Backfiring causes?

[Allie]

I had my bike apart for some catch up maintenance, I replaced the intake o-rings, thoroughly cleaned the carbs and replaced all o-rings, checked valve clearances and measured all valve shims. When I got everything together I tried to start it up but it won't start. It kind of wants to catch but it wont, and it pops and backfires. I thought perhaps my cam chain jumped on the sprockets, as I'd turned the engine checking clearances a bit before I remembered to put the tensioner back in. I checked the cam timing as per instructions today and everything seems dead on. The #1 mark on the exhaust cam points straight at the front gasket surface when 1-4 on the advace are TDC. There are precisely 20 pins between the specified marks on the cam sprockets.

It seems obvious that I did something wrong though (who knew??). I'm pretty sure I didn't cross the plug wires, I double-triple checked them. Of course that's not always enough with me, but assuming they are right that leaves fuel, right?

I know that lean conditions will cause popping and backfiring, but can it be bad enough to make it fire directly out the tailpipe, like a gun shot? All the fuel screws are at about 2 1/2 turns out, the air screws are at 1 1/2. The carbs were all dipped, rinsed with spray cleaner and blown out with compressed air. all jets and stuff were thoroughly cleaned as well. All new o-rings from Mr. Barr. The petcock is working and all bowls fill with fuel. The airbox is pretty well sealed. I did the 'bench balancing' as per instructions. I just can't imagine what I'm missing. The carbs are off it again from checking the cam chain, I'm not sure if I need to open them up again, or not. I thought of a possible test (haven't tried it yet), if I put it all back together and cranked the engine with the plug wires disconnected for a few seconds, should the plugs be visibly damp? Any tips or hints? Help? Please?


<<<Final Update>>>

Just because this post ended up being such an odyssey I will put the spoiler here. The main cause of my backfiring seems to be that somehow I left all the ignition timing adjustment screws on the breaker plate loose, so there was quite a bit of 'break dancing' going on under the points cover. Funny, I don't remember touching it before all of this. What was I saying again? Oh yes - By the time SteveN mentioned 'checking easiest things first like ignition timing (always good advice), I'd already discovered a cylinder with no compression, so obviously the bike would not have run well, backfiring or no. The cause of THAT was that our heroine neglected to (okay she was scared to) check and adjust her valve clearances for many thousands of miles. She was damn lucky not to have burned an exhaust valve or two, that's how tight the valves were. After putting MUCH smaller shims in, setting the timing, and bench adjusting the carbs the bike runs FANTASTIC! Today I fine tuned ignition timing with a timing light and it runs even better, I guess I'm ready to vacuum sync the carbs and it should run even better yet. Still pretty smooth though.

So service those valves regularly, kids! It's very easy using Basscliff's tutorial, which includes several variations of other members. It's also a good idea to do a compression check every tune up, to keep tabs on your engine's health.

 

[Steve]

Couple of thngs jumped out at me.

You say "There are precisely 20 pins between the specified marks on the cam sprockets." Make sure you count carefully. Pin #1 should be over the #2 mark on the exhaust cam, pin #20 should be over the #3 mark on the intake cam. That means there are only 18 pins between the marks. That part caught me a few times, it's worth going back to re-check.

Another thing, your fuel screws (the ones on the bottom) should only be out about 3/4 turn, not 2 1/2. That will make it too rich to the point that it will have trouble starting. The air screws (the ones on the sides) are correct at about 1 1/2 turns.

Try those and let us know.

.

 

[Allie]

Yes pins one and twenty are directly over the marks. That's interesting about the fuel screws (the ones under the carb by the bowl, right?) because they definitely were all in the area of 2 1/2 turns out when I took them out. I will definitely try 3/4. The boots did look a bit moist with fuel when I took the carbs off again. I'll try tomorrow, It's a lazy day and I'm full of pie . Thanks muchly!

 

[Steve]

One of these days we are going to hunt you down to get some of that pie.


By the way, how big is your hubby? :-\\\

.

