Hi all.  Subject vessel is a 1972 Commander '41 with mighty 427's and closed cooling.  I am performing some deferred service to hopefully resolve some cooling and drivability issues.  In a separate thread, I de-Bubba'd my raw water cooling systems and finally have proper flow through both engines, each idling at 165F; which is right in spec with the manual.  Now, once I bring her up to 2000 she creeps up to about 175 (both engines).  If i bring her up to 3000 she'll lay over, almost like in a car when you have a backfire out the carburetor and the engine goes flat.  I back off the sticks and ease her slowly back up to 3000 and she'll go.  In parallel, the port engine stays at 175F and the starboard creeps to 185-190F.  Back her down to 2000 and both engines return to 175F.

I wasn't able to have anyone monitor the engines while underway, so I had only my gauges, eyes and ears to collect this data.  I can't be sure if both engines laid over or just one, but as you can imagine losing power on one side while attempting to get on plane probably feels pretty close to losing both.  The boat just nosed down.  The engines never stalled however.

My first thought is fuel system.  I have holley electric pumps, a Racor dual canister system with vacuum gauges for each engine.  The carburetors are marine Edelbrocks.  No sign of an anti-syphon valve on either tanks.  Choke wide open.  Could the secondaries not be opening?  I believe the late Commander Morland says the original carbs open secondary metering around the 3000 RPM mark, but I'm not sure if that applies to the Edelbrocks.

Second thought is the ignition system.  Mallory electronic ignition conversion, with new pickups, caps, rotors.  New spark plugs.  No miss, hesitation or anything on or off idle, no vibration (which is like a jackhammer through those Paragons)...very smooth through the lower power range.

Now on the way home, basking in my glory of my successful raw water impeller replacement and subsequent cooling benefits, it came to me that could my distributors not be advancing?  Could they be stuck at base timing?  If so, it would explain why the engines run so good at lower RPM, but so crappy at cruise.  Also, I wonder if the excess heating I'm seeing is a result of retarded timing?  At 3000 RPM the distributors should be adding like 10 degrees of advance.  If its adding none or a negligible amount, could this be my problem?

I know the next suggestion is to go check the timing on these engines.  That is my plan, but I can't get back to do so until next Saturday, so I'm trying to harness the brain trust of this forum to formulate a plan of attack.

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Captain John

You have described a classic performance issue under load ( peak torque ) with electric fuel pumps and Racor filtration .

If you have original fuel tanks the factory anti -syphon device is inside the tank at the top of the pickup tube .

That is a designed air leak that electric fuel pumps " do not " work with . That along with the unknown fuel flow rating of those electric pumps installed and restrictive Racor filtration is probably an issue you need to consider . It appears that your engines are starving for fuel under load with your current setup . You are heading to uncharted shallow water with your engines needing more fuel flow (  lean / detonation ). The original mechanical fuel pumps were rated @ 120 gph free flow for comparision .

The original Carter AFB carb secondaries "started to open " @ 3600 rpm . The replacement Edelbrocks may open secondaries earlier than the Carter spec .

Maximum timing advance is 34 degrees @ 4000 rpm per Chris Craft / Mallory spec . Full timing advance at 3k rpm ( cruise rpm ) is very risky with todays fuel quality .

You will get a backfire thru carbs @ 2800 -3000 rpm if your valve lash is too tight .004 cold or less . That rpm range is where the engine is getting maximum cylinder filling ( torque ) . I would consider a correct valve lash a priority also .



I agree with Tim. You likely are having a fuel starvation problem.

Couple of things to check.

First, make sure the anti-siphon bleeds are removed and plugged.

Second thing is to check how the Racors are plumbed. The Holley fuel pumps do not like to operate under a vacuum. You should go from the fuel tank to fuel pump to Racors to carburetor. It concerns me if the vacuum gauges are monitoring the Racor filter restriction as the Holley fuel pumps will not develop much suction to even allow the gauges to display a restriction. Likely that the PO plumbed the Racors before the Holley pumps.

You should set the valve lash. Excellent procedure found in the files section. Your 427s likely still have the soft valve seats so you will experience a tendency for the valve lash to get tighter over time and require occasional adjustment. This is a function of hours and rpm.

Your distributors have mechanical advances in them. Two things that can go wrong with the advance, broken springs allowing the weights to be all out at idle and the advance plate getting rusty and not allowing the advance to move. It a minimum you should verify that the idle ignition timing is correct and that full advance is occurring.

After you address all of these issues, the engines are running better, and you still have an overheating problem, you will need to verify that the heat exchangers are not dirty.

