Yamaha 4 Strokes

[JimR]

We all know older Yamaha 4 strokes can be a pain to sort out ... leave a comment here & I guess it will get fixed

[78xt500]

Hi I have a 1978 Yamaha xt500 that seemed to run fine after I rebuilt the carb but now it has problems running right. I replaced all the gaskets and plungers and cleaned the carb a bit recently. The bike seemed to run fine but now it is starting to have problems. The first thing I noticed was fuel spewing out of the overflow on the carb. Not to long after that had happened, the bike started running funny, The bike wants to bog out when given throttle. It does seem to idle decent though. When I first start it I can give it throttle and it will rev up all right but after a few revs(like 2) it starts to bog out bad. Does anyone have any suggestions? Do you think the float stuck??

[JimR]

hmm I guess you have rebult the carb incorrectly
Take it off again and check the jet needle (you may know it as the valve seat assy) and the float for the correct height as it ran ok before I guess you have 'manufactured' a fault. Does the carb have an accelerator pump ? if so did you replace the diaphram there ?
Regards Jim

[cosmo]

Hi, i was wondering if you could tell me if haynes manual on xs 500 is correct on
the subject of alignment of the camshaft(page 71.). The reason i wonder is that i should check if this was correct on my bike , but it is opposite on the bike than what it says in the manual. So could the cams be one tooth off? i noticed that the cams rotate counter clockwise so this would mean that the cams are retarded? If i put the alignmentmarks on the cams approx where the manual says, the v mark is well past the timing mark. The bike starts, but it idles rough and the strange thing is that left cylinder gets hot and right one you can put your hand on and not get burned. I found out that the nipples on the intake for left cyl was loose , this could cause lean mixture? Glued it with ms polymer. This is most likely not the problem. Noticed that the manual said that the timingmark should be aligned to the F mark on left cyl when cyl on top dead centre. Sorry if i spelled anything wrong , im norwegian

[JimR]

Originally Posted by cosmo

Hi, i was wondering if you could tell me if haynes manual on xs 500 is correct on
the subject of alignment of the camshaft(page 71.). The reason i wonder is that i should check if this was correct on my bike , but it is opposite on the bike than what it says in the manual. So could the cams be one tooth off? i noticed that the cams rotate counter clockwise so this would mean that the cams are retarded? If i put the alignmentmarks on the cams approx where the manual says, the v mark is well past the timing mark. The bike starts, but it idles rough and the strange thing is that left cylinder gets hot and right one you can put your hand on and not get burned. I found out that the nipples on the intake for left cyl was loose , this could cause lean mixture? Glued it with ms polymer. This is most likely not the problem. Noticed that the manual said that the timingmark should be aligned to the F mark on left cyl when cyl on top dead centre. Sorry if i spelled anything wrong , im norwegian

Hi Cosmo
welcome to the forum ....

what are 'the nipples' are they the nuts and screws for adjusting the valves ?
the Mark should be at T when the piston is at top dead centre (not there is LT for left & RT for the right and the F mark should align when the points open (when the spark occurs).
Valve Timing L/H cylinder at top dead centre the marks on the cams should line up with the marks on the cam caps.
thanks cegan09
Method to Diagnose
Check & adjust valve clearances intake 0.01 mm exhaust 0.015 mm

Do a Dry compression test - note the readings
Do a Wet compression test - note the readings
the dry compression test you should get 125 psi easy and upto 146 psi
if the wet compression test shows a higher compression reading you have piston/cylinder wear, if it reduces you have valve problems
don't forget to warm the motor before doing the test and hold the throttle wide open and use the kick start.
if this is OK
Remove the clutch (r/h engine) cover and check that the gears (see picture) are set up correctly
this will ensure that when you use the timing marks they are in the correct position. adjust if required
Check and adjust the dwell angle (22½ degrees) and ignition timing LF - left RF right remember to adjust the set on the back plate first and follow with the set on the sub plate.
if this doesn't fix the problem remove & clean the carbs ... refit and balance them.
Other things to check :- both fuel taps work correctly, crankcase breathers are ok and the air filter is in good condition

[cosmo]

Hi, thanks for reply, the niples are mounted on the intake between the carbs and cyl head, this is because they create wacuum on the fuel taps so that they open and deliver petrol. In the manual it said that the alignment mark for intake cam should be 1mm before the cam cap(towards carb) and the exhaust mark should be 1.5mm before the cap. Will try the vet comp test, the dry test were around 120 lbf/in2 and totally equal. Do you have a prosedure to controle the ignition mark pointer, it has been tinkered with. I have fitted the bike with electronical ignition from piranha and i use a test lamp to set the ignition manually. But why is one cyl wery hot and one hardly any temp?

[JimR]

Originally Posted by cosmo

Hi, thanks for reply, the niples are mounted on the intake between the carbs and cyl head, this is because they create wacuum on the fuel taps so that they open and deliver petrol. In the manual it said that the alignment mark for intake cam should be 1mm before the cam cap(towards carb) and the exhaust mark should be 1.5mm before the cap. Will try the vet comp test, the dry test were around 120 lbf/in2 and totally equal. Do you have a prosedure to controle the ignition mark pointer, it has been tinkered with. I have fitted the bike with electronical ignition from piranha and i use a test lamp to set the ignition manually. But why is one cyl wery hot and one hardly any temp?

Vacuum Take Off's:
I know understand you
Valve Timing :
set left cylinder to TDC align the marks (with no cam chain tension) when you tension the chain you may have the marks move to ½ a tooth out on recheck which is fine. if you get a bigger difference you may need to replace the chain and that awful tensioner
The Dry test was at the bottom end of expected so I guess you have some engine wear
You did not say electronic ignition was fitted so we now have a much bigger problem to work out. Have you checked that the read outs from the electronics are correct ? if you have the full CDI set up you may find the pickup coil has failed but if you are using the luminition you then have futher problems ! (Luminition was a very early electronic ignition which required the pickup to be in total darkness)... so which system do you have

[WR450F]

I have a 2006 wr450f, the timing chain has worn out and i am in the middle of replacing it. but before i assemble it i was curious if any one could let me know how much side to side, up and down play is ok in the rod bearing?

[cosmo]

Originally Posted by JimR

Vacuum Take Off's:
I know understand you
Valve Timing :
set left cylinder to TDC align the marks (with no cam chain tension) when you tension the chain you may have the marks move to ½ a tooth out on recheck which is fine. if you get a bigger difference you may need to replace the chain and that awful tensioner
The Dry test was at the bottom end of expected so I guess you have some engine wear
You did not say electronic ignition was fitted so we now have a much bigger problem to work out. Have you checked that the read outs from the electronics are correct ? if you have the full CDI set up you may find the pickup coil has failed but if you are using the luminition you then have futher problems ! (Luminition was a very early electronic ignition which required the pickup to be in total darkness)... so which system do you have

I have opticalpickups.

[cosmo]

Finally found the time to check the marks on the camchain and they all lines up. The timing mark and v mark lined up too. BUT, when i drained the oil i got the sensation that it smelled like gasoline. I put it in a can and it looks like it is about 4 liters. Whats your verdict on that ? Deep shit? This bike is my cousins ride and she got a bit scared of it because it would rev up out of control. This was due to poor springs on the advance timing so it "jammed" in advanced timing and wouldnt come back down. New springs and electronic ignition was installed. So now i got it running fairly good , but it has a little misfire and the right cyl that doesnt get as hot as the left cyl. What to do. I was hoping to not have to split the cam chain to get the cylinders off and rebuild them.