VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 1925 engine idle woes

so looking into advice on my 1925 engine series K.
ill try to be as detailed as i can be. also sorry if terms are wrong or i'm missing something obvious. i am brand new to cars and engines with this car and just started learning about 3 months ago.

got the car non running (was fuel related and remedied)

the engine runs well and idles a bit high with a slight loss of power every 10 or so seconds only at idle. i have it set high so it wont stall. if i lower the idle screw it is just about right on the speed/rpm but that loss of power every 10 or so seconds will cause it to stall. it runs better warm but does not remedy the issues.
i seem to grind first gear way too often with the higher rpms to be happy with where it is set.
i cant retard the spark all the way without it stalling to test the RPM to set it at about 300. at full advance it gets about 600RPM using method in the manual (12 seconds timing = 60 lifts of valve arm) so 60x10=600


i've tried nearly everything besides pulling the pistons.
compression is good, new head gasket, new oil, rust flushed from engine and radiator. carbon scraped from cylinders when head removed.
spark is good. tried Autolite 3076's which worked well and Champion W89D (since 3077s arent made) as well as autolite t7 which ran terribly in the car but its what it came with. all gapped at 0.30 as per my repair manual but i see alot of people state 0.25 on here.
valves have all had a lapping in the cylinder head and stems polished to 2500grit
i have one spring that is ever so slightly shorter but does not resolve the issue to lift up on it with screw driver as per manual while running. it is spring #5 exhaust of cylinder 3 (planning to replace soon anyway.)
all intakes are set to .008 and exhaust set to .010 as per repair manual. (seen some sources that say should be 0.006 intake and 0.008 exhaust.

timing is set, distributor breaker point is set to 0.020 at full retard, cap is fine and no cracks. mild pitting on the contacts in the cap but overall good condition and trued.

carburetor is a refurbished one and float set to 9/16 as per manual (carter rak-xo) slightly floods when car is off after a few min and it is definitely not dirty.
valve needle has been ground / lapped with valve lapping paste to try to get a better fit/seal (it is a metal one)
air adjust screw is set to approximately 1.25 turns out. (1.5 from manual gives a bit of carb popping)
idle screw set as low as i can get it without it stalling

refurbished vac tank thanks to ChevyChip (float had a few issues but been patched)
no vacuum system leaks that can be found.

i have a bit of an exhaust leak at the manifold joint going into the exhaust pipe but i don't think that would be too much of an impact. maybe i'm wrong. best way to seal it? i have a muffler weld that goes to 1500F but have not applied it yet

You have the wrong carburettor. The carby you have is off a 1928. I have had it before on a 1926 not running properly because of this.

what would be the correct / compatible carburetor for the 1925?


correction to the carb i have (Carter C RX-0) the old one i had was the rak-xo

I think you have the correct carby. Have you got the air intake tube from the carby to exhaust.

yeah i i have it it just kinda sits in the carb with a screw pressing on it. to hold it in place.

The loss of power every 10 seconds is likely due to the vacuum fuel pump drawing fuel. There should be a fitting on the intake manifold that has a much smaller hole that the tube size to minimize the loss of vacuum when the pump is drawing fuel. If that is not in place there will be a much larger loss of idle speed when pumping. The float on a 4 screw vacuum tank has a straight section on the float wire just below the upper bend. That gives a longer time between pumping cycles as the float can move further up and down before it activates the valves.

You need to adjust the carburetor idle mixture to the maximum idle speed without hesitation on acceleration. The wear on the throttle shaft and carburetor body usually results in fewer turns out on the adjustment screw. Many RAKX-0 carburetors have the screw bottomed out or nearly so to idle well. The RX-0 not so much.

i tried swapping out the connection on the intake manifold to one with a slightly smaller inner diameter, that seems to have partially resolved the issue but not completely.

what should the inner diameter be?

i could also do a temporary jerry rig with a petcock and dial in how much its closed!

thanks chipper!

The hole is small like .020 or .030To get a quick orifice I have used a pop rivet just remove the part that looks like a nail