Another thing, make sure that you are using the correct engine oil for your flat tappet camshafts. Todays popular oils are formulated for roller cams so they tend to be light on the zinc. Tons of threads on this item. Best bet is to use something like Shell Rotella T or equal.

Secondaries on the carbs come in at 2900 +- on those engines. Just a thought that they are not.. 

Thank you all for the information.  In the spirit of Dick Morland – “This is going to be a long one…”

My plans are as follows (both engines):

  1. Change oil with Rotella T, new filter
  2. Adjust valves.
  3. Check base timing and advance.
  4. Verify secondaries are opening (I know they are mechanical, but the top plate is vacuum I believe)

The jury is out on number 5, which is all things fuel related.  First and foremost, I think there is truth to all the comments made surrounding this topic.

My current fuel system is as follows:

Stock fuel tanks.  Each engine is fed with a set of twin Racor S3232TUL filters (in parallel, rated at a 90GPH each with a clean pressure drop of 0.6psi at max flow) between the tank and the suction side of each Holley Red marine pump.  There are vacuum gauges on the Racor systems to theoretically display delta P across the elements for each engine.  A pressure regulator with gauge is installed between the outlet of each pump and the inlet of each carburetor, with no additional filtration.  All lines and hoses are 3/8” in diameter. 

I concur that these pumps prefer a flooded suction.  What I cannot ascertain through research is a pump curve for those pumps based upon inlet pressures.  That would be handy to determine the net output of the pump based on the static head of the fuel tank. 

What’s more is that Holley and just about every other marine electric fuel pump manufacturer recommends the fuel tank outlet to be at the bottom of the tank.  Following the installation instructions to a tee (no pun intended), electric fuel pumps would be doomed for any application where fuel is drawn off the bottom of the tank and exits the top.

A closing thought regarding the anti-syphon valve.  If it is as it left the factory, some amount of air is drawn into the suction side of any pump – mechanical or electric.  I’d expect that where this is air, there is not fuel, so the total pump output would be proportional to the amount of air introduced to the fuel stream by the anti-syphon valve.  I’ll further speculate that if the pump meets OE specifications for pressure and flow, the pump selection becomes academic. 

I recall reading that Matt Cowles is running electric pumps on his mighty 427s with good results, so I’m inclined to understand the secret to his success. 

Could a quick and dirty diagnosis be as simple as watching the gauges while stabbing the throttles (at port, in neutral) looking for excess vacuum on the Racors, and/or reduced pressure on the pump outlet?

I am all for finding the right answer, and if it is a design flaw with my fuel system, I’ll be the first to make it right.  However, redesigning a fuel system without unequivocally diagnosing the condition not only makes me a parts changer, but it also leaves root cause questionable.     

I appreciate the help with making me a technician and not a parts changer!

Captain John

A quick test would be to pull the fuel tank pickup tube out a bit and plug the air jet orifice and then do a sea trial to check performance under load .  

The original mechanical fuel pumps were rated at 7 psi & 120 gph free flow going to the Carter AFB`s . If you are seeing less than 7 psi under load with your pressure gauge that should help you correct the fuel supply with those electric pumps and the Racors that are installed . 

Be sure to read the SAE rating on the oil you select . Do not use SM / SN rated oils  they do not have the correct antiwear package for your 427`s  . A straight 30 wt of SJ -SL rated oil will be best and never use a Fram oil filter ! 

Did your sea trial produce 4k max rpm ever when you bought your Commander ?

Keep us posted 



You guys have me worrying about yet another gotcha.  Every electric fuel pump I know of is after the main filter, to protect the pump from debris.  What is unique about this one?  Holley has instructions for mounting their pumps close to and below the tank, after the fuel filter.


The air bleed is the next worry.  My best friend has been restoring and building boats for over 40 years, hasn't had a problem with the bleed on carbureted engines with electric fuel pumps, does have problems with fuel injected engines, and has to take out the bleed port.  This is over thousands of old boats, many of them get electric pumps.  But these are mostly older wood boats with newer engines, if you guys say this is a problem on Commanders, I need to be ready to address it.  Plus, it just bugs me to have that air bleeding into my fuel pump.  Air bubbles can quickly destroy impellers on big industrial pumps, I don't want that possibility.

So, this is the pickup from an older Chris Craft woody, what is the thread on this bleed screw?  Tapered or straight?

Sorry to instill widespread panic among the Commander community!  My worst fear is blowing my mighty 427s due to something totally preventable.  I want to let them eat, but I’m not totally convinced that my fuel system is poorly designed. Partly because I’m in denial (haha), partly because everything that was done by the previous owner was meticulously executed.

