Idle / acceleration issues

[bigmanisme]

I have an 1989 R3500 with a 1993 454 that has been converted with an Edelbrock 1411 750 carb. It just started popping randomly through the exhaust and deceleration. Gently take off from a stop and it sputters as if going to die. When I gas it a little more it catches and takes off. I have the fuel pressure set at 5 psi and the initial timing is set at 4 degrees BTDC. I have replaced the fuel filter and lines. Put the engine at TDC and retimed to 4 degrees BTDC. It idled fine and revved fine until I got out of the driveway. Hesitation and popping through the exhaust resumed. Help!

 

[bigmanisme]

I have also checked Cap/rotor/wires for damage and they are in great shape.

 

[rich weyand]

Did you set the timing with the vacuum advance disconnected?

 

[bigmanisme]

Yes.

 

[bigmanisme]

I don't know what shape the carb is in since it was on the truck when I got it. I have everything to go through it so is that a viable solution to my problem?

 

[Boone83K10]

Deceleration pop is almost always timing. Damper slippage causing bad timing??... maybe. Sticky valve staying open?...maybe. Crossed plug wires??...maybe. I find that when most people say "randomly" it usually means they were messing around and something got changed. I am not saying you did that but it's hard for the timing to just suddenly change. Does the 93 454 have a computer?

 

[350runner]

Or running lean

 

[rich weyand]

x2 on running lean. The other thing that occurs to me is, to which port is your vacuum advance connected? It should be connected to manifold vacuum, not timed vacuum. With only 4* BTDC base timing and vacuum advance on timed vacuum, you are running awfully retarded on idle, such as when you first give it gas off idle, or when you take your foot off the gas.

Connect the vacuum advance to manifold vacuum, and run more like 12* BTDC base timing and see if that clears it up.

As for mixture, do you have access to an A/FR meter?

 

[bigmanisme]

No on the damper slippage and sticking valve. I thought lean as well, so I fattened up the Idle / Air mixture with an additional 3/4 turn out and the idle cleaned up as far as the popping goes. Deceleration cleaned up quite a bit as well. Does seem a little undertimed upon take off. A little bit of a bog at part throttle but does clean up with more throttle. I do not have access at this time to A/F meter. I am running the timed port of the carb. 12* BTDC seems a little high to me though. I was planning on bumping up to 8* BTDC.

 

[bigmanisme]

In the morning I will switch to manifold vacuum and bump timing to 8* and let you know how that works. Should I fatten up the idle mixture a little when I bump up the timing? Thanks for your help guys!

 

[bigmanisme]

Whoever had the truck before me was running the stock 12psi pumps to the carb without a regulator. I ran new lines to a bypass regulator. After this is when this all started.

 

[MonsterZ]

So, if I'm reading this right, it was running fine without the regulator, and started acting up with one? Are you sure you are flowing enough fuel out of the regulator? Second on the timing, somewhere around 8*-12* sounds a lot better. As far as idle circuit goes on the carb, until you are totally awesome with timing I would back off (the two screws in the front) until you hear your engine speed start to slow down again, and then take it back in just until it stops gaining speed. Do them one at a time, exactly the same technique on both. then I'd set your idle screw back down to about 800 rpm. This will put you just on the rich side of ideal, and should help with getting your timing down. Is your motor otherwise stock and are you sure you don't have any open vacuum leaks? A missed vacuum leak, especially on the manifold, can throw off your advance very badly and can be most frustrating.

 

[rich weyand]

Whoever had the truck before me was running the stock 12psi pumps to the carb without a regulator. I ran new lines to a bypass regulator. After this is when this all started.

Yeah, so to get the right mixture, the carb was adjusted way tighter on fuel than normal because it was being over-pressured. When you cut the pressure back, the setting was way lean. That's where your problem showed up.

Timing shouldn't affect the desired mixture much is at all. Right now you are just wasting a lot of fuel/air mix because the pressure wave is getting to the piston late. Timed port on the carb is making it worse at idle, will foul your plugs, and is probably adding an off-idle sag as well.

For best power, you should set a stock Chevy V8 (sbc or bbc) to 36 degrees with the mechanical all in and the vacuum disconnected. So at 3000 rpm it should read 36 degrees. That probably results in base timing around 16* BTDC. More conservative is OK, and won't ever knock, but is leaving horsepower on the table.

 

[bigmanisme]

No vacuum leaks anywhere. It was running ok without the regulator, come to find out the feed hose was coming apart on the inside so full flow was not getting to the carb. Fuel pressure is now at just over 5psi. Engine is stock as far as I know (previous owner wasn't too helpful).

 

[bigmanisme]

Bumped inital timing to 10* BTDC. It would only advance out to 30*. Switched to manifold vacuum and it idled better but ran worse. Switched back to timed vacuum and it idled a little rougher than the manifold vacuum but ran really well except it still has that off idle stumble. Took out on the freeway for the first time to clear out the carb. Ran really well but it reminded me that 12" glasspacks are not very good when you are coasting down the offramp. After that it still sounds like it is breaking up upon takeoff and then it accelerates fine. I will bump timing up to 34* total regardless of initial. I am starting to think it is the carb.