Decel Backfiring - Jetting?

I have an HRC kitted 2000 XR650R, converted to supermoto.

Ever since installing the HRC kit, along with a full exhaust system, filter, air box mods etc. The bike has been backfiring on deceleration. . . a lot. I have checked the obvious things like exhaust and intake leaks, had it on a dyno to check the fuel mix (a lovely 13:1 to 15:1 all the way through the revs). But it still backfires. It's backfiring mainly at mid to high revs but only when i close the throttle completely, if i open the throttle a little it stops.

Problem is, being a supermoto, i need the compression of deceleration to brake into corners. So i can't just raise the idle to get rid of it.

I am now thinking that the problem is that it's running too lean when the throttle is closed (or there is a problem with the spark plugs, but i'll try that one last cause the HRC plugs cost a fortune here).

So my question is (finally, sorry for the long babble), will increasing the pilot jet size richen my mix with the throttle fully closed and hopefully stop the backfiring? And, if it does richen the bottom of the rev range, will this kill the power at low revs and take off? I'm currently using a 68s pilot and a 185 main, and i have tried 3 jet sizes either side of the main with no success.

Any help is greatly appreciated, i've just about run out of ideas to fix this.

Tom.

I had the same problem - I got rid of my problem by screwing the pilot screw on the carb a half turn at a time, and decreasing the idle revs if required. It now idles perfect and now i hardly ever get backfires.

You say you need the compression for supermoto - have you done the mod that will increase your engine breaking - as the xr650r has a diaphragm that softens the engine braking.

ice.

plug the decceleration softener on the carb

do a search on it you should find enough info and a few links :thumbsup:

In my limited jetting experience, the backfiring is from a leaner bottom end. Bottom end as in your pilot jet (0- ~1/16 throttle) and needle jet (1/16 - ~1/4 throttle). As mentioned previously, note your pilot screw and idle screw positions. Then open up (richen) your pilot screw a little bit at a time.

If that doesn't fix it, then look into a richer needle jet (not needle or needle clip, but the tube that the needle moves in and out of, the same tube that the main jet screws into). Most people only change the pilot, main jet and needle height. However, the needle jet is an important part of the air/fuel system, especially with a beefed up motor. I was thinking that there was an "HRC needle/needle jet," if you don't have it, look into it.

Try a 70 pilot jet. If you're flowing enough air to need a 185 main then a 70 pilot sounds reasonable. Might as well pick up a 72 while your at it, just in case. Sharp popping on closed throttle deceleration is almost always a lean pilot circuit.

Also:

The best way to find the proper setting for the pilot screw (aka fuel screw) is the following. This method can also indicate if your pilot jet is too big or too small.

1) Warm up the engine to full operating temp.

2) Turn up your idle a few hundred RPM (basically you want a fast idle). This will make it easier to hear small changes in RPM. Watch for overheating--pointing a big shop fan at your engine will help it from getting too hot during the fast idling.

3) Turn the fuel screw IN until the idle starts to drop and miss.

4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.

The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to drop at 1 turn out and starts to drop at 2 1/2 turns out then 1 3/4 of a turn out should be the correct setting.

If the peak/smoothest RPM is reached somewhere between 1-3 turns then your pilot jet is correct. If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one. If the bike runs with the screw all the way in then your pilot is way too big. If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up (bigger) on your pilot jet.

Typical fuel screw settings are in the 1 1/2 to 2 1/2 range.

Thanks for all the advice, looks like i have some work to do this weekend.

I have ordered a #70 pilot and will try it out. I do know of the mod to block the deceleration softener, but i haven't done it yet (for some reason i was thinking it would make the backfiring worse?). I'll do that mod too.

The fuel screw is set at 1/2 a turn out, this was set when it was on the dyno. It made about 30% less power at the bottom end with the fuel screw set at 1 1/2 turns out. Now i'm confused cause this would mean that the pilot is too rich? But then again the dyno run was only done on acceleration, not deceleration.

I have got the HRC needle and needle jet installed so theoreticaly they should be right. And it never backfires when cruising along at mid throttle, only when i shut the throttle off completely at mid to high revs.

I may as well recheck the exhaust and intake joins again. I used a gas torch (not lit, and listen for increased revs) to check for intake leaks, and a match (lit, to see if it gets blown out) to check for exhaust leaks. Is there a more accurate way to check for leaks?

If your at 1/2 turns out right now, I'd try setting the fuel screw per the above routine and see if the popping is reduced to an acceptable level.

Regarding disabling the air cut-off valve--remember to only do one thing at a time so you know which things are doing what. What I'm getting at is don't disable the ACV and change the pilot circuit at the same. Do one, test, then to the other.

BTW, I'd love to see your dyno charts.

Haven't got the air fuel graph on the computer, but heres power/torque.

I've been told that a Vortex Ignition will get me another 6 to 10 horsepower, but i'm not sure if i really need it.

Good point on doing one thing at a time, i'll do the cut off valve first, the pilot screw then pilot jet.

XR650SMR_Dyno_Graph_1_small.jpg

Blue line is pilot at 1 1/2 turns out, red line is pilot screw at 1/2 turn out.

if that vortex ignition is going to produce that kinda gains i would definately buy one.. i would love to see some dyno charts with it installed to see what the real world data is.

When (and if) i get it, i will definately put it on the dyno for a comparrison run against the last graph.

Supposedly, the vortex offers 4 to 7 horse power increase on a stock XR, but really comes into it's own on a bike with high comp piston and high flow parts right through (carb, cam, exhaust etc). But you also need a damn good spark plug too, the ones that came in the HRC kit are $80 (aussie) each here. And you need to lighten the flywheel to get the gains down low too.

