backfiring

Hello, first time XJ owner here when I bought my 82 maxim 750 the floats were not working properly so i cleaned carbs, etc. My oil level light was also always on, after putting the carbs back on and removing exhaust to check sensor, went out for a ride only to find it now has a lack of power and backfires out exhaust almost everytime I let of throttle (shifting, comming to stop or just driving). I pulled a idiot card and forgot which air jet was the main (small or large hole). If I reversed them would it cause this? I have also read that not getting the headers seated well could cause air into exhaust and cause back fire? hoping someone here can help me out. Thanks

 

Yup... sounds like it's running way too rich. Confirm this by looking at the plugs.

Backfiring is gas getting to the exhaust and getting ignited there - that means it's getting too much gas to burn with the air that it's getting...

 

Thanks woot, I beleave I used the larger holed jet as the main air jet assuming i need to swap them, I also noticed in the Haynes manual I have that it says there is a plate that covers the air jets, however when I took them apart mine had no plate covering them, since it did not backfire before am I safe to assume that the plate is either not needed or was not used on my carbs?

 

If it wasn't in there and it worked... well I'd guess it was fine...

Keep us in the loop eh?

 

will do and thanks again

 

Backfiring is usually a sign of running too lean. You may have the jets misplaced, or more likely, you have introduced an air leak into the system when remounting the carburetors.

Lean Backfire

If you Google backfiring rich or lean you will get a lot of sites which can explain why this happens.

You can initiate a rich backfire by turning off the ignition while you are riding in gear, then switching it back on. I had a grand time in the summer of 2000 running an old carbureted Chevy pickup at work, turning off the ignition going down steep hills then turning it back on at the bottom. The best results were obtained when the other mechanic with me was asleep in the cab, or I was passing a herd of horses. Damn muffler was shaped like a watermelon.

 

thanks MAC when I was riding it did feel as if it was lean, wasnt positive if the air jets being reversed would cause that, but looks like from here and others ive asked it can, going to check headers too, was a tight fit and cold here in Minnesota so hurried a bit and may have cockeyed one or two the way the day has gone a double dose of uh oh sounds about par LOL thanks again guys. Glad I found this site for sure

 

Thanks Mac... my mistake...

 

Woot, I'm coming across as a know it all I fear. No disrespect intended, since I always assumed too rich as well until somebody explained it to me once.

 

and that is how I came to say - too rich...

Not a know it all - it's all in the presentation, I understood what you meant and learned something myself

 

Hey, speaking as one of the biker newbies here, I think we need as many know-it-alls as we can get on this site!

 

hrm - a bit incomplete... but a wiki link

http://en.wikipedia.org/wiki/Back-fire

I don't understand how a lean backfire works... I'm looking for a gif.

Ok this made sense of it: http://faq.f650.com/FAQs/BackfiringFAQ.htm

Basically - the cylander gets so lean that the spark doesn't fire it ( can be caused by timing too) so the unburnt gets blown into the exhaust where the heat sets it off.

 

Never heard of a lean backfire. New one on me! I had always understood the condition occured with a rich mixture and a timing issue. Thanks MAC.

 

Hey thanks to everyone for there help, think I finally got it (think because it went down to 28 degrees today so only got a block or so of a test drive) I went to change the air just and found that according to the Haynes manual I had them wrong, also noticed that one of the diaphrams was not seated properly, switched the jets and fixed diaphram and then could not get bike to hold an idle at all!! UGH! longer story short found a site from another post here and found that the HAYNES manual actually has the main and secondary air jet locations wrong, ***take note for anyone else using this manual for carbs*** So bottom line having that diaphram not seated caused enough air gap to lean out motor, no back fire and power back atleast on a very very short ride, thanks again guys for your help

 

As always, we are at your service. I believe someone did post the Haynes error somewhere on our site. Been a while since I've seen it and I didn't notice the issue with my carbs. I just wrote down what came out of which hole on the diagram in my book and have been golden since. Got lucky I guess. Seems no one ever messed with the carbs on my bike (I think the guy I bought it from was the third or fourth owner). Glad you nailed the gremelin. Happy trails!

