Stromberg bypass adjustment

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lowact
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#1 Stromberg bypass adjustment

Post by lowact » Sun Jan 28, 2018 7:42 am

Hello, can anyone suggest regarding this?
It seems that one consequence of playing with ignition timing is that my car now does not idle and is very slow to reduce revs when I back off, so I am thinking to adjust my carburettor bypass valves. All published procedures seem to originate from documents such as this:

https://www.triumphwedgeowners.org/uplo ... retors.pdf

However, this procedure requires the bypass valves to have external adjustment. Stromberg 175 CD-2’s have internal adjustment, you must take the carb off the car and then the bypass valves off the carbs and dismantle the bypass valves to access the screws that alter the spring tension. Instead I am planning to adjust each bypass valve to an appropriate vacuum setting before refitting the bypass valves and carbs back onto the car …

So, what would be an appropriate setting (vacuum) for the bypass valves to open? According to Z-S the valves are normally closed and are opened by “high manifold vacuum that is present in the manifold when the throttle plate is suddenly closed”. This would suggest a vacuum of around 25” Hg would be appropriate? Currently my bypass valves are opened by a vacuum of 11” Hg, this seems so low that I’m suspicious that it is a “Jaguar special” that only works with (is integral part of) the overall emissions control scheme including retarded ignition (at idle)?
Regards,
ColinL
'72 OTS manual V12

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MarekH
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#2 Re: Stromberg bypass adjustment

Post by MarekH » Sun Jan 28, 2018 10:10 am

Dear Colin,

The bypass valves are there to admit a small burp of air into the cylinders at very high manifold vacuum only. This will happen mostly during overrun when the throttle plate has been quickly snapped shut. At this point, there is a huge amount of fuel still wetting the insides of the manifolds which is sucked into the cylinders and if the throttle is closed faster than the needle can drop back into the jet, then the mixture becomes overly rich. This doesn't fully ignite in the cylinders, but is expelled into the exhaust, where it ignites with a popping sound so beloved by boy racers. The sole objective of the bypass valve is to admit a tiny amount of air when the throttle is snapped shut so that the mixture can burn in the cylinders and not in the exhaust.

As such, working (or even non working) bypass valves will have no effect on idle whatsoever as they they won't move against their springs until the manifold vacuum is down below 30kPa (0 being absolute vacuum and 100kPa being air pressure at sea level).

A car will typically idle at about 40-45kPa, WOT throttle will equate to 90+kPa, cruising ~40-60kPa, pulling uphill 60-90kPa. At no point in normal progress will the engine see a persistent operating point which is "easier than idle", except when coasting downhill, with the throttle clossed by the driver. (100kPa equates to no vacuum (0inches) at sea level; ~29inches equates to full vacuum depending on atmospheric conditions. "The scale goes the other way.")

Bypass valves can have an influence on running conditions in two ways however when they fail. Over time, with heat, they can (will)become brittle and inflexible. In this state, their first failure mode is that they now can become a permanent slightly random bypass for air as their diaphragms are broken internally. The second failure aspect is that to make them work as designed, they have to sample manifold vacuum and so, if already broken, or the vacuum tubing is compromised, they can be a source of vacuum leak which naturally manifests itself most when the throttle is closed, i.e. at idle.

You have two choices:- 1/ Blank off the bypass valves where the diaphragms currently sit and blank off their source of vacuum signal at the manifold(s). Doing this eliminates one (four) source(s) of vacuum leaks at the manifolds and four sources of inequality of air quantity at the carburettors. Unless you are in the habit of using your right foot as an on/off switch, you are unlikely to notice any difference in designed performance of the car; 2/Replace the old/broken/brittle bypass valves with new ones and replace the vacuum hosing back to the manifold which controls when they activate.

If you want to check whether thay may bre leaking, use a plumber's torch to dose their exteriors with neat propane. If any of it is getting into the cylinders at idle then the car should run richer than before.

