loss of power / cuts out / surges - 1978 DT175E

[DT175 1978]

Hello again. So, I mentioned in another thread that I'm working on restoring a '78 DT175 that's just like what I had when I was a kid. Here's what it's doing

As the title says, it cuts out at around 4000 RPM and just kind of struggles and then just surges up to about 6000 RPM and generally runs rough. It was also spewing gas out of the carberator overflow whenver I reved up the RPMS high, so yesterday I replaced carberator. It doesn't spew gas and runs nicer at an idle, but it still does the thing where it struggles as soon as you get into the power band and then it surges almost to the red line. Basically, it runs almost exactly the same as before and maybe a little worse.

I wanted to clean the air filter when I put the new carb on as it was kind of ugly and who knows how old. It ws dripping with oil, I mean dripping and was actually disintigrating when you touched it. I really couldn't have cleaned it without destroying it. So I did use this old, bad, air filter when I tested the new carb. Could that be my whole problem?

The very next thing I want to try is replacing the air filter. What are the chances that's the problem? At this point, I'm not holding my breath (I thought the carb was the problem)

If that doesn't get it, what is the next logical thing? Reed valve?

Anybody have any idea what's going on here?

Thanks,

Matt

[flyday58]

There'll be an extensive list of suggestions shortly, but details are the key on this forum. Most knowledgeable peeps on the planet about Yams in general, DTs in particular. But we need details.

Does the frame and engine serial number begin with '2A7'? That's correct for the '78 E model, anything else needs more research.

Where did the bike come from? Was it running, a barn find, in a box? Sitting under a tarp for the last 20 years? We could use whatever history you have to help get you sorted.

So you dropped some parts. Unless you stepped on the needle it should be fine. Is the clip still on it? Important as it will just stick in the nozzle without it. The slide will only go in one way, there's a plastic pin sticking out inside the barrel for a guide so as long as it's lined up, again you're safe.

The carb off ebay, is it a new aftermarket or OEM? OLDBIKEREHAB and myself bought non-OEM and both of us found damaged o-rings on the idle air screw:

Didn't think I would have to disassemble a 'new' carb. With problems I kept having I decided to go back with the Mikuni carb and I'm glad I did. I'm pretty sure OLDBIKEREHAB is still running his aftermarket unit without any problems.

The problem I had that made me shove the DT in a corner of the shed for about 6 years was it wouldn't rev over 3000 rpm no matter what I did. I could putt around the yard but it absolutely refused to go over 3Gs. Long story short, the high speed coil in my magneto was bad. Once replaced she revved to the red-line no problems.

Yours could be a fuel prob with the carb, air leaks (Airhead steered me and others to the crank seals), magneto issues, even a loose frame ground/ earth point. And as much as you want to get riding, we will all tell you the game has just started; grab a cold and settle in for the long haul. Better to sort things now than to have to redo things later. Witness my thread on heat seize, which turns out to be a wrong piston/too little oil/ too small main jet deal. I rushed so that's what I get. Hey, I'm American, what else could I do?

Here's mine:

Taken with my iPhone, pictures so life-like...if you need glasses!

[DT175 1978]

Thanks flyday58.

I have checked the serial number on the frame and the engine and they match, and begin 2A7. I believe it was early / mid production run based on the chart I looked at for '78 (not the lowest serial number for '78, but far from the highest).

I bought the bike recently from a guy on Craigslist. I'd say it was a "semi-survivor". It was mostly all there and would start and run as described above, but was visually pretty rough. A classic case of a bike that's been "rode hard and put away wet".

It's a sunshine red model, all of the plastics were oxidzed / faded to light pink and some places all the way to white. It had what appeared to be the factory original battery in it (completely dry and crusty, off course) at least I assume it was original because it was old and had Japanese writing all over it. There's a few bent parts (the chain protector, foot peg bracket) and I really think the air filter is original from '78 based on how it looks and how it'll kind of turn to liquid if you rub it between your fingers. So I believe it has just over 4,000 original miles and feel it has recieved maintenance / repairs, at least from the second owner, only on an "as needed" basis. The speedometer and tach. work, which makes me think they've worked all along and it shows aprox 4,300 miles.

Other than just cleaning it up a bit, I have added a new battery, reproduction left rear blinker and right front stop switch as well as the carberator I mentioned. The carb is a reproduction. I bought it from eBay (zoom zoom parts) because it was the cheapest one I could find.(free shipping) I'm pretty sure I have the throttle valve and clip installed correctly at this point. It starts and runs with the new carb, actually idles nice, dies when put the choke in after it's warmed up (like it should) and the throttle does work now that I've reinstalled the valve.

