'75 DT175 Carb settings

[roadking67]

Hello fellow Yamahamers, I have need of your assistance. I am resurrecting a dead dirt bike and need to know all the proper carb settings. I can't get this thing started. It pops and sputters but no joy. The air screw has no retainer spring, should it have one and does its absence affect the setting of said screw? What about the throttle stop setting?

I know nothing of the history of this engine because it came via ebay. It took some time to realize that the cylinder does not have the autolube input hole, this cylinder may be from a YZ. The bottom end is from a '76.

The reeds seem O.K., it's sparking. The carb seems to be working. Gas leaks out of the overflow pipe. The airbox is disconnected. How exactly do I check/set the timing? How do I check for TDC? How do I dump this turd before it kills me?

I need help, I admit that freely.

[kobascle]

Hello fellow Yamahamers, I have need of your assistance. I am resurrecting a dead dirt bike and need to know all the proper carb settings. I can't get this thing started. It pops and sputters but no joy. The air screw has no retainer spring, should it have one and does its absence affect the setting of said screw? What about the throttle stop setting?

I know nothing of the history of this engine because it came via ebay. It took some time to realize that the cylinder does not have the autolube input hole, this cylinder may be from a YZ. The bottom end is from a '76.

The reeds seem O.K., it's sparking. The carb seems to be working. Gas leaks out of the overflow pipe. The airbox is disconnected. How exactly do I check/set the timing? How do I check for TDC? How do I dump this turd before it kills me?

I need help, I admit that freely.

I just went through a complete refurbish of a 1975 DT400. These things are hard to find parts for. Took me 6 mnths. If you have the time and drive it`s a good bike. Don`t know what you paid for it. I paid $700.00 for my 400 and wouldn`t sell it for less than $3000.00 You can find what you need if you search for it. If you complete the project? GREAT.. If not? Sell it on EBAY

[Airhead]

Cant be far away from running, you need to sort out that carb though, seems that the float height needs adjusting or perhaps a new needle valve is needed, fuel level is much too high if its coming from the overflow If you do split the carb, dont forget to clean the pilot jet while youre in there, this could also give poor starting if its blocked

[roadking67]

Thanks guys. I was looking for a cheap old thumper to ride the trails when I found this bike for $95. That's about 47 British pounds. I have owned 2 strokes in the past and prefer to avoid them, however, this seemed like a good deal so I bought it. Many parts are missing, the original engine was K.O.'d by the kid who sold it to me. I need to know carb settings and timing procedure. The turd question was rhetorical.

How does a malfunctioning needle valve cause gas to leak out of the overflow pipe? Does it leak up into the venturi area and then drain back down to the bowl? I tested the float valve by holding it shut with my finger and letting the gas flow, it worked fine, no leakage. I put a radical bend on the float level adjustment tab to ensure that the float was shutting off before overflow. No leakage overnight but in the afternoon sun all the gas leaked out. Perhaps the heat causes the valve needle to leak? The needle has visible scratches running down the taper.

Can I compensate for the leaking needle by setting it at a leaner (lower) setting? It may still leak but at least if I can get the bike to run that would be a big step forward.

There seems to be a wealth of knowledge on this board, I hope to learn a lot here. Thanks.

[roadking67]

I really need to know where to set the throttle stop screw and how to check the timing. Could the timing be so far advanced that it is turning the engine backwards? The reason that I ask is because the kick starter kicks back hard every time that the engine pops. It only pops once each time. The nut that holds the magneto flywheel on was loose, could it have been removed and then replaced incorrectly?

I need to get this bike running so that I can sell it. I cannot afford to buy a manual for a bike that I have to sell. I have two other bikes that I can't even ride until I get rid of this one.

[Airhead]

I really need to know where to set the throttle stop screw and how to check the timing. Could the timing be so far advanced that it is turning the engine backwards? The reason that I ask is because the kick starter kicks back hard every time that the engine pops. It only pops once each time. The nut that holds the magneto flywheel on was loose, could it have been removed and then replaced incorrectly?

I need to get this bike running so that I can sell it. I cannot afford to buy a manual for a bike that I have to sell. I have two other bikes that I can't even ride until I get rid of this one.

