Back Firing

It is worth checking into. I know it is wired exactly as it was before I got the carbs done. It started most of this when I got the carbs back. The thing is that I orginally took them in because they were leaking gas out of the bowl. We found out there were a lot more problems with them than we thought. That is how I ended up with these smaller mekuni's. I will check the wiring, but I am still curious about any other ideas someone might have. Do we synch the 2 and 3 carbs together, and could they just be running extra rich? From what I have found, black plugs usually point to a rich issue. I also thought that those plugs fired on an exhaust stroke and the compression, so would it matter if they are switched? I am just about ready to give up on the old KZ . I will check the wiring and make sure everything is right.

Could it be a problem with the coil for the 2&3 cylinders?

I don't think you mentioned the spark color. Try pulling a plug and laying it against the engine (put the plug wire back onto it, of course) while turning the bike over. You should see a spark jumping the gap. Blue spark is good. Check all four cylinders this way, one at a time, putting each plug back before moving on.

Also, I can't stress how useful my Clymer manual has been when trying to test different components (including my coils).

Timing is worth checking, as good spark at the wrong time is as useless as bad spark at the right time.

Sometimes observations can help you to decide what to look at first. If the two cylinders which blacken your plugs happen to share a coil, I'd look at electrical issues first. Get a multimeter and make sure that you have good connectivity between the distributor (under the points cover) and your coils. Check the coils themselves. Make sure that there's connectivity between the ends of the plug wires through the coils (it'll be tens of thousands of ohms) -- Clymer will help you with testing the coil. There are many things that can go wrong. Your points may be bad/old/dirty/whatever, there may be several connectors between the distributor and the coil which can come apart (one came out on my bike and I sometimes wasn't getting spark), the plug caps (the very end of the plug wires -- they screw on) can corrode internally, and that's just the start.

If the two cylinders which are blackening plugs don't share a coil, I'd start with fuel delivery. Make sure your carbs are synched correctly -- you can build a homemade manometer with a $3 stretch of 3/16" ID clear tubing from Home Depot and some ATF. Use it to see if your carbs are in sync (between a "good" and a "bad" one).

Just a start, but don't get discouraged. You'll be glad when the bike is running as well as it should!

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I can recheck the timing tomorrow to make sure something didn't slip when I was resetting the cams. I was very carefull to make sure I left the T right on the mark when I got the cams adjusted correctly. I did notice that when the T was on the mark, the oval part of the metal was past the sensor on the pick up coil by about 3 degrees or so. Should the oval be right on it when the crank case and the mark are at T?

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Yes, that's correct. It's pointed directly @ the sensor when the engine fires ("F" mark)

So, when you checked your valve clearances, did you have to change any shims? and when you torqued down your cam-cap bolts did you find any stripped or loose? how did the cam bearings look ?
Reason I ask all this is because I just went thru what you are doing. I found 3 loose cam cap bolts, and stripped one out myself ! Discovered if the c-cap bolts (all of them) are not drawn down slowly/evenly the cam lobes that are pointed down will be pushing on the valves and have an unequal pressure on the cam caps as you tighten them. They only take 12 ft pounds of torque.
Anyway, my problem was zero valve clerance on my intake valves, even tho I had pretty good compression, also my cam chain had run out of tensioner pressure (streched beyond the point where the auto tensioner could no longer push the cam chain tight).

If you have a lot of miles on the engine and an automatic c-chain tensioner you may need a "APE" aftermarket adjustable cam chain tensioner, that allows you to get the c-chain tight.
With the valve cover off and the spark plugs pulled, crank the engine w/the starter, watch the c-chain action, should be tight with no slack, smooth and no bounce.

Backfiring out the muffler ? big bangs ? can you get your palm (or a piece of thin cardboard) over a carb intake while its cranking? you should feel suction. If it pushes gas or air out? The intake valves (clearance) is too tight and/or the valves/seats are shot.

Post edited by: ~DW~, at: 2005/10/16 00:10

Thanks for the advice so far. I would guess that 1 and 4 are ok since the plugs looked ok. I will try all these ideas next. At least this should give me some kind of idea on what is going on with this. I think the carbs are ok, because I got them done by old WG. I know that the oval on my pick up coil is always a little bit past the middle of the coil when the F or T mark is lined up. That is what the book said needed to be done. I don't think 2 and 3 share a common coil, but it is fustrating because I didnt have this problem with my old carbs. Then again, I found out that the number 3 carb wasn't running at all. I also found out they had shoved a nail in number one to keep it from draining gas out of the bowl. Any more hints and trick someone can give me?

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"I don't think 2 and 3 share a common coil"

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Your bike is a 4 cylinder, right?

Mine is also and the left coil supply's spark to cylinder's number 1 and 4.
The right coil supplys spark to cylinder's number 2 and 3.

Post edited by: ~DW~, at: 2005/10/16 00:35

#1 and #4 are definitely common on my SR. Definitely check spark, just because it's easy to verify. I like easy things.

Good luck!

#1 and #4 are definitely common on my SR. Definitely check spark, just because it's easy to verify. I like easy things.

Good luck!

When I drive back home today, I will see about testing the coils. How hard are they to get these days? I do remember testing them and I think the spark was blue, but I will check the coil either way for those intermitant failures.

ok, so it didnt do it before but it sounds like it was only running on three to start with, since the carbs were messed up, will it run long enough to put a timing light on all four wires, or better yet get a digital pyrometer, you can look at the pipes with it and see which ones are firing and not. I still think it is in the ign timing or coils, could even be in the spark pulg wires

Check to see if you are getting at least 12.6 volts to the coils with the engine running. Make sure the advance is working. If you need to sync the carbs use a drill bit. also check resistance through the spark plug wires and spark plug caps.

Neotrion wrote:

When I drive back home today, I will see about testing the coils. How hard are they to get these days? I do remember testing them and I think the spark was blue, but I will check the coil either way for those intermitant failures.

As hard as checking out Z1 or Bike Bandit . Z1 has a great reputation, and Bike Bandit is REALLY easy to search once you've told the website which bike you have. The hardest part is shelling out the dough!

I'm not sure about your coil, but on my 650 I believe that the resistance was 23k ohms between the ends of the spark plug wires, and 4 ohms between the wires feeding it (yellow and black or green depending on which coil). Of course, you should let the Clymer manual be your guide, but if your numbers are anywhere close to those I wouldn't suspect the coil.

G'luck!