View Full Version : 08 Blast backfiring under acceleration

jet pilot
04-06-2013, 07:28 PM
I have a 2008 Blast that I bought from a friend over the winter. He used it once, layed it down and sold it to me.

So, over the winter I put on a Firebolt fairing with HID lights, painted it to match and the wife is very happy (its for her). When I went to pick up the bike from the seller, it would turn start but not come off of idle without dying. Knowing that it had sat in his garage all summer, I figured the carb needed to be cleaned. Back at my house, I removed the carb and took it apart. The inside of the carb showed alot of "varnish" and gunk in the carb from sitting. The DAS boot was deteriorated too. I replaced the boot and jetted the carb according to this forum. The bike has a Jardine exhaust on it, I did the airbox mod, Iridium plug. The jetting that I have is:
Pilot 48
main 175
Needle is the 27094-88 sportster N65C. I went with the needle instead of shimming the stock needle.
Air Screw 2.5
Also has: Airbox mod and Jardine exhaust.
Im at 800' elevation

I did the carb work and the airbox mod and it fired right up. Idled perfect. When I blipped the throttle it would respond instantly. I never held the throttle at a steady hi rpm though. This was the middle of winter and I couldnt ride. After the carb work was done, I continued to work on the Firebolt fairing retrofit. By the time I got that done, It was nice enough to go for a little ride. Again, it started great, was on hi idle (choke) and I let it warm up for a minute or two until it came down to the normal idle. I went out of the driveway and started giving it gas and passed about 1/2 throttle it started to back fire.

I brought the bike back in the shop, tore the carb apart to check everything even double checked the float). It was clean inside. I put the carb back on and same thing. This time I was running the bike on the lift in neutral and when it goes past 1/2 throttle it will start to backfire. I took the carb off again, put all the stock parts back in....same thing. I took the carb off AGAIN and put the other jets (mentioned above) back in as the jets were not the problem. This time when it was running, I was watching the gas flow in the carb. At idle and up to about 1/2 throttle the gas sprays evenly out the jet needle. After 1/2 throttle, the fuel begins to spray out of the carb (away from the engine). Im assuming this is because the backfiring is creating a "wind" that is blowing the gas out. I should also note that when it is backfiring, there are "sparks" that can be seen where the exhaust pipe goes into the head.
I checked the static timing next. The red light was not on on the LED. I moved the module about 1/16" of an inch and the light came on. Next I timed it dynamically. Other than getting a nice oil bath, the timing light is showing the two dots on the crank at 1200rpm.
I checked the TPS next. The voltage is supposed to be .5v. Mine was 1.247v. The lowest that I can get the TPS to read is .85 so i set it there. I started the bike, same problem.
Next I started the bike and sprayed starting fluid around the carb intake. No change in rpm. Then I did the base gasket. No change in RPM. Next I did the head gasket. The engine died! It started back up and when I sprayed the same spot, It dies again. I thought that this was weird considering that I did a compression test and the compression is at 130psi. A head gasket will usually yield a lower reading. So I went to my local Harley dealership and picked up the head gasket and a new rocker box gasket. Changed them both. Started the bike. Same backfiring. I ohmed the coil next. The primary coil is supposed to be .4-.6 ohms. Mine is at .8 to .9. The secondary coil test was within spec. Plug wire tested within spec. Just for the heck of it, I pulled off the cam cover and verified that the cams are lined up correctly. After I got the cam cover back on, I did the timing again just to make sure I put the module back in the same spot. I did notice that my timing light does not show a consistant pattern of flashes. There is a "dead spot" in the flashes every so often. Could be my light, but i thought id through that in here too.

The only thing that I cannot test is the ignition module. Is it common for them to act up at higher rpm? From what I read, it does advance the timing in three different steps. So, I am thinking of replacing the coil as that was just outside of tolerances. Im also thinking of replacing the ignition module, but at $150.00, I want to do that last. And if I can find a tps, id change that too.

At this point, Im convinced that the problem is electrical. Tonight I was looking on the forum again and see that I missed something. Someone said that if you are using the Iridium plug, to use an Autozone 15N plug wire. I didnt do that. Could the wire be jumping spark at higher RPM?

