1973 Westy - Brakes up rpm momentarily...

[fido]

Previously I had the servo (booster in US speak) line hooked up to just one manifold on my dells, and I never noticed any rpm change when I put my foot on the brake or let it off again while idling.



Following a little top end rebuild, I decided to hook all 4 manifolds up to the brake booster (partly to get a better more stable signal to the autobox, and also to help smoothness at idle). Now when I put my foot on the brake, for a brief fraction of a second, the rpm needle flicks up 20rpm or so. Does it again when I take my foot off the brake. Is this normal? I can convince myself of how it could be normal (small amount of extra air to the engine, so increase momentarily in power), but I have little trust in the engine currently (until it takes me a few thousand miles without playing up), so I'm suspicious the one way valve is duff, the servo is on its way out, etc......

I'm running a 1700 W case, with Mallory unilite (12/28 timing, thus may make more of an impact having a little more air compared with standard retarded idle timing?) DRLA 36's on CB manifolds, 003 type autobox


can anyone join me in this paranoia or at least say theirs does the same thing?

[vdubyah73]

Normal. Servo uses stored vacuum, so to speak, to help apply brakes. so the servo asks for vacuum when you apply brakes and then asks for more vacuum when you release, to restore the vacuum that was just used. It's not normal if idle goes up and stays up when you hold your foot on the brakes.

[fido]

I should clarify.. it blips up then returns to normal 800 rpm idle when I either put foot on the brake, or remove foot from the brake. Its only a small increase and only momentarily. From an old Colin post, it seems this may be an indication of an over rich mixture... I'll go and investigate.

Thanks for some confidence that my booster isnt on its way to the big scrapyard in my shed!!

[vdubyah73]

better to be a bit rich rather than a bit lean.

[Amskeptic]

I should clarify.. it blips up then returns to normal 800 rpm idle when I either put foot on the brake, or remove foot from the brake. Its only a small increase and only momentarily. From an old Colin post, it seems this may be an indication of an over rich mixture... I'll go and investigate.

Thanks for some confidence that my booster isnt on its way to the big scrapyard in my shed!!

It sounds like engine is running excessively rich.
Colin

[fido]

it was indeed. Makes sense I guess that joining all 4 manifolds would make a difference to the tuning of the engine. The lean best idle routine had me screw the idle mixture screws back in to around 2.5 turns - were at 4 previously in independant manifold mode. I might drop down an idle jet size as 2.5 turns is not much for a dellorto (it has twice the thread pitch of weber carbs, so this is equivalent to 1.25 turns on one of those).

Anyways, its still a little rich, but the brake-induced mini surge is gone :)

[Amskeptic]

it was indeed. Makes sense I guess that joining all 4 manifolds would make a difference to the tuning of the engine. The lean best idle routine had me screw the idle mixture screws back in to around 2.5 turns - were at 4 previously in independant manifold mode. I might drop down an idle jet size as 2.5 turns is not much for a dellorto (it has twice the thread pitch of weber carbs, so this is equivalent to 1.25 turns on one of those).

Anyways, its still a little rich, but the brake-induced mini surge is gone :)

Joining all manifolds reduces the back pulses that normally travel back up the manifold when the intake valve closes. This fools the carburetors into thinking there is more actual airflow than there is. In racing lingo, this is known as stand-off where the back pulse that occurs when the intake valve closes causes air to travel backwards up the carb. Your carbs are calibrated for stand-off. When you smooth out the pulses with your new brake booster intake manifold damper system, well, a tad rich as you noted. . .

Colin

[fido]

that makes alot of sense. I was thinking the same things from a different direction: when the piston has just sucked in its load, there is now a continuing vacuum in the manifold due to the coupled intake manifolds, so the fuel/air mix continues to be drawn into that manifold at a much more consistent pace than a brief suck every now and again. Therefore, the heavier fuel is likely to move in a more regular fashion, rather than the thin air taking up the emergency call for flow that every now and again...

same things, I think. So, if this is the case, I imagine an idle jet rejigging is probably not a bad idea, and I may even care to look at the main jets too, as I imagine this effect will last beyond the start of the progression phase.

One large benefit I noticed was that on putting the engine back together after the head swap, she fired up just fine and idled smoothly. When I measured the airflow to synchronise the carbs, the LHS carb was pulling just 2 (cubic feet/min?) and the right 7! But she was smooth as silk - no doubt why VW decided to run with this arrangement in the first place!