weak spark, 2 and 3 I think, especially when hot?

My 85 xj700 was running pretty well after syncing and replacing the voltage regulator. Went to the UK for two weeks and when I got back, she fired right up. About a mile down the road, when I stopped at a red light, the bike bogged way down and seemed to be running only on two cylinders. After further inspection I determined that the two problem cylinders are 2 and 3. I think the spark is always fairly weak because the bike bogs and surges upon takeoff. It seems like they lose spark altogether only intermittently. I tested the pickup coils and they are to spec. The ignition coil for 1 and 4 reads within 20% of spec (12kOhms). The coil for 2 and 3 is reading about 3.1 Ohms which is a little more than 10% higher than the 2.7Ohm specification. Could this be my problem? My connections seem ok. I would think if it were the TCI, my problem would be occurring all the time as opposed to intermittent. I have replaced the spark plug wires/boots with dyna cables. The bike fires up and idles well but once it gets warm it starts to miss. I have yet to do a compression check but last I checked, it was good. I also need a valve job but as of right now, I think it is spark. I need to get this solved ASAP because I commute on this bike. It is almost impossible to get off the line without slipping the clutch and if I'm on a hill...forget about it, pretty embarrassing to say the least.

Thanks in advance.

 

I know, stupid question, but how old are the spark plugs and are they gapped right? I learned this one from experience. I found mine runs best on the tight side of spec.
The other question, how long ago was the compression check? I'm thinking compression because you only have the issue when the engine is warm. Are you sure it's an ignition and not compression problem?
Maybe even the issue is a bad tank of gas?

 

I know you are saying, spark, spark, spark, but I wouldn't rule out fuel.

 

I'm not sure of anything with this bike. If it is a compression problem (and I won't know until tomorrow) could that be as simple as a valve adjustment? How is compression affected by the engine temperature? I have had an issue with bubbles stopping fuel within the inline filter but the bike is idling well until warm and this problem is largely recent. My plugs show no signs of getting too much fuel either.

hmmm

 

Compression increases with temperature normally because the pistons expand and seal tighter against the walls.

 

Can you temporarily switch the coils? If the problem follows the coils, then...

 

I had a similar problem with my bike. It seemed as though two of the cylinders had a weak spark - only when it was warm. Turned out to be the petcock. Evidently, when the diaphragm that draws the fuel into the carbs got warm it expanded the rubber enough to open up the tiny cracks in it so that it would not draw the needle open to allow fuel to flow and it was starving for gas and only acted like a spark issue. The thing that confused me is that even on prime it didn't run good. But there were actually two problems going on - the prime side was clogged up from non-use. I changed the petcock and it runs beautifully again (I also cleaned the carbs again while I had the tank off but they were still nice and clean).

 

Interesting Mirco. That's making me wonder if my filter bubble issue might be the problem after all. The petcock is brand new from Chacal and is actually an on/off valve only. I think I'll try bypassing the filter for a ride and see what happens. I hate to ride without it but...

Thanks all.

 

No, the compression decreases when the engine is hot, because the oil thins.

 

Going to revive this one; I've got the same issue. Bike bogs from a stand still - picks up around 3-4k RPMs. Issue is only affecting #2 and #3 (both off the same coil). If it were gas flow (petcock) I would assume that the issue would start at #4 since its the last in line for gas.

Checked the spark today with a spark tester - cylinders #1 and #4 had consistent spark, while #2 and #3 seemed to show inconsistent spark at low RPMs, and often the light would completely dim when giving gas, until about 3-4k, where both would then suddenly show strong consistent spark.

I have a second set of coils, which I'm planning to swap out the 'suspect' one with. Should I be looking somewhere else for the problem? Everything else seems just fine except the lack of spark on 2 and 3 at low RPMs.

 

How about just swap the leads and see if the problem follows the coils?

 

wow, i just finished my bike and took it our for the first ride and im experiencing the same problem once the biked warmed up on red lights it just wanted to bogg out, unless i played with the throttle, and then on take offs it had next to no power untill i revved it to 3 4 grand then once it picked up it felt like it fired on all 4 sounded great it just takes off... almost left my gf behind lol !!

i will check my spark and swap some coils as i have extras...

mfchapman
yes very embarasing i even didnt ride it after my first ride it was so shitty...

 

im sure once this bugg is worked out its gonna be a blast ....

 

The "Bogging-Out" isn't the fault of the Coils, ...
That's a problem with Mixture.
Probably too Rich

Tweak the Pilot Mixture Screws on your Darkest Plugs IN a >> TINY << bit and keep at it until the Bogging-Out is cured.

 

the mixture screw dont seem to have an effect for me. can choke it out or wide open, spark is still intermittent