Howdy folks, I appreciate your continued help getting this 74 dt250 sorted. It has spark, the timing is set...pretty darn close, and the carb is very clean. It will run for a couple seconds with some gas in the cylinder, but then nothing. I've tried it with no filter on and just a uni style foam filter.

Here's a pic of the vm28's slide at rest. It sounds like the slide is "bottoming out" and the throttle cable is as loose as possible. should the vm28 slide leave this much of a gap at rest?

conditions when trying to start:

air screw - 1.5 turns out.
idle screw - turned in just enough to lift the slide a tad
Uni fuel filter attached to carb
Choke on. 
Main jet: 140 or 150, clean
Idle jet: seized in place, but clean. Unsure of size. 

Thanks for your help! 

 

Has the carb been totally cleaned. Ok pilot is stuck in but can you spray something through it or poke a fine wire brush strand right up to see it in the venturi. Choke is a big possibility which is a fixed jet down a deep hole in the bowl--tube fits down in there. If blocked then opening the choke only give more air making a bad situation worse. Gotta take the needle jet right out as can block up. Slide needle jet that is. Mine after years of sitting had grown up inside which i didn't expect & hugged the needle all the way down. Put a scratchy split pin up there & worked away. Metered part is only near the top & can see the step looking up from the bottom. looks like it has the side main jet & stuff gets behind it as flow back to front so take it out of the holder. Oring that seals the bowl onto the needle jet is important except if not sealing you'd have an overly rich mixture--opposite to what you have.
Possible but unlikely its broken a reed petal or blown out a crank seal. Could try starting again & restricting the inlet somehow to create a rich mixture. Cap the carb mouth with something that fits with a small hole just to prove a point. Upside down tacho drive looks interesting. Gotta be a story there. While raving on i clean forgot how's the fuel flowing into the carb--unresticted?

Thanks for the reply, it's nice to go through the basics again.

some things I forgot to mention: compression is 135-140 and fuel is flowing with no in line filter from a hanging aux tank.

carb was totally disassembled and cleaned. Both jets (main and pilot) are free and clear. The o-rings are in good shape.

The only thing I didn't do was run a jet cleaning rod down in the choke passage. I just removed the choke plunger and sprayed through it. The rubber on the bottom of the choke plunger was also a bit torn.

So i'll try pulling the choke off again and ensuring it's clean.

I'll keep the thread updated!

There's more that "both jets".  RT325 mentioned the starter jet in bottom of float bowl and the needle jet (aka main nozzle aka emulsion tube). 

My thought is you think the pilot jet is clear but it may not be as clean as it seems to be.  There's also air passages and without removing all the removable jets you really can't get the carb clean...  and even then, sometimes you need to clean it multiple times.  Ultrasonic cleaner is sometimes the only way to get the crud out. 

What you're seeing at back of carb is the slide cutaway.  If you have the slide out this will be obvious. The front of slide has little to no gap.

Starter jet: 

 





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Thanks for the input guys. When I mentioned that it was clean I think I failed to mention just how clean it is. I had everything in the ultrasonic cleaner, used carb cleaning rods where appropriate, and sprayed compressed air through all the passages to ensure they are open and clean. Here's two pics of the carb:

 
 


One thing I did notice on my most recent tear down, however, was this torn gasket:



  


I'm sure it encircles that tube for a reason, so it being torn away would do what???

Thanks again!

 

The torn gasket could well be your problem .
Without that sealing , your enrichener circuit might just suck air , rather than fuel .

New one just ordered! I'll keep you guys updated once it comes in within a few days hopefully. 

Can't say it's causing your issue but the choke plunger needs to seal when off...  float level looks pretty high...  and critical jet setter o-ring looks bad in the picture... could be optical illusion I guess.

All that said, my gut and experience says something with pilot jet and/or starter jet is most likely the issue... possibly that gasket.

 


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Thanks for the info and input. The critical o-ring certainly isn't perfect, but it seems like it would seal. I'll replace it anyway, just in case. The float height seemed a bit high to me too, but I set it to what the manual listed for the dt250: 17.3mm. 

lastly, I ordered a NOS choke plunger with fresh rubber on the bottom to ensure it will seal off, regardless of it being an issue now. 

between the o-ring and the float bowl gasket, hopefully it'll fix the problem. If not, the next step will be to extract the pilot jet, even though it's clean and clear, and then dunk it all in the ultrasonic again. 

Choke jet--call it start jet MarkT points to, compressed air is ok but put a spay nozzle in there to see spray passing out into the bowl, then do it in reverse from the bowl coming up through the jet. Choke plunger rubber is probably the least of your worries considering you're looking for lack of fuel rather than too much choke--choke not shutting off. Hope for some good news soon. Think i said somewhere just hand choke the carb when it starts, or something restrictive across the mouth.