OzVMX Forum
Clubroom => Tech Talk => Topic started by: Vince H on October 19, 2014, 04:23:23 pm
-
hey guys been doing a part resto on my XR75 but cant get it to fire has spark fuel and compression can anyone help me out with what might be wrong ??? ??? even tried start u bastard cheers
-
You don't have something blocking off air to the carby rag in airbox? if you have spark it would fire if you have fuel but you need air maybe your timing is right out?
-
180 out on the cam?
-
no there's nothing blocking the carby that i know of. also have gone over the timing twice myself and had a mate look at it that knows more about bikes than i do and still wont go any other ideas
-
Check you key way on the crank shaft behind the flywheel ,that it hasn't back fired and partly cut the keyway in half.This is a common problem as you still get spark but your about 20 Deg out on your timing.
-
It has some but how much compression?? bent or non seating valve, still check the flywheel key as Hardex has answered first, agree they can cause grief in the baby XR's
-
non seating valve,
The most common problem on small Honda 4t's ;),check inlet valve clearance :)
-
Try a new spark plug and fresh fuel.
-
Birds nest in the exhaust pipe??
-
wires wires wires....ol mate reassembled his xr 500 and spent two days kicking and re building the rebuild till he figured the coil wires were cross :oed. check the wires...
-
Will check these ideas out can someone tell me how to check the inlet valve clearence exhaust is new so should have no blockages I can feel the compression with my finger over plug hole don't have a tester so can't tell exactly how much it has
-
Will check these ideas out can someone tell me how to check the inlet valve clearence exhaust is new so should have no blockages I can feel the compression with my finger over plug hole don't have a tester so can't tell exactly how much it has
Get the piston to TDC. Both rockers (inlet and exhaust) should have some play at this point. Not sure on what the clearance is supposed to be but your manual will have that info. It's just a matter of loosening the nut at the rocker and adjusting the screw until the correct feeler gauge can move ever so slightly between the screw and valve tip. From memory, I think the gap should be 0.04"
-
0.04" both valves cold. ;)
-
Well checked what has been mentioned and all seems to be fine but now I'm not getting spark at the plug and the spark I can get by disconnecting the wire to the coil and hold it against the motor only throws an orange spark now instead of a bright blue like before. Side as has been off while sitting around for couple of days could it be some rust starting to form on the magneto ????
-
Maybe it is time you told us all about what has been done to the bike since it last ran or did you buy it not running.
Most of what has been suggested won't happen to a bike just sitting for 6 months.
Does an XR75 have pionts? If there is no spark make sure the pionts are clean.
-
brought it not going yes has points and they are clean was one of the first things i checked when wont produce spark after it was before. all that been down is a pull down and clean up
-
What do you mean by pull down? Is that stripping the motor or just taking the tank and seat off to give it a clean?
Assuming the motor is sound, check that there are no insect nests in the carb vents or in the fuel tank vent hose(if it has one) and give the carb a good clean out, making sure nothing is bloked. Most faults with points igntions are condensors , resistor in plug cap , coroded points(clean them with emery tape not a file) sometimes top coil and bottom coil occansionally also check kill switches. Check correct stator wire is going to the coil and points, on a most hondas I have seen it is the black one. I'll say it again, the spark plugs with unleaded fuel can seem to go off after a period of sitting and go black. Make sure you have a non-resistor spark plug(that "R" on the side doesn't mean racing) It is quite common that you can see a spark at the plug but when it is inside the motor under pressure it will track down the side and not fire. The same can happen if the spark is week. If you put a timing strobe light on the lead when you kick it over you should see it flash.
-
Remove (unscrew) the plug cap and cut the last 15 mm of the high tension lead off.
This last bit seems to develop high resistance.
-
The motor was stripped down cleaned and put back together car by is clean no blockages it had spark the other day pulled motor out to take off head to check valves and now back in frame and has no spark at plug. Has spark coming from stator wire when grounded to motor( orange in colour) but not at plug once all hooked back up.
