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HELP! Break down diagnosis needed....

  • Thread starter Thread starter BTreadz
  • Start date Start date
I put the compression tester to it tonight. Not good. Cylinder 1: 155psi, 2: 130, 3: 90, 4:95.
Plugs 2 & 3 were white. Ouch! I checked them all 1k miles ago about 1k after top end rebuild, and they all had good color. What happened? My plan is to check valves to see if they were too tight, and see if I can adjust to get compression back. Good plan?
 
JTGS850GL said:
You did do the compression check with the throttle wide open, right?
Click to expand...


Thanks JTG850GL! I forgot to hold the throttle WFO. Everything is within allowable limits. Cylinder 1: 162, 2: 135, 3: 142, 4:150


That's a relief and what I expected since I never heard any engine noise...What's my next step in the diagnosis? I haven't rebuilt my petcock or lined the tank (although it looked pretty good)...
 
Quick update. I have spark to all four plugs. Must be a petcock/tank/carb thing....
 
If you think it's petcock related, then don't try to re build it. The success rate on rebuilds is very low. Just pick up a new petcock.
 
Last edited:
Macgyver said:
First thing I would do is get rid of the inline filter. Our bikes don't need them, and they tend to restrict fuel flow since we run on gravity and not a fuel pump.

Get rid of the inline filter, I learned this the hard way on my 650. I ditched the inline, and it cleared up my fuel delivery issues.
Click to expand...

Have you followed up with this yet? Best to eliminate all bad things that could be causing problems.
 
I will get some new plugs and try eliminating the inline filter. Wouldn't that be an easy fix.
 
Clean your gas cap vent, a plugged one will cause your problems. If your tank can't vent a vacuum is formed and no not much gas can flow out the gas valve, it is only a few screws and only takes a couple minuets. I have had this happen on a couple bikes now.

V
 
It's fine advice to get rid of the inline filter but don't worry about it right now as the bike has been running fine for a year with the inline filter there...so it's not the issue (unless it's plugged with fine rust sediment in which case you should have a look at the inside of your tank). It's pretty simple to check if your petcock is working properly. Take the tank off the bike, make sure the petcock is in the "run" position, of ANY gas is dripping out of it the petcock is toast. If no gas is dripping out attach a hose to the vacuum port, again make sure the lever is in the "run" position and suck on the the hose a bit (mimicking the vacuum your bike makes), gas should pour out of the petcock in a good stream, if it doesn't your petcock is toast. No need to test the "prime" setting but you can if you want to, gas should simply pour out when you turn it to "prime". This test will at least eliminate the petcock as being your problem.
 
Least with compression numbesr that good youre relatively assured the piston and valves are unharmed. If they were holes the compression would show itself as way down..usually.
 
Petcock is fine. Fuel tank vent is not clogged. I pulled the tank apart because I can see what looks like rust that someone poured a coating over that didn't disperse well. I'll have the tank lined and painted anyway. The petcock filter screen was clean. I pulled the carbs and drained the bowls, and all four had gas. Could it be a clogged Pilot Jet? It would die as I rolled on throttle, but would not idle.
 
So 2 and 3 are on the same coil...HMM. Wonder if the coils bad maybe?? Got a spare coil you can swap in to see??
 
Check for spark on 2&3 again. Make sure the spark is a good bright blue. Make sure that you're getting proper voltage to the coil hot wire (O/W). Should be close to battery voltage with the engine running. Measure it and let us know what you read.
 
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