The Stovebolt Site Forums The Shop Area Engine Shop Black sooty tailpipes evisited

Some time ago I posted about my black sooty tailpipes. good advice came back from a number of bolters including Hotrod lincoln. He mentioned that I had too much carburetor on the 283. I was running a 650cfm edelbrock and have switched to a 500cfm AFB. the tailpipes are still black and sooty and the truck uses a LOT of gas. Maybe I need to swap inyake manifolds and go back to a 2barrel carb. Its obvious even to me that it is running way too rich but I don't know what to do about it. The truck starts and runs well and idles good and has good acceleration What should I do next?

Any advice is greatly appreciated


Weeds

I'm certainly no carb expert, but can't you adjust the rich / lean ratio, and /or change out the needles / jets, and metering rods ?. Edelbrock has a good tech section on-line for their stuff. I assume AFB has similar.

First thing I would do is take it to a garage and have an emission tester put on and see whats going on.That will tell you if it's the idle,mid range or high speed jets that need to be worked on or maybe a combo of things.You got to have a starting point when it comes to carbs.You could spend a ton of time and money buying jets and parts and never get it right.The only other way is to get a setup "factory" carb and manifold and install it.

Pete

Pete has pretty good advise, but a easier way is to take a drive. Get on a long straight road with little traffic. Get in second or third gear and run flat out for a decent amount of time, 1/4 mile or more, full rpm, just as if you are racing someone. When you are at top speed, push in the clutch and shut off the engine ASAP, and get off the road as quickly and safely as possible. Pull out a spark plug or two right there on the side of the road. If they are black, your carb is jetted to rich, if they are brown, just right, if they are white, to lean. You have to check them right there, any driving will alter the high rpm check.

For idle adjustment, get the truck up to temperature and remove the air filter, it does have a decent size filter doesn't it? not one of the 3" or 4" lawn mower filters, but a good sized factory filter. Anyway, adjust the idle screws in till it starts to shudder, a little on each one 1/4 turn at a time. Once both are in where it doesn't want to run, back them out 1/4 at a time till its running smooth. If you go to much it becomes rich yet still idles OK. Stay on the leaner side. It will take a few tries on each screw till you find the right spot. 1 1/2 turns out is usually pretty close.

You can lean a carb with bigger rods, smaller jets, or a combination of both. You can also lean down a jet size or so just by lowering the floats a bit. Thats a quick and easy fix, float adjustment. Go to low and you can get a high speed lean out.

Be sure the secondary's are full closed,
check float level,
check timing,( advanced more for better fuel economy),
check vacuum advance ( should be full time vacuum),
check fuel pressure ( to high will cause a rich condition).
Stock fuel pumps, OK, electric should be regulated down ,4-6 psi is all thats needed.

Since you changed carbs, have you had the truck on a long hard drive? It will take some time to dry out a whole exhaust system. Find you a long steep hill and make a bunch of high speed runs up it, thats a good way to really work a engine and get it really hot. If it still has dirty exhaust, after really working it, then its still rich and needs attention. A long 100 mile trip would do it some good also.

Joe

The A500AFB that I swapped to had been overhauled by the Quad Shop in Rockford IL so I just installed it assuming it was set up correctly. These 283s when they first appeared in the mid 50s I seem to remember that they were equipped with 2 barrel carbs although some I recall had tri power setups.Maybe I just need to go back to the simple 2 barrel setup. what do you all think?

Weeds, some versions [Power-Pak] did have 4 barrels [Roch 4GC or Carter WCBF] and properly sized and set up should do OK, but most of the originals I've found have the 2 barrel intakes on them - I've always run 2 barrels on all the many 283's I've had and haven't had any problems

Bill

Pull the linkage off the secondary's, then you have a two barrel. Its worth a try, you may be into the four barrels more then you think.

Joe

now, dont forget all cars now seem to have black, sooty looking tailpipes due to the "new" unleaded gas. You cant make the tailpipes "chaulk over" like they did on leaded gas no more. When swapping carbs, it is easy to have stumbles, and bogs due to the carb not responding to the signals from the smaller engine. But you dont seem to be experianceing any of those signs--although over richness would also prevent them. Over richness is sometimes a crutch/coverup to make a poor size swap work (at economy's disadvantage). JoeH is from good automotive stock-his dad is a famed Pontiac builder-tester guru. Good way to test whats really going on. I know that a 500 AFB is a dandy size for a 283 and have near stock 300 hp/327 AFBs (whatever cfm size they are) on 265s before with decent driveability. Remember the quote is "better hot than stock" not "better stock than hot" as a lot of old timers like to chime--I are old now, but still like the better Hot version best.

