It is going to be something simple when you find the problem, you watch.
If you are covering the throat of the carb that acts as a choke. If the carb is supplemented at high speed by the low speed jet. If the pony only starts and runs with you covering the throat of the carb or will not run without some or all choke. Most likely the LOW or idle SPEED CIRCUIT (jet and passages) are not allowing enough fuel by for the carb to operate correctly. Or the low pressure (vacuum from venturi(s) side of the Low Speed Circuit don't have enough vacuum to pull the correct amount of fuel air mixture. Gaskets, seals on the throttle and choke shafts, new o-rings, jets and seats help there. Super simple carbs here is a breakdown of a 955C D315 w/a down draft carb. See how the fuel pick up on this model pulls from the bottom.
A good set of Cat manual for your model are a must. First Cat tool I bought. 😉👍 op manual,,service manuals for chassis and engine(s) and a parts catalog for your serial number . Cat manuals only.
 
Good luck

I'm sure it will be something simple, rebuilding the carb will probably fix the issue but I'll have to wait for the parts to arrive in the meantime I'm just glad I can get it running by hand choking and hope making a better gasket tomorrow may make it easier to get off the trailer. I have the manuals for the machine, parts, service, and operators. Still need to buy one for the CCU but I haven't gotten around to verifying which unit is on the machine dealing with this pony motor. I'll need to figure out the right brake and the slight water pump leak after its off the trailer. Hopefully there is enough packing left that I can just tighten the water pump and the brake is a minor adjustment but we shall see.

dont use any of those silicone gasket makers. they are often a WOFTAM. get some actual gasket paper. and dress the gaskets with a little grease or aviation no3 form a gasket.

the only gasket maker to use is loctite master gasket 515,518 or 510.

silicone based ones are problematic as the excess breaks off and blocks things like fine galleries and valves.

Someone recommended that stuff to me or I would have just bought gasket material instead for much cheaper, figures it's not good stuff. I'll just try and use it again for the day and then once the rebuild kit arrives I'll have a good gasket.

if you find you cant just nip the gland up and stop the leak. (note the gland should weep just a little to lube and cool it a tiny drop every few minutes is fine) you can buy gland packing at any irrigation pump service and most industrial suppliers.

This may help it's on the zenith k style carby. 

 
 

regards 

 

I believe the pony on my '39 D8 is the same as yours. The only way it would run is with the choke closed all the way. The choke plate had the little spring valve that opened to let air in. I would dump gas down the carb and it would start and run good enough to start the main. I had taken it off to try to fix it. I could not find anything plugged up or otherwise wrong with it. While it was off my shop burned to the ground in the Oak wildfire and the carb was ruined. There seems to be heavy competition for any that come up for sale. saw a rebuilt one offered for $800. I found an imported knock off for $90, it is a good looking carb and looks like it willl fit right on. I have not installed it yet. If it works out I will post results on here.

I wouldn't be surprised if it was a blocked breather. Went to mess with it yesterday and while I was hand choking it I saw the fuel leaking out wasn't from badly made gasket, there was so much pressure it was seeping out of the idle adjustment screw. I ordered some tip cleaners someone else linked and the rebuild kit should be here early next week. I hope that will fix the issue and not need a new carb.