blown SBC with too little fuel flow below 3k rpm

In a 29 A Tudor streetrod, I run a 355 SBC with a Weiand/Holley 250 blower and two boost referenced 600 cfm Holley 4150 carbs. Timing is set at 16 initial and 34 total. At idle and up to almost 3000 rpm the only fuel flowing from the carbs into the engine comes from the idle circuits, and the PRIMARY boosters don't even start to flow until 3k rpm. The result is that the engine runs way too lean up to and including freeway cruising rpms. At 2500, A/F ratio is 14.0. Idle in neutral at 1400 rpm yields 5 % CO and in drive at 900-950 yields 2 % CO. My initial thought was that the carb venturis are too big, so that manifold vacuum is not pulling fuel through the carbs. However, Holley recommends two even bigger 750 vaccuum secondary carbs when used on a 350 with the 250 blower. So how do I increase, below 3000 rpm, fuel flow from the primary boosters in these 600 boost referenced carbs ?


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Are these blower specific carbs or are you trying to make something else work? What are the carbs list #'s?

Have you done any work to the idle feed restriction or any of the air bleeds?

You don't want fuel coming from the boosters at idle or cruise rpm or it will be pig rich. At cruise you should still only be on the idle/transition circuit.

The carbs are built by Holley for use with superchargers and bear Holley part number 0-80575S. I have made no changes in these carbs as they came from Holley, so no alterations in air bleeds, the idle circuit, etc. The only clue I have, beyond the data I have reported, is that these carbs are calibrated for use with a 6:71 blower on a SBC while I am using them with a 4:71 ( the swept volume of the Weiand/Holley 250 is comparable to the 226 swept volume of an original Roots 4:71 and well below the 330 or so swept volume of a 6:71). Given the larger swept volume of the 6:71, I would not think that
calibration for a 6:71 would cause the particular problems I am experiencing below 3,000 rpm with this 4:71. Also, we have checked the engine vacuum at and below 3,000 rpm and no vacuum leaks appear.

What makes you feel it is too lean? Does it have a drivability issue at cruise rpm?

I would think that your cruise A/F would be about perfect at 14.0.

Make sure the accelerator pumps are adjusted to .010 of an inch, on both carbs. The rear carb is the primary carb at idle through, say 2 grand. This could also be a problem with the pump lever eccentrics. Both carbs may require a pink, or blue eccentric, but the fine tuning is an art.

We need to know if both primary floats are adjusted, and what jets are in the front jet plates on each carb, plus which pump cam eccentric (most likely white), and the clearance between the pump rod on, and pump on each carb are at now.

At idle, the headers begin to glow red within minutes, even when the engine water temp otherwise is at only 180 degrees. Glowing headers usually evidences in a blown motor either not enough initial timing (we currently are using 16 degrees initial) or too lean a fuel mixture. On some pipes, we are seeing temperatures of over 1450. As for the helpful questions about the accelerator pump and jettings, I will not be able to provide that info before Monday. Thanks, by the way, for all of the assistance from, and excellent questions by, to all who have joined this thread.

Idle mixture screws is what controls idle and up fuel mixture. What do the plugs look like? With a blower motor going by header color is not a tuning method, plugs are the golden rule.

I'm suspicious of the power valve circuit- either the opening point being incorrect and/or the vacuum supplied being incorrect.

The power valve needs to activate as soon as you are into the throttle and the engine is under load. Depending on what the PV "sees" you will need to choose their opening point accordingly.

The boost referenced PV ports needs to be supplied with manifold vacuum from under the blower. The vacuum supply to the PV's should drop with rapid throttle opening and when the engine sees a load- just like on a NA set-up.

If the referenced PV ports are plumbed wrong, it may be seeing pressure not vacuum or not enough vacuum, depending. If this isn't correct, the A/F mixture will be LEAN.

The engine needs to be under load to determine what the real A/F ratio is at various RPM points.

