Carb emission test

[mgtf328]

I have an old Gunsen gas analyser, had it for years so I don't think it's very accurate. Just for curiosity I tried it out on my recently acquired BJ8.

I tried it out on each exhaust pipe in turn. The front carbs gave a reading of 9.5% CO but the rear carbs gave 6.8%. As I said I don't think the numbers mean much but the difference between the readings should be reliable. The timing is set at 16 degs with a strobe and the plugs do look a little dark.
The rear carbs are much leaner than the front which might explain why it takes a little while to start from cold in the morning when it pops and bangs a bit. Otherwise the car runs well and starts first time without the audio effects after the initial start up.
Does anyone have any suggestions as to whether I should enrichen the rear carbs or back off on the front. It has a fast road engine in it and the garage I bought it from said it likes to run slightly rich.

Thanks

AJ

 

[bdcvg]

The CO reading is directly relative to the air fuel ratio. 14-7 is ideal and I believe a CO reading over 2 % is rich. Those numbers are a perfect world however. There is probably a website some where that explains it all more accurately but initially it would seem leaning out the front carbs to match rear may be a start. Just keep in mind too lean is a killer.

 

[Deleted member 19315]

I disagree and would richen up the rear, then plug chop it to see what I had, as mentioned too lean can cause problems with detonation , overheating and ultimately engine damage if unresolved.

 

[mgtf328]

Thanks for that guys. I've been playing around and found one thread that said 3% was the figure to go for. That's quite a drop and I'm not sure if I'm brave enough to tune them down that far, especially as the garage I bought the car from said it runs better when it's rich!
I've balanced them with the jet adjusting nut and got them both down to 5.8% give or take 0.5%, it varies up and down quite a lot. I had to adjust the fast idle as it slowed to 500 rpm after adjustment, it's at 650 now. It seems to perform better (picks up on the throttle better), but I still get a bit of popping on cold start and it needs a bit more cranking than it did before from cold, it starts first time when it's been warmed up just a bit.
Can you please tell me what "plug chopping", is?
I have checked the plugs by running the engine for a minute or 2 at 2500rpm and shutting down when it's still running. They're all chocolate coloured and look OK to my eye. I did notice that if I check them after it's sat idling for a while then they are a bit sooty. I'm thinking maybe I should have adjusted the slow running screw instead of the fast idle to get the idling speed right. I find the guidance on slow running a bit confusing. The idle screws are both screwed out 1.5 turns some sources say 3.5 turns and I've seen other's say to screw them right in and ignore them. Still some more research to do to get it right!
Thanks again
AJ

 

[bdcvg]

Sounds like You have hit a good number, I'd drive it for a week or 100 miles and check it again. The choke can probably be adjusted to cure most of the cold start issue . If there is ethanol in U.K. gas that does reduce the energy that is produced during combustion and will not help cold starts.

 

[Bob_Spidell]

... I find the guidance on slow running a bit confusing. The idle screws are both screwed out 1.5 turns some sources say 3.5 turns and I've seen other's say to screw them right in and ignore them. Still some more research to do to get it right!

Too true! I have some literature that says only the idle circuit should be used, and some that says forget 'em and use the fast idle! I think the only argument for using the slow run valves exclusively is precision; theoretically, since the fast idle screws are connected to the choke circuit you may get a different 'resting place' every time you use the choke. I like to turn the slow run screws out a couple turns, then use the fast idle to get desired idle speed (around 750RPM for my BJ8). Since I take long road trips, often changing altitude a lot, if the idle starts to bog down at higher altitudes I can back the slow run screws out a half-turn or more, then screw them in when I get back to near sea level.