Midrange Missfire

[conquest87tsi]

So now I am some '80 special carbs with stock jets. Got it to idle very well, can even start it with the kicker! The problem is that I have a miss fire over 2,000 rpm. It responds to throttle very well, but if I hold it at say 4,000 I can hear it sputtering. Not terrible, but it;s there. It sounded to me just like it does when your about to run out of fuel. So last night I lowered the float heights to 21.75mm to see if that would help. It was almost midnight when I started it up to check it and I really didn't notice a change, better or worse.

Does anyone have any tips? It has the same miss all the way up to redline, but doesn't have any hesitation. I am noticing the miksfire with a load even. Like I say it isn't horrible, but if it's lean I don't even want to go down the highway at 4000 with it like this.

[natemoen]

Your floats are pretty rich, check you plugs and I bet they are pretty dark.

I am.guessing.the miss is the puckup coil wires. Have you checked them.at all yet?

[DGXSER]

Could be a couple things. I doubt the float height will cure your issue. Did you personally clean the new carbs? If not, go through them carefully, you may find some dirty needle jets or a little dirt left in the pilot circuit.

I do agree with Nate on the pick-up coils also. Another member here has an 80 Special I have been helping him with for a few weeks now that has a miss in the low end it seems, but much worse under power. We checked the headers and all are equally hot even after a mile or two of missing. Checked the plugs, colortuned one cylinder that looked lean. Replaced that plug as it was coated in ash. None of it helped. Then I pullled the timing cover and started tuggin on wires to the pick-up coils. Pulling on the one wire the bike dies. Obviously a bad pick-up coil wire, so the switch will be made this weekend.

[conquest87tsi]

Misfire

I completely disassembled and cleaned these carbs when I bought them. I haven't checked the pick-up coil wires. I will be doing that tonight. I am pretty confident it is not a spark issue though. The bike runs fairly well, it's just that I can hear it misfire when I hold it at a constant rpm. Will see what it sounds like in gear.

So do you guys think I should set the floats back to 23mm?

[motoman]

I completely disassembled and cleaned these carbs when I bought them. I haven't checked the pick-up coil wires. I will be doing that tonight. I am pretty confident it is not a spark issue though. The bike runs fairly well, it's just that I can hear it misfire when I hold it at a constant rpm. Will see what it sounds like in gear.

So do you guys think I should set the floats back to 23mm?

Actually , like Nate, fairly confident you DO have a spark issue. Old school rule of thumb: Pri./sec. ignition before fuel when diagnosing.......and especially holds true in these or any scoots cases. Conquest, I'd reset those fuel levels at the 23-24mm range, getting them ALL identical of course.........would not particularly want to see another member showing pics of a ventilated piston..........seen two already this year. Rollng down the road at a steady 4-5K rpm has these motors and fueling leaned out alot already........just sayin'.....

[James England]

I am pretty confident it is not a spark issue though.

Why is that? What is your confidence about this based on?

[conquest87tsi]

Misfire

Motoman,
I raised the float heights to 21mm from 23mm. It is my understanding that this will richen the mixture, not lean it out. By raising the height, the floats will require a higher(richer) level to close the needles. I am pretty sure of this.

As far as the ignition, the only thing that has changed on my bike is things with the carb to make it run this way. Checked pick-up coil wires and they are fine. I rode for an hour with new plugs after the adjustment to 21.75mm float height. Outside plugs were almost completely whit. Inside were a little grey. Which is strange because someone installed 110 jets all the way across. So I removed and adjusted floats to 21mm even.

If I am incorrect on the float heights please let me know. Could be an issue on my 3hr ride tonight.

[GLoweVA]

Motoman,
I raised the float heights to 21mm from 23mm. It is my understanding that this will richen the mixture, not lean it out. By raising the height, the floats will require a higher(richer) level to close the needles. I am pretty sure of this.

As far as the ignition, the only thing that has changed on my bike is things with the carb to make it run this way. Checked pick-up coil wires and they are fine. I rode for an hour with new plugs after the adjustment to 21.75mm float height. Outside plugs were almost completely whit. Inside were a little grey. Which is strange because someone installed 110 jets all the way across. So I removed and adjusted floats to 21mm even.

If I am incorrect on the float heights please let me know. Could be an issue on my 3hr ride tonight.

The float height adjustment is done with the carbs upside down, so the higher the setting (23 vice 21mm) means that the actual fuel level is lower.
In other words, if you want more fuel in the bowl, then the float needs to be able to rise further towards the top of the carb which means the 'float level' needs to be smaller as measured to the bowl gasket surface.

I do agree with those above that the pick up coil wires may be an issue if you've got a stumble. As the vac advance assembly moves during the revving, then the wire(s) may 'open' up and cause no spark.

[natemoen]

The lower the float height measurement means that the actual fuel height will be higher, so his floats being at 21.75 means his fuel level is 1.25mm higher than stock making for a richer tux.

[conquest87tsi]

Misfire

So yesterday I decided to embark on a 3hr ride back home. The bike was running awesome. Started great, idled great, ran great. About half way in it started to pour rain out. I stopped for gas and waited for it to pass as I didn't have a full face helmet or goggles. Well I got back on the road and rode for about 35miles without a problem. Then I passed a slow moving van, as soon as I got around the van the bike started running like crap. When I tried to accelerate it would fall on it's face. I switched the petcocks to prime and reserve..nothing. With the choke all the way out it would run pretty good, and thats how I made it to my folks. The problem is that I have to make the 3hr ride back home on Sunday and I can't figure it out.

If the rain damaged a plug wire, richening the mixture with the choke wouldn't help would it? My plugs are greyish with a touch of tan.

It ran great before I passed the van, and the road has those stupid damn rumble strips in the center. Not sure if the vibration did something. When accelerating to pass I didn't give it anymore than I had done previously in the day.

I don't know what to do.

[GLoweVA]

I've had issues with the rain in the past too.
It's obviously grounding/shorting out.
In my case it cleared back up after it dried out.
It could be anything from pick up coil wires, to a cracked coil or even I've heard about the neutral switch wire causing similar symptons.
Hopefuly after yours dries out, it'll work okay again.

The problem is that during that time that the bike is acting up, you're probably only running on 2 or 3 cylinders and not all 4.
This causes the other (working ones) to work much harder trying to keep up those highway speeds.
I found this out the hard way last year.
I limped mine home on 2 and ended up melting a hole in #4 piston because it ran so lean and hot for too long.

[conquest87tsi]

Misfire

I really thought it was an ignition problem caused by the rain. But then I kept thinking back to when it started acting up. I had ridden for almost an hour in dry whether when it started running like crap. Plus I just past someone and went over some rumble strips. Then I got to thinking what I did differently when I last had the carbs apart, and it dawned on me. I noticed that one of the float bowl gaskets was fuel saturated. So my dumb ass used rtv to seal it. Well guess what was clogging 1/3 of that main jet.

[petejw]

I really thought it was an ignition problem caused by the rain. But then I kept thinking back to when it started acting up. I had ridden for almost an hour in dry whether when it started running like crap. Plus I just past someone and went over some rumble strips. Then I got to thinking what I did differently when I last had the carbs apart, and it dawned on me. I noticed that one of the float bowl gaskets was fuel saturated. So my dumb ass used rtv to seal it. Well guess what was clogging 1/3 of that main jet.

yep, silicone and petrol dont mix,
but hopefully uve sorted it all out.