Pony motor help

My pony motor on my 1955 catapillar 955a traxcavater is giving me problems. I cleaned the carburetor and now the needle where the float stops is leaking. Where would I find that part?

Also if any body knows how to tune the carburetor "setting all the screws" I could use some help. The motor has A hard time revving high enough to start the diesel engine. I stall the pony motor often before I can get the diesel engine started.
thanks
joe

Hi Joe,
there are a few places that sell carb kits and parts for the TU-4C. I bought my last set of parts from Roberts http://www.robertscarbrepair.com Look for the Zenith TU3X1C for the 730 diesel etc. Slightly different carb model to the Cat one but all the same parts http://www.robertscarbrepair.com/piccategory.asp?id=362 You want parts 7 and 8 (or just the needle).
Before you get to that step though, get some fine paste of some description and try lapping the needle into the seat - you might just need a quick clean-up. Also, remember that you need to run the carb dry once the main engine has started, by turning off the gas fuel valve underneath the pony gas tank. This is to drain the fuel bowl but also to stop the needle valve from bouncing and flooding, and thus diluting the pony oil with fuel.

Regarding tuning, 1.5 turns for the main jet and 1 turn for the idle air screw is where mine sits, but you'll have to experiment. I had a #15 main jet and swapped that out for a #24 so my adjustment changed.
Two things to note:
- if you have the main jet screw that comes in from the side (rather than straight down), then screwing it in opens up the jet because it pushes on a bell-crank under that tiny cover. Remove that cover and verify that the bell-crank is a solid 90 degrees - if whaled on, they bend easily upsetting your adjustment
- the idle air bleed controls the amount of air mixing in to the idle fuel. So winding the screw out introduces more air thus leaning out the mixture

In both case, err a little on the rich side, so you don't suffer lean overheating syndrome.

For the high idle speed, you adjust the spring tension on the pony governor. Get a tach on it and aim for 3350 rpm (correct me here guys). That gets the main engine turning over about 350rpm-ish. Many guys these days use a contact-less tach where you aim and point at a strip of reflective tape that you stick on the pony flywheel. Open the throttle right up so the governor is controlling the speed and adjust the spring tension (more tension, higher speed I believe). Do this with the pony turning the main. Then, quickly check high idle with the pinion disengaged and check that it's about the same 3350 rpm.

Report back on your results : )

If you're still having trouble getting it to run right, you can thoroughly clean the carb. We'll get to that step if necessary....

thanks a lot Neil.
I will get to it and let you how it goes.
joe

If your pony motor has no power, could be you have a restricted gas flow out of the tank and into the carburetor. If you remove the top of the bowl, connect the line back to the tank and open the valve to ensure you have more than a trickle of gas getting through. If your tank has rust, do yourself a favor and clean it and re-line with one of the products made for that purpose. That stops the rust that makes its way into the lines and carburetor over time and you can head off the day when it rusts through and starts leaking.

If you have to run it with some choke all the time to keep it going, you probably are getting enough gas, but most likely need a good carburetor cleaning. This involves drilling out some lead plugs and carefully reaming the passages with a correctly sized drill bit to cut out all the old varnish and rust that accumulates over the years. The passage that was the problem on mine is the one across the bottom of the bowl from the main jet to the high speed metering well. High speed metering well is held in by a hex head plug on the corner of the bowl next to the cast iron carburetor body. That passage runs straight across the front edge of the bowl and once you get the soft plug out, you can do a good job of cleaning it. Replace the plug with lead shot, bead of soft solder, BB shot coated with sealer, or even thread the hole and put a short screw in it...........lots of things workl

I didn't even remove the cast iron part of the carb from the engine, just removed the bowl from the side of it. Some of the ponies won't idle, Those may need the idle jet cleaned and removal of the carburetor is required for that.