Relevent specs: 312, modified b intake, 1" phelonic spacer, Demon Jr Carb, MSD dist. (connected to ported vacuum), MSD ignition, MSD adj. timing control, PCV valve connected to carb

So here I go:  I have a hesitation with the car when the vacuum drops below 12, it clears up at about 6-7 (which I would assume is the power valve opening).  Speed doesn't matter, gear doesn't matter, RPM doesn't matter.  Always the same thing.  Accel hard, Accel soft - either way it bogs in that range.  I'm pretty sure my damper has slipped so no good for timing.  I've checked TDC with 1 cyl and a screw driver.  Set timing using both vacuum and RPM.  I've got it set now at about 21 vacuum at 700 or so RPM.  Timing control allows me to adjust on the fly, so I've checked a wide range of settings w/ no change.  The only thing I'm not 100% on is the springs/bushings in the dist.  But I didn't have this problem before I slotted the intake and changed the carb from a street avenger, so assumed that the vacuum/mech curves were ok.

- John

John: Lake Forest, IL



'57 Fairlane 500 - Looking better every day.

- John

John: Lake Forest, IL



'57 Fairlane 500 - Looking better every day.

Ideas from my own Demon Jr (a 525) -

Needle and seat assemblies that came with it didn't like the Tucson gas and I changed them out for a pair of .130 stainless parts from Holley. Hoosier was right - fixed sticky problem immediately - keeps bowl levels solid. Added bigger FE style fuel pump.

Flipped the carb over and made sure the transfer slot position relative to the primary throttle blade was exactly like the picture in the Demon literature (the remaining opening looks "square"). Made sure the choke was absolutely open and off the high idle cam - then adjusted the low idle speed with the secondary throttle blade (with the engine running). There is an Allen Hd set-screw on the choke side that you have to access from the front with a long (really long) hex key wrench. 

Reduced accelerator pump squirter to .025 from .031 (made no change so I left it - uses less gas). Figured it might be going too rich for the amount of timing I had in the motor at low revs/low vacuum.

The bird always starts in second (stock Fordo set-up) - which meant drive it like you've got an egg under your foot to avoid a bog (Demon primaries are big bore). Starting in "low" no problem. Raising the primary float level made a big difference in this - on mine - dunno about others. Built more low end torque is all I can figure. Amount I raised it was very slight so a little must go a long way.

Good Luck - Steve

- forgot one other change - put "Black spring" (stiffer) in vacuum secondary housing.......  

 
 Steve Metzger       Tucson, Arizona

I'll have to go through that list of checks.  The one thing that strikes me though is the secondary springs.  I had problems with that when I had the street avenger carb.  My Demon Jr. didn't come with the spring selection so I'll need to order them. 

Thanks as always for the assistance!

 - John

John: Lake Forest, IL



'57 Fairlane 500 - Looking better every day.

As a general rule, Demon carbs like considerably more initial ignition timing than other carbs.  This has to do with the leaner or better emulisified idle mixtures associated with these carbs.  Don’t be afraid to bump the initial timing to 15° BTDC or more and give it a checkout drive.  If that helps quite a bit, then shorten up the ignition lead or curve in the distributor so you have less total timing available while maintaining more initial.

I have a Road Demon Jr. 525 on my 272 and runs just about flawless while providing good fuel economy.  And like Steve mentions, float level is important.  I run mine with the fuel level in each bowl essentially in the middle of the sight glass.  Running a lower float level raises the rpm or vacuum signal it takes to start and maintain the fuel flow into the main fuel circuit.

Be forewarned that many of the Demon carbs have a lazy ported vacuum signal which will both delay and give an inadequate amount of vacuum to the distributor under light cruise situations.  This can also create a hesitation issue.  I’ve remedied this issue in these carbs by simply drilling a new ported vacuum pickup hole located just above the primary throttle blades at idle and then putting a new external vacuum hookup port at the outside of the carb for a distributor connection.

Lorena, Texas (South of Waco)

As a secondary note, there was a mention of starting in 2nd gear in an above post.  I have a FOM/FMX so I start in 1st.

I think I'm going to have to pull my msd distributor and revisit the curve / total advance that I have set.  I was messing with timing the other day (w/ my timing control,) troubleshooting and I had the car running really well at idle / WOT.  However then the car had a really hard time turning over / starting.  Could be the problem all along?

- John

John: Lake Forest, IL



'57 Fairlane 500 - Looking better every day.

Although I’ve modified several Demon carbs for an improved ported vacuum signal, this is something that’s not done lightly and without experience with carbs. For myself, it’s about an hour job and I’m considered pretty quick on these types of things. Although I have no pictures of that particular modification handy, the pictures at the following link will give a rough idea of the ideal hole placement. The fifth picture down in that thread shows the ported vacuum hole to which I refer.

http://forums.y-blocksforever.com/Topic23772-3-1.aspx

Once the hole is drilled in the appropriate location, a variety of methods can be employed to get the vacuum path to the outside of the carb where a hose can be hooked up to it.

.

But I have to ask first what is your initial timing set on and what curve attributes do you have in the MSD distributor? What spring combination and what color advance stop bushings are you using? A good starting point are the two light blue springs and the blue bushing. Your description sounds like an excess of initial igntion advance.

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And the next question is which port on the carb are you hooking the distributor to? On the Demon Jr., there are two closely spaced nipples on the passenger side of the carb with one of them being direct vacuum and the other being ported vacuum. If memory serves me correctly, the most forward nipple is for the ported vacuum signal; this is easily verified with the carb off of the vehicle and tracing the circuitry. But in your case, I’d suggest hooking a vacuum gauge to each port with the engine at idle and the carb connection that has no vacuum signal will be ported vacuum. If both nipples have vacuum, then the throttle blades are too far open at idle and that’s a symptom of some other issues taking place if the engine is not idling too fast.