Backfire

I have been working on the bike this summer and have done quite a bit. I have another problem. It backfires. The bike would never idle so I cleaned the carb- wasn't very dirty and adjusted the butterfly valves. For the first time it idles. Sure it idles at 1600 but it idles ... Yea!

Even before I did this the bike hesitated on start and backfired. It does not backfire at idle only when moving and all the time.

Other problem. Pilot screws (these are brass right) have had their head striped out. Arg!

Any help?

 

I went for a little longer ride and found that the idle started in increase. I turn off the bike and immediately start it again and the idle drops. WD-40 on the manifold intake boots changes nothing. I wanted this fixed by August. Very frustrating.

 

Can anyone jump on this and let me know what they think?

 

Idle will increase as the bike warms, that's why you run the choke for extra enrichment until it warms up.

Backfiring could be extra fuel. If you can't adjust your mix screws you are going to need to replace them. These are wasted spark ignitions so you get a spark at exhaust time. Most likely you still have unspent fuel or you are pulling more in at exhaust time thus the backfire. Could be mixture could be air leaks.

 

Thanks Bill. I have tried to check for air leaks and don't find any. If the heads on the mixture screws are stripped out, how do you get them out? Also, how do you adjust the idle speed. Is it easy to do without taking the carb off?

Thanks again for any help. I am determined to get this thing.

 

well once you have the idle mix right the idle can be adjusted with the knob between carb 2 and 3 bowls. you can give it a try and see if it gets you at least a reasonable idle. If you have air leaks or your mix is way off it won't help a lot.

Getting the screws out may require drilling them out or using an easy out. you want to be careful not to damage the threads of course. Chacal has replacements.

 

I had to replace one of my mixture screws for the same reason. Drills easy and mine came out easy using a craftsman straight splined easy-out. Have to be V_E_W_Y V_E_W_Y careful to drill straight into the center of the screw or you will be.

 

I wanna tack on a question: how do you determine if there is an air leak at the carb boots?

 

One way is to probe with an unlit propane torch - if you have a leak bike will rev up.. You can use starter fluid or carb cleaner too but propane is cleaner.

 

I have none of those things... What if I can barely get the damn thing to idle (gotta keep it revved or it dies)?

Lemme ask you this too: does it sound like a gas or air problem that my bike will idle at 1000 RPM on full choke and that's only after I give it the gas to start it. Three weeks ago if I had put it on half choke and pushed start, she'd start and jump up to at least 2000 RPM (more than 3 at full choke). Can a choke break?

 

That is a really interesting way to do it. Everyone has told me to use WD-40. I like the idea of a torch.

This is a little off topic; however, I think important. When I cleaned my carbs (they were already very clean) I was unable to remove the Needle Jet(main nozzle) Is this very important to remove and, if so, how do I do it?

 

hehe... sorry for hijacking the thread

 

Wait, wd40 will work? Do you spray it on the boots as the bike is running? Please explain method.

 

You can do that. The WD40 will suck into any air/Vacuum leaks and change the idle. I like to use propane since it leaves no oil for dirt to stick to.

MN

 

Backfiring (or "popping") generally indicates a LEAN condition.

You very well could have an air leak; carb-to-manifold or manifold-to-block especially or worn out throttle shaft seals.

The fact that the mixture screws are buggered doesn't bode well for them maybe being halfway right...

Before we get too upset about the mixture screws, lets check for air leaks-- an unlit propane torch is generally the most effective.

I'm also a bit perplexed by your statement "adjusted the butterfly valves." What exactly did you adjust, and how?

 

To adjust the butterfly valve I used the throttle screw stop and the throttle adjuster screws I made sure that the butterfly valve on each of the carbs had the same gap. I used a thickness of a notepad paper folded in half to get the distance. Does this make sense?

 

Yes, very much so. What you did is commonly called a "bench sync."

That means the butterflies (and the carbs themselves) are "close" to being synchronized; so now we need to ascertain the cause of the lean condition. While it COULD be related to your torn-up mixture screws, I'm betting there is a bigger problem that needs to be fixed first.

First off, let's be sure it really is lean and not HORRIBLY rich, enough to cause "afterflash" in the pipes, as Bill suggested.

Plugs: White or sooty? I'm betting white.

Have you checked your float levels (using FLUID) yet?

Plus you should go ahead and check for air leaks as described; with the bike idling, direct an unlit (but valve open) propane torch at the junctions between the carbs and the boots, the manifolds and the motor, the ends of the throttle shafts (where the c-clip is) etc., and see if it makes the idle change.

Can you get the idle down below 1600 if you carefully back off the big idle adjustment knob?

