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Bike's sick, backfires and idles terribly

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This is my third season with my 450 EXC, as most others it always had a case of deceleration popping. I lived with up till about a month ago when it really worsened from slight popping to plain backfiring. (My friends pointed out that it shot a few flames out of the exhaust even though it has a spark arrestor...)

My bike is far from home so I can't ride/wrench it every day. On my last visit I decided to take my carburetor back with me so I could clean it and see if something could explain the new problem.

As pointed out in other threads, I've found only 2 issues with the carb. First I still had a bit of a stopper on the throttle slide (obviously nothing more than performance issue). Second my fuel mixture screw was mixed up quite a bit : I counted 3.5 turns on it, the o-ring was destroyed and o-ring and washer were in reverse order (probably explaining the destruction). The rest of the carb was pretty much top shape. I did clean it up and put it back together.

For the mixture o-ring I went to an auto part shop to get the "closest" matching replacement. The original was in pieces so I couldn't compare exactly, but it was a bit bigger in width and I knew it wouldn't go in easily. I've managed to cram the o-ring in the carb and fit the other pieces along with the screw. It felt a bit tighter than before but turned correctly all the way to the bottom.

I went back to ride this weekend, but it was a disaster. I put back everything together, fire it up and ... POW bratapata POW powww pop pop .. awfull! it backfires like crazy just by opening and closing the throttle. While ridding, the deceleration pop is now a sickening series of explosions.

I fiddled quite a bit with the idle and mixture adjustments and it turned out the only way to stop the problem was to bypass the idle altogether by setting the idle screw high enough (easily over 2000 rpm).

Note that the top end has no problem, aside from the throttle now opening all the way, it did felt more alive than before. It's just the idle that's totally gone.

Some people suggested it might be a timing or spark plug issue?

I tend to doubt the mixture o-ring I used.

repair manual suggests the following :

- clogged idling jet : did that

- incorrect adjustment of carburetor : tried everything and more on that part

- defective spark plug

- defective ignition system : whaa? 👍

I really doubt it's a plain plug issue, after running a whole day in those conditions, it was healthy brownish, and nice clear sparks... I've cleaned it and it didn't change anything. But granted mine is 3 year old, I'll replace it.

I need help on this one

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seems obvious, you found the bad o-ring- but then you put in an o-ring that was the wrong one and crammed it doun the Fuel Screw hole- that is not going to make for an accurate fuel screw metering. It is probably plugging or is torn. I would be concerned about getting that out of the carb... Get the right parts and start over with diagnostics- may cure it may not- other issues, check valves, seal head pipe, adjust pilot circuit...

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valves, seal pipe, get correct o-ring(I just bought a JD fuel screw and it came with new o-ring and washer $18), throttle slide plate in upside down= the hole should be at the bottom and can see it thru the back of the carb when assembled.

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agreed, I'll get the right o-ring ;-) shouldn't be too much trouble to take out with a small pick

throttle slide plate is okay, triple checked

valves have about 30 hours, they're due to be checked but last time I only had to tighten the exhausts a bit.. I'll follow advise and do them.

I'll go on and seal the pipe, but only after I get back to the "normal" popping behavior

.. so, don't blame the ignition system yet?

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agreed, I'll get the right o-ring ;-) shouldn't be too much trouble to take out with a small pick

throttle slide plate is okay, triple checked

valves have about 30 hours, they're due to be checked but last time I only had to tighten the exhausts a bit.. I'll follow advise and do them.

I'll go on and seal the pipe, but only after I get back to the "normal" popping behavior

.. so, don't blame the ignition system yet?

I think it was Connie francis that said: "One thing at a time sweet Jesus!"

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lol yes of course... but as I mentionned bike is far up north (3h drive from home) and I won't see it again before 3 weeks.

I just want to get as much knowledge as possible so I can diagnose properly and fix this on the next visit

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lol yes of course... but as I mentionned bike is far up north (3h drive from home) and I won't see it again before 3 weeks.

I just want to get as much knowledge as possible so I can diagnose properly and fix this on the next visit

nah, it sounded like it ran good in the throttle range accept when the throttle gets closed or is idling- I would concentrate on what you are- I don't know about betting money on it but I will bet your time on it:busted: Its hard to see all issues when you have one that is obvious and is making substancial problems- but sounds like you checked the spark and that looked good- beyond that make sure the plug cap to wire connection is good- they get worn- but don't touch anything besides the carb and the valves before testing...

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i have an 08 450 with jd jet kit it has 20 hours its was recently serviced and the valves just got checked and they were bang on. My bikes been back firing when i let off the throttle with the flames shooting out, ever since i got it. I have a small leak where the head pipe meets the end pipe and from what i hear theres a good chance that's not helping at all. I havnt sealed it with high temp silicone yet but i am hoping that will help.