That's about all I can think of to tell you about it, but if anyone has more questions - ask away.

Your bike looks great, by the way. Mine looks almost exactly like that of course, but your's is looking good!

[Airhead]

Hello again. So, I mentioned in another thread that I'm working on restoring a '78 DT175 that's just like what I had when I was a kid. Here's what it's doing

As the title says, it cuts out at around 4000 RPM and just kind of struggles and then just surges up to about 6000 RPM and generally runs rough. It was also spewing gas out of the carberator overflow whenver I reved up the RPMS high, so yesterday I replaced carberator. It doesn't spew gas and runs nicer at an idle, but it still does the thing where it struggles as soon as you get into the power band and then it surges almost to the red line. Basically, it runs almost exactly the same as before and maybe a little worse.

I wanted to clean the air filter when I put the new carb on as it was kind of ugly and who knows how old. It ws dripping with oil, I mean dripping and was actually disintigrating when you touched it. I really couldn't have cleaned it without destroying it. So I did use this old, bad, air filter when I tested the new carb. Could that be my whole problem?

The very next thing I want to try is replacing the air filter. What are the chances that's the problem? At this point, I'm not holding my breath (I thought the carb was the problem)

If that doesn't get it, what is the next logical thing? Reed valve?

Anybody have any idea what's going on here?

Thanks,

Matt

Yes you should replace those old old crank seals

[flyday58]

Full agreement with my illustrious colleague. Being from the sounds of it, all original, all the organic bits will be shot or nearly so. It would be worth it to do a full disassemble and replace all the seals you can get, most of which the parts houses have. The ones they don't tend to show up on ebay frequently enough, so a bit of patience is in order to wait for them to show. Again I can personally vouch for changing the crank seals, as can many others on the forum.

We live for this stuff, so take lots of pics as you go, both during the disassemble and reassemble. You will have a fairly permanent record and we all dig seeing an old duffer brought back to how they looked way back when.

[DirtyDT]

Experience shows that while some people can get away with just new fuel and a carb clean, most of the time there are other things that need to be at least looked at. I guess it depends on what you want to do with the bike, how much cash you want to spend on it and how prepared you are for it to, quite literally, go bang.

Where applicable, timing, plug, points, filter, carb etc will all have an effect but without knowing if the seals and compression are right you may never get the bike to run correctly and risk damaging parts which will cost you additional money.

A lot of old bikes have been bodged (made to work using more Heath Robinson innovation than technical expertise). Investigating and confirming what should be on the bike is the start. There are lots of experts here who can help. Next is how deep you want to go, and when. I recently rode 200 miles on an engine whose history is still an unknown. It ran like a pig but did get me there and back. I have also used it in the same condition to take me to work and back. A 42 mile round trip. It still runs like a pig, even with a new carb from a good friend on the forum.

The engine I have mentioned is my spare engine, the original is stripped. This is going to be rebuilt over the winter and that will then give me the confidence that certain parts are OK and if there are issues, they must be elsewhere. Stripping the original engine has already flagged up that both 5th gear cogs (Drive and driven) need replacing. Obviously the engine will be fitted with all new bearings and seals on the rebuild.

Good luck.

[DT175 1978]

First of all – thank you ALL. You guys sound like you really know what you’re talking about and you have all made some good points.

It sounds like you guys are all in agreement that crank seals would be the next most likely thing to cause the issues I’m having?

I was wondering if a problem in the reed valve would cause this, but haven’t heard that mentioned, so probably not I’m guessing?

I think DirtyDT made some points that really spoke to my situation.

Experience shows that while some people can get away with just new fuel and a carb clean, most of the time there are other things that need to be at least looked at.

I think I was hoping to be one of those people who got away with just new fuel and a carb clean.

and how prepared you are for it to, quite literally, go bang.

This one, I didn’t fully think about until I had the bike home and had started messing around with it. At first, my thought was to keep costs down and if it blew up on me, who cares. Then I started wondering, if I’m flying down the road at 45MPH and the engine blows and hard seizes, will I be able to pull in the clutch and roll, or will the rear wheel be locked up no matter what I do? I really don’t know. How dangerous is it to blow the engine in your bike while you’re riding? I have a lot to learn…

I guess it depends on what you want to do with the bike, how much cash you want to spend on it

My original thought was spend as little cash as possible. Money is tight and I don’t want to overspend. My intent was to use the bike just around town, maybe one or two miles to work, generally light use. With that said, I guess I still need it to run properly.

