Restricted curvy exhaust backfiring

[otaylor38]

hello to all (:

first of all, i would like to say hello to everybody. iv been reading various topics on this forum for a while now, but have only just got round to making myself a member.

now then, I have a 2001 sv. when i first heard the bike, it didnt have a restrictor kit in, and was running sweat as a nut.

however, when i purchased the bike, i came to an agreement with the bike shop that they would fit the restrictor kit free of charge, if i bought the kit. so i went on ebay and bough the washers from a company called 'kickstart engineering' for £40

since its been fitted, the exhaust has been backfiring and popping. the shop says this is normal for a restricted bike, but is it?

iv had the bike apart today, first of all the front carb wasnt on properly at the manifold (where the restrictor is, as im sure you all know). i initially though the bike was pulling in too much air from this point. after putting the carb on properly, it still pops alot.

i have checked both spark plugs and they are both looking a healthy colour, no suggestions as to it running lean or rich.

so. has anybody else had this problem? should the bike do this while restricted? is their an air leak, or is it just the restrictor itself?

im happy that its not doing the bike any harm, but its just annoying to be honest lol.

sorry for the essay, and any help very much appreciated.

thankyou in advance.
oli

[Spank86]

I don't know but I would think so, of it's a straight washer restrictor it's simply restricting flow without changing the ecu so there's Unburnt fuel which is causing the popping in your exhaust.

Hopefully someone else will chime In and either correct or support me.

[Scythe92]

As Spank86 said, it's popping because after a certain rpm, air flow into the carb becomes choked, however the same amount of fuel goes in, making it run rich. It is fine, just learn how to ride the bike restricted, using all the low down power (before 8k or so)

[Dunn-y]

I had my Curvy restricted for well over a year and had no popping of any sort even with a straight through exhaust. (Slip-on with no baffle) So I would think something is not quite right.

The restricting washers go between the engine and carbs so restrict the air/fuel mixture not just the air. (I maybe wrong) Beat me to it.

[timwilky]

OK, I have never looked at restrictions, how they operate so pardon the ignorance.

But I thought the washer sat in the inlet manifolds, as in after the carbs. Therefore I do not understand the assertion that they restict or choke the airflow. That would cause the engine to run rich unless it is rejetted.

If they are positioned where I suspect, then they restrict the volume of fuel/air without altering the mix ratio.

The popping/backfire is usually as a result of air leaking into the exhaust allowing detonation of unburnt fuel.

Or have I got my understanding of the washer position wrong and they sat in the carb intakes?

[Spank86]

You could be right, there's various ways of restricting bikes, sv's could well be the way you stated, of course it's still possible that the restriction is affecting the fuel air mix since choking down any pipe will make at least some difference to the ratio however slight.

[yorkie_chris]

No, they fitted them wrong.

The fact that the carb wasn't on right would cause popping, and is a worry about their competence...

Sounds like an air leak still, check the rubber cap over front vacuum nipple and all vacuum lines.


The restrictor should not make them pop. The restrictor has exactly the same function as not opening the throttle all the way.

[Scythe92]

OK, I have never looked at restrictions, how they operate so pardon the ignorance.

But I thought the washer sat in the inlet manifolds, as in after the carbs. Therefore I do not understand the assertion that they restict or choke the airflow. That would cause the engine to run rich unless it is rejetted.

If they are positioned where I suspect, then they restrict the volume of fuel/air without altering the mix ratio.

The popping/backfire is usually as a result of air leaking into the exhaust allowing detonation of unburnt fuel.

Or have I got my understanding of the washer position wrong and they sat in the carb intakes?

My mistake, they were in the inlet manifolds, it had been a while since I looked at them

[otaylor38]

Hey. thanks for all your help guys.

yeah, sorry, should have said earlier. The restriction its self is in the manifold between the carb and the engine. Therefore it should be restricting both the air and fuel ratio as a whole?

i agree Yorkie-chris, youd have thought a fully trained bike mechanic would be capable of pushing the carb into the rubber and tightening the little clip. when i found that, i was convinced that was the problem, until i started the bike and it still did it.

Cheers. i shall check all the little pipes and that again. see if i can find anything this time round.

if not, i guess it could be the restrictors themselves? as im sure you know, they are a circular washer with a sort of triangular whole in the middle. may be a silly thing to say, but could that be in the wrong way?

i wonder if this is the price to pay for buying a £40 kit off kickstart engineering, and not paying the full £200 off FI international.. hmm.

thanks again for all your help :). i shall see what i can find :)

[yorkie_chris]

Don't go down that train of thought, FI international are the most incompetent "engineers" (pah) ever to make *rses of themselves by churning out badly thought out sh*te with hilariously marked up prices. Their only skill is churning out glossy leaflets with scaremongering lies and intimidation all over them. C***s.

The triangle hole is bollox as well, it's in there for marketing reasons, earlier FI kits had round holes and they're fine.
Kickstart engineering have obviously copied FI cnt without thought. £40 is still a pretty steep price...

[otaylor38]

ah right. i see haha. yeah i had heard that your just paying for glossey leaflets. not what you want really. ill forget that idea then lol.

oh right i see. i couldnt quite understand why its triangular, but cheers for that. i guess its just to make people like me believe its more complicated than it actually is ;) haha.

hmm agreed. £40 isnt cheap. alot happier paying that than the full £200 though. tbh, id be alot happier having an unrestricted bike, but hey hoe, thats a seperate discussion and not really an option (N).

cheers again

[otaylor38]

hey. i have checked all the lines, and checked the front nipple. all the lines appear to be inplace, with no cracks that i can find. really confused and stuck now =/.

it sometimes seems worse than others, but i cant understand why this would be? also, an occasional mis-fire on what i think is the front cylinder. any more thoughts? cheers again

[otaylor38]

Sorry to bring up an old thread, however I thought you lot might be interested in what I eventually (several months later) found to be the problem.

It was the fuel mixture screw on the front carb. It was way out, so it was over fuelling.

I know it seems like a simple thing and probably amongst one of the first things to check, but I couldn't and still can't understand why it had changed to start with. It must of been something the dealer had done while putting e restrictors in I guess?

Anyway, it's back to the 2.5 turn factory setting now and I can't fault it. Runs so much better at low revs, and also no popping or spluttering.

Mystery solved

Thanks all