hey newbeatle---just for reference sake---if valve adjustment were responsible for backfiring, what would you look for, loose valves, or tight valve adjustment??

 

I would have thought that it would have been typical of an over tightened valve, however I have no reason to doubt the poster that I copied the previous post from, so if they were too loose, it will also appearently do it... ( I'm just trying to envision what was happening here )... the inlet or exhaust valve must have still been opening, one or the other ???

 

My exhaust is stock. Also i am turning the right screws. But...... what i forgot to mention..... I took drain plug out of exhaust to get water out one time, and i went to turn it back in no more than two turns and it was going hard so i backed out. I stripped it. So i got a 3/8 inch pipe plug, which is the same size as the drain plug(measured with micrometer) and put it in my exhaust very hard. What i'm wondering i could have also stripped the threads inside my muffler, and for some reason air is coming out there???? I don't know. How would i check leaks in my muffler?? Maybe i should take it to dealer and have them look it over. I surely don'w want a rocker arm in a cylinder. Thanks for replying and words of advice everybody. Diesel

 

DIESEL... That plug is far enoughback, that I doubt it is the cause of your problem... I have heard of guy's running with out that plug... but just to be sure, try a little high temp silicone on the plug... that should seal it up good enough with only a few threads... you could use a tap to clean out the threads, if you are concerned, but IMO, that is not the source of the back firing...

 

Dieseltripp, Heres your fix. Go one up on the pilot jets.

The only thing that causes backfiring on letoff, is a lean condition in the idle circuit. Either exhaust leak, intake leak or pilot circuit. The big clue, is you said you were already 4 turns out on the mixture screw. Trust me, been there done that.
The way you can tell if the pilot jet is too small, is to adjust a warm motor at idle. start at the recommended turns out on the mixture screw. Then turn in, until you get a drop in rpm, count the turns the whole time. If going in doesnt cause an rpm drop...the pilot jet is too big.
Now turning out, count the turns, til you get another drop in rpm....and if you dont get a drop going out, or you get to 4 turns, and no drop....the pilot jet is too small. When you do this, im betting you wont get the rpm drop going out, before 4 turns.
After the jets are in, try this procedure again, if there sill isnt a drop going out....you need to go another one up on the pilot jet. But im thinking youll be ok just going one up from here.

 

I've read that its hard to find one size larger pilot jets for the P650/P700 carbs. Does any know where to find 1 size larger pilot jets for the Prairie?

 

BUSTER... you'll notice that I posted a response about the Idle circuit, but later was think about it & thought that a basically stock quad ( under normal circumstances ), should not need a larger pilot jet... then I saw the post I copied in my last post...

MIKEY... most shops will typically just drill out your Idle jet to the next size larger... you may need it with increased inlet & exhaust air, but I do, for sure, with the cams, head work, & increased displacement... I just haven't gotten around to it yet... besides it sounds pretty radical when I let off from WOT, back firing through the Big Gun...LOL...

 

Did you try idleing it up a bit. I had mine pop going down hills using engine braking right after I let off with the throttle closed. It was just needing a tad more air.
Try it and let us know. A simple thing to try.

 

Okay, i put some high temp exhaust sealant on the plug in the exhaust. I also put new plugs in. Now i noticed i had a carbon buildup on both the plugs, and also a bunch of carbon came out when i took my exhaust plug out. I don't get it. Either it's too lean or too rich. Don't know. I tried turning the idle up, doesn't help. So i'm wondering if something is majorally wrong with the jetting. But the dealer i got it from knows his stuff, he's been in business a long time etc.., as i'm sure his mechanics are top notch. So i would think they would set it up right, considering they sell 3 to 5 P700s on a decent day. Also........ i noticed it is starting very hard. I have choke all the way on, then when it sounds like she might die i shut it off, then it barely idles. Dang, i know a lot about wheelers but anything more than basic routines i fall flat.

Now for the unreal part. I got some sparks in my eye on monday. I finally went in today to get it looked at so she pulled out metal piece and buffed eye, so she's sore and i get to miss work. Which i don't like at all. Anyways i had to miss work because of dust and other foreign materials getting in my eye which would infect it. SO i can't even see if the backfiring stopped. I will tomorrow.
Dang, i don't even have 300 miles on my baby yet. I just hope i can dial her in and all is good. I know it will be worth it. Thanks again

Diesel

 

heres from the mikuni website
""



4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires. ""

So i still think that youll need to get bigger pilot jets, youre at the linit of travel ( 4 turns) on these).
Most atv/motorcycle carbs have 3 or 4 circuits (paths), which can be independantly rich or lean. 1can be lean and 3 rich, or 2 rich and 2 lean, and so on.
Going by the symptoms, backfiring at let off, you have a lean pilot( idle circuit). As far as the carboned plugs, thats a separate problem. The needle could be too rich,or the main could be too rich. But they are separate and independant issues.
Heres some sites for carb info,

jetting
jetting 2
Hope this helps.