spitting/missfire at low speed and warm up.

judokia · March 20, 2009, 12:37:20 PM

Hi Guys

My GS500f 2005 7000 miles has developed a missfire/spitting when starting and at low speed. Oil and filter along with oil pressure switch were replaced on Sunday along with Iridium plugs and i had a great day chasing my brothers K1200 that afternoon. Ran perfectly on 20 mile motorway journey home most of which was 70-90 mph. On the way to work i noticed that when warm the idle was a little lower than normal but thought nothing more about it. When staring on a little chock to go home the revs shot up to 4000rpm when i pulled the chock in to 2000 rpm it ran lumpy/hunting and would make a spitting sound. Riding home it would spit occasionally at low rpm but once warm idle would be stable but low 1100. Riding with revs above 2500 seemed fine but a little lacking in power.
I changed back to the original plugs as i though the may be faulty but no change.

Any suggestions

lawman · March 20, 2009, 12:54:10 PM

It sounds like a one track mind I have, but that's what my '04 was doing with badly adjusted exhaust valves. I mean really, really badly adjusted, like 4 changes off.

judokia · March 20, 2009, 01:03:48 PM

Hi lawman would that happen all of sudden as in my case / would a days hard ridding have brought this on. Was you exhaust valve too tight or too loose. I guess too tight,not closing fully.

lawman · March 20, 2009, 01:15:24 PM

Quote from: judokia on March 20, 2009, 01:03:48 PM
Hi lawman would that happen all of sudden as in my case / would a days hard ridding have brought this on. Was you exhaust valve too tight or too loose. I guess too tight,not closing fully.

It was too tight. I had 275s in and ended up with 255s. Now it runs like a charm.

I'll be honest, I bought the bike with the problem, so I don't know if it came on gradually or not. I imagine there would likely be a gradual onset, and then a tipping point where the problem started to perhaps cause premature wear and got exponentially worse. A day's "HARD" riding is likely to speed that up to noticable if the problem is already there.

For the 2 hours, tops (probably 1 hour including reassembly), it will take to check, I would start there. When were they last checked?

I'd also like to know what the plugs looked like when you pulled them - I am not a fan of plug fanciness generally (iridium my hairy balls - this is a 25 + year old engine design. They couldn't SPELL iridium when this thing was designed), but I'm wondering about whether maybe your mixture was/is off.

Always start with the simplest solution. Engines run on gas, compression, air/vacuum, and spark. I'm guessing from the limited info I have and my similar experience that you have an exhaust valve (air) issue and a gas/air mix issue combined.

judokia · March 20, 2009, 01:30:03 PM

Thanks lawman your comments make sense and are much appreciated. I bought the bike second hand with 3000 miles on the clock and clearances havent been checked since i have had it so youy may well be right. I'll do some more checking this weekend. As for the iridium plugs they did seem to make the bike more responsive.

lawman · March 20, 2009, 01:43:06 PM

4,000 miles is the checkup/service interval on the valve clearances. I'm betting that's your issue. If they weren't done before you got it, I'm willing to bet a fair amount you're off because wear accelerates if they're tight. Get a good set of gauges - I had a hard time finding small enough ones.

I really don't buy the iridium plugs bit. Sorry. With electronic timing and a fancy computer to adjust, maybe, but this setup is ooooooold and has none of that. I doubt they helped much. I bet you replaced old plugs with new ones, iridium or not, and it was new plugs - whether iridium or made of paperclips - that make the difference.

judokia · March 20, 2009, 02:20:51 PM

Can any of you expalin to me why the clearance should tighten rather than looses with age. Please forgive my ignorance.

lawman · March 20, 2009, 02:29:32 PM

http://gstwins.com/gsboard/index.php?topic=38827.msg436387

The valve eats into the valve seat over time.

sledge · March 20, 2009, 04:32:51 PM

Check the air-filter isnt clogged, run the tank down to reserve, fill up with fresh petrol and try some of this. http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_170703_langId_-1_categoryId_165750
If there is no improvement then go looking at the carbs/valves.

judokia · March 28, 2009, 03:11:17 AM

Hi guys just to let you know that the bike is running fine now. Turned out that i had a air leak to the carbs. Had the tappets reset as well so she is running sweet now.

Thnaks you all you helpfull suggestions.

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spitting/missfire at low speed and warm up.