Straight Pipe Tuning !

[gs850bobber]

Hi i have a 81 gs850G, that i recently turned into a bobber, and in the process cut the exaust off underneath the rider foot pegs, resulting in having the system be straight pipe. Right now it back fires when down shifting and it is running a little rich is there any adjustment to the timing or the carb that i can make to improve the performance of the engine. Without placing a baffle in the pipe to create some sort of back pressure.

Thanks
GS850 bobber

[makenzie71]

Running a little rich? Cutting the exhaust would lean it out. Backfiring...you can't do away with that. You'll need to, at least, come up a size on the main jets in the carbs.

[renobruce]

Yes, backfiring is usually from a lean condition. You need to do chop tests and read your plugs on the pilot, jet needle, and main jet, and adjust accordingly.

[Commodus]

Hi i have a 81 gs850G, that i recently turned into a bobber, and in the process cut the exaust off underneath the rider foot pegs, resulting in having the system be straight pipe. Right now it back fires when down shifting and it is running a little rich is there any adjustment to the timing or the carb that i can make to improve the performance of the engine. Without placing a baffle in the pipe to create some sort of back pressure.

Thanks
GS850 bobber

Well, you made a really big change to your motorcycle. Since your machine works as a system....now you get to learn about carburetion! And, I'm guessing, the intake side as well since you surely didn't leave on the airbox either.

Step one, search the internet for a good primer on carburetor theory. Then you will be better positioned to decide your next step.

[tone]

If you are running with open pipes & pod filters on stock jetting its going to be very lean carry on running like that without going up on the main jets & you will soon be saying bye bye to your exhaust valves & possibly pistons

try 25% bigger mains as a starting point & see what it does but dont expect this to make it run correctly, i see plenty of messing around with jets & needles in your future

[Steve]

Please don't get the impression that all of us are just trying to blow you off. Quite a few of us know a bit about carburetion, some of them, quite a bit, but with your sawed-off pipes, you are going into unknown territory. If you had an unbaffled header, I would suggest something in the 125 to 130 range for main jets, but I have no idea how the open pipe would change that.

Commodus made a good point. The bike is designed as a system. Any changes you make to the exhaust will affect the intake, as well. As a 'bobber', you probably went minimal on the intake, so I will assume pod filters. Hopefully you have not chosen to go without any filters for a couple of reasons. One, they will get most of the garbage out of the air before it damages the inside of the engine. Two, the CV carbs on your bike need at least some restriction ahead of them in order for the slides to operate. Otherwise, you will not get to use any throttle above idle.

Good luck with your project, and be aware that Z1 has very good prices on main jets, just $1.20 each when I got mine last week. Also, there is good information on BassCliff's website about how to do plug chops to read the spark plugs to see how the engine is running.

.

[Richharr]

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=170287370254&sspagename=STRK%3AMEWAX%3AIT&viewitem=#ht_2412wt_1201

these should work, but measure yours first

[chef1366]

Get a Dynojet kit. You'll have trouble getting the needle right without the taper and notches. You'll also have to drill a larger hole in the slides.

[earlfor]

Also, keep in mind that with cut off, open pipes, every single person you ride past or anywhere near, is going to be ****ed off at you and try to run over your ass with an F350. You aint gonna make any friends and you arent helping the image of motorcyclists anywhere. Just what we need, more noise legislation.

Earl

[Nessism]

Also, keep in mind that with cut off, open pipes, every single person you ride past or anywhere near, is going to be ****ed off at you and try to run over your ass with an F350. You aint gonna make any friends and you arent helping the image of motorcyclists anywhere. Just what we need, more noise legislation.

Earl

I concur with this sentiment. Noisy bikes are a bane to the motorcycle populace at large since they build ill will with the regular people.

[Hammered]

Well he didnt ask for anyones fells on the volume the pipes will make.

My question is, did you leave the pipes the length at where you said you cut them? If yes, rejet. If no or you are willing to add on and have a long opem pipe exhaust, you can solve some of the issues with a slip in baffle. Ive seen them as small as 1 1/2". This should restrict a good bit of the backfiring. Now what is said is true about intake and exhaust working as a unit. If you dont take in as much air as youre trying to output, the bike runs poorly. Same goes the other way around. And just for the record. Sometimes loud pipes are what makes the difference between that minivan driver watching out for you or what little suzy in the backseat was crying about. Just my opinion.

[trippivot]

you are going to make the primary portion of the straight pipes longer. all 4 pipes treated individually. next a 8~10 degree megaphone at 12~14 inches long with a 45 degree reverse cone welded onto the end of the mega phone opening. the outlet hole in the reverse cone will need to be about 1~1.25 inches in diameter. this will reverse the aggressive sound waves that create the backfiring you are hearing.

not a fuel problem you are dealing with .


what straight pipes do is smash sonic moving air into stationary air without diffusion. so you get to hear the sound waves in action.

this is just the way it is straight pipes do that .. make them longer and put a slash cut on the end it will really rattle the windows!!!

[mpls-ugly]

you are going to make the primary portion of the straight pipes longer. all 4 pipes treated individually. next a 8~10 degree megaphone at 12~14 inches long with a 45 degree reverse cone welded onto the end of the mega phone opening. the outlet hole in the reverse cone will need to be about 1~1.25 inches in diameter. this will reverse the aggressive sound waves that create the backfiring you are hearing.

not a fuel problem you are dealing with .


what straight pipes do is smash sonic moving air into stationary air without diffusion. so you get to hear the sound waves in action.

this is just the way it is straight pipes do that .. make them longer and put a slash cut on the end it will really rattle the windows!!!

Dose a slash cut really have an effect on the sound?

[trippivot]

slash cut does deflect the sound waves in an aggressive wave flip.

try a straight cut pipe and put a slash on the same engine you hear the difference clearly!

[rudy]

slash cut does deflect the sound waves in an aggressive wave flip.

try a straight cut pipe and put a slash on the same engine you hear the difference clearly!

It's true. I have turnout slash cuts on my 400. It's silly loud. Good thing I don't even ride it. Flies like a scalded cat though! Considering mufflers if I can find some that will fit nicely.