Author Topic: Backfiring? Any ideas? (Read 4681 times)

interfuse · « **on:** June 28, 2005, 06:58:42 PM »

Lately my bikes been backfiring. It's not too bad driving around the city, but on the highway its doesn't sound very good.

In the city I get a solid pop when up shifting from 1st to 2nd and slight popping under deceleration.

On the highway last night in the mid rpms when I would roll off the throttle it was backfiring quite a bit. There was also a wierd suction sound. But I couldn't tell where it was coming from.

I'm thinking it could be either:

1. Exhaust header needs tightening and/or
2. Needle clip needs to be raised (running lean on the needle)

Anyone else have any other ideas?

TheKillerB · « **Reply #1 on:** June 28, 2005, 08:39:01 PM »

Backfires come from incorrect ignition timing. I would imagine this could result from a worn cam chain or tensioner or the chain jumping a tooth on one of the cams. It could also be an ignitor issue if the ignitor is firing off the spark at the wrong time. Is it backfiring out of the pipe or through the carb. That would tell you if detonation is going past the exhaust valve or the intake valve. Does it run good when it's not backfiring?

gsxr400 racer · « **Reply #2 on:** June 28, 2005, 09:45:37 PM »

lean condition

TheKillerB · « **Reply #3 on:** June 28, 2005, 09:59:40 PM »

Does the lean condition cause it to burn so hot that it causes preignition?

gsxr400 racer · « **Reply #4 on:** June 28, 2005, 10:10:43 PM »

it could but preignition is a different monster your thinking deisling thats preignition. he has a lean pop going on sometimes when jeting you get this wich is sometimes cured by your fuel screws. on decel. i would suspect a intake boot or oring missing from carb top, or petcock vacume line hole!

interfuse · « **Reply #5 on:** June 29, 2005, 12:28:33 AM »

Quote from: "TheKillerB"

Backfires come from incorrect ignition timing. I would imagine this could result from a worn cam chain or tensioner or the chain jumping a tooth on one of the cams. It could also be an ignitor issue if the ignitor is firing off the spark at the wrong time. Is it backfiring out of the pipe or through the carb. That would tell you if detonation is going past the exhaust valve or the intake valve. Does it run good when it's not backfiring?

What I "think" is happening is that air/fuel mixture isn't igniting in the chamber (not enough gas, too muh air). Instead it gets pushed out the exhaust valve and then ignites when it hits the hot pipes. Cars behind me were probably getting a pyrotechnics show.

Another symptom: my engine is producing crappy power from about 4000-7500 rpm (where abouts the backfiring occurs). I'm hoping raising the needles will fix the backfiring and smooth out the bog.

interfuse · « **Reply #6 on:** June 29, 2005, 12:36:26 AM »

Quote from: "gsxr400 racer"

lean condition

I'm not looking foward to ripping into the carbs again. But its getting close now.

Carb jetting sure is fun.

Red01 · « **Reply #7 on:** June 29, 2005, 01:14:19 AM »

exhaust leaks can also cause unburned fuel in the exhaust to light off and pop or boom.

gsxr400 racer · « **Reply #8 on:** June 29, 2005, 01:15:10 AM »

Quote from: "interfuse"

Quote from: "gsxr400 racer"
lean condition

I'm not looking foward to ripping into the carbs again. But its getting close now.

Carb jetting sure is fun.

Time consuming!

echomadman · « **Reply #9 on:** June 29, 2005, 09:03:42 AM »

yeah, make sure its not an exhaust system leak, i had to put an aftemarket baffle into my sv's can and i didnt put it back on tight enough, air leaking into the can oxegenated the unburnt fuel and it ignited, esp at low-mid rpms and under decelleration.

interfuse · « **Reply #10 on:** July 01, 2005, 05:35:33 PM »

Today I finally had a chance to look into the backfiring problem. Here are the two problems and there solutions.

1. I adjusted the carb needles two steps richer. It's like a new bike now. It pulls steadily throughout the RPM range and no backfiring on deceleration. The morons who fixed my bike lost one of the little o-rings from on top of the carb. So I replaced that. They also cracked one of the plastic carb roofs by over tightening it. Luckily I had a replacement on hand. Carb problems solved.

2. A piece that holds my mid-pipe in place rattled off. So my mid-pipe and can were coming loose. I'm not quite sure how it managed to stay attached on the highway last week but thank God it did! I think this was causing the sucking sound and louder backfires. For the time being I've attached some bailing wire to hold the mid-pipe in place. I'll try remaking the metal bracket because I'm still getting the occasional backfire when going over bumps.

Thanks for everyones input. You've all helped keep my bike on the road! If it weren't for this board I probably would have thrown my bike off a bridge by now.