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Discussion Starter #1
Hi all,
Working on a 01 SV650S with 35k miles, backfires at idle but runs like a gem above 2k rpm. Stock headers with a yoshimura Tri Oval dual exit pipe, stock intake, tank lifted, 17.5 pilots and 136 mains. Pilots are fresh, mains were pulled and cleaned, and stuck some fresh CR9-E plugs in.

I'm open to ideas, air getting into the exhaust somewhere? needs valve job?

Thoughts?

Thanks

Gitter
 

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First thing....check valve adjustment. (never hurts even if it's not the problem...process of elimination) Being in CA you likely have the PAIR system which can do similar things if one of the reed valves malfunctions.
 

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Discussion Starter #3
valves are on my list of things to do, that said, thoughts on removing pair system first? Its still in tact, but chucking it seems a bit less time intensive in comparison.
 

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Checking (and perhaps adjusting) the valves is a routine maintenance procedure while PAIR removal is a modification.....I'd do the maintenance first and only if that doesn't cure the problem (which there is a good likelyhood that it will) then dig deeper into other things.
 

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Hi all,
Working on a 01 SV650S with 35k miles, backfires at idle but runs like a gem above 2k rpm. Stock headers with a yoshimura Tri Oval dual exit pipe, stock intake, tank lifted, 17.5 pilots and 136 mains. Pilots are fresh, mains were pulled and cleaned, and stuck some fresh CR9-E plugs in.

I'm open to ideas, air getting into the exhaust somewhere? needs valve job?

Thoughts?

Thanks

Gitter

The three main causes of backfires on CV carbs are

1;Air leaks into the Carb intakes.Look for damaged/distorted seals.

2;Exhaust leaks

3:Air cut off diaphragm damaged/distorted.

The ACV diaphragm is number 36 on here,

https://www.motorcyclespareparts.eu/en/suzuki-parts/1999-sv650ns-motorcycles/carburetor

It's designed to enrich the mixture when the throttle is closed to avoid the,"Lean Mixture" situation and avoid popping on throttle closing but if the Diaphragm becomes damaged/decayed it can leak fuel into the system when it shouldn't.

For the leaks try spraying some WD40 or Carb/Brake cleaner around the carbs when at Idle and if the idle speed changes you know you have a leak.


HTH :)
 

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I really can’t see this is anything to do with valve clearances (and I’d be somewhat circumspect about delving too much into an 18 year old bike’s interior unless you think there is something wrong - a single snapped stud or rounded off bolt might make it beyond economic repair).

Because it only has the problem at low engine speeds the other contributor’s suggestion of a carburettor leak, exhaust leak or air cut off valve are much more likely causes. I note it has an aftermarket exhaust, so that would be a suspect - and would be easy to find. It sounds like the ACV is pretty cheap and easy to replace, so it would be a good idea to do that and make sure there are no leaks on the inlet side at the same time.

Good fortune,

Alan


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You guys are ignoring the Basic Rule of Diagnosis! That being 'make sure ALL maintenance has been performed before proceeding'. Trying to always match a symptom with a cause can be frustrating with so many things overlapping and especially on modern engines where you're actually working on a sentient being that is working diligently to mask or compensate for another shortcoming elsewhere. The Curvy isn't as sentient as modern engines but still a too tight intake valve can leak enough at low speeds to backfire through the carb. It's possible, and it's basic maintenance that should have been done, so no harm in tidying that up first...right?

If you have a 35,000 mile old SV and have never checked the valve clearances.....shame, shame upon you.:(
 

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If you have a 35,000 mile old SV and have never checked the valve clearances.....shame, shame upon you.:(

No Sir, I have a 135,000 mile SV650S K6 that has (to the best of my knowledge) never had its valve clearances adjusted.

:)


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No Sir, I have a 135,000 mile SV650S K6 that has (to the best of my knowledge) never had its valve clearances adjusted.

