2001 Buell Blast - starts and idles fine BUT sputters/backfires/dies when on throttle

[blastnovice12345]

The post's title is the TLDR but here's the full story for context:

Bought bike last year (3,700 miles and no mods) in April and noticed discoloration around the float bowl. Bike ran fine all riding season except it seemed to have this problem near the end of the season--especially after the bike had been sitting for a couple weeks when it rained for several days (bike stayed outside but with a cover). However, I could still get the bike running after letting it warm up, although it might sputter in the first 10-15 minutes of riding.

I rode the bike to my dad's in November to ultimately store it for the winter in a non-heated garage. He started the bike a couple of times between then and January, 2023. At that time, I took the tank off and drained it. I also took the air box and carb off because I wanted to clean the carb and fix what I suspected was a failed float bowl gasket (it was).

Since I had the carb off and float bowl off, I decided to clean the carb inside and out and kinda followed this video (https://www.youtube.com/watch?v=_YbDLFoXtLI). I did NOT take the float off, nor did I do a jet kit. I took the idle and main jets off and soaked them in carb cleaner, although the holes on them looked fine. I also took the auto-enrichener plug out and sprayed carb cleaner in it. There was also some discoloration on the diaphragm plastic near the auto-enrichener.

I had some problems with the clutch cable and replaced that, as well as problems with the gas cap (but that's for another post altogether). I did an oil change and transmission fluid change and then finally got the carb, air box, and tank reassembled enough to start the bike a week ago.

First, the starter would turn it over, but it would barely fire. After adding more gas to the tank, it would start, idle, but run really lean (even after adjusting the idle on the carb itself). It started running well enough that I was riding the bike in the backyard to test the clutch cable, but then I dropped the bike on its right side when I went into a skid on grass (broke front right turn signal). Got the bike running again but it was very rough. I also saw the carb move violently during idle and noticed that it had come off the rubber boot with the manifold. After I properly seated and secured the carb to the boot, it ran even worse.

I figured that I must have done something to the carb to ruin it, so I bought a new one off of ebay motors. It's not manufactured by Harley and when I got it, there were two things different about it: the throttle cable bracket was not the correct shape used the one from the OEM carb) and some valve on the side that has a plug on it was different, such that I had to fashion my own plug for it. However, the rest of it was identical to the OEM carb.

I installed the new carb and it would start and idle fine, but would sputter/backfire/die when given a little gas. It would seem to run OK if the throttle was opened up quickly to high RPM, but only after some sputtering.

I disassembled once again and noticed a tiny pool of gas at the mouth of the rubber boot. I put the idle jet (45) and main jet (165) from the OEM carb into the new carb. However, I still have the same problem: bike will start and idle, but will sputter/backfire/die when you start on the throttle.

I'm super frustrated and I'm not sure what else to troubleshoot. The only other things I can think of to check are the spark plug and/or replacing the o-ring between the manifold and the engine (see 8:00 of this video https://www.youtube.com/watch?v=6bOa6QAZm7o). I guess one other thing would be to try different jet sizes for the main jet (new carb came with 190, 180, and 176) I don't think I need to do anything with the idle jet since it seems to idle correctly.

PLEASE HELP!

UPDATE, 6/10/23: Worked on the bike yesterday and here's what I tried.

I confirmed that the fuel is flowing steadily from the tank to the float bowl.

I inspected the spark plug. It's gapped within specs (0.041 in) and shows a little soot with what the manual says is consistent with too rich a mixture.

I inspected the rubber boot between the carb and the manifold. It had a small crack inside, but nothing that went all the way through, such that there's no evidence that air is getting in.

I had already bought new o-rings so I went ahead and took the manifold off and replaced it's o-ring, even though, upon inspection, the existing o-ring looked good and that no air was getting in there.

However, when I started to reassemble the bike to test it, I noticed that one of the wires in the 6-place Deutsch connector had been severed. It's the wire associated with pin 4, which is the TP Sensor Input. It seems like someone may have replaced this in the past because the wire colors didn't match those listed in the service manual. Anyway, I temporarily used some wire clips to reconnect the severed ends and reassembled the bike, except that I left the stock air box off as it was getting too cumbersome to always reattach it to test the bike. However, I did reattach that hose with the three-way fitting which I believe is associated with the emissions. As before, the bike started and idled well, but still backfired or sputtered frequently when given some throttle. Putting my finger over that air hose made the idle run slightly faster. I disassembled everything and once again, there was a little pool of gas in the rubber boot when I took the carb off.

My dad soldered the severed wire and I reassembled to test. Again, I left the air box off. It ran great for several minutes with only a couple of backfires. But then it seemed to backfire more frequently and my dad, who was straddling the bike and testing the throttle said he felt some gasoline blow out of the carb onto his leg.

I really thought that this severed wire to the throttle position control was the cause as it seemed to make sense to me: if the TP sensor is not sending a signal to the ignition module, the spark plug won't fire correctly, causing the backfire/excessive gas between the carb and the engine. And since the TP sensor has nothing to do with the idle, it made sense that the idle was fine. But I'm still having the problem, and what confuses me more, is I found a video on youtube where someone was adjusting the autoenrichener on a carb where he had removed the TP sensor completely, which implies the bike can run properly without the sensor.

Here's a recap of where I'm at with parts: new aftermarket carburetor, but with the original fuel line (not the plastic one they sent with a fuel filter) and the main (165) and idle (45) jets from the original carburetor. To me, it's intuitive to use the smallest numbered jets I have since the mixture seems to be rich. Also, it's my understanding that the carb has to be 'jetted' properly depending on the air intake and exhaust. Since these are both the original, it further makes sense to keep the original jet sizes and it looks like they don't make anything smaller. Just like the original carb, I've capped off some kind of valve on the side that's close to the fuel inlet. The new carb does not have a plug over the idle jet but I haven't adjusted it because I can get the bike to idle fine. I've replaced the manifold engine o-ring, although it didn't appear to be needed.

And again, I'm now testing it without reassembling the bike to test it without putting on the air box, but I feel like that shouldn't make too much of a difference.

I'm heading back tomorrow but I'm not entirely sure where to start. I was thinking of checking this kickstand sensor and BAS, as suggested below. I was also thinking of checking the timing. But aside from this, I'm not sure what else to do and I'm hoping that my long update above sheds some additional light on the problem for the experts out there.

[cvc]

Check your carb boot/coupler for cracks, pretty common if the carb was moving around. Vacuum leaks will burn a piston on those bikes. I have replaced several pistons and top end gaskets on Blasts with 2000 to 18000 miles on them and not had further problems.

[blastnovice12345]

Thanks. I checked the carb boot for cracks and it has split a little on the inside, but nothing that goes all the way through. I've added a detailed update in my original post. If you have a minute, please review and let me know what you think.