Well if the worst vice is advice you better ignore this

The ignition timing is 1.8mm BTDC

The throttle stop screw is merely a way of adjusting the engine idle speed (tickover) and should be set to about 1500rpm

Since this bike has not got electronic ignition it might have a mechanical advance unit on the points cam. If it has you need to check it is functioning correctly and not jammed in the full advance position as this would cause the kick back you describe

The magneto flywheel locates on a woodruff key so cant be put on wrong and in any case it would not affect the timing even if it was

[roadking67]

Well if the worst vice is advice you better ignore this

The ignition timing is 1.8mm BTDC

The throttle stop screw is merely a way of adjusting the engine idle speed (tickover) and should be set to about 1500rpm

Since this bike has not got electronic ignition it might have a mechanical advance unit on the points cam. If it has you need to check it is functioning correctly and not jammed in the full advance position as this would cause the kick back you describe

The magneto flywheel locates on a woodruff key so cant be put on wrong and in any case it would not affect the timing even if it was

[roadking67]

Well if the worst vice is advice you better ignore this

The ignition timing is 1.8mm BTDC

The throttle stop screw is merely a way of adjusting the engine idle speed (tickover) and should be set to about 1500rpm

Since this bike has not got electronic ignition it might have a mechanical advance unit on the points cam. If it has you need to check it is functioning correctly and not jammed in the full advance position as this would cause the kick back you describe

The magneto flywheel locates on a woodruff key so cant be put on wrong and in any case it would not affect the timing even if it was

Thank you, this is good info. I looked inside the flywheel where the advance device is and all that I saw was rust. Now I need a tdc indicator/dial guage to set the timing. Is the timing set by locating tdc and then backing the piston down to 1.8mm before tdc, and then setting the points to close? I have never set the timing on a bike, I sure miss that pointless ignition now, but as my mom used to say, if it's pointless then why bother?

I didn't log in to make this post, is that normal?

[Airhead]

Thank you, this is good info. I looked inside the flywheel where the advance device is and all that I saw was rust. Now I need a tdc indicator/dial guage to set the timing. Is the timing set by locating tdc and then backing the piston down to 1.8mm before tdc, and then setting the points to close? I have never set the timing on a bike, I sure miss that pointless ignition now, but as my mom used to say, if it's pointless then why bother?

I didn't log in to make this post, is that normal?

but as my mom used to say, if it's pointless then why bother?

If you think that the chunk of rust you have seen is the auto advance unit, then it is extremely likely that this is the sole cause of your problem, having said that!...I dont know for sure myself, am just guessing and trying to remember back in the days that all ignitions had points. You need to remove this and leave it to soak overnight in some easing oil, or WD40, or even diesel oil, give it a good clean up, .

They work on the principle of centrifugal force which pulls out the bob weights against spring tension, this then turns the points cam slightly which advances the ignition.

As you can see its important that this functions properly and pivots and cam are not seized with rust

Once you have done this and got it working, Re assemble and put the flywheel back. Look on the magneto flywheel and you may see two marks. lines

One will be TDC and can be verified by removing the spark plug and holding a pencil or something so that you can feel TDC, you will see that this lines up with another mark somewhere on the crankcase. The other of the two lines is the point of ignition ie 1.8mm BTDC, the points (contact breaker) should just begin to open at this mark. The easiest way to test is to use a strobe light. You will need to power your strobe light from a car 12V battery though as you DT is 6v. Run the engine in a dark area and look if the lines match up, if you then rev the engine you will see the effect of the auto advance unit in operation.

[roadking67]

Thanks for all of your help oldgit, I was looking forward to fixing this bike but when I got it open, no joy. The advance mechanism is nowhere to be seen on this machine. It should be mounted on the flywheel, yes?

A very curious thing, your description of the ad. mech. fit exactly with the diagram (tech bulletin '74 dt175) on the Yamaha website. Here's where it gets strange: the picture shows an access hole to adjust the advance mech. on the mag flywheel, but this hole does not exist on my engine, nor does it exist in the parts catalog diagram on the same webpage! The parts catalog makes no mention of the advance mechanism, other than in a tech. bulletin.

Here's what I found in the real world. The magneto coils look different from the diagram, no advance mech. anywhere. The woodruff key is gone (sheared off), the flywheel spins freely on the shaft. There is a deep groove inside the flywheel that runs 1.5 inches and then stops dead. It looks like something came loose in there and caused the flywheel to seize, which sheared off the woodruff key. There is much scoring/grooving on the inside face of the flywheel where it meets the shaft. There should be none as they should turn in unison. The timing mark on the flywheel aligns with the timing plate a few degrees before TDC, at least according to my oak twig TDC indicator. This is where it sparks, even as I spin it freely on the shaft by hand, with the engine not rotating.

If I can find TDC on the engine I can align the flywheel with the engine and get a rough timing set. (done) If I can clean out the key way on the flywheel and the shaft, as they still have the sheared key metal in there, I might be able to get this thing running.

I will still need a new needle valve, I hope that I can find one.

Damn this engine, every time I think I'm out it pulls me back in!

