Exhaust Mod Now Backfiring

[sdparis1988]

Hi my name is Scott, I live in Boonville near Columbia Missouri and I recently bought my first xs650.

I would insert a pic but I am new and could not figure out how to do it quickly.
It passed inspection and runs great. I bought it to make an old school bobber but that will have to wait till winter because Im having to much fun riding it.

Here is my current problem. I cut the back plate out of the stock exhaust this weekend to give it a "better" lower sound. I saw this in another thread here on xs650.com. I finished it up and road it around yesterday with no issues.

Today on the way in to work i noticed it was backfiring on deceleration. To be clear, in 5th gear, mid/cruising throttle - then - no throttle and still in gear, it backfires.

What do you think? Where do I start? Remember I am brand new at bike mechanics. I am a fair auto mechanic but it has been years since i worked on anything that had a carburetor and had to have its timing adjusted.

Ive heard a lot of comments on here about rejetting and float clip but it is all foriegn to me. I'm reading like crazy but can only take so much in at once. I'm also planning on switching over to the cone air filters and running high mid length drag pipes.

Should I do something now, wait till I make all those changes, rejet with each change?
I certainly don't want to do any damage to the bike. Let me know what you think. i am seriously looking to the voice of experience since I have none here.

 

[ippytattoo]

This would be a good place to start as to tuning the original carbs. Also a little more info about the bike would help such as year and any other mods that have been done. You can put all of that in your signature like I have to help others help you.

 

[sdparis1988]

thanks! Ill look over the carb info tonight but i know it will be the weekend before i have the time to tackle it. I'm looking forward to the learning experience. like I said, it has been 20+ years since i have owned anything with a carb.

 

[sdparis1988]

i will work on my signature too.

 

[weaselbeak]

You got rid of a good deal of your muffling, so now you are hearing what was always there, it's just not being muffled out now.

 

[DEMON DAVE]

back firing due to a too lean condition , pull the plugs and check the color very light or near white way too lean , a light tannish color is near perfect , blacker coloring is rich.
open pipes , open air filters lead to lean condition 99% of the time

 

[XSLeo]

An engine is an air pump. It draws air in on one end and pushes it out the other. The carbs add the proper amount of fuel to this air as it passes through.
When you change the amount of air flowing through you have to adjust the carbs so they mix the proper amount with the air flow.
The carb guide ippytatto provided a link too has a section on carb tuning for mods. Gives how to test, what to change from what the testing indicates.
Follow the steps.
Leo

 

[just_puttering]

I have a 79 xs650 special with shorty open pipes and mikes xs carbs. I called mikes and he said I should not have to adjust the jetting, but it backfires like crazy. Any thoughts? I have heard exhaust manifold leaks contribute, so I am replacing whatever sealing rings are between the exhaust and the engine outlet...

 

[grizld1]

What you have is knockoffs of 28 mm. Keihin PWK 2-stroke carbs bored to 32 mm. because only one needle jet per venturi size was offered in that series and punching out the 28's was the only way to bring the cruising range mixture sort of under control. There are many threads on these carburetors detailing the grief they've caused most owners. Do a search and you'll find plenty. BTW, "shorty open pipes" are performance killers. All they're going to do for you is to make an obnoxious amount of noise--unless of course you get some kind of kick out of annoying others, in which case they'll get the job done just fine.

 

[ANLAF]

Welcome to the forum, sdparis.

Have you got anything on your engine air breather outlet(s) just above the carbs? XSLeo recently told me how power brake valves work when fitted on these breathers - they stop oil leaks and also help to correct the vacuum conditions in the engine modifications can alter.

They are inexpensive and worth fitting.

Anlaf

 

[just_puttering]

grizld1` Well I can see why you have that moniker. I bought the bike with the carbs and shorty pipes, not my choice. Just learning, like eveyrone else. Figuring stuff out is the fun part (really)

 

[1974jh5]

When all else fails, check the exhaust carefully for air leaks. A leak between the pipe and the head will suck some extra O2 in there and lead to backfiring. It's not usually violent like the kind that comes from really bad jetting, burnt valves etc it's more of a 'pook' sound.

 

[XSLeo]

Quite often even a good running engine can back fire through the exhaust, On deceleration it is most noticeable. With some mufflers this is what gives you that nice rumble in the mufflers.
On most exhaust systems the mufflers quite this backfiring down to a not to noticeable amount. As Grizld1 mentioned several people have had some luck with those carb sets, most have had large amounts of trouble. With a bit of research you will find the threads.
I think Grizld1 was one of the people that had trouble getting a set to work. spent something like $600 worth if new parts and tinkering for several weeks to get them half way right. As I recall they are now very expensive paper weights.
Leo

 

[grizld1]

No, Leo, I'm the guy who warned about exactly what those carbs were as soon as they were offered because they'd been notorious for trouble on Brit bikes for years, and I was called a liar and worse for my trouble. Mrriggs is the man who put the time and effort into trying to straighten out a pair of Pewks and gave us the benefit of his research in an extensive and well-written thread. JP, don't take my comments personally. What short straight pipes will do to your performance is to narrow the rpm range in which your motor makes power and move the power peak into an rpm range that your motor can't reach--way out past red line.

If the carburetion and ignition are right, straight pipes should not cause popping from the exhaust under engine braking. Watch and listen at a flat track or road race event sometime. Most of those machines will be running unbaffled megaphones, but as far as the sound under engine braking goes it's the same difference. If you lean out the fuel screws on a machine with OE muffs on it you'll hear that popping loud and clear. I guarantee it.

To clear up any confusion, "grizzled" means "turning gray." I should probably change the handle now, since I don't have much hair left that isn't gray, except for what's turned stark white.

 

[XSLeo]

I apologize for the mistake on who had the real troubles. It was a while ago. I do remember enough about the trails and tribulations some went through to not try a set.
Leo

 

[just_puttering]

Ha ha, good post griz. I want to do more research on proper tuning, including exhaust applications. but I'm working on re-wiring the whole bike right now. I did read the rriggs post, great info on these funky carbs. I could tell the minute I tore them down I didn't like the design. Chrome slides and poor castings, two piece carbs with funky gasketing, oh well, stuck with them.