I need the forums help here. I think that I have included everything that I have done. I have about 15 hours into this issue and its driving me crazy. I am an experienced mechanic so I will understand the technical stuff, if you have any ideas.....


04-06-2013, 10:00 PM
You know the TPS isnt set right, so why havent you addressed that further?

Is this fault RPM specific or throttle specific or combo of both, can you slowly accelerate up to redline? How fast what gear does it happen?
Im no expert on the blast, but i do know a if its a legit back fire like pops out the intake its timing related. Youve already verified your fueling soooo a carb, with TPS means more throttle, advance the timing, tooooo advanced... Pop out the carb... Hmm TPS .85v spec is .50v i bet it advances the timing too much for rpm.

jet pilot
04-06-2013, 10:14 PM
As I mentioned, I cannot get the TPS to go as low as .5. the voltage does climb smoothly though from idle to WOT without exceeding the 3.7v max. I ordered a new one today. Im with you on the timing issue as the TPS talks to the ignition module for advancement.

I can slowly accelerate to about 1/2 throttle without backfire. Once it starts to backfire, the rpm wont pick up anymore so I can even get it to redline. It will do it in nuetral or in gear. Makes no difference.

04-07-2013, 12:29 AM
Then as long as you are sure no mechanical interference is causing the minimum 0.85v then id suspect the TSP. if a new sensor produces same results then back to mechanical interference.
Sorry for sounding like a dick. Pet peeve to look past small obvious faults, cause those are often the entire issue.
Im also learning about the blast, see as my girl has been beating hers up for a few months now, itll be due for an over haul before 10k -.- damn begginers

jet pilot
04-07-2013, 07:49 AM
No offense taken. At this point, Im ready to change every electrical componant....but then I would be a parts changer, not a mechanic. Every now and then, there comes a problem that stumps a person and he has to ask for help, so I welcome all suggestions.

jet pilot
04-09-2013, 07:13 PM
Any Ideas.....Please...

04-09-2013, 07:48 PM
TPS didnt do it?

jet pilot
04-14-2013, 04:06 PM
Update. I tried a brand new coil as it was slightly out of tolerance. Not any better. Tried the plug wire..not any better.

The TPS should be here this week so that's next. I also picked up a new ignition module in case the TPS doesn't do it.

Crash, I'm with you on the tps...I just didnt have one to try so I was trying different parts as I receive them. Should have it this week though. Found a new one. I like new electrical parts versus used.

04-14-2013, 04:29 PM
By the time your done you should have a new bike it sounds like.( Good luck)

04-14-2013, 05:03 PM
I like new electrical parts versus used.
This has always been my philosophy will all parts, as well as quality manufacturers. None of that value crap from napa or duralast build to explode ina fiery ball of death.

jet pilot
04-27-2013, 07:11 PM
I tried the new TPS. Still cant get the voltage to .5. The idle screw is backed out all the way and there are no obstructions stopping the throttle plate from closing. I can only twist the TPS to .947v.

Tried a NEW ignition module---no Different.

I brought the bike to the HD Dealer who is well versed in the Blast as they still use them as trainers. Time (and $$) will tell, I hope. Keep the ideas coming though if you figure this out..

06-16-2013, 03:58 PM
Just out of curiosity (and in part because i had similar issues, and was going to try a few of the things from this thread before i figured it out)

when you take the carb apart, and you take the larger jet out, theres that little silver tube that sits on top of the jet, and into the throat of the carb. if that one little peice is in upside down, it will cause issues pretty close to what you are having. when it is in the right way, it sticks out into the throat of the carb more than the other way.

06-16-2013, 06:15 PM
1 - 48 pilot is too rich for your modifications and elevation, put in a 45.

2 - Put the stock needle back in, shim it. Your mods don't require an N65 and coupled with your rich pilot could flood your chamber. An N65 has a different taper, it's not the same as shimming the stock needle.

3 - This sounds like a classic case of upside-down needle-jet (the silver thingy).


This is why we tell everyone to NEVER "clean" the carb. You don't need to, especially if the bike runs. I don't care if it sat for a year, try running it first.

EVERYONE always screws up when "cleaning" their carb.

All that a "cleaning" should entail is:
1. Remove fuel bowl
2. Spray carb cleaner through all orifices until it flows nice and clean
3. Put fuel bowl back on.