 

[Allie]

Weeeeell everything is back together again. I turned all of the fuel screws from the seated (gently) position out 3/4 of a turn. Now when I crank the motor it very faintly feels like it's thinking about firing. No pops or backfires so that's good. After a bit of cranking a peeked at the plugs, and two and three are a smidge moist, one and four look pretty dry. That could mean a couple of things, or nothing, I guess, but what comes to mind is a spark problem originating from either the coil or the points. I'm pretty sure I didn't jostle anything with the points, but hmmm maybe the coil wire for 2 and 3 got bumped when I was messing with the gas tank.

I'm so done working on it for now, so either later tonight or tomorrow morning I'll check ignition (and take my protien pills!) before doing anything else. 3/4 definitely seems better, if ignition is okay should I just back each screw out another 1/4 turn and try again? It's gonna be a real PAIN turning those screws with the carbs installed .

Thanks muchly. I think it's a fivegone conclusion that if I ever meet GSers in person baked goods are mandatory. My price for opening my mouth!

 

[Skateguy50]

lol, love the extra numbers and talk of baked goods in exchange for wrenching help.

Anyways my 650 is a bit different of course but using a small screwdriver for computer repair I was able to adjust those mix screws with the carbs on the bike. If it is the ones I think the process was to get it to idle as high as it would then move on to the next, and if it got to high set the idle set screw back a bit and continue.

I always try to think about the issue and work backwards stopping at the easiest fixes / checks first. Backfiring is usually from unburnt fuel igniting in the pipes, could be from one cylinder not firing and expelling the fuel to mix with the hot exhaust from the other cylinders, or could just be extra fuel from all the cylinders. My old Monte Carlo did that when one of the wires went and I was running essentially without one of the cylinders.

You need spark to even try to set the carbs so check that they are sparking, usually I just remove 1 plug and hit the starter and see if it arcs. Then repeat and make sure they are all firing. If that is good move on to the carbs and most likely that adjustment Steve was talking about.

Good luck!

 

[Allie]

I lied, I went back out and checked for spark - all okay. While I was at it I checked for compression - NOT okay. Now I realize it should be warm for accurate readings, but 2 and 3 are LOW. 3 is like 50 psi or so, 2 is pretty much nil. The others are 100 psi cold. I don't like THAT... I guess I take the valve cover back off and turn the engine slowly to see if anything looks sticky. I'm really stymied and frustrated. Every shim went back in its proper bucket, and they're still there. I don't see how I could have bent a valve, as the timing chain appears to never have jumped, and appears perfectly timed. Apparently, I'm a mechanical menace.

 

[Skateguy50]

Ouch... that would do it... I am just in the process of buttoning up my 650 top end. If you dig into that level just do yourself a favor and plan to use OEM gaskets on the rebuild. The piston rings ran me $130 and cylinder hone at the stealership another $45, some say pricey but they did a great job and charged me 1/2hr labor for about 90 minutes of work.

I didnt take a lot of pictures of the process but my little rebuild thread has a few... but before you fear the worse can you post up your shim before and afters to see if maybe its just a miscalc leaving some gaps?

(not saying you would but adding one to everything does NOT apply to shims)

 

[Allie]

I always fear the worst! I'm picturing a hole the size of nebraska in my piston, and valves bent into pretzels... . I didn't change any shims, I just measured them and put them back. Nothing should have changed there. Coincidentally, cylinder 2, the one with no compression, had a very fouled plug when I first pulled them out. Something may have already been up there... still before I touched it, the bike's worst offense was that it was getting very cold blooded, and the idle was getting a little hard to pin down. It ran fine when it was warm. Classic signs of intake woes. Somehow I made it all worse.

 

[Buffalo Bill]

No, please no!

No, please no!

Hey I don't think you did anything wrong.
But for comp. test cold put some oil down the plug holes, keep the plugs out and open the throttle wide, or mo-betta have the carbs off.
Read some of this guy's advice: http://www.dansmc.com/mc_repaircourse.htm
Bill

 

[Allie]

Thanks Bill, WOW Dan has lots of good info for noobs like me! I guess you're right, I don't seem to have flubbed anything, but really even the 'good' cylinders are borderline low. Things are really pointing to the valves now, as in it's WAY time for an adjustment. I'll squirt some oil in the cylinders tonight and see what happens, but I bet it's valves. I'm thinking the decline in running prior to my working on it was more than just intake o-rings. I'm also betting that I knocked some carbon loose in #2 when I checked the shims (zip tie method) and it's wedging the valve open enough to kill compression. I did peek in each spark plug hole and verify that all valves at least open and close visibly, so I can rest my mind about bent valves and holed pistons. If I'm lucky I don't have a burned exhaust valve for #2 or 3, and won't need a valve job...

