Backfire during accel -- too lean?

[corksil]

Hello I have done way to much crap to this bike to explain all of it, and it's far far from stock.

Am I correct to assume that a backfire during acceleration is due to a lean condition?

After lots of troubleshooting, I've come to the conclusion that the bike needs a bigger pilot jet.

If I crack the throttle open just a tiny bit, the RPM climbs and climbs and climbs up to 6k. If I open the throttle moderately, the bike often backfires and hesitates and stumbles a little before the engine spins up to higher RPM. This is with the bike sitting motionless.

General description of the problem -- mis/hesitation/stumble on acceleration off idle when accelerating. Above 4krpm this bike pulls surprisingly hard for a 250. If I'm starting on a hill I have to rev the bike up a bit and burn the clutch to get moving. If I'm starting on flat ground, I can open the throttle and dump the clutch at which point the bike hesitates, stumbles, falters, and then "catches" and takes off.

I suspect I need a bigger pilot jet.

Messing with mixture screws hasn't improved anything. The proper main jet is installed after lots of trial and error and this bike screams above 4k.

Any thoughts? My next thing to try is opening the choke to make the bike run richer and seeing if that helps with acceleration from a stop.

[choneofakind]

Yes, backfiring on decel is from a lean condition.

Check the simple stuff first before driving yourself batty. Check vacuum leaks in the carbs, especially around all the hoses and coasting enricher. I had one get loose and drove me nuts on decel with tons of pops and bangs. Fun but annoying.

I also had the same symptoms of the bike either stumbling or revving up like mad without much in between. It's annoying.

[Linkin]

Sounds like a vacuum leak and lean condition.

Warm up the bike and idle it. Crack the throttle sharply. If it bogs or stumbles - too rich. If it hangs above the idle rpm, it is lean or has an air leak on the intake somewhere.

Popping etc on decel is also a lean condition but also indicates exhaust leaks somewhere before the muffler. When the throttle is on, air is forced out, once on decel, the leaks then suck in cold air which ignites and causes the popping/burbling

[Somchai]

By the way, those who'd ever had the chance to listen to the older Porsche 911 race engines then those will understand what it means when an engine during accelerating is popping / backfiring - they run very rich...

[tgold]

Is the bike truly backfiring?

Backfiring is technically fire going back through the intake system. It is a more serious and dangerous issue than what many people mistakenly call afterfiring which is the burning/exploding of gases in the exhaust system. True backfiring can be a very serious issue that can be violent enough to pop carbs off the intake manifold, cause fires, etc, (not to mention this all happening between your legs).

Just want to get a clear picture of what's going on. If what you are talking about is actually afterfiring, then that is usually caused by an overly rich condition.

[jkv45]

I would say if you are getting true "backfiring", as tgold mentioned, you may have an issue with a burned intake valve or one hanging open.

Is the air filter still in good shape? If it's truly backfiring it will probably show signs of being burned.

If it's just popping, it's most likely carb-related problems. I would check for an intake leak first, but if the idle mixture screws are not effective you may have plugged passages or jets in the carb - which is what I'm thinking might be the cause of the problem.

[Motofool]

Originally Posted by corksil

......... Any thoughts?.........

Don't limit your troubleshooting to the mix.
When some of the combustion persistently happens beyond the limits of the valves, it is always a timing or sealing issue.

Timing involves slow burning mix (lean or rich), cams and sparks.
Sealing involves valve seats and springs.

If you have CV carburetors, the slides are not returning down quick enough, due to some problem with vacuum input, diaphragms, springs or guides.

[dcj13]

Along the lines of tgold's comments...

Is it backfiring out the tailpipe? Or out the carb(s)?

[corksil]

To be perfectly honest I hoped that people would agree with me in needing a bigger pilot jet.

Here are the modifications
-area P full race exhaust
-crower race cams
-K&N pod filter intake, no airbox, no snorkel -- just pod filter attached to carbs
-dynojet 110 main jet
-dynojet carb needles (clip on the second to lowest of 6 positions -- RAISED needles)
-mixture screws around 2.75 turns out from bottom

I just adjusted the valves and they are a thousandth or two on the loose side of the clymer maual 'spec.' This is due to the 0.05mm incrementations on the hotcams shim kit and the fact that the race cams were ground down to a more aggressive profile. I had to make up the difference in grind with shims. The cams were not welded and built up to raise the lobes, they were simply ground down in all areas except for the lobe to produce the more aggressive profile.

