Stalling on braking

[JohnFol]

Afternoon all, some progress towards silky smooth running of my '79 spyder but could do with some help on "where to look next".

Main problem is the engine runs a bit lumpy and cuts out under braking. Brake servo tests ok and disconnecting vacuum from intake manifold causes the engine to die.

• Car fires up instantly (has new WOSP starter, magnecor leads, battery, cables, Huco pump. . .)
• New coil but some question about exact model number / using with or without ballast
• Cold idle a bit high as I found the screw to throttle was missing and my replacement needs some adjustment
• Hot idle "ok"
• cannot adjust mixture as screw blanked off
• Tank drained of old fuel and partially filled with fresh super unleaded
• floats checked, reset, and new carb gaskets
• new cam belt

Happy to go back to basics such as timing alignments / distributor etc. but reluctant to add in more variables. Any thoughts, ideas, suggestions welcome

[Neil-yaj396]

Does it stall as soon as you use the brake pedal (in motion) or when you come to a stop?

[JohnFol]

When coming to a stop. I get enough notice to either coast to a halt or try and recover it.

[Nigel]

Hi John,
I think there's something going on inside the brake servo.

I suggest you disconnect the vacuum pipe from the inlet manifold, blank off the manifold
spigot, and do a road test, [remembering that your brakes will be compromised].

It doesn't sound like fuel or ignition to me.

Nigel

[Neil-yaj396]

I was going to say that it does sound like fuel, the dreaded micro dirt in the carb, but certainly worth doing Nigel's test in the circumstances.

[JohnFol]

I had the engine running and basically "jumped" on the brake pedal several times and the engine note went down. Also noticed as the engine warms the issue is worse.

Neil, coincidentally I took both jets out just today, cleaned them and also stuck a bike foot pump via suitable nozzle into the holes. Also stuck on new spark plugs yesterday.

Will be trying the idea of blocking off pipe to brake servo as this would rule out / identify one more thing.

As a side note, I found out vacuum advance only moves with a bit of direct force via a screwdriver, and the distributor is seized in place so cannot be adjusted

[JohnFol]

Hi Nigel, partially tried your suggestion  . . .

Disconnected vacuum pipe at servo end and blocked it. Engine fires and runs. Unblocked it and engine dies instantly.

Not sure if this helps prove or disprove anything

[Nigel]

Hi John,
Your partial test has shown that a massive vac leak will cause your issue.

My plan is to totally isolate the braking system from the engine, eliminating
any possible vacuum leak when the servo push rod and internals are in
any part of their operation.
[the servo is a fairly complex unit, seals and valves, an inlet air filter, etc]

Therefore, remove the vac hose at the manifold,and blank that spigot.

Again, 'be wary of heavy brake pedal during test'.

Hope you get a result of some sort.

Nigel