Reincarnated! Weird (and dangerous!) engine cutting out - what is it?

[trunt]

Tonights conundrum is why in the gasket sets are there two square carb-to-pedestal insulators, but one has a cut-out which - if positioned correctl - exposes a hole in the carburettor body which seems to lead nowhere? Is this an "appendix" which has become useless over the CD175 evolution?

It’s for the bypass valves on the emissions version, those slots are for the port that bypasses the throttle disk.

[dasadrew]

It’s for the bypass valves on the emissions version, those slots are for the port that bypasses the throttle disk.

Ahh, ok. Thanks.

[trunt]

If anyone is interested.. here are the bypass valve drillings. There is a spring loaded valve that seals the path normally but when the vacuum is high enough it lifts the valve and allows fue/air around the throttle disk.

Yes, its raining and I'm bored!!

IMG_6373.jpg

[Philip Wardle]

Although not raining here it is a rather dull and rainy afternoon ... the Haynes "Zenith Stromberg CD Carburettors'" manual says the by-pass valve limits "the very high manifold depression which occurs when the engine is in the over-run condition", this condition being "not conducive to efficient 'clean' combustion". Haynes mentions the poppet valves found in some throttle plates are an alternative (and rare) means of achieving the same thing. I assume there's no variations of the CD 175 that have both the by-pass valve and poppet valves, and all the carbs I've seen so far, which from 1973-74 UK Stags, have neither.

If your carbs do have the by-pass valve the holes marked 'bypass to manifold' hiles in the picture above would be drilled out to connect, and so if you wanted to ensure the valve works as intended you would need the carb-to-pedestal insulator with the cut-out. One possible source of these, which I came across during earlier research is: -


156249-OE.png

[dasadrew]

Ahhh - that triangular thingy with 3 screws is on my carbs, so I'll use the insulators with the cut-outs. Had used the standard ones up to now as that was what was fitted when I first disassembled the carbs.

[dasadrew]

Although not raining here it is a rather dull and rainy afternoon ... the Haynes "Zenith Stromberg CD Carburettors'" manual says the by-pass valve limits "the very high manifold depression which occurs when the engine is in the over-run condition", this condition being "not conducive to efficient 'clean' combustion". Haynes mentions the poppet valves found in some throttle plates are an alternative (and rare) means of achieving the same thing. I assume there's no variations of the CD 175 that have both the by-pass valve and poppet valves, and all the carbs I've seen so far, which from 1973-74 UK Stags, have neither.

If your carbs do have the by-pass valve the holes marked 'bypass to manifold' hiles in the picture above would be drilled out to connect, and so if you wanted to ensure the valve works as intended you would need the carb-to-pedestal insulator with the cut-out. One possible source of these, which I came across during earlier research is: -


156249-OE.png

Exactly! Your post popped up as I was typing! I have two new ones as I'd always put them in the parts box thinking they shouldn't be on my carbs

[trunt]

I found the bypass valve annoying, at least on a manual car, maybe you wouldn't notice on an Auto? It was causing the revs to take longer to settle down to idle, its adjustable but I just couldn't live with it.. I just screwed the things in enough to almost keep them closed all the time. Others have made gaskets that completely seal the ports under the device.

[richardthestag]

I found the bypass valve annoying, at least on a manual car, maybe you wouldn't notice on an Auto? It was causing the revs to take longer to settle down to idle, its adjustable but I just couldn't live with it.. I just screwed the things in enough to almost keep them closed all the time. Others have made gaskets that completely seal the ports under the device.

you can buy them new from Burlen, have only seen one set of uk carbs with them fitted and followed instructions to disable them by blanking off the ports.

If you must insist on getting them working then this chap has detailed how they work https://static1.squarespace.com/stat...bs+Part+II.pdf

[dasadrew]

slowly getting there .... I thought! Hoping that I can post pictures again, I was checking the function of the relief valve (or overflow valve) and discovered that, although the throttle butterflies are synchronised left-right, the lever which opens or closes the valves is significantly different left - right. Can't at the moment see how that happens since it sits on the flanged part of the throttle shaft. Any ideas?

Vergaser 1.jpg

[wilf]

Is it fitted back to front?

