What is the relationship between advanced timing and HC?

2056 with Dellorto 40's, new cap, rotor, wire, no points 050 dist. Timing set at 27 DG at 3200 RPM, 4 DG at idle, idle set at 900 RPM. mixture screws set at 1/2 turn out from lean.

Test results are:

Idle Test: 835 HC with a standard of 400 HC PPM.

4.18% CO with a standard of 3.5% CO.

Cruise Test: 1118 HC with a standard of 400 HC PPM.

4.74% CO with a standard of 5.5% CO.


So should I retard the timing to 0 DG at Idle and set the mixture scews lean or ??????

What else will cause high HC? In years past the HC has been as low as 90 PPM. Can't figure out what is wrong with it. Seems to run fine. Double checked valve lash & carb sync.

You're likely running quite rich. HC goes up as the mixture gets rich, as does CO (but CO rises more slowly). Lean out your idle jets, not just the screws. Buy or borrow an LM-1 wideband meter and tune the idle and cruise to be around 13.5:1. I'll bet you're running more than 12:1 now. It's VERY easy to tune things to be way too rich, ask Mark Henry. If you can find a CO meter instead of a wideband, tune it to be between 2-3%.

Another potential issue is you've leaned out the idle so much that it's misfiring, which causes HC to soar. However, since your cruise numbers are also way up there, that's not likely the problem.

Mark - maybe check for a vacuum leak? Anything done to those carbs since the last time, besides idle mix adjustment?

By the way - I have an exhaust gas analyzer you're welcome to use if it can help any.

is this the year you decided to run premium "so it would run better" ?
CO isn't *too* bad so maybe it's not that; but run the lowest octane you can without pinging, especially in the regime of the test.

high HC is typically poor ignition; check/change plugs.*
izzat a stock cam? you can open the valve lash *a little* to "take out" some lift and duration; that'll help.
reducing the timing couldn't hurt...

position of the mixture screws is meaningless without knowing your idle jet sizing...
but if you're running rich, you may have to back down a size.

___________
*unexplained high HC... if you "didn't change anything" and it started doing this, look to ignition...

Engine has like 10 miles on it. Fresh plugs cap & rotor.

Running 65 idle jets, same 65's that always passed in previous years.

Running 8.5:1 compression ratio. So it has 91 octane fuel in the tank.

Checked for vacuum leaks with carb cleaner and couldn't find any. The idle mixture scews are all responsive. The carbs sat with no fuel in them for 9 months, so maybe I need to take them apart and freshen them up.

There seems to be a slight stumble just off of idle.

I have a CO meter. Haven't tried that yet.

Best to break it in more and then put it on the scope....tune with the scope. Run as low an octane as you can w/o pinging. That cam may be the biggest issue besides the lack of miles on the motor. The rings are probably not seated yet and you are getting some crankcase blowby.

QUOTE (MarkV @ Sep 22 2005, 09:54 AM)

Engine has like 10 miles on it. Fresh plugs cap & rotor. Running 65 idle jets, same 65's that always passed in previous years. Running 8.5:1 compression ratio. So it has 91 octane fuel in the tank. Checked for vacuum leaks with carb cleaner and couldn't find any. The idle mixture scews are all responsive. The carbs sat with no fuel in them for 9 months, so maybe I need to take them apart and freshen them up. There seems to be a slight stumble just off of idle. I have a CO meter. Haven't tried that yet.

Empty the tank and put 87 or 89 in it for the test. Loosen the valves to .016". Leave the timing where it's supposed to be. Give that a shot and report back. The Cap'n

I can get a 30 temporary permit without passing emissions. I think I need to get the rings broken in and sort things out some more before I have it tested again.

If I fail again they may force me to take it to their referee, who knows what kind of can of worms that is likely to open.

QUOTE (MarkV @ Sep 22 2005, 11:54 AM)

Engine has like 10 miles on it. Fresh plugs cap & rotor. Running 65 idle jets, same 65's that always passed in previous years. Running 8.5:1 compression ratio. So it has 91 octane fuel in the tank. Checked for vacuum leaks with carb cleaner and couldn't find any. The idle mixture scews are all responsive. The carbs sat with no fuel in them for 9 months, so maybe I need to take them apart and freshen them up. There seems to be a slight stumble just off of idle. I have a CO meter. Haven't tried that yet.

There's part of the problem right there. Get some break in done on the motor. And as was said previously, run the lowest octane you can for the test.

Aha. Fresh motor! You change the cam? What kinda cam you running? The Cap'n

Is the float height correctly set on the carbs? Are they getting only 3-3.5 psi of fuel pressure? When I made those changes to my dells, the smell of unburned fuel out of the exhaust almost completely disappeared.

If this was done in the past, maybe a float flooded?

Put some miles on it and change the oil right before the test too. Dirty oil can make it smell foul to the sniffer..