optimally.......

[mlinder]

Just wondering what I'm missing in my calculations.

At sea level, the amount of air that can be let into the chamber, not under compression, is 14.7 psi.

When you take that volume and compress it at, say, 9.0 to 1, you should have 9 x 14.7, which is about 164 psi.

But there must be something I'm missing in this admittedly simple calculation, some factor I am not allowing for.


Off to research.

[mlinder]

OK, I think I'm getting it. Will post more as I get it.
9:1 CR pistons do, in fact, give you about 11:1 CR, trying to figure out why....

[tbpmusic]

Mark, I can't answer your question, sorry.
Sometimes this stuff just seems illogical, but it's real enough.

All I can do is assure you that a freshly rebuilt 450 with good rings and valves will indeed show 180+ psi........the kind of compression gauge you use doesn't seem to matter that much on these old twins.

And Honda does throw up a red flag at 164 psi (on a 450)........

[mlinder]

Right. I'm real close to figuring this out. PSI is measured with air, not fuel.

The heavy duty CR computations give similar oddness.

So far, I've found that adding about 22% to the factory listed CR gives you your CR and PSI during compression testing. Some of it has to do with fuel (CR testing is not done with fuel), and some of it is something else, but don't want to say yet until I know for sure.

So far, a dry (no fuel) CR SHOULD give you a PSI (and CR), as I said, about 22% above listed CR, at least in the 8:1 to 12:1 range.

Still trying to figure out if it is a linear increase.

[david 750f]

Compression ratio's and the PSI reading on the gauge do not yield a 100% corelation.
 
For example:
Standard pressure at 4000 feet is 12.69 PSI which would yield a much different reading than at sea level (14.7 PSI).

Compressing any gas results in an increase in temperature, which results in an increase in pressure due to the "Ideal Gas Law"

http://en.wikipedia.org/wiki/Ideal_gas_law

This is why the gauge reads higher than the theoretical value of Compression ratio * 14.7.

Valve clearances and valve timing also have an effect on the readings on the gauge. A long duration cam would yield a lower reading than a short duration cam.

As mentioned before, different gauges would also produce different readings.

 
Hope this does not make the issue more confusing

Cheers,
David

[mlinder]

Yep, this is what I've been finding.

However, boyles law states "For a fixed amount of an ideal gas kept at a fixed temperature, P [pressure] and V [volume] are inversely proportional"

That means that a gas compressed into half the volume has twice the pressure. I've been trying to figure out how much heat is created at 9:1 and how much expansion that causes. I'm guessing it's about 22%  

Ideal gasses can turn liquid and start having different, non-linear compression characterisitics, but this is at about 75 atmospheres. That would be 1057 PSI. Not happening in our motors.

[tbpmusic]

Ideal gasses can turn liquid and start having different, non-linear compression characterisitics, but this is at about 75 atmospheres. That would be 1057 PSI. Not happening in our motors.

It would on Jupiter !!!!!

Seriously though, things related to physics/chemistry are rarely truly linear - that's why natural logarithms were devised...........
That's one of the reasons it's called the "Ideal" Gas Law.

[mlinder]

Yeah, but the compression characteristics of an 'ideal gas' are linear until 75 atmospheres at 31 degrees celcius
Nitrogen is very close to being an 'ideal gas', and the air we breath, and our engines breath, is more than 70% nitrogen

Gawd, seriously, I need to do more research, because as far as I can tell, if a bike, geometrically, is supplying a 9:1 compression ratio, the math says in actuality its about 12:1.
This is, obviously, true. The honda manuals call for it, the calculations say it, I just want to know why. I'm not content with saying "PSI for a 9:1 engine should be 176.4 at sea level because these are the numbers we plug in".

I want to know why we plug those in.

[w1sa]

I think the main reason we see different PSI specs for engines with the same CRs, is the differing 'induction efficiency' that exist in all engine designs.

Efficient engine design calls for the most power from the smallest capacity. That's why we increase compression ratio to start with. But apart from compression ratio increases, designers already build in the highest degree of induction efficiency appropriate for the design.
The target is to always get the most (usable) A/F charge into the cylinder, at the desired CR to produce the maximum usable power. (That's also why forced induction is used.)

So, along with the other variables contributing to cylinder pressure output (when compared to its CR) you have the ratio of induction volume to standard volume.

