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Discussion Starter #1
On my recent trip, I had a heck of a time with hot restarts at the fuel pump.
The consensus on the forum has been electrical issues, as in the heat causes a weak spark during cranking. The proposed fixes have been Iridium spark plugs, direct ignition coil wiring, and reducing the pick up coil spacing. I have done the first two of three to my current bike, and the third one to my old bike. My new bike seems to have the greater issue with hot starts, but they both definitely have (had) issues. I'm starting to think it's not electrical at all, but fuel related! I think it's vapor lock!

According to this article, "modern fuel blends can boil at temps as low as 100 degrees".

With my new bike, a hot start sounds the same as when I try to cold start without a choke. Crank-crank-crank-POP(backfire)-crank-crank.... Adding choke during a hot start (as Spockster suggests) would force more fuel through, clearing up the vapor lock sooner. I started to think about what else was different on my old bike. I did the ear shave, and in the process I replaced the fuel lines with generic fuel lines from Farm and Fleet. These happened to be thicker rubber, and perhaps somewhat better insulating and reducing the chances of the fuel boiling.

Older fuels that were designed for carburetors (circa 1985) had a higher boiling point. When the VN700/750 was new, this wouldn't have been a problem. Otherwise, I think they would have changed something, given how common of a problem this is. The article suggests using ethanol free fuel as ethanol lowers the boiling point of fuel. That would be a worthwhile test. If using ethanol free fuel reduces hot-start issues, it's probably vapor lock causing the problem!

There's my problem suggestion. I don't have a solution as of yet. I just wanted to throw that out there to see what you all thought.
 

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I've struggled with the term all my life, and I'm still not sure it exists, I've just never seen it or had to fix it.
Where would the 'lock' be in this fuel system?

In the old days people swore by clipping a wooden clothespin onto their steel fuel line to cure vapor lock. But if you tossed the clothespin, it still started just fine.

I've seen gas boiling in a plastic gas tank, bubbles and all, but there was never any lock on those particular engines.
 

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Grumpy Old Man
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I always thought on hot starts… hit button , crack throttle simultaneously… no start stop cranking
Wait a few & repeat.
Excessive cranking I think is what leads to the backfiring.
 

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Grumpy Old Man
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Just thinking out loud here…. Shut off petcock before you shut down the motor … then fuel up and try to start before re-opening the petcock. Should be more than enough in to bowl left to restart.
 

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Normal hot start procedure for just about anything with a carb is part throttle or half throttle and crank it.

But every time I've done it on this bike, it's no start and 99% of time, a backfire.

When I first did the Two-Wire mod I thought hot starts were better, but I was right back to hitting the choke in no time.

Immediate hot restarts don't seem to be a problem, it's after sitting about 4-7 minutes. On the immediate start, mine is like turning on an electric motor, touch the button and it's running.

So it seems heat soak might be a factor. Is vapor lock happening inside the carb, but the enrichment circuit isn't affected, so it's able to supply fuel?

Still trying to imagine how that could happen.

Try heat shield under the carbs? Aux. Fan to run while you're in the store?

I suppose a temp gun would be handy.
 

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Is it possible for the fuel level in the carbs to drop that much in the time it takes to pump and pay for gas?

Carbs are vented, so it shouldn't be any pressure buildup from boiling gas. But it seems there's no fuel available for the pilot jet.

I still want to find out what the white/red wire does. Maybe if I disconnect it, hot start problem goes away?
 

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Grumpy Old Man
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Spockster … I was thinking too much fuel initially, and flooding out.
Maybe shut petcock off & let carb run dry. Fuel up & then start from scratch?
 

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Spockster … I was thinking too much fuel initially, and flooding out.
But then you could just pin the throttle and crank until it fires. Unless the gas is gushing out of the throat, a flooded bike will always start that way, as long as the plug still fires. And at least in my case, I have to use the choke.

If I try throttle it just backfires, add fuel and it starts.

If I ever get around to testing the red/white wire, I would slip it out of the CDI plug, then reconnect while watching the timing while it's running. (would have to hit the start button) If that's interesting then I might vary the voltage to the connection and see what happens. I'm not sure if that wire is advancing or retarding the timing, but I think it's one or the other. When the voltage from the start circuit hits that CDI terminal, I think it retards.

The Ford Model T has the spark advance on the steering column, I believe you retard it to avoid breaking your arm when you hand crank it. If you're strong and brave you can advance it some for a quicker start, less cranking.
 

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Grumpy Old Man
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Maybe for the possibility of vapor lock, you would be refilling the bowl with cool fuel you just pumped into tank? Just spit ball’n here.
Spockster … You do have a good point on the timing👍
 

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I don't know what's going on, maybe we can all figure it out. It's a head scratcher.
 

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Grumpy Old Man
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Spockster … yep , maybe just take one tank trips, no problem then 😀
the carbs could not be in a worse location to soak up some motor heat while it is parked
 

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Could just disconnect the wire and ride it. If the problem stops, stub that #$%#^& off.
 

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I'd use a temp gun and watch the carbs as soon as it's parked. Just keep reading it all over.
 

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Get to work Jason. Haha

You can install a volume knob on your handlebar and dial up the timing. Or use a capacitor and make a "Push to Pass" button.

push to pass - Google Search
 

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Grumpy Old Man
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O’boy … I don’t what to see Jason messing with his new bike. If it ain’t broke don’t fix it.
Jason are you listening? Lol
 

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I’m curious to what y’all find on this one. I bought a set of feeler guages and diamond burrs but never did the pick up coil mod. I can get the bike to start hot with either with the throttle open or the choke. It seems to start most of the time, but I’d say a 3rd of the time it gives me trouble.
 

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Discussion Starter #19
Wow, this thread blew up overnight! Good grief!

I'll admit my lack of carb knowledge, but if the gas in the jets themselves were to boil off, the venturi would pull only on the fumes, rather than pulling gas up the jets. It would be fuel starved until the vapor is cleared out and fluid is pulled up the jet.

Scooter 007, I plan to be a bit less ambitious with this bike. It ain't broke! I'd still love to figure out the cause of this.

Does anyone run ethanol free fuel? I also found this "To fight vapor lock, adding a small amount of combustible oil to the fuel (¼ cup to 10 gallons) will reduce the volatility of the fuel; common additives are Marvel Mystery Oil or Diesel Fuel. Adding too much will cause the oil to carbonize during combustion and increase the chance of pre-ignition (knock/ping)." Doing both ought to really make a difference. I don't think I'd do this as a long term solution, but it would be a worthwhile experiment to determine if it is actually vapor lock occurring.

I just filled up this morning, and forgot to get ethanol free fuel. I'll pick up a bottle of Marvel Mystery Oil and add it to my next tank of ethanol free fuel to see if it helps. If the issue goes away, THEN we can start talking mods to fix the issue.

More web research:
"This problem is particularly acute after running the engine for a sustained period of time at highway speeds, then suddenly coming into stop-and-go traffic. The associated heat sink effect will elevate the temperature of the carburetor and the fuel will boil in the bowl. This can lead to serious flooding on some cars and hard starting problems when hot in other vehicles, while many other cars may notice rough idle characteristics."
 

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If nothing else, the MMO will clean some carbon out of the combustion chamber.

I'm disappointed JP, I thought you would dive in and start experimenting. :)

The jet is practically just a tube, the vacuum pulls the fuel up through the jet, but fuel level has to be high enough.

Guess I'll take a look at this again when I can ride and tinker around.
 
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