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yamaha ag 175

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well i've been going crazy. I have 2 complete motors with bad mainshafts, and I plan to pick up a complete parts bike in a week or so which will give me 4 top ends, and hopefully one useable bottom e

so today made the frame loop for the seat and brazed it on, now I just have to paint and mount the rear guard!

Nice! That'll make the commute tons more fun!

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ok so I picked up 2 x ag 175 bikes, a 79 model which is pretty rough and really just a parts donor and a 76 model which is going to be the one I ride.

I also got a complete 3rd engine for free and a frame...

after a fair bit of research I discovered the ag is pretty much a dt 175 with some cost cutting and some durability agricultural/framer style modifications (ie silent block swing arm bushes (dont need grease) etcetc

I want to get one on the road as cheaply and quickly as possible!


this is how I got it - this is the one I have been doing up


terrible photo of the parts donor - thing is rusted to death


stripped down the frame to degrease and see what needed replacement


bare frame after degreasing - new tyres at this stage - dunlop k70s


here shows the bike with the degreased engine back in

so what i've done so far

- new k70 tyres

- new wheel bearings, wheel bearing seals + sprocket bearing

- painted frame & shocks and various other bits black

- chopped off the heel toe gear selector so its just a toe selector

- cleaned carb - reset float height, needle jet height, and mixture

- adjusted throttle cable

- emptied oil tank, flushed bled and adjusted autolube system

I need

- rear brake rod

- silencer for exhaust (will probably just make an alloy can with 38mm perforated tube, glasspack and then 60mm overlaid aluminium tube

- handlebar mirror

- turn signals

- chain

- 16 or 17t front sprocket

- chain guard

I have ordered

- flat bars - will use standard levers throttle etc etc for now

- 43mm pod filter

the lights and wiring is all good which is great! I was stoked to see a front brake switch!

problems at this stage

- bike doesn't want to idle, it will idle but then after blipping throttle it will bog down or idle high... to me this is the classic sign of bad crank seals... so it looks like I'l be tearing down and rebuilding a spare motor lol. lucky i have 2x spare motors... I have gone over the engine a few times to ensure there is no exhaust or intake leak.. I suppose a head leak would cause that but I would expect to hear foofing..

future plans

- full engine rebuild

- boyesen dual stage reeds

- port work

- expansion chamber

- cdI perhaps ?

if anyone in aus has any bits I need in particular

- rear brake rod

- chainguard

for dt 175

please let me know :)


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ok so I decided to tear into the spare ag (read dt125 motor overbored to 175cc) motor I have. its a 78/79 model with CDI magneto which is pretty cool!

so this one semed ok - barrel isn't scored and conrod is good but thinking rings are worn as the compression is fairly average (90 psi dry). I chose the spare motor to keep one motor complete/running on the bike.

anyway I couldn't get it to select gears and upon turning the sprocket over by hand It would reach a point of serious grindy/lumpy resistance. I was concerned that perhaps this gearbox had been run out of oil and the gearbox had seized leading to stripped teeth (as I was given a 3rd engine in parts that had been pulled down to reveal an exploded gearbox) , so I tore it down. anyway what I found was this >



there was a very rusty seized chain on the bike, and what I imagine happened is the PO got the bike running and tried to go for a ride. the chain seized and locked up the mainshaft sprocket which then transferred the load to the gears (which miraculously survived) what didn't survive was the mainshaft splines. They have been twisted off axis. This jammed the gears on the bent splines and prevented you from being able to change gear

so what I did was cut the shaft and remove the gears - all the bearing surfaces and the dogs are in great condition. it is just a new shaft that I need!

so the spare motor will get

- new seals

- new bearings inc the cylindrical roller shaft support bearings

- gasket kit

- rebuilt autolube pump

- new stainless engine case bolt kit

- ported and case matched cylinder

the crank is good

once I do this full rebuild I will swap the cylinder head and piston from the other engine over onto this one as it has much better compression and less little end slop.

I'm not going to rebuild the conrod as its in remarkably good condition!

I initially didnt even think to check to see if the crank seals could be replaced without totally tearing down the motor... well it seems like they can quite easily be replaced by just removing the primary cover and a few of the components in there...

so while Im trying to source a new mainshaft for my full engine rebuild I'm going to slap a seal kit into the engine thats in the bike... I am pretty sure the crank seals in that are gone because its chewing oil and the oil its burning smells like crap (gearbox oil) not like beautiful fresh synthetic 2t oil!

I was pretty slack with the photos but may do a few more when I slap the seal kit in...

in the mean time I am trying to decide whether to pay just for a shaft (quick and small to post) or to pay a similar amount for a whole other engine... I suppose it would be safer to get another engine for spares... but at the same time I dont want too much crap lying about!

