Solved? - Rough idle issue

[Who.Me?]

I'm trying to diagnose a couple of idle problems that I've put up with since I got the truck. I'd appreciate any pointers as to how to diagnose and fix this...

Symptoms - The engine fires up strongly from cold* but the revs drop and it dies straight away unless I give it gas for 20 seconds or so. It idles very slowly and rough until it starts to warm up. It drives fine with no flat spots, backfiring, surging or pinging. When warm it idles without stalling, but the idle RPMs jump up and down by up 100PM. This isn't a progressive loping or looping change like a typical vacuum leak. It's a constant, random jumping. It sounds fine at high RPMs, but the idle is rough/uneaven.

(*If the truck has been sat for a week or so, it'll take some cranking before it fires up. I think that's normal though and is due to evaporation from the carb?)

I'd appreciate any pointers for diagnosing the idle issue. I'm getting to the point where I'm thinking it might be a distributor problem, but I'd appreciate any pointers. It doesn't seem like a vacuum leak. I've had that problem before and the revs 'hunted' up and down more slowly and the carb didn't respond to tuning at all. The is carb responding to adjustments to the volume and bypass screws as I'd expect at the moment.

More info below:-

Engine = 1600 dual port built by a previous owner from a B code case with Autolinea G01 heads. I checked the compression a couple of thousand miles ago and was 115/110/115/110. I will check this again, but the idle issue has been a problem for as long as I've had the truck. I can't detect any vacuum leaks with carb cleaner, but I'll pressurise the manifold and check again with soapy water (this worked for me when I did have a vacuum leak a while ago). The valves were checked last weekend and were all fine. The plugs are gapped correctly.

Carb = Solex 34PICT3, rebuilt about three years ago. Main jet = 130, Idle/Pilot jet 57.5. I think I've got it tuned OK. The plugs are all dry and are an identical, graduated light brown colour with no soot on the electrodes. The exhaust doesn't smoke, but it is a little smelly/fuely. I've always thought that was normal for aircooled engines though.The tailpipe is dry and black w ith no fluffy deposits. Choke seems to be set right and stands fully open when warm.

Fuel pump = rebuilt pierburg square top. Fuel pressure= 1.5psi. I thought that might be a little low based on posts here, but googling 'Solex carb fuel pressure' returns a bunch of results saying they need 1.5 - 2psi.

Distributor = Bosch 043 905 205L SVDA (0 231 168 015, JFUD4) This is the correct distributor to match the code on the carb flange. Running points and condensor. Timing and dwell are within the specs described on the VW Resource pages. The vacuum advance holds a vacuum with the tongue test. The history of this distributor is unknown. It was bought used and I've done nothing other than replace the points and condensor and lubricate it. It cured the flat spot that I had with the 009.

I'm going to go back and re-do the compression and vacuum tests, but I'm stumped as to what this could be.
_________________
Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786

[FritzCP]

I bet you have crap in the overly complicated idle circuit of the carb. the 34-3s are especially finicky that way. I would remove the idle jet, bypass screw, volume screw and idle cutoff from the carb, and blow carb cleaner and compressed through every hole back and forth. Wrap a towel around the base to keep the mess to a minimum.

also, you may have a hole in your intake on the inside into the preheat pipe.
_________________
Fritz Custom Products
Fritz 10 - Tin Screw Torquer
Fritz 124 - Crank Pulley Puller
Fritz 70 - 6306 Bearing Puller
Fritz 44 - Dizzy Puller

[BarryL]

Check the dual port boots clamps and integrity.

Maybe try plugging off the carb suction to the distributor just to eliminate that then put it back if that's not it.

[Who.Me?]

[flemcadiddlehopper]

I agree with Volksheime. However, before going to the trouble of tearing it apart to try an find the issue try this.

Take the air cleaner off, start the engine and warm it up a bit, 'til it runs well. Then fold up a clean rag until it's about the size of the top of the carb. Now rev the engine up full throttle and then cap the top of the carb with the rag. Hold it at full throttle while the rag makes the revs drop. Remove the rag before the engine stalls out. Rev it up and repeat a few time.

The idea is that the high suction pulls any dirt out of the circuits. It's a shade tree mechanic trick. Check the wind direction before doing this as it does create some black smoke that you don't want blowing into your kitchen window.

Gordo.
_________________
Everybody Dies....Some Never Live.

Retrograde Garage. Vintage Aircooled, and others.

[FritzCP]

Could also loosen the Idle jet just a tad as an easy test. Ive had 34-3s that wouldnt idle of choke at all, and loosening the jet 1/8 turn did the trick to allow it to idle as no amount of carb cleaner through the jet passage would clear it.

also the SVDA 34-3 hole in the butterfly is about 3mm, the DVDA carb hole is about 5mm diameter. you said it was rebuilt, and though unlikely they changed the butterfly, its worth a check to make sure its the correct smaller SVDA hole size.

is your heat riser clear?
_________________
Fritz Custom Products
Fritz 10 - Tin Screw Torquer
Fritz 124 - Crank Pulley Puller
Fritz 70 - 6306 Bearing Puller
Fritz 44 - Dizzy Puller

[Who.Me?]

Well I think I've found at least part of the problem - I was wrong about vacuum leaks.

I last checked with carb cleaner a few weeks ago and couldn't detect any leaks.

I re-checked this evening by pressurising the intake through the carb with a vacuum cleaner set to blow (I took precautions against blowing dust down the intake). I sprayed the intake and all the joints liberally with soapy water and found a few small bubbles at the outer ends of the throttle spindle where the nuts are, but more significant amounts at the #1-2 manifold-to-head joint. I tried tightening the nuts down some more, but it made no noticeable difference.