Exhibit A is my fuel system: 

You fuel pump should always be BEFORE the filters. Electric fuel pumps push, they can’t pull, and I can’t imagine how that setup isn’t starving your carbs of fuel. For example, even in a perfect world, the red Holly pumps pushes 7psi, and those filters have .95 (or thereabouts) psi drop, so that’s no better than 5psi at the carb. Even if you switch to the Blue pump, you’d have to regulate it down to not more than 10psi for those filters, and with the psi drop, that puts you at about 8psi at the carb which is too much. IIRC, those Racor housing you have max out at 90psi, you need the next size up that gets you to 120psi (they only have one inlet/outlet).

After years of wrestling with this and talking with Racor, Edelbrock, Holley, and the smart guys here, there’s really only one setup to get the right flow:

- Shutoff on tank
- 100 micron Holley billet filter
- Anti siphon valve
- Red Holley Marine Pump
- 120psi Racor housing (w the filters you have, but I wouldn’t double them)
- Carb (put a pressure gauge as close to the carb as you can)

This get you just about 6psi consistently at the carb, which is what Edelbrock (or even the Carters) want. This setup does create a situation where you have to clean that screen inside the filter once or twice a season, but it’s really the only way to balance psi and fuel supply that the 427 needs.

Hope this helps
Also, the thing you have to remember about those Holley instructions and filtration...they’re talking about a 100 micron before and a 40 micron after. Those Racors are 10 EACH, so you’re likely getting a lot of cavitation out of those pumps.

This is great information. Thanks for chiming in Matt. For the sake of not messing with too much, if I were to rearrange the flow direction and route the discharge of the Holley Reds to the existing Racors, what is the issue with keeping the housings if I am running two in PARALLEL.  If I am reading the filter spec correctly, the pressure drop across those filters are at full flow. In my arrangement I have a total of 180 GPH of filter area at that the same delta P.  Also, with the anti-syphon, did you plug the factory one and place an aftermarket one between the 100 micron filter and the pump inlet?

Edit:  The 660RACC02 flow 60 GPH, not 90 GPH.  I have two in parallel (not series) so the total throughput is 120GPH, which is equivalent to the single larger unit you have.

Do not confuse good craftsmanship and workmanship with proper design. The system as it is would work very well if you were using the mechanical fuel pumps. Also do not forget that your raw water pump had a worn out cam plate. Meticulous care can only go as far as what there is an understanding of. There is also the issue of a thorough understanding of the total application design issues.

The issue with many electric fuel pumps and the Holley fuel pump in particular is that the pump is not designed to operate under a vacuum and in particular a vacuum with a fluid that contains air bubbles.

There is an engineering relationship, NPSH, of pump head pressure to suction pressure that accounts for the vapor pressure of the the liquid being pumped. Holley does not publish these curves and I suspect for good reasons in that the curves would be very very bad. It appears to me that the Holley pumps and other similar designs have impellers and impeller housings that simply have no design consideration for operating at a vacuum. They are designed for a limited application designed to a specific sales point.

The Holley pump design is very sensitive to air bubbles in the fuel. What happens is that the air bubbles collect at the impeller disc. Once the impeller disc is covered with air, the impeller is air locked and it will simply stop pumping until the pump head pressure drops low enough to allow the air bubble at the pump disc to dissipate or go through the impeller and start moving liquid once again. Unfortunately when this happens with an engine, the engine will exhibit fuel starvation but never suffer a total lack of fuel.

The quantity of the air bubbles present is not a proportional relationship and pump flow. It is more of a logarithmic function with a very sharp cutoff. It is also important to not confuse the air bubbles with cavitation issues.

One thing that would help but not totally alleviate the problem would be to mount the pump so that the motor armature is horizontal, inlet looking down and the outlet looking up with no P trap loop present in the pump intake line. Yes, this is rather impractical but it is technically a partial solution. Eliminating the air bleed needs to be the first thing on the list.

There is also really no good reason to be using a fuel filter of less than 40microns on a carburetored engine. The 100 micron strainer is also adequate to protect the Holley fuel pump from debris. 100micron is basically a strainer, not a filter and will tolerate the accumulation of considerable debris before blinding off or significantly reducing flow.

This discussion is a good example of people adopting specific designs without an understanding of the overall design requirements and these designs becoming adopted as acceptable. Just because it is commonly found or done does not make it correct.
John Mario said:

Sorry to instill widespread panic among the Commander community!  My worst fear is blowing my mighty 427s due to something totally preventable.  I want to let them eat, but I’m not totally convinced that my fuel system is poorly designed. Partly because I’m in denial (haha), partly because everything that was done by the previous owner was meticulously executed.

Exhibit A is my fuel system: 

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