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  #1  
Old 12-03-2014, 03:04 PM
Tobalcain Tobalcain is offline
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Engine wont start

I recently stumbled across 79 sf that was sold to me with a burnt piston. Therefore, I felt that it was running when it" blowed " up. By the looks of the oil sprayed out of the left exhaust, it kinda figured.
Looking for a project and confident that "I can fix that". so I took a compression reading and number 1&2 had zero compression. So off came the head and I found both pistons where nuked...I pulled some parts from a spare engine that I had that was an unknown quantity( and wanting to keep matching #) and purchased new rings and gasket kit.
followed the manual with great care...turned the key, pushed the button and it cranks over but just wont lite up...Not even with ether ...Anybody out there run across this and any ideas. I can tell you that it has drank some fuel from the test bottle and that I have tried reversing the the leads that go to the coils and still nothing. The carbs that where on the bike when purchased have been set aside and a set from a running engine have been installed and still nothing. Just for clarification every part that I used was mic'd and the rings where gaped. Everything torqued per specification and I have spark.
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  #2  
Old 12-03-2014, 03:53 PM
jetmechmarty's Avatar
jetmechmarty jetmechmarty is online now
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OK. We need three things for this machine to run.

1) COMPRESSION 2) FUEL 3) SPARK

One of them is missing. You tested the compression since the work? You say you have fuel and spark. Is the spark happening at the right time?
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  #3  
Old 12-03-2014, 04:03 PM
Tobalcain Tobalcain is offline
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Good question, i thought , could it possibly be 180 out, i switched coil leads thinking that would swap the firing of the coils. I also thought that even if the carbs where junk it would still fire of on the ether. what do ya think
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  #4  
Old 12-03-2014, 07:49 PM
CaptonZap CaptonZap is offline
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A couple of things to check:
Are the cams timed right? You do have a manual to get the procedure, don't you?
The timing disc on the left end of the crank has a slot in the back side. The slot is off center, so that it only goes on one way. It can be put on backward and screw up the fit of the parts, but if you are anywhere near as talented as your namesake, it can be remedied.
Check those two things and let is know if they are in order.
CZ
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  #5  
Old 12-03-2014, 09:23 PM
Tobalcain Tobalcain is offline
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The dots on the cams are aligned with the arrows on the cam bosses. As for the timing, I have not disturbed it from the factory settings. All the paint marks are in place so I feel confident that the prior owner has not messed with it. Im wondering if the burnt pistons could be a clue as to the problem. I just related that issue to leaning of the carbs. The fuel tank was plumbed up so that each petcock feed a set of carbs. I wondered if it could have been as simple as a low fuel situation or maybe an extreme overheat. As I understand it, as long as both cams are on their marks then the cam timing should be correct and the pointer is on the T mark on the timing wheel. Is that how you see it. As for my namesake, I only wish i had his talent...
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  #6  
Old 12-04-2014, 08:47 PM
CaptonZap CaptonZap is offline
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OK, assuming those two things are good, what compression readings do you have on the cylinders since the rebuild?
And what sort of valve clearances do you have?
I like to use a differential pressure gauge to check the cylinders, because you can find out more than just if you have compression. But if you don't have one, just make sure the readings are close to the same across the board.
Take four old plugs and break off the ground straps. Take out your plugs and use the modified plugs plugged in and laying on the cylinder head to check the output of the coils. You should see a healthy spark across all four when you hit the starter. If you don't, you have ignition problems.
CZ
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  #7  
Old 12-05-2014, 09:59 PM
Tobalcain Tobalcain is offline
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Talking

I will give it a try and get back to you.
I work weekends 12 hrs shifts so probably be Monday before I get a chance to play with it again
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  #8  
Old 02-05-2015, 02:33 PM
Tobalcain Tobalcain is offline
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Unhappy

Quote:
Originally Posted by CaptonZap View Post
OK, assuming those two things are good, what compression readings do you have on the cylinders since the rebuild?
And what sort of valve clearances do you have?
I like to use a differential pressure gauge to check the cylinders, because you can find out more than just if you have compression. But if you don't have one, just make sure the readings are close to the same across the board.
Take four old plugs and break off the ground straps. Take out your plugs and use the modified plugs plugged in and laying on the cylinder head to check the output of the coils. You should see a healthy spark across all four when you hit the starter. If you don't, you have ignition problems.
CZ
I did the pressure test and found all the cylinders were holding, what I did not account for was that I did the test prior to installing the cams. So by now i bet you guessed it. After I re-shimmed the valves all was fine.Almost.....

