I tried tuning the carb both with the air lid off and on. Did every half turn from 1-3.5 and I get loud pops everytime. When it pops I'm still getting a blue flame, clearly visable, from the exhaust. So, do you think the idle jet would make a difference? It idles fine, and runs fine when started for a minute, but takes like 5-6 times of starting and cutting off before it'll idle fine. I've seen people with more mods than this use the stock pilot jet, so I didn't think that was it. I took the exhaust off and re-tightened it, still the same problems. Very annoying...


If you suggest replacing the pilot, what size is stock, and what size to you suggest to go up to with an HMF full, no snorkel, Outerwears lid, and ProDesign w/foam filter? Thanks guys.

 

Sorry to hear your having so much trouble.[img]i/expressions/face-icon-small-sad.gif[/img]
I have installed 2 hmf full systems and had no trouble.

Is this popping on accelleration or decceleration? If it's just on deccel, both of mine do it sometimes.
If your quad ran great before you started, The only thing I can think of you may have messed up while installing jet is the float setting or connections to carb. Did you bump the float while you had the bowl off? Did you tighten up intake clamps after loosening? Did you change the jet inside the"little plastic cup" or the one below it? ( I don't know if pilot and main jets have the same threads or not)
I know these may sound like silly questions but if it ran great before you started working on it, these are the only things I can think of that you might have messed up.

 

This is from the mikuni website, and might help you get it fixed.
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.

Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.

 

Let us know what you find out strubby, I'm curious what the solution is once you figure it out.

 

Ok, just to eliminate some of the possibilities: Exhaust system is tightened to the max. Both at the cylinder and header/muffler junction. There's really not many places to look, but the intake system is very secure. I did have the backfire screen out, but put it back in and it did nothing. It really sounds to me like the ignition problem, and my friend's dad said the same. Although it's a bit different than the machines he works on. I get crackling pretty much every time I let off the gas if I'm revved above 1/4-1/2 throttle. But if I rev it higher, I get a LOUD bang. Almost broke a window in the garage [img]i/expressions/face-icon-small-confused.gif[/img] The jet I changed the main with was the center in the splash dish. I touched nothing else in the bowl. It runs perfect on acceleration. Runs a little uneven when coming down, or on the very low end. On the trail, it stalled a few times coming down from acceleration to a stop. I may have to take it to a shop for the ignition check, unless you can tell me what to do. I'm limited because I don't have a manual yet. The reason I'm reserving the shop for last is there's a 2 week plus wait, unless I take it to the shop and it'll be mucho dinero. Since I added all the aftermarket parts, I doubt this would be covered under my warranty right? Still don't know guys.

 

You should get a DJ needle and try that in there.

 

I don't know if I missed it in one of the posts but I didn't see any indications that you messed with the needle at all. Before you mess with the pilot jet at all you should try adjusting the needle first. Either shim the stock needle or get an after market one.

 

Nope, I haven't messed with the needle. What would you suggest I do with it? And does it sound like the needle could be causing the off-throttle backfires? I didn't see it specifically mentioned in Hondabuster's post, but I guess it could be a cause for running excessively lean off-throttle. If you think I could adjust the stock one, what would I move it to. And if you think it'd be better to get aftermarket, where could I buy it from, and what needle exactly? I didn't see individual needles for the Mikuni carb on magicracing. Also, I still don't have a manual so if you could tell me what to do, it'll get done. I'll also run a search. Thanks for the continuous help guys.

Edit: Ok, I read how to do it and realize it's not exactly adjustable. Would putting the top washer on bottom really make a difference, or am I basically left with having to buy a new one? If so, I'm not sure where or what to buy yet.

 

I use the dynojet needle, the taper is a little different. I've heard where people use a shim or small washer to raise the needle but I've never done it. I would suggest either raising the stock needle(lower the clip) or buy an aftermarket one and set it up per the manufacturers specs. I think DJ recommends the third clip(from the bottom)

 

So, the needle would affect the off-throttle fuel mixture? I just thought it didn't affect idle mixture. I'm thinking I'll just but a Stage 3 from Alba since it's like $20, I'll get it in about 4 days and should take care of everything. It includes a 155 main, what kind is it? I have a 175 Mikuni (I think) in there now from HMF, so should I even change that? And a 25 pilot, Alba LTZ needle on 3rd clip. It's not a problem to buy that, but I want to know if you guys think it would do the trick.