The whole thing relies on the vacuum in the manifold peaking when you lift off the throttle sucking the air through the one way valve to empty the air from the hose to the servo. The peak manifold vacuum may be lower because of different carburettor design. The peak flow of air may also be restricted by the balance pipe..
But presumably, if the supplied carbs and balance pipe/ T junction etc for those carbs are all part of “ a kit” then they should be compatible. The only parts outside of the kit ( in my case ) are the original servo and valve. Both of these were working before the new carbs were installed so I think it unlikely either have “ failed” but the valve may accidentally have been installed incorrectly ? Does this make sense
I'm running twin Weber IDF40s and did away with the balance pipe and the servo works well. One advantage is less clutter in the engine bay. As Zed said, arrow on the one way valve pointing to the engine.
My money's firmly on the valve being the wrong way round. The servo does NOT rely on you taking your foot off the gas pedal, that's just Mike being Mike using his non engineering background theorising and presenting it as fact. I had ICTs/balance pipe on the small connections, T'd off for the servo just like yours and it worked just fine. If there had been any doubt I'd have fitted bigger fittings on the manifolds and like yourself did wonder before I tried it to see.
Its a kit of parts that fit together and provide some servo action. No more no less. It doesnt have a type approval so has not been scientifically tested. Some vacuum below the throttle plates as you shut the throttle is used to pull air from the servo pipe through the non return valve. If the new duals produce less vaccum then a sticky non return valve could amplify the effect. Say ..500mb vacuum from a single carb with all the air flow through one idle bypass circuit Say 400mb vaccum from a dual carb with all the airflow through two idle bypass circuits. Say.. 100mb needed to open the valve. Result from this thought experiment.. .. single carb you get 400mb at the servo Dual carb you get 300mb at the servo A 20% drop in manifold vacuum becomes 25% at the servo. You need to measure the vacuum you have before and after the non return valve.. If the valve is backwards then you shouldnt be getting any servo action at all unless its leaking. For reference my manifold pressure is 560mb absolute at idle under a progressive carburettor. Or 450mb of vacuum..
It's not helpful to present your "thought experiments" as fact Mr Mike. In this case based on a a pin sized advance take-off inside the carb? If you must do it, which is fine and entertaining, please make it clear that's what you are doing. Do you even have a servo? If you did I would suggest pressing the brake without releasing the throttle and you would find out that the brakes and servo work just fine. The engine sucks the whole time or it wouldn't get fed.
Aside from the workings of the servo ( beyond me ) I am with you on this. It is interesting as it appears that you are the only person that has replied so far that has run the same set up as mine, used the “ small “ balance pipe etc with no problem. @DubCat had different carbs but only connected to one ( I think)
So do I now - works fine on one cylinder's worth of vacuum (bigger fittings as my manifolds didn't have any). I tried it with the ICTs / type-1 engine as it's actually a royal PITA finding a spot thick enough that won't hamper plug access or point downwards and fill with petrol. Worked - left them like that. The only thing worth a mention is not all barb fittings are equal - for a given hose size some have bigger holes through than others. I would hope your kit maximised here but there's every chance they didn't even consider it in this day and age. Within the confines of what you had it does make sense to have them as big as possible. I should dig out my steel type-4 balance pipe, it's not all that fat.
I actually don’t think I am getting any servo action. When I say about a “ kit” . I bought the set up from a well known individual who re- jetted the carbs etc etc, rather than from a supplier direct. He specialises in Weber ICT carbs hence why I don’t think it has anything to do with the kit, but mor likely valve installation error. I am awaiting a reply from the supplier ( it’s only been a day ) but posted this thread simply due to curiosity .
If you are concerned it looks like there's plenty of meat to drill and tap for a bigger fitting, though obviously that would have been easier before they started!
I really hope this won’t be needed. There is an extra port on each ( as per your up close shot ) that presumably could be enlarged to take a bigger balance pipe….the other one, I think is behind the shroud. The lads who installed the carbs followed the detailed instructions meticulously ( but May have put valve backwards ) Fingers crossed it’s a simple error .
