Ford 4100 carb issues...fluctuating RPM

COE Dan

COE Dan

Senior Member
In my Falcon, I have a newly rebuilt 1966 289 V8 (<2000 miles) with a Ford 4100 carb. The engine and setup is stock. I have several problems:

1) I cannot lower the RPM below about 650. Ford specs say 450-500. The hot idle adjust screw disengages from the throttle stop before 500 RPM is reached.

2) I've hooked up a vacuum gauge at the vacuum reads 16-17 psi in neutral and 14-15 in gear (auto trans). Ambient air temperature during this test was abou 93F at sea level.

3) There is a rythymic 1/2 psi drop ever couple of seconds. This happens at all speeds (idle through max). It is usually not noticeable under acceleration but only at constant RPM idle or cruise. When driving it feels like the engine hiccups.

4) The indications on the vacuum gauge are not consistent with a vacuum leak (not enough of a psi drop)

5) At idle, I can plug both primary venturis and the secondaries are open sufficiently to provide enough fuel and air to maintain idle RPM and keep the engine running.

6) I've checked timing (8 degrees BTDC); dwell (30-31 degrees at idle).

7) The card does not show signs of leakage. However, the interior of the intake manifold appears to get wet with fuel. The gasket below the carb spacer above the manifold is wet with fuel and fuel is seeping out from it.

What I need to know is:

a) what is causing the hiccup?
b) should the secondaries be able to power the engine like this?
c) is fuel siphoning off the secondaries and powering the engine so I can't get the idle on the primaries low enough?
d) does the seepage of fuel indicate fuel entering the intake from the secondaries and accounts for the hiccups?
e) normal vacuum should be 17-21 inches - could 15-17 be normal at high ambient temps?


Just looking to see how to fix this...any help is appreciated.

Dan
 
5) At idle, I can plug both primary venturis and the secondaries are open sufficiently to provide enough fuel and air to maintain idle RPM and keep the engine running.
Click to expand...

This to me indicates a vacuum leak of sorts in the carb or intake. The secondaries should be closed and when you cover the primaries the engine should lower rpm and quit. Something is not right with the rebuild, Did you have a shop rebuild it?
 
Adjust the secondary plates to shut but not tight enough to stick then set the idle with the primaries.

Check for a leaking power valve or power valve gasket.
Remove the carb and remove the power valve cover and see if it drips.
It may take a while for it to show up as it has vacuum pulling on it causung the leak to be more severe when idling than when just setting.
 
Good tips so far! Plus, if the rebuilder took the throttle plates loose from the throttle shafts, he may have not centered them properly in the throttle bores. Back off the idle speed screws to let the plates close completely and hold up to the light. You will see any mismatch. Loosen the throttle plate screws slightly and futz the plates into position and retighten the screws.
 
Thanks all - several good ideas to try. The next step for me is to pull the carb.

I did not do the carb rebuild on this one...kind of wish I had, then I'd know what I'm dealing with. I've rebuilt 2 Strombergs and after trying those, figure I could have done a decent job with the more complicated 4 bbl.

Dan
 
Dan
Don't hesitate to tie into it.
The Ford 2100 and 4100 carbs are probably the simplest carbs out there.
Very few places to go wrong with them.
Be sure the power valves are good, get the float level and drop set right.
The hardest part is getting the external choke and fast idle linkage adjustments right.
 
Some kits had incorrect power valves in them and wouldn't seat properly. Also check your vac lines to the trans vac modulator (auto trans) and the vac advance unit on the dist.
 
Remove Carb
Back off idle speed screw
Adjust throttle plates so that they close completelt
Adjust idle speed screw until the plates are open 1/2 turn
Turn idle mix screws in until lightly seated
Back out 1 3/4 turn
Reinstall carb
It should idle reasonably well.
Adjust idle speed to about 550 in neutral
Turn one mixture screw in slowly until engide begins to falter.
Back out 1/2 turn
Adjust idle speed if necessary
Repeat mix adjustment on other mixture screw.
Adjust Idle speed if necessary
If not idling properly apply the adage that 90% of all carburator problems are in the ignition system.
 
Check all of the above and also check power brake booster and line to booster, pcv hose and auto transmission vacuum line for leaks , if problem still exists take w-d 40 and spray around intake gasket and carburetor base at idle.
Good Luck!
 
Just one additional thought, while you are spraying carb cleaner
at the gaskets, be sure to spray around the throttle shafts, my 2100
just had idle problems and it turned out to be a leak around the
throttle shaft.
Larry
 
If you need further help on the 4100 you may want to contact Alex Denysenko @ MoneyMaker Racing. He is an absolute guru on 4100 carbs. He runs a 65 K-code Mustang in Super Stock drags and is required to run a 4100 carb. He currently has the 'stock' 289 mustang going in the 10's on the qtr mile and is able to pull the front wheels. He is a moderator at mustangsandmore.com website.
 
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