Significant power loss on sudden acceleration: Yamaha DT175

[pauloffner]

Hi, new here on the forum, any help is greatly appreciated.

I've got an old DT175 that has had a few problems over the years. Since fixing all the basic things; cleaning the carby and air filters, and putting a fuel filter in the line, the bike is running very well again. However, the bike has a really significant loss of power under load when i suddenly rev it. It's fine when the acceleration is gradual, and the bike can get going quite fast, but its the sudden acceleration that causes the bike to lose power almost to the point of stopping. All i have to do to regain the power is to back of the revs, and the bike will take off again. The problem is consistent every time.

Does anyone have any idea what this problem could be? I would have guessed that it's due to problems with the seals or rings, but i haven't ever pulled an engine apart?

Is there any way of testing to figure out exactly what the problem is?

cheers,
Paul

[Fizzer Thou]

My first guess would be the seals either side of the inlet manifold.After that check that the timing and the advance/retard timing is working properly,together with the points being set correctly and a clean air filter.

Start off with the basics before assuming that it might be something to do with piston and rings.
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Just talk bikes.What else is there?

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[pauloffner]

have checked the inlet manifold and reeds and they were alright. I haven't checked the timing though.

how would poor timing affect the power under sudden acceleration? slow acceleration is fine.

by the way, thanks for the really quick reply.
cheers,
Paul

[MattWadz]

wrong mixture? main jet too big?
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[pauloffner]

By mixture, do you mean the setting on the carby? I'm pretty sure I've set the mixture correctly there. Not sure if you meant fuel/oil. the fuel is 30:1 ratio.

Not sure about the jet. I'll check.
thanks

[Robby]

It'll be a slide carb, and for whatever reason it's leaning right out when you whack the throttle open.

If it has an accelerator pump then I would be looking at this, in case the seal O-ring between the two halves of the carb is knackered.

Most likely though is water in the carb. Drain the carb into something like a cup and see if there is any water in there. If there is, I would add a dash of meths to the fuel. Meths binds with water and makes it vaguely flammable, and more likely to get picked up with the fuel and carried along out of the engine.

[Blau Zedong]

[pauloffner]

Pretty sure my year model didn't come with an oil pump. The pump was only included in subsequent year models. The carby I have now doesn't have an intake for oil. It's not the original, but I believe the original did not have an intake either.

I've already pulled the carby out many, many times and it doesn't have any water in it. I'm not sure what a slide carb is. Mine looks like the one in the following link https://yamahaclub.com/forums/topic/15457-dt175e-carb/
(Sorry, my computer wont let me add an attachment), but doesn't have the oil intake as i said.

Robby, I did have the though that it may be leaning out, but the float level is set really high and I couldn't understand how it could be lean.

thanks

[Fizzer Thou]

If the float height is set too high this makes fuelling at smaller throttle opening too rich.It is best to set the float height to whatever the manual specifies.
If the timing is set by points there is normally a tolerance that you can set the gap to - say 25-28 thou.The same can be said for the spark plug gap.But if you were to set the relevant gaps at mid-way between what the manual suggest,this will give some leeway.

I have experienced some problems with the resistor element inside of the spark plug cap.This normally reads between 5-10k ohms.If it goes open-circuit this causes quite a delay in building up the charge in the ignition coil before there is adequate voltage to jump the faulty resistor.I have had this on my Exup1000 and when all four cylinders suddenly chimed in together it took me a bit by surprise.
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'

[Blau Zedong]

[pauloffner]

Really?
where is this - next to the magneto? I'm not sure how it attaches to the carby though, Unlike other DT models i've seen, there is no oil intake on mine, only an air breather.

After taking the bike for a ride, something really strange happened which has not happened before. The bike would not stop . Everything seemed to be quite normal when in gear travelling; however, when i put the clutch in to stop it in the shed, the revs would pick up till the motor was running flat out, and I was putting no throttle on. The kill switch wouldnt kill the bike, and the bike was steaming hot, so i stopped the bike by blocking the exhaust.

I understand that this is most likely caused by running very lean as a few people have already recommended, but could also be caused by a bur on the piston. If the problem is running lean, is the most likely solve to increase the size of the main jet? (I think this deals with throttle in the 80-100% band - is this correct?). How many sizes up should i go?

