Back firing from carb?

[awsomeaussies]

I just got my 416 rebuild done today and when I went to fire it, it just back fires. Even through the carb:huh Surprised my airfilter didn't catch on fire. lol Timing should be good. I made sure it was on TDC. I got the cam chain sprocket lined up perfect. My hotcam lobes are facing down. Adjusted the valves to specs. So I don't think it's a timing problem. Could it be my jetting? I have an 39mm FCR that I bought used. Maybe I should go back and clean it really good. I have the main jet at 170, needle in the middle. I'm at sea level. My mods are a powroll 416 kit. Stage 2 Hotcam, Sparks exhaust, k&n filter. Don't know if that helps any:confused:

[jeremy_283]

take a look at the carb, i dont know much about backfiring but mine did it be4 to, it was related to the carb

[awsomeaussies]

Wouldn't you still think it would start though? I'll give it a shot and see what happens. I'm just hoping I put it together right:o

[jeremy_283]

put a leather glove on, if your afraid of the sparks, so tha tit acts like a choke, and see what happens...... i am just throwing out suggestions now tho

[bwamos]

When mine did that it was because I installed my cam and set the timing w/ the timing chain slack on the wrong side. aka.. the cam timing was actually off. Slack needed to be on the tensioner side of the chain (obviously, ;)).

You did get the slack on the correct side of the chain, right?

Outside of that I'm no help, sorry.

[awsomeaussies]

Sorry I don't really understand what you mean:confused: I'm kinda slow:p How do you know what side the slack is on? Because I just slapped the chain on the cam sprocket and made sure the sprocket marks were dead on. I didn't even realize much, if any slack in the chain.

[REDRIDDER]

Originally posted by bwamos
When mine did that it was because I installed my cam and set the timing w/ the timing chain slack on the wrong side. aka.. the cam timing was actually off. Slack needed to be on the tensioner side of the chain (obviously, ;)).

You did get the slack on the correct side of the chain, right?

Outside of that I'm no help, sorry. x2 what he said check.....

[bwamos]

Sounds like you did what I did the first time. ;)

You tensioner is on the intake side of the engine. It pushes against the chain. So the chain is pulled tight on the exhaust side and in an arc on the intake side.

When you set the cam timing simple rotate the cam backwards until the chain is tight on the front side of the motor (exhaust side). That's where the timin should be set at.

Once you release the tensioner I always cycle the crank around a couple times to make sure it's still in time.

If the slack happened to be on the front side of the motor when you released the tensioner you cam timing could easily be off 2 teeth, causing your intake valves to open early causing blowback through the intake system.

To check it now though. Just pull the head cover and flywheel timing plugs, and make sure everything is lined up correctly since there is already tension on the chain.

Also make sure you don't have 2 different marks on the flywheel. I know the 300ex has a |T mark and a || mark. On the 300ex you have to use the |T mark.

[maddmatt02]

did you release the tension before removing it or was the chain stretched so bad that you were able to get it on/off without touching the tensioner? and like was said earlier, after you get done with something liek that,, always turn it over slow by hand to make sure theres no contact, and after a couple rotations check to see if its all still lined up.

[awsomeaussies]

UPDATE: I checked the cam chain and the slack was on the correct side. Tested my tensioner just to make sure it was working ok and it was. Went over everything again just to make sure everything was lined up right. Put it on TDC and my piston was at the top, my hotcam lobes were facing down and my cam sprocket was lined up perfect. Next step I just tried some obvious stuff. Put new fuel in the tank. 93 octane. Took out my FCR carb and through my stocker back on there, just to make sure nothing was wrong with the FCR. So tried to fire the bike again and nothing:( It trys cranking over, but never starts. No backfiring this time... but just puffs of white smoke from the exhaust. I remember reading in the manual... when you first put the engine on TDC make sure your valves are loose, if not then rotate the crank around until they are. I can't remember if I did that when I first took the bike apart. Could the engine possibly be on the wrong stroke? Which means it would be 180 degrees off right? Other than that I have know idea:scary:

[maddmatt02]

the crankshaft rotates 2 times for every one camshaft revolution, so if you had the piston at TDC, and had the camshaft timed correctly, you could turn the crankshaft one revolution, loosen everything, and turn the camshaft to the correct timing and assuming spark occuredath the correct time it would run. alls that would happen is what used to be the exhaust cycle is now the compression cycle, and what used to be the power cycle is now the intake cycle. as long as you had the piston at TDC and the camshaft correct then it should run. I dont remember how the gear goes onto the cam in a 400, but only other thing I could think of is, is there any possibility you have the cam 90 degrees off? so when the engine wants to fire, which is when the piston should be at TDC and all valves closed, your exhaust valves are opening very early?

