1975 DT175 Enduro - Carburator Settings?

[bb3924]

I had this bike starting, but running poorly. Removed and cleaned carburator and all jets. Initial reinstall, the carburator was spilling fuel out of the bowl overflow drain. So, I bent the tang on the brass float and reinstalled. Now it is no longer spilling fuel, but will not start.

So, the current set-up is;
- pilot jet in bowl is bottomed out
- air jet is at 1/2 turn out.
- idle is at 1 1/2 turns out

Plug is wet and there is good spark.

The float tang is set so when carburator is upside down, without pressure, the floats are slightly inside the carburator housing. But, again, the carburator is not spilling fuel through overflow and there is fuel on the plus, so the float is likely not an issue, I would think....

Anyone have a service manual to share the jet settings...

Thanks

[Cynic]

Introduce yourself in the newmembers area. Just say high and let people know a little about you.

Plenty of folk on here know the little DT but without an intro you may not get any replies.

[blackhat250]

Hi bb" try a new plug,,, if thers fuel ,it must be poor spark,,, Air screw is correct, what do you mean by " pilot jet in bowl is bottomed out,"

[bb3924]

Based on other posts, I believe my terminology is correct calling it the pilot jet. Inside the float area of the carburator (bottomside of carburator housing, not float bowl) there are three jets. A long wide jet in the center that the end of the throttle cable extends into from the top side, another that I believe is the pilot jet. The pilot jet screw access is "bottomed out", meaning it is turned completely into the carburator. The third orifice in the bowl side of the carburator is the gas supply "shut off", which is actuated via the float. I adjusted the tang on the float and it no longer is spilling fuel via overflow. And, the plus is wet, so I know I'm getting fuel.

I did try a different plug too and it didn't solve it - just saturated another plug.

My understanding was always, gas, air and spark. I have gas, have spark, have air - so the mixture must be off.

Again, this bike was starting and running rough prior to the carburator removal, cleaning and reinstall.

thanks for the assistance.

[nayruf]

Hi

Try to do a net search for 'Mikuni VM tuning manual' you should find a link to a PDF from mikuni USA, I found it useful when I set my DT enduro carb up,

[Airhead]

you need to adjust the tang on the float to achieve 21mm from the float to the bowl gasket surface, this is best checked with the float inclined rather than inverted...inclined to the point where the tang just begins to touch the spring pin in the float needle (not allowed to compress the spring)

[bb3924]

So, adjusted the float tang to 21mm (although it may be +- 3mm from 21mm as float "bulbs" are not the exact smae shape), verified idle screw at 1.5turns and air jet at 1/2 turn.

The DT starts on first or second kick, cold with choke on. It revs up, sounds good - UNTIL it warms up and choke is turned off. At that point the engine runs irratic... meaning the rpms jump up to 7k rpm, then nearly cuts out, then back up and finally it stalls.

I'm getting to be a carburator removal expert. But, I know I'm not a carburator expert. Manual suggest that "engine performs worse as it warms up" is a rich mixture problem. Frankly, I don't know whether the float is still off, another jet needs adjustment, or???

The only portion of the carb I did not tear down was the choke. This carb has the side choke and when disassembled I can see the hole/choke mechanism engage and disengage the opening.

Thanks

[DirtyDT]

Sounds like crank seals may be shot. It is important that the carb is set correctly however, the symptoms you are describing sounds like seals. If it is them then - depending on how bad they are - the following will happen;

The bike will start cold with choke. When revved the bike will rev high but not return to idle quickly. It just sits at a high rev rate and then slowly returns to idle but tends to cut out if not "blipped" by using the throttle. It will tend to get worse when the engine heats up. It may run better when hot with the choke on for short periods.

The left hand seal will suck in air causing a weak mixture and you will never get the bike to idle correctly. The right hand seal will suck in gearbox oil which will be burnt during the combustion phase.

Some left hand seals can be replaced without splitting the engine.

[dt502001]

First verifie that the left seal is not leaking by spraying wd40 at it while running and see if the revs change.

If no change then start with the air mixture screw 1/2 a turn is just a base setting and no 2 bikes will run the same so turn it out a 1/8 or a 1/16th of a turn and see how it runs or in if it get worse. You will most likely need to blip the throttel as your adjusting to see how the bike reacts and keep it running,,, as well you will need to adjust the idle screw 1.5 turn again is just a base setting

If you cant get it to run better with adjusting the air screw then I would suspect that the choke plunger seals are bad.

You said the bike ran before this carb cleaning but you didnt say how well,or what prompted you to clean the carb.

The reference made in the manual would be how the bike actualy performs not this situation,that would be refering to fine tuning and your a long way from that yet.

[bb3924]

Well, thanks for the heads up, but I think that's bad news.

I played with the carburator and it runs well when cold - revs up and will idle down to a steady 1500rpm idle (attempts to idle lower than that and it stalls out).

So, the bad news is once it warms up, it starts to smoke, white smoke. And, then it will not run, it just stalls out and goes back to the up/down engine rev. From your diagnosis, I'm guessing the right seal is the issue.

Given that I'm "advanced beginner" mechanic, is this job within my skillset? I'm not afraid of the task, but this appears to be an almost complete teardown to repair.

Bottom-line, what kind of timeframe are we looking at for a professional? (Which I would guess requires about 3X as long for me.)

Thanks for helping me...

[Cynic]

With a pretty average tool kit and a bit of common sense its a simple enough engine to diy.

Even with genuine seals and gaskets and changing critical bearings like both crank bearings and the output shaft bearing. Which will be knackerd, they always are. You will have decent change from 100 quid.

If you have the parts ready, I would reccomend a bolt kit too. You could do it in a weekend, prob best to allow one weekend to teardown. Then the next to assemble.

Only special tool will be a flywheel puller, thats about a tenner.

[DirtyDT]

These little strokers are easy to work on but you need to be systematic when they give you problems. Before you do anything you need to assure yourself that the carb is rebuilt properly and set to factory specs. Make sure the spark plug is the correct one and is gapped properly (always good to have a few set up). White smoke from a 2 stroke is quite normal but it will smell different if it is burning gearbox oil. So with the carb set and spark plugs at the ready you can check the next item; the airbox. I am assuming that the bike has the standard airbox and the filter has been cleaned/replaced and oiled with the correct oil? If not, read no further and get this sorted next. If all of the above is fine you can then move on to checking for air leaks. With the engine running you need to spray around several areas to see if air is being sucked in, the engine note/speed will change as you spray if you have a leak. Using something like WD 40, spray around any rubber part that connects the airbox to the engine so airbox to inlet tube, inlet tube to carb, carb to engine barrel. If when spraying the engine note changed then you will need to sort out where the leak is. The rubbers get hard with time and heat. Ok so we have the carb in spec, some spark plugs in spec, we know the air filter is clean and oiled and we know that there are no air leaks on the inlet side of the engine. Time to tackle the seals. Generally the left seal keeps out air and the right seal keeps out oil. They too get hard and stop doing their job over time and heat. Take off the left crankcase cover (left and right references are with you sitting on the bike ). With the cover off you will normally see the rotor. This will spin with the bike started so, being safe, start the bike. You should see the rotor spinning. Carefully spray some WD 40 behind the rotor whilst listening for a change in engine noise. If there is a change then this seal will need to be changed.

That should keep you busy for a while but make sure you complete the steps because if you just throw the carb back together and it is out of spec or the plug you are using should be for a tractor you will never get the enormous happiness that is riding and smelling a 2 stroke. It is something to be savioured.