Trouble Starting When Warm

[Chokse]

I am having a problem and I am hoping that someone has a suggestion/solution.  In the morning, my bike starts just fine.  However, after the engine is warm, it starts to have trouble starting.  If I ride for a bit, and then turn off the bike for more than 10 or 15 minutes, it becomes almost impossible to start.  It will crank all day (so the battery and battery cables seem fine) but it won't start.  It almost seems like it is flooding the engine.  A few weeks ago, I moved from 105 main jets and 40 slow jets to 110 main jets and 42 slow jets.  I had been having lots of trouble with the 105/40 combo and my K&N filter/Vance & Hines Exhaust, so I went with larger jets.  I also added a Pingel PowerFlo gas valve because my OEM petcock was broken and would no longer shut off (A real problem when you want to remove the tank!).  The move to larger jets did improve throttle response and completely eliminated the popping on decel, something I was never able to eliminate with the 105/40 combo.  However, with the larger jets, I started to have trouble starting (the trouble mentioned above), and the bike would often have a loud backfire when I finally did get it started.  So, I kept the 110 main jets but went back to the 40 slow jets in the hope of fixing the starting issues, but that didn't work, so I am now back at 105 main and 40 slow and I still have the problem.  So, I don't think it is the jets.  I checked the plugs and they look fine (I replaced them about 6 months ago), so I am out of ideas.  The only additions I have made to the bike (performance additions) are the extra shims, the K&N filter, the Vance & Hines exhaust, the Pingel PowerFlo fuel valve, a Dyna3000 ignition module, and Dyna coils and wires.  However, all of these items have been on the bike for quite some time and have never caused a problem before.  And finally, before you ask, yes, I did very thoroughly clean the carbs and the rubber gasket has been removed from the air cleaner box.  So, does anyone have a suggestion as to why I am having so much trouble starting the bike when it is warm, but have no trouble when the engine is cold?  Any help would be greatly appreciated!

[L J VFR]

It sounds sort of like it may be vapor locking.  I'm not sure what causes it, someone else can chime in.  I know this only occures once a bike is warmed up and like you are describing. Check all vacuum lines, make sure one isn't cracked.  Possibly your ignition system isn't working properly even though it has in the past.  You said you have a new ignition module, maybe when it gets hot, it is causing issues.  Change this out with the original one and see if it does the same thing when it gets hot.  This is what I would do.  Hope this helps.  Lance

[ezrydr]

I've had problems with bikes not acting right when they got hot - not with the Magna, but with a couple of others - and in both cases it turned out to be a wire that had a bare spot that didn't touch anything when the engine was cold, but when it warmed up the metal expanded enough to make contact with the exposed bit of wiring.  I spent a lot of money and labor both times before locating the problem (and fixing it with a couple of inches of electrical tape).

Now what you're describing doesn't sound like the same thing, but possibly the same physical phenomenon - metal expanding when hot - could be what's triggering your problem?

I realize that's something of a reach but it's all I can think of.

[ezrydr]

A bike mechanic dies and is greeted in the next world by a white-robed figure who consults a huge book and says, "Ah, it says here you were a motorcycle mechanic."

"That's right," the man says.  "All I ever did, all my life."

"Well," the robed guy says, "maybe you didn't know this, but we don't just sit around here.  Everyone works at the same trade he or she followed in life.  Come with me."

He leads the way down a long corridor to a big room, which turns out to be the most amazing shop the mechanic has ever seen.  The walls are covered with all sorts of tools, obviously of the best quality; the lighting is excellent and there is a big hydraulic rack and in short everything that a shop owner could ask for.  "Wow," the mechanic says.  "This is great."

At the end of the room is a long row of motorcycles of various makes.  "When you get them all fixed," his guide tells him, "there will be more.  There will always be more.  This is eternity, after all."

The mechanic walks over and looks at the tag on the handlebars of the nearest bike. It reads:

INTERMITTENT ELECTRICAL FAULT

He moves on to the next bike, and then the next and the next.  Every single one says the same thing:  INTERMITTENT ELECTRICAL FAULT.

"Hell!" the mechanic says.

"Well, of course," his guide says.  "Where did you think you were?"

[L J VFR]

      One up for William!!   

[lragan]

Indeed!  You wind up replacing all sorts of stuff that is perfectly OK, then when you finally find the problem, are tormented with "Why didn't I think to look there?  It was right in front of my eyes!!"

Great joke, William.  I will use it, be assured. 

[Greg Cothern]

I would personally go back to the stock petcock....  I know it sounds simple and that it ran fine with no problems but I would slowly start taking changes out of the puzzle.
When it does this have you pulled a plug at that moment to see if it is flooded?  Can you smell fuel as though it is flooded?
When it does start what does it act like?

[Chokse]

The new petcock does not seem to be the problem.  It was on at least 2 months before the problems started.  Besides, the point is moot (no pun intended) as the old petcock was broken and is now somewhere in a landfill, and I am not going to buy a new one just to test it out.  I'll have to try all other avenues before I finally do that.  I'm going to take the bike apart today to see if I can find anything I might have missed (like a bare wire).  I am also going to try changing out the K&N filter for a Honda one to see what that does.  I have read on several forums that people have not been happy with the K&N filter.  We'll see what happens.

[lragan]

I am happy as a clam in sand with my K&N.  While my bike requires some "choke" when cold, when it is warm, it fires on the first cylinder -- click, vroom, just like clockwork.  I have had riders of other makes remark on how well it starts.

[TLRam1]

I would also have said check your float valves but that would cause flooding whether warm or cold, I would check them anyway just because. As mentioned, looking at your plugs for a wet one might give you some clue. Breakdown of the plug, these are hard problems and no fun to problem solve.

Keep us posted on what you find out. I have a few ideas but I like to narrow it down by elimination.

Swapping coils out is good, if it's not too much work take the electrics back to stock and start over. 

[Chokse]

I think I found the problem.  I went back to the Honda OEM air filter (it was a royal PITA putting those two rubber gaskets in the air filter housing!!!) and everything seems to be better now.  It started up very easily when both cold and warm, so I think this is the solution.  I haven't had enough time to give it a good testing, but I'll get quite a bit of riding in next week and we'll see how things go.  Thanks for all the suggestions.

[Chokse]

I'm happy to say that the air filter swap seemed to work.  I rode it quite a bit today and it is running perfectly.  It started up each time with no throttle (warm and cold) and there is almost no decel popping, and absolutely no backfiring when I start it.  I guess some Magnas just seem to not like the K&N filter in them.

[roboto65]

Might also depend on the jet size and weather you had jetted or not. Running a K&N on a stock bike that most likely came lean from the factory would not help !!!

[Chokse]

Like I said in the original post, I have run every combination of main a slow jets from 105 to 112 on the main jets and 40 to 42 on the slow jets.  Right now, the bike has 110/40s in it, but now that it is running better, I am going to try and switch to a 107.5/42 combo.

[Chad in Michigan]

Hey I just thought of something, maybe nothing. I remember having trouble with the k&n and all the jetting i did.. Do you have the screws on the bottom of the float bowl pointing to the center of the engine? they will go the other way (with the screws on the outside) but it is not the correct way. I remember running out of fuel or having a lean problem with them on the outside. the only thing I can think of is that when they face the outside there is not enough fuel in the bowels or the floats hit the sides or something. it made a big difference for me on my bike anyways. I have looked at the service manual for the vfr750 and those are on the outside, but they also have two different size jets (mains and slow) on two of the carbs. I dunno.. Just rambling.. lol