Backfiring into intake

[mdr]

OK, back to the backfiring?

Update:

Got the carbs on, hooked up a temp gas tank, and no joy.  Still backfiring into the intake.  #$%@!

Bright side, the air box held up so far. <sigh>

Getting desperate, I got out the compression tester.  Results, cold engine, going from #1 (left knee) to #4 (right knee)

142 - 194 - 202 - 204

Double checked #1 just to make sure I didn't forget to hold open the throttle or something.  Double #$%@!!

I figure the next test is for piston height (aka hydro lock test).  It locked up a LONG time ago - 8 years or so - and has run well since then.  I don't remember any lock recently (CRS?) so I doubt it, but I'm gonna check it anyway.

What to check after that?  Leak down to see if the intake valves are hosed?  I did just do a valve adjustment, but that was to rule out  valve adjustment as a problem.  On #1 the intake valves measured 0.18 and 0.17 before and now measure 0.15 and 0.16.  That's loose - so it wasn't the adjustment before either.  So unless I didn't get the lock nut on well, the valves should be good.

This is a sudden onset condition, so with out a hydro lock recently, I'm thinking something broke.  A valve, seat, or something.

I just want to get an idea what I'm in for before I start tearing her down.  May not be possible without a tear down inspection, but...

Other suggestions?

Triple #$%@!!!

[mdr]

K.  Checked the piston heights.  2&3 are as close as I can measure repeatably, under 0.5mm, maybe better.  1&4, not so much.  1 is about 2+mm lower than 4.  All measured at TDC.

I gotta R&R a rod, don't I?

I did notice carbon deposits coming back on the screwdriver tip from #1 too.  Could be the source of the backfiring - hot spots in the deposits igniting the incoming mixture.

I'm going to read through Colin's article (Concourier Spring 2010 - over at the COG site - membership required for the full article) and figure out what I'm doing from here.

Before reading, I'm inclined to button it up and run it on 3.5 cylinders since this probably happened about 9+ years ago and I've been running it for, not as many miles as you'd probably think, but enough if there's going to be cylinder wall damage it's already happened.  There were a small amount of deposits on the old plugs when I pulled them, so I believe the piston rings are sealing ok since there's no obvious oil fouling.  Maybe just clean the carbon out of the cylinder, run it, and know I need to clean it out before every oil change.

Thoughts?  I got some reading to do.

[connie_rider]

You don't "have" to do the rotor replacement.
If it's been running good, it will probably continue to do so.
I suspect there are more bent rods out there than we know. running and running, and running.........

My bike had a bent rod and I rode it for sometime before replacing with another engine. The only thing I noticed on mine was it was a bit down on power.

As far as backfiring into the intake,, the bent rod is not the problem. Keep looking.

Ride safe, Ted

[mdr]

OK - updates.  Tried cleaning the carbon out last weekend, figuring maybe it's got a hot spot that'll occasionally set off the incoming fuel / air charge.  LOTS of Seafoam smoke.  Feed in about 3oz using the vacuum port.  I put a ball inflator needle in the end of the hose to restrict the intake of the Seafoam (trick from the VW forums) and it did a good job of restricting the take up of the Seafoam.

Let it sit the 5 minutes, fired it up, and did I mention lots of smoke?!  I think I lubricated the muffler bearings doing this too.    There was oil dripping from the weep holes.

It's still backfiring but I ?believe? it's a little bit better.  I'm going to try it again, a bit more aggressively, when time allows - hopefully this weekend. 

Also cut off the coil ends of the plug wires and replaced the one (#2?) that was a little short for the OEM sleeve after trimming.  A spark plug pull on that cylinder is upcoming too, and I may again replace that one spark plug just in case it's a little weird or something.  Doubt that has anything to do with it since this was happening before R&Ring the plugs.

[connie_rider]

If you have an infrared gun, check the temps of the pipes and try to see which cylinder is giving the problem.
(It can be hotter or colder) Hotter could be from an overly rich condition. Cold would be missfire.
If hot, check float levels. If cold, check float levels. If nothing appears bad, CHECK FLOAT LEVELS....

If you don't have an infrared gun, you can remove 1 wire at a time and see if you can find a problem cylinder.
PS: Harbour Freight has the infrared/temp. guns,, cheap....

If you find a problem cylinder, concentrate on that cylinder....

(I think) You mentioned carbon in #1. Have you tried changing the plugs? If you carbon foul a plug, the plug can "die". Clean or not, it can missfire. Try replacing the plugs...

Last dumb question. When you did the valve adjust, did you put "any" wires on wrong? Including; do you have the correct spark plug wires on the correct plugs? Did you re-install the ground wire on the coil?

Ride safe, Ted

[jim snyder]

OK - updates.  Tried cleaning the carbon out last weekend, figuring maybe it's got a hot spot that'll occasionally set off the incoming fuel / air charge.  LOTS of Seafoam smoke.  Feed in about 3oz using the vacuum port.  I put a ball inflator needle in the end of the hose to restrict the intake of the Seafoam (trick from the VW forums) and it did a good job of restricting the take up of the Seafoam.

Let it sit the 5 minutes, fired it up, and did I mention lots of smoke?!  I think I lubricated the muffler bearings doing this too.    There was oil dripping from the weep holes.

It's still backfiring but I ?believe? it's a little bit better.  I'm going to try it again, a bit more aggressively, when time allows - hopefully this weekend. 

Also cut off the coil ends of the plug wires and replaced the one (#2?) that was a little short for the OEM sleeve after trimming.  A spark plug pull on that cylinder is upcoming too, and I may again replace that one spark plug just in case it's a little weird or something.  Doubt that has anything to do with it since this was happening before R&Ring the plugs.

Mark,
   Have you checked the valves lately? This sounds like its backfiring into the intake due to a tight valve on number one, and the low compression on # 1 seems to confirm that. If not that then may be a small piece of carbon build up on the valve seat. I know how we used to get carbon deposits off of valves on car engines, but never tried it on a motorcycle engine.

[mdr]

   Have you checked the valves lately?

One of the first things I did since I couldn't remember the last time I checked 'em.  All cylinders were in or just out of spec.  Brought all of them to mid-range.  #1, the problem cylinder, was just in spec.

TODAY'S UPDATE: Backed 'er out of the garage and injected the Seafoam into the carb's vac port again.  I noticed this time the needle has a hole in the tip, and a slot on one side.  If just the needle tip touches the Seafoam, I got a air/SF mix.  In further and it pulls just the SF.  Feeding just SF in this way for a couple seconds didn't seem to phase the motor in the least (wasn't too worried about hydroing it - for obvious reasons ;P).  Well, it did have one effect.  I didn't get hear of feel a backfire afterwards

Still need to hook up the carb sticks to verify there's no back pulses, and to sync the carbs, but after the bike quit smoking I declared victory and put it back in the garage.   

[shibby_cbs]

  IMO that's the only reliable way to repair the airbox - once and for all - Steve

have you guys tried plastex. its the same stuff i used to repair/recreate the tabs on my upper fairing. i sealed my box with this stuff and its holding like a rock.