81' Backfires at idle

Judotrip · Posted: July 10, 2007, 11:48 am Post subject: 81' Backfires at idle

I just rebuilt the carburator and added new carb boots. The bike sounds like it is running stronger, but it seems to have lost some of the peppiness... I does run with no choke, but seems like a weaker idle
I have not replaced the main jets because they will not come out....
Also it seems one side is burning clean and the other side is burning richer....
What steps should solve the backfire and give me the pep....
Thanks,
Judo

xsjohn · Posted: July 10, 2007, 4:11 pm Post subject: Re: 81' Backfires at idle

Did you remove the brass plugs over the low speed mixture screws and reset the air screws..........Did you remove the pilot jets in the bowl and clean them out......Tiny hole has to be clear......Hole is about .0016 thousands.......

John

Judotrip · Posted: July 10, 2007, 4:25 pm Post subject: Re: 81' Backfires at idle

I replaced the pilot jets, but have not touched air mixture screws.... this is my experiment tonight...

trailguide46 · New Member Joined: Jul 10, 2007 Posts: 3

Doesn't backfire at idol indicate lean? Mine did that and I went up on pilot jet one size (from 42.5 to 45). My mains were at 137.5, so I left them alone. I went out 3 turns on mixture screw. Bike runs really smooth now.

xsjohn · Posted: July 10, 2007, 4:43 pm Post subject: Re: 81' Backfires at idle

Careful not to go deep if you drill.....If you slip a bit the drill will damage the screwdriver slot in the mixture screw below the brass plug.........Been there and done that.......I wouldn't take the mixture screws out there is orings in there that can fall apart.....would try them first......Bit of oil down the hole and turn them carefully back and forth at first......

John

trailguide46 · New Member Joined: Jul 10, 2007 Posts: 3

Also, mixture screws have a tiny o-ring at bottom, and a small washer underneath the mixture spring. Won't get a clean idol without all those in place. Should look like: o-ring/washer/spring over the screw.

Judotrip · Posted: July 10, 2007, 5:10 pm Post subject: Re: 81' Backfires at idle

Thanks for the help. I'll let you know how it went tomorrow...

Judotrip · Posted: July 11, 2007, 10:57 am Post subject: Re: 81' Backfires at idle

New spark plugs and 2 1/2 turns on the mixture screw seemed to bring the bike back to life and stop the back fires....

Could a bad spark plug or poor spark cause backfires?

I don't know, but I am back on the road again and am running like a champ...

I would recommend the iridium spark plugs, they seem to do the trick.
THanks,
Judo

Xumi · Posted: July 11, 2007, 1:10 pm Post subject: Re: 81' Backfires at idle

Probably not the plugs.. I'm guessing it's the adjustment to the mix screws.

Glad to hear you're back on the road!

Judotrip · Posted: July 11, 2007, 4:30 pm Post subject: Re: 81' Backfires at idle

Yes, you are probably right....I'll see how the mileage is affected by the changes...

Judotrip · Posted: July 17, 2007, 5:44 pm Post subject: Re: 81' Backfires at idle

I am now having some little sputters/backfires when deaccelerating. I have the mixture screws at 2 out, and rebuild the carbs. What else is there? The backfires aren't the monsters they were, but it never did this before.

I also have an over rev when the choke is pulled out...
Any ideas?
Thanks,
Judo

jayel · Posted: July 17, 2007, 5:56 pm Post subject: Re: 81' Backfires at idle

choke out = more fuel = higher idle, backfiring from deacceleration = exhaust air leak (or even short pipes) or lean condition or retarded timing

Judotrip · Posted: July 17, 2007, 6:24 pm Post subject: Re: 81' Backfires at idle

What is used to limit fuel? Is it the float level or needle length?
I am getting different results from both ends. Choke all the way out is too much fuel, and choke in is giving lean behavior...
Judo

jayel · Posted: July 17, 2007, 7:02 pm Post subject: Re: 81' Backfires at idle

yes to all of the above - what size main jets and pilot jets do you have, and do you have the adjustable needles (series of E-clip slot rings at the top of the needle?) what kind of exhaust?