Also not to forget, even the 265s also came from the factory with twin WCFB Carters (and a bit of cam), but the 283s could be had at 245 hp with twin WCFB Carters and a SMOOTH idling cam. So I feel its not over carbed.

Joe H and edski, thanks for the replies, much appreciated. Now I am really confused. I will unhook the secondaries, drive it awhile, and see what happens.In the meantime I will go to my local boneyard to se if they have any 2BBL manifolds and carbs.

Joe H, I could use some Pontiac advice also. I have a 1935 flathead 6 with a stuck engine that I can't free up. Will a38 6 bolt in? thanks.


Weeds

First, I am not convinced this is a carburetor issue.

My standard checklist in this situation is:

(1) compression test

IF compression is good then

(2) complete ignition test

IF both compression and ignition are good then

(3) fuel pressure test

IF the fuel pressure is below the allowable limit then time to look at the carb.

A 500 CFM is too big for a 283. Too large a carburetor NORMALLY leads to a LEAN mixture, as the air velocity is too small to create the necessary negative pressure in the fuel wells. The engines with two four barrels used carburetors that were approximately 385 CFM, and they were run progressively. That is, under all but wide open throttle application, the engine was running on ONLY the two front barrels of the rear carburetor, which were significantly smaller than the two front barrels on a 500 CFM AFB.

Also, you did not state which 500 CFM AFB you are using. The Chevrolet fuel curve is 180 degrees different than the fuel curve for a Cadillac, Chrysler, Dodge, Ford, Pontiac, etc., etc., etc. If the carb was not designed for Chevrolet, have you changed out the jets, rods, idle tubes, airbleeds, and especially the secondary airvalve?

Jon.

Jon, thanks for the very informative reply.

1. What kind of compression numbers am I looking for?
2. What is a "complete" ignition test?
3. How do I do a fuel pressure test and what numbers am I looking for?

The .
500CFM AFB came off of a Mopar 318. I have not attempted to open up the carb
as I would have no idea what I was doing.

The engine has only 4200 miles on it since rebuilt as new. I would be surprised if the compression was bad unless the rings are not yet seated.I drove it 1100 miles to KC and back and did not baby it for the reunion in 2011. It's no wonder this thing is messed up if all the things you mention are wrong.

Thanks

Weeds

Weeds,

Cannot tell you exactly what compression numbers you are looking for, as it will depend on your compression ratio and camshaft. There are others here which are far more advanced engine builders than I.

In general, I would be looking at:

(1) compression on first revolution
(2) deviation of each cylinder max compression vs average for all cylinders
(3) ratio of average compression vs recommended average for cam and compression ratio

For ignition, I would try to find a local hobbyist with an oscilloscope (I bought a new old stock one that was obsolete many years ago for $5.) This instrument is fantastic in its use, and you don't have to know anything about it. Get the picture book, and compare the picture on the screen to the picture in the book. I once found a bent distributor cam when not a single Chevrolet dealership in central Missouri could find it!!!

Fuel pressure is easy. Acquire a fuel pressure gauge, and temporarily install right at the carburetor. Again, I can not tell you what pressure to look for, as I have no idea what fuel valves the builder put into your AFB. I run 7 1/2 PSI on my twin AFB's but I run 0.093 inch fuel valves. If your builder put in the 0.111 inch, maybe 4 1/2~5 PSI. You have to know what has been done.

And a 500 CFM AFB from a Chrysler on a Chevrolet engine is a TERRIBLE idea. Check the second and third lines in my signature!

Jon.

Jon, I'd really like to hear a bit more about the "oscilloscope"
that you bought for ignition diagnostics. This is one subject
that I've never seen discussed on the forum since I've been on
it. Maybe you could elaborate on another thread, I'm sure it
would generate a lot of interest and a lively discussion.
For instance, is it a dedicated automotive scope like a Sun vs
a service grade radio/tv scope like an Eico, Sencore, Heathkit,
etc. What sort of book are you using and would they still be
avalable today and what sort of hookups are needed.

Thanks
Denny Graham
Sandwich,IL

x2 on the O-scope info!

Jim

Denny and Jim - I just started a thread. Hopefully, some of the electronic whizkids will jump on it!

Jon.

What intake manifold are you using? Is it a single plane type?

A 500 cfm carb on a dual plane intake is a excellent setup for a stock to moderate 283 build. HOWEVER, the carb may require metering rod and/or jet size changes for best results.

It is also important to heed the wisdom shared by others in this post.ie fuel pump pressure, float setting and timing (also using the correct carb port for distributor advance.

I recommend working with what you have instead of a 2 barrel setup.