 

Kelly again

The ignitor box can couse a backfire too. (did on mine) but I don't know what all has been done to you bike. I really feel like you have an ignitor box problem (I know that's the root cause of your spark problem)

Kelly

81 XJ 550 Maxim

 

thanks bikerlife, hopefully wont have to replace ignitor box, tight budget (would love to be able to have $$ to renew all, the bike deserves it LOL I like it more than alot of the new ones Ive looked at.

 

I just finished a project of using JB weld (fun stuff that is) to cover the few holes in my mufflers just before they hit the cones. I knew that by changing mufflers with shorter leangth/less back pressure could create the lean effect, but I didn't equate holes in the muffler to do the same thing. Since fixing the leaks, I don't notice backfiring to be a problem, or should I say it is much more rare.

However, from the sound (kind of a sputter/stutter) of the engine (almost the same sound like when you hit that zone that says switch over to res. tank or pull over) I really think that my cylinders aren't burning everything that they should be. To be even more specific, my engine only seems to do this sputtering thing in 2nd gear between 2-3K rpms. Should I be looking for more leaks in the mufflers or does this indicate a fire issue?

 

Time:

You should examine your Plugs to see what is happening "Mixture Wise"

That "Flat-spot" could be a few things.

Pilot Mixtures not at the right Ratio for supporting Ignition in the Low-Band and Off-Idle.
The backfire would indicate you have one or two that are too Lean.

Sticking Diaphragm Piston.
Not rising well enough to allow the Main Jet Supply to keep-up with the Fresh Air Intake for a moment.

Plug Gap too narrow.
Not a strong enough Spark to support Ignition of the changing Fuel Mixture
as it transitions from Pilot Jet Supply to Main Jet Supply.

Wrong Heat Range.
Check spec's on the Plugs your are running and make sure that you don't have a set of mismatched Plugs.
(Sometimes a Plug Fouls and the bad Plug gets substituted with a different Brand or Heat Range ... worth checking.)

 

Thanks Rick.

 

I have been trying to start my bike for a week now. I had a bad coil and managed to get a good NOS one for 60 bucks.

Now the bike will not start, sometimes it backfires really bad, I just saw two huge flames (7 inches) coming from the two pipes of the exhaust. I am, almost deaf also.

Anyway, The coils are good and I am wondering if the wrong connection of the coil wires to the spark plugs can cause this. I understand that:

Coil 1 (Left)
- Cable 1 (left) goes to cylinder 1
- Cable 2 (right) goes to cylinder 4
Coil 2 (Right)
- Cable 1 (left) goes to cylinder 2
- Cable 2 (right) goes to cylinder 3

Is this correct? It wasn't firing this way (also backfired, but from only one cylinder I believe) so I switched clinders 2 and 3. Now it seems to backfire from both pipes.

Do I have to clean my carbs? I opened carb bowl screws of cylinder 1 and 2 and no gas came out, but I could see it through my clear gas line...

I will try to find a spark tester, but they are fairly new.

Before the coil replacement I could run the bike on cylinders 2 and 3 without problem.

What should I check next?

 

Gene:

If you opened a drain screw and didn't get fuel out of a fuel bowl you have a fuel delivery problem to investigate.

The fuel bowls should have fuel in them for the carbs to supply fuel to the system.
You might have stuck closes floats or contaminated beenie-screens.
Either way, you'll have to get that situation fixed before going on to other matters.

I'd day the Carbs have to come off and be cleaned. If they sat all winter that's S-O-P.

When you get them cleaned-up invest in some insurance and install an Inline Fuel Filter.

 

Thanks Rick, I will get those carbs out.

I already have an inline fuel filter, one of the XJBikes great tips!

I worry that the backfire might be caused by something more serious. I mean, fuel must be getting to some cylinders someway...

Is the coil connection correct? Coil cables left to right 1-4-2-3?

 

You have the connections right. You don't have Fuel going to a couple of Carbs, though. Once you get that worked-out, we'll get on to Tuning.

Did you try putting the Petcock in Prime to see if those empty Fuel Bowls would fill?

 

Dont' huck the bad coil, most can be rebuilt. I'm working on a write-up. There is a link to a rebuild that is outlined in our links section. It isn't an XJ coil but the process is the same. One catch, the coil can be salvaged so long as the primary winding is intact. If you don't read 2-3 ohms on the coil, say perhaps infinitive ohms, the coil is shot.

 

^^^ Are you rewinding the secondary on these?