Personally I'd take option 1/ and if taking 2/ I wouldn't be too fussed about the exact point where they cut in, other than it should err on the nearer to vacuum end of the scale such that they don't cut in except in the extreme circumstances of going downhill with the throttle closed.

kind regards
Marek

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malcolm
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#3 Re: Stromberg bypass adjustment

Post by malcolm » Sun Jan 28, 2018 10:53 am

Marek, you continue to amaze me with your knowledge and ability to explain matters. That is not a sarcastic comment by the way. :salute:
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
2009 Jaguar XF-S
2015 F Type V6 S

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MarekH
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#4 Re: Stromberg bypass adjustment

Post by MarekH » Sun Jan 28, 2018 11:43 am

Dear Malcolm,

I see you are in Fleet. Feel free to pop over for a cup of tea anytime. I am at south side of the Hog's Back just before Guildford.

kind regards
Marek

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#5 Re: Stromberg bypass adjustment

Post by lowact » Sun Jan 28, 2018 3:06 pm

Thx Marek. I note you are quoting absolute (abs) pressures whereas I was quoting vacuum gauge (vg) pressures.
You advise that the bypass valves should not move against their springs until < 30 kPa (abs). My valves are in good condition (not brittle or leaking) despite this they are opening at 11" Hg (vg) = 37 kPa (vg) = 100-37 = 63 kPa (abs), i.e. well before thy should? Would explain why I'm having to drive thru towns with my foot on the brake, despite that throttle plates are adjusted to be hard shut at idle (idle and balance screws backed right off) ... Also vacuum tubing is without leaks, I note leaks would tend to prevent the valves opening, not cause them to open as is evidently my problem?

Don't want to just blank them off if I can help it, I'd prefer to solve the problem instead of accommodating it. 30 kPa (abs) equates to 21" Hg (vg) so 1st I will try and set to at least this.
Thx again.
Regards,
ColinL
'72 OTS manual V12

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#6 Re: Stromberg bypass adjustment

Post by MarekH » Sun Jan 28, 2018 3:23 pm

I'd blank them off completely and then set my idle the way the manual says it ought to be set. Then, if I were to want to reintroduce them, I'd make sure that is done in a manner which doesn't affect the idle. Doing it this way removes a "maybe" from the methodology of "fixing things" which, given you have doubts as to their efficacy, can only be a good thing.

kind regards
Marek

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#7 Re: Stromberg bypass adjustment

Post by malcolm » Mon Jan 29, 2018 9:30 am

Thanks for the invite Marek, I've PM'd you.
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
2009 Jaguar XF-S
2015 F Type V6 S

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#8 Re: Stromberg bypass adjustment

Post by lowact » Wed Jan 31, 2018 8:27 am

Might be hard to reinstate the bypass valves without altering idle conditions if I must remove the carbs to do it … If I doubted bypass valve efficacy I might be less hesitant about blocking them off. It’s not the bypass valves I doubt, it’s my understanding; whether, specifically on our carb’d V12’s, these valves might have other/additional function besides deceleration bypass? I'm confused as follows:

1. The carb’d v12s have monstrous gulp valves (P/N C.34018) specifically for decel bypass. So, throttle plate bypass should not be required for this function?

2.
MarekH wrote:
Sun Jan 28, 2018 10:10 am
if the throttle is closed faster than the needle can drop back into the jet, then the mixture becomes overly rich
The bypass is from downstream of the piston/jets to downstream of the throttle-plates. So, if the throttle plate closed faster than the needle could close the jet, bypass opening would make the mixture richer, not leaner?

3. When these valves were used (on other cars) solely for decel bypass they were externally adjustable with instruction on how to adjust. Ours have internal adjustment and our service manuals instruct against adjustment including “… adjustment will be lost if any attempt is made to separate by-pass valve from body. If it is suspected that diaphragm is damaged complete by-pass valve assembly must be replaced.
Also “…The following items must not be adjusted in service … 7. The by-pass valve spring loading”.

4.
MarekH wrote:
Sun Jan 28, 2018 10:10 am
they won't move against their springs until the manifold vacuum is down below 30kPa
I cannot stop the valves opening at ~50 kPa (abs); the hex spring-carriers become screwed out of their hex-shaped housings. This means that the bypass valves must, by design, always be open when the throttles are closed, including when idling? Also I note that the bypass valves are metal-on-metal so can never be vapour-tight; even when closed a modest vacuum can suck a significant flow thru them.

