1972 41 Commander with 427s

I'm looking for a sanity check as I'm a bit baffled with my port engines miss at idle, and pop through the carburetor when I stab it in neutral.  I can usually avoid the pop if I ease the throttle forward, let the hesitation happen, then once on the main metering circuit she runs beautifully.  Up to this point I've just dealt with it.  Its a pain when docking because the throttle response is delayed, but I've gotten used to it.

This past weekend after a 2 hour cruise we approached our slip and as I always do, I bumped the gears in reverse (at idle) to slow down and prep for my stern-in spin maneuver.  This time the port engine stalled as soon as I bumped into reverse.  I had to pump the throttle to get her to light again, and long story short, I could not keep her running for more than 5 seconds in reverse/idle.  If I cracked the throttle she responded just fine - like always.

New this season was plugs, cap, rotor, wires and carburetor rebuild on both engines.  Other engine runs absolutely perfect.

My suspicion is a vacuum leak.  Earlier in the season I attempted to diagnose the off-idle stumble/pop by plugging the PCV port on the rear of the carburetor as I think we can all agree this sounds like a textbook vacuum leak.  I sprayed carb cleaner around the carb and intake and heard no change in idle speed or quality.

I haven't checked timing because it involves removing my air conditioner (don't ask - long story) to gain line of sight to the timing mark.  I plan to do this anyways, but could timing be causing this whole mess?

I'm open to suggestions - I've been working on engines my whole adult life and this one is eating my lunch :(

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A couple thoughts:

vacuum leaks usually have more effect at higher speeds when vacuum is higher, so that's not likely the cause.

the last thing you did is probably the cause. in this case, carb rebuild. check out that carb again.

backfiring is usually a timing issue. if the carb checks out, then check the timing.

just my .02, I may be way off base. 

Thanks Jim - actually vacuum is highest with the throttle blades closed, so at idle speed the engine is at the highest atmospheric differential pressure.  The engine draws air through the path of least resistance.  If the throttle blade is open, then it will get its air through there.  If the throttle blade is closed it will draw from the idle air circuit and just about anywhere else it can get air from (i.e. vaccum leaks).

This issue happened before the carb rebuild and all other work specified.  I did all this work for good measure as I wasn't sure how old all these components were so I wanted to to start from scratch.

Another thing to add is that the carburetor shooters are spraying a nice stream when I pump the throttle, and no leaking observed in the venturis at idle.

Captain John

What type of ignition are the engines using , points or electronic conversion ?  Also what type / brand of coils are you using and what resistance rating  ?  Are you using the original Carter afb # 4157 carbs ?  The idle mixture screws at front of carb should be set to 1 turn out max from fully seated or 7/8 setting which is best for air / fuel mixture on the idle circuit .

Tim

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Hi Tim - 

My setup is as follows:

  • Edelbrock 1409 carburetor (rebuilt this year)
  • Mallory crab cap, rotor, coil, electronic ignition (all new this year)
  • New plugs gapped per spec

Same exact setup on the stbd engine which runs flawlessly.  Both carburetors idle tuning was performed using the rpm method, then fattened up 1/4 turn after max rpm was reached.  Idle set at ~900 on both engines which is around 650 in gear.  Electric choke adjusted and confirmed fully off once warm.

Lastly, this is not a fuel delivery issue as I have low fuel pressure alarms set at 4 psi at the carb inlet that are not activating.

Timing!

I'm hoping thats it - that would be an easy one.  On a separate note, this past spring I serviced my advance weights by cleaning, lubricating and verifying all springs were in place and functioning.

Tim Miller said:

Timing!

As others have said, verify the timing.

I would also be looking at a vacuum leak. How is the PCV hose and the PCV valve? Does it rattle?

When was the last time you adjusted valve lash. A tight lash setting on an intake valve can cause your symptoms. This issue usually comes on slowly and gets progressively worse. The 427s do not have hardened valve seats unless you have had the heads done so the original configuration with no lead gas does tend to pound out the seats.

I disconnected the PCV hose from the carburetor and plugged the port temporarily - no change.

Heads were done with hardened valve seats 100 hours ago.

I'm going to try spraying some carb cleaner around the intake runners and carb base and check for a change to idle quality.

I will also pull my AC so I can get a timing light on the flywheel dot.

Captain John

You mentioned a re-build of the Edelbrock carb . Did you replace the check valve located at the bottom of the accelerator pump chamber ?  That is the main cause for a weak pump shot , hard starting cold or hot issue on many engines . I have seen many guys replace Carter carbs after a re-build and they never new about the pump check valve that keeps fuel flow / volume correct for cold / hot starting . The Edelbrock carbs should be checked for that item .

Most carb repair kits do not supply that check valve . 

You also mentioned you can ease into the throttle ok and  deal with the issue . That procedure uses the accelerator pump chamber . 

What brand of electronic ignition are you using ?

Tim

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sounds like a wire backwards, check firing order. Dave

Hi Tim

The only check valve in the 1409 accelerator pump circuit that I know of is located just prior to the shooters (highlighted in yellow).  These are included in the kit and I used them.  If I snap the throttle I get a good squirt out of them.  I don't think this issue is fuel related.

I am using the Mallory electronic ignition on both engines - stbd runs perfecto!



Tim Toth said:

Captain John

You mentioned a re-build of the Edelbrock carb . Did you replace the check valve located at the bottom of the accelerator pump chamber ?  That is the main cause for a weak pump shot , hard starting cold or hot issue on many engines . I have seen many guys replace Carter carbs after a re-build and they never new about the pump check valve that keeps fuel flow / volume correct for cold / hot starting . The Edelbrock carbs should be checked for that item .

Most carb repair kits do not supply that check valve . 

You also mentioned you can ease into the throttle ok and  deal with the issue . That procedure uses the accelerator pump chamber . 

What brand of electronic ignition are you using ?

Tim

----

Captain John

Yes you are correct for the Edelbrock carbs you have  .  The Edelbrock design did change the accelerator pump discharge circuit from the Carter design .

Tim

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