Carb throttle pot - SV650.org

embee · 29-05-05, 08:13 PM

just a tip for anyone with a carb engine which suffers "hunting" at very light throttle openings steady cruising.

After prompting from my mate who has the twin sister to my bike (we bought a pair), I finally got round to checking the throttle pot setting. My bike had tended to do the "hunting" thing on trailing or very light throttle at say 60mph, if you crack the throttle a touch more it disappears and the engine pulls strongly. My mate's bike never suffered that.

The manual says set the full throttle reading on the pot to 78% of the "track" value (it's a resistive track with a moving contact which sweeps round it). Mine was set at around 82%, not a big deal you might think.

Having set it to 78% (3.92k out of 5.03k) the hunting no longer exists, and it has indeed subtley altered the engine feel at small throttle openings especially round 3-4000rpm. It has a "sharper" character.

Basically it is now telling the ignition box that the throttle isn't open quite as far as before, so it will be getting slightly more ignition advance. This seems to have cured the hunting (on mine anyway). Costs nothing and worth a check.

29-05-05, 08:18 PM

hunting?

Scoobs · 29-05-05, 08:34 PM

Quote:

Originally Posted by Grinch

hunting?

Keep going above and below a mean or nominal point, or the ideal point. Difficult to explain. In this case the throttle is partially open, but the fuelling is on (above nominal) or off (below nominal), giving a jerky feel to the bike.

29-05-05, 08:37 PM

Do you mean the throttle position sensor (TPS)?

embee · 29-05-05, 09:09 PM

Quote:

Originally Posted by THV

Do you mean the throttle position sensor (TPS)?

yes, but here specific to carbs.

Sorry, "Throttle pot" is just a common terminology in the industry.

On the carb engines the ignition ECU takes the engine speed and throttle position to determine the ignition value appropriate for the "load".

At very light load you need more ignition advance to get good stable combustion. If the throttle pot (potentiometer) is out of adjustment the ECU will select an inappropriate ignition value, and if that's retarded it'll tend to make it somewhat unstable, hence the hunting.

I know others have commented on this phenomenon in the past, though I guess carbs are a bit "passe" these days.

I don't know the sensitivity of the ignition calibration and I'm not going to check. Last time I had a timing light on the SV everything got covered in oil mist out of the timing hole!

02-06-05, 08:50 AM

Seems a good idea I'll have a look while the rest of it is in bits.

02-06-05, 10:24 AM

I have a 01 SV650s (22K), which is exhibiting similar symptoms.

When filtering through traffic at low speeds, it is very jerky, like riding a bucking-shetland pony. Not enough to throw me off, but makes for a wobbly ride. It also seems to lag in the middle of accelerating, at about 60/70, but will then suddenly pickup again. Could this be the cause? If so, where is the throttle pot located?

TIA

Ian

johnnyrod · 02-06-05, 10:32 AM

The pot is on the right (off) side of the rear carb, you need a manual anyway cos it has all the details of testing. Be warned, the two screws mouting it are made of chocloate and tightened by the Devil with both hands. You might have to use a hammer to shift them. When I checked both mine and my mate's (1999 and 2000 models resp) they were both a mile out. Dunno if it made a noticeable difference, I didn't notice anything to be quite honest, but was happy it was now as it should be. The bike did used to stutter below 3000rpm but the ignition advancer sorted that anyway.

As usual, something that is supposed to be set at the factory but/so is never checked and often wrong.

Ian, could be the problem, while you are at it I would also disconnect the retarder bits which work in 2nd and 3rd gear, this should help the bit round town. As for when you're going faster, not sure from the description. Could be lumpy carburetion, or all sorts of things. If you ride in traffic alot I'd get the ignition advacer key, it gives you a hell of a lot of bottom end, which is just what you need.

embee · 02-06-05, 10:46 AM

the low speed stuff may be simply other set up stuff, like chain adjustment, carb balance, idle mixture settings, but the higher speed bit sounds like the thing I was getting.

The throttle pot will affect things mainly at lighter throttle openings because I'd assume it'll get to a "full load ignition" curve by mid throttle opening somewhere. Anyway, well worth a check if you have a multimeter to hand.

Throttle pot is a small black plastic device on the righthand side of the rear carb, just lift tank and there it is, 2 phillips screws. It has a triangular connector which is the usual thing of push in the little tab to release the catch and pull apart.

It's in "Fuel & Lube" service manual section 4-33.
http://www.sv650.org/sv_manuals.htm

Measure resistance of track (Resistance#1, you'll find it's very close to 5k Ohms), then hold throttle open and measure resistance#2.

