2001 750SSie - horizontal cylinder airbox backfire after fuel pump replacement
It's hard to explain this issue without a background essay but I'll attempt brevity.
TLDR version: I replaced a dead fuel pump on my '01 750SSie, and now the horizontal cylinder is backfiring in the airbox, presumably running lean. I've confirmed it's the horizontal cylinder only, because I watched it happen with a Carbtune hooked up. It's a little spit-pop backfire every 3-4 seconds at idle which appeared right after the pump replacement and a stupidly simultaneous airbox & ecu mod. I then undid the mod, which mitigated the popping somewhat. Then I followed Brad the Bike Boy's TPS reset procedure up to the butterfly-balancing step, but stopped there because when I added throttle on the horizontal cylinder with the butterfly balance screw, the backfire came back.
My WAG/hope is, maybe there's some kind of contamination/blockage in the fuel circuit, because I think the old pump was killed by crud in the tank, possibly from a tank repair patch that went bad sometime while the bike was sitting.
I don't suspect belts or valve damage, though I haven't checked these in-depth yet. I've had no indication of anything else being wrong or mis-adjusted prior to the pump change.
My bike is an '01 750SSie that I got "cheap"; it ran fine at sale but had been neglected & unused for a couple of years. I spent a year fixing it up in my spare time. I put a hand-polished 800 header on it and a set of new-made Termignoni cans that cost as much as I paid for the bike, because a little twin Duck with Termis was my red Italian bike dream (Do I need to explain Red Italian Bike disease to this crowd?). When I got it back on the street last July, it rode pretty much perfect but the idle was a bit low.
Then I was laid off & I moved to a different state, so the bike got a 1,200 mile cargo van ride to coastal islands Georgia, promptly rode out Hurricane Matthew with no damage (stored under a friend's porch out of the weather, no flooding, thank goodness), sat for the "winter" while I got a job and got my life in order... I would occasionally start it up, keep the battery charged, refresh the gasoline, etc. until a couple of months ago, I went to take it for a ride and it wouldn't start at all.
Long diagnostic story short, the fuel pump was dead. I wasn't surprised, because it was literally the one item on the bike that I hadn't replaced, cleaned, or fixed during the "repair year". (I HAD found back then that some previous owner had removed the fuel filter and used shitty clear plastic tubing as tank "fuel line", which had since disintegrated into crumbly orange jell-o... I cleaned the tank, put Permatex repair paste over a little patch of surface rust in its back corner, added good submersible fuel line, a new Mahle filter, and new clamps, but didn't replace the pump at the time. The filter sock was orange and ratty and felt delicate, but due to the impending move I didn't have time to source and replace yet another thing)
NOW, the original filter sock had disintegrated, and the old tank repair paste had come up, and was loose and cruddy in the back corner of the tank. Needless to say, I drained and cleaned the tank as best I could before doing the following, over the course of several weeks:
1. I replaced the pump with a Bosch 69544 pump & Bosch 68015 filter sock, which are the current-production direct-purchase equivalents of the Bosch OEM stock pump.
Mistake 1: While I was replacing the pump, I didn't think hard about replacing the fuel filter, because it was still "new" in my mind, even though it might have been subjected to unfiltered crud.
Mistake 2: While replacing the pump, because my bike is kept away from home & I can't work on it easily, I compressed the mod work I had planned months ago, and did an airbox mod, BMC filter, and stuck in a Ducati Performance ECU I got from eBay, all at the same time. (I thought that since the ECU, cut airbox, BMC filter, and my already-installed Termignonis basically are the complete Ducati Performance package, I could "plug and play" and it should all work together reasonably well right away; Yes, I am Sometimes The Idiot and have badly misplaced faith in things that Italians did years ago)
2. I added a gallon of Ethanol-free 90 Octane from the Enmarket over on Wilmington Island and started up the bike with what I hoped were "Performance" mods and the new pump; there was a little hiccup-pop backfire into the airbox every couple of seconds. Frankly frightened of possible damage and the consequences of weird shit I didn't understand, I shut off the bike, did some research, and undid the mods to return to "last known good configuration":
3. I put the stock filter and a stock lid back on the airbox and ran the bike; the airbox backfire still showed up, but now much less, maybe a little burp every 4-5 seconds while running at idle. I shut off the bike and thought some more.
4. Wanting to eliminate throttle body issues, and thinking of how the bike has historically wanted to idle at 8-900 RPM (and how it needed the throttle goosed to start the first time I test-ran it after the "repair year"), I went through Brad the Bike Boy's 2V TPS reset procedure (I've skipped describing some steps below). I found that the TPS was reading ~235mv with butterflies closed (not 150mv), and the bypass screws were almost three turns out each, which seemed high. I set the TPS to 150, and opened the master butterfly to 326mv/1.65 degrees, but because I had slipped with my screwdriver when backing out the balance screw I had lost count of its original position, so I had to test-start it with the balance backed off.
5. With bypasses closed and TPS reset, the left/vertical cylinder pulled on the Carbtune gauge but the right/horizontal did not. Running "vertically biased" with a touch of Fast Idle to start, the bike actually sounded smooth and started without any trouble and did not backfire/pop at all. I swapped hoses and ports on the gauge to make sure the non-reading from the right intake was the bike's fault and not the gauge or its hoses (it wasn't the gauge), confirming that I needed to add more balance screw to get the right intake "on the board".
6. I turned on more right/horizontal butterfly with the balancer screw to get the horizontal cylinder "on the board" and started the bike again. This time both sides were pulling on the gauge, but the airbox backfire was back! I watched the Carbtune while the bike ran, and when there was an airbox pop, the right/horizontal gauge reading would drop off. So, it's the horizontal cylinder that's not running right.
Now at this point I'm unsure what to do first. I'm wondering if some crud got into the fuel circuit when I replaced the pump, and maybe there's a fuel delivery blockage somewhere; but how could that affect only one cylinder if the fuel runs in a loop? Could the horizontal cylinder's injector be crudded up? Is there a way to check that? Should I backtrack and look at all the hoses, etc. in the tank, after draining the fuel?
Could it be the horizontal cylinder has an intake valve problem? Belt/cam problem maybe? I gave them a visual once-over and nothing is obviously amiss...
There's no sign of anything catastrophic like a bad noise or shaking, just this intermittent "pup" in the airbox from one cylinder. Every 3-4 seconds at idle, no bangs, no clanks, no grinding, thank goodness.
(Thanks for reading!)