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When I first put my full FMF pipe on my bike I was to excited to fire it up and take it for a spin so I didnt seal it. Just a few weeks ago however I had my pipe off my bike and I sealed it up and with the combo between that JD kit and an R&D powerbowl almost ZERO backfiring... ZERO:thumbsup:

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I think if you (anyone with a stock header) look at the pipe collars meeting the flange on the head, you'll see it's either cocked or tipped at an angle (up or down) or they are out of alignment from side-to-side. I don't think KTM has improved the sealing quality up through the '08 models as there are still complaints on line. If they are out only a little the sealant works. The further out of alignment, the quicker it starts popping again after sealant application.

I experimented with heating and bending for a true alignment after bolting it down at the kickstarter with success. Ultimately I bought the HT header and powervane for more bottom end plus it lines up using a heavier pipe collar. The best part of the HT mod - It lets the stock jetting work w/o modification on the '05

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I didnt think of plug cap , good suggestion. They thread on to the wire and sometimes you need to cut off 1/4" and thread it back on.

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I think it may be an exhaust leak too, check all of your exhaust pipe connections and seal with high temp. sealant. I would also check your air box and rubber boot that holds your carb to the cylinder and look for air leaks which could cause a lean condition. IMO trash the o ring you bought and buy the correct size. Sometimes KTM owners put the carb slide in upside down, just my .02

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Here are values for my TPS :

Rges = 4.96 ohm

Rvar closed = 1.05 ohm

Rvar open = 4,16 ohm

Rvar closed should be closer to 0.84 ohm, but I didn't reset the slide to 1mm, so these look valid.

Exhaust leaks are a problem on this bike, even the canister could be sealed better. I wonder if the header could have been warped by temperature contrast, we got so much rain this year there is water everywhere... Is it possible to straighten/flatten the collars? I suppose the sealant wont do miracles if the gap is too big.

And speaking of sealant, is that the stuff :

http://www.permatex.com/products/Automotive/automotive_gasketing/gasket_sealants/auto_Permatex_Copper_Spray-A-Gasket_Hi-Temp_Adhesive_Sealant.htm

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nope, use this Permatex_Ultra_Copper_Maximum_Temperature_RTV_Silicone_Gasket maker

I guess, since we are thowing suggestions into the bag- check your air box boot and carb flange for air leaks. The head pipe sealing is important and causes some back firing- but does not make your bike idle like crap or really run bad- so that to me is a side issue- it can magnify the issue though- Get the carb sorted- Pilot Circuit/ correct O-ring- I think that's where your main problem lies- then take care of the other issues- which will just be a part of the things you check/ maintain...

as far as tweaking the pipe- anything is possible including making things worse- don't play with that unless you are confident- could also check your head pipe springs- maybe lost some preload??

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It seems labor day is early than I thought, I'll see my bike this weekend!

I got a new plug, new o-ring and ultra copper silicone. I should have everything to cure the problem...

I've read D.H.'s faq on pipe sealing (http://www.thumpertalk.com/forum/showpost.php?p=4305222&postcount=8) and although he goes in depth on why popping happens, I'm not so sure on how to actually apply the silicone. Anyone share your experience? How much you put on, where, and do you tighten everything right away or let it sit for a bit? Sorry it's not obvious for me :-P

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Problem is mostly fixed now.

Within 5 minutes I've fixed the o-ring issue, it was actually harder to cram the bigger o-ring than to take it out. I went on to check my valves, the right intake was a tad tight, readjusted to spec. Replaced the spark plug with a freshy. Sealed the pipe at the manifold, and at the muffler connexion. Couldn't quite wait 24 hours to start it up, it rounded to about 18 hours cure time.

From the moment it came alive, idle was buttery smooth with no hiccups or whatever. I yanked the throttle a couple times, engine responded precisely with no bog. As for backfires, they were pretty much gone until the whole thing got warmer. I think the header seal is a success, the behavior is completely different and I have much more power in the low rpm range. I'm wondering if my muffler couldn't be causing the remaining pops. I see carbon leaking out of all bottom rivets and my fiber packing is probably long gone in smoke. I'll tackle this during the winter season.

For now my bike is fully usable and my buddies get all the roosts 👍

Thanks all for the advise!

BTW, unless someone begs for it, I won't post a guide on how to seal the pipe... the job I did was pretty much guestimated and sure looks like it :worthy: I found it hard to access the manifold and properly spread the silicone on it. Considering the mess I did and the positive result after an incomplete cure, I see it's no rocket science anyway.

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