You're not going to get this thing perfectly tuned by any stretch until you get the carbs FIXED fixed which means getting the idle mixture screws 'remedied' so you can fine-tune the mixtures. However, unless they're WAAYYY off, it shouldn't cause backfiring. There IS another issue at play; hopefully once it's fixed the buggered mixture screws will be close enough to where they should be for the bike to at least run enough to figure out how much THEY need adjusting.

 

Yup very important - under the main jet is the emulsion tube with tiny holes that has to be clean. Once you unscrew the jet it should fall out the top side but dirty ones have to be pressed out quite often.

Fitz maybe we are hung on terms a backfire (bang - not pop) has been from pumping fuel then get a bang in my experience. I would agree a popping type could be lean.

Jon what type are you experiencing?

 

You beat me to it Fitz...

 

yeah, you usually type faster than me...

 

Alright. I did the check with an unlit torch. Yep it is leaking. Idle went up. Didn't go up with the WD-40. As soon as the idle went up it started popping. I wouldn't say it bangs. It is like someone is doing popcorn in a metal pan. Very loud popcorn.

I also cleaned the plugs during this latest maintenance -- sooty not white.

I have not checked the fuel level on the bike. That was going to be the next thing. Although with the boots leaking, it may be best to wait.?.?

Lastly, you said it should fall out. Definitely not. So how would you recommend pressing it out of the top? And by top I assume you mean the top the way the carbs sit on the bike.

So... recommendations on order to fix this thing and can it be run while popping with out damage? It is not much fun.

 

I use a screw drive to carefully press it out - you don't want to damage it. I use a small bit of cloth or rubber to pad the screwdriver and use a flat bladed driver that spans the hole well.

You press from the bottom when you unscrew the jet. One caution there is a washer - make sure you have it out before you try getting the tube out.

 

The easiest way to REMOVE a Stuck (Nozzle) Emulsion Tube is to DRIVE it Out.

Using a Cap Screw of the same Thread Pitch as the Main FUEL Jet
Screw the Cap Screw Bolt into the Bottom of the Stuck Emulsion Tube.
Strike the Cap Screw with a Small Hammer and Dislodge the Tube.

(For this Job ... I keep a Ball Peen Hammer Head in with the Hammers with Handles. Having the Hammer Head in your hand will lessen the chance for an errant blow.)

 

OK.. I will take the carbs off ... again and make sure that piece is clean. I am also going to check the fuel level height. I may not take them off until I get the new boots. Any other suggestions for now?

 

Jon, where did you detect the leak? which boot junction?

I know you don't want to hear this but you asked; I would strongly suggest not trying to ride the bike with it "popping" it WILL eventually damage something. You don't want to cause more expensive repairs by trying to save a buck or two and getting anxious about riding it until it doesn't "pop."

If you KNOW there's an intake air leak then you know it's popping because it's lean... don't burn a valve running it before it's fixed.

You got the bench sync right by the way; and checking fuel level/float height is another part of the "while the rack is off" adjustment regimen.

Slow down, take your time, proceed logically, don't jump to conclusions.

If it were me in your situation, I'd be looking for another (better or same shape) rack of carbs on eBay as a backup. I have TWO spare racks, one a true "spare" and one for parts, but then again I have two operational XJs. I paid $20 for one set and $55 for the other, so they don't have to break you.

 

But its not the carbs right? It is the carb to manifold boots. It is one or both of the two inside boots. When I put the torch in, I am not sure I can be specific enough to know exactly which boot.

I figured that might be the case. I won't ride it. Just makes me want to fix it more.

 

Jon, we know you have a carb issue to deal with down the road.

THIS isn't it. You've got either a cracked boot; an carb that isn't seated properly; or the manifold-to-block gasket/o-ring is gone and it's leaking there.

BE CAREFUL. Intake manifold bolts tend to break if you so much as look cross-eyed at them; replacing the intake boots is NOT something to be taken lightly. Make sure it's not something way simpler to fix.

+1.

 

So, how do I go about diagnosing whether it is the manifold boot or the o-ring? I took the manifolds off and did not see any o-rings. There was a gasket. It didn't tear and everything seemed fine. Where is the o-ring?

 

The O-ring is supposed to be in its locating channel around the back-side of the Manifold.
If the O-ring is so "Mashed-Flat" that you can't see its Outline ... Probe (Poke) around the Back-side of the Manifold using a Large NAIL onto which you have Grinded an Extra-Sharp Point.

The Point of the Nail will "Tink" on the Back of the manifold and will likely "Stick-in" the O-ring or NOT go "Tink" because it found a Non-solid surface.

Wipe the back of the Manifold with a Tissue damp with Armoral.
Then, hit it with a Hair Dryer.
The O-ring will be revealed by the Armoral that remains on the Rubber.

 

Ok I just got wd40 it's the best I could do, could someone explain to me in detail what I should do? Get it running, get my brother to hold the throttle, spray one boot at a time until it spikes in RPMs?