 

[Allie]

Rough compression test results, taken with the carbs on, plugs out, wide open throttle.

No oil:

#1 - 100 psi
#2 - 0 psi
#3 - 60 psi
#4 - 100 psi

Oil squirted in cylinders:

#1 - 150 psi
#2 - 0 psi
#3 - 100 psi
#4 - 230 psi !!!!!!!!!!

It seems fairly obvious that I need a rebuild before it ever sees the road again, and that's just not going to happen right now. I guess it's going into storage and I'll think about it again in the spring

 

[nutsandbolts]

You may want to check you timing marks under cover the at the points. I had to adjust mine slightly after doing a valve shim adjustment. If you don't have a manual, let me know, I can paste the instructions.

 

[Steve]

Before you start a tear-down, I would tap on each of the valves in #2. Use a wood or plastic dowel and a small hammer, place the dowel on the closed valve, tap it so it goes down about 1/8" and slams shut. That might be just enough to unseat any carbon that might be lodged there. Be careful, though, how far you tap the valve open. If the piston is at or near TDC, you might remove all doubt as to whether a rebuild will be necessary.

.

 

[Allie]

I will try it, but if I understand correctly it looks like I need rings no matter what. Still, hopefully that will give me some compression in #2.

 

[almarconi]

I find it hard to believe that the bike was running fine and now there is no compression and it will not start. More than likely something is not assembled correctly. I would remove the valve cover and check your valve clearances and your cam timing. Check over everything you did.

 

[Skateguy50]

Okay, keep it simple Ali and focus on what you did so far.

It ran before you got into this mini project, so start with what you did and what could cause the outcome. It sounds to me like the "zip tie" method has caused an issue with the valves seating, this is my big fear with it and why I fuss around with locking them on the top with a drill attachment on the cams... I should really buy the tool.

Anyways the zip tie isn't that hard and should not have been able to bend the valve or scratch it, that's the entire point of using them I guess. However as you said maybe it dislodged some carbon or even cleaned up some carbon but only in one part.

Now the question becomes can you right that wrong without 1) damaging the engine or 2) doing a top end rebuild. I am not so sure there is a good answer to that.

If it was me (I know its not) I would say if you think you are going to park it for a while anyways... find a good spot to park it and take a peak a bit deeper in the engine before you buy any gaskets or spend any money. If you can fix it up you will need a new base and head gasket to button it back up and the whole (while I am here mentality) can kick in and you might consider the rings and honing as well as the valve stem oil seals. Again start small to big and if you get the head off and remove and store the shims and buckets in order to not mix them up, then flip it over and see what the valves look like. If you are getting 0 compression it will be obvious if its the valves fault.

as a ball park on my 650 I am in about $300 for gaskets and 4 sets of rings, then another $45 for the shop to do the cylinder honing (you can order them up for like $35 online and do it yourself). oh and about 3 weeks of not riding and maybe 8 hours of actual wrenching.

 

[Buffalo Bill]

No Way!

No Way!

Rough compression test results, taken with the carbs on, plugs out, wide open throttle.

No oil:

#1 - 100 psi
#2 - 0 psi
#3 - 60 psi
#4 - 100 psi

Oil squirted in cylinders:

#1 - 150 psi
#2 - 0 psi
#3 - 100 psi
#4 - 230 psi !!!!!!!!!!