Started the engine and it had poor spark in 2 & 3 so I did all the tests.

Pick up coils 720 ohms

primary side ign coils 1.5 ohms
secondary side on both coils was 11. 5 k although the book called for 15 k.

cdi box was taken out and tryed out in my other f model and it started up just fine.

all the wiring was tested through out the ign circuit and every thing was jeehawing..

put it all back together again expecting a differant result and I got one,,now no spark in all 4 cyl.

scratching my head and for no other reason than what else could i do and I un-plugged the resister and guess what weak spark in all 4 cyl..

so weak it will not fire off the engine..

Any ideas??

Oh side bar...I noticed that the the previous owner had swapped out the glass fuse panel for a blade fuse panel.

He tied all the browns to the central buss bar with the exception of the heavy gauge brown wire that went off to the cancelling devise.
He then fused off each of the rd /ws, rd/ys and the brown wire with the proper fuses. the 30 amp main is in its own fuse holder separate from the others. I assume that this is fine since the starter motor does engage , the turn signals and brake lights work and the horn blows.
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  #9  
Old 02-05-2015, 06:48 PM
CaptonZap CaptonZap is offline
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Quote:
Originally Posted by Tobalcain View Post
I put it all back together again expecting a differant result and I got one,,now no spark in all 4 cyl.

scratching my head and for no other reason than what else could i do and I un-plugged the resister and guess what weak spark in all 4 cyl..

so weak it will not fire off the engine..

Any ideas??



.
Yep, when you say "unplugged the resistor", do you mean you jumped around it?
Use you licorice meter and check the voltage at the red/wht wire that goes to the resistor. With key on, you should have battery voltage. If not, jumper a wire from the battery "+", to the red/wht wire leading to the resistor and see what sort of spark you get. If it is good, check every connection between the battery and resistor, which is covered by many people here, under the "Clean all the connections on the bike, as well as the contacts in the ignition switch, etc. etc." advice column.
If that doesn't cure it, jumper to the other (coil) side of the resistor and try again. If that cures it, replace the resistor. If that doesn't cure it, check the ground wire to the TCI, and continuity of the orange and grey wire from the coils to the TCI, which includes all the connectors.
We'll wait with bated breath, so hurry.
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  #10  
Old 02-05-2015, 07:06 PM
CaptonZap CaptonZap is offline
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Crazy Steve poked one of my retarded brain cells, (thanks Steve) and it occurred to me that the reduced resistance of your coil's high tension windings might be indicative of some shorted windings in the coil secondary.
I don't know of any easy, i.e. DIY at home, way to test for that. So you are left with substitution. I see that you are looking at a coil for sale, so, if you get it, let us know how that affects things.
CZ
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  #11  
Old 02-06-2015, 06:55 PM
Tobalcain Tobalcain is offline
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Thanks for getting back to me, I did check the voltage at the coil lead and had 11.8 volts even though battery voltage was actually 12.3 volts on a freshly charged battery. Battery sits on a tender the whole time it has been in the garage. I did not as of yet but I will test the lead going to the resister and I will get back with you on a reading. I did jump out the coils with a direct battery line to the coils but had no resulted change. I have started leaning toward the coils idea only out of desperation. If there is one thing that drives me nuts is when you become so desperate that you just start throwing parts at a problem.
Another vintage bike (BMW) enthusiast told me that there could be a problem with the spark plug boots. He said that resistance on the cap itself should be 5 ohms. Remove the ends, test them and test the coils again without the ends.
Is there a proper ohms reading for the resister? I do have a spare and I tested it and it had the same reading as the one on the bike. I don't remember off hand what the reading was but I doubt that its the problem. Tomorrow I will due a battery draw test on the starter motor but if it started on 1 and 4 , it doesn't explain why 2 and 3 had weak spark from the first firing.
From the very beginning I knew nothing about this bike and bought it with 2 burnt pistons. I gotta figure the bike was running when it burnt the pistons so I figured that electrically everything should be Okay..
Man, I refuse to let this thing beat me.
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  #12  
Old 02-06-2015, 09:26 PM
CaptonZap CaptonZap is offline
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Well, I don't know your level of electrical savvy, but here are some more things to try.
Measure the voltage at the red/wht coil wire when you crank the starter. (It runs the current through a diode inside the TCI, and if that opens up, you could have problems, although it would affect both coils. And they are healthy diodes, so I don't think that is the problem.)
The voltage at the red/wht wire, while cranking, should read at least battery voltage minus half a volt at most. If it doesn't, quit stalling and clean ALL the connectors on the bike, including the switches on the handle bar, and in the ignition switch. Check ALL the grounds. AAnd for sure the one from the battery to the engine case.
By the by, 12.3 volts at the battery is low, so if that is all it has after charging, you might want to jumper from your car and see if that helps.
The caps should measure 5K ohms, and there are little ceramic pieces inside the caps that are the resistors. They do go bad, but all four at once is unlikely. Make sure than none of them are broken. The one on the bike, under the tank, that the red/wht wire plugs into, should read about 1.5 ohms

