Stromberg Bypass Valve setup

[Holeshot]

Hi guys.

I’m servicing my carbs and each of the bypass valves on all 4 carbs have differing length springs and screw adjustment lengths.
I realise there’s a strong argument (made in every thread I’ve read about these valves) for just blanking these off but I’d like to run with what I’ve got because I just want to get back on the road quickly without a load of fuss so please bear with me :) I know.... maybe I’m just an old fool!!

So...How do I set the correct pressure for the valve without special equipment? Is it even possible?

Otherwise, is there a blanking kit available?

Thanks very much.

Cheers, Hilton.

[MarekH]

Atmospheric pressure at sea level is 100kPa or 1bar.
The bypass valve seeks to admit additional air into the carburettor when the engine is on overrun and manifold vacuum is high. I would estimate that the air pressure in the manifold would be at or below 30kPa in such circumstances, i.e 70kPa below 100kPa.

As an opening shot, I'd set the bypass valves to move at pop-off pressure differential of 75kPa. If you have a bicycle track pump or an air compressor then you ought to be able measure at what pressure they open and close and adjust accordingly, if need be.

Then I'd blank them off. Based on the datalogs taken from my car, you need to be snapping the throttle shut or coasting downhill with a closed throttle to achieve manifold pressures below 30kPa. On my last trip out driving through the Surrey hills, this happened 15 times for a total of less than 15 seconds and that's from over one hour's driving up and down dale.

The other measure is to ask when do modern fuel injected cars cut the fuel and retard the spark? This happens roughly at below 35kPa on a closed throttle.

kind regards
Marek

[lowact]

Hilton, what is the issue you are trying to resolve? If you cannot satisfactorily adjust as per Marek's advice (I couldn't, one bypass would be open at idle, even when adjusted for maximum spring closing force) quickest way to blank them would be as per Marks suggestion at the end of this previous post, make a solid gasket?
viewtopic.php?f=6&t=12929&hilit=bypass+valves

I am now believing that, whereas the original purpose of such valves was for overrun vacuum relief, not so for S3 E's, they were effectively repurposed, primary purpose is to improve control of air fuel ratio during starting and at low throttle openings, this is why there is temperature-controlled vacuum assist, springs are very soft, adjustment is internal so novices or even licensed mechanics can't mess up the settings only the factory, gulp valve used instead for overrun relief.

[MarekH]

A closed throttle is not the only one factor in generating vacuum - engine revs also matter.

The engine has to want to be consuming more air than air can leak past the closed throttle - that's what vacuum is.

At low rpm (e.g. at 120rpm cranking speed), enough air bypasses the throttle for the engine to get all of the air it needs to accelerate beyond cranking speed and thus start the car. At idle, the engine has just enough air to run but not do any work (the clue is in the title "idle") and this generates moderate vacuum , but at high rpm on a rapidly closed throttle, a lot more air is momentarily still being pumped by the cylinders than can get past the closed throttle and that's what generates the high vacuum signal to pull against the spring.

The bypass valves won't have any function at starting. Starting a car is a very heavy loading of the engine (i.e. low or zero vacuum generated) and is the polar opposite of overrun where the load is light (i.e. super high vacuum). We also know that we need to add more fuel to massively richen up the mixture at start, which is not what adding more air does. At overrun (or gear change), the mixture is momentarily too rich to burn as the air supply is snapped shut and the bypass valve's sole function is to ignite the mixture rather than allow it to flush through and maybe ignite in the exhaust. No fuel is saved and no performance benefit is derived. It aids with noise and/or unburnt fuel emission. (Modern cars when they sense this abnormally high vacuum at high rpm simply cut the fuel to th injectors and then reinstate it as revs drop down or engine load goes back up.)

The bypass valves can only come into play at much higher rpm than that at startup or idle, which is when much more air is being consumed by the cylinders than can be supplied by a closed throttle plate. That is what generates the vacuum and that's what opens the valves.

Back in the day when everyone ran carburettored cars and mileage was high, this may have been a sensible feature, but now, with very limited mileage and very few carburettored cars remaining, it isn't a big factor in the overall emissions equation anymore.

kind regards
Marek

[Holeshot]

Thanks for such comprehensive answers guys - I can see the sense behind just blanking off the valve completely. It’s probably what I’ll do eventually to be honest.
Colin, there’s no real issue as such, just that after having been laid up for some time, the car barely runs and basically, it’s the carbs so thought I should strip them down and give them a long overdue service. Finding that the bypass valves’ internals aren’t consistent, I thought it best to find some way of checking whether they’re set up correctly.
I recognise I’m most likely on a fools errand in trying to set up a flawed device with the added randomness of 50years of wear, but I’d like to give it a go and get the best out of what the car was born with.
Marek, I’m not sure what adaptor would be best when I use my track pump - can’t work it out I’m afraid. Any advice?

Thanks again for indulging me

[MarekH]

I'm sure you can jury-rig some rubber hoses and duct tape it to death. It only has to hold 1.7bar assuming you are at sea level. You could get creative with a bicycle tyre I suppose. I'm sure it'd be nice to make it work as designed, even if it really doesn't do much. Good luck.
kind regards
Marek

[lowact]

Since you are thinking to eventually remove them, why not for now, just screw them fully shut?

