I am trying to get my truck smogged and I'm running into problems with high HC at idle.

The engine is a 400, stock compression with a Wolverine hydraulic cam, 280 degrees intake/exhaust, .528 lift. With 105 degree intake centerline. Edelbrock 750 carb, headers etc.

The cam is a bit lumpy and has a pretty rough idle. My problem is that even though it holds 14"hg of idle vacuum. I can't seem to get the HC below 700ppm at idle. I tried advancing the timing and leaning it out and they went to 1400ppm. Even though the idle was much smoother. Could I have gone too far lean. I know how to tune for power, but this is the first time I've had to tune for a number on a smog machine. It's got a recent tune up and everything is in good shape.

I tried looking here and other places for info, but I must not be using the right words for my search to get the info I need.

Thanks for any help.

 

Although you mention the cam lobe centerline, you didn't mention the LSA, but from the description (lumpy idle) I'm guessing that you've got a bit of overlap (the LSA is somewhat low, like 110^). That could be the root cause of the trouble.

You also didn't mention what temperature the thermostat is. I'd recommend putting in a 195 thermostat, it won't really hurt performance and it will help vaporize the fuel. I'd also guessing that since you have a 4bbl, you have an Aluminum intake manifold. If the exhaust crossover is blocked off, you may want to get that opened up again. Unfortunately, none of the aftermarket intake manufacturers have heat shields on the bottoms to prevent oil coking on the exhaust crossover. I think that stinks. If you were ambitious, you could pull the shield off the old 2bbl intake and find a way to install it on the bottom of the Aluminum intake. I'm doing this on a 3 deuce manifold for my C#&*y.

And you may want to paint the outside of the manifold so that it retains more heat. Magazines make a big deal out of the idea of keeping the intake manifold cool to make more power, that would only matter at the extreme top end of the RPM band. That's just stupid. You would have to rejet the carb every time the weather changed just to keep it in tune. Try to find a way to keep the intake temperature stable.

Set the timing near TDC.

Are you using extended tip spark plugs? I've had excellent results with them.

And you may want to get a MSD because the multiple sparks will help burn the fuel up.

Unfortunately, I don't think 400's had thermactor ports. Too bad. Putting one of those air injection manifolds on the headers is probably out of the question.

 

I have a couple articles that show some tips and tricks for passing a smog test. I can attach them to an email.

 

as you have found out,too lean=hc.you also don't want too much advance.i disconnect the vac and set as low as i can and still drive it,but watch the temp.i got a dual quad tunnel ram cleveland through like it wasn't even there.do you have access to a gas analizer?(local tune up shop)

 

[updated:LAST EDITED ON 29-Dec-02 AT 04:00 PM (EST)]Correct me if I'm wrong, BUT...
I thought that high HC was too much unburnt fuel passing through, either too rich mixture or missing. Lumpy idle would be concidered missing IMHO. High CO would be incomplete combustion, i.e. too lean. High CO and HC could be missing caused by being too lean.
That said, try setting your timing on the slow side, then adjust your idle mixture to the lean side of best idle. Set your idle speed high enough to start to get into the main circuit of the carb and cover up a lot of the lumpy idle, the faster the better. You can always set everything back after it passes.
Just my $.02
Greg

'77 F-250 Camper Special 400ci driver
'76 F-250 Supercab 360ci current project
'71 Mach I 429CJ in storage
'79 F-150 for parts
huct on foniks wurkt fer me

 

What about setting the idle a little higher, and leaning it out, just for emissions? Also, using an adaptor and a small 2BBL can work wonders.

Jared

 

Emissions are an extremely tricky area. There's so many ways you can screw up on an emissions test and it depends on what kind of test you have to pass. Is it a basic 2 speed (idle & cruise) or do you have to put it on a dyno? Do they only measure HC, CO and CO2 or do they measure NOx as well?

In general high HC at idle means that you're passing unburned gas through the exhaust due to too rich a mixture but it could also be due to a lean misfire or the idle being too high and you're into the main jets. Most emission tests require a limit on how high the idle is. If your idle is too high and you're into the mains then you'll be on the ragged edge of passing the high HC test as well. Then there's valve overlap which is basically a measure of how long the intake and exhaust valves are open simultaneously which determines how much unburned fuel passes into the exhaust before the exh. valve closes and the intake stroke begins.

You can try to mess around with idle mixture, speed and timing but you may just be better off finding a local place that specializes in tuning vehicles to pass emissions and taking the truck there.

 

I ran my '71 Mach 1 through emissions last year with a 110 LSA lumpy bumpy cam. In order to pass, I leaned out the carb big time, as well as RETARDING the timing. Also bumped up the idle quite a bit. It barely wanted to run and the headers started glowing, but it passed the idle test.

 

Thanks for the advice, I'm going to try some of those tricks at the shop. Maybe I can buy some time on an analyzer and figure out what's up.

It's only a two step test. Idle from 350-1250 rpm and cruise 2200-2500 rpm for HC CO and CO2.

 

Don't forget the basics like changing the oil and air filter. They can have a significant effect on emissions as well.

 

Just thought I'd share my recent experience trying to get my truck past the smog *****. I went in on Tues and ran my bone stock 400 with all the smog equipment except for the cats thru the 2 speed dyno test. The past 3 times I've ran it, it's gone thru without a hitch but this time it failed on the idle CO by 0.91% CO is a measure of partially burned fuel so basic logic would tell ya that it's running just a tad rich. I took it home and leaned it out a little (about 3/4 of a turn on the idle mixture screws) and took it back for a retest. This time the idle CO was fine, 0.11% with 3.0% being the limit but the HC went off the scale by 1000 PPM, total jump was around 1300 PPM. So I went way too lean...

Since only the first retest is free ($15 a pop for each test after that) I decided I'd better take it to a local repair place and get them to adjust it on a scope so that I wouldn't be wasting time & money turning the screws 1/8 of a turn and getting it retested. Ended up having to replace the distributor to get it to maintain timing, the old one was allowing a 9 degree swing while the engine was running. When it was replaced the truck flew thru the test with flying colors.

The good news is that this is the last test this baby'll be having to pass. Since Washington state has the 25 year limit in effect and the smog tests are good for 2 years I'm done with the smog *****. Now it's time to get the new engine dyno'd and installed!