HarleyTechTalk

After putting on 15000 miles, the bike decides to start a backfire the instant I touch the start button. Started out just the 1st start of the day, then every so often. Then almost all the time. Replaced all the O rings and injectors. Still did the same thing. Then replaced the intake-to-head seals and I could see where it was flashing past the seal ! Replaced the seals, worked perfect foe 3 weeks. Now it's doing it again. Seriously !? WTF?  Is there some kind of sealer/lube I'm supposed to be putting on them ?
  Is ANYONE else having this problem ?  This is not my 1st rodeo, so I have NEVER seen this "Potty mouth" before.

Well, we have to guess at what your riding, we assume an M8 based on where you posted.

All seals should be lightly lubricated during assembly of components. Is your intake the OEM plastic unit? Look there for issues as well.

Yep like Ohio said, the plastic one can cause issues, a tuner down here told me he had replaced quite a few now.

I've found much less aggravation by switching to the aluminum manifolds from SE (27300167) or S&S (160-0241A).

Yes it's an M8 107. It did start this with the plastic intake. I did change over to the aluminum. So, I don't think it was the plastic one since they both do it.

Check the intake flanges for damage and that they're flat. Lubricate the seal, finger tighten the bolts evenly. Do not perform the final tightening until the TB and TB support are on.

I did it that way. Was VERY careful with alignment.

They give you the correct seals? You would of checked that though, very odd, I hope you find the reason soon.

Well,... they fit perfectly. hmmm
Not sure, but I think there is only 1 seal for the M8 at this time.

Well, I have now changed injectors, O-rings for the injectors and intake seals. I have the aluminum intake. I changed the fuel filter and the pressure regulator. It still does it !  Makes NO sense !  Worst part is, no-one else seems to have EVER had this problem.

An old adage: when chasing a fuel problem, it's electrical. When chasing am electrical problem, it's fuel.

So, here it is a year after this whole thing started. Still having the same problem. I have also changed the MAP sensor. no change. Can't find any bad wires (yet). If it didn't run perfect, maybe I could find something to fix. Any suggestions ?

Assuming you are 100% certain you have a recurrent intake leak:
Have the heads been off ever? Or milled? Are the surfaces where the seals hit still flat and true?
What air cleaner you running? The fitting of the backing plate can "pull" an intake and the manifold away enough to blow the seal(s).

Maybe your chasing the wrong gremlin.  Because the issue has worsened and increased it's frequency with the number of start cycles it would make me consider the compensator.  You call it a backfire but that's a subjective description.  Is it sending fire and smoke into the breather?  As the pre 2014 twincam compensator aged and got weak, the first crank (or more) would kick off like a little explosion and spin the engine back (often resulting in a grind / bang / stall like reaction and a subsequent intake or exhaust loud firing back and puff of smoke).

Have you checked the compensator health? You still haven't given a full description of the model, year etc. of the bike.  IIRC the earlier M8 compensator was problematic.

I also recall some posters saying something similar to your basic description was happening and if they tapped the start button and clicked the bendix in once then on the second tap held the button in it would start normally.

Check for head gasket leak, some have had this problem.

Quote from: guydoc77 on April 24, 2024, 07:59:44 PMAssuming you are 100% certain you have a recurrent intake leak:
Have the heads been off ever? Or milled? Are the surfaces where the seals hit still flat and true?
What air cleaner you running? The fitting of the backing plate can "pull" an intake and the manifold away enough to blow the seal(s).

I wouldn't say it has a recurring intake leak because it runs perfectly at any RPM. The heads were never removed. No leaks or blow-by. Everything is flat,true, and in alignment.
I have the SE ventilator air cleaner. But this started when the stock one was still on. But,... I will look closer at that area and see if there is something with that.  thanks

Quote from: kd on April 24, 2024, 09:14:14 PMMaybe your chasing the wrong gremlin.  Because the issue has worsened and increased it's frequency with the number of start cycles it would make me consider the compensator.  You call it a backfire but that's a subjective description.  Is it sending fire and smoke into the breather?  As the pre 2014 twincam compensator aged and got weak, the first crank (or more) would kick off like a little explosion and spin the engine back (often resulting in a grind / bang / stall like reaction and a subsequent intake or exhaust loud firing back and puff of smoke).

Have you checked the compensator health? You still haven't given a full description of the model, year etc. of the bike.  IIRC the earlier M8 compensator was problematic.

I also recall some posters saying something similar to your basic description was happening and if they tapped the start button and clicked the bendix in once then on the second tap held the button in it would start normally.

  (not being snotty or defensive)  The compensator wouldn't have anything to do with the "pop" I get when starting. Literally, the very INSTANT you touch the start button, it happens. Like a millisecond before the motor starts to turn over. I can put my hand in by the top of the intake and feel it pop.
 It's not a "kick-back" likr a bad comp would do. I had that issue with my twin Cam bikes.

Even though I didn't find anything electrical,(yet), I'm very much thinking KD and Hossamania might have something there.
I did contact Powervision to see if they had any suggestions. I did get a response. Waiting for additional info from them.

Sticky valves will do that.

It's been about 9 bikes ago that I had to check spark and compression. so this (2018) has the compression releases on it. When I went to check the spark, I had 1 plug out of each cylinder. Put the plugs in the wires and turned it over to see how the spark was. But there was none. I'm like ? Found out if there's no compression, there's no spark. Searching forums and internet, I find this out. OK. So put plugs in, grab a couple new ones and put them in the wires. Great spark! Something I read was (don't quote me) "until the crank reaches approx. 50 RPM, there won't be any spark either".  Is this true ?  Does the ECM not enable the coils before 50 RPM ? I never heard this before. If this is true, then I shouldn't have any spark there to cause this "POP"/ backfire in the intake.
 Thinking about this, lets ignore that thought for a second. The "POP" is instantaneous. If the coil can send power to the plug, the intake valve on 1 of the 2 cylinders would HAVE to be open on the firing plug in order to ignite the fuel in the intake manifold. I don't know what those odds of that happening are, but seems highly unlikely.
  Now, if the ECM tells the plug to fire at or next to 1 RPM, do you think this is a possibility ?  In which case the ECM "could" be the culprit here. 3 different dealer service managers,"never heard of this", so "we'll get back to you".  Not sure how long this will take. Been 2 weeks.
  The plug wires checked out perfect. But I replaced them.  The plugs looked good. 2 of them had a little wear. so I changed them too. Strangely. I started it 5 times and it didn't do it! But I'll find out this weekend. Last year it could go a week or 2 before it would act up. Fingers crossed. But I know better. 
 
 Anyone else EVER have this issue ?  Been looking to talk to you for a year now.
Thanks 

I think you're the guinea pig on this one, sbeamer.

Hmmm....did it "pop" when you were testing for spark? (both plugs installed but no coil wire attached). If yes, then.....(enter your lightbulb moment here).

It did pop a couple times but, I still had the wires connected to the plugs under the tank. BUT,.... I honestly can't remember what it did when I took the tank off and checked it the opposite way. I'm pretty sure I didn't have any wires connected to any of the plugs at that point. I don't think it was Popping during that set of spark tests. hmmm   

If you really can't recall, do the re-test to confirm. you'll eliminate a lot once you confirm.

Re-did the test. With all plugs in, "no wires" on plugs, did not "pop". Only with wires connected. Today went for an extended ride. About 5 or 6 starts, no POP. Yesterday it popped twice in 10 starts. ?? just bazaar

Are you talking about a backfire at startup only, runs fine once started? Or is the pop coming from somewhere else?

At start up. Not every time. Just and only when you touch the start button, and only for that instant.