Starting procedures

Starting your engine equipped with IDA3C type carburetors is not difficult assuming the engine, ignition and carburetion systems are in good working order.  Triple throat Webers do not have a classical choke feature, they rely upon the accelerator pumps to squirt raw fuel down the throttles to provide an enriched fuel mixture for cold starting.  The following procedure assumes the squirter nozzles are functioning correctly and delivering something like 0.7ml per nozzle per stroke of the throttle lever arm.  If they are not functioning correctly then work must be performed to correct the issue.  Also assumed is the ignition system, engine and carburetors are in good condition and tuned correctly.

The following procedures are offered as guidelines for starting your engine.  Each engine is unique in its own starting requirements so adjusting these procedures to suit your engine are recommended.

Cold starting

The Weber triple throat carburetors as used on the Porsche 911 engine do not have a choke device, they rely upon the accelerator pumps to squirt raw fuel down the throttles to provide an enriched fuel mixture for cold starting.  The following procedure assumes the squirter nozzles are functioning correctly and delivering something like 0.7ml per nozzle per stroke of the throttle lever arm.  If they are not functioning correctly then work must be performed to correct the issue. 

When started from cold, fuel will not atomize or combust very well resulting in a “lean” condition although the mixture will be good once the engine warms.  Spitting up through the intakes is typical after a cold start.  Once the combustion chambers warm up a little, atomization and vaporization of the fuel mixture becomes more fully developed thereby creating a more combustible mixture.  AFR readings will be in the 17s (a lean condition) and after a minute of idling the AFR will be 11.5, this is without adjusting any setting on the carburetor.

Each engine is unique in its own starting requirements so adjusting these procedures to suit your engine are recommended.

• Turn ignition on

• Let fuel pump run for 5 to 10 seconds to assure fuel bowls are full.

• Fully depress accelerator pedal three to four times in a methodical but quick actuation and maintain full throttle for two seconds after each depression to allow time for the accelerator pump to complete the priming squirt. 

  • Rapid pumping of the throttle pedal does not achieve the same end; the two second hold time between pedal depressions allows the spring in the second chamber of the accelerator pump body the necessary time to displace the fuel out the squirter nozzles above the main venturis. 

  • A quick pedal depression is preferable as you want the squirted fuel to enter the intake manifold, a slow depression results in the squirt hitting the partially opened throttle valves or the wall of the main venturi.

• Wait approximately 3 seconds with the throttles closed which enables the fuel in the intake tract to evaporate, liquid fuel will not combust without evaporation.

• Initiate cranking while slowly depressing pedal about 1/10 of travel.

  • Larger throttle openings decrease intake vacuum which is needed to draw fuel from the float bowl into the idle/progression circuit, the engine will not idle until this is achieved.

• The engine should fire and come to an idle, but a little ‘nursing’ or additional ‘blipping’ of the throttles may be necessary to get a little more fuel squirted into the throttle bores before everything settles down and begins to run smoothly.

• If the engine stutters without starting then it will have burned off the "prime" so repeat quick pedal depression one time, allow 3 seconds for evaporation followed by the cranking procedure.

• If too much pedal actuation occurs without the engine starting, then it may be flooded with too much un-burned gas.  To clear this situation, hold throttle to floor (without additional pumping) and crank to allow excess fuel to evaporate and try starting again with one quick pedal depression followed by opening throttles to 1/10 travel while cranking.  If the engine starts while clearing the flooded condition, then be ready to release the pedal to prevent over-revving the motor.

• When it does catch, hold the idle to 1500-2000 RPM while gently "massaging" the throttle (give it little squirts from the accelerator pumps) until it smooths out and idles on its own, about 10 seconds during this phase.

• Start driving immediately, don’t let it sit and idle like your Honda.  Keep RPM below 3500 RPM until the temperature gauge begins to indicate the oil is warming up.  The main reason to drive upon starting is to minimize fire issues in the air cleaner due to fuel vapor and spitting up through the intakes which could ignite this vapor, driving will efficiently snuff out fires in the intake tract.

Hot Starting

Restarting after the engine is warm from recent running may be challenging; this is the result of today’s fuel having a lower boiling temperature than the fuel of 50 years ago.  High engine compartment temperatures combined with heat from the cylinder heads being conducted via the intake manifolds to the Webers may cause the fuel in the float bowl to boil or “percolate”.  Hot ambient temperature and high altitude magnify this boiling which may result in fuel escaping the carburetors and flooding the cylinders with fuel or worse, dripping onto a hot exhaust header which may cause a fire in the engine bay. 