-
Are you sure the igntion wire isn't the black one? The plug coming out of the motor normaly has the black wire with a short extra wire connected to nothing. I use it as a test wire for setting the timing.
Make sure that the points wire isn't shorting on anything and disconnect the kill switch and check again for spark.
I would connect it again it no fault before starting.
Have you any resistance readings on the top coil, primary and secondary?
Try replacing the condensor, just cut it of the coil and wire in a anything you can get to fit. VW bug or early holden will do the job.
-
the spark is orange is what i meant the wire is black like u were saying sleepy might be a short like u said will look tomorrow
-
So have spark up to where coil plugs in but not at plug or plug lead. It's a 77 model so condensor is attached to coil can it be taken off and replace or is it easier to replace whole coil if q does anyone have a coil they are selling??
-
Just cut the wire to the condensor and do as I suggested in the previous post. You can buy aftermarket coils with condensor but my money is on just the condensor.
You can test the coil with an OHM meter if you have one. Primary is about 3.5 ohm secondary is about 180k ohm.
If you are near the southside of Bris I could lend you a coil to test. but change the condensor first.
-
will try that sleepy cheers
-
Did you get it going?
-
no not yet have given up fpr the moment will be trying again at the weekend
-
well think i have finally found the problem its compression got my old boy to show me how to properly use a pair or vernier calipers today and turns outs the barrel i have with the standard piston is at 48mm. does anyone know of where i can get my hands on a reasonably priced over sized piston kit or a barrel and piston and rings that will let me get the old girl running again that wont cost me and arm and a leg cheers
-
what is the standard bore size service manual says 47 mm but piston kits on net i found say 48 is standard for a 77 model
-
Hi Vince, looks like your bore and stroke are 48mm x 41.4mm
Below are the specs taken from all 3 mini bikes from 1977 including yours....
(http://i40.photobucket.com/albums/e249/alexbrown64/p19_zps53827243.jpg) (http://s40.photobucket.com/user/alexbrown64/media/p19_zps53827243.jpg.html)
Cheers,
Alex
-
any bike wont go=fuel compression or spark so simple, yet we get cases that do our heads in
-
cheers alex that has cleared up a few things i guess it could be the valve guide then maybe????? ??? ??? ??? or the actual piston and ring will check them next
-
To do a proper diagnosis you should probably do a compression test, and a leakdown test, on the engine before ripping it apart and spending money.
Most little bikes will push start and run on pretty low compression, but not keep running if you let the revs drop.
Someone more familiar can tell you what compression is usual for that motor, but I'd guess anything over 90psi might work ok.
If you've got spark and enough compression, then push starting the bike with a capful of fuel directly down the sparkplug hole can tell you a bit about if it could run.
-
well ended up being a bent valve fixed that problem now my spark plug has died can someone tell me what plug i need as the one stated in the manual which is NGK C7HS i cant seem to get and the one that my local Honda shop told me to use a C7HSA is too small (just falls in the plug hole) :o :o :-\ :'(
-
maybe "po" tapped the spark plug out to next size up? D series??,take the old plug to the shop?? too easy ::), :P
-
hey guys got the xr to start but it wont idle and doesn't seem to rev all the way out any suggestions seems to me like its getting to much fuel cause if i turn off fuel it will pick up in revs right before it runs out of fuel any other thoughts would be helpful tho as its sending me insane trying to get it to run :o :o :o :o :'(
-
Float valve would be a good place start
-
make sure there is a main jet in there as well!
-
Float valve would be a good place start
Yep, mine sticks every 2nd time its been standing around for a while and not poop in the bowl either oh and the points timeing is the other thing that likes to have a tickle from time to time with a nice cleaning of the contacts ;)
-
those carbs on xr200's down to the xr75's normally suffer needle/needle jet wear and come good with those parts being replaced