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Misfire after a few miles cruising. Kinda scratching my head.

  • Thread starter Thread starter howardshane2
  • Start date Start date
H

howardshane2

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Alright so I will attempt to describe the problem I've been having. I have a 81 750 with 4 into 1, pods, dyna s, and a dynojet stage 3. I just got the bike all back together and running after a full rebuild put 100 miles on it with no issues and then the other day I was riding it and it just died. I got it to start again but it was definitely misfiring and I limped it home. Checked my plugs and one of them looked carbon fouled so I figured I must be running to rich. Got new plugs leaned it out and it fired right up. Went for another ride the next day and everything seemed fine and a few miles in exact same thing. On the side of the rode I pulled all plugs and checked spark and everything checked out so I started messing with the petcock and gas started dripping out of my no. 3 carb so I took it back to run and put the plugs back in and tried it one more time. It started but was misfiring and I held the throttle for a bit and it cleared up and I was able to get home no issues. So that incident made me think my floats were not right. Checked and adjusted float heights and put it back together and everything seems fine but I haven't ridden it for an extended period of time yet. Just curious if anyone else has experienced something like this? Oh and it's a new petcock and I put an inline filter in so I good visually see the fuel, and the vacuum line is new.
 
To clarify... Your new petcock has a vacuum line connected to #2 carb??
and you have those rubber plugs installed over the pilot jets?

Pull the carb rack , hold it in normal bike position and feed fuel to carb fuel tee (auxilliary tank, etc) to see if your floats are working right and not leaking excessively.
 
Check the points gaps, be sure the cam timing is spot on, and then be sure then ignition timing is on using a timing light. Incorrect timing gets you poor fuel burn and fouled plugs quickly. May have absolutely nothing..or very little..to do with your carbs.
 
The vacuum hose is on no. 2 and all the rubber plugs are in but they weren't new and kinda dry rotted but seemed ok enough. And when I got to prime for a few seconds the no. 3 carb will start to drip, are the floats supposed to stop all fuel flow when after they are filled in the prime position? If so that may be my problem. I don't think it's ignition related because I have the dyna s that I set with a timing light and I made sure the cams where in the right spot before I put the engine back together and the problem starts to happen after a few miles of riding with no other indication that something is wrong. Spark is good on all four plugs. Could I possibly be flooding out that number 3 carb while cruising around?
 
Sounds like you have a stuck float or needle.
Maybe a speck of something keeping it from closing.
Tom gave good advice with bench testing the carb bank.
I mean once the carbs are off just check that carb float/ needle carefully.

Could be weak spark on #3 also. First check for spark....then unscrew all 4 plug boots, nip a quarter to half inch off each wire and re screw them on. That will expose some fresh wire to the boot.
Just a couple thoughts.
 
Yeah I am going to be checking the floats out really well and doing the bench test. Just to clarify though, when I do the bench test and fill the carbs with fuel if the floats are working properly I should get no leaks correct? And if I had a weak spark on one of the cylinders wouldn't the problem be immediate? If I go out to the garage and start the bike everything will be fine, it will sound healthy, and if I go around the block it pulls great but the minute I get a few miles from my house is when it has started acting funny. It will start to miss and die and then if I start it back up and keep on the throttle for a bit it will clear up. Makes me think spark is ok but I could definitely be wrong and when I checked the plugs they all had a nice blue spark.
 
First, with the bike turned off, check the voltage at the battery. I know you think you have a great spark but I doubt you can tell how great a spark is by looking at it arcing. If the voltage is under 12.5VDC with the bike off, the battery is not fully charged. A weak battery could be a bad battery. If this is the case, I can talk you through testing the battery's health. If the battery may not be charging, you can check by running the bike at about 4K rpm and see what voltage you have at the battery. Should be a bit over 14 VDC. If it isn't, the charging system may be the culprit. Time to talk about that.

OK, if battery and charging system are OK, then time to check voltage at the coils. With the bike NOT running, put your meter probes on the orange/white wire where it plugs into the connector directly before a coil. If you see the same voltage as you saw at the battery when you check or it is no more than a volt less, this isn't an issue. If you see 10.5 VDC or less, the coils are not getting full power and I can explain where to go from there. If coil voltages are inadequate, you will get plug fouling and some backfiring.

If the coils check out, timie to check out your valves. If valve clearances are too small, then the valves may not be sealing and you will some plug fouling and backfiring.

If ALL these things are OK, then it is time to talk about your carbureteors... not until then. By the way, the float valve and seat assembly are intended to meter gas while the bike is running and not be a fuel shut off exactly. They do shut off but if gas overwhelms them while the bike is not running, they LEAK. This is normal. Gas will overwhelm a needle/seat if more gas is going in and that points to petcock issues or the bike is on sidestand and gas in the bowls will travel downhill to the #1 carb and it will leak. If the #3 carb is leaking, take off the carbs and set the FUEL LEVEL using the clear tube method as the float may have been bent improperly in the past and the float level set is allowing too much gas in the that bowl.
 
I will check all of those things when I get home today. The stock wiring went in the trash when I was rebuilding the bike and I re wired it myself to something more simple and easier to work with. I'm running an anti gravity 8 cell battery, new stator and voltage regulator from electrosport, and the dyna s ignition with the dyna coils and wires. I just followed the dyna wiring diagram when I was hooking everything up so it should be easy to troubleshoot. I'm not the best at electrical stuff so it's possible I got something wrong.
 
Starting to think it could be the dyna ignition. I read some instances where people with that ignition have had similar problems where everything is fine until it warms up and then a few miles down the road it will cut out some cylinders. So I'm gonna follow the dyna check outs when I get home just to rule that out.
 
Could this problem also be related to the inline fuel filter I put in? Just trying to brainstorm here.
 
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