If I crack the throttle suddenly and hold it open, the bike hesitates a bit then revs up.

If I barely open the throttle and hold it open, the revs up to 6k in a second or two.

If I try to launch hard (race-type start) the bike falters/hesitates/sometimes-emits-a-popping-sound, and then "catches" and takes off with lots of power.

Before installing the race cams I could let the clutch out and roll on the throttle to get moving. After installing the race cams I have to rev it up a bit and then let out the clutch at 3-4krpm to get moving. Once I'm moving at 5mph I can roll on the throttle to wide open and the bike really takes off.

My thinking was that installing race cams (designed for more mid/top-end performance) would hurt my low end up until mid-range. This was the case until I raised the needles a clip and it felt like I suddenly had 10% more low-end. I raised the needles (lowered the clip) a second time and achieved maybe 5% more low-end. Somehow I doubt raising the needles to the highest position would achieve more power gain.

The backfire sounds like it's in the tailpipe. Not firing back through the carbs.

Thanks for the help.

[Motofool]

The exhaust valves could be floating due to the aggressive cams with original springs.

[choneofakind]

I mean... if you wanted us to say bigger pilot jets, by all means give it a go. All you're out if that's not it is a couple hours of your own time.

[Somchai]

Originally Posted by corksil

Before installing the race cams I could let the clutch out and roll on the throttle to get moving. After installing the race cams I have to rev it up a bit and then let out the clutch at 3-4krpm to get moving. Once I'm moving at 5mph I can roll on the throttle to wide open and the bike really takes off.

At our race engines with 'hot' cams we'd setup the idle at 2,500 rpm, while the normal street engine was at 900 rpm...

[Ghostt]

Keep in mind the CDI unit is primitive, and dumb.

How so? ..

The EX-250's CDI uses a very simple advance curve: 2-dimensions, X and Y, with no compensation for load. It's pre-historic, rudimentary.

At about 4,000 or 4,200 rpm, depending on the year-model of the EX-250, the CDI has the ignition timing at full advance (38 degrees BTDC for the older engines or 42 degrees BTDC for the newer engines).

When you roll-on full throttle at about 4,000 rpm you suddenly make the air/fuel ratio much richer. In a modern car or motorcycle engine the ECU knows when this happens and it retards the ignition a bit because richer air/fuel ratios burn faster than leaner mixtures.

Because the EX-250 CDI has no way to compensate for the sudden rich (faster burning) mixture the ignition advance stays way up there at 38 or 42 degrees and you end up with too much of the burn occurring before the cylinder reaches TDC.

Instead of instantly making a lot more power (as you're anticipating when you open the throttle wide) you make only a little bit more. And the engine slowly lifts itself out of the situation.

I know that the Ninjette's bigger brother the EX uses a PAIR system, which recycles the exhaust gas, to be reburned. While a good idea it's leads to popping out the exhaust, if I remember correctly the NewGen Ninjette has something similar.

http://www.ex-500.com/wiki/index.php?title=PAIR_Removal

A far as messing with jet size, hope you have an exhaust gas analyzer handy, otherwise you'll be guessing.

[Linkin]

^ Thought that was EGR? Kleen air?

You don't just stuff a bigger jet in to stop decel burbling. Completely sealing the exhaust system will help. After that, I think the EX250 carbs have the idle circuit mixture screws capped, meaning the carb body has to be drilled to access them. Modern emissions bullshit for you.

[tgold]

I agree with Ghosst. Put it on a dyno with an exhaust gas analyzer and you'll know exactly what your AFR is at any rpm. This will help you get the best power and drivability. To me it'd be worth the $$$ to save the time and headache.

As for what Linkin said, it's not on deceleration where the problem is.

[jkv45]

Originally Posted by corksil

-dynojet 110 main jet
-dynojet carb needles (clip on the second to lowest of 6 positions -- RAISED needles)
-mixture screws around 2.75 turns out from bottom

If I crack the throttle suddenly and hold it open, the bike hesitates a bit then revs up.

If I barely open the throttle and hold it open, the revs up to 6k in a second or two.

If I try to launch hard (race-type start) the bike falters/hesitates/sometimes-emits-a-popping-sound, and then "catches" and takes off with lots of power.

It's not unusual that an engine with CV carbs will hesitate when you whack open the throttle abruptly.

I would still look at the carb diaphragms closely to see if there is any damage. Also lift the slides manually and let them drop - do they both drop at the same rate? Do they lift smoothly? Are you using stock diaphragm spring? Are others available?

