Carburetor Problem

[QQBlue1]

I am working the bugs out of a 383 that I did not build and a smallish edelbrock type of carb that I did rebuild, less than 600 cfm.

1. The truck (67 D100) was starting and idling fine at lower altitude. I drove it out to Colorado and leaned out the fuel by changing out the jets and matched them to metering rods, and changed to the yellow step up springs. The truck was running much better, plugs looked good etc . . .

2. The next day it would not start even if I primed the carb. I did a series of checks, suspected a spark issue and put an old single point distributor back on. It fired up three times in a row but then failed to start again. I put the new distributor (a magnetic pick up unit from summit) back on and now the truck will start as long as I prime the carb. The charging system is fine.

3. The fuel pump and filter are new, the floats are adjusted to edelbrock specks (which may be my problem since it is not an edelbrock). The bowls are not dry prior to cranking, they have fuel in them. There were no choke issues at either altitude.

4. I messed with the air fuel mixture and discovered that if I wind the screws all the way in, the carb keeps running. I must be pulling air in from somewhere, but that said, I'm fairly certain that problem was present prior to this incident.

5. I feel like buying a new carb . . . . but maybe I'm missing something obvious? I haven't replaced the starter, but I don't suspect it is bad. Any advice would be appreciated!

 

[Ranger16]

Is the choke adjusted properly? I have found that my 383 (with an Edelbrock 750) is rather temperamental to proper choke adjustment.

 

[69Runner]

I messed with the air fuel mixture and discovered that if I wind the screws all the way in, the carb keeps running.

This indicates you are not running off the idle circuit. Sounds like the throttle plate is not fully closing and/or the idle circuit is blocked.

 

[QQBlue1]

Is the choke adjusted properly? I have found that my 383 (with an Edelbrock 750) is rather temperamental to proper choke adjustment.

I have been keeping an eye on the choke and it seems to be working, but am tempted to switch to a manual.

 

[QQBlue1]

This indicates you are not running off the idle circuit. Sounds like the throttle plate is not fully closing and/or the idle circuit is blocked.

Thanks for that tip! I will look into this and see what I find.

 

[Ranger16]

I have been keeping an eye on the choke and it seems to be working, but am tempted to switch to a manual.

Working is one thing, but proper adjustment is critical.

 

[miller]

Is the carb base gasket in good shape?

 

[HALIFAXHOPS]

This might help you.
http://jgronoj.weebly.com/

 

[QQBlue1]

I finally had a chance to look into the carb this weekend again. I pulled it apart, blew air through the passages and was able to get the butterfly under the primaries adjusted better and inspected the gaskets. It starts much better, and the choke is working, and the idle circuits are behaving better. I put about 200 miles on it over the last couple days and it ran pretty good. It still runs even if I close of the air fuel mixture screws (I'm pretty sure those butterflies aren't closing all the way), but considering were things were last week, I feel as though I made some progress and I appreciate all the tips you gave me. Thanks!

 

[miller]

It still runs even if I close of the air fuel mixture screws

That could be a sign of damage to the mixture screw tips, from being over-tightened.

 

[AR67GTX]

When you took it apart did you inspect the idle mixture screw tips for any damage? Possibly they are scored or blunted and don't seal well when lightly turned closed. Is this running a stock cam? What are you setting the idle speed at? At idle, have you looked down the primary throats to see if any fuel is dripping from the boosters? Do you get any change in idle speed response at all when turning the mixture screws in and out?

If you haven't seen it yet the Edelbrock owners manual with a good explanation of all the operating circuits is available on line here. http://www.edelbrock.com/automotive/mc/carburetors/

 

[QQBlue1]

I have another carb in storage that I'll be able to pull in a couple weeks and will compare the condition of the idle mixture screws. That is a good suggestion as there is only a little change in the idle speed when I turn the screws in and out - maybe three pounds of vacuum, from 11 down to 8. I looked down the primary throats during idle and could see a little fuel at the bottom of the butterflies. When I revved the throttle it looked like a fairly consistent spray. It tends to bog out a little under a load, but all things considered the drivability and idle of the carb is getting better. I am not running a stock cam, it is the wild card on this engine. Thanks again for the suggestions.

 

[miller]

Might also consider looking at the float valves, what kind of shape their in, and float setting.

 

[oldbee]

Sure sounds like "69runner" has a good guess with your "wild card cam".. You're probably past the idle circuit slots in the primaries. "google" holes in carb primary plates to compensate for throttle plate position. ( it's usually a band-aid fix though)

 

[QQBlue1]

I have the idle worked as good as good can be and have been happy with the results, but the carb gradually started chugging a little again and lost power. I checked the plugs and sure enough the # 8 cylinder plug was completely fouled and covered with oil. This is the second time it has happened. Plugs 1,3,5,7 all look good. Plugs 2.4 & 6 look good on one side and as I rotate them that are nearly white. I had been thinking this was fuel distribution problem perhaps something with the # 8 valves? I cleaned up the plug and it ran a touch better. Any suggestions?

 

[miller]

It would be worth a look at the compression on #8, comparing it to the others. If you've got a mechanical problem in that hole, it will show.
If it's good, start back-tracking from the plug, checking that plug wire, contact inside the dizzy cap. Can pull that plug, and holding it to ground, rotate motor, looking to see what kind of spark your gettin'. (Just bite down on a stick when you do that...or, use a little insulation.)

 

[AR67GTX]

Try switching a couple plug wires around or substitute another one on plug 8. Sometimes bad plug wires are hard to diagnose.

 

[QQBlue1]

It was the wire. And I probably only got a thousand miles out of it as it was relatively new. Thanks for the tip!

 

[miller]

Good deal! Glad you got the ole gal fixed up.

 

[AR67GTX]

Good news. There are some pretty crappy plug wire kits for old cars on the market. On my GTX and 66 Corvette I bought Taylor black wire kits and made my own wires up and used stock boots where they showed and they have been absolutely solid. At least making up my own connections and crimps I feel confident they are good.