SuperMoto Junkie banner

1 - 20 of 57 Posts

·
Registered
Joined
·
29 Posts
Discussion Starter #1
I am a motorcycle mechanic and I am currently stumped by an issue I am having with a 2002 Suzuki DRZ400S (~10,000miles on it) that I am working on. The customer brought it in with the complaint that he was riding it, shut it off and it wouldn't start back up. I brought the bike in and after checking spark, plug condition, carb/air filter condition and compression I decided that it needed a piston and rings (compression was 90psi. and it should be 137psi). After replacing the piston and rings, re-timing the cams, replacing the plugs, and completely cleaning the carb, the bike will not fire. It sounds like it wants to, but will not fire up. I rechecked the cam timing, rechecked spark, rechecked the carb (removed, inspected and cleaned all jets and passages) and I still can't get it to run. If you have any insight or ideas as to what I have overlooked I'd love to hear what you have to say. Thanks

-Matt-
 

·
Mullet Lord
Joined
·
3,302 Posts
I've seen bad coils that would produce spark with the plug out, but not fire under compression. This is based on my experience as a Stihl certified 2-cycle equipment, so it's not entirely accurate, but I think it's worth a shot. Do you have access to another coil and/or CDI that you could swap out and try? Is it backfiring, popping, or doing anything to indicate the presence of spark? I'm not familiar with the carb on the S model, but does it have any vacuum plugs/hoses that could be leaking?
 

·
Registered
Joined
·
29 Posts
Discussion Starter #3
The S model has a CV carb, there is a vacuum line but I made sure that it is not leaking. The plug does spark when grounded (I don't know if it is sufficient spark, but it's visible). The bike acts like it wants to fire, occasionally backfires both through the exhaust and through the intake. This made me look back to the cam timing (since I was getting reversion). But it right at spec, i checked for tight valves, but they are all within spec. I have an E model so I tried the CDI box out of my bike (even though they have a different part number and probably is set with a different timing advance, the E model has higher compression than the S model) and still no luck. I might try swapping the coil out of mine next and see if that works. If you have any other ideas, shoot em over. Thanks for taking the time to think about it with me.
 

·
Registered
Joined
·
29 Posts
Discussion Starter #5
I did double check to make sure that it had the correct plug in it, and then I did replace it with a brand new one (double checked gap as well). As for it being 180 out....i know for a fact that it was at TDC on compression due to the timing marks on the rotor and cam sprockets. I spoke with another tech who races an LTZ400(same motor as DRZ) and he was baffled too. It just doesn't make sense. I've been working on bikes for almost 10 years....and I've never come across anything this difficult to diagnose. I'm starting to think that it has to be electrical. I've checked and rechecked everything mechanical two or three times. I guess monday, I'm gonna try a new coil and start inspecting the wiring harness, ignition pick up, etc.
 

·
Registered
Joined
·
781 Posts
do some electrical tests
thats my next best guess.
maybe your not getting enough volts to the coil or something
charging system?
 

·
Registered
Joined
·
781 Posts
just throwin stuff out there
 

·
Registered
Joined
·
716 Posts
Did the piston/rings solve the low compression issue? you retested afterwards correct? You check valve clearance but did you inspect cam lobe condition/valve lift? Ive seen cam lobes(automotive) ground done so evenly that you couldnt tell it was destroyed by looking at it, it was polished smooth but no longer offered proper lift.
 

·
Registered
Joined
·
29 Posts
Discussion Starter #10
Yes, the new piston and rings increased the compression above the servicable limit. As for the cam, I did not measure the lift on the cams, but being that the bike only has 10,000 miles, and the condition of the buckets over the shims I do not THINK that that is the issue. It is something to look back into though. Thanks for thinking with me.
 

·
Registered
Joined
·
29 Posts
Discussion Starter #12
No, I'm positive it's not an issue with the quality of gas. I was attempting to run it on an auxiliary hang tank with fresh gas in it. Thanks for the thought though.
 

·
Registered
Joined
·
8,628 Posts
Is it a nice Blue spark?

I agree it sounds electrical. Normally electrics either work or they dont but I have seen a few that went wonky before dying.

Have you checked/charged the battery? I have seen low batteries do some really strange things.


I just had a friends daughters CRF that was doing about the same thing and the gas just didnt smell 100%. $500 in parts later I drained & refilled and it ran perfect. When I go to drop it off he tells me his weed eater would not run either......stick my finger in the tank and......Diesel. Some one had put diesel in the gas can.


.
 

·
Registered
Joined
·
723 Posts
Do you have TIMED spark?
and when riding, I'm thinking insulation break down on maybe something going to the coil
 

·
Registered
Joined
·
29 Posts
Discussion Starter #18
CHecked the flywheel key and it is in tact and where it should be. Still nothing! Put a manual cam chain tensioner to assure that the timing isn't going out. Put another coil off a drz on...still the same thing. Any more ideas? I'm f**** stumped!
 
1 - 20 of 57 Posts
Top