Ok i have a 86 IROC-Z, it came with a 305 TBI, but that was swapped out for a 350 Carb. Its a stock 350 with a Weiand Intake and a Edelbrock 650 carb. Its acting up on me.

If i run it hard (after letting it warm up), when i shut it off the engine makes bad noises, almost like a diesel shutting down. If im nice and calm it doesn't do it. The car is timed, just did it last weekend, carb isn't tuned in perfect, still tweaking it day to day, but i don't see that making a huge difference maybe thought, i dont know a lot about carbs. The car never really starts up fine either, i always have to crank it over several times before it will fire up and then still sometimes it kills right away and have to do it over again.

Any ideas?

 

TBI wasn't available in '86. However, TPI was.

Assuming it had TPI originally, what are you doing for fuel pressure regulation and distributor?

 

alright TPI whatever. Its a general basic stock Distributor, nothing fancy and the fuel is feed by a manual pump, thats bolted to my block.

 

"Whatever"? Big difference, actually.

The stock '86 distributor was computer controlled. Unless you're running a computer controlled carb (which an Edelbrock carb isn't), the distributor won't work properly.

If the car originally had TPI, it had an in-tank electric fuel pump. Did you do anything with that?

Where did you set the timing?

 

I know there's a difference I just said whatever cause I assumed it didn't matter since I was carb'd. As for the distributor I don't know. It's got a vacum hose that runs to the carb, but I took it of and messed with it and it didn't seem to matter much, when the car was running if it was plugged in or not. Didn't do anything!

 

Okay, sounds like it has at least the right type of distributor.

Where did you set the timing, and was the vacuum hose from the distributor hooked up to the carb when you set it?

What did you do about the electric in-tank fuel pump?

 

The hose may not have been i don't recall, we were messing with it and it didn't seem to matter weather it was plugged in or not, it didn't change anything, not even a difference in RPM's

The electric in tank pump was taken out, all that's in there now is the floater for the gas gauge.

 

check your timing again, this time having someone looking at the timing mark prior to unplugging the line at the distributor, I believe you should see a change, if not... then you'll have to probably disconnect something else, which I am not sure of...

 

If the vacuum advance is hooked up to a timed vacuum port, it won't make a difference at idle whether hooked up or not.

But, the timing should be set with the hose disconnected and the port capped. Set the timing at 8-12 degrees BTDC at idle. After tightening the distributor clamp, keep the timing light handy and check for more advance as you increase RPMs by slowly opening the throttle. It should advance beyond the timing tab. Now, hook the vacuum advance hose back up and repeat that. You should see a lot more timing suddenly come in as the throttle gets a little off idle. Suddenly opening the throttle should drop it down as vacuum drops with the suddenly opening throttle.

 

Here are picks of my Distributor and Carb.

Distributor:
You can see the clamp on the bottom left corner, that the vacuum line from the distributor


Carb:
Theres 3 port on the front. The far left hose thats yellow and black. Thats the one that runs to the distributor. The middle hose runs to a breather on the valve cover. And the right one is capped off. Now when we took the cap off the right one, there was a lot of vacuum going on. But not on the left one where the distributor is hooked to. Could that be the problem, im on the wrong port on the carb.

 

That looks right to me what number is stamped on the carb?

 

im not sure, where is the number stamped?

 

bottom front left pad as your looking at the carb it looks like 1406 but not too sure

 

yeah its 1406, whats the point of that?

 

You have the vacuum advance hose hooked up to the "ported" or "timed" vacuum port. That is a pretty common one to use. I prefer the other one, the one that has vacuum at idle, but I doubt that's your issue.

Going back to your timing, where did you set it? If you went by the factory setting for electronic control, you won't have enough advance to run properly with a non-electronic system. 8-12 degrees BTDC with the vacuum advance disconnected is usually good.

Dieseling is typically either hot spots in the combustion chamber continuing the ignition process even after the engine is shut off (often caused by improperly set timing), or the throttle blades being too far open at idle due to improper idle adjustment. Using the full vacuum port often allows an engine to idle better, eliminating the 2nd cause.

BUT, timing has to be set properly before you mess with anything else.

 

i believe the timing is set at exactly one, or close to that.

 

this should help on the tuning which could eliminate your problem.

 

That is not enough for a non-computer set-up. Bump it up to between 8 and 12 degrees BTDC at idle, vacuum advance disconnected. Reset the carb idle with the vacuum advance reconnected (like I said, I prefer the full vacuum port, but if you leave it where it is, you should still need to readjust the carb idle settings).

 

how do you change the carb idle? The two screws on the front of the carb?

 

by the book

1. fully warm the engine make sure the choke is fully open

2. air cleaner in place

3. set desired speed with the air screw (the one on the side for idle)

4. adjust the IMS (screws on front of carb) on ONE side to get max possible RPM. Do not go rich beyond the maximum speed point.

5. If the above changed the idle speed more than 40 RPM, then readjust the speed.

6. adjust the side OPPOSITE of that in step 4 to get maximum RPM.

7. reset the speed.

8. carefully trim each IMS to again get the maximum idle RPM

9. go leaner just enough to get a 20 RPM drop in speed.

10. reset the speed to the desired RPM.

11. this is a lean-best idle set. setting richer than this will not improve idle quality or proformance, but could tend to foul plugs.

 

Alright, i printed off a book thats got all that info, but i dont really know a lot about carbs so im not sure what it means when it says. Air screw and when it says reset speed?

 

reseting the speed just refers to the idle speed
the Air screw if you look at your last pic is on the far right on the throttle linkage
side of the carb

 

Alright, i re-timed it last night and the dieseling has stopped. Car runs a lot better and is quite a bit faster. But i think i still need to tweak the carb a little more to get it perfect.

Thanks for all the info!!