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Discussion Starter · #1 ·
Hello so I've got a very frustrating problem that I've been working on for the last few months. Bike will cold start and runs perfectly and idles, but after a while once the engine really gets warmed up (I'm assuming close to operating temp) the rear cylinder will start to bog out more and more until it just completely cuts out. Here is everything I've done to try and fix it:

- Originally thought it would be fueling issue. Complete disassembly of carburetors, cleaned out as per instructions of the service manual and used new gaskets. Adjusted screws 2 3/4 out. Carbs are always synchronized after being touched.

- Replaced all fuel lines since the original ones were cracked and showed a lot of wear. Cleaned out the fuel tank as well and did a petcock rebuild using a kit from All Balls Racing.

- Replaced spark plugs, plug caps, wires and ignition coils. When the front and rear are swapped the problem is always at rear cylinder.

- Checked valve clearances and replaced 6/8 shims that were out of spec. Although the bike seems to be running happier when cold, the problem still occurs.

- Checked for vacuum leaks anywhere. Carb intake boots still looked good. Also sprayed some carb cleaner at and around all vacuum hoses both when cold and warm when the rear cylinder starts to cut out completely.

Pretty stumped here. I've made it this far and I really don't feel like giving up. But I just have nowhere else to look other than the wiring I suppose. Also should probably add that last summer the bike's R/R died on me while on the highway and took a lot of components with it it basically blew up anything that was electrical including my CDI box. Replaced everything and got a MOSFET R/R kit I wired in from roadstercycle. Just felt like I should include that info too just in case. Tonight I took a look at the stator and pulse generator and they seem fine. Tested the pulse gen and resistance was in spec. Gotta head back out to put it all back together but if anybody has any ideas or can point me in the right direction I'd really appreciate it.
 

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After it cuts out, do you see spark on the rear?

If you don't I would suspect your shiny new CDI. Maybe some friendly nearby SV650 owner would let you swap to test.
 

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Discussion Starter · #3 ·
I haven't really been able to check that yet. Should I let it warm up and when it cuts out just pop the rear plug out while it's running and place it on the head? Or should I play it safe and cut the engine first before I check? I wanna be able to do it fast enough to see because as the rear cylinder cools off even for a bit it kinda kicks back in for a bit and might make it confusing.
 

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Or you could simply substitute a known good CDI. It's the only thing I can think of that you haven't swapped left-to-right.
Rectangle Slope Schematic Font Parallel
 

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in the spirit of troubleshooting... grab 2 longish wires and jump the harness from the front to the rear coil. they both fire at the same time anyway.
Circuit component Schematic Rectangle Font Map


Rectangle Font Parallel Schematic Pattern
 

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Discussion Starter · #7 ·
I'll try to see if anybody locally has a known good CDI. I'd be very surprised if this CDI went out on me in less than a couple of months, are CDIs known to just go out like this unreliably even with MOSFET r/r? Kinda strange considering where it's located as I didn't think it would be affected by engine thermals all the way in the back...

I'm getting ready to head out and troubleshoot some more, I'll give that a try with jumping the harness. I didn't know they fired at the same time that's interesting lol.

Yeah I still have some older NGK plugs that are known to be working, just got some wear on them. I'll try these out and reply with my findings.
 

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. . . . they both fire at the same time anyway.
If they both fire at the same time, why are there two lobes on the signal generator cam?
 

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Discussion Starter · #9 · (Edited)
Welp, I did the bypass harness trick, and the fattest fireball I've ever seen shot out the exhaust and now my ears are ringing. Am I also supposed to have all the wires plugged in as well on the HT leads?

Edit: It eventually started up and idled around 2000rpm instead of around 1300 where it usually is. But it only held for maybe about 3 - 5 seconds and it died on its own every time. Also checked for spark as fast as I could with a spare plug, I could see a spark but that could just be with how much it cooled off enough on the cylinder to bring a weak spark to life on a 40 degree day.
 

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there are only 2 wires on the pickup sensor on the flywheel. so they must fire both simultaneously (waste spark) right?
Yes,, there are only two wires. However, there is processing inside the CDI which is able to differentiate between close pulse spacing and long pulse spacing, and fire the appropriate coil for each pulse.
 
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