another thing I am not clear on, is in a car, the crankshaft rotates twice for the cams one revolution, and you have a distributor that also rotates slower than the crank so that the #1 cylinder only gets spark on one of its appearances at TDC, on a quad where the igniton is just a pickup off of crank rotation, what makes it not fire everytime the piston is at TDC, something in the box maybe? in that case then seems like it could matter if you set the cams to the correct TDC? but what would indicate which one would be correct, the mark on the crank doesnt change on each revolution, neither do the marks on the cams?

I probably just confused you with that, cuz honestly I started out like I knew what I was talking abotu and ended up doubting it and confusing myself at the end.

one thing you could try though wpould be to set it to TDC until all marks line up, take off timing chain, turn crank one complete revolution so it is back to TDC, realign everything, turn it over by hand a few times to make sure there is no cantact between anything, and then try and start it, because maybe somehow something is telling it to fire when the exhausts are closing and the intakes are opening, instead of when both should be closed?

Ive been sitting here debating on not even posting this in case I look like a moron but I am gonna go through with it and see what happens, lol.

I need to go research how the cdi knows to tell the coil to fire only every other crank rotation...

[bwamos]

another thing I am not clear on, is in a car, the crankshaft rotates twice for the cams one revolution, and you have a distributor that also rotates slower than the crank so that the #1 cylinder only gets spark on one of its appearances at TDC, on a quad where the igniton is just a pickup off of crank rotation, what makes it not fire everytime the piston is at TDC, something in the box maybe? in that case then seems like it could matter if you set the cams to the correct TDC? but what would indicate which one would be correct, the mark on the crank doesnt change on each revolution, neither do the marks on the cams?

It does fire on both strokes. TDC of the compression, and TDC of the Exhaust (technically a tad before, I know someone would be tempted to correct that, lol). So as long as his timing marks on his crank are lined up and the cam is in correct time, it should not matter. Since it fires on both strokes there's no 180deg out.

My 2nd thought is to check the flywheel key to make sure it isn't broken.

[awsomeaussies]

If heard it both ways. It fires no matter what stroke it's on and I've also heard it has to be on the correct stroke for it to fire. So I just tried flipping my cam and sprocket around, just to check. Figured it was worth a shot. But still nothing:( After doing this the exhaust wasnt smoking as much, but the backfiring from the carb came back. So I'll check the flywheel key. Not exactly sure what the "key" part is. lol But I'll check to see if the manual shows it.
These pictures don't show much, but maybe you can get an idea...
It's on TDC and you can see the lobes are facing down.
http://i14.photobucket.com/albums/a329/awsomeaussies206/DCP_1421.jpg
http://i14.photobucket.com/albums/a329/awsomeaussies206/DCP_1419.jpg

[maddmatt02]

theres a notch in the shaft coming off the crank, and theres a groove int he flywheel, and there is a key that fits in there to make sure the flywheel stays spinning with the crank at the right timing. if there was no key there would be a possibility of the flywheel moving around and it would change the ignition timing.

[awsomeaussies]

I got to thinking.... when I was installing the new timing chain and trying to get the lock nut off that holds down the clutch basket. Which was a pain to get off btw... maybe I spun the crank the wrong way from doing this? I was trying to keep it as still as possible, but it was moving around a little bit trying to pry that little metal tab away, so I could get the bolt off. I don't know how much of a difference it would make though?

[bwamos]

Not much on the clutch side that could throw off timing. The flywheel is on the stator side, FYI. (I'm sure you knew that though).

If the flywheel ke was sheared the ignition timing could be off anywhere between 5-90 degrees. If it were firing a ways off the plug could actually be sparking while the intake/ehxuast valves are open. Thus you'd be seeing a lot of blowback, and backfiring when trying to start it.

[awsomeaussies]

Yes I new that:p I just thought the basket spun with it. Sorry I'm not very good with with this stuff and being blonde doesn't help either:devil: I'll take a look at the flywheel key tonight.