Take the plug caps off and then apart, (and BMW.s, back in the day, had some that did not come apart since they were crimped together with a metal outer case, so your'e in luck there), and poke a straight pin into the plug lead core wires. Leave the caps off, and clamp the wires so that there is exactly 1/4 inch between the pins and the engine case. (Or pull the red/wht wire off of one coil and test the other one's leads) Crank the starter. If the coils are able to put out a spark across the gap, they are good.
If you have a car with a coil ignition, you can take it's coil and hook it up, the positive terminal to the red/wht wire, and the negative to the grey or orange wire, and try the same test with the 1/4 inch air gap. That will test the TCI and power to it. (All of the forgoing can be done with the plugs out, to save wear and tear on the starter).
Since it looks like you have a stable of 11's, try the coils off of one of the other ones.
And you aren't beat till you quit.

Last edited by CaptonZap; 02-06-2015 at 09:28 PM.
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  #13  
Old 02-07-2015, 09:26 AM
Tobalcain Tobalcain is offline
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Red face

I have cleaned and tested all the electrical connections, but to be safe I will run through all one more time.

I will get back to you this week as I work overnights on weekends....
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  #14  
Old 02-07-2015, 11:23 AM
motoman motoman is offline
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Quote:
Originally Posted by Tobalcain View Post
I have cleaned and tested all the electrical connections, but to be safe I will run through all one more time.

I will get back to you this week as I work overnights on weekends....
The voltage drop from batt. to coils you stated was the dead give-away they are not clean. Your ignition switch and kill switch contacts ARE the most likely culprits. Removal, dis-assembley of both and drop ALL pieces(including switch itself) in some Evapo-Rust. Let set overnite, remove, wash, blow-dry, a dob of dielectic grease and re-assemble. Remove ground wires from regulator mount, clean ends of them AND frame mount contact area. Un-plug, clean, dielectic grease the five plug-ins behind fuse panel.
Fully charge battery and if 12.5v and below, replace it.....period. (another subject, but batt. tenders shorten battery life and cause early cell sulfation of lead acid wet cell batt.).
A new or good fully charged wet cell batt. voltage will show 12.7-13.1v.
FWIW, what 'key on' batt. voltage shows should be same at pri. coil connections. Any voltage drops indicates poor connections.

BTW, CaptionZap is spot-on.
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