A couple other thoughts are:

If you raise the float level, this will initiate an earlier transition from the idle circuit into the mid-range, etc.

Be sure the fuel pressure is 5 to 5-1/2 psi while under boost.

Larger main jets may be needed.

If you have adjustable "jets" for the air bleeds, try smaller ones or restrict the orifice in some other way.

A temporary "fix" to see if this will help can be fashioned from a small piece of wire to act as a restriction to the air bleed. Another non-permanent way is to use epoxy and re-drill smaller holes w/a pin vice.

Once the correct size is found, the same thing can be duplicated w/solder for a permanent fix. This probably won’t be needed in your case. Before doing this, check the PV.

The typical 0.110-inch-diameter needle and seat can be replaced with as large as a 0.150-inch assembly. Again, this probably won’t be needed in your case.

Hopefully this will give you a couple things to look into.

Good luck!

Headers glowing red means too retarded timing or too rich , not too lean. The "lean makes headers glow" is a common misconception. The headers are glowing from a rich or retarded situation that is burning late into the combustion cycle and out into the header pipe. This can also be aggravated by a too tight lobe seperation angle on the cam allowing the blower to blow some of the incoming mixture out the exhaust during overlap.

I have always tuned the carbs to run the exhaust temps, at most 1350 degrees, at wide open throttle under load. I also like to set the timing, at a locked out base of 36 degrees, or so. Are you running a 'boost' retard? If so, what is it set at?

I would suggest increasing the main jet's by, at least, 2 sizes. This may help, depending on your 'quench', and initial timing............tune up. Quality/Octane of fuel will also effect this. You may need to run, or mix 50/50 higher octane fuel, depending on your 'quench'. What is your final compression ratio?

Using Ross forged blower pistons, static compression is 7.6:1 which, with a maximum of 9 pounds of boost on the 1.33:1 pulley, yields right at 12.0:1 final compression. Lobe separation on the Crane hydraulic roller HR-230 IG is 112 degrees with advertised duration of 292 intake and 300 exhaust. The aluminum heads are Dart Pro 1 with 72 cc combustion chamber and 230 cc intake runners. Cooling comes from a Walker "S" line 3-3/4 inch thick radiator designed for use with a blown small block and, complete with a full-radiator shroud, a 2800 cfm electric fan. Water pump is an anti-cavitation high flow Edelbrock. Ignition system includes an MSD 6 BTM (boost timing master) which currently is set on "0", allowing the engine to go to full boost. That will be dialed to retard boost as the temperatures warm. Distributor is an MSB Pro Billet 85551 with mechanical advance only.

Confused on your last post. The Boost Timing Master doesn't control the boost, it controls the timing, relative to boost pressure. It should be set to control the timing based on engine parameters, not the weather. You should be retarding the timing some at higher boost levels if this is a pump gas motor.

You might be better served with the timing locked at 32-34 degrees and have the Timing Master pull 3/4 to 1 degree per lb of boost.

With your final 'quench'/'compression', I would suggest some higher octane fuel, all be it a 50/50 mix.(take your pick better than 100 octane) Your boost timing should be adjusted to retard between 1 and 1 1/2 degrees per pound of boost also.

I've been there, and done that!

Yes, that is what I meant to say. It is currently set at zero retard, and as the weather warms up, we will be able to retard the timing as needed for warmer weather under boost conditions. We may also very well dial in a small amount of retard under boost at all times, but will see how it all comes out on the dyno.

The note about thinking we were lean at cruise was due to the glowing header tubes, and what causes that on only the center cylinders on BOTH sides? AF is showing 14.0 under those conditions which should be perfect, hence the confusion as to what is causing those 4 cylinders to have the header tubes so much hotter than the rest.

At this point, I think we will continue with the tuning, as we have yet to get into the power circuits and WOT tuning yet. We were just trying to get the idle and cruise nailed down first.


Thanks for all the help!

retarded timing and/or rich mixture makes the headers glow red.