Gen1 SV650 Stuttering/Bucking at high RPMS

[DeskJockey]

Sparknotes: Gen1 SV650 stutters/bucks under heavy load at higher RPMs. Where to start diagnosing? I’m thinking I'll start with my exhaust leaks, and then plugs/coils. Carbs are cleaned.

Story:

LRRS Round 2, nhbubba lapped me out of T2, and when we got back to the garage chided "Hey, you know you can get on the gas out of T2, right?". I was actually WOT… so this was concerning. I ran a Qstarz GPS on my bike for a race, and compared that to some data kindly offered by another SV650 racer (#365, Petorious). Check out the attached graph, showing speeds on the straight.


(The splits are about 150ft each, just eyeballed in the Qstarz software, so not very precise).


My acceleration was terrible, so something was clearly wrong. I’ve since cleaned the carbs, where I discovered that one of the main jets was fairly gunked up. That seemed to increase power, but now it’s sputtering dramatically under heavy loads. It doesn’t seem to happen right out of the corner, but rather only after 1-2 seconds of acceleration. The sputtering and bucking scared me enough to get off the track and try to diagnose it.

I’m going to repair my exhaust leaks, then check plugs and coils, but any suggestions, insight, or motivational ridicule are appreciated.

[freezinvt]

When you cleaned the carbs did you pull the screens behind the float needles and make sure those were clean? What are you running for main jets and air filter?

[Ductard]

Sparknotes: Gen1 SV650 stutters/bucks under heavy load at higher RPMs. Where to start diagnosing? I’m thinking I'll start with my exhaust leaks, and then plugs/coils. Carbs are cleaned.

Story:

LRRS Round 2, nhbubba lapped me out of T2, and when we got back to the garage chided "Hey, you know you can get on the gas out of T2, right?". I was actually WOT… so this was concerning. I ran a Qstarz GPS on my bike for a race, and compared that to some data kindly offered by another SV650 racer (#365, Petorious). Check out the attached graph, showing speeds on the straight.


(The splits are about 150ft each, just eyeballed in the Qstarz software, so not very precise).


My acceleration was terrible, so something was clearly wrong. I’ve since cleaned the carbs, where I discovered that one of the main jets was fairly gunked up. That seemed to increase power, but now it’s sputtering dramatically under heavy loads. It doesn’t seem to happen right out of the corner, but rather only after 1-2 seconds of acceleration. The sputtering and bucking scared me enough to get off the track and try to diagnose it.

I’m going to repair my exhaust leaks, then check plugs and coils, but any suggestions, insight, or motivational ridicule are appreciated.

I'm interested to hear what people have to say. Having similar problems with my KTM 520SX. Since both are carbed I'm guessing the causes/solutions might be similar.

[DeskJockey]

I did not pull those screens - could that cause a fuel shortage?

Running 150 mains jets, not sure about the air filter.

[freezinvt]

[QUOTE=DeskJockey;1414338]I did not pull those screens - could that cause a fuel shortage?

Sure can. Before you pull the carbs again, though, I would check for good connections at the coils and plugs, might as well check those off the list before you pull stuff apart.

[Jim]

Run a compression test on the cylinders?

[The Snowman]

What type of carbs? I've had high-end sputtering on my old G1 SV when I swapped over to flatslides and was running too rich. I've also had stuttering issues with spark. G1's are notorious for eating coils. Try first changing spark plugs. ONLY use NGK CR9E, not CR9EK or Iridium plugs. They like nice, big, fat spark. Do a spark test to make sure that you have that nice big spark. If one plug wire w/plug doesn't have a big spark, change out that coil.

[GingahNinjah]

Do you have full jet kits with needles or did you just do main jets? are the Needle valve clips adjusted correctly? if the issue persists I highly recommend bringing the bike for a dyno tune at Souhegan Valley Motorsports, Scott Mullen is a genius with the Gen 1!

[DeskJockey]

Stock carbs.

This is good stuff guys! So far the list looks like
- Compression Test
- replace spark plugs
- check spark, replace coils as necessary
- do a better job cleaning carbs (I have a very mechanical friend who I will convince to oversee my carb cleaning, as I obviously didn't cover all the bases last time).

I can't wait to get this sorted out. I was getting out-motored by 300's last round lol.

And if I can't get this fixed this weekend, I'm definitely taking it to a pro.

[freezinvt]

Good suggestions by the others. Might also be worth checking for a pinched or kinked fuel line. Remember, the main line from the tank to the fuel pump will move/twist some as you set the tank down.

[maxim_X]

Do you have full jet kits with needles or did you just do main jets? are the Needle valve clips adjusted correctly? if the issue persists I highly recommend bringing the bike for a dyno tune at Souhegan Valley Motorsports, Scott Mullen is a genius with the Gen 1!

If it doesn't work out for you fixing it yourself then I would second this advice. He will get it right. In fact if there is any real question on the state of tune on the bike the it would be a smart idea to bring it to him regardless. Thank us later.

[Dayon]

try replacing the plugs and plug caps at the same time. This has happened to me multiple times and the SVs are notorious for it. I just had to replace mine at the last race. They get a carbon track inside the plug cap and causes the spark travels done the carbon track to ground (path of least resistance) instead of arcing across the spark plug. When this starts to happen it occurs at high RPM, above 7500ish. If that doesn't do the trick, I'll third this advice to have Scotty take a look at it.