Carbs help please (twin Solex)

[mr_nunn]

Bit of background - over the Easter holidays, while sitting in Easter traffic, the van starting misbehaving - rough idle, mifiring/stalling at idle etc, some backfiring etc. Ran well at speed though. My first thought was vaccuum leak and I replaced some dodgy looking rubber (brake pipe elbow, and on the balance pipe between carbs) but problem has persisted and I can't find any further leaks with WD40 spray. Have also excluded (I think) plugs, leads, and compression. I think it's a problem with the right hand side of the engine (looking from the back) as pulling the leads on those two plugs made little difference at idle, whereas pulling the two left leads kills the engine.

Well, lo and behold on investigation of the right hand carb I found fuel emanating from the fuel bowl part. Can't tell exactly where from but perhaps from that gold bit sticking out in the pic (what is that gold thing by the way?)...



My current working theory is that the float valve has gone causing (a) the leaks; (b) the bad behaviour (flooding?). I have an electric fuel pump and no regulator (I hadn't heard of them till yesterday but will be purchasing one now!) and I see from other posts that this can be too much pressure for the carb.

First question - does this sound like a sensible theory and/or any other things I should look into?

Second question - what to do about it. I presume I need to get inside the carb. Can this be done on the vehicle and without messing up all the settings? I had the carbs refurb'd and set up about a year ago and has been running nicely, so am reluctant to mess up all the adjustments. However, it looks to me like to get the top off, I'd have to take off the choke (and indeed open up the choke mechanism to get at the screw). Am I just going to have to take the whole thing off at the manifold first and do this off the vehicle, or is it not that complicated?

Finally once I get in is the valve likely to be repairable or can I get a new one? Never opened one of these up so not sure what I'm going to find or how complicated to fix.

Couple more pics:

presumably need to remove the 4 screws on top corners of fuel bowl section, and the one white-ish screw bottom left?


but choke unit looks like would get in the way and complicated to remove - will all the adjustments get messed up?


Any advice much appreciated!

(p.s. these are the stock Solex PDSIT 34 2-3 carbs)

[bigherb]

The brass thing a thermostatic valve that governs the amount of fuel injected by the accelerator pump.
The top of the carb can be taken off to replace the float valve you don't have to remove the linkage you can turn the top over with the linkage still connected.
Original 1.2mm float valves are NLA but you can use standard Beetle 1.5mm ones, measure the original height of the float valve above the carb top and fit the correct thickness sealing washer, normally 1mm thick for the CU carbs.
What fuel pump did you use? It would be better with a fuel pump with a max pressure of 4 psi such as the Facet Posiflow.

[mr_nunn]

Thanks BigHerb - very helpful; in that case I'll try and get in there over the weekend then and see what's going on.

I couldn't honestly say what fuel pump is on there - it was fitted before I had the van. I'll investigate and if too much pressure either invest in the Facet pump or a pressure regulator.

[mr_nunn]

Well my theory was not really corroborated...opened up and float moving freely and needle valve seems to stop flow.

I'm more sure now that the fuel leak is coming from that brass thermostatic valve thingy mentioned above. Can i just unscrew this and check washer (is there one?)/seal with ptfe or something?

How exactly does the thermostatic valve work - would a leak there explain my dodgy idle symptoms or do i need to look elsewhere?

Thanks as ever for any more help!

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[bigherb]

The valve works by altering the displacement in the pump chamber there is an alloy seal 15x12x1mm between the valve and the carb body which will be the one to cause a leak, and an O ring inside to seal the port. Be unlikely to be causing an idle problem that will be somewhere else, air leak, bad adjustments etc.

[mr_nunn]

Thanks bigherb. Took out valve and just as you described. Will replace sealing washer..then on with the hunt for the real problem...

I initially thought an air leak but if it's that i haven't found it yet... any other tricks to find air leaks apart from the "spray common leak zones with wd40 and listen for idle change"?

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[bigherb]

Just a matter of blanking off pipes to the vac diaphrams to make sure they are not split. Make sure the carbs and manifold nuts have not come loose. Then check the fuel cut off valves on the carbs click when you disconnect and reconnect the wires with the ignition switched on, then remove the valves and make sure the jet in the end is not blocked and the rubber seal is intact.

[mr_nunn]

Thanks again bigherb. Currently waiting to pick up new washer for that thermostatic valve and some gasket material to put carb back together.. then I can fire her back up and continue the hunt for this pesky air leak if that's what it is...

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[mr_nunn]

Hi - update and more questions after some further investigations today.

I replaced the washer in the thermostatic valve which seems to have cured the fuel leak on the RH carb.

Seemingly no vac leaks after extensive searches.

Then I checked the timing with a strobe - it was reading 20 deg BTDC at idle with both vac pipes connected - should be 5 deg ATDC (and I know it was set right not long ago by Campershack). On further investigation, the vac retard **CORRECTION - I meant the vac advance [as pointed out later in thread]** (upper one one distributor) seems to be producing a vaccuum at idle - you can feel it with a finger, which I think is messing up the idle timing - the idle changes when you disconnect that pipe (in fact the timing then goes to ~5deg ATDC).

I further investigated the left hand carb (where that vac pipe comes from) and it was visibly dribbling fuel out of the nozzle and down the throat at idle.

This vid shows the change in idle with the vac pipe, and also the dribbling fuel: https://www.youtube.com/watch?v=90SA8Q5Ah4I" onclick="window.open(this.href);return false;

I presume none of this is as it should be? I have investigated the float needle valve and it still seemed to be stopping the flow; I also tried changing it for a new one but the same.

Any ideas what the issue is/what to do next?

[bigherb]

If you have vacuum at idle then it is usually the throttle butterfly is not closing.

[mr_nunn]

Ok will check thanks. Any ideas what is going on with the dribbling fuel out the main nozzle?

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[bigherb]

Ok will check thanks. Any ideas what is going on with the dribbling fuel out the main nozzle?

Usually too high fuel float chamber level. May be caused by someone incorrectly fitting a 0.5mm thick washer under the float valve rather than the correct 1mm

[mr_nunn]

Thanks BH you're a star I'll get onto both of these things and report back if/when any progress!

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[ajsimmo]

Are you sure you're checking the timing when fully warmed up and off choke? If it's still on choke it'll be riding on the fast idle cam, holding the throttle plate on left carb slightly open which opens the vac port to apply vacuum to the advance side of the can on the dizzy, hence the timing jumping to advanced rather than retarded. If it hasn't been disturbed it should still be correct @ 5°ATDC. The vac retard applies around 12° of retardation so without it you should see timing is at 7°BTDC, + whatever the vac advance is applying on choke (and if it's revving quite high you may also be seeing a little centrifugal advance on top).

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[ajsimmo]

Just watched your video and that is the advance pipe you are removing, not the retard (which comes from the idle circuit port behind airbox via a t-piece). The throttle plate may be held open by play in the linkage and cross shaft? Has the adjustable link rod (vertical from cross shaft to RH carb) come loose? That balances the airflow of both carbs. If it's far out it will cause the symptoms you describe, as could one of the choke housings coming loose, as they also carry the pivots for the cross shaft (stoopid design, Solex!).

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