Backfires thru carb

[MLG]

Engine starts and idles fine. When I give it throttle it spits and back fires thru carb. I replace the carb. Replaced stator, spark plug. Check gas flow. Checked voltage 13.6, please help

[Mech]

Well..  lean mixture can cause backfires into the carb.

A blocked exhaust could possibly do it but it would probably have some subtle symptoms as you tried to ride it.. It would start to go sluggish before the backfires started.

What make and model is this thing .. just so we know what sort of carb it will have.

[MLG]

1992 LT-Fwd-xn 300

[MLG]

I tried almost everything.

new carb too

does exact thing

idles perfect

[Mech]

Well if you give it a run for a few minutes, pushing it till it just starts to falter, then pull over and turn off without idling, and take the spark-plug out it might look lean, a light colour or white.

You could try checking the exhaust isn't blocked. In the manuals they say to take a bung out of the back of the muffler every so often and rev it to clear it of gunk. I've done it though and never got anything out. It's best to test for a blocked exhaust by loosening it at the head and muffler brackets so it can leak real bad at the head. If it runs a bit better like that then it's blocked. but it has to leak real bad. It needs to be ten mills away from the head.

Mainly though I'd suspect a lean mixture. The old carb may have needed work, and new aftermarket carbs are almost never set up right for a bike. Aftermarket carbs are tuned in a sort of average way to run on any bike, but not well on any.

I'd suggest cleaning and setting to original carb myself.. it's always a better option than buying a new one and then having to do work on it anyway to get it going right.

Edited July 26, 2023 by Mech

[Gwbarm]

If it has the original OEM carb on it, it seems every ATV i have bought recently had an aftermarket on it and their good carb trashed.

[Mech]

Yeah well that's a point... 

[Gwbarm]

Also, all the kits are built for OEM carbs and wont work correctly in aftermarket carbs.

[Mech]

I've never used an overhaul kit in my life Gw..  I've bought diaphragms and O rings but that's it.

Oh and a couple of float needles..

[Gwbarm]

Thats good practice, probably most carbs dont need new jets and needle, sometimes the float needle needs replacing, and i use the new bowl gasket. I think kits are mostly for peace of mind, i have rebuilt my carb, diaphram doesnt come in the kit anyway. I usually buy kits for carbs i know nothing about, dont really know if the parts are original or from a cheap kit, or have been drilled out.

[Mech]

In the olden days, when cars had carbs, then I used to buy kits, but jap bike carbs are so damn good they just always seem to come right with a clean and adjust. Wear in the slides more likely to be the death of them than anything else I find.

 

[MickeyD]

Many good answers above.

‘Another might be a burnt intake valve causing a backfire into carb off idle or under load. Take a compression check.

Good luck with your findings.

[Gwbarm]

I have not really experienced that , but good point. Thanks for the input.

[Mech]

For the record...

If there's a problem with the inlet valve it shows up worse at idle than when the throttle's open or it's revving. Same with inlet/vacuum leaks, they show up bad at idle but become almost un-noticable once the revs get up. If either thing is so bad it can cause a miss or backfire at revs, then it will be bad enough to almost prevent idling..

Possible exception being a very weak inlet valve spring.. which is pretty much a thing of the past..

[MickeyD]

9 hours ago, Mech said:

For the record...

If there's a problem with the inlet valve it shows up worse at idle than when the throttle's open or it's revving. Same with inlet/vacuum leaks, they show up bad at idle but become almost un-noticable once the revs get up. If either thing is so bad it can cause a miss or backfire at revs, then it will be bad enough to almost prevent idling..

Possible exception being a very weak inlet valve spring.. which is pretty much a thing of the past..

I beg to differ. As a mechanic for 40+ years, I’ve seen plenty of burnt intake valves behave the way the OP describes the symptoms.

A flat exhaust cam lobe will also cause a spit back through the carb.

A compression check is needed.

 

Edited September 4, 2023 by MickeyD

[Mech]

I've never seen a burnt inlet valve in my life. I've seen them distorted, and with distorted seats, and with tight tappets, but never burnt.

Regardless, how did these machines with burnt inlet valves idle ?

In my experience anything that causes compression to be forced back into the inlet plays havoc with the induction, and so the idle. Once there is a greater amount of, and faster, air flow the effect becomes less pronounced.

 

And a spit back of unburnt fuel through the carb is not the same thing as a back-fire through the carb.

[MickeyD]

It’s obvious you haven’t seen everything. 

[Mech]

Obviously..  So how did these machines with burnt inlet valves idle ?

[MickeyD]

Why do you feel the need to question my comments and knowledge? I made a statement for the OP. 
I’m going to remain polite and just let you be. Good night. 

[Mech]

Fair enough.

[Dan Kroes]

Maybe take a look at your valve adjustment and carb float level. Is this a new condition or old?

[800CCBRAAAAAAP]

here's a trick.  Put a piece of tape around your throttle, and another piece o the handle bar.  Next, mark out idle, 1/4, 1/2 , 3/4and full throttle with a marker on the tape.   With the bike running, make the throttle go to each position.  You will then know which jet is incorrectly jetted.  The bike'sgas is metered by different overlapping jets in the carb.  You need to figure out what position you are running lean on, and then richen it up one jet or two sizes up.

[MickeyD]

1 hour ago, 800CCBRAAAAAAP said:

here's a trick.  Put a piece of tape around your throttle, and another piece o the handle bar.  Next, mark out idle, 1/4, 1/2 , 3/4and full throttle with a marker on the tape.   With the bike running, make the throttle go to each position.  You will then know which jet is incorrectly jetted.  The bike'sgas is metered by different overlapping jets in the carb.  You need to figure out what position you are running lean on, and then richen it up one jet or two sizes up.

Trying to pictured this procedure with a lever style throttle.

[Gwbarm]

Intersting, do you go by sound in the mid range speed to deterrmine if the jet is lean or rich, or exhast smoke, or performance.

[Mech]

Performance.. Under load.

[Gwbarm]

I understand that, but how do you determine which jet it is, they work together to a point and pin pointing that point when they are not together is difficult. Thats why i have always stuck to the OEM recommended jets.

[Mech]

Oh ok..  well to set a carb up from scratch you set up the idle fuel and air jets so the adjustment of the idle mixture screw is working right in the middle of it's operating/adjustment range, then you pretty much jump to the main jet if the carb/vehicle is able to run up to there and you adjust/change the main and main air jets so it's running right or slightly rich at full throttle, then you test/change slide needles and discharge tubes or emulsion tubes till it runs nicely with a steady throttle or slow opening throttle, through the needle's range, then you change the emulsion tube for one with more or different air holes, and/or the slide cutaway, to overcome any flat spots on hard acceleration.

And hopefully, if you've done it all right, it comes together pretty quickly without too much fiddling around.

Edited September 20, 2023 by Mech

[Mech]

That's the order you do it in anyway.. Hopefully the carb, and the size it is, is already set up by the carb manufacturer to be right for the size engines those are normally fitted to, and any changes are only to suit a new cam or hot-up.

People have trouble when they decide a bigger carb must make their bike go faster..  haha..  and so they fit something that lacks air velocity through it..  then the fun starts.

[Whiterabbit]

Sounds like a lean condition. Look for an air leak at the manifold or manifold/carb area with some carb spray first before tearing into your carb. Unless you know what you are doing with carbs I doubt you will spray some cleaner at it, throw on some new o-rings n gasket and fix the carb.  You have to thoroughly clean the air mix, the pilot jet, the main jet, the emulsion tube, the off idle passages and a few more to get a carb up to snuff. It's doable but ya' need to know what you are doing.