Yamaha Big Bear 350 carb problem

[Frank Angerano]

Well thats a good sign as far as the compression goes. Any readings on a compression test ? Should be about 150 ish 

[JacobSlabach]

the compression was 60psi.  I adjusted the valves to spec with no change.  I'm gonna pop the oem carb back on to see if the aftermarket is indeed the issue.

[Frank Angerano]

Ok well you can spray the engine down with a soapy solution and crank the engine and see if any bubbles show up. Maybe a head gasket leak.  If not then maybe a new piston and rings may be in order. 

How do you plan on putting the old carb back on with a broken float strut? 

[JacobSlabach]

It was running before with the oem carb, just also flooding and filling the airbox with gas.

[Frank Angerano]

Can’t see the float operating properly with that break in it but give it a shot.  Just don’t burn the bike if a backfire happens with all that fuel. 🥵

 

[JacobSlabach]

yea, am concerned about hydro locking it and doing some damage...

[Frank Angerano]

What year is that bike ?

[JacobSlabach]

90s.  not sure exact year.  thats another thing I should probably do is find the vin and get that nailed down.  anyone know where the vin is on this bike?

[Frank Angerano]

Yamahas left front lower frame. 

[JacobSlabach]

ok.  I got the vin, but left the paper in my shop lol.  I put some WD-40 in the cylinder and retested compression.  now it al 80-90psi.  So it is rings, but also valves prolly.  I'm thinking it ran rich (carb flooding) for long enough that it built up crud in the cylinder and the valves arnt sealing properly and it blew the rings.  I just thought it would be smoking if the rings were shot....

but yea for those out there wanting to know if their low compression is due to rings, squirt/poor 1 ounce of oil into the spark plug hole and hook up the compression tester, then turnover once or twice by hand to make sure it's not going to hydro-lock, then test compression (with electric start).  if the reading is better, your rings are shot.

[JacobSlabach]

owner gave me permission to dig in.  Head is off.  Inside looks exactly like the bayou.  I'm thinking rings, valves, and a gasket set?  Should I replace the cylinder and piston also or just rings?  Trying to do it as cheaply as possible for this owner.  But if its not a good idea just do rings and valves, I'll get the whole sha-bang

[06kfx440]

Inspect the cylinder walls. Look for scratches up and down as well as cross hatching. If nothing deep comes up then go ahead and get a ring kit. If the piston is okay then just clean it up.

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[JacobSlabach]

ok.  how do you clean the piston, and also the head?  Its got black build-up and I'm worrying about the new valves sealing correctly.  I tried carb cleaner on the bayou with no effect... maybe oven cleaner?  I herd that stuff will cut through some baked on crap..  Maybe I could try it on another piston first (I still have the old bayou piston with crud on it)

[06kfx440]

Try a soft wire brush. Nothing to crazy because you don't want to scratch the metal. I've heard simple green works wonders. Also you can try this stuff called awesome. You can get it from dollar general

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[Frank Angerano]

The cylinder should be honed pit with a honing tool. I’ve attached a pic for you.  The pastor can be cleaned with starting fluid.   Def put a new set of rings in as well. 

That honer goes into a drill and you go up and down the cylinder nice and easy at a slow speed.   

[JacobSlabach]

Ok, so should I hone it only if its scratched or just in general?

[Frank Angerano]

In general is always good.  

[06kfx440]

Oh sh** Frank. I didn't even know those were made. Can you bore with those too?

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[Frank Angerano]

No sir. Just for honing and somewhat bringing a cylinder back from a minor warp condition.  

[JacobSlabach]

btw, thanks to quad crazy for making this forum easy to use!  my bike's derailleur hanger broke and I was trying to ask for help because I couldnt find the part online....  first the forum wouldnt let me post a pic till I had 10 posts (whats the purpose of that????)  then it wouldnt let me post more than 5 posts (and I was being as helpful as I could) in 24 hours...  it's like they are trying to make it as hard as possible to use their forum...  THANK YOU to quad crazy for an easy-to-use forum!!

[JacobSlabach]

getting parts in- I ordered a valve compressor but it hasnt come in yet...  So I have new valves, a top end seal kit, new rings, piston, and wrist pin.  I have the honer too which I will use on the cylinder (it does have some minor scratching from the rings)

[Frank Angerano]

Sounds great just don’t go crazy on the cylinder.   A few passes is all it takes.  On another note the valve springs, when you put the retainer clip in to hold the valve make sure it’s in right.   They fly off and it’s gone! 

[JacobSlabach]

haha thanks- yes I almost lost one on the bayou...

[JacobSlabach]

update:  Rebuilt the bike with:

New piston

New rings

New top end gasket kit

New valves

Honed cylinder.

Compression rose from 60psi to 120psi

Runs great and a lot stronger.  Owner pleased.  I guess this is my first successful rebuild lol- I think I was losing compression again with the last one because I didnt replace the valves or hone the cylinder.

[Frank Angerano]

Nice work @JacobSlabach  any pics to share?

[JacobSlabach]

oh shoot.  I didnt take any because I knew what I was doing this time.  apologies to the forum!  Will take pics of the warrior as I get to it.

[Frank Angerano]

It’s always good to post pics for the forum.  A picture is worth a thousand words. Plus you should be proud of your work! The next guy with the same machine with the same problem will have something to look at down the road.