Carb Boxer CO setting and HC

Finally got my new toy working, I have purchased as a package 2 Sun MGA 9000 Cali edition smog machines,these machines read CO, Co2, HC and O2.

For starters anyone know what I should shoot for concerning CO and HC readings?




The headers have ports for each cylinder so I can diagnose and fine tune each carb. I started by putting it just in the exhaust tail pipe and it clear the carbs are way off, the engine was not fully warm but my CO readings were around 6% which is very rich and the HCs were around 4000 as well, so I am dumping raw fuel out the exhaust basically as the car is catless also. I am going to try and get around 2.5% CO as I know carb cars like it rich unless suggested otherwise, don't know what is acceptable for HC on a carb car though.

Thanks

Hi Jeff,

The numbers I shoot for, as I got from Ken McKay at Ferrari NA many years ago are:

CO 2-4%
HC <300 ppm

If everything is spot on, the HC number is achievable.

Regards,
David

Thanks David,

Gives me something to shoot for concerning the numbers, the HC readings seem like a lofty goal from where I am starting, I'll know further in the next few days whats happening with each cylinder.

Concerning the Co at 6%+, so much for my lean drop adjusting skills, I did run the idle up to 1200rpm as suggested in the Porsche manual and adjust the mixture so i had the max rpm for each mixture screw then lowered the rpm back to 900 and this was the result, Ha!

The gloves are off with this gas analyzer though concerning my flat spot and carb tuning issues.
Time to stop messing around !

Regards

Jeff, I obviously find this thread most interesting! Carb tuning is something I'm going to need to learn, and using a Sun machine sounds like a good idea.
Curious too, what RPM range is the 'Flat spot'?
Best regards,John

Hi John,

The flat spot is off idle around 1500 rpm, but it will happen at say 2200 rpm if I am shifting gears and just on the throttle again, it really happens after getting on the throttle a bit, it's more of a total bog or it acts as if the car wants to die, so more than just a simple flat spot. I have been learning more on the Webers through some books and this could be from too much fuel from the accelerator pumps so I have adjusted the volume down below 4cc of fuel per 2 throttle pulls, will see what happens after I get the carbs in tune with the gas analyzer which will tell me whats happening all through the rpm ranges which is key.

BTW I bought these 2 machines for $400 from a shop N of Bakersfield found him on the internet, one was supposed to work but didn't, I did get one working by replacing the motherboard, you can get these cheap since Cali has gone to the 5 gas dyno vs these 4 gas testers. Don't bother with the little cheap Heathkit and Gastester types that just read CO only, you bump the unit and it gives different readings for the CO and they don't read HCs which is whats most important.

Regards

Jeff,

Davids numbers are right on the money. With some work you CAN get to 300-400PPM HC. When you do I think your flat spot will be gone. Anything over about 1000-1200ppm HC indicates a dead miss. At 300-400 (on a carbed car &amp; below 200ppm on injected) indicates a nice degree of cylinder firing efficiency across the entire bank.

Thanks for the tip JRV, makes sense concerning the flat spot.

Regards

With no hands on yet, I can't speak from experience, but it seems logical that by tuning all the carbs to equal and good emmissions, that the engine should run better. Although not necessarily the lowest emmissions being the best running conditions. Be sweet to connect all 12 cylinders with sensors, then be able to switch between them as you tune.
If the machine looks like it has promise, I might have to find one myself.

From what I have gathered on carb cars being a little richer is better, less chance of a lean condition on the pistons and valves, the rich mixture keeps them cooler, clearly my car is way off though.

Any quality 4 gas analyzer will work, some are better such as the digital computer based models vs the ones that have the old analog needles which are moving about alot when testing and hard to get an accurate reading, the one I have is overkill since it did smog tests as it has many lockout devices and shut down features on it to prevent tampering unless by a Snap On tech.

If you know what you are doing the gas readings can tell all about the health of the engine and its combustion efficency, when I had my 930 checked over for a PPI all the mechanic did was put it on a smog machine as he said it told him the complete health of the engine components such as the compression and valve condition.