[awsomeaussies]

ok, get this.... last weekend the neighbor hood kid cameover and while I was working on my bike he hit my starter button:mad: I had my timing chain tied up on my handle bars with the top of the cylinder off. It made my chain fall down into the case, so I took my side cover off to and it was bunched up alittle bit. I got it loose without having to take my clutch basket off... so I figured everything was ok. So I just got it back together to see if it would fire again. Before I tried hitting the starter, I spun the crank around a couple times to make sure everything clears and lines up ok. Everything did... but when I would spin the crank, by the time I got to TDC the crank would kind of spring back. Which it never did that before. I went ahead and put some oil in the bike, just to see if it would start. When I went to start it now the crank isn't even moving!!!:grr: WTF! Now it's even worse than before. So would this be a sheared key? I never got the chance to look mainly because I don't have the right tools to hold the flywheel steady. This friggin sucks. I noticed more slack in my timing chain. He probably stretched my brand new hd chain also.

[the Z Man]

Do you have an automatic compression release?

[awsomeaussies]

No I don't:(

[the Z Man]

I can't tell from the pics but is the piece for the compression on the cam?

[awsomeaussies]

You mean the spring and plunger thing? If so, it's a hotcam so it's not needed.

[BigBore24]

sounds like the starter clutch went out. the one way bearings that engage the crank when the starter is turning can come apart. were you turning the engine counterclockwise when you were turning it by hand? my manual says only to turn it ccw, and someone correct me if im wrong but i believe the reason is for the starter clutch. as for the not running problem. is it possible you could have bent valves? why did you do the rebuild in the first place? just wondering if the timing chain didnt come off and bend some valves. i speak from bad experience:(

[awsomeaussies]

Nevermind figured it out. I had the spacer on the wrong side of the starter gear. So that's the reason why it wouldn't spin. But it still wont start. The bike sounds like it wants to, but it just wont fire. Also I always turn the engine counter clockwise;) Maybe your right about a bent valve. But I had the valves replaced by C&D. But maybe I bent them doing something. I'm going to try to borrow a compression tester from work. If the valves are bent, there wouldn't be much compression would there? Also anyone know off hand what the compression should be? I'm starting to regret the rebuild. I figured it should of been pretty easy, but I'm a newbie to this engine stuff. But if anything at least I'm learning a thing or two.

[awsomeaussies]

Ok so I did a compression test last night. And there's nada:( So I'm guessing I should check the valves? Anything else?

[awsomeaussies]

The first time I put this thing together I had the piston in backwards. The stock piston had the arrow pointing to the intake, but later finding out that aftermarket pistons normally points to the exhaust.... The piston had 2 marks on it, probably less around 1/16"-1/8" deep. So I switched it around and figured it would be alright. So maybe I f'd up my piston or would it still be ok? Since I'm not getting compression I'm assuming I bent those 2 valves that hit it. So now I'm thinking that's my problem... because I can't figure what else would not give me any compression at all. When I took the head off to check the valves, they looked fine to the eye, they don't looked damaged or anything. So I dunno:o Could the valve be possible damaged inside and is there anyway to check without a spring compresser?

[boosted3g]

Turn the head upside down and fill the combustion chamber with kerosine. If it stays full then the valves are good and did not get hit. You are probably fine there is plenty of piston to valve clearance unless you have a huge cam or your timing is off. Check your vavle clearance and make sure they are not too tight, that will cause bad compression numbers. Since its a fresh engine the numbers will be low to start with but they will climb when the engine is broken in.

[awsomeaussies]

A friend of mine cameover to take a look and he thinks it's the intake valves. So we'll try that tip and see what happens. Another thing I noticed... I hope someone might be able to explain this. I posted that the neighbor kid hit my starter button when I had my timing chain loose. Anyways it bunched up in the case... I got it loose by wiggling it a little, but when I took a look at my piston I notice that it was higher than normal when it's at TDC. Isn't there supposed to be like 1/4"-1/2" from the piston to the top of the cylinder? Because now there's only like an 1/8" or so. Which seems really weird why it would have change:confused:

[maxamillion]

There should only be around .040 inch from the top of the piston to the top of the cylinder. An 1/8 inch sounds alot more realistic than 1/2 inch.

[awsomeaussies]

Hmm... maybe it was more like 1/4" then. All I know is that there's not as much as there was for some reason:ermm: But we figured out the intake valves are bent. We didn't even have to do the kerosine test, because you can see the gap.

[Polaris 450MXR]

If you have the money to buy a repai manual I would recomend that. And buy the time you get it figured out you'll know how the engine works! Also I think that the Flywheel has two marks a F and a T. It sounds like that was the original prob if it was lined up on the F it would be just out of time a little. and the homey that hit the start button bent your valves. But thats just how I read what has been stated.