I would try going up a size on the Pilot and see how it feels. You have some adjustment (idle mixture screws), so it's not critical.

What gas are you running? How old is it? What do the plugs look like? When it stumbles, is there any black smoke?

[Ghostt]

OEM airbox? Pods?

Dyno jets are not the same size as OEM.

How-to tune idle mixture screws

Your initial setting should be 3 full turns out from the bottom, remember just slightly tighten them, don't go all Conan on them.



Then warm it up, slowly turn 1 side pilot screw in until the rpms drop, then back out till RPM's peak (and no further)* Twist throttle...see how it reacts...tweak to taste.
Repeat on other side... test ride. You're looking for best throttle response and transition from just cracked to 1/4 throttle opening or so.*

Thats a quickie, 10 cent tutorial on setting pilot screws.**

[corksil]

Quick reply because my mind is a little tired from working on other projects.

Messing with mixture screws didn't seem to help. I suppose I could give it another go but I don't think that would help much, if any.

Pod filters, no airbox -- as stated previously.

I agree with the notion that the factory ignition timing is not ideal. With that said, I'm not willing to spend any more $ on an EFI setup or programmable ignition timing. In the past I looked into switching over to the older 86-07' CDI box due to the timing advance differences and their cheap availability on ebay. I don't remember coming to a conclusion if it would be an upgrade or not -- because I couldn't find information on the exact ignition timing difference between the pre-gen and the 08-10' models. 11-12' were EFI unless I'm mistaken, so those are out of the question.

Somchai has the best and most economical solution --- simply raise the idle to 2500 where the bike has a lot more torque to get moving from a stop.

The gas is a few months old and the climate recently is non-stop monsoon rain. I drained some gas out of the float bowls to check for water and didn't find any so bad gas MIGHT be a contributing factor but not likely. It was 91 octane pump gas. Race gas is not in the budget.

Diaphragms in the carbs are not torn and look to be in good condition. OEM slide springs, I doubt an aftermarket slide spring would help with acceleration. Might make idle throttle blipping a little more responsive, but not help when I open the throttle and try to accelerate.

EGR/kleen-air is disabled/deactivated/capped/deleted. There is no exhaust gas recirculation on this bike.

Dyno is NOT an option. I have an AFR gauge but it's somewhat permanently installed on my car and removing it just to diagnose this issue would be a several hour long hassle.

Thanks for the help guys, I really do appreciate it.

[corksil]

Does anyone know the specific differences between pre-gen and new-gen CDI boxes?

(Capacitive-Discharge-Ignition) -- the little black box under the rear seat that controls ignition timing advance. If the 86-07' CDI had more aggressive timing advance or a more ideal advance curve, I could swap my 09' CDI out for an older model.

Maybe I'm not old-school enough, but I think this is the last carb'd bike I'll ever own.

[Somchai]

Originally Posted by corksil

Does anyone know the specific differences between pre-gen and new-gen CDI boxes?

(Capacitive-Discharge-Ignition) -- the little black box under the rear seat that controls ignition timing advance. If the 86-07' CDI had more aggressive timing advance or a more ideal advance curve, I could swap my 09' CDI out for an older model.

Maybe I'm not old-school enough, but I think this is the last carb'd bike I'll ever own.

Before looking for the pre-gen box (the ignition of the pre-gen was up to 42 degrees) maybe this could be worth a test: RTR-KAW-5-05 $69.95
Link: http://www.factorypro.com/prod_pages/prodk04.html
Scroll down to the ignition advancer kit, this will give you 5 deg. advance and so your bike will have 40 degrees at least.
I hope it works, good luck

[jkv45]

You shouldn't need any new ignition parts to get it to run right. What's there isn't great, but if it's working I don't think it's causing the problems.

First thing I would do is get some new gas in there. A few months can easily be too long, especially with ethanol in the gas. Unless you have raised the compression ratio you don't need anything over 87 octane (unless you have to in order to get away from ethanol). 87 without ethanol will give you the most power. Does the gas you are running contain ethanol?

Because you said the idle mixture screws aren't having much of an effect, I would consider cleaning the carbs.

[tgold]

Stop everything and do this ^^^^^^^. To be honest, you are wasting time chasing a possibly non-existent problem if you do not eliminate the water or other debris in your gas as a source of the issue. Just draining your float bowls won't do it. You will have to remove them and clean them. While you have them off and before you clean them, check for signs of fine red residue (rust) as this will play havoc with your jetting and cause the symptoms that you are describing.