1978 yamaha dt 175 knocking

onebadcaballero · 07-13-2011, 10:03 PM

Hi there, I just purchased a 1978 yamaha dt 175 two stroke for 300 dollars. It's a little rough and almost street ready (but I still ride it off road). Either way I rode the bike around for the first time and it wound out to about 9 grand when I shifted (power band felt around 5500-8000). No bumps no misses.
So I buy the bike two days later and i notice it isn't idling right and is now taking two or three kicks to start and it feels like it hits a rev limiter at 7 grand. The guy told me he added some more mix to the gas tank (he disconnected the oil mixer so he mixes manually) so I figured he fouled the plug. So I replace the plug and dump out the old fuel for some 93 octane mixed 32 to 1. It runs better as the old plug was fouled but it still has that bad miss @ 7 grand. So I rebuild the carb and after messing around with the mixture screw got 8 grand out of it. The bike defiantly had more grunt but it wouldn't rev past 8 grand in gear.
Then as I'm taking it down the road after further carb adjustment the 33 year old bike makes a funny noise and I feel a loss of power. I pull over and listen to a knocking coming from the crank case. I think to myself I got the crank on my 350 small block chevy turned for 90 bucks, with the matching rod and main bearings for another 60 bucks, this thing is gonna be cheap to fix!
So I call the first machine shop and they say they don't turn cranks

. The next machine shop tells me that two strokes have hardened steel cranks and are impossible to turn so you need to buy a new one. Next the Yamaha dealer tech guy's like "78 dt huh... never worked on one of those I'm not sure". So now I'm here.
Is the miss strictly a fuel issue? Can I turn the crank? What's a safe size overbore. Thanks for your time.

fradykatt · 10-26-2011, 06:33 AM

no u can not turn a crank on a 2 stroke yamaha dt 175 crank is in 2 pieces conected buy a pin with a roller bearing I can fix for you u will need a 20 ton press and a fixture to aline it when re assy stan

fradykatt · 10-26-2011, 06:35 AM

sounds to me like a piston slap and u r over reving it also. i have never had mine to 9 gran about 7500 is max on mine but i have massive torque due to porting on my cly

07-13-2011, 10:03 PM	#1
onebadcaballero Join Date: Jul 2011 Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts	1978 yamaha dt 175 knocking Hi there, I just purchased a 1978 yamaha dt 175 two stroke for 300 dollars. It's a little rough and almost street ready (but I still ride it off road). Either way I rode the bike around for the first time and it wound out to about 9 grand when I shifted (power band felt around 5500-8000). No bumps no misses. So I buy the bike two days later and i notice it isn't idling right and is now taking two or three kicks to start and it feels like it hits a rev limiter at 7 grand. The guy told me he added some more mix to the gas tank (he disconnected the oil mixer so he mixes manually) so I figured he fouled the plug. So I replace the plug and dump out the old fuel for some 93 octane mixed 32 to 1. It runs better as the old plug was fouled but it still has that bad miss @ 7 grand. So I rebuild the carb and after messing around with the mixture screw got 8 grand out of it. The bike defiantly had more grunt but it wouldn't rev past 8 grand in gear. Then as I'm taking it down the road after further carb adjustment the 33 year old bike makes a funny noise and I feel a loss of power. I pull over and listen to a knocking coming from the crank case. I think to myself I got the crank on my 350 small block chevy turned for 90 bucks, with the matching rod and main bearings for another 60 bucks, this thing is gonna be cheap to fix! So I call the first machine shop and they say they don't turn cranks . The next machine shop tells me that two strokes have hardened steel cranks and are impossible to turn so you need to buy a new one. Next the Yamaha dealer tech guy's like "78 dt huh... never worked on one of those I'm not sure". So now I'm here. Is the miss strictly a fuel issue? Can I turn the crank? What's a safe size overbore. Thanks for your time.

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10-26-2011, 06:33 AM	#2
fradykatt Join Date: Oct 2011 Posts: 2 Thanks: 0 Thanked 0 Times in 0 Posts	no u can not turn a crank on a 2 stroke yamaha dt 175 crank is in 2 pieces conected buy a pin with a roller bearing I can fix for you u will need a 20 ton press and a fixture to aline it when re assy stan

10-26-2011, 06:35 AM	#3
fradykatt Join Date: Oct 2011 Posts: 2 Thanks: 0 Thanked 0 Times in 0 Posts	sounds to me like a piston slap and u r over reving it also. i have never had mine to 9 gran about 7500 is max on mine but i have massive torque due to porting on my cly

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