Bike wont start after top end rebuild.

[Suzguy01]

Im back after not being able to work on my project bike for the whole winter. Bought the 2007 used only running on a single cylinder. Valve was busted, swapped out the top end and timed it according to the manual. Ran a compression test and got 0PSI in either cylinder, switched the timing 180 degrees so that the LT mark was on the front pickup, put it back together and am getting around 140PSI in both sides, so that seems about right. It turns great manually with a wrench and the starter seems fine. The carbs were deep cleaned about 6 months ago. Put in new plugs and battery. It was sitting for around 6 months with a very small amount of gas in the tank, drained the floats, put in fresh gas. I put the petcock on prime for a min then to On. After about 2 mins of trying to start her I got a gunshot loud backfire, but still wont turn over. Checked the air box and had no fuel leaking into there as I had previously. Does this seem like a carb issue to you guys? This has all been a big learning experience for me as it is my first bike. I really want to be able to ride this summer. Ideas and input would be greatly appreciated.

[Big Rich]

Have you tested for spark? With that bang, you're getting some spark but it might not be the proper spark for constant ignition.

And next time you try and start it, pull the plugs if it doesn't start again. If they are wet with gas, you have a spark problem. If they are dry, you have a carb problem.

[Suzuki Stevo]

I changed JUST the head gasket (4 bolts) on my '76 Yammi MX125C 2-Smoke and it wouldn't pop with the kick starter, had to compression start it from a hill, don't know why. My dad said he had the same problem after doing head work on a '37 Chevy   

[lucas]

Hey so, from your post it seems like you may have set the timing wrong. 

Help me understand, you set the timing according to the manual with the RT marks lined up (as the manual instructs) but then you reset the timing with LT lined up?  If so I don't think you will ever get it to run like that.

You may need to reset the timing according to the manual and search for any other problems like disconnected electrical bits, failed coils, clogged jets, closed gas tank petcock, out of gas, fouled plugs, weak battery, faulty clutch safety switch, faulty side stand safety switch.. The list of common no-start causes.

[scotty110788]

I have just this past couple weeks had same problem, replaced top end after one cylinder wasn't running. Was my timing, dismantled million times and fixed it by fluke lol even though all my marks were lined up perfectly, I still don't know what problem was


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[gsJack]

.........................Bought the 2007 used only running on a single cylinder. Valve was busted, swapped out the top end and timed it according to the manual. Ran a compression test and got 0PSI in either cylinder, switched the timing 180 degrees so that the LT mark was on the front pickup, put it back together and am getting around 140PSI in both sides, so that seems about right...................

Front pickup?  There is only one pickup on a 2007 GS and cams are timed with the notches pointed inwards and the rotor aligned with the RT mark towards pickup.  If you turned them 180* and compression went from 0 to 140 psi cams must have been turned with notches outward first time.

With good compression now I'd put a jumper from a non running car battery to it to start.  Backfire is probably from unburned air/fuel collecting in exhaust and finally firing due to weak spark.  I'd get that big bang backfire occasionally on very cold winter days when cranking slowly with weak spark.

[Suzguy01]

Ok, so for the spark, the plugs are literally brand new fresh out of the box, I don't know how to check the spark other then holding it connected into the coils up the the block and press the starter in which case I see a spark. I have not tried push starting since I don't have any hills near by, I guess it is worth a try. I should clarify on the timing, I do have only the single pickup, howvever when I timed it with the RT mark on the pickup, with the cam notches inward I had zero compression, when I did it the same way with the LT mark on the pickup then I got 140PSI. I also want to note that on the diagram it says that the right piston is TDC, on my bike that takes place with the LT mark on the pickup. I don't know if this is something that was done mistakenly by a previous owner or what. My battery is new and is almost always on the trickle charger, I will try it hooked up to a car though.

[gsJack]

.....................I do have only the single pickup, howvever when I timed it with the RT mark on the pickup, with the cam notches inward I had zero compression, when I did it the same way with the LT mark on the pickup then I got 140PSI. I also want to note that on the diagram it says that the right piston is TDC, on my bike that takes place with the LT mark on the pickup. I don't know if this is something that was done mistakenly by a previous owner or what...................

Here we go again, LOL.  Another member or two have said the same, that the left piston was up when the rotor RT mark of a single pickup coil GS was pointed at the single pickup coil with cam notches inward.   

My diagram above was made with info/diagrams taken from 2 places in the 89-99 shop manual and put together to show engine timing.  The single coil pickup was shown for a 98 French GS where it was first used.  I have since found and downloaded a 89-09 shop manual and it shows the same thing for 03 and later GSs so I believe my diagram is correct as shown.



Best to recheck everything to make sure you have it correct.  Your 140 psi compression is good but barely by Suzuki specs, I was delighted to have 140 psi on both cylinders on my 02 GS at 100k miles.  Suzuki specs:

[gsJack]

[Suzguy01]

Lol is the right answer, but when you say check everything, in regards to the timing I have set that chain and cams at least 15 times, 18 pins between 2 and 3 mark with 1 level with the front of the head and cam notches inward. I even have all the torque specs memorized for everything on the top end. I guess now my question is, is it more vital that the Right piston is TDC or that the pickup is aligned to the RT mark, because I cant do both. It was running on one cylinder prior to this rebuild so I know it had to of been timed one of those two ways. Could it have such high compression with the timing being wrong? I know that is a pretty obscure question just not sure what else to do.

[lucas]

You are lining up the RT with the rear pickup, correct?

If you are using the front pickup that might be causing confusion.

[gsJack]

It's a 2007 GS with only one pickup.