Missing on top under load. | Yamaha SR500 Forum

stevea
New Member

Posts: 8

Missing on top under load. Oct 9, 2008 12:30:38 GMT -5

Quote

Post by stevea on Oct 9, 2008 12:30:38 GMT -5

Hi Guys,
New memeber here. My bike is a '78 with a 535 jug, supertrapp and a K&N. Stock carb. My problem is when I'm in 5th and I get on the throttle to get that last 10% out of it the motor starts missing. I have to back of the throttle and tickle it to get back up. On a flat I'm lucky to get 65-70mph out of her. The problem is at it's worst when under a load, i.e hills or strong head winds. It bucks and bleches and even backfires once in awhile. I can't figure out if it's spark or a fuel problem but it's pretty agrivating. Any ideas on helping me trouble shoot to the bottom of this?

Thanks Guys,
I've tried some of your suggestions already fuel filter, plug wire and such. The highest octane we can get here in California is 91 unleaded. The bike pings on it but I mitigate it somewhat with a fuel additive. There is a local gas station that sells 100 octane but it would be cheaper to put whiskey (cheap whiskey) in the tank! The plug is a nice toasty brown but I'm going to replace it anyway. Cheap piece of mind to eliminated it and it can't hurt. I've thought about replacing the plug wire but it seems to be integral with the coil. Is this true? Anyway, you guys have given me some ammo for the fight this weekend.
Thanks and I'll report back.

Last Edit: Oct 10, 2008 10:21:10 GMT -5 by stevea

wotavidone
Senior Member

Posts: 582

Missing on top under load. Oct 9, 2008 18:51:43 GMT -5

Quote

Post by wotavidone on Oct 9, 2008 18:51:43 GMT -5

Could be heaps of things.
Check ignition timing.
Try enriching the mixture - if its too lean up top, it would require a bigger main jet. Have a look at your plug, plenty of guides around to tell you if the deposits on the plug indicate anything out of the ordinary.
Also, run the highest octane fuel you can get, I'm wondering whether it might be preignition/pinging. I run Australian octane number 98, with a bit of octane booster in it.
I think US octane numbers are different, but in anycase, go as high as you can get and see if it makes a difference.
Maybe your plug is too hot. With NGK brand plugs your standard would be a 7, maybe an 8 (colder) might help, if pinging is a problem.
Is your plug lead OK, weak spark can cause missing under load.
Haven't got a partially blocked fuel filter limiting fuel flow at high revs?
Like I said, could be heaps of things, but try a few of these that are low cost and could help. Always do the easy things first before you pull the engine down for a rebuild.
Mick

frankyb
Full Member

Posts: 101

Missing on top under load. Oct 9, 2008 19:54:58 GMT -5

Quote

Post by frankyb on Oct 9, 2008 19:54:58 GMT -5

Same thing happened to one of my dirt track bikes, lots of head scratching and carb dismantling to no avail. Started on the electrics next [as Mick said, simple things first] by changing plugs, no more problems and motor running fine again. What i didn't understand was normal start, idle and run up to about3/4 throttle before problem became apparent. Hope the fix is as easy for you, as my symptoms were pretty much as you describe them. Cheers Franky b.

stevea
New Member

Posts: 8

Missing on top under load. Oct 13, 2008 10:16:11 GMT -5

Quote

Post by stevea on Oct 13, 2008 10:16:11 GMT -5

I changed the plug this weekend and that was the problem all along! I was happy and pissed at the same time. I'd pulled the plug two months ago as a diagnostic tool and it was nice and toasty brown just like the picture so I put it back in. At least it was a cheap easy fix. My little girl is running great again.
Thanks Guys,
Steve