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Gregness
09-03-2013, 12:39 PM
So I tore the D2 apart and pulled the Pony carb again, and rebuilt it with the kit I paid $100 for. I put it all back together hoping for good news... but nothing seems to have changed. It still start running too rich and just dies off. New float-needle valve assemly, new gaskets, everything cleaned, tried adjusting the new high-speed mixture screw, nothing worked. Damn. (FYI, I have rebuilt many carbs over the years)

So it is starting to look like "find a replacement carb" time. Any feel for cost and availability for these items, for a 4U machine? Any ideas, something I might have overlooked?

G

snowshoveler
09-03-2013, 12:50 PM
Hello
I rebuild carbs every day its my job.I am a small engine mechanic.
However these cat carbs were a different animal. I really had to sit down and figgure where the fuel went and when. Then the fix was easy...I was missing a small gasket...yikes.
Found gasket on bench and installed it...all was good.
Regards Chris

Old Magnet
09-03-2013, 01:31 PM
Get yourself one of these reprint manuals. It's the only one that tells exactly how these carbs are supposed to work. Suspect yours has some clogged passages.

http://www.ebay.com/itm/Zenith-TU-Cranking-Engine-Carburetors-/360619665791?pt=BI_Heavy_Equipment_Parts&hash=item53f69b697f#ht_973wt_890

boaterri
09-03-2013, 02:40 PM
I have not worked on Cat pony engines but when fixing running problems on my outboard motors, lawn mowers and chain saws I have found that a complete disassembly of the carb and overnight soak in mineral spirits followed by a couple of cycles thru an ultrasonic cleaner OUTDOORS with mineral spirits usually cures any gunked up carb running problems. There are a million little passages you can't get to any other way.

Rick

Garlic Pete
09-03-2013, 02:59 PM
I've done quite a few of the horizontal pony carburetors, both from J series and U series tractors.

Old Magnet's advice is good, those Zenith manuals give you all the theory with charts of the passages and the whole works.

I haven't had luck with boaterri's approach. The key is what snowshoveler said. Logic out the passages, find the ones that are plugged up, gaskets that are missing, jets that are wallered out by previous rebuilders, etc.

The most common problems are running too lean and needing choke. This problem is usually one of two things. There are several very small passages that run under the bowl. These seem to accumulate dust, lint and varnish no matter how careful you are about running the carb dry every time you use the engine. I've had a bowl which soaked in one of those paint can carb cleaners for a couple of months with a shake or two every few days.

When I drilled out the plugs and used a small bit to ream out those bottom passages, they were not hard as a rock, but they were plugged almost their full length with pasty, sticky material. Once cleaned this way and the plugs replaced or filled with a bead of solder, that carb worked great.

The second cause of lean running on certain U series carburetors is the tricky ninety degree internal angle, which converts the screwing in or out of a metering jet into movement of an internal needle. I don't know if your carburetor has this, but it could be the cause of your problem. Usually, someone bends this linkage by adjusting the screw too far, resulting in lean running. If your carburetor has this linkage, it could be opened up too far, resulting in permanent rich running, even at the extreme of adjustment.

Another thing to note is some of these carburetors adjust the opposite of how you'd expect. Some of them get richer as you screw the adjustment in and leaner as you screw it out. The carburetor with the ninety degree crank inside the top cover uses that to convert movement of the high speed adjustment screw into internal movement of the meter. On this carburetor, you screw the adjustment in to make the mixture richer and out to make it leaner. This is counter intuitive, although I'd assume that if you screwed it in and the engine ran worse, you'd try turning it the other way, even if that didn't seem right.

edb's very helpful descriptions and attachments about the middle of this thread were really helpful. http://www.acmoc.org/bb/showthread.php?18985-Adjusting-Carb-on-pony-and-other-pony-problems

Just some thoughts that might help, or might not.