Retiredgentleman · Posted: July 17, 2007, 11:47 pm Post subject: Re: 81' Backfires at idle

I recommend you make up a level gauge to make a direct measurement of your float bowl levels.
Drill a hole in a spare drain plug, insert a piece of brass or metal tubing to make a barb. Attach a clear plastic tube to the barb.
This is a cheap and accurate level gauge. You can even use it with the carbs on the bike.
When I got my 78 SE 5 months ago it was running real bad! .............hard to start, plugs fouled with carbon and raw gas. Now after carbs rebuilt and float levels set using the clear plastic tubing, the bike starts easily everytime, accelerates really well and plug colour is good.

brasco18 · Posted: July 18, 2007, 10:50 am Post subject: Re: 81' Backfires at idle

I am having some similar issues with performance. I recently leaned out my mix and have now lost a bit of pep... bike doesn't seem to have the same acceleration as it did when it was running super rich. My floats are set to 23mm I believe. Does this mean they are "high"? Is there too much fuel in my bowls? Before I adjusted my mixture I was getting about 30mpg, maybe less. I thinned out the mix and am now waiting to see what difference it made. If my floats are off as I think they are, could this be the performance issue and mileage horribleness?

xsjohn · Posted: July 18, 2007, 11:29 am Post subject: Re: 81' Backfires at idle

Tin Tops should get 50mpg if tuned correctly..........Too many people try to tune these with just the mixture screw....Although the fuel from the mixture screw is coming in even at mid throttle the mixture screw is not a tune all screw.........Just for low to low moderate speed......The main needle (jet needle) is far more important for correct fuel mixtures as is the main jet for high mid to top end.......The mixture screw shoud be set correctly and work done on the main needle and main jet.........Also to many people just throw in a huge main jet without tending to the correct mixture setting of the main needle..............

Save yourself and get my main needle kit.........It works like it is supposed to...........

John

Retiredgentleman · Posted: July 18, 2007, 11:57 pm Post subject: Re: 81' Backfires at idle

brasco18; The fuel level in the bowls is the starting point for the overall carb operation.
Its so easy and cheap to make up a plastic tube level gauge. Once that is set up correctly you can go on to make the other adjustments as needed.

xsjohn · Posted: July 19, 2007, 1:27 am Post subject: Re: 81' Backfires at idle

When 1980 rolled along Yamaha needed a new carb to satisfy the new EPA requirements......Cover the mixture screws ....set the main needle (jet needle) marginal and a too small main jet...........

brasco18....sorry I thought you had the later carbs..,.....

Had a 77 and think the floats were 25mm

John

brasco18 · Posted: July 19, 2007, 9:40 am Post subject: Re: 81' Backfires at idle

damn it... I am still getting horrible gas mileage (less than 30mpg). I don't understand it. So how does this tube level gauge work? I understand how to make it but not how it helps.

'75 xs650B Motor
'77 xs650D Stock carbs

Retiredgentleman · Posted: July 19, 2007, 1:26 pm Post subject: Re: 81' Backfires at idle

You can check/set your fuel level either with the carbs on the bench or it could be done with the carbs on the bike...........I set up mine with the carbs in a bench set-up with a temporary small gas tank suspended above carbs.
y 78 SE needs to have the fuel level at the bottom of the float bowl flange which is 5 mm below float bowl gasket surface. Carbs/bike must be level for the set-up. Install your clear plastic level gauge by screwing into bottom of float bowl. Simply hold or tie the tubing along side the carb. When you turn on the gas the level is shown in the tubing. No need to run engine. This is the most accurate way to set fuel level. If level is not correct you have to drain gas and remove bowl to bend the float tang...........then repeat.
Be careful as the float needle valve will fall out when you remove the float to adjust.
I replaced the float bowl phillip screws with allen head cap screws....................makes life easy!

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