Solex 40 PII-4 throttle shaft wear

[wildtexas]

Afternoon all.

I am in the middle of pulling my carbs apart on the 912.

I'm aware of the issues around worn throttle shafts or throttle bodies at the shafts.

My question is what is deemed excessive movement?

Fore and aft, side to side?

I have a dial meter to take measurements if anyone can give an idea.

At present it doesn't seem excessive to me and there are no gouges in the the throttle bore.

But acceptable to me could be excessive to someone else. The car has been sleeping for a while so I can't really comment on whether it had a rough idle or not.

[911hillclimber]

Suggest a call to Chris at Fenn Lane Motorsport.

[fetuhoe]

You can't really measure shaft clearance in the manner you are suggesting although it may be a rough guide to worn shafts.

You really need to measure the diameter of the shaft and the bore of the housing but again this can be tricky if the wear is uneven.

The force you apply to the shafts will have an impact on the measurements you make and as clearances should only be a few microns using a DTI is unlikely to be very meaningful.

A Class IV fit of an 8mm diameter shaft in a bore (which is the best fit that still allows hand assembly) would be around 5 microns minimum with a maximum of around 25 microns.

At running temperature of 70 degC this could increase up to 30-35 microns which should still work quite well.

We normally make shafts to an h6 tolerance which is around 10 microns and have a reamer to re-size bushes to an H6 hole size.

This can result in clearances of around 20 microns at room temperature.

If the shaft moves back and forth then the throttle plate isn't centered correctly in the carb body and this can also cause leakage problems which can involve re-sizing the carb bore and fitting a slightly oversize plate.

The best way to check both the bearing and throttle plate sealing is using a vacuum test and a synchronometer.

A typical Weber/Zenith fitted to a 911 will need a flow rate of between 4 and 5 on a typical synchromometer - This is kg/hr and for air at a vacuum of 23 inches of mercury Hg which is equivalent to 8-10 CFM.

We would normally expect a carb with the idle mixture screw closed to show a figure of around 1 kg/hr using a test vacuum of around 6 inches of mercury - Typical Dyson performance.

[Ashley James]

You can buy shaft, butterfly, accelerator pump linkage and spring repair kits. I bought mine from US EBay and it turned up in two days.

I’d buy a couple and fit them very carefully. The quality is superb. If you’re lucky this will reduce wear to inconsequential but make absolutely sure there’s no tendency for the butterflies to stick closed and make sure tickover is reliable hot and that it always falls back to the same revs.

If you’re unlucky I have someone who can rebush and re-sleeve and I doubt he’s unique, so there maybe someone near you.

Here’s just the shaft and butterflies. I needed the pump linkage spring as well, but this should do you.

40 PII -4 Solex Carburetor, Throttle shafts set complete, THROTTLE SHAFT https://rover.ebay.com/rover/0/0/0?mpre ... 1549333149

[911hillclimber]

When I did my Webers way back, the repair kit had Teflon sleeves included to 'wind' into the oversize body bore and the shaft without removing the shaft at the end where the linkage/return spring locates which is where most wear occurs.
The Teflon was about 5 thou thick (if I remember right, could be very wrong) and was a tube, about 10mm long and the length cut to create a 'scarf-joint'.
I assembled them with some grease to help things, and it all worked a treat for 3 years when the engine was sold to a guy with a splitty.

[fetuhoe]

There are two types of Solex P40ll-4 carbs fitted to 912s - there are both Twin Shaft and Single Shaft designs.

Early carbs had a single shaft design and later models had 'split' shafts which were introduced to improve emissions by providing better tuning capability.

The Split Shaft design also has a superior progression circuit so the transition between the idle and main jet circuit is better.

The Split Shaft design does allow the individual barrels of the carbs to be better balanced than the Single Shaft designs but the weakness is that the rubber blocks used in the linkages deteriorate with age and generally need to be replaced.

The 'recommendation' for single shaft carbs is that they should be balanced by 'twisting' the throttle shaft - a procedure that I am not too keen on using.

The ideal method to eliminate shaft and/or bore wear is to use a slightly oversize shaft and line ream the body to suit.

We have a 'long' carbide reamer with an 8mm pilot that we had made specifically for this job. It is long enough to repair either single or split shaft carbs.



The pump links are also quite often badly worn and need replacement otherwise the accelerator pumps just don't work well.

[Kneeslider]

I used Carburettor Exchange in Leighton Buzzard to recondition my Split Shaft Solexes. They seemed to do a thorough job, with everything replated, no wear obvious on the bores or throttle plates etc. I did about 1k miles of setup fiddling, and took them and the car to a Dyno, to get the setup just so.

I was rather surprised when I found that there was quite a bit of air leaking through the throttle shafts, enough to kill the combustion on that cylinder if the base of the carb was sprayed with carb cleaner. I had made sure that the carb to manifold joints were spot on, and there were no leaks from this area. I proceeded to set up the jetting and mixture screws to compensate for the air leak I wondered if I should have taken them back to Carb Exchange, but they were done about 3 years ago, and the performance and econonmy are more than acceptable, except for a very minor hang up very occasioinally when returning to idle, so I figured that I would just go with what I had. Maybe they are all like that, even when new?

I haven't personally had my carbs apart to the extent of removing the throttle shafts, so I don't know if they had been bored to fit the shafts, or if there were any seals in place

[Kneeslider]

Hmm. According to this, the throttle shaft area etc should all be as good as new.

http://www.carbex.demon.co.uk/page2.html

Its the sort of thing that you take for granted when handing components to a specialist, but having seen the effect of spraying carb cleaner at the throttle shaft, it looks like a case of 'they all do that'.

Incidentally, my engine is in fine fettle after new 1720cc AA Piston and Cylinders, and freshly lapped valves, with plenty of compression, and a SC cam that looks like new. I kept on playing with mixture and jets, but couldn't get rid of a 3,000 rpm flat spot, and thought that I was going way rich with the jets, from looking at setup advice on the Registry forums in the USA, so took it to Bogg Brothers to put on the dyno, see the CO at various RPMs and see what's what. The Solexes ended up with a 1.4mm bore on the main jets (as an aside, the Solex jet numbers didn't seem to logically correspond to the bore diameter of the jets, i.e. 1.3mm drill size is the same as a #122.5 Solex jet) #60 Solex idle jets, and 2.5 turns out on the mixture screws.

It now runs really nice, except for the slight idle hang up.

[wildtexas]

Chris et al,

Many thanks for the very useful information.

One of the down sides of being way out on the western extremity of the U.K. Is that there aren't many very knowledgable specialists over here. So I tend to read a lot and try to do my best.

After recent issues on the forum with carbs being put in quarantine by postal services, I am reluctant to parcel them up.

As I say, they don't seem terrible to me, I have seen far worse.

My intention is to go through them and make sure they are clean and the pump works appropriately as Chris recommends.

If I do experience symptoms when it is all back up and running, I know I can call on the right people at that time.

Thanks again Matt

[911hillclimber]

I remember seeing soft closed cell rubber washers sandwiched between the carb body and the linkage lever (where the return spring goes) on a rally 911 to keep the dust out (quote).
The washer was about 15mm dia and 5mm thick approx. relaxed. It was crushed down so compressing.
It was dry and the throttle came to tick over reliably (quote)

It also became a seal of sorts.

The PMO carbs have the throttle shafts run in sealed ball bearings, and the radial play is really very little indeed, or at least are on my PMO's.