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Nice work so far. Can you really have too much? :biglaugha:

I've acquired enough crap to need another shop. :eusa_think:

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so just popped down to the local bearing guy - man he is good and cheap!

picked up left crank seal and right crankshft O ring, had to order double lipped right crank seal -- this is for the motor thats in the frame at the moment... the other motor is getting a full rebuild but I'm not splashing out on bearings etc until I find a replacement gearbox axle (p/n 315-17421)

I found it rather annoying to have to find the seal and bearing numbers on the strip down.. the 175 ag uses a 175 bottom end with a 125 clutch (clutch rod in the side cover etc) and a 5 up shift drum, but in terms of the bearings they seem to be the 175

crank bearings

6205 c3 $11

6304 c3 $11

mainshaft + countershaft bearings

15x22x12 - needle roller bearing - shell type (IKO TA1512)

15x25x12 - either a hardened case needle roller bearing (NKI etc) or a cylindrical roller bearing - Yamaha call it a cylindrical roller (also IKO but may be Yamaha only part)

2x 6304 $8

engine seals (pushrod, main sprocket, gearchange, kickstart and crankshaft) all around $3-4 each

14-24-6 mm

20-30-7 mm

25-40-8 mm - crank

28-40-8 mm - crank RH side (DOUBLE LIP!)

26-38-5 mm

22-34-7 mm

o ring for crankshaft

18x1mm 50cents

I LOVE this engine, its so easy to replace the crank seals.

pulled the magneto and the stator plate off in about 10 minutes, pried out the old crank seal and there was fuel and residue in the bottom of the magneto cover and around the seal so it was definitely blowing a little bit of crankcase mixture out of the left crank seals... used a steel drift to seat the new seal after packing the running lip with grease.

the bearing guy had to order the RH crank seal as its a double lip.. will have it by the end of the week, will have to drain the oil, remove the gearbox cover and then spin the primary crank nut, pull off the spiral gear and remove the collar then replace the O ring, coat with grease, remove and replace the crank seal and then it should really rip and hopefully not smoke out the neighbourhood! probably about 30 mins work...

the puchs I have are a full strip down for the seals... :madflame:

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Always be careful of pattern seals on the crank. They can cause a world of pain if they let go prematurely.

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pattern seals?

Ive been looking at my reed blocks on this engine....

so the ag 175 motor has the smallest of all series reed block,s it seems like it would be ideal to go to the later (1980s) dt 175 reed block


not my picture

ag 175 block on the left (also dt 125, or early at 125 /ct 175)
middle is 1980s dt 175

right is rz 350 block...

I think the dt block should fit my cylinder without modifications, that block with boyesen reeds would be prime I think

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after more browsing the forums I've discovered that the ag 175 is an early dt 125 bored to 175cc... essentially the same setup in terms of carb + reed valve + porting as the earlier yamaha CT 175 series which apparently hit 60mph fairly reasonably.

The DT175 has bigger carb (28mm) bigger ports on piston, bigger reed assembly and a slightly higher comp ratio, so its a big gutsier and can do 70-80mph with sprocket changes

so I am not sure now whether I want to go to DT piston + cylinder for better starting ground or whether I should just rip into porting the heck out of the ag 175 cylinder! I have 3 ag 175 cylinders 3 pistons, 3 cylinders ... so plenty of bits to play with...

I am thinking I'll just work with what I've got at the moment, I don't want to blow a heap of cash on this project I should probably throw in some new rings in.... how much compression do they normally have? this bike had about 150 cold.

in other news, I replaced the right hand side crank seal... I noticed the PO had done some serious dodgery with the clutch... so I used my spares to get that right, a spacer washer was missing so the whole unit was slopping back and forth.

LOL this is what I found when I pulled off the gearbox cover.


erm that seal doesn't look like its in place :o (the seal cover was on I had removed it by that stage)

that explains why it was drinking gearbox oil hah! anyway so I removed that seal and installed a new one and a new crank O ring.

the bike runs much better with new crank seals!

My replacement handlebars also came today, they're 2" rise. I wanted to flip them but the controls hit the tank at full lock so I will have to make up a new steering block to prevent the bars hitting the tank. For the moment I have them normal way up.... I shoulda got drag bars lol

also ordered some turn signals - set of 4 turn signals for $8 dollars is a good deal. I will throw in LED bulbs and use an LED flasher unit to take the load of the battery etc...

things left to get

- mirrors - thinking handlebar end mirrors

- hand grips

- chain

think I am going to make my own chainguard out of aluminium bar... It will polish up nicely and will do the job well.

I also need to figure out a seat arrangement for the bike

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ya there is

I said I removed it before I took the photo...

probably why the seal is cocked out of place and not out completely, the retainer doesnt hold it in all the way around.

this time I just seated the seal in about 3mm past the lip to reduce the likelihood of it popping out again...

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Be carefull if your gonna do the porting , if you go too wide on exhaust port , you"l get ring rattle ,then broken rings :yeah: , ask airhead :hah: ,

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what porting have people had the best results with.

I think I'll start off with an underbarrel spacer to increase the transfer and exhaust duration because the top end of these bikes feels a bit weak. - that could also probably be helped to by going to a 28mm mikuni... then I'll case match and open out the transfers at the base

the intake port doesn't look great either so I may just clean away the lips from cylinder to sleeve

I probably wont go about widening the exhaust just yet the first thing I noticed was that there is a 45 angle in the port - now I wonder if yamaha intended that to reduce short circuiting at low rpm and if removing it will hurt performance.

