Carb Shooting Flames! Wicked!

HI all! Fairly new to this whole classic bike restoration deal(so go easy on me). Bought an old '74 KZ400D last november for dirt cheap. Well finally got it running today with the :huh: help of my friend. It ran a little rough at first with the choke fully on and a little throttle. After about 15 or so secs I tried to open the choke up to let it breathe. Also the air filter is not connected to the back nor is anything. Anyway, I would give it few good smooth throttle turns then randomly make a loud pop and shoot a small flame or puff of smoke out the back of the carbs. Then die. My friend says it could be the timing or the wrong fuel/air mix because of the open air back. I already cleaned all the gunk out of the carbs and soaked the jets overnight. Someone else said it could also be the points. Not sure where to start, but any input would help this newbie. but amazed that it actually runs! Thanks for any replies in advance.

As your friend said, it could be the timing, but you should also check your valve clearance to make sure the valves are closing all the way.

Do a compression test. Check the valve clearance. Then check ignition timing. (especially the advance using a strobe.) Then start twiddling with the carbs.

Do a compression test. Check the valve clearance. Then check ignition timing. (especially the advance using a strobe.) Then start twiddling with the carbs.

yep......when you have fire out of the carbs, that means the spark plug is trying to spark while the intake valve is partially open....usually that is a sign of either the ignition timing is too advanced...or the valve timing is off and the valve isnt closing at the right time....

also if it is a twin, swap the plug wires for shits and giggles....

Id hope it is an ignition problem if i were you...much easier to fix...

Good luck man.

I recently went through almost the same thing on the '79 KX750 I am building and your friend, loudhvx, and 550A2 are all correct. On mine it was the cam timing, and if the 400D i like my 750B it is not that hard to do, just make sure to use the manual. Hope it is just the ignition time too far advanced though.

Well the cam timing would make perfect sense, since i did take that apart as well. Suppose I'll check that first this weekend before messing with anything else.
Another beginner question:
Is there an easier way to move the timing slightly with the points plate?
Again, thank you all for your input.

KZJAT wrote:

Well the cam timing would make perfect sense, since i did take that apart as well. Suppose I'll check that first this weekend before messing with anything else.

Absolutely.


KZJAT wrote:

Another beginner question:
Is there an easier way to move the timing slightly with the points plate?
Again, thank you all for your input.

You can't move the valve timing with the timing plate.

The ignition timing can be altered by rotating the timing plate or by altering the gap at the points, however, altering the point gap also alters dwell and will affect running. Rotating the plate is the best way.

If the points have a pit, the other side usually has a bump. In order for the timing to be stable, the bump has to be completely filed flat. Even then, it won't be as good as new points, but should work ok.

Are you sure you have the plug wires correct?...Russ

The plug wires can go to iether plug. They both fire at the same time like each pair in the inline-fours.

440's and 400's were also wicked lean from the factory. If the airbox isnt' connected, it's probably not going to run much above idle without carrying on and dying.

On 440's there was a plastic extension on the airbox, under the seat. Without that, they were tempermental.

So it's eithe full air box or you'll have to rejet the carbs. Best bet is full air box.

Larry C

Yup Larry's right. I worked on one 440 and it would run to 4 04 5 grand and got up there slow without the air box sealed to the carbs. Way too lean

New successes and failures today:

I started with the cam timing which involved ripping the
top off and checking the marks. It was spot on. Guess that's not the issue. Then Went to home depot's plumbing section to fashion some airbox boots, they fit quite well actually with a little wiggling.

Here's how it ran:

Started up rather fine with choke fully on and a 1/4 throttle. Let it rev for a little bit then gave the carbs more air, revved it a little more then backed off to see how the idle was. Couldn't hold an idle so stayed on the throttle. Started to notice it couldn't keep chugging without more and more throttle until I had it fully throttled. Then struggled, then died. Waited a little bit then tried to start it multiple times with no luck at all. Maybe the airbox is starving the bike now. Will keep trying until i get bored or give up for today.

Did you put in an inline filter. There have beem a lot of problems with those. They sometimes just stop flowing, even when they are new.

Verify that you indeed have good fuel flow to both carbs. Sounds like they may be running out of gas. This can be the filter mentioned above, or the petcock is clogged, or the tank vent (in the gas cap) is clogged. And of course make sure you have about 1/2 gallon or so gas in the tank. You may have to be on the reserve if it's less than about 3/4 of a gallon.