Jump to content
Sign in to follow this  

79 mx175

Recommended Posts

my brother and i picked up this little 175 the other day for 150$, it had been sitting in a cattle trailer being neglected for 5 years its was a bit rough (seat torn, mouse nest in airbox, carb gummed up, rusty tank etc) but i got as much rust as i could out of the tank and put a filter in line to catch the rest, cleaned out the carb with some chemtool, bought a new airfilter for it and the thing fires right up and runs really strong for a bike thats been sitting for 5 years.

only one issue with the bike it wont idle right and takes a while for the revs to go down. everywhere i look it says this is an air leak but i cant seem to find where it is.

also when i first started it it smoked really really bad and smelled like burning 4 stroke oil and ran kinda choppy but im 95% sure thats because i coated the tank in 2 stroke oil after cleaning it and it just had way too much oil in the first tank

another thing I'd like to know is what common problems (if any) to look out for on these bikes?

thanks for any information i can get on this great little bike

Share this post


Link to post
Share on other sites

I have a 74 mx175 I'm working on. The entire fuel system was full of rust from the tank to the reed cage. I treated the tank and cleaned the carb then removed and cleaned the reed assembly the reeds were metal and were pretty rusty also. Maybe your reeds are giving you a problem if they are rusty?

Share this post


Link to post
Share on other sites
I have a 74 mx175 I'm working on. The entire fuel system was full of rust from the tank to the reed cage. I treated the tank and cleaned the carb then removed and cleaned the reed assembly the reeds were metal and were pretty rusty also. Maybe your reeds are giving you a problem if they are rusty?

nope i checked those too and they were in great shape amazingly i was thinking it could be one of the crank seals i hope its not tho

theres a number on the piston too its 25 does this mean its bored out?

Share this post


Link to post
Share on other sites
nope i checked those too and they were in great shape amazingly i was thinking it could be one of the crank seals i hope its not tho

theres a number on the piston too its 25 does this mean its bored out?

Yes that 25 mean it is bore over call 1st over .25mm, 2nd over .50mm, 3rd over .75mm and all to 4th over 1.00mm

More likey it the Left crank seals that is the problem area

Share this post


Link to post
Share on other sites
only one issue with the bike it wont idle right and takes a while for the revs to go down. everywhere i look it says this is an air leak but i cant seem to find where it is.

Spray some carb cleaner around the boot and see if the revs change. Then the seals/bushings for the throttle shaft. If the revs change, that's the problem area, or part of the problem.

Share this post


Link to post
Share on other sites
Yes that 25 mean it is bore over call 1st over .25mm, 2nd over .50mm, 3rd over .75mm and all to 4th over 1.00mm

More likey it the Left crank seals that is the problem area

ok and is there anyway i can check this without cracking the cases

Share this post


Link to post
Share on other sites
ok and is there anyway i can check this without cracking the cases

You don't need crack open the case

You do need a tool get Yamaha flywheel puller which behind magneto flywheel then you see the crank seal behind the magneto stator plate then take the stator plate off and just replace the seal by using a small seal hook puller.

Some time there are no other option other then Splitting the Crankcase there just no around that if case was made with casting lip that way where you have install the crank seal first before assembly the two half, I think that Yamaha motor it can be done on outside.

Becuase it been siitting for so long it best just replace the both rubber seal don't wastes your time just look at it just replace it.

Edited by shspvr

Share this post


Link to post
Share on other sites

ok so i cleaned up the head and it now has an idle and the revs come back a little faster now and it seems almost normal, just a little carb adjustment and it should be good (i hope) now my next question, could the problems have been caused by carbon build up in the head maybe raising the compression enough to make it ping?

Share this post


Link to post
Share on other sites
ok so i cleaned up the head and it now has an idle and the revs come back a little faster now and it seems almost normal, just a little carb adjustment and it should be good (i hope) now my next question, could the problems have been caused by carbon build up in the head maybe raising the compression enough to make it ping?

Usely with air cooled 2 strokes do rattle a fair bit more the older they get, the worse it is.

Possible the piston is damaged or extreme worn out.

The piston make slapping sound in cylinder walls.

Other possible the connection rod bearing is worn out and the top of piston hit on the cylinder head. I would pull the cylinder head off and recheck inside the piston to see any damaged. If not then take the cylinder off and check the connection rod.

If the ring are stuck to the piston it best to replace them and do ligth hone on cylinder.

Besure clean a round piston with brass wire brush

Ping causes of pre-ignition or detonation

1: Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust like round the piston out exhaust pipe or port.

2: A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges can eliminate this cause).

3: A lean fuel mixture.

4: An engine that is running hotter than normal due to a cooling as let it run to long sitting still.

Edited by shspvr

Share this post


Link to post
Share on other sites

i already did check everything the only thing that looks out of place is a very small ding in the compression dome on the head, probably from the previous piston, im pretty sure it was rebuilt not too long before it sat. i dont think it was pinging but im not sure, ive never experienced bike pinging. but it is running quite well now the revs still hang a bit but i think thats just the bike i had a pe175 that revved almost the same way this does now

Share this post


Link to post
Share on other sites

As long you spark plug tan or ligth gray at idle test and then ride it a round then recheck it if all look good then your spot on but if it black but dry that a tad rich fuel mix then it still ok but you need change them out when they foul out so keep more on hand here a tip bump up spark plug heat range by 1 like let say your stock is B8ES go to B9ES will solve that.

Share this post


Link to post
Share on other sites

my can am 175 had a detonation problem from high compression due to carbon build up,remove the head and scrape the carbon off the piston with the back of a hacksaw blade,new head gasket and you should be good,shell v-power gas doesn't hurt either :smashpc:

Share this post


Link to post
Share on other sites
my can am 175 had a detonation problem from high compression due to carbon build up,remove the head and scrape the carbon off the piston with the back of a hacksaw blade,new head gasket and you should be good,shell v-power gas doesn't hurt either :smashpc:

i already cleaned the piston/head everything is all good now

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...