It seems fairly obvious that I need a rebuild before it ever sees the road again, and that's just not going to happen right now. I guess it's going into storage and I'll think about it again in the spring

0 compression? You have to find out why. It don't go to 0 on it's own.
Bummer, remember it was running. A little sleep and in a week or so an idea will come to you about how and why. Kind of like figuring out a melody in your sleep.
I think the cam chain is off. Because one cylinder is way high and another is why low, are you dyslexic? Bummer to take it all apart again, but when you're curious you'll unbolt it. It don't cost nothin' to unbolt the top. Try to save the gaskets.
Bill

 

[raistian77]

I find it hard to believe that the bike was running fine and now there is no compression and it will not start. More than likely something is not assembled correctly. I would remove the valve cover and check your valve clearances and your cam timing. Check over everything you did.

+1

you are missing something.

When I used to rebuild diesel and gas engines for a living I made a habit of tapping each valve spring after assembly. Valves are notorious for hanging slightly open. Using the zip tie method is great but has a bad habit of loosening up carbon and lodging valves open just enough to throw compression in the garbage.

Good luck

 

[Allie]

0 compression? You have to find out why. It don't go to 0 on it's own.
Bummer, remember it was running. A little sleep and in a week or so an idea will come to you about how and why. Kind of like figuring out a melody in your sleep.
I think the cam chain is off. Because one cylinder is way high and another is why low, are you dyslexic? Bummer to take it all apart again, but when you're curious you'll unbolt it. It don't cost nothin' to unbolt the top. Try to save the gaskets.
Bill

Well yeah, I'm trying to figure out why it went to 0, and the 230 has me worried too, that just doesn't seem right.

Okay forgetting all about the carbs for the moment, the ONLY thing I did under the valve cover was measure all the clearances (all but two were under .05mm, the smallest I can measure atm), and then using the zip tie method I removed one valve shim at a time, measured it and wrote it down, then put the shim back in its bucket. Then moved on to the next shim. All valve shims went back in their buckts and were seated flat. I did not botch this, period.

Nonetheless no one has more trouble believing this whole thing than me, and obviously I AM missing something. Something happened between starting the project and now to make the bike not run, and I'd love to know what it is.

As I said, when I started doing this job I had the cam chain tensioner out, and turned the crank by hand a few times. All of this really does have the smell of the cam chain being off, and Bill has me really believing this. I want to believe that's why cylinder number 1 is so high. So here's what I did to check and double check the cam timing, and believe me I hope somebody says (lovingly if they ever want baked goods) Look ninny, you missed a step go do it again.

First I rotated the crankshaft so that the number 1 arrow is pointing straight ahead directly at the front gasket face, the area next to the tach drive. The T mark for cylinders 1 and 4 on the advance mechanism is lined up with the timing mark under the breaker plate (with no slack down the front of the chain between the exhaust cam sprocket and the crankshaft sprocket). If I understand correctly, this is what I want. It means the exhaust cam is properly timed. Step two, I removed the idler from the head revealing the entire top run of the cam chain. I counted the pins between the number two arrow on the exhaust cam and the number three arrow on the intake cam. Starting with the pin directly over the number two mark I counted twenty (20) pins; pint number 20 being directly over the number 3 mark on the intake cam. I counted several times, I did not miscount. According to what I understood from the manual this is exactly what I want, 20 pins means the intake cam is properly timed. The reference seems to be starting from the crankshaft, up the front of the chain to the exhaust cam, back to the intake cam. These distances must be precise; the run from the intake cam down the back of the engine to the crankshaft is taken care of by the tensioner. This is the way I bolted everything back together. Again if I did something wrong please tell me because I'd love to go out there and change the cam timing and have the bike run again.

As far as the carbon on the number two exhaust valve, I did a little banging with a plastic toothbrush handle today, to no avail. I'm not quite done trying that though, I'd like to find something a little sturdier to bang with. I also switched the shim from a 2.70 down to a 2.65 to see if I could relieve the clearance a little - still tight, and still no compression.

I also may be a little hasty with the assumption that it MUST need rings right now, though it seems obvious there is some wear there. I think ideally it would be good to get all the valve clearances within spec, solve the mystery with the cam timing, and take another set of compression readings.

Bill you're right sleeping and thinking is good, that's why I didn't mess with it too much today.

I sincerely thank you all for your interest, patience, ideas and advice. It's hard to do those things when you can't see the bike I imagine. Sooner or later someone will hit on it and the bike will at least run again, because it ran before.

Gnight,

Allie