Pete.

edb
09-03-2013, 06:34 PM
Hi Gregness,
sounds like you are starving for fuel flow from the tank.
If you are still using the edge disc type screen in the sediment bowl of the tank tap, they are notorious for clogging up and, short of dismantling and properly cleaning each disc. soaking etc does not seem to work.
Sometimes the float bowls can be distorted from repeated abuse to unstick the needles and hang up the float thereby restricting the needle opening and hence low fuel flow.
Fuel flow can be checked by removing the bowl drain plug and observing the flow from the drain plug hole--bit messy and dangerous due to fuel split but as long as you are careful to not light up nearby for a while all should be well.
Cheers,
Eddie B.

Tim Matthews
09-03-2013, 08:26 PM
My pony was giving me a few fits. It would start right up rev and then seem to run out of fuel or flood out. did the carb thing with almost same results.
Turns out the rotor burned a pathway from the plate on top down through the center to the stem where it would ground out, only after it ran for bit
breaking down the resistance.
Check the rotor with a ohm meter ,better with a megger shows up real good .
After I found it was shorted .I peeled the top plate off with my pocket knife and you could see the black path it was taking ,looked like tooth decay.
epoxyed it back together for a short term fix till I got another rotor. Tim

Gregness
09-04-2013, 09:22 AM
I guess I suck at writing... my problem is not starvation or leaning out... it is incurable richness! I did use ridiculous quantities of carb cleaner and high-pressure air on every passage way I could find, BTW... and if a too-lean condition were the problem I would assume that clogged passageways were indeed the likely culprit. For lack of a better idea, for the moment I plan to pull the float bowl (again) and double check the operation of the float and valve assembley (again).

However, it seems likely to me that a jet is hogged out in there and allowing too much fuel through. The rebuidl kit did not have any replacement jets, unfortunately.

G

Old Magnet
09-04-2013, 09:34 AM
Wrong assumption....a clogged passage can effect the flow in another.

Jim Davis
09-04-2013, 10:41 AM
On my D4, I used to just barely crack the fuel valve open. Hadn't remembered that until I read your post. It's a makeshift method, but I could run the pony and start the diesel.

Jim

edb
09-04-2013, 07:09 PM
Hi Greg,
if you have the early style main jet that screws into the top cover vertically then check and see if the thread on the jet is free to screw all the way in. If the jet screw thread locks up before it can affect the mixture--too far open--then obviously it will run rich.
Same goes if the end of the needle was rust pitted and someone re-pointed the needle it will be too short to reach the jet and affect the mix--do some measuring to suss it out that the pointy end of the needle can get to the jet--from memory it should actually seat in the jet--evidenced by a polish line when the needle seats lightly in the jet.

For the later horizontal main jet screw the screw works in reverse (IN to Richen, OUT to Lean) because of the bellcrank lever.
If the lever is distorted--not a 90 degree bend-- then it will also run rich because the jet needle is held too high when the screw bottoms out. The bellcrank levers often get bent.

Also check that the gasket is under the main jet. and that the compensator jet, #9 in the scan below, is fitted in the bottom of the bowl near the main jet as if it is not there the mix would be rich also.

Just somethings to check.
Cheers,
Eddie B.

Gregness
09-04-2013, 09:43 PM
Hi Greg,
if you have the early style main jet that screws into the top cover vertically then check and see if the thread on the jet is free to screw all the way in. If the jet screw thread locks up before it can affect the mixture--too far open--then obviously it will run rich.
Same goes if the end of the needle was rust pitted and someone re-pointed the needle it will be too short to reach the jet and affect the mix--do some measuring to suss it out that the pointy end of the needle can get to the jet--from memory it should actually seat in the jet--evidenced by a polish line when the needle seats lightly in the jet.

For the later horizontal main jet screw the screw works in reverse (IN to Richen, OUT to Lean) because of the bellcrank lever.
If the lever is distorted--not a 90 degree bend-- then it will also run rich because the jet needle is held too high when the screw bottoms out. The bellcrank levers often get bent.

Also check that the gasket is under the main jet. and that the compensator jet, #9 in the scan below, is fitted in the bottom of the bowl near the main jet as if it is not there the mix would be rich also.

Just somethings to check.
Cheers,
Eddie B.

Thanks Eddie, this is helpful.

G