Topic: CB750 exhaust gases values

[leovich]

G’day everyone, this morning I rode my ‘70 CB 750 to the inspection ( every 2 year here in Italy, I don’t know in US if you have this same need ) by the way the bike was all ok on brakes and lights but they want more than 10% CO value at steady 40 km/h. Do someone have same common value for this bike? I mean, my bike is all stock, air screw 1 turn out, good 900 rpm idle and ride ok, but I want try the next step for carburettor with an exhaust gas analyser. Does someone know the correct CO value for this bike? Many thanks and enjoy your ride! 😉

[bryanj]

Dont think that spec was ever posted by Honda

[Deltarider]

[...] I rode my ‘70 CB 750 to the inspection

4-4 exhausts?

[...] but they want more than 10% CO value at steady 40 km/h.[...]

I cannot give you the correct values, but 10% CO is very, very high. Are you sure 10% is what they want? I mean... you will be sooting your sparkplugs real quick.

[calj737]

[...] but they want more than 10% CO value at steady 40 km/h.[...]

I cannot give you the correct values, but 10% CO is very, very high. Are you sure 10% is what they want? I mean... you will be sooting your sparkplugs real quick.

I’d think it’s just the reverse? 10% is likely a lower emissions standard than his bike currently produces. To know for certain, he should post what the current reading is. Modern emission standards are intended to lean the fuel mixture, not enrich. Given Italy’s recent efforts to produce less vehicle emissions, I’d think he will struggle to get by…

[Airborne 82nd]

Italy’s way to stop the use of your bike. Next will be California if not already? I'm sure the Feds want to do this. Next they will want us to put a electric motor on our bikes or park them

[Deltarider]

10% CO!? 10% CO and I don't want to be even near that bike. Have a look below in what A/F ratio 10% CO translates in and you'll see it is not even listed. Neither do oldstyle analogue exhaust gas analyzers of the 70s have it on the scale. Why the Ttalian MOT wants such toxic level instead of an environment friendlier output, is a mystery to me. At 40 km/h it would mean you'd have to lower the choke flaps at least halfway.
Something must be wrong in the communication.

[Deltarider]

Ha, I found one (see pic). So the Italian MOT wants 10% CO? Really?

[leovich]

Thanks at all and sorry for this terrorism 🤣 They want a 10% min CO2 value, not CO. These 2 values are inversely proportional each other. Someone here know I’m a triumph dealer mechanic and on the latest carb fitted bonneville/legend and the earliest triples with open loop EFI we set CO ( CO carbon monoxide )value to a max of 3%, this is a value given by shop manuals referred to euro-spec vehicles. They don’t specify a CO2 value, but a max of 3% CO. I have this knowledge for this motorcycles but I’m really curious to know if someone here have a value referred to the 750 four. I mean if someone use the exhaust gas analyser for setting carbs or has tried on a freshly tuned bike a gas test

[leovich]

[...] I rode my ‘70 CB 750 to the inspection

4-4 exhausts?

[...] but they want more than 10% CO value at steady 40 km/h.[...]

Yes yamiya 4-4 no number with baffles fitted

[HondaMan]

In the past I have lowered the CO and CO2 values by just turning the air screws inward a little bit. At sea level this was 7/8 turn instead of the standard 1 turn: at altitude here in Colorado it is 3/4 turn and raised idle speed to 1200 for the test. Then it went back to 1050 idle speed when I went home again. The air screws are in between 3/4 and 7/8 turn now, with #40 pilot jets. Mine has HM341 pipes, though, which are slightly more restrictive and will help lower the CO as a result.

[Deltarider]

These bikes will not have a good driveability with 3% CO, I'm afraid. Expect to need 6% for that. What that would result in @ idle, I'm not sure but I have seen 4,5% been suggested. 
In The Netherlands we don't have these tests. For cars yes, not for bikes. The policy with old vintage cars is that, what was accepted back then, is also to be accepted thereafter for that model.
What you could do, when the testers insist, is prepare it so it reads 3% CO at a constant 40 km/h and after the test, readjust it to a percentage that restores the driveability you were used to.
What many mechanics do, when they suspect a car will fail the emission test, is to, prior to the test, have it idle on a mix of gas and an additive (like Forté or similar brands), to clean the fuel system and remove carbon, and then go to the test. It is a common strategy in many workshops.

[PeWe]

I had an old car thad had high values due to the Stromberg carb needle and emulsifier tube.

One trick to get better readings was to use ethanol in fuel. Antifreeze carb ethanol used back then.
10% or so which is used in todays fuel, E10.
Helped for a while until needed to be replaced.