Additional issues contributing to difficult, hot re-starting are:

• Webers that have been modified using the PMO drilling feature for “anti-percolation” cause the idle circuits to flood with percolating fuel escaping the float bowl via these drilled holes.  A flooded idle circuit will require excessive cranking of the engine to clear the flooded situation before the engine can restart. See information regarding an alternate solution for the PMO modification.

• Heat that is conducted into the Webers from the heads through the intake manifolds will result in fuel in the galleries feeding the accelerator squirters to percolate in advance of percolation issues within the float bowls.  The fuel in the galleries will be discharged via the accelerator squirter nozzles, directly down the throttle bores.  This contributes to a flooded situation which must be cleared by cranking the engine before the engine can start.

• The fuel in the galleries feeding the mixture screws from the idle jet will drain raw fuel into the throttle bores each time the engine is shut down. 

Mitigating actions for percolation issues are provided following the hot re-starting procedures below:

 Hot re-start after a one-minute rest time: 

• Turn on the ignition and immediately crank the engine while slowly opening the throttles to fully open.  It should fire after about five seconds of cranking.

• When it fires, rev the engine a couple of times and immediately drive.

 Hot re-start after a 15-minute rest time: 

• Turn on the ignition and immediately crank the engine while slowly opening the throttles to fully open.  It should fire after about five seconds of cranking.

• Allow engine to rev to redline and rev it to high RPM several times and immediately begin to drive it.  Continue driving using high RPM and full throttle for the first few minutes after the re-start.  This is to prevent a “backfire” past the intake valves which could cause a fire in the air cleaner.

Mitigation actions for hot restarting:

• Install insulator plates between heads and intake manifolds.  By installing longer studs, it is possible to install multiple sets of insulator plates.  The limit to how many may be installed is headroom in the engine bay for installation and removal of the air cleaner housings.  Three sets is a reasonable maximum stack of these plates.

•  Incorporate fuel delivery system revisions such as:

  • Fuel pump installation on front cross member

    • This avoids fuel percolation issues in the fuel pump if located in a hot engine bay

  • Return to tank fuel line installation

    • This avoids fuel percolation issues in the fuel lines located in the engine bay

  • Incorporate use of a “Throttle Stick” to depress throttle pedal when parking.  See the below discussion regarding this action.

Throttle Stick for hot re-starting

My contribution to the issue of hot re-starting is to use a “Throttle Stick” to hold the throttles Wide Open after shutdown.  The purpose is to allow fuel that enters the throttle bores from percolation within the fuel galleries feeding the accelerator squirters to evaporate instead of ponding on top of the closed throttle valves and eventually seeping into the throttle bores where it is unable to evaporate thereby creating a rich combustion tract during re-starting efforts.

Typically, hot re-start procedures (above) are effected by slowly opening the throttles to wide open during the cranking process.  This is intended to clear the flooded situation caused by fuel escaping from the fuel galleries feeding both the accelerator circuits and the idling circuits in addition to float bowl percolation.  Opening the throttles adds more fuel to the engine resulting from accelerator squirt contribution.

The recommended procedure to for mitigating hot re-start issues is to use a “Throttle Stick” to hold the throttles wide open at shutdown to allow for fuel evaporation:

Shutdown for “Throttle Stick” procedure:

• Come to a stop and set handbrake

• Rev engine to 3000+ RPM

• Immediately kill ignition and simultaneously, fully depress throttle pedal to the floor

• Insert “Throttle Stick” between top of throttle pedal and back side of lower dash

Re-starting for “Throttle Stick” procedure:

• Remove “Throttle Stick” while maintaining throttle pedal fully depressed

• Crank engine a few revs (3 to 5 seconds) with pedal fully depressed

• Reduce pedal depression to about 10% to 15% open and continue to crank

• Depending upon engine response, adjust pedal “judo” to re-start

Theory of procedure:

By revving the engine to 3000+ RPM before shutdown, you will have discharged some of the available fuel from the accelerator pump and it will be burned by the running engine.  Then, by shutting down the engine and flooring the pedal you will completely discharge what remaining fuel there is to be delivered from the accelerator pump.  That fuel will be quickly atomized and evaporated by the spinning of the hot engine while at wide open throttle.

The “Throttle Stick” will then maintain the throttles at fully open which will help atomize and evaporate any additional fuel that percolates into the throttle bores.

When re-starting, the fully open throttles will not activate any slow speed circuit or accelerator circuit fuel contribution thereby assuring a clearing of excess fuel from the intake tracts.  Once a few revs at wide open throttle have been completed, the throttles may be closed, and a more normal starting procedure may be attempted.

NOTE: Heat soaking may purge the fuel from the accelerator galleries so a couple of pedal depressions may be required to re-charge them before they regain effectiveness.