Tillotson JR1A Carb - need specs on adjustments

[Chris Coppi]

I have this Tillotson JR1A carb on my 1928 Chevy sedan.  Been on it for about 60 years.  No history beyond that.  I need help understanding how to adjust it.  There are 3 places that I see that I can tweak... The screw where the throttle linkage connects, a screw with a spring that is in the intake section below the throttle plate / blade and the venturi tube thing on the bottom near the bowl with the T bar on it.  The exhaust is running rich and burns my eyes and smells bad... black carbon in plugs and exhaust pipe.  I know this is not the factory carb, but it's what I have and I don't care if it's original or aftermarket...  Any documentation or help would be appreciated.  The engine runs fine with as is, but backfires if the choke plate is 100% open.

[Oldtech]

Basically:  The screw by the throttle arm is just the idle speed. The horizontal screw by the fuel inlet is the idle mixture, and the one on the bottom is the main jet. You have conflicting symptoms though. If the carb is too rich you need to turn the bottom screw in, but... you say it backfires with the choke open, which would mean you need to open the main jet about a quarter turn.  So... I suspect there are other problems and you need to overhaul it.  If you decide to take it apart there is a jet BEHIND the brass screw in the middle of the picture that must be removed to get it apart. 

[ojh.]

It sounds more like raw fuel is dribbling into in the manifold, like there is trash in the needle & seat, float too high or something along those lines that will defy a cure by adjusting idle richness or main jetting.  Best take it apart.  I believe I have an original carb for that car, the Tillitson might be the better choice when rebuilt.

[carbking]

Before throwing rocks at the Tillotson (although throwing rocks at a Tillotson is not a bad idea 😞

 

Rich running:

 

Defective ignition - 95 percent

Defective carburetion - 5 percent

 

Backfire through carburetor:

 

Defective ignition - 50 percent

Defective intake valve, or valve timing - 50 percent

 

Would suggest:

 

(1) compression test

 

IF compression is good, then

 

(2) ignition test

 

If both compression and ignition are good, then is the time to worry about the fuel device.

 

Jon

 

[Chris Coppi]

I appreciate all the feedback.  I recently had the carb re-built and I'm pretty certain the fuel is clean.  I recently had to rebuild the fuel pump - yes, it has one - probably put in when the after market carb was installed since original setup would have been gravity / vacuum feed right?  I checked the sediment bowl and it was still clean from the last time I cleaned it too.  So, I'm not inclined to tear that carb apart again, but will if it comes to that.  As for the ignition, what should I be looking for specifically - the coil, the rotor and points?  I know the wiring is mostly original and in need of replacement, but it only started back firing since the carb was rebuilt which could be an adjustment on the choke flapper since it stops when I coke it ever so slightly.  The foul exhaust also only started after the carb rebuild...  What should the compression be?  I'm not a mechanic, so my apologies... 😬