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DT175mx poor running


Davefad
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Hi Everyone and thanks for reading this or even suggest a cure!

My DT175mx hasn't run right since I've had it. It was a full refurb job and have been doing it up over the last 3 years.

When I finished it and first started it, it wasn't running right. Since that day I can't tell you how many times I've tried different things.

When its first started, for the first 10 seconds it sounds ok. Then it starts choking on fuel. It wont rev past 4000rpm and just smokes. It won't tick over and the tick over adjustment screw on the carburettor has no effect whatsoever. If the petrol is switched off, it will eventually run absolutely fine for a brief period until the petrol is used up in the carb.

Things I've tried.

*Replaced the float valve as this was causing the carb to overflow. The old one has a ridge in it and the small spring action was really weak.

*Set the float height to 21mm from the gasket face

*Every jet removed and cleaned, cleaned, cleaned again. Every orifice in the carb was cleaned with carb cleaner. A torch or daylight was observed through the jets to make sure nothing was blocking anything anywhere.   I have taken the carb off so many times to check, measure and clean everything. I'm certain there are no blockages.

*The needle was replaced with an original Yamaha needle from Fowlers. the old needle had a slightly deep scratch on it. So I replaced it thinking this could be causing it to stick.

*Both crank seals have been replaced.

*Exhaust was replaced with 'A Big One' (Made no difference so the original exhaust is back on the bike)

*Removed the autolube pump system and ran it on premix. No difference.

It just won't rev out. 

I have checked the spec and compared the size of the jets, needles and valves and they are all to spec. 

Could it be the reed valves? They look ok. No daylight through the cages...

Thank you

 

 

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How old is this bike? Is it old enough to be wanting to run on leaded petrol?

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On 3/27/2022 at 10:56 PM, Snakebite68 said:

How old is this bike? Is it old enough to be wanting to run on leaded petrol?

Not sure about that. I never considered it. The bike is 1978 so its 42 years old...Its one of the early ones with monoshock, round tubed swingarm. 

I would of thought that unleaded would be fine. 

I tried to start the bike tonight.

Runs sweet for about 5 seconds then it sounds like its gargling on petrol.

Float height definitely 21mm

Needle valve replaced as the old one was leaking.

The new needle valve was not from Yamaha but I wouldn't have thought this particular part would make that much difference. Same numbers engraved on both needle valves.

There was a groove in the needle and the tiny spring was completely useless. The float compressed it under its own weight so I replaced it. Then set the float height to spec. 21mm.

Everything else is standard

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Throwing a slight curve on this, double check the wiring, i know, i know but bear with me, the early monoshock bikes use a 7 wire cdi which uses 2 source coils. One for low speed, one for high. The later bikes had a cleverer cdi and use 1 coil.

Its not uncommon for people to switch the ig parts about thinking everything swaps but it doesent work, well it does badly and if the red is broken or not connected but generally it wont rev out. 

Something for you to look at, have had a dt175 since i was 16, yep bought for 2 quid in the pub (yes at 16!), still in my garage under a blanket and im past 50 something the dt can barely do lol into the wind. 🤣

When you do get it running properly you want to richen things up a touch, e5 and more so e10 run hotter than normal unleaded, (yamaha's are already happy with unleaded as japan never had leaded fuel). So you need to richen the mixture to prevent burning a hole in the piston when riding fast, (go on ask... still have the piston somewhere).

Hope it helps.

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On 3/29/2022 at 8:49 PM, Cynic said:

(yamaha's are already happy with unleaded as japan never had leaded fuel).

Did NOT know this!!!

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  • 3 weeks later...

Easter weekend spent finding the problem.

A very long story short. The carburetor turned out to be intermittently overflowing.

Changed the float needle.

No matter where the float height was adjusted to. I checked to see if the float was actually floating, it was! So, I bought a Chinese carb just to try it. It runs so much better BUT not quite right due to unknown jet sizes etc in the Chinese carb. 

Turns out the body of the carburetor where the pin goes through one of the float posts,  has worn oval shape.... hence intermittent overflowing so I need a replacement good carburetor.

Anyone know anyone with one please? 

The original carburetor is a Mikuni 2H5 00. 

All jets etc are to spec.

Thanks in advance

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Can they be sleeved?

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I think i have an old carb body you can have, you will need to put your parts into it and such. Let me know if you dont find anything.

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Thanks everyone for your help.

I went to Staffordshire classic bike show on Saturday and eventually found one carburetor. Just one....

No identification markings but a mikuni nevertheless.

I had a closer look at the carburetor yesterday and turns out its brand new! No petrol varnishing or muck in it whatsoever. I set the float height and fitted it.

Absolutely brilliant result, the bike runs as sweet as a nut. Chuffed to bits that the problem is now sorted. 

Cost of carburetor from the autojumble £35. Bargain😎

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Yey!

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