So it's not as easy as bolting on a CDI and getting 10 horsepower. But then again salesmen always make "qualifications" to their estimated horsepower gains.

What will it take to crack 50 lb/ft of torque with these machines?

4186322-L.jpg

What will it take to crack 50 lb/ft of torque with these machines?

4186322-L.jpg

Money and lots of it..........

Nope.... Not enough.

17908592-L.jpg

Tom,

Where abouts are you in aussie?

Block the air cut off thats your fix for no back fires.

If your running and getting a good air to fuel mixture, 3 ways to tell, exhaust temp, CO2 sensor up the pipe, plug reading, exhaust temp is better.

Your pilot is on the money and a 185 is a bit fat.

Who did the dyno test for you?

Who installed the HRC kit?

Who told you about the Vortex giving you that much gain?

Other than the potential sound annoyance. Why does it matter if it pops on decel? You obviously don't want any power from the engine (throttle closed). So the idea of optimum mixture for power is out. Honestly, you shouldn't care if the engine jumps out of the chassis, kisses the trophy girl, then hops back in the chassis at the apex. Do you think you are doing some engine damage or something?

popping on deceleration usually indicates a lean condition which should definately be looked at.

Okay, so it is lean. so what. The throttle is closed, cylinder pressures are low, there is little airflow through the engine and the engine is decelerating. so what if it is lean. You guys do realize moder fuel injected bikes and cars do a decel fuel shut off under certain conditions. I wouldn't worry about it unless the noise drives you nuts. You aren't hurting anything on the engine.

:thumbsup:

Basically XR Stamm you've got it in one. . . The noise is driving me crazy.

I'm not talking about one or two pops here, but a flurry of loud bangs. None of the other bikes on the track backfire on decel (except one suzuki), and most of them have aftermarket pipes, pistons, cams etc. So i'm pretty sure it can be tuned out some how. I'm also not completely convinced that it's not hurting the engine/exhaust.

That being said, i blocked the air cut off. Still backfiring. I turned out the pilot screw. Still backfiring. I changed the pilot for a #70. Still backfiring. I rechecked the exhaust and intake for leaks. No leaks.

Got onto a new line of thought while testing it though. The backfiring mainly happens after getting on the gas for a bit and then backing off the throttle. Or when i am nearing an empty tank of fuel it gets really bad. So i'm thinking maybe the float bowl is draining and then leaning out. I think i'm going to try taking out the reserve petcock and mesh filter and put on a new piece of fuel hose too. Might be wrong, but it's cheap and easy and i think my only other option is to buy a new pumper carbie and exhaust to be sure i get the problem.

Atleast it still goes fast. It's just the noise that's shitting me, it brings a little bit too much attention from the police when you have an illegal exhaust on the bike.

Tom,

Where abouts are you in aussie?

Block the air cut off thats your fix for no back fires.

If your running and getting a good air to fuel mixture, 3 ways to tell, exhaust temp, CO2 sensor up the pipe, plug reading, exhaust temp is better.

Your pilot is on the money and a 185 is a bit fat.

Who did the dyno test for you?

Who installed the HRC kit?

Who told you about the Vortex giving you that much gain?

Kasuyaho,

I'm in Sydney, Have had CO2 censor up the pipe on the dyno and all was good, plug looks fine, Dyno run was done at Sydney Dyno in Kensington, HRC Kit installed by Dirt Bikes R Us, Vortex information is from Nick at Teknik Motorsport. Nick builds the XR650's that race in (and win) the Australian Safari, so i trust him (a bit, he's still trying to sell me something though). His labour is expensive but he does really good work, he did my suspension too and it's perfect.

Update:

Took out the fuel strainer and reserve tube and used a larger fuel hose. Magic, atleast half of the backfiring has gone away. It's down to 3 or 4 pops when going from about 6000rpm down to 2000rpm into a corner and not virtually no loud bangs. This i can live with. Now i'm going to try and go back to the 68s pilot and original pilot screw setting to see if it gets worse again. Will also try a 180 main jet as spoke to Teknik the other day and he says that the Aus safari bikes run on a 178 to 182 depending on temperature. So you're probably right about the 185 being a bit fat.

Kasuyaho,

I'm in Sydney, Have had CO2 censor up the pipe on the dyno and all was good, plug looks fine, Dyno run was done at Sydney Dyno in Kensington, HRC Kit installed by Dirt Bikes R Us, Vortex information is from Nick at Teknik Motorsport. Nick builds the XR650's that race in (and win) the Australian Safari, so i trust him (a bit, he's still trying to sell me something though). His labour is expensive but he does really good work, he did my suspension too and it's perfect.

Update:

Took out the fuel strainer and reserve tube and used a larger fuel hose. Magic, atleast half of the backfiring has gone away. It's down to 3 or 4 pops when going from about 6000rpm down to 2000rpm into a corner and not virtually no loud bangs. This i can live with. Now i'm going to try and go back to the 68s pilot and original pilot screw setting to see if it gets worse again. Will also try a 180 main jet as spoke to Teknik the other day and he says that the Aus safari bikes run on a 178 to 182 depending on temperature. So you're probably right about the 185 being a bit fat.

Yeh i got Nick to do my suspension, it is brillant, no comparison to the stock setup, did he raise the rear of yours when he did the suspension?

(lengthen the stock internally???)

I got Nick to lighten my flywheel and put in his ignition map in a vortex, best bang for ya buck, have not got it back yet, should be soon.

172 jet here open side cover stock motor.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now