If you induce a higher volume of gas into the cylinder (than would exist under zero atmospheric differential) you automatically increase the 'induction ratio'. That will provide a relative increase in cylinder PSI under compression.

Gas velocity has momentum or inertia, and that is what can enable a cylinder to be charged beyond a standard atmospheric volume of  1
Engine designers maximise this effect with varying degrees of valve timing overlap, intake design, inlet and exhaust port design etc.

Anyhow, that's how I believe it works, but I dont have a specific equation to support it.
 

[mlinder]

That would be all well and good, but the momentum of the gas isn't present during compression testing.

[w1sa]

I believe it is.
It's part of the reason faster cranking usually provides better PSI results.

[mlinder]

Nope, not really. Charging effect won't happen until you have some serious velocity on the intake.

[markjenn]

Motors that have sat for a long time seldom show compression specs that meet book.  Get it running and then start worrying about whether the compression is a problem.  It's got plenty enough to start.

- Mark

[w1sa]

Motors that have sat for a long time seldom show compression specs that meet book.  Get it running and then start worrying about whether the compression is a problem.  It's got plenty enough to start.

- Mark

+1     
After some search and read, I found wikipedia references to CRs, cylinder Pressure, Volumetric efficiency etc.

Essentially, the raw ‘compression test’ pressures in the cylinder(s) are determined by dynamic compression ratio (DCR), specific heat ratio(between 1.2 and 1.4 power), atmospheric pressure (say 14.7lb@sl), , and volumetric efficiency as % or factor of 1.
DCR is the effective closed cylinder CR after allowing for valve timing/actuation, and is almost always less than static CR.
Specific Heat Ratio is a value representing the increased pressure produced due to thermal expansion of the a/f mixture when compressed.
Volumetric efficiency is the expression of closed cylinder a/f volume as a percentage of closed cylinder volume.

Expressed as     (DCR ^1.3 x 14.7 x 1)
Or, using 9:1 DCR
      9^1.3 x  14.7  x  1
      = 17.4  x  14.7  x  1
      = 256psi
For an equivalent pressure gauge reading, you deduct the atmospheric pressure (14.7 psi). The actual ‘compression test’ figure as seen on a gauge would therefore be 241psi.

The real world numbers have to incorporate actual values to get close to spec pressures on a gauge.

For example, with a static CR of 9:1, a DCR of 7.5:1 and other factors remaining the same, the equation becomes
      (7.5^1.3  x  14.7  x  1) – 14.7
      = (13.73  x  14.7  x  1) -  14.7
      = 187 psi

For comparison to factory test spec numbers, all of these values should be considered variable to some degree in any calculation attempted. e.g a V Efficiency of 96% changes the above result to  179 psi  etc.

The main difficulty arises when attempting to identify Volumetric Efficiency. Everything else can be assumed, calculated or measured to a close value.
I suppose at least with all other main values known and a good test gauge attached to a well built engine, the Volumetric Efficiency could be reasonably derived.................

[mlinder]

It's the specific heat ratio I'm looking at. Why is is it 1.2 to 1.4? How do we calculate it's actual number? i.e what is specific heat ratio for a volumetric compression ratio of 11.5 to 1?

[w1sa]

The 'specific heat ratio' is a scientifically concluded exponent value applied to gas behaviour. The 1.2 to 1.4 simply provides a range to cover different conditions/assumptions included in any calculation.
 
In scientific speak, the 1.2 value is a polytropic value and the 1.4 is a dry air adiabatic value. The known value for air/methanol is apparently 1.3 (approx). The exponent value of 1.3 is also (apparently) highly favoured by individuals that theorise/design.
I did read one argument that stated the value of 1.2 (approx) was probably more relevant, since it served to cancel out other optimistic valve closing calculations etc and provide a more real world figure.

The exponent value chosen does not itself change for different calculations based on different CRs.

It's impossible to know what values Honda used originally. The calculations for engine design may well have been different to real world test results, which would dictate the 'workshop test' figures we see published.

[mlinder]

Are you saying that the specific heat ratio does not increase with higher compression?

[w1sa]

How long is a piece of string? Is there a carrot at the end?
Is the string attached to a stick?     

Are you saying that the specific heat ratio does not increase with higher compression?

That's correct. Not for calculation of cylinder pressure (cold/no ignition compression tests) at standard/room temperatures, using DCRs etc

Additional pressure in the cylinder requires significant additional heat (which occurs during ignition), at which point the effective/absolute cylinder pressures are far greater......