Back fire through carb at WOT

[Canty]

I'm still trying to exercise the demons out of my 03 GTI LE. Today was the 1st time on water since 2015. On the trailer it seemed fine. I did put an RPM reading timing light on before I left to set idle at 3k. The readings were all over the place. 3700, 5400, 9999. The ski idles smoothly so not sure about the sparatic readings. Oh well, off to the water. Hard to start even with choke. Once it ran a while then it was quick starting. Okay so in the water it idled fine. Slight throttle was fine. Mid range throttle it would die out. If I went past mid throttle to WOT it ran fine. When I let off WOT it would back fire through carb. So bad in fact that it blew the cone shaped exhaust off the round exhaust. Sorry I dont know the names of the pipes. When it blew the pipe apart it was a loud pop then the motor struggled to stay alive. It sounded terrible then died and would not restart. I only tried for a few seconds though since I did not know what had just happened. When I got home it fired up right away. I reassembled the pipe, started it and it revs fine. But when I let off the throttle at high revs it still backfires through the carb. Any ideas? When I had the motor out on the work bench this past winter I did verify timing with a guage. I have had a TON of issues with this carb. Time to get a new one?

 

[Canty]

Let's just say that I timed it incorrectly, a few degrees off. Would that cause my issues? Hard starts, no mid range, etc.?

 

[mikidymac]

The ignition timing is set by aligning the timing marks and typically isn’t an issue.

The bigger issue is rotary valve timing which is done with a degree wheel and finding TDC. I would suspect rotary valve timing on yours coming out the carb.

A backfire out the exhaust is typically too much fuel from leaking needles and seats.

 

[Canty]

Yeah, I'll double check the rotary valve. I have the degree wheel. It who knows I may have been wrong. Would timing cause it to die at mid throttle?

 

[mikidymac]

Hard to say.
The other thing that can cause all kinds of weird is a bad voltage regulator.

 

[Canty]

Is there a test for that? Aside from measuring voltage during engine revs that is.

 

[mikidymac]

That’s about it. Check the voltage while reving it up.

 

[Canty]

Okay. I'll check the voltage regulator tomorrow. If it's good I'll verify my rotary valve as well.
Thanks

 

[gazzaskirepair]

I'm still trying to exercise the demons out of my 03 GTI LE. Today was the 1st time on water since 2015. On the trailer it seemed fine. I did put an RPM reading timing light on before I left to set idle at 3k.

Statement above “1st time in 5 years “ I would say you should clean out the fuel system , carbs , old fuel and replace the grey fuel lines if you have any

 

[Canty]

Fuel system has been totally cleaned or replaced. Tank cleaned, fuel selector cleaned, new filter, new lines, carb has been rebuilt.

 

[gazzaskirepair]

I would rebuild carbs with genuine Mikuni kits and recheck the rotary valve timing while it is off

 

[Canty]

Genuine kits were used and the sticky followed closely. I'm thinking rotary valve.

 

[gazzaskirepair]

misread
Yes recheck valve timing and confirm correct angle rotary disc if it was never run properly since you had it

 

[Canty]

Okay. Will do!

 

[Canty]

I did not get a chance to check the rotary valve but I did test the voltage regulator. I started with a battery reading 12.4vdc. During revs it went no higher than 12.74vdc. Is this okay? Shouldn't it be higher but not over 15vdc?

 

[mikidymac]

At about 4,000 rpm you should see 13.5-14 volts.

 

[Canty]

I saw nothing over 12.7 and that was revved high. As high as I though was safe, high enough to make the backfire threw the carb again. Would this also signal a bad regulator?

 

[Canty]

So I checked the rotary valve tonight and it was off. I lined up my degree wheel and found 147 degrees. My ski was off. I cant remember how many degrees but I can say that one tooth one way gave my ski a 139 degree reading and one tooth the other way and I was close to 155 degrees. So to fix this I flipped the valve over and lined it up to 144 degrees. That was the closest to 147 degrees I could get. I didnt run the ski yet. I started it but that was all. I'm waiting for the new voltage regulator to arrive.