The bubbles are coming from the inboard side of the flange.

Last summer I noticed that the accellerator jet had gone missing. I thought I'd lost it down the manifold and ended up having to take the whole manifold off with the engine in situ (the jet wasn't there - it turned up under the bench a few months ago).

I used new gaskets and a bit of permatex high tack for good measure when I refitted it, but I guess it wasn't good enough for #1-2.

From memory, that was the easy side of the manifold so I'll pull it off and see if I can sort it with the engine in. If the flange is bent, I can always lap it. I guess it depends if the problem is with the mating surface on the head.

I'm not so concerned about the few small bubbles around the throttle spindle nuts. The vacuum generates a fair amount of pressure when set to blow (more positive pressure than I reckon the engine creates negative pressure). There is no detectable wobble or play in the spindle and it was re-bushed with metal bushings when the carb was rebuilt a few years ago.

Out of interest, is there a trick to accurately torquing the rear-most manifold-to-head bolt? There isn't enough clearance for a torque wrench, or even to get a socket on the nut. I can only get the open end of a spanner on it.
_________________
Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786

[Who.Me?]

[Who.Me?]

Well, fixing the air leak and cleaning the carb had bugger all effect. It still won't idle properly. This is getting depressing

When I dissasembled the carb I found it was clean as a whistle. It was professionally rebuilt less than 2000 miles ago, shortly after I'd fitted a brand new fuel tank.

I checked the volume and idle screw settings. The bypass was one turn out and the volume was 5.75 turns out (pretty rich?). The plugs were clean, so I'm guessing the manifold-to-head leak was leaning it out somewhat.

I lapped the manifold end where I'd detected the leak with wet wet-and-dry paper laid on to a sheet of glass to make sure it was absolutely flat, and I thoroughly cleaned the mating surface on the head. I used a new gasket with a smear of permatex in case there were any fine scratches.

I reassembled and pressurised the system and sprayed it with soapy water again. Apart from the same small amount of bubbles around the right hand end of the throttle spindle, there was no hint of a leak.

I don't know whether the spindle bubbles are significant. Could it ever be entirely airtight under significant pressure? The spindle needs to be able to rotate within in the bushes. It was re-bushed with brass bushes when it was rebuilt. I'm suire the pressure that I'm using is much higher than the vacuum drawn through the carb when the engine is idling.

I set the volume and bypass screws at 2.5 turns out and filled the carb bowl. It fired straight up and the idle quickly built to 1300 RPM. BUT - the revs didn't drop when the choke started to open and the throttle came down off the fast idle cam. Playing with the bypass and volume screws had no effect. It just 'hung' at 1300 RPM.

I was concerned there might be a major vacuum leak that I'd missed, but after shutting it off and restarting, it responded to the screws. I don't know what was going on there.

I re-tuned it as best I could to around 850-900 rpm (revs still bouncing up and down), then I went for a drive that included a fast road.

When I returned to town, idled pretty steady at 1300 RPM (remember - I'd tuned it at 850-900RPM before I left).

I reset the idle speed down to 850-900RPM and it idled like crap again.

I've tried spraying varying amounts carb cleaner along the whole intake from the carb to the heads. Apart from when I sprayed it directly in to the air cleaner, it had no effect on the idle.

On the off-chance that the issue was with the throttle spindle, I smothered the end with grease to form a temporary seal, but it made no difference to the idle issue.

It does seem to run relatively smoothly if I set the idle to 1200RPM or higher, but as soon as I set it to a normal idle, it runs rough and the revs bounce around again.

I'm stumped. Anyone got any bright ideas?
_________________
Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786

[BarryL]

Have you tired to insulate the distributor from the carb yet? It's just to make sure the distributor isn't making the advance cause your fluctuation. I guess your bubble test would show if the rubber boots were a problem. The throttle bushing "leak" thing is most likely perfect and as new normal.

[Who.Me?]

[BarryL]

[Who.Me?]

[BarryL]

[Who.Me?]

[BarryL]

On the start-up and die issue are you sure the choke is correct and not too lean? Does it do it like that every time even after it's warmed up?

On the floating rpm thing are you sure the pilot jet is tight in the hole and clean inside on the little tip hole?

Your fuel pressure should be closer to 3psi (2.8psi to be exact). If it runs ok going up a hill getting on it then I doubt fuel pressure is the problem.

How is your float level?

When you say it smells gassy but you thought that was just a VW smell, does it smell rich-gassy VW smell?

Do you have another distributor just to disprove any electrical/mechanical flaw in that part?

Can you feel the fluctuation in everyday driving around?

[Who.Me?]

Thanks for giving it some thought. Answers below...

[BarryL]

Far far away on my computer reading your symptoms I'd say if you aren't starving for fuel then the pump is ok as is. It reads like your needle and seat aren't closing all the way.

Safety first with the moving belt and gasoline but can you look down the throat of the carb with a mirror and bright light with it idling? Look for fuel coming from the angled pipe or any fuel dripping inside after you shut it down.

[FritzCP]

try setting your choke to only 3/4 closed or 1/2 closed (inst4ad of full closed) at stone cold.

is the idle cutoff solonoid functioning properly? maybe pull it and take a look to make sure the plunger isnt distorted and hotwire it to make sure its not sticky and is retracting the way it should instead of just the old 'does it click' test

maybe a valve is a little sticky when cold.
_________________
Fritz Custom Products
Fritz 10 - Tin Screw Torquer
Fritz 124 - Crank Pulley Puller
Fritz 70 - 6306 Bearing Puller
Fritz 44 - Dizzy Puller

[Who.Me?]