Here’s the deal – I did grow up riding dirt bikes, but I never really learned to work on them or really to work on anything much at all. I’ve always wanted to be more mechanical, the interest and desire have been there, but I haven’t ever learned. At this point in my life I have fixed a few things here and there, just enough to know that I just need to dive in and give it a try if I’m ever going to learn. Nobody is going to come by and teach me, I have to learn it myself.

My thought on this bike is that it would be relatively cheap project for me to learn on. I guess my hope was that I could clean it up, and get it running better (had high hopes just carb would get me there) and then go through it bit by bit, picking out the next thing that bothered me the most, research it, fix it, move on to the next thing that bothers me the most and so on. I was definitely going to apply a “if it’s not broke, don’t fix it” approach. The idea was to figure out what’s broke, and fix that.

I do know that a complete tear down and rebuild would be best. I’d like to do that someday, but for now was hoping I could get it in good running condition as is. I really don’t know. Maybe I should rethink the entire project and go ahead and do a complete restoration, but I’m afraid that I’d get in trouble due to lack of mechanical skills, lack of tools, lack of funds, and lack of spousal support for the project.

Back to the CRANK SEALS: I have questions about this. Crank seals can be done with the engine in the frame, right? I probably have to drain the crankcase oil first? Then just remove the crank case side covers and clutch and I can get at them? There’s at least some chance that the crank case seals may be the issue causing the problems I’m experiencing? I glanced at my Clymer manual regarding this, but it was a little vague (as usual) so I thought I’d ask.

[flyday58]

You're going to need a flywheel puller for the left side. They're widely available on fleabay and other places for not too much scratch.

[DirtyDT]

Oh............................I wasn't saying a full engine strip was the only option. What I was saying was, if you can live with it's foibles, run it until it breaks. See if you can pick up another engine and run that till it breaks.

On a positive side, parts in the US are a lot more plentiful and cheaper than in Blighty. I buy most of my new parts from the US and even with the postal charges, import duty and the postal service charging about 12 bucks for a card saying you have to pay charges it is still cheaper than buying in the UK.

I would not try and tackle the seals without a proper strip down Some seals can be removed with a couple of self tapping screws and a hammer but if you get into difficulties then you will have to strip it.

As Flyday says, you will need a puller. they are cheap.

As I said previously I try and run one engine in and one out. The current bike has the original engine out at the moment and is almost ready to start rebuilding. I just have to get a troublesome stud out and remove a blind bearing. The engine looks like this at the moment.

[Airhead]

Before you dive in any further lets look for evidence that may go someway to proving the crank seals have gone bad. You say you have an issue with the engine revs climbing dramatically even with the throttle closed?

This behaviour would suggest the left hand seal is passing air to the crankcase...so theres a simple test to confirm this

start the engine and with the straw attached to the aerosol spray some WD40 through the slots in the flywheel, take aim at where the crank seal will be in the centre. It this seal is faulty you will see the engine revs drop back down, you will certainly see or hear a change in the engine behaviour, give it a try

[DT175 1978]

I'm sorry, I may not have been clear... I'm not very experienced, and I don't know all the lingo. I'm not sure that we're talking about the same thing...

You say you have an issue with the engine revs climbing dramatically even with the throttle closed?

By throttle closed, you mean "not giving it gas - not twisting the grip" If so, then No, with the throttle closed everything is fine.

It starts and idles just fine.

When I try to go for a test ride, as I twist the throttle slowly and ride away in 1st gear everything is fine and as I slowly increase the throttle, the engine RPMs slowly increase - just like normal... Up untill I hit about 3500 RPM or so, then, it stays at 3500 RPM and does not respond to the throttle for just a few seconds and then surges up to 6500 RPMS (or just below the red line).

If that's what you meant, then I applogize for my lack of understanding.

Thanks

[Airhead]

Yeah thats what I meant, now it looks like its worth investigating other things first although we cant discount the seals on such an old bike!

Having read what you just posted, Perhaps there is and issue with carburation or fuel supply

Take off the fuel line from the carb and open the petcock, you should have a good strong flow of fuel...have you?

Have you stripped and examined the carburettor?

[DT175 1978]

Take off the fuel line from the carb and open the petcock, you should have a good strong flow of fuel...have you?

I have removed the fuel line and opened the petcock to drain all of the fuel from the tank. Unfortunately, I do not have much frame of reference to know if the rate of flow was strong or not. I recall thinking it could have come out faster. The petcock does leak a little when I turn it from "off" to "on".

Have you stripped and examined the carburettor?

