Hunting at idle

[Who.Me?]

Hi

Sorry - I know this is a very long post, but I needed to explain all the steps I've already tried...

I've been trying to tune the carb on my single cab for the last few weeks, but it's behaving oddly. I rebuilt the carb (a German Solex 34-pict-3) using a mixture of old and new parts (some of the bits in the generic Solex rebuild kit wouldn't fit, or just broke, so I re-used original parts where possible, cleaned everything and replaced the gaskets).

The engine is a 1600 dual-port, with a VW block, but with Brazilian heads. The inlet manifold looks to be a genuine VW part. I've checked the valve gaps and timing (looks to be an original Bosch 009 distributor, but it's been upgraded with a Compufire ignition module).

After rebuilding the carb, I followed the tuning instructions on vw-resource, step by step, including setting the Fast Idle Adjuster gap and set the Volume Control Screw (the small screw) to 2.5 turns out.

Initially, the carb wouldn't respond to any adjustment of the Bypass Screw (the big screw). I suspected an air leak around the throttle spindle and the carb to manifold flanges both turned out to be rather warped, so I pulled the carb, fitted new throttle spindle bushes and lapped (flattened) the manifold flange on the carb. I also filed the inlet manifold flange flat (also warped).

Despite doing all that, when I re-fitted the carb again, the engine wouldn't tune and was 'hunting' at idle, cycling from 600 rpm to 1400 rpm while I was trying to tune the carb. I had to set the idle to around 1400 just to keep it running.

Last night I tried checking again for air in-leakage around...

the carb base
throttle spindle
along the manifold
at the manifold boots (it's a dual-port)
at the manifold-to-head joints
at the Bypass Screw
at the Volume Control Screw

I wedged the throttle to run at about 1500 rpm and sprayed carb cleaner at those points, but got no conclusive results. At a couple of points, e.g. the carb base, the revs blipped up momentarily, but I couldn't recreate the effect when I tried spraying the same area again. (Would carb cleaner cause the gasket to swell and seal any leak?)

The only time I had a significant change in revs throughout the test, was when I sprayed carb cleaner directly in to the air cleaner, which proved that it could affect the revs (it stumbled and started to die).

When the engine was well warmed up, I managed to set the idle speed to 850, but blipping the throttle screwed up the idle setting and it 'hunted from 650 to 1000rpm.

I finally got it roughly stable at about 800 rpm but when I adjusted the Volume Control Screw, it ran faster the leaner I set it, which was the opposite of what I'd expected. It ran faster as I screwed it in, until it finally died when the screw was all the way in. For now I've set it back to 2.5 turns out, but I know the mixture won't be right.

The engine ran surprisingly well afterwards, but I know that the mixture won't be right because the Volume Control Screw is at its default/starting point. I need to check tonight to see how it starts and runs from cold.

Can anyone think of any diagnostics I've missed? Am I doing something wrong?

One other thing, the Volume Control Screw is the original from the carb. The o-ring on the replacement screw in the kit shredded, so I re-used the original, but as above, I couldn't detect any air leakage at the screw.

I'm stumped.

[olliehank47]

[Who.Me?]

Thanks.

[Who.Me?]

[Who.Me?]

I can buy a rebuilt Solex 34 pict 3 as a part-exchange for mine in the UK, but the cost may be higher than getting one shipped in from the US.

Can anyone recommend an advertiser on here who sells properly refurbished, genuine, Solex 34 pict 3 carbs?