Thanks so far for all the helpful responses everyone

[Blau Zedong]

[pauloffner]

thanks
I was looking in exactly the wrong spot then!

[HD]

Reference to the bike screaming, have you checked the throttle isn't caught in the throttle housing on the carb bit. Like the cable is pulled out and put over the side. I've done this quite a few times!

And stop saying carby
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Rusty '02 Vito Camper + CBR600F3

[pauloffner]

Yes, no throttle cable stuck. The bike was running fine and normal when in gear travelling. Its only when I stopped that the revs would climb .

Should it be "carb" instead .

[HD]

Thats better

And hmm, don't know then. Maybe the slide is stuck up or something or you may have bent the needle and it isn't seating properly when shut off.

I'm just guessing. Best bet would be to take it apart and put it back together.

Although when my inlet rubber cracked my bike was revving stupidly high so check it is on/isn't cracked.

HTH
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Rusty '02 Vito Camper + CBR600F3

[Teflon-Mike]

https://yamahaclub.com/forums/forum/32-yamaha-dual-sportoffroad-bikes/

OK, you are by use of the term 'Carby' an antipodean colonial... confirmed by your location in your profile! Welcome!

Right; link above is font of all knowledge DT175 wise.

You dont say what model you have. I am presuming, and I guess most here are also presuming DT175MX.

But you could have pre-78 DT175 'Twin-Shock'; with five port barel and points ignition. Different kettle of fish.

Of the MX's, we have the 78/79 and early 80 model, with tubular swing arm, then the later box section arm model, that lasted until 1984, in the UK and western markets.

You antipodeans however carried on getting the 'Third World' MX well into the 90's, with revised DT-LC styling; peak seat and triangular tank.

For your problem, first thing to check is the exhaust. These engines ALL of them dont like poor exhausts and they tend to rot in the elbow beneath the manifold. Silensers also get oiled up. Rubber connector also perishes.

Check the pipe well, and possibly treat it to a new exhaust gasket.

Ignition wise, if points model, replace the condensor and clean the points. Condensors are often neglected and can 'go' under load and stop sparks.

If its an MX ought to be CDi ignition. Curiosue arrangement; has two seperately timed ignition coils on the stator, and at revs or under load CDi switches circuits. Could be CDi unit is duff or the 'advanced' primary winding is shot. Or just a loose or mis connected wire!

They are all auto-lube. Often blanked off, but check, if the oil injection line to the front of the carb hasn't been blanked or blanked properly, or the hose is perished, you could under load be dragging air.

Also check the oil pump. Often run on pre-mix, the oil pump not removed, but without oil, lacks lube and siezes stripping the nylon drive gears.

There's very little advantage running pre-mix, TBH and a lot of hassles. If you have the pump, best to use it.

Running pre-mix; 30/1 seems a bit thick, TBH, but you DO need to re-jet to use pre-mix as you are reducing the fuel content of whats flowing through the main-jet, it dont care whether its pure petrol, pure oil or mixture of the two; delivers save volume of fluid per volume of air sucked, regardless, and going fuel rich makes the charge ratio fuel lean, for any jet size. Its now a 3 way ratio....

If we are talking MX's, there are also significant differences between the tube-swing arm MX and the box arm models.

Carburettor and air-box is one of them. Tube arm models dont like Box arm carbs or vica versa, likewise neither is that happy with T-Shock carbs or vica versa.

Also air-boxes on early models was considered 'restrictive' and often butchered. And they aren't TOO happy running 'open' or on cone filters. If you dont have an air-box, or you have badly 'modified' air-box, probably wont help.

Do some part no checking, see what you got and try and find a 'standard' to work from.

Meanwhile.... old 2T's, suffer gummed rings if left derelict, especially if piston at BDC when left stood, the exhaust port open.

While bore & ring wear, and perished crank case seals really are about the only engine troubles these things can give internally, short of bearings colapsing on the conrod & crank!

May be worth lifting the barel. Iron liner they are easily & cheaply re-bored, and replacement oversize pistons widely available.
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[Teflon-Mike]

[Teflon-Mike]