:)


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But....somewhere down the road when/if you start having problems....will you continue to ignore basic maintenance and go off chasing your problem, or will you verify the tune just so you're not ignoring something simple? It's funny how many people come here with problems...and we offer solid basic advice, but it's not what they want to hear so they keep asking 'what else could it be'? Then later many just disappear without us ever knowing what their problem actually had been.:(

I fix things for a living...and have for over 30 years with some degree of success and always start with the basics if they've been ignored. Might be surprising how many weird problems have been cured just by doing the basic maintenance that didn't seem related to the problem. Even if it doesn't cure the problem....you at least then know your foundation is solid before you start climbing the sometimes precarious ladder of 'what if's'.
 

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We will have to disagree on this matter.

I am not ignoring maintenance (in the case of opening the motor to adjust the valve clearances on my 135,000 mile 13 year old K6S). Rather I have made a conscious decision that more harm than good might be done by invasive surgery than might be gained by fixing something that I don’t believe is broken.

My bike doesn’t use any oil between regular and frequent changes, is not mechanically noisy and the compression is good (150 PSI on both cylinders). The lack of symptoms suggest the valve gear is in rude health. The bike came with a large history file (oil and filter changes, filters, chains, tyres &c) but nowhere can I see any record of work inside the engine.

Since bought the bike (it had been a London commuter previously) I’ve replaced the brakes, returned the exhaust back to standard (much better), replaced the radiator and hoses, replaced the front header pipe and carried out a myriad of servicing and repair jobs to get it running really nicely. Whilst working on the bike I’ve come across several bolts and studs that have either rounded off or snapped, just through work hardening via age and use. My considered opinion is that if I stripped the top ends to adjust the valve clearances there would be a significant chance that at least one more might break in the process rendering the bike beyond economic repair. In this respect my decision not to do this has been like a doctor treating a patient, the first principle being: do no harm.

Here is an example, it probably took 3 days (off and on) trying various things to get these two Allen bolts that hold the header pipe onto the front exhaust port off the bike:



I am conscious that it remains a high mileage bike, older bike that has a low residual value (perhaps £1000). I bought cheaply it as a project (it was still a runner and came with a new MoT certificate) and it has turned out rather better than I expected. I am managing its dotage: it is now safe and runs really well (particularly having carried out the various adjustments to the fuel injection system, which were free and have improved running enormously), but I’m aware it is worth more as parts than it is as a whole entity. I’ll enjoy using my 135,000 mile, 13 year old SV for what it is (I have 4 other bikes) - cheap, cheerful and good fun. I won’t go looking for problems that aren’t there though, that would be daft. If a major component fails I’ll break the bike for spares and easily recoup the small investment I have in it. Saying that though, I suspect I may well see my K6 to 150,000 miles in about 5 years time.

I forget the age of the OP’s bike, but it was probably similar. I think the ailment he described (a misfire at low speed) is almost certainly a leak on either the exhaust or intake sides that will be fairly easy to fix without invasive surgery (which is what I said above). I don’t think it is likely that valve clearances are causing the issue, and I think stripping the top ends to make adjustments with no evidence of a problem would be a ridiculous risk on an older bike and completely disproportionate in expense.

So Sir, I return to my original point: we disagree. I don’t think advising the OP to strip the top ends in an attempt to fix a low speed misfire is sound advice (it would be a ridiculous thing to do at this stage). I think he should look for simple and cheap fixes first, and in doing so not risk doing more harm than good to his older bike.

Alan


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It has been several years since I had my SV so take this with that in mind. Several times in the 10 ish years I did have him the rubber caps on the vacuum port on the front cylinder intake boot cracked and leaked. Just something simple and easy to check, replace. Also in 104k miles and 2 valve clearance checks there was 1 valve that was approaching needing attention, tending toward the tight side of in spec. Your results may vary......
 