[roadking67]

Update: The engine from ebay seems to be made up of parts from several other engines. The bottom end ser. # is a '76, but the mag. flywheel has no advance mech. access hole like it should. The cylinder has no auto lube oil input hole. Cylinder ser.# is 43000. Does anyone know how the engine will run without the advance mechanism?

[Airhead]

Been reading through this thread again, you really need to say a bit more, unless i missed it i cant remember if you did actually say this bike has points (contact breaker), I just kind of assumed that with the age of it, so has it got points and a cam that operates them?, Photo might be useful too !

[roadking67]

The '76 engine has points, the ignition works but it is not the stock ignition for this bike. The coils and flywheel are different, no advance mech. The ignition on the '75 engine is the correct one with the advance mech.

The points are opened by the collar on the flywheel, I guess it is out of round at the right spot.

I will try to post some pics, I have no idea what bike this ignition is from, perhaps a YZ? The mag. flywheel is not steel like the correct one, it is magnesium or aluminum or something, and very thick. It needs no puller to remove. The coils are narrow and straight. I am in doubt about this thing ever running again.

Yesterday I dropped the float valve on my gravel driveway. I found it after an hour on my belly, picking through stones. Surprisingly relaxing, very Zen.

[Airhead]

The '76 engine has points, the ignition works but it is not the stock ignition for this bike. The coils and flywheel are different, no advance mech. The ignition on the '75 engine is the correct one with the advance mech.

The points are opened by the collar on the flywheel, I guess it is out of round at the right spot.

I will try to post some pics, I have no idea what bike this ignition is from, perhaps a YZ? The mag. flywheel is not steel like the correct one, it is magnesium or aluminum or something, and very thick. It needs no puller to remove. The coils are narrow and straight. I am in doubt about this thing ever running again.

Yesterday I dropped the float valve on my gravel driveway. I found it after an hour on my belly, picking through stones. Surprisingly relaxing, very Zen.

My god, the more this goes on the more i'm confused, when did a 2nd engine appear, which engine are we working on? because if its got points it will need a working auto advance unit wont it?

What's a collar on the flywheel?

Any flywheel should need a puller to remove if its on a tapered shaft

My head hurts

Think i'll get my driveway gravelled

ok i've had a little lie down...Taken my medication, And read this post again!

Looks to me like youve got this bike and the previous owner ko'd the engine

so along came another engine from ebay (Turd engine)

From what you say about this, this turd's a lemon and have my doubts it will ever work as it is

So......

a - Can we fix the 75 engine?

b - Can we use the flywheel and auto advance and any other gubbins from the 75 engine to sort out the lemon turd engine?

[roadking67]

In post #4 I mentioned the original engine was wrecked by the prev. owner. I failed to mention that he bought a '76 engine on ebay and bolted it in the frame. That's where I came in.

The '75 engine is useful only for parts. The left side case is busted out at the rear of the drive shaft. The ignition looks intact and proper for the bike. It may be possible to transfer the '75 ignition to the '76, but that will mean cleaning out the woodruff keyway on the crankshaft. I don't know if I can do this, the keyway is barely recognizable on the '76 shaft, almost invisible.

I am trying to find a cable to connect my phone to this pc to upload pics, in the meantime the collar on the flywheel is the part of the flywheel that rests on the shaft, it has the keyway on the inside to hold it in place, and the points rest on the outside of the collar. As the flywheel turns, there seems to be a high spot that opens the points at the right time, just before TDC. Perhaps this is the cam that you mentioned? It is cast integral to the flywheel, it is not a separate part. This entire ignition is from some other bike. Made by Hitachi. There seems to be no method of timing advance on this unit.

I actually like this bikes chassis and suspension, it's better than my '75 XL125, but I have a history with 2 strokes, mostly wrenching, rather than riding, which is why I prefer thumpers. Not so exciting, but much more reliable.

I failed to properly introduce myself, I will remedy that situation soon. Thanks for your attention.

[roadking67]

It appears that the engine is flooding badly, I think that the carb nozzle is worn out. Local dealers can not supply. I found a used carb at a wrecker from a '79 MX175. It has the same body, it's almost identical to the DT175, the main jet is smaller-130, the slider cutaway 2.0 instead of 3.0. Would it be suitable?

[Airhead]

Cant see a problem with using another carb, but check the needle jet isnt worn oval on this one too. I would first use the main jet out of your original carb (is it a 140?) and the slide too. Have you checked what specs are right for the engine, jet sizes, Cutaway etc? Try to stick to these specs, you can always 'Plug chop' later

[roadking67]

Re: Mx175 carb for DT175.

