TB42S idle issues

This is slowly doing my head in, I have a 1989 GQ with the good old TB42 carburetor engine. It has around 250,000 km on it.

A month or two ago it rusted out the top of the centre muffler/resonator box and the whole exhaust was replaced with headers and a 2.5" system. Just before the muffler blew a big chunk out the idle started to play up.

What do I mean by play up, well the car starts cold with the auto choke without issue. Even at 2 degrees. Press the throttle down to the floor, count to 2. release, turn key and broom. It's running fine, idles on choke around 1800 rpm till it warms up and you blip the throttle...then it has the step down and eventually it idles at 650 in gear (it's an auto).

This is where it get's weird. Go for a short drive, think 10 minutes and it is all good. Go for a longer drive, think 20+ minutes and the idle is now 1500 RPM in neutral and round 1200 in gear. And it just pushes forward. After a good long highway run a 100 - 110 when you stop for fuel it usually diesels and smells weird when you turn in off....

Just to check I even started it on the hard stand from cold and let it go through a full choke cycle to no choke, then with the occasional rev up for a few minutes and then back to idle....sitting reading facebook and the idle just slowly climbs to 1500 rpm (neutral) and stays there....checked the throttle is fully closed and nothing has changed.

You have your idle and idle mixture wrong I think. So yesterday I finally get some time to go out an play with the truck. Got it all warmed up, idle is 1500 rpm so I tune it down to 650 (in Drive) which is about 750 in park. get the mixture set (all the way in, out 2.5 turns and then tune to fastest idle), reset the idle and redo the mixture until the process yields no change. Sounds good, is idling warm at 650 in drive.

Cut to this morning, heading out to rescue a Tawny Frog Mouth that has been injured, starts fine and runs fine on the choke as per normal. Heading out on the short drive to the bird, no issues and stops perfect when I get there. Started to come home, 2nd pawl on the choke (the slower fast idle pawl step) and started to drive home. As I turned into the last corner before home the car stalled....bang it into N and restart and stops again...won't idle. A bit of hand throttle, restart and it starts OK. Use the hand throttle to set the 'idle' to 650 in gear and I drive the rest of the way home....pull up at home and the idle is now at 1100 rpm in neutral. sigh

Haven't had a chance to do a long run since yesterday but this is doing my head in.....

It has had in the last 6 months, new plugs, new leads, new fluids, new exhaust (inc. headers), new filters and most of the vacuum lines replaced. I can't find/hear any leaks and all the lines are plugged in.

I am thinking a new carburetor, but I don't really have the $600 if it is not going to fix my issue.

Any advice is welcome.....

Cheers

Greg

Check the choke break diaphragm is not split or has a hole in it. Maybe have a look at the float level and that the needle and seat aren't get stuck when it gets a bit of heat.

Thanks Mudnut, I'll check the diaphragm on Thursday when I get time. I'll have to find some time to take the carby off to check the float level though...oh well.

Going for a highway run today so I will see how it goes at 110 for 40 minutes or so.

It might also pay to put in or get a mechanic to put in a carby kit, as over time the rubber parts can go brittle, or shrink, crack, etc.