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I recently did a top end rebuild on my 1971 CB500 because it leaked oil. The procedure went fairly smooth (replaced all gaskets and o-rings, lapped my valves; and put in new piston rings/ cleaned up the pistons) until I went to start the bike.
It sounded like it wanted to start but wouldn't and would back fire through the carbs/ airbox.
My first thought was timing is off. I double checked my cam timing and it was fine.
I then checked valve/tappet clearance, spark (brand new plugs), plug gaps; cleaned my points, checked point gaps, as well as coil to point connections, and everything checked out. I even added fresh fuel for good measure. I also played with the advancement and retardation of the ignition timing; I still get the same result--which is only cylinders, 2 and 4, are firing.
What is strange to me is that both of those cylinders have different coils and points. I think the only correlation they share is the same intake valve sequence.
It still sounds like timing, to me, after everything is said and done. I lined up the notch on the cam rode with the gasket surface (to the right) while at T.D.C. (1.4) like the manual said though. I lined the surface up with the center of the notch. Is it suppose to be lined up the top or bottom of the notch? It could totally be something else though. Just not sure what at this point.
I just had a similar issue on my cb350. Check that they (the points connections) are making a good connection and check the continuity of those wires that go to the points. Mine had a crack under the cloth sheath, not visible when you looked at it and it was doing a similar thing, only firing one of my two cylinders. Make sure your points wires aren't grounding out on the cover or case. Not familiar with the cb500, but maybe that gives you a starting point.
The more I think about it the more it seems like a bad connection. If it's backfiring it means maybe it's getting fuel and only sparking a little every so ofte, therefore creating a backfire with a surplus of fuel and then not firing again.
Backfire through the carb generally means you're running lean - check for air leaks in the intake boots or at the carb; could also be that your timing is retarded.
Check if ya getting spark from both plugs and that ya cylinders are getting fuel (plugs should be wet ) if ok then ya looking at intermitant fault from ya points thru to coils also check for arcing cross ya points turn ignition on open ya points with a say a pencil if ya getting a large spark cross em ya condensors could be suspect try swapping condensors over see if fault moves with them am here for any questions mate good luck i know am havin same probs with my cb350f
Instead of adapting a relay for the turn signals, they do make brake strokes specifically for this purpose. I prefer the brake strobe option because it flashes faster (more likely to get attention) and then holds steady after a number of flashes. They can usually be had for around the $20 mark.
@elduderino: Connections appear to be good and nothing is grounded I agree about why it's backfiring though.
@Rich Ard: There was no problems with the carbs, intakes, or boots before I took the bike apart. The carbs and everything else were stored and I didn't make any changes to them. I played with advancement and the problem continued. Thanks for the ideas.
@yorkie350: Plugs are sparking and getting fuel. I'm not sure what an "arch" would look like; I did notice some spark at the points but isn't that normal? I'll also try swapping condensers to see if that makes a difference. Thanks for the advice.
@Sonreir: I actually have no idea what you're talking about haha. I'm not much of an electrical guy, can you elaborate at all please?
Thanks for all the responses guys. I have off work tomorrow and will try out some of this new insight.
Get a multimeter and check the continuity of your wire from where it connects to the points to your and where it connects to your coil. The connection might be good at the points, but the wire might have a break in it somewhere from point A to B.
EDIT: I just read you were getting spark, so maybe not the issue, but I'd still check.
@yorkie350: I swapped condensers on my points and I got the same result. What does arching look like? I did notice some irregular sparking at my points.
Hiya mate yeah in the manual it says if it not runnin /backfiring and you get a arc ( sparking from points ) you could have problems with ya condensors ive swapped mine over too made a difference one side to other so jumped in for new condensors hope this helps mate keep us posted see if we can't get to the bottom of ya prob
Forgot to mention, and this could open a whole new can of worms, when I lapped my valves I didn't put the valves back in the cylinders they were lapped with; nor did I test them for leaks. It was my first time and I didn't really know what I was doing (this feels just like that one night during my senior year of high-school).
Anyways, a friend of mine suggested, maybe a couple of my intake valves are staying open just enough to cause a back-fire/ all the problems I'm having. Solid theory?
Tomorrow I'm going to pull my valves off and test them for leaks.
Yup ya mate's right doesn't take much for passin thru valves to happen only suggestion is to open her up relap the valves BUT go steady they aint got too much lip before you've gone too far then its gonna be new valves agh$$$ unless ya know which went were . you could try to back off the adjusters on ya rockers slightly mite be under pressure enough to hold ya valves not quite seated had that with mine supposed to be 0-02mm i went 0-07mm sounds like there ticking but got it firing better worth a try before opening her up mate
I live in Jax so I swung by his place last night to hear for myself what it's doing. It's get a nice solid blue spark at all four cylinders and wants to start but putters and pops. The two cylinders that do not fire are 1 and 3 which do not share a coil or points so it makes me think its something other than the ignition. Possibly mechanical timing but I didn't get into all that while I was over there. The only other possibility (and I didn't check, I was only there for about 20 min) could be the spark advance mechanism is stuck or not functioning and causing it to start in an advance position? Does that seem logical? He also had his valve adjustments quite a bit too loose, so I told him to go back and get a bit more drag out of his feeler gauges. Loose valves can delay the mechanical timing a bit.
@yorkie 350: I'm gonna give it a shot. I also noticed my tappet clearances where off by a smidge.
@Wee Todd: everything is firing correctly and my wires go where they're suppose to go. I have really strong spark as well. I'm starting to lean back towards a mechanical error and not electrical. I think my valves (crossing my fingers) are the source of the problem.
I'm going to wrench on it all day today. Ill let y'all know how it goes. Thanks guys.
@904cafe: Just saw your post. Thanks for stopping by the other night. Hoping to have this thing running tonight or at the very least have more insight to what's going on.
No probs mate here if ya need any idea's / answers just make a note of any settings etc or take photo's . (TIMING) Have ya checked the line on ya top cam sprocket is horizontal to ya cylinder head when at TDC of no1 cylinder must be no1 cylinder not no4 when ya points for 1-4 is at the T mark this should make sure ya cam is spot on if its more towards no4 cylinder rotate the motor by the large nut in the centre of ya points one complete turn so the 1-4 T mark is at TDC that should be no1 cylinder at TDC check ya horizontal line against the top edge of ya cylinder head again if its level then ya cam shaft is spot on if not take tension off ya cam chain and slightly move cam shaft while not moving chain till every line is horizontal then pop ya camchain back on after this put ya top rocker cover on and again starting at no1 cylinder at TDC at 1-4 T mark reset ya timing hope ime making sense will try forward the pages from the manual for initial set up mate
I learned a big and expensive lesson today. Check your valves before putting everything back together! a couple of my valve keepers got caught up and a couple of valves didn't close all the way. On top of that, when I first put my cam back in I didn't know about the whole timing thing (I was following a youtube video and not my manual) so when I tried kicking my bike with the wrong timing and unsealed valves I bent a couple of valve stems :'(.
I'll order new valves soon and get this thing back on the road. Thank you guys.
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