- How is the ignition?
- Advancer springs sloppy as usual?
- Cut both 1 wound, reshape and attach.
- Advancer might have a play on crank. I have noticed that TDC will be ok when advancer is set max counterclockwards on 2 different engines.

T and F will then match case mark.
Adjust ignition so F align with case mark at ca 1000-1200rpm.

You'll see that full advance will now happen not before 2700 rpm. Idle is better and throttle response from low too.
This might affect the exhaust measurements.

We do not need to check our old bikes in Sweden anymore.
Last time was 2018 for my K6. Before that every 2nd year if in traffic.
I do not remember if they measured exhaust gases before on motorcycles.
Sound level yes.

[leovich]

I had an old car thad had high values due to the Stromberg carb needle and emulsifier tube.

One trick to get better readings was to use ethanol in fuel. Antifreeze carb ethanol used back then.
10% or so which is used in todays fuel, E10.
Helped for a while until needed to be replaced.

- How is the ignition?
- Advancer springs sloppy as usual?
- Cut both 1 wound, reshape and attach.
- Advancer might have a play on crank. I have noticed that TDC will be ok when advancer is set max counterclockwards on 2 different engines.

T and F will then match case mark.
Adjust ignition so F align with case mark at ca 1000-1200rpm.

You'll see that full advance will now happen not before 2700 rpm. Idle is better and throttle response from low too.
This might affect the exhaust measurements.

We do not need to check our old bikes in Sweden anymore.
Last time was 2018 for my K6. Before that every 2nd year if in traffic.
I do not remember if they measured exhaust gases before on motorcycles.
Sound level yes.

Thanks for reply, ignition is ok, all genuine honda parts, timing spot on, I modified the advance springs, F mark spot on at idle and full advance mark is reached before 3000rpm, bike is idling good at 900-1000 rpm

[HondaMan]

I had an old car thad had high values due to the Stromberg carb needle and emulsifier tube.

One trick to get better readings was to use ethanol in fuel. Antifreeze carb ethanol used back then.
10% or so which is used in todays fuel, E10.
Helped for a while until needed to be replaced.

- How is the ignition?
- Advancer springs sloppy as usual?
- Cut both 1 wound, reshape and attach.
- Advancer might have a play on crank. I have noticed that TDC will be ok when advancer is set max counterclockwards on 2 different engines.

T and F will then match case mark.
Adjust ignition so F align with case mark at ca 1000-1200rpm.

You'll see that full advance will now happen not before 2700 rpm. Idle is better and throttle response from low too.
This might affect the exhaust measurements.

We do not need to check our old bikes in Sweden anymore.
Last time was 2018 for my K6. Before that every 2nd year if in traffic.
I do not remember if they measured exhaust gases before on motorcycles.
Sound level yes.

Thanks for reply, ignition is ok, all genuine honda parts, timing spot on, I modified the advance springs, F mark spot on at idle and full advance mark is reached before 3000rpm, bike is idling good at 900-1000 rpm

A couple of hints that might help if you have to do more changes to it to make it 'pass' emissions (without changing the carbs, which would make it NOT a K0):
1. This was important in certain US States to pass emissions - make the spark advance even slower. I've removed coils from their springs to accomplish this, making the spark reach full advance at 3000 to 3300 RPM instead of before 3000 RPM.
2. Use a hotter sparkplug with longer reach, like the X22ES-U series from Nippon Denso. If you can get the NGK D8ES-L (or the DR8ES-L version) this can also help a little bit. Both of these plug have longer, hotter tips than the D8EA or X24ES-U plugs.
3. Make sure the float bowls are not running too deep (I think you have already done this?). About the only way to verify this in real-time is with the so-called "clear tube" method. The floats should be set using the tiny sliver edge reference (which I think you have already done) in those early carbs.
4. Make sure the exhaust seals are good the the pipes where they enter the head. There are small, thick copper ring gaskets there to ensure a good seal, but they can usually only be used once: if the exhaust spigots are removed and reinstalled, then these gaskets must usually be replaced, too, to make them seal. If these leak then the pipes can pull in extra air during the period in-between exhaust pulses, and this turns into higher unburned hydrocarbon count (i.e., higher CO2 numbers) at the exhaust once the pipes are good and hot. This is counter-intuitive, but was proven to me with cars that had dual exhaust systems and V-8 engines where the left side and right side of the engine had its own exhaust pipes.

[MauiK3]

I’m sure glad Maui does not do this type of testing.
We just go for “safety check” once a year.
Lights, tires, horn, insurance, no smog.

[scottly]

CO value at steady 40 km/h.

This is only about 25 MPH. Is the test in 1st gear, and is there a load applied?