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Discussion Starter #12
If you have a 35,000 mile old SV and have never checked the valve clearances.....shame, shame upon you.:(
Perhaps I should have prefaced my post with "it has 35k on the clock, and I just bought it". At no point would I run a motor up to 35k miles without checking the valves. I cant speak for what the previous owner did or didnt do.

Also, its not popping up through the carbs, its popping out the exhaust.
 

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Perhaps I should have prefaced my post with "it has 35k on the clock, and I just bought it". At no point would I run a motor up to 35k miles without checking the valves. I cant speak for what the previous owner did or didnt do.



Also, its not popping up through the carbs, its popping out the exhaust.


I don’t believe this misfire problem is anything to do with valve clearances OP, that was a red herring. Check for air leaks both sides - and the fuel cut off valve someone mentioned above.

Here is an idea: if you check the plugs you may find one is running very lean, in which case there will be a leak on the intake for that cylinder. If the plugs are okay perhaps there is a leak on the exhaust side.

Don’t feel guilty about not checking the valve clearances, my feeling is lots of riders never bother. By the time their SVs need the check their residual values are so low it is not worth going to a Suzuki dealer for the work, and it is a fiddly job even for an experienced mechanic to do at home.

Good fortune,

Alan


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It's funny how many people come here with problems...and we offer solid basic advice, but it's not what they want to hear so they keep asking 'what else could it be'? Then later many just disappear without us ever knowing what their problem actually had been.:(
I usually take it as a sign that one of us hit the nail on the head with the "Duh!" suggestion and the OP is too embarrassed to come back and admit it!
 

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I usually take it as a sign that one of us hit the nail on the head with the "Duh!" suggestion and the OP is too embarrassed to come back and admit it!
Agreed! But...also must give thanks to those Members who come back with the final solution to their problem which really does enhance the database for everyone to benefit.:)

There is always an element of competition when trying to remote diagnose a problem...and some get more competitive than others, while I find it frustrating that good basic diagnostic procedures are often ignored just because some of them require you to unbutton the engine a little. If checking valve clearances is too 'intrusive'....then motorcycle mechanics might not be for you. No harm in that, just that everyone needs to know their limitations (thanks Inspector Callahan :))
 

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Discussion Starter #16
while I find it frustrating that good basic diagnostic procedures are often ignored just because some of them require you to unbutton the engine a little. If checking valve clearances is too 'intrusive'....then motorcycle mechanics might not be for you. No harm in that, just that everyone needs to know their limitations (thanks Inspector Callahan :))
Again, I'm not entirely sure who this is directed towards. At no point did I say I was going to skip checking the valves. Simply mentioned that removing the pair system was a 10 min job compared to a valve check which is more time intensive.
 

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... backfires at idle but runs like a gem above 2k rpm.

...17.5 pilots and 136 mains. Pilots are fresh, mains were pulled and cleaned, and stuck some fresh CR9-E plugs in.
As a quick test, can you make the backfire go away by adding choke?

Does the backfire go away when the bike warms up?

"Fresh Pilots" means brand new?
 

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Again, I'm not entirely sure who this is directed towards. At no point did I say I was going to skip checking the valves. Simply mentioned that removing the pair system was a 10 min job compared to a valve check which is more time intensive.


... it was aimed at me - some petulance on the part of Smug and Smugger methinks.

Ho hum.


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Discussion Starter #19
As a quick test, can you make the backfire go away by adding choke?

Does the backfire go away when the bike warms up?

"Fresh Pilots" means brand new?
Yes the backfire goes away when the choke is on and it almost completely goes away when the bike is warm. And yes fresh pilots meant new ones.
 

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Yes the backfire goes away when the choke is on and it almost completely goes away when the bike is warm. And yes fresh pilots meant new ones.
Check the Choke plungers.See if they are in good state and operating correctly.The screws that hold them in can easily be damaged so try to only use the proper JIS quality screwdrivers if possible to remove them.
 
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