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ok so i picked this bike up for a song and a dance, it runs though barely. i started checking things over and found it has super spark on all 4, but 2 and 4 have yellow spark instead of good blue spark like 1 and 3. is this a sign of a weak coil or something else? also while looking through the clymers (bleh) i got w/ the bike i found a note saying cylinders 1 and 2 have a little over 85lbs compression and 3 and 4 have over 100lbs. none of these are good and im sure this cant have that much wear and tear on it @ 48k so im thinking it probably has valve issues. what is the best way to compression test these horizontal motors and determine whether its a piston and ring prob or valves, ive heard and read "wet" testing isnt conclusive on them.
 

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1993 gl1500, 1976 gl1000
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A leak down test is the way to determine if the valves have a problem. Harbor Freight has a cheap tester the works for me. Compression test is done on a warm engine, all plugs out, choke off, throttle wide open. That will tell if there is low compression but not why. The wet test is to determine if the rings are seating properly or worn. If the bike has sat for a long time the rings could just be stuck and not seating.
 

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That's too low, but many people fail to check at wot. I would do that check first. I've read of many who have low readings but get acceptable readings when checking with wot. Also many flat engines have rings that reseat after driving a couple hundred miles.
 

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First of all download a factory workshop manual.
Now, are you sure that you properly numbered the cylinders? #1 is right-front; #2 left-front; #3 right-rear and #4 left-rear.
That's because you said that #2 and #4 have yellow spark while #1 and #3 have blue. If that is really the case then it's most probably not related to either coils or ignition points because the coils (and respective points) feed #1&2 (one coil) and #3&4 (another coil). So I would suspect the plugs/caps/leads (in that order). Maybe first of all try to clean up the plugs and set the gap to 0.65mm.
Compression should ideally be around 174lbs but the book says everything above 145 and difference less then 14.5 is tolerable. 85 to 100lbs would be way too bad. However, with that mileage, I don't think that's the case unless some valve is stuck or badly blown head gasket or some other mess of that kind. But I doubt the engine would even start then.
Here is what I would do: FIRST check the valve timing (see the manual) belts. Check that the marks perfectly align. If one tooth off the engine will run terribly. Two teeth off - good bye (pistons will hit valves). Change them if they look bad. When you're sure they are ok then adjust the valve clearances (0.1mm, refer to the service manual). Then measure the compression using a common compression gauge. Remember to keep all the plugs out, the choke off and (important) throttle FULLY OPEN while measuring compression! Then you will have more relevant results. However if the battery is low or if the bike was sitting for a long time (stuck piston rings) the results will be lower, but I bet it will be way more than 100lbs.
Good luck!
 

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Discussion Starter · #5 ·
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So i put new plugs in and that resolved my yellow spark problem. I also found the PO hadthe 1 and 3plug wires crossed, i fixed that, put fresh gas in it and it still didnt want to run but was firing on all 4 w/o backfires. I sprayed her w/ a little ether and she roared to life, till it runs out of ether, so im fairly confident its not pulling its own gas as it should and a good carb cleaning is in order. It is smoking a bit so im guessing the rings are stuck but a few of you seem confident that will resolve itself. My next step is to check the timing belts and timing, do these have any tell-tale signs of needing new ones, or should i just change them regardless? Thanks for the help, im loving this forum already :cheesygrin:
 

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Actually i have another question before i go tearing the carbs off, the airbox assembly isnt on it + CV carbs = not proper vacuum for the carbs to function correctly, right? Is this likely my fuel feed problem? Did i just answer my own question? :ROFL:
 

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Yep, I've heard some people say that these won't work well without the air box, though I've never tried it myself.
Maybe the easiest way for you is to put the air box&filter back and see if that makes any changes.
You might also have clogged petcock, fuel passages or floats set too low. You can try unscrewing the fuel drain screws at the bottom of carb bowls to see how much fuel will pour out - it should be continuously leaking for at least 20 seconds if all is OK. If it quits leaking much sooner than that carb starves of fuel. As the gasoline will be pouring down the engine be sure to do this while the engine is cold and not working to prevent fire!
As for the timing belts, well you can look at them and check for any obvious signs of wear but if you're unsure then go ahead and change them, it is always better to know they are good.
 

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1993 gl1500, 1976 gl1000
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The belts need replacing. You probably won't see any signs of wear at all. The problem with timing belts is the breakdown of the internal belts not the outside rubber.
 

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Yep, change the belts. They don't cost much(much cheaper than a new motor), you can get them at a auto parts store or can be ordered by them, and you can rest easier because if you break one you can likely kiss the motor goodbye. There is a tutorial located on this forum that takes you step by step with great photos. I've done it myself and it doesn't require any special tools. It just takes time and attention to detail.

On the fuel issue you need to see if your fuel pump is actually doing it's job(after you've checked the fuel filter.Replace it anyway. And you can use low pressure air, with the tank cap off to see if the line is clear). Take the hose off the carb inlet, put it in a glass jar and crank the engine. Does it pump fuel?

Good hunting!

Hobie
 
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