5. Since these valves must be open and are always open when the throttle plates are closed, I wonder if they are somehow supposed to be part of an idling emissions control scheme? Do we know anyone with a US emissions spec car who is successfully running with these blocked off, all I can find is stories of people wanting to rectify that these have been blocked off?

6. Mine is an outwardly original US spec car, I imported it in 2012 and have always had this problem, initially to a much lesser, almost tolerable, extent. All I’ve really done that could have made it worse is fix a LOT of vacuum leaks. I can directly relate fixing leaks to a worsening of this problem.
So what might be the cause of my problem, if it is not mis adjustment of the bypass valve spring loading? There is one other possibility, that I was not considering could have such a dramatic effect by itself, but maybe it does. Same as reported by Sven Baganz in another post, I have found I had two types of by-pass valve mounting gaskets, one type has no gasket material separating the ports, i.e.:
https://1drv.ms/i/s!AiSPBKa26IcchJQ2E2RYoThN1ohQ_Q
Also, when I purchased a by-pass valve refurb kit from SNGB it had one of these gaskets. The effect of these is to cause a quite large (the gasket is thick) and permanent leakage path around the throttles, so this time I did query the efficacy. SNGB replied that this was what they always supplied and there was nothing else available. I contacted Burlens (OEM). They replied that it was the wrong gasket and promptly sent me 4 new ones, that I am currently installing:
https://1drv.ms/i/s!AiSPBKa26IcchJYe2tbuNXKjfa8RUw

Still leaves me with 3 unknowns: Do I also alter the bypass valve spring loading, what to? Why does the service manual impress that the bypass valve should not be fiddled with under any circumstance? Why do the gaskets with no material separating the ports even exist? These aren't questions. I guess I'm just feeling a need to caution against blocking these valves off - sorry. EFI has never looked so good ….
Regards,
ColinL
'72 OTS manual V12

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#9 Re: Stromberg bypass adjustment

Post by MarekH » Wed Jan 31, 2018 12:12 pm

I have assumed that the bypass valve is not operating at idle and cuts in only at sub-idle pressures. What you are describing is a bypass valve which has a join between the inlet and outlet to permanently allowing a small air bleed at idle and then operate by opening more fully in the manner already described at manifold pressures below that, i.e. deceleration. In this case, the permanent air bleed is augmented at sub-idle pressures with the following result:-

The bypass valve's input is pre-throttle plate but post metering needle, so it will alter the manifold vacuum when called to open (to deliver more air than it's basic idle air bleed) and so promote combustion in the cylinders as a result of not being at such low vacuum as to make combustion sub-optimal, but it doesn't alter the mixture strength.

The gulp valve on the other hand, takes its air inlet from the air cleaner and so whilst it appears to be doing the same job of bleeding air and in the same exceptional low pressure circumstances, it alters the mixture strength because it is injecting air only but no fuel.

The two will also act at subtly different times in that whilst both sample manifold vacuum, the other sides of their respective diaphragms are at air cleaner pressure (very close to atmospheric pressure) for the gulp valve and post venturi pressure (i.e. subject to the depression of the metering needle/piston) on the part of bypass valves.

It sounds as though you won't be able to totally block off your bypass air bleed at idle but I'd still suggest you stop them operating to admit further air at low vacuum in trying to set up the carburtrettors.

You can of course, still go for the option of blanking them off, but your throtle plate will want to be set slightly further open in that scenario.

kind regards
Marek

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#10 Re: Stromberg bypass adjustment

Post by lowact » Thu Feb 01, 2018 11:08 am

I've belatedly thought to check the Burlen website. Found an updated service bulletin for the CDSE & CD-2 carbs that also describes the purpose of the bypass valves:
http://zenithcarb.co.uk/zencdsecd2/?___store=zenith
Proves to me that u r correct, blanking off these valves is the practical solution for me, so that is what I am now doing. Sincere thanks for helping me to solve this, for persisting with your advice and suggestions. It has been extremely helpful and I am very grateful.
Regards,
ColinL
'72 OTS manual V12

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