[dasadrew]

Scrub that last question. Measured the angles of the throttle shafts and they were different. Released thm to move freely andgained abou0.001mm movement at the shaft which resulted in the missing 3mm at the valve lever.

I'll have to slightly release the butterflies and start over.

I must admit that I'm still learning that, although the Strombergs make the impression that they came out of Heath Robinson's blacksmith, they sometimes need a touch of watchmakers care!

[richardthestag]

just going to say, the float chamber vents are going to vent regardless of where the pin is located, you can get them adjusted but I do not think this is root cause.

Now I have been chasing a stag stalling issue over the last couple of days, when it does stall it inviably needs a 30min break before restarting. been happening for a while now but the car arrived in the workshop this week.

Carb rebuild not needed as a set of fully refurbished carbs from gauge shop (bought a few years ago but done barely any miles since). just needed to reset the jet heights, fit decent throttle butterfly discs that actually closed and then baseline. 4th set I have had like this

Before starting up connected my inline pressure gauge and quickly found the robsport supplied pump was pushing 5.5psi, swapped back for the original SU electric (non points) pump and 2.8psi was restored.

Then found that the distributor baseplate (piranha / newtronic) was not connected to the vacuum advance

Then found that the ignition timing was wrong, could get it to 10dbtdc but no further, removed distributor which was a faff in itself because of the piranha wiring and got the ignition sorted. another one of those "rotor pointing to the nearest coil bolt and not the furthest" situations fixed it. certainly caused by lack of attention to the jackshaft orientation last time the chains were worked on.

Then the engine started and ran beautifully, until I revved it! then it stalled instantly on settling back to idle. Ah I though, running too lean, cursing myself for not checking ignition timing at the start of the job! enrichened each carb half a turn and it stalled again after returning to idle. ffs, stinking headache possibly because of exhaust fumes clouded my brain.

Enrichened each carb another 1/4 turn, this time when it stalled the car would not restart. Piranha module box was hot, almost too hot to touch. let it cool for 5 mins and the engine restarted. the box was loose fitted by this time, lifted it and the engine hiccuped. ah! drop the box 2cm and the engine cut out and wouldn't restart.

I had the old newtronic p8 box from my stag so swapped the box and it was running again but now hunting like a *******. leaned each carb off 3/4 turn - see how ignition fault makes you think fuel?

Hunting still there though. so switched engine off, restarted and smooth as you like. would have passed the 50p test on start I reckon, revved it and it started to hunt again. you rotten ungrateful bugger you.

Touched the hitachi red ballast coil and too hot to touch. ah

Swapped for a 12v coil, restarted and smooth until the engine was revved. Then it started to get all lumpy, almost hunting but no fuel smell any more

where I am now; cust original piranha logic box was root cause for his car stalling randomly?!? and needing a 30 min rest before allowing any further progress. I am not convinced that vacuum advance on the piranha is working correctly, probably need to rebaseline the carbs again even though they seem spot on. I can leave it idling for 5 mins, then rev it and it responded quickly and smoothly, no fliutter no misfire just instant and smooth.

I am now leaning towards another rogue pair of B1AQ meter needles. I had an issue a while back almost exactly them same, engine would start smooth but as soon as revved the carb dashpot would rise and the drop too far and the idle was affected. only swapping out meter needles for a very ancient, serviceable, used old stock items fixed the issue. That took me almost a day to figure out

A long post but hopefully you might have seen that ignition faults and carb / fuel faults can get very inter linked. Absolutely impossible to advise via air mail correspondence on root cause, any success here will likely be down to a best guess on what might be causing the issue. In this whole process above I have to be methodical and structured in my approach, try one thing, find no issue then move on. In this particular instance though there were many issues, but I believe tonight that root cause for the stalling when returning to idle was a **** piranha box.

I am rather hoping that when I finish tomorrow morning this car will drive better than it had for decades

[dasadrew]

Very aptly described. I've got a Pirhana set up ready to go in the box, new DD Distributor cap, new spark plugs etc., et. but if I do all of that at once I'll still not know what the fault was ..... or potentially still is!

All-in-all having seen the scoring on the oil damper tube and the stickiness of the piston to drop out of the top housing, I really feel that it might have been the problem. Got new hoses and elbows delivered today, so hope to be driving it next week - weather permitting!