 

Pseudo, What they told me was start the engine and get an idle. Spray the Wd-40 on the manifold boots. You need to make sure you are conscience of where you are spraying. Spray the boot and especially where it connects to the engine and carb. If the idle changes, you know that there is a leak.

This did not work for me. Maybe I was not a good sprayer. I used an unlit torch and found the leaks very quickly.

Hope that helps. Guys... let me know if I missed something.

 

Thanks, will do te test now

 

Nothin. And now my bike is soaked in wd40 lol.

Now that ya'll mention it, my exhaust is making a popping noise too, like a dull popping though more of a pup or even a puff-puft noise???

 

YOU GOT the manifolds off (unbolted from the motor???) Reeaaalllyy? And didn't break any bolts? YOU DA MAN.

However, if that's the case, one or both may have been a tad loose. Replace those gaskets; peel them away from the manifolds so you can replace the o-rings too; use NEW BOLTS (please use new bolts) with anti-sieze compound and a torque wrench when you reassemble.

While they're off, flex the manifolds and ensure that any cracks don't go all the way through. I pack the cracks with black high temp silicone RTV; others use liquid electrical tape. Since you were apparently able to get the manifolds off, I would replace any that are badly cracked.

If you get all that fixed and the issue persists, do the propane test again. If you get the same apparent result, it COULD BE (not saying that it is, but could be) the throttle shaft seals on the inner carbs.

Lets hope it was a loose manifold.

 

My manifolds have cracks especially next to where the clamp is tightened. I found a pair of used ones on ebay that they say have no cracks. I hate to buy used but if they're in good shape, I would think about it. What do you think?

 

Also a confession and a question (OK, 2 questions). My son deserves the praise. He actually got them off and one was loose.

Why new bolts?

Can that type of black silicone be found at a Lowes or that type of store?

 

Auto parts store for the silicone for sure - Black high temp RTV.

New Bolts - many of these have been over tightened which stretches and weakens them.

 

Thought of something else. I softened my airbox to carb boots with Berrymans Chemtool (think thats the name). It was recommended to me by someone that said it would work better than WD-40. It worked great! Should I soften the manifold boots?

 

I wouldn't. just spread a thin coat of lube on the insides to help installation.

Tell me about the Berrymans - I need to soften my XS400 boots. Does it leave them soft? Any harm to the rubber? I usually just heat stiff rubber to get it installed.

 

I talked to a guy at Lowes that said his dad swore by it. I was worried about harm to the rubber also. I saw a post on here that talked about it eating through the diaphrams in the carb. I decided to try it.

It did soften the rubber. When done, I washed them really well and then rubbed a coat of silicone lube into them. I wiped them off and they seem to be staying soft. It has only been a couple of days. I guess we will see over time.

One recommendation. I put paper towels around them and then poured the berrymans. I left them for 10 hours turning them over in the middle (sounds like a cooking show). I don't think they need this long. My guess is 4 to 5 hours would be enough.

Hope that helps.

 

Cool thanks I may give that a whirl.

Did you baste them between turns?

 

be careful with the ChemTool. It's great stuff but it DOES attack some things and not others.

Be careful with eBay parts of that nature. If your manifolds aren't cracked all the way through, repair them with black RTV and fit new gaskets and o-rings.

New bolts because those bolts were garbage to begin with, and you're quite lucky to have gotten them out without breaking one. No sense tempting fate.

 

Thanks Fitz and Bill,

I will replace the bolts.

Bill, I put them in a glass bowl, Made sure I used enough to get the paper towes wet with a little at the bottom. I then covered the bowl with tin foil to keep it from evaporating. I didin't use a plastic wrap because I didn't know what the chemicals would do to it. I only basted when I turned them.

 

Maybe we could sell "This Old Bike" to the Food Network...?

 

If you have ALL the Manifolds off ... It would be a very good idea to run a Tap into the Threaded Holes on the Head just to clean them up.

Throw-out the Cap Screws that you successfully removed.
Replace them with Qualiti Hardeware or Stainless.

Most Importantly, ... TREAT the Threaded Holes on the head with Permatex
Anti-Seize Compound.
Place some Compound in the Hole
Run a Bolt in-there and spread the Compound evenly.
You're one of the Lucky ones!

 

Looks like another order to XJ4Ever. Glad they are here.

 

Jon as much as I love Len you should be able to get the stainless bolts and antiseize at ACE hardware - they have a pretty good metric selection and you won't have to wait.

 

Brother, aren't we all. Len carries stuff that Yamaha themselves "missed" even, like brake caliper sliding pin boots.

Speaking of which, I gotta put together an order myself...

 

Still need the gaskets and o-rings. So my order will go in too. Unless there is a better way?.

 

Len is hte man for the gaskets an orings - I didn't realize you needed them.