You'd have to take the manifolds off to do it, you wouldn't want it in the one pictured, it would be untidy let alone getting swarf inside.I expect your kit is from WOW, I'd have faith. If it was a kit JK had cobbled together from whatever they could source cheapest it wouldn't have those nice manifolds or the spacers or the heat stoppers, the take-offs would be tiny and block access to the plugs, the casting would prevent you getting a washer on the head studs and/or getting a spanner on them, the base would be too thick to get a nut on the now too short studs, you'd have to grind the sides of the manifold base to stop it clashing with the tin etc. There's more to fitting twin carbs than random piles of parts. :.
I might be wrong, but I thought the type 4 balance pipe, is the same size as the servo pipe, which is 13mm ish isn't it?
I have a MAP sensor connected where a brake servo would go. Its connected to a dead headed 5mm dia. tube. Below the throttle plate the pressure rises to near atmospheric at WOT,( or towards the ported vacuum pressure at least, which is also measured for the ECU driving the EDIS module). Its the throttle closed /light load scenario that provides the high vacuum. Why you need the one way valve, and "storage" in the long pipe to the front, so the servo begins work before the throttle shuts when you hit the brakes and let off the throttle.
Ok, I'll give you WOT IF your carb was designed for the max an engine could take. Anything less and it's throttled creating enough vacuum. The valve is backfire protection. The Brazil servos have the valve built into the servo connection so bang goes your "storage" guess. I know when something goes a bit wrong you love to come up with an alternative theory to the plainly obvious, something that could just about be true if you squint and ignore practical reality, but... does this help anyone troubleshoot or just amuse anyone with common sense and confuse and worry, waste time (and possibly money) for the unwary? .. oh well, carry on.
Hi Huyrob I currently have 4 busses , three with servo and one without. These servos are installed with DRLA twin carbs , FRD ( just like you ICT’s but from Delorto) and a stock setup. I did a lot of testing 20 years back when I built a 1745 with the DRLA’s as I was worried that a single manifold take off for servo with the twin carb wouldn’t be enough. If you put a vacuum gauge up by servo you get a good feel for what happens and this is what I learnt from the testing. Firstly the non return valve is critical ( some servos have this built into servo ) . The servo does ‘pump up’ ( well pump down as it’s a vacuum) over time. At closed throttle it pumps down quite quickly ( few seconds) but when cruising along it can take a bit longer as there is less manifold vacuum ( suck) being created by the engine. This is true of all the different carb servo combos I have tested. Peak vacuum gets built up when throttle closes. The one way valve ensures that the build up of vacuum only goes one way and builds up to a maximum vacuum and stays there until you press the brake pedal. When you brake, the vacuum in servo reduces the pedal force for you and then vacuum gets dumped after you let off the brake pedal. The system then builds a vacuum over again ready for next time you press pedal. Not 100% sure this is exactly how a servo designer would explain it but it is what you see in a vacuum gauge when you use the servo From what I’ve seen the quickest recovery ( servo vacuum build up ) comes with a fairly big vacuum pipe and barb. It also appears to be slightly better with stock carb and stock cam. Both a hotter cam and twin carb appear to be a little less good at building the vacuum at idle in ‘ real world’ I’m not sure how much this matters but I have fitted a fairly large manifold barb and pipe where I could as I like the idea that the servo vacuum recovers a quick as possible The non return valve is critical to system as this allows the vacuum to build up . Without it you would have almost no assistance. With regard to system you have I’m sure it will work as long as there is a vacuum in the balance pipe. I suspect the recovery time might be less quick than using a larger manifold barb and hose size but what you have should work. The fact that you appear to have almost zero servo assurance says something more significant and probably simpler is wrong As Zed originally suggested my ‘guess’ is also that the non return valve is either broken or more likely installed the wrong way round. It could be the balance pipe is not connected properly, however I think you would have spotted this already good luck and let us know what you find
Great description. I tapped off the vacuum from #4 inlet on the manifold for my Dells with a 1/4" NPT fitting, which has an ID of maybe 10mm. The servo recovers quickly after braking, so wide as poss for the vac connection is likely a good thing. The one-way valve is absolutely necessary. The vac is incredibly "pulsey" without it (measured with a vac gauge).