The standard for this bike: main jet is 160, slide cutaway 3.0. Would the 130/2.0 carb choke the bike badly? I have inspected the old nozzle, I am not sure if it is the problem. There may be some wear at the top, I can't really tell. Would that be enough to cause the bike to inhale enough gas to flood it badly? The plug always looks black and wet, yet it ALWAYS sparks when I remove and test it, which is every time I try to start it.

I cannot get this bike started.

With the exhaust pipe off, it pops more often, when I hold the throttle open, likewise. The fuel flows liberally when I kick it with the throttle closed. It seems like too much fuel. The float needle has visible grooves in the business end and the carb still overflows. Could a leaky float needle cause the engine to flood while starting?

I tested the air jet, it seems to work, I put a new o ring on the nozzle, I will replace the spark plug. Does anyone know the correct plug gap? The plug is B8ES, I believe.

My main question is, what is causing the carb to dump so much fuel into the engine, nozzle or float needle? When I kick it it pops once each time if I hold the throttle open. If closed, almost never. When push starting, it seems to flood instantly, no firing at all. The plug is sparking, plenty of gas but no fire.

The pilot jet is always clear. I just reset the float height from 20mm to 21mm.

The old girl is perpetually wet but she has no fire for me.

I am actually considering ether, it's that desperate.

The ether is for me, I don't know what to do with the bike.

[Airhead]

Gas leaks out of the overflow pipe.

is this still the case...surely you fixed this after all this time

How does a malfunctioning needle valve cause gas to leak out of the overflow pipe? The needle has visible scratches running down the taper.

Possibly answered your own question there

Can I compensate for the leaking needle by setting it at a leaner (lower) setting? It may still leak but at least if I can get the bike to run that would be a big step forward.

No you need a good float needle shut off and to have your fuel level float height set correctly

The float needle has visible grooves in the business end and the carb still overflows. Could a leaky float needle cause the engine to flood while starting?

so from august 9th until august 31st you still need a new needle, It needs replacing...not saying it will solve the problem, but it is a problem in itself so fix it and move on from there.

Oh and to answer your question about How does a malfunctioning needle valve cause gas to leak out of the overflow pipe? ..Well fuel gets past the needle valve, float rises, needle valve closes, fuel gets past visible grooves in needle valve, fuel exits carb onto gravelled driveway!

[roadking67]

Hey oldgit, I appreciate all your help but I think there is some miscommunication here. The valve needle (or needle valve as I called it) and float needle are two different items, but when you call them by the same name I just get confused. I understand how the float needle having grooves would leak, but it did not leak when I tested it. When it leaks gas should come out the overflow pipe, and it does. How does this cause the carb to flood the engine? I understand from others on this site that bikes still run with leaky float needles.

Second issue: The valve needle has some scratches down the taper. The scratches are insignificant compared to the space between the needle and the nozzle. I don't know how much trouble this causes. There seems to be some difference of opinion as to how these things work. Some people say that when the slider is closed that the needle shuts the gas flow off. I wonder how, because after much internet searching I have found diagrams that show that the needle is not supposed to stop the nozzle up completely, indeed it cannot, ever, because even a new needle is not nearly thick enough to fill the nozzle completley. If it did, say goodbye to the air from the air jet. The air jet supplies air through the nozzle even at idle, yes? Scratches might let some minute extra fuel flow by, but this cannot be very significant to the overall flow. The needle is not supposed to even touch the nozzle according to the diagram I found. This fits with the fact that the needle is free to move laterally when removed from the slider and dropped into the nozzle.

The kid who sold me this bike swears that he rode it until breaking the cases, meaning the carb should work as is. He may be lying, I'll never know. There is simply no way for the valve needle to have worn down while NOT operating the carb, so it must have been working.

Yes a new valve needle and float needle would be great, I don't know what the parts market is like in Europe, but around here there is NOTHING available. Only one local Yamaha dealer would even respond to my parts request, only to tell me good luck, they cannot supply any new parts for old bikes. E-bay is the only source. I cannot purchase from E-bay, as they require a credit card just to be a member. The local wreckers are my only source, hence the question about the MX175 carb as replacement.

If anyone can contribute knowledge about how the slider valve needle works in carbs in general, and specifically how it shuts off the fuel flow into the venturi that would be great.

I just had an idea. I could remove the valve needle and stop up the nozzle and replace the slider sans needle. If the needle is causing the flooding this should prevent it and allow the engine to start and run on the pilot jet. Wait, that will prevent air from the air jet to enter the venturi, yes?

There is a hole in the carb barrel nearest the manifold side, just past the pilot jet hole. What is this for? Is it a bypass outlet for the air jet? Should air flow from there at idle? If so I think that it might be blocked.

I am still learning the basics of the wonderful mystery that is the carburetor.