No I have not. I didn't think it would be necasary as it's brand new. The problem existed when I had the old carb on, and I went to the new one and the problem still persists, so I thought it must be something different. Do I need a rebuild gasket kit to take the carb apart, or can I just reasemble with the same gaskets (it is new)?

[Airhead]

I have removed the fuel line and opened the petcock to drain all of the fuel from the tank. Unfortunately, I do not have much frame of reference to know if the rate of flow was strong or not. I recall thinking it could have come out faster. The petcock does leak a little when I turn it from "off" to "on".

No I have not. I didn't think it would be necasary as it's brand new. The problem existed when I had the old carb on, and I went to the new one and the problem still persists, so I thought it must be something different. Do I need a rebuild gasket kit to take the carb apart, or can I just reasemble with the same gaskets (it is new)?

Forget the carb for now if It's new

Describe the rate the fuel flows from the disconnected carb feed pipe, It's hard for us to 'guess' what you saw

[DT175 1978]

Forget the carb for now if It's new

Describe the rate the fuel flows from the disconnected carb feed pipe, It's hard for us to 'guess' what you saw

I removed the carb feed pipe at the petcock, so the fuel flowed from the petcock. The stream that came out of the petcock was about the same diameter as the opening it flowed out of... the fuel came out at a slight arch. I recal thinking it may come out at quite an arch and be hard to catch in a jar as the petcock opening points to the side, not strait down, but it came out at a moderate arch and I was able to catch it just below the petcock. Maybe a rate of 3 ounces per minute.... I don't know, that was a wild guess. It slowed as I got near the end. I may have to check again later and give a better explanation, it's hard for me to describe the rate of flow.

[Airhead]

Thats a different petcock to my Europan one then, does the bowl on the petck have a hexagon or square projection at the bottom?

Fuel starvation will also be caused if your airfilter isnt sound (with no holes) and well oiled...this is a priority too

[DT175 1978]

Thank you once again.

I will check the petcock again when I get home and report back - I do believe it has a hexagon shape at the bottom.

So, this situation sounds like fuel starvation?

regarding the air cleaner - it is bad. It is very bad and I know it, and haven't got a new one yet. I plan to get a new air cleaner very soon and install that, and I will report back as to whether that changes anything or not.

[Airhead]

Yes fit a new cleaner, it will just muddy the waters, you will lose vacuum if its holed and that will affect fueling

meanwhile, loosen the petcck bowl just slightly, then drain the tank and remove the petcock completely take care not to lose or break the fiber sealing washer, you may find a filter gauze on the part that goes inside the tank...clean if necessary

I cannot remember if the petcock holding nut undoes clockwise or anti-clockwise so with care try each way but dont over force it

now remove the bowl you loosened and you will see another gauze held in place by the bowl sealing O ring, carefully remove that one and clean if necessary

Tip*** do NOT try undoing the bowl using a 12 point socket or open ended spanner, the hexagon is soft material and will round off easily...use a 6 point socket

[flyday58]

All good stuff. I'll just add: When the revs stall at 3500, does the engine sound like it is 4-stroking, firing every other or even every 4th time? Mine was rescued from a junkyard after 20 years; it wouldn't accelerate much over 3 grand. Ended up water had been getting into the magneto for awhile and had compromised the hi-speed coil. This is it, the small one:

If the fuel issue doesn't pan out, keep this in the back of your mind. Since you haven't had the flywheel off yet, I'd pursue Airhead's route and sort out your fuel supply first.

[DT175 1978]

All good stuff. I'll just add: When the revs stall at 3500, does the engine sound like it is 4-stroking, firing every other or even every 4th time?

I just went and messed with the bike on a break from work, trying to get my details strait on exactly what it's doing.

It does stall at about 3500, and when it does it, it sounds bad. Hard to describe, but doesn't sound right (it strugles and kind of sounds more "farty" sounding than a good running engine, metalic in it's sound) and then all at once it jumps up to high rpms. (like past the shift point, almost to red line)

Just sitting in neutral, and slowly turning the throttle, the RPMs don't to go up past maybe 5000 (and it struggles to get there starting at about 3000). If you turn the throttle quick in little short bursts you can rev it up to and easily past the red line.

By the way - mine is also being rescued after sitting a long time.

I didn't get enough time to check it out just now as I somehow dropped the cable off the throttle valve again - I don't know what's going on there, maybe I dropped it by lossening the throttle cable too much and messing with it as I don't know exactly where that should be set, but, again this time I think I have it right now... but I'm heading soon to check it out some more, try to get clear details on exactly what it's doing.