[MarekH]

Because 1/ they may not have an adjustment screw - not all did, 2/ the diaphragms may be brittle and cracked inside, and 3/ the vacuum hoses may be the problem, not the bypass valves themselves.

kind regards
Marek

[lowact]

They all can be adjusted, ref link in post #3. If diaphragms or vacuum hoses are damaged the bicycle pump idea would likewise not work, until you fixed this.

There are 2 types of bypass valve. On earlier Strombergs there was external adjustment and internal vacuum source, locally in the carb. The concept of adjusting the valves to reduce backfiring is applicable to this type. On V12 e-types it is completely different, I'm not sure we can presume they have the same function/purpose?

For our cars there was external vacuum source and access to the adjustment screw was blanked off. Here are pics of dismantled bypass valve showing the protected adjustment screw. Knocking the little brass plug out of the front cover (spring housing) would give access to the screw making it externally adjustable, there is even an o-ring on the screw for sealing... However all credible, authoritative advice is to not attempt to adjust these.



Mixture control when starting, I was thinking about how the thermostatic valve (C37430) is used to inhibit operation of the bypass valves. When the engine is below temperature the bypass valves are allowed to be sucked open by high intake manifold pressure. When the engine has warmed up the thermostatic valve vents the vacuum lines, therefore the bypass valves (should) never open?
Or so the theory goes. In my case I couldn’t stop just one of the valves opening when idling, even with the springs compressed as much as possible. (Yep, dismantle, adjust the screws, reassemble, test, repeat ...). So balancing the carbs was impossible. So I blanked them off. This enabled me to balance the carbs really nicely and have a low, smooth idle. Downside was, fuel economy was worse. So, having tried it, my advice is now to persevere with the bypass valves if you can.

Suggestions by others on how to adjust them:
https://www.triumphexp.com/forum/spitfi ... t.1688038/

How I would now try to do it:
Start with the screws all tightened the same, as far as possible which is one thread before the nut is out of the hex housing.
Note which carb bypass valve is open at idle.
Loosen the others a turn, recheck.
Repeat until they all don't open at idle.

[MarekH]

They all can be adjusted

I think you can read my reply as "unadjustable unless you take it apart"

I'm not sure we can presume they have the same function/purpose?

The ones with the external vacuum port allow you to take pick and choose how and when you get a vacuum signal but the internal type doesn't give you that choice, as the vacuum signal is coming from an internal connection to the carburettor. Other than that, the principle is the same. I seem to remember that there are two gaskets for these. One has a gap in it so there is a small but permanent air bleed. If using those, the throttle plate would be slightly more shut by default. This doesn't affect bypass valve operation.

When the engine is below temperature the bypass valves are allowed to be sucked open by high intake manifold pressure.

You meant "low intake manifold pressure".
Very very low pressure, e.g. >1500rpm and throttle snapped shut, or when unloading the engine with a gear change. Pressure will drop below 30kPa, i.e. a spike of 70kPa of vacuum.

When the engine has warmed up the thermostatic valve vents the vacuum lines, therefore the bypass valves (should) never open?

Correct. It also means when water temp is above 100'c, there ought to be a small air leak into the manifold as the air cleaner is always close to outside pressure and this is higher than manifold pressure (except at WOT, when they are both at close to atmospheric pressure) so air must be creeping into the manifold when this happens, causing a higher idle after the car starts. The bypass valves themselves can't operate at starting/cranking however, as there is no vacuum at startup.

Suggestions by others on how to adjust them:
Note which carb bypass valve is open at idle.
Repeat until they all don't open at idle.

The vacuum signal acts in opposition to the spring, so you need to adjust the spring to make the diaphragm not pop open as easily. At idle, you are probably at ~40kPa with standard camshafts, so you want to go further than just "don't open at idle" - that's an absolute minimum. (Take note: -fast road cams ~45kPa at idle so you don't want these bypass valves set to open when cruising at 2500rpm, you want them set to poen well below the vacuum you see at idle. I wouldn't want to set by reference to idle as this is the wrong benchmark - it is influenced by distributor retard/advance, engine tuning etc. I'd set them at >=70kPa of vacuum.)

Maybe I should just make some clear carburettor body parts so you can have fun watching them....

kind regards
Marek

[ReconPilot]

All great replies. I was confronted with setting these valves several years ago. Made a specific test box with calibrated vacuum gauge and variable vacuum source to calibrate them.

Bypass Valve - Original from 1974
Bypass Valve repair kit (minus diaphragm)
New diaphragm.
Original tension spring

Here's what I found.
Disassembled each valve keeping original screw adjustment setting intact. Replaced small o-ring, Bypass diaphragm and installed new gaskets.

On testing set up all four to open at the same vacuum. Recorded ambient temperature for database.
Waited 30 minutes then repeated the test. ALL WERE OUT OF CALIBRATION! This went on for several days of testing! The results became clear.
Final assessment: the old tension spring(s) did not hold their tension value. Temperature affected them also leading to being inconsistent in repeat opening vacuum. Failed springs!
Burlen Fuel Systems does not sell the spring alone. You have to buy the complete valve.
Result: Unless they are required to meet an emissions standards test, blank them off, plug the vacuum line (with a BB) and be done with a vacuum leak source. Since the internal adjustment springs become unreliable (tension) due to age, heat, cycle operation and other factors it's a waste of time to attempt adjustment. Replacement valves are very costly.

My advice - make a suitable blanking gasket thereby eliminating an unneeded component for each bypass valve.

Happy Trails,

Dick