I'd really love somone with an early style DT 175 to have a port map they'd show me (stock) I want to know how differet the (ag175) dt 125 barrel is from it. (airhead?)

the only thing that saddens me is there is not a lot of meat on the cylinder to open out the transfers.

In saying that a lot of the threads comment on how great an improvement they get by swapping to boyesen reeds and getting the squish down to 0.03-0.05"

the best gain will probably come from making a proper expansion chamber which I'll start doing the calcs for.

It was cool to stumble across the brochure for the 1970s old noguchi racing kits and how much power they added to the dt 125s !

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ok so today I spent (wasted lol) a bit of time mocking up a seat. I think I am going to buy one of the repop vinyl seat covers from ebay that are about $60, and use a low profile foam with a wood base ( the original steel base has fallen apart from rust) $T2eC16NHJIYE9qUcM76YBRk6jHchBQ~~60_12.J

I also looked at the rear guard which is rusted through in spots, I'll need to figure something out here as it needs a rear guard for roadworthy

these re cool too


80 bucks a pair, cheap and probably better than my shot originals but very likely to still be garbage lol

i wont be getting them unless I really like the bike ;)

I really want a tacho, the ag motor has a blanking spot where the tacho goes but from what it looks like no gears or anything.. spose i'll need a whole donor bike to get all the bits cheaply lol

I've also gotta lower the front forks a bit. I think I'll modify the spare set, that or I suppose I could poke the tubes through the top triple crown

good to see the bike isn't smoking now the crank seal is fixed - it puffs a little when cold, and smokes a bit on hard squirts but I thin that may be crap in the exhaust burning off... the exhaust is still very oily inside and smokes after shutdown so I'd say theres a lot of gearbox oil in there.

are there walkthroughs anywhere on conversion to CDI. I imagine I need to swap the stator, magneto and the little CDI pack in, do I need to swap the coil - at first glance the coils looked the same... I have the bits - Is it even worth it>

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thats a good point it does run pretty well, I should clean the points with emery they do look a bit unevenly worn...


this was my first go at making a seat, materials cost me $15 (ply, high density foam, black marine vinyl) lol I am cheap.

I didn't really have much idea or inspiration as to how to make it look cool without sewing a side liner around it etc. This was just staple gun lol

I thought about cable tying the foam to make a brat seat but wasnt sure ? haha

I feel like I should have made the first part longer lol I still have some spare foam and plenty of vinyl..

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the rear brake rod came from ebay usa so threw that on... the brakes aren't half bad, I may sand the drums and throw new pads in... I need to properly setup the front brake etc.

I had another go at the seat with the leftovers from the first attempt.

think it came out better bt still looks sh** - for $15 for both attempts I cant complain... my plan is to get this bike going and see if its worth doing a proper job on, if so I will, if not I will sell, I'm into it for barely any dollars so I am not worried about the sale factor.
also built a silencer for it, it was very loud, its got a barbed baffle inside the main muffler then it has a 20mm diameter insert with glasswool around it covered by the 38mm aluminium pipe... may still be just too loud for roadworthy but we will see... cost me all of $10 lol... I am not sure if 20mm for the end pipe is too restrictive, dont want to cook the piston.


I cut up the rear guard as it was rusted and I am just going to use a short fender at the back to mount the brake light/tail light/number plate

the clutch is slipping so I cant tell how well the bike is performing, - slips like a dog in all gears but worst in 4th and 5th... anyone know what I should do. there is free play in the clutch lever and the clutch seems to be properly adjusted -the springs are tightened all the way down. the plate order is correct per dt 175 manual.

it seems to be revving out nicely, but the engine and exhaust feel very hot after a short ride so perhaps the timing is too advanced. I set it with verniers in the plug hole lol not the most precise... anyone got a better method - I dont have a spark plug mount for my dial guage

other than that most things are getting done still need to wire in blinkers, mount rear guard, make a chain guard. flip the bars and make a steering lock, and do a bunch of general clean up on the bike motor etc etc

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so I think I'll need to teardown the clutch and check all the clutch specs.

measure the spring free height 34mm

check the steel plates are flat on a sheet of glass

check the friction plates are 4mm (or no less than 0.4mm worn) as per manual

ensure the rubber rings are seated properly.

file the grooves in the clutch drum (noticed some minor grooves on first teardown but didn't worry at the time - my picture above you can see them, they loo kworse in the photo than they are. )

what oil do people use (anyone in aus) I use a cheap JASO MA (wet clutch spec) specced 20w-50 but I remember the clutch in my Puch used to slip so I am wondering if its the damn oil and not the clutch setup. should I go to a semi synthetic JASO MA (

could I add another friction plate or would that just be stupid?

The manual shows the order as being
- clutch basket- steel - oring - friction - x5 then clutch cover plate directly riding on top most friction.

I see plate kits are about $70 and spring kits $35. I'd like to avoid buying new at this stage but I guess I might have to.

I have 2 spare clutch drums complete with discs so I could feasibly go through these and make up a clutch with the best parts and see if it still slips.

Cheers for any ideas :)

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