Suzuki DRZ 400 SM - carb problems - can you help an overlander make it home?
 

After travelling with my faithful steed (DRZ 400 SM) from London, through Europe, central Asia, India and Nepal, I'm on my return journey and have run into some carb issues whilst in Iran.

The issues started with some dirty fuel purchased in Pakistan, this crippled the bike - losing power sporadically at medium rpms. After limping over the Iran border, I cleaned the filter in the petcock and the one on the float valve seat in the carb; finding a ton of crap.

She seemed to be running nicely for a few thousand kms until a related problem developed, at high rpms she consistently began to misfire meaning I could only do 93kmph max to avoid any issues.

I removed the carb again and inspected the plug. The plug was pretty black and I assumed that she was running rich. So, I reset the pilot mixture screw to what I thought was the factory setting - 3.5 turns. But, I think I made a mistake with this - I undid the screw and then turned it in 3.5 turns. Rode around town with this but the problem persisted - when I removed the plug later it was completely black.

So, I corrected this - turned the pilot screw in completely and then loosened it by 3.5 turns. This time, no carbon on the plug but she was backfiring like crazy at high rpms. Guessing that she was running lean, I loosened the pilot screw another turn - same problem, I kept repeating this test and am now at 6.5 turns but she keeps backfiring. Am I doing the correct adjustment here? Am I turning the pilot screw the right way? Is there something else I should be looking at? The airfilter is cleanish, she has compression and there are no visible leaks from the exhaust I also checked the float height by eye and it looks ok.

Any advice I can get here would be really appreciated - I can't explain any of this to a local mechanic here. I've taken the carb out of the bike so many times that I'm going crazy.

See post number 2 in this thread for the technique that I understand is used for all carburettors of similar construction:-
http://www.horizonsunlimited.com/hub...-setting-49379
I don't know the standard setting (start point) for your particular bike but 6.5 turns outward sounds like too much; having moved on to use FI some years ago I have only vague memories of doing this type of adjustment with carbs.

2.5 turns
 

All Mikuni and Kehin carburetors are TURNS OUT from (gentle) bottom.

Try it again and you'll likely to be fine.

I have one of those bikes and find it reliable as it is uninspiring. YMMV.

Thanks for the advice - will try fiddling with it again.

The technique for adjusting it on the XT sounds good, however the pilot screw is very tricky to turn whilst the carb is on the bike - my fat fingers and long screw driver don't help - I might need to bodge together a small tool to try that method out.

I just want to make sure - do these symptoms sound only like the pilot screw is in the wrong position or could something else be causing the problem? Perhaps an air leak in the ducts the carb is attached to, or something else?

Cheers,

David

Like the XT thread, it never seems to be easy to get at these adjustments - I used a small right angled screw driver IIRC which helps with the burning sensation in the finger tips from the hot engine. It's awkward but not impossible.
You need to have the carb in good adjustment before considering more aspects.

If your problem occurs at high RPM (and I'm assuming also high load) then adjustments to the pilot screw will have almost zero effect on the problem. In this situation it is the main jet which dictates how much fuel the engine gets.

I would also be hesitant to assume your current problem is related to your previous problem. Could it be something else altogether?

Totally agree, but 6.5 turns out for the pilot screw still sounds too much.
To understand more about your carb and its settings, assuming that this one is fitted to your bike:-
Mikuni carburetor operation and tuning
To have the carb "in good adjustment" is explained in there.

As per Henns' post:-
What has changed recently?

"until a related problem developed" - is this fact, or assumption?

I know nothing about the carb in your bike, but every carb I've had my hands on has the standard pilot air/fuel screw setting of between 1 and 2 turns from gently seated to bottom.
The way I set it is to get the bike running at a slow steady idle on the idle adjuster, then adjust the pilot screw until the point where you get maximum idle speed. Re-adjust the idle to a slow steady setting, and re-adjust the pilot to the point where the motor runs fastest. Do this one more time and you should be right.

What the pilot air jet does is introduce some air into the fuel in the needle jet (you should see a few small holes around the side of needle jet tube) which turns the fuel in that tube into an air/fuel foam. This foam is much easier to atomise as it comes up the venturi and hits the airstream in the carb throat.

I think you need to strip and clean your carb again, and see if you can find an in-line fuel filter to fit. A larger filter is less restrictive to the flow. Small ones can let too little fuel through when the level in the tank gets low.

Cheers

Nigel in NZ

6.5 turns is waaaay too much.

What mods have you done ? 3x3 mod ?? Sport exhaust ? Are you running the standard jets ???

2.5 turns out on the pilot should be a good starting point. If the bike used to ride fine with your current set up then it should now.

The pilot jet is really just for idle and small throttle opening.

Remember, rpm and throttle position are not directly related.

My drz gave me lots of running problems with a filter and pipe routing. Make sure your fuel pipe is clear and kink free. You want it with no kinks at all.

What tank do you have ?? There is a filter in there too.

Also, check your stator pickup isn't coming lose which is a problem.

Also, the stator itself is a known weakness on this bike and can cause running problems... do you have a multimeter to check it ??

Ted

www.touringted.com

Many thanks for all the replies - just got back to the bike after travelling for a week to Mashad by car.

Working on the bike now - I've managed to fiddle a way to adjust the pilot screw without removing the carb from the bike - will get the position set correctly shortly.

Also, found a hole in the fuel line - have trimmed this bit from the line. Likely this was affecting things.

If the issue is still there, I will strip and clean the carb and will update you guys on the progress.

The bike is standard except for a sporty exhaust.

Thanks again,

David

Hi David

According to the Clymer manual in front of me, for the SM

The Pilot Jet is 3.5 turns

Thanks Fern and thanks to everyone else for replies.

I have cleaned the carb, reset the pilot screw using Nigel's method, replaced the fuel line and the spark plug. After each of these the problem still was there - idles fine but backfires at high rpm.

So, I moved onto the stator - I disconnected the cable and retested - problem gone! Then, I reconnected to test again and the problem still seemed to be gone, strange. So, I think it is either a problem with the stator (maybe it is starting to die) or with the connector.

I have a multimeter - is there a simple way to test the stator without inspecting it by draining the oil and opening the case?

I have a kick-start on the bike - do you think I can rely on this for the journey home and just leave the stator disconnected?

Thanks again - I owe you all many beer

David

No... the stator powers the whole bike. Not the starter. No stator, no bike...

It looks like its starting to break down. Its a very common Dr problem. Grab one off eBay asap :cool:

www.touringted.com

Thanks Ted - i see what you mean - the bike would only be running from battery when it is disconnected.

I'm not 100% sure but I think the ignition system is powered by the stator, not the battery....... I can't remember

Once you've got that side apart, be sure to loctite the stator and the pickup bolts while you're in there.

www.touringted.com