car burbling/coughing after 3000 rpm

[BluePaulie]

Hi,
I have a 1978 1800 with dual 40 IDFs, headers, electronic ignition and slightly beefed cams.
Car was running great (the WAAAAAA sound at full open throttle in 2nd gear was glorious), then suddenly it couldn't rev past 3000. It bogs down, burbles and backfires after 3000 rpm.
Strange thing is that in the driveway it will rev past 3000, so it has something to due with high throttle, low rpm, under load (too rich?).
I figured it was time for a carb rebuild, but I don't want to waste my time chasing the wrong ailment.

Any Ideas?

[18Fiatsandcounting]

I'd start with checking the ignition timing, verifying that you start at around 5 or 10 degrees BTDC at idle and then advancing smoothly up to around 36 BTDC at 3500 rpm. Do you have vacuum advance on the distributor? Could that vacuum line have an issue?

If the problem appeared suddenly, it's likely that either something went wrong in the ignition system, or you developed a significant vacuum leak somewhere. If the car idles fine, that latter possibility is less likely.

-Bryan

[Nut124]

Will the engine pull at WOT for any length of time?

Did you make any changes or do maint before this started?

Could be a few things:

1. Fuel pressure. If this were the cause, I'd expect the engine to pull at WOT for several seconds before the bowl go dry. This would affect all 4 barrels.
2. Dirt flakes in the carb. I had a spec of dirt get sucked into a main jet anytime I floored the pedal. The engine would start running on 3 cylinder until I got back on the idle circuit and the spec fell back to the bottom of the bowl. This would typically affect only one cylinder.
3. Retarded ignition. Like Bryan states, retarded timing makes an engine gutless above mid rpm.
4. Could be a failing ignition module or pickup.

[BluePaulie]

Fixed,

Before I ordered the carb rebuild kits, I decided to start from simple to complex - I removed the emulsion tube/main jet and blew compressed air through them.

Presto - the Waaaaaaaa came back and the calls pulls like crazy up to 5500

On a side note: When I was staring down the carb throats, I noticed that I could see the edge of the intake manifold, so I'll add porting to the to-do list...

[Nut124]

Glad you fixed it. It's likely you have dirt in the carb bowl and this could happen again unless you clean it up. I'd try to suction the bowl empty via the jet assy hole.