Hi

I purchased a new Throttle cable from HV cycle . It is stated to fit the 1974 DT250. On installation I am have the carb cable to set right and snap back but the oil pump cable has  slack and needs a small turn at the handle bar to  seat back. i cannot adjust the nut anymore at the pump side to increase tension. Can any one help how to fix  this issue. if  keep unscrewing the nut it will come out of the case. 437-26311-02

 from near Salisbury, UK.

Is it possible to slacken the carb adjuster right off, set the oil pump cable first and then re tension the carb cable? 

I have no adjustment nut on the carb side. It is spring tensioned at the carb and will snap back at the handle . But i need to turn the handle further to close the pump cable. Looks like the pump cable has slack. Will this any way effect the working

The pump spring does not have the kind of tension snap back .

I would expect the cap of your carb to look like the one shown in this link. The longer hexagonal "tube" is a threaded adjuster that the cable sits in and below that, on the same thread, is a small locking nut.

Vergaser carburetor Mikuni Yamaha DT 250 512 75-76 | eBay

On my phone so mite be misreading a bit. Is the pump cable thingy returning right back, not binding etc. If it starts from say halfway up then you'll have heaps of slack.  OK not that. Did you have to assemble the lower cables into the junction box slide & the pump cable popped off when you weren't looking. Should always be enough pump cable adjustment at the lower end. I don't think this is relevant but back off the adjuster at the throttle control then take up the carb cable slack at the lower end if it has one in that short cable then set the pump cable adjuster so the pump drum aligns with the pin & mark then take up the top cable slack. All a bit of a guess from experience & my own mistakes--lots of them lol.

My carb cap is flat .

Could you post a photo of it? You'll need to use a third party host such as Imgur.

You should have a metal 45 degree elbow with adjustment on top of the carb cap.  Sometimes it can be found still on the old cable. 

Looks like this one fits onto the elbow. By memory the elbow is retained by a circlip into the cap. Locknut on the elbow with limited adjustment.
If you have that setup minus elbow you'll have miles of play you can't get rid of.
Mind you i thought the original issue was getting rid of excess cable at the pump not carb.
Is the pump cable cam spring returning the cable cam fully back.
hvccycle.net/yamaha-dt250-dt360-dt400-th...-cable-437-26311-02/