 

[Canty]

Any advice guys and gals? The rotary timing is at 144 degrees and the new voltage regulator is installed but it is still backfiring through the carb. The high speed screw is at 0 turns and the low speed is at 1 turn out. It starts right up and idles well and if I am slow on the throttle it revs fine, if I hit the throttle hard it chokes and almost dies. If I go to WOT it revs but when I release the throttle it backfires through the carb. Could it be something to do with the accelerator pump? I have had trouble with this carb from day one. Is it time for a new carb? I have three weeks to get it lined out because I have a week at the lake with the family and I am sure they are wanting to drive it.

 

[mikidymac]

Did you set the rotary timing per the manual marking the degrees at the top and bottom of the MAG intake port?

On my 03' GTI it would die when hitting the throttle and no matter what I did the carb could not be fixed so I spent the $210 for a new one from OSD and it ran perfectly ever since. Mine never backfired so I can't say for sure it is your issue. Typically a backfire out the carb is a timing issue either rotary or ignition.

Are you sure your stator is aligned and the flywheel key isn't sheared off?

 

[Canty]

I timed it with a degree wheel by placing 360/0 degree mark at the bottom of the front port. I marked the 147 degree angle and got it as close as I could. I placed the piston at TDC by a screwdriver since I didnt have a gauge. I read your post about your gti and the new carb so I just ordered a new one from OSD too. Not sure about the flywheel. I can check that this weekend. The only reason I can see it backfiring through the carb would be a timing issue or a fuel issue. I know its timed right, so it has to be a fuel issue right?

 

[Canty]

I removed the flywheel and verified that the stator was not broken some how. It was not mounted with the screws in the middle of the slots, about 3/4 up the slot. I hooked a timing light to it and started it. My light RPM guage read 9999 even when I dropped the idle some. Also, my timing light did not show the timing mark on the PTO. From what I have read on here is that it doesnt make much difference the clocked position of the stator, it's more of a rotary valve timing. But I feel I should have seen that mark on the PTO right?

Correction. I just re-did the timing light and the mark is at about 2-2:30 position. That's pretty far advanced. To much advance and it could backfire through the carb right?

 

[mikidymac]

You have a mark on the cases and a mark on the stator. All you do is line those two marks up, shouldn't have to do anything with a timing light at the PTO.

And yes, having the ignition timing off is a big deal.

 

[Canty]

I looked on the forum and Google for where those marks are. All I found out was they are around the 9 o'clock position. My stator isnt far off from center of the screw holes, and this forum has posts saying the stator doesnt make much difference. But I'm curious. If I move the stator from one extreme to the other, how many degrees of timing can I change? The manual had the index mark on the PTO and timing light procedure, so I just did it to see where mine was.

 

[Canty]

Okay, here is the update. I moved the stator from one extreme to the other and saw very little difference. I also replaced the coil and wires since that could have been a cause of the back fire. Now the back fire isn't as bad, more like just a puff. I am wondering if it is possible that that puff could be caused in part from the motor just starting (less than 15 seconds) and it just being choked. I am thinking maybe the choke gives it more fuel than it needs in the few seconds of running or the cold carb is running rich and 15 seconds isn't enough time to warm it. Or is that only an issue on automobiles?

IF that all is not the cause then here is what I am thinking. And I know some of you will say this is not correct but here is my idea. My current timing is set at 144 degrees. What if I moved it to around 154-160 degrees. Here is my thinking. Back in 2003 when this ski was new everything had tight tolerances. Now, 16+ years later, there may be some wiggle room here and there. If the back fire/puff is a timing issue then that would mean the exhaust is getting into the intake which tells me the rotatory valve is starting to open on the intake while the combustion is taking place. If I move it back just a little bit that should keep the intake closed for just a bit longer allowing the exhaust to exit properly.