If you need to alter it to get 78% of the track value, make sure you have a good condition phillips/crosshead screwdriver (NOT pozidrive) which fits the screws very well because they're the usual cheese, and a bit of WD40 or similar first might help. There's not a huge amount of adjustment, mine's almost at the end.

EDIT - johnnyrod beat me to it!

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29-05-05, 08:13 PM	#1
embee Member Mega Poster Join Date: Jul 2003 Location: Warwickshire Posts: 2,801	Carb throttle pot just a tip for anyone with a carb engine which suffers "hunting" at very light throttle openings steady cruising. After prompting from my mate who has the twin sister to my bike (we bought a pair), I finally got round to checking the throttle pot setting. My bike had tended to do the "hunting" thing on trailing or very light throttle at say 60mph, if you crack the throttle a touch more it disappears and the engine pulls strongly. My mate's bike never suffered that. The manual says set the full throttle reading on the pot to 78% of the "track" value (it's a resistive track with a moving contact which sweeps round it). Mine was set at around 82%, not a big deal you might think. Having set it to 78% (3.92k out of 5.03k) the hunting no longer exists, and it has indeed subtley altered the engine feel at small throttle openings especially round 3-4000rpm. It has a "sharper" character. Basically it is now telling the ignition box that the throttle isn't open quite as far as before, so it will be getting slightly more ignition advance. This seems to have cured the hunting (on mine anyway). Costs nothing and worth a check.

29-05-05, 08:37 PM	#4
BURNER Guest Posts: n/a	Do you mean the throttle position sensor (TPS)?

02-06-05, 08:50 AM	#6
BURNER Guest Posts: n/a	Seems a good idea I'll have a look while the rest of it is in bits.

02-06-05, 10:24 AM	#7
iffy Guest Posts: n/a	I have a 01 SV650s (22K), which is exhibiting similar symptoms. When filtering through traffic at low speeds, it is very jerky, like riding a bucking-shetland pony. Not enough to throw me off, but makes for a wobbly ride. It also seems to lag in the middle of accelerating, at about 60/70, but will then suddenly pickup again. Could this be the cause? If so, where is the throttle pot located? TIA Ian

02-06-05, 10:32 AM	#8
johnnyrod Member Mega Poster Join Date: Jul 2003 Location: Doncaster, oop norf Posts: 2,122	The pot is on the right (off) side of the rear carb, you need a manual anyway cos it has all the details of testing. Be warned, the two screws mouting it are made of chocloate and tightened by the Devil with both hands. You might have to use a hammer to shift them. When I checked both mine and my mate's (1999 and 2000 models resp) they were both a mile out. Dunno if it made a noticeable difference, I didn't notice anything to be quite honest, but was happy it was now as it should be. The bike did used to stutter below 3000rpm but the ignition advancer sorted that anyway. As usual, something that is supposed to be set at the factory but/so is never checked and often wrong. Ian, could be the problem, while you are at it I would also disconnect the retarder bits which work in 2nd and 3rd gear, this should help the bit round town. As for when you're going faster, not sure from the description. Could be lumpy carburetion, or all sorts of things. If you ride in traffic alot I'd get the ignition advacer key, it gives you a hell of a lot of bottom end, which is just what you need.

02-06-05, 10:46 AM	#9
embee Member Mega Poster Join Date: Jul 2003 Location: Warwickshire Posts: 2,801	the low speed stuff may be simply other set up stuff, like chain adjustment, carb balance, idle mixture settings, but the higher speed bit sounds like the thing I was getting. The throttle pot will affect things mainly at lighter throttle openings because I'd assume it'll get to a "full load ignition" curve by mid throttle opening somewhere. Anyway, well worth a check if you have a multimeter to hand. Throttle pot is a small black plastic device on the righthand side of the rear carb, just lift tank and there it is, 2 phillips screws. It has a triangular connector which is the usual thing of push in the little tab to release the catch and pull apart. It's in "Fuel & Lube" service manual section 4-33. http://www.sv650.org/sv_manuals.htm Measure resistance of track (Resistance#1, you'll find it's very close to 5k Ohms), then hold throttle open and measure resistance#2. If you need to alter it to get 78% of the track value, make sure you have a good condition phillips/crosshead screwdriver (NOT pozidrive) which fits the screws very well because they're the usual cheese, and a bit of WD40 or similar first might help. There's not a huge amount of adjustment, mine's almost at the end. EDIT - johnnyrod beat me to it!