Author Topic: new exhaust causing backfires (Read 6271 times)

j4hfr34k · « **on:** March 11, 2009, 02:43:49 PM »

i have searched online and on the forums but havent foundd exactly what im looking for. My stock muffler was rusted out so i replaced it with an exhaust tip with two walls with fiberglass between similar to a glasspack muffler and mounted it to the header it is a stock 4-1 header. the bike is a 1976 cb750a. i figured it might need the air mix changed but the manual i have, a chiltons motorcycle and atv repai manual that covers everything from 1945-85 doesnt list the air screw on it, it has the pilot screws, throttle stop screws, and vaccum gauge fitting. im no mechanic but learn fast and can follow a manual. i build computers for a living and motorcycles are a passion im finally being able to get into. any advice/ help would be appreciated.

lrutt · « **Reply #1 on:** March 11, 2009, 02:59:02 PM »

2 things cause a backfire, exhaust leak at the head allowing air in at the head / pipe junction, or an overly lean mixture.

So.....check the pipe attachment to make sure it's sealing well, maybe new exhaust gaskets.

Once you eliminate that then you might have to adjust your air screws or possibly go up a size on the low speed jet. 76 runs lean anyhow but I had some very free flowing pipes on mine with a KN in the stock box and never backfired. Just had to screw the adjustment screws out a bit more.

j4hfr34k · « **Reply #2 on:** March 11, 2009, 03:09:16 PM »

thanks, i hadn't thought of that we had removed the entire exhaust to get the muffler off, 30+ years of rust was pretty tough lol, had it strapped to a table with my fiance standing flat across the junction and fifteen minutes of pulling twisting etc to get it off. but i hadn't thought to check and make sure there wasn't a small leak at the heads, and i didn't know that they ran lean already thanks for the tips. Ill be posting photos and a video or audio file off the new exhaust, when i update my info and join the club with my numbers. if anyone can help me find the air screws on the carbs to adjust them id bee very appreciative as well. it doesn't backfire when idle but does a lot when shifting down from drive to low, and when letting off the throttle after high revs, (i figure that's when it would normally backfire if it was backfiring anyways so i didn't mention it in first post.)

Spanner 1 · « **Reply #3 on:** March 11, 2009, 03:19:52 PM »

The air-screws your looking for are under the carb. throat, forward of the carb. bowl, i.e. under between the bowl and the motor. Normal setting is one-and-a-half turns out from being seated ( lightly ) on your bike with PD 44A carbs.

j4hfr34k · « **Reply #4 on:** March 11, 2009, 04:01:34 PM »

thanks, will check them tomorrow. i appreciate the help this forum is really fast, and on topic too, ive been working with computers and the internet for a long time and have never seen a forum with such timely responces on so many different topics, and knowledgeable answers at that.

fmctm1sw · « **Reply #5 on:** March 11, 2009, 04:35:53 PM »

Quote from: lrutt on March 11, 2009, 02:59:02 PM

or an overly lean mixture.

+1. That's what happened to me. Idle screws were way out of whack...

j4hfr34k · « **Reply #6 on:** March 11, 2009, 05:06:03 PM »

thanks, i figured it was probably something to do with the fuel mixture since the new exhaust is alot more free flowing than the old stock muffler. will post pics/vid tomorrow when i get out to check the air screws. thanks again

griff6-5-zero · « **Reply #7 on:** March 11, 2009, 06:53:12 PM »

flames are cool

j4hfr34k · « **Reply #8 on:** March 16, 2009, 04:45:26 PM »

update: posted pics on the gallery of the exhaust i changed it to, but have since added a cherry bomb muffler to make it a little quieter. still backfires a little and noticed unburned fuel when revving as well, do you have to remove the carbs to adjust the air screws, and which direction would make it leaner in or out. also any other suggestions would bee appreciated as well. thanks yall are really informative and have helped solve nearly every problem i have come across really helpful to someone with minimal mechanic experience

http://gallery.sohc4.net/main.php?g2_itemId=308142

MCRider · « **Reply #9 on:** March 16, 2009, 05:02:29 PM »

Quote from: j4hfr34k on March 16, 2009, 04:45:26 PM

update: posted pics on the gallery of the exhaust i changed it to, but have since added a cherry bomb muffler to make it a little quieter. still backfires a little and noticed unburned fuel when revving as well, do you have to remove the carbs to adjust the air screws, and which direction would make it leaner in or out. also any other suggestions would bee appreciated as well. thanks yall are really informative and have helped solve nearly every problem i have come across really helpful to someone with minimal mechanic experience

http://gallery.sohc4.net/main.php?g2_itemId=308142

Air screws pass air. Counterclockwise is out and more air, leaner. Clockwise is in and less air, richer. Not too familiar with your carbs, just in general. You may need to install richer low speed jets as well. I'm not sure if they are replaceable. Bike was set up lean for emissions, usually.

Spanner 1 · « **Reply #10 on:** March 16, 2009, 05:20:07 PM »

Whoops, mc !, on his carbs. the idle/low mix. screws are out for richer..stock set is 1-and-a-half turns out from lightly seated..opposite to the earlier carbs. !

j4hfr34k · « **Reply #11 on:** March 16, 2009, 06:02:41 PM »

thanks for the info. the adjustmenys can be made without taking carbs off right?

MCRider · « **Reply #12 on:** March 16, 2009, 06:14:32 PM »

Quote from: Spanner 1 on March 16, 2009, 05:20:07 PM

Whoops, mc !, on his carbs. the idle/low mix. screws are out for richer..stock set is 1-and-a-half turns out from lightly seated..opposite to the earlier carbs. !

Sorry. I'll bow out. Over my head on that one.

Spanner 1 · « **Reply #13 on:** March 16, 2009, 06:20:19 PM »

Bro j4, see my earlier post for adj. screws location !

j4hfr34k · « **Reply #14 on:** March 16, 2009, 06:31:44 PM »

Thanks! I will update tomorrow after I have done the adjustments or changes.

j4hfr34k · « **Reply #15 on:** March 21, 2009, 10:37:05 AM »

update: fixed the idle mix no more black smoke or backfires thanks spanner and everyone else. i have another isssue now though that started yesterday beefore i adjusted the carbs, the throttle jumps to fully closed now even at idle, and it obviously tends tio kill the engine, it didnt do this until just the other day it normally stayeed bout a quarter of a turn or so from fully closed, and had a little slack, beetween the two throttle cables is this normal or is it normal to havee a small midway between fully closed? the manual has a small diagram but doeesnt go into detail about the throttle grip. thanks again

Spanner 1 · « **Reply #16 on:** March 21, 2009, 12:16:20 PM »

The cable adjuster at the twistgrip is just to adjust the cable slack only.
The idle adjuster is between the carbs and the motor, a big knurled knob, turn in or out as required...

j4hfr34k · « **Reply #17 on:** March 21, 2009, 03:26:34 PM »

thanks again spanner got the idle fixed now and everythign is working top notch i just have to make a bracket and weld it to the new muffler and everything will be peachy. no more backfires

will post pics when it is not half way ziptied and ghetto rigged together as well as new pictures when it is painted etc. thanks again for the help

Spanner 1 · « **Reply #18 on:** March 21, 2009, 07:57:46 PM »

Great, youdaman..hope you enjoy riding your bike.

j4hfr34k · « **Reply #19 on:** March 22, 2009, 09:34:17 AM »

well i couldnt have done it without yalls help

so i guess that makes yall "damen"

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Author Topic: new exhaust causing backfires (Read 6271 times)