Disheartened after work on carb/HG, advice needed

[Thrasher07]

Thank you for the link!

Bolts all went in fine and were greased. The water pump was new last year and I can recheck. The only other thing I know happened was that my partner decided to blank off the breather that comes off of the carb base because the hoses were all loose, could this be having an effect?

[bogbasic]

Did the thermostat go in the right way round ;-?

[mac]

Afternoon girls - me been thinking! - and my conclusion is that I am wrong in saying that there shouldn't be a problem combining "old" top and "new" bottom - in fact there will be!.

Spent the afternoon taking carbs apart and fiddling (I must be on a register of some sort by now) and, well see below.

Firstly the filter was incorporated in the idle circuit very late in the life cycle of the 300 when the Z10 being used was effectively a 400 series unit with an idle solenoid installed. The pic.below shows the circuit with the top/bottom interface joint arrowed - on most cars this is a brass tube sealed with an 'o' ring compressed between the top and bottom halves of the carb body, later cars have the filter in place of the brass tube and again sealed by a (much thinner) 'o' ring.


The pics below show both old and new versions,










The pictures should be self explanatory but if you use the late bottom and shorten the filter you loose the seal where the idle circuit crosses the top/bottom face (the filter 'o' ring fits inside the top housing) at the very least the idle circuit will be compromised. (You cannot rely on the top housing gasket to provide sufficient sealing for a circuit under potentially high levels of vacuum. Potential also exists for venturi depression to draw excess fuel across the gasket face directly into the throttle venturi. Under "normal" sealed conditions manifold depression act on the idle circuit orifice below the primary throttle plate to draw metered fuel from the primary well (just above the main jets). If an additional vacuum source is created at the housings joint by the missing seal around the filter results would be unpredictable.

Disconnecting the crankcase breather pipe from the carburettor insulation block is a no no I'm afraid - not only will it result in over/under pressure problems in the engine (carbon build up, oil leaks, oil seal lip evulsion etc.) but the inlet system will loose the unmetered (by the throttle plates) air flow from the crankcase into the inlet manifold which is in fact metered by a calibrated orifice fitted the pipe from the carburettor to the three way pipe junction at the rear of the head.

Hope this helps! - Mac.

[bogbasic]

Awesome work - you should write an almanac ;-0

[Thrasher07]

Pleased to say, with the help of Mac's advice and rebuild guide, we had the carb off for the umpteenth time, put it back on and twiddled and it's running the best it ever has in my ownership... for the minute! Will definitely get my other carb machined and set up as a spare. And thanks to Mac for the info on the breather hoses too. Hopefully, this is all, until something else crops up!

[Ride_on]

If you block the pipe in the insulation block the engine should run ok, but this should only be used as a temporary measure to check things, as the crankcase needs the breather system.

The insulation block pipe is normally restricted in the breather system, ie there is a plug with a small hole somewhere. Early 340s had this in the rubber pipe near the insulation block, later one used a modified T piece at the valve cover. You can get (minor) problems using the wrong or missing parts. 1.7s I'm not sure about but the principle is the same. You can't have too much air comming in there.

[Chris_C]

.. and it's running the best it ever has in my ownership...

Great news!

[Thrasher07]

Thanks for the advice guys. The breather hoses were all loose, so I have ordered new ones.

[Thrasher07]

Just to be double-sure, whereabouts does the valve go when putting the breathers back on? Noticed the car is a bit sluggish and lumpy on acceleration, I guess this could be down to needing more carb-fiddling?

[mac]

Pipe from oil separator on block goes to three way junction at the rear of the inlet manifold.

From the junction one pipe goes to air cleaner housing and the other goes to the brass pipe that is part of the insulation block between the carb and manifold. It's this third pipe that should contain the restrictor valve (a small plastic pellet with a hole (approx 1.5mm) through it).

Your poor running might be carb issues but the B172k is notorious for failing dizzy caps and especially rotor arms - can give identical running issues to carburation faults.

Mac.

[Ride_on]

I'm not sure about 1.7s but I thought the seperate in-pipe restriction was only in early 1.4s, later ones (in 1.7 era) used a restriction built into the T piece.

[mac]

Yes and no,
The early B14's used a restrictor in the pipe from the 'T' piece (on the valve cover) to the carb but there has only ever been one ' T ' piece - part no.3277001 and it's never had a restrictor. In later cars (B14.4E) no restrictor is shown in the parts listing.
(But I have found them fitted on later cars, in the 'to carb' pipe).

The B172k (subject of this thread) has always had a restrictor in the 'T' piece to carb pipe.





Mac