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Back firing and after firing at the same time.

#1

StarTech

StarTech

Here is one for you guys to figure out.

Kawasaki FX751-AS04

After operating for 30+ minutes, the engine will after fire on shut downs and will back fire thru the air cleaner on start ups. Luckily I already repair one that was backfiring out the air cleaner a month ago so I had inking of where to look for the problem. Problem was confirmed so the parts are on order; although, the distributor said they are on back order for nearly a month.

It is bad as I had already had to replace the starter due a hot restart problem. The starter flat refuse to operate when it was hot. Not good when the client uses the mower commercially. And it also took me nearly 5 weeks to get to mower due to severe backlog due health issues here.


#2

I

ILENGINE

First thought was faulty ignition modules. Spark advance not working correctly causing it to fire too soon or too late. Or valves out of adjustment


#3

StarTech

StarTech

My initial thought was that because they were aftermarket coils but I had already swapped them out using OEM coils. Valves rockers were in spec when checked but it is a valve related problem.


#4

T

txmowman

Check the valve lift. Might be a worn lobe on the camshaft.


#5

StarTech

StarTech

Cam lobe fine.


#6

Tiger Small Engine

Tiger Small Engine

StarTech said:
Cam lobe fine.
Click to expand...
How many hours on Kawasaki FX engine? Have you pulled engine shroud to check for debris buildup on cooling fins, etc.?


#7

StarTech

StarTech

3000+ hrs. And the engine shrould has been removed twice already and I have remove yet again to effect the final repair but those parts are on B/O currently. No debris found and the both after market coils have been replaced with OEM coils. But after what I found I know those after market coils are fine.


#8

G

gearz

Probably not the issue but a flywheel out of time? Maybe the key way is sheared or worn. Tight valves will also cause the issue but you said they were good to go. I had Iridium spark plugs installed at one time and they caused a cold back fire like the spark was to strong so I installed the old ones and it never happened again.


#9

H

hlw49

It may not be an engine issue at all. Had one with a similiar problem and it was due to wet grass packed around the engine control module wiring harness. He had taken the grass chute off the deck and it was blowing grass all over the mower and it got packed around the control module and was shorting it out.


#10

Tiger Small Engine

Tiger Small Engine

hlw49 said:
It may not be an engine issue at all. Had one with a similiar problem and it was due to wet grass packed around the engine control module wiring harness. He had taken the grass chute off the deck and it was blowing grass all over the mower and it got packed around the control module and was shorting it out.
Click to expand...

3000 plus hours is a lot of hours (pretty much maximum for a Kawasaki FX series engine). Has this engine been rebuilt, to your knowledge? Even if it runs, it will be way down on power.
Perform a compression and leak down test and go from there.


#11

StarTech

StarTech

Well one customer here has over 4000 hrs on a Kawasaki FX850 engine and it is still running strong. The only thing than a complete carburetor including throttle bushings that has done to the engine is an oil Gerotor replacement. This seems to a common problem the carburetor where the throttle and choke shafts wear in two. The last it took a sch 80 brass pipe to make the bushings.

And doing a compression test on the the FX751 is basically useless due ACR. The engine passes the modified leak down test fine with < 20% loss which surprises me consider the #2 cylinder problems. If the customer keep running it would had gotten to be a cold cylinder problem but currently it is just a hot cylinder problem. I do have the needed parts on order but the distributor has them on back order.

I kinda lucky to have ran into this just a month ago on another Kawasaki FX-T00V with over 3000 hrs on it too except it had already gotten to the cold stage failure. Slightly different resulting damage so he got lucky on the repair bill. Now of his previous repair shop told him last year he need a $4000 plus engine, which I repaired for $300 last summer due high oil usage. That #1 cylinder but this #2 cylinder went down. Same root cause but did some other damages.

With Kawasaki engines I am seeing several with over 3000 hrs still running strong but of course the customer are doing regular oil changes and using good oil.


#12

Tiger Small Engine

Tiger Small Engine

StarTech said:
Well one customer here has over 4000 hrs on a Kawasaki FX850 engine and it is still running strong. The only thing than a complete carburetor including throttle bushings that has done to the engine is an oil Gerotor replacement. This seems to a common problem the carburetor where the throttle and choke shafts wear in two. The last it took a sch 80 brass pipe to make the bushings.

And doing a compression test on the the FX751 is basically useless due ACR. The engine passes the modified leak down test fine with < 20% loss which surprises me consider the #2 cylinder problems. If the customer keep running it would had gotten to be a cold cylinder problem but currently it is just a hot cylinder problem. I do have the needed parts on order but the distributor has them on back order.

I kinda lucky to have ran into this just a month ago on another Kawasaki FX-T00V with over 3000 hrs on it too except it had already gotten to the cold stage failure. Slightly different resulting damage so he got lucky on the repair bill. Now of his previous repair shop told him last year he need a $4000 plus engine, which I repaired for $300 last summer due high oil usage. That #1 cylinder but this #2 cylinder went down. Same root cause but did some other damages.

With Kawasaki engines I am seeing several with over 3000 hrs still running strong but of course the customer are doing regular oil changes and using good oil.
Click to expand...
Yes, Kawasaki engines can run 3000 plus hours if well maintained. The leak down test of less than 20% is good. Compression test with compression release is just another test, not definitive, but a piece of the puzzle to figure out issues. Have you checked for intake leaks on gaskets?


#13

StarTech

StarTech

Not necessary as I have already found the #2 cylinder problem base on previous experience with a very similar problem except that one only backfire out the air cleaner because the operator let get that bad.


#14

R

RevB

StarTech said:
My initial thought was that because they were aftermarket coils but I had already swapped them out using OEM coils. Valves rockers were in spec when checked but it is a valve related problem.
Click to expand...
The Kawasaki coil wires are carbon track units and they have had problems with continuity before. Ask me how I know. The plug boot connectors are just crimped around the cable with a small spike penetrating the sheath to contact the carbon coated string. Check these as well. I have replaced mine with these....much more secure connection.

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#15

StarTech

StarTech

Hmmm. I don't use those type terminals here as the high tension leads are strip back and conductor folded back and the terminals are then crimped in place. Matter of fact I replace high tension leads whenever mice chew them in two.

Using those spike type of terminals are just asking for problems.


#16

R

RevB

StarTech said:
Hmmm. I don't use those type terminals here as the high tension leads are strip back and conductor folded back and the terminals are then crimped in place. Matter of fact I replace high tension leads whenever mice chew them in two.

Using those spike type of terminals are just asking for problems.
Click to expand...
Damn hard to replace carbon track when they are moulded into the coil housing.

There is a screw in the cap that screws about 3/4" up into the cable...nothing gets "peeled back".


#17

R

Red Good

Loose valve seat ?


#18

StarTech

StarTech

RevB said:
Damn hard to replace carbon track when they are moulded into the coil housing.

There is a screw in the cap that screws about 3/4" up into the cable...nothing gets "peeled back".
Click to expand...
Most high tension lead are not molded into the coils. They are either screwed in or push in (depending the pin used) and while epoxy holds them in place afterwards. It does however requires very sharp skinny blade knife to do this.


#19

StarTech

StarTech

And well there is enough guessing the problem. It is a valve sticking due to valve carbon fouling caused by a slightly blown head gasket. When hot the valve is sticking monetarily in the valve guide which is also carbon fouled.

Normally the solution to clean or replace the valve and clean the valve guide if not worn or displaced along with new head gasket.


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