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'93 VN 750
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23 Posts
Discussion Starter #1
Background:
Recent carb cleaning, synced carbs, lots of Seafoam, removed the 2 small red diaphrams in the gas cap and sprayed cleaner to clear vent line under gas cap (after reading about POOGS), replaced the spark plug cap on front by throttle cables, sprayed cleaner on plug wires to see if any problems (no problems - kept running fine with no stalling), replaced vent hose from left carb to petcock, right carb vent hose is plugged (bike has been coastered).

I have easy cold start in the mornings (with Choke ON), smooth idle, smooth and fast acceleration, easy warm start after a drive, but around 5000 rpm and higher it feels like its struggling for enough fuel...sputtering, almost stalling, etc... :( (this will happen in any gear) It almost sounds like only half the motor is running, because I can pull in the clutch and rev it but it does not sound like it should (sounds weak - hard to explain in writing). Usually I can twist the throttle to open up all the way and keep going around 60 mph in top gear. At times this will continue for a mile or so with the bike sputtering and jerking the whole way, until I close the throttle and slow down. Then after I slow down to around 40 mph, I can speed back up to 55+ until the problem returns (which may take only a minute or two but sometimes longer).

Keep reading for more weirdness...
Here's one of the main things that has me puzzled as it relates to POOGS. This problem occurs regardless of how much fuel is in the tank. It happens as I'm leaving the gas station and the next day when I'm at 3/4 tank or lower. (I've read that POOGS is common around 1/2 tank)

The other thing that has me confused is that this only seems to happen when its really HOT outside. I live in Florida. This "almost-out-of-gas-sputtering" problem has NEVER happened when I'm riding early morning or late evening (when its cooler outside, 70-80 degrees), but it happens every day when I'm going home from work around 5pm...(yes its still 90+ degrees then) I've only had this bike for 1 month so I haven't had the chance to see what happens when the weather is colder (guess I'll have to wait until November for that to happen :D


I need some help and insight on this one, PLEASE!!
 

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Old Truck Junkie
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4,133 Posts
I used to have that problem when hot weather as you say. I don't know what fixed it.
What I would try is to open the tank and let the pressure out before starting the engine.
I am thinking that the high pressure is pushing extra fuel into the carbs and flooding the carbs.
 

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'93 VN 750
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23 Posts
Discussion Starter #3 (Edited)
Thanks, I'll try opening the gas cap today before I start it up. How long does it need to be open? Would it matter if I open it and then start it and let it run for a minute with the cap open?

I have been riding with a spare key in the tank and tried opening when it started to hesitate, while going down the road. I'm not sure if that helped or not because the problem persisted even with the cap open, but it never fully stalled as long as I held the throttle wide open.
 

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Old Truck Junkie
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4,133 Posts
Just open it and close it.
Mine would run like it was flooded. I had to open up the throttle to keep the engine from stalling. As long as I had the throttle open all the way it would run. After a little bit it would settle down and run right.
 

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900 Posts
I know I keep bringing this one up.. but I missed it before..well acctualy 2 times now... the little hose that goes into the right ear(filter box).. insure that it is in the bow and that it is not bottemed out...if its is not in the box.. like mine was.. twice...it seems to get to much air while riding.. and almost sucks the fule up into the hose.. crazy I know.. but think of it like a straw in a soda can.. you can blow accross the top of it and it will suck soda up the straw... just a thought.
 

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'93 VN 750
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23 Posts
Discussion Starter #6
I know I keep bringing this one up.. but I missed it before..well acctualy 2 times now... the little hose that goes into the right ear(filter box).. insure that it is in the bow and that it is not bottemed out...if its is not in the box.. like mine was.. twice...it seems to get to much air while riding.. and almost sucks the fule up into the hose.. crazy I know.. but think of it like a straw in a soda can.. you can blow accross the top of it and it will suck soda up the straw... just a thought.
I do not have a hose going into my right ear box. Should there be? I'm guessing that was something stock, before it was coastered?? My vent hose on my right carb is about an inch or so long and is plugged.

Is there another hose that I need to be concerned about?

Also, my vent hose in the middle of the carbs from the "T" is connected under the tank to the black plastic tube on the box that appears to be a cover plate for the gas level sensor area. I tried routing it other ways (under the seat, etc..) but it caused more rough running problems and made things worse. I put it back to where it was when I got the bike.
 

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its the vent hose that goes to the carbs.. I dont belive this should be pluged.. there should be a T fitting between the carbs for the vent hose... sound like this might be missing or is redirected (I did this while doing some other work and forgot to put it back)...I however am not costered or have an ear shave... someone costered may need to step in here.
 

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Premium Member
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4,054 Posts
It sounds to me like there are some vacuum hoses/vent hoses crossed somewhere. I would try to get with someone in the area who has the 750 and see if you can identify the problem. There is a difference between vacuum hoses and vent hoses, and there should be a vent hose connecting to your right ear that lets the carb bowl stay at a nominal pressure. If that hose is free wheeling or is bottomed out or plugged, it will effect the way the carb fills with fuel and how the engine runs. I'd do a shout out to other riders in your area on here and see who's around...
 

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Also, my vent hose in the middle of the carbs from the "T" is connected under the tank to the black plastic tube on the box that appears to be a cover plate for the gas level sensor area. I tried routing it other ways (under the seat, etc..) but it caused more rough running problems and made things worse. I put it back to where it was when I got the bike.
That is the hose... sounds like it has been moved.. I think what is happeneing is that once you get up to speed it gets too much air and that causes your issue...try and move that hose back to the right ear.
 

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'93 VN 750
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23 Posts
Discussion Starter #10
what size is that hose?

That is the hose... sounds like it has been moved.. I think what is happeneing is that once you get up to speed it gets too much air and that causes your issue...try and move that hose back to the right ear.
I'll be sure to try this tonight, but I'm sure the hose is not long enough to go all the way over to the ear. Anyone know what size that hose is because I'll have to stop and buy one on my way home today? (5/16 or 1/4 or ???)

Why would it be bad to have that hose from the "T" going under the tank as opposed to going inside the right ear box?
 

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mine runs along the side of the breather box (big plastic in the way air box under the gas tank)and out to the right ear... not sure of the size.. but I would say 5/16 maybe... if the end of the hose is exposed then it will get to much air flow..most all of us have also cut the end of the hose at a 45 degree angle to keep from "bottoming out" the hose causeing it to become plugged...also not a good thing...carbs require the right amount of presure to work right... if yours is running under your tank and is paritaly blocked from the on comming wind then at that speed it would be fine.. but its when you get up to 60 or so that you start picking up more air and it causes the hose to chock out basicaly.
 

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'93 VN 750
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23 Posts
Discussion Starter #12
Thanks for all these helpful suggestions, but can anyone answer why, if its vent/vacuum hose related, does this problem ONLY occur when its really HOT outside???? I ride early mornings with no issues whatsoever, but in the afternoon it happens everyday for most of my ride home, which is all back roads at 60+ mph.

So please, can anyone shed some light on why a mis-routed vent hose would only be affected during HOT weather?
 

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... It happens as I'm leaving the gas station and the next day when I'm at 3/4 tank or lower. (I've read that POOGS is common around 1/2 tank)...

...This "almost-out-of-gas-sputtering" problem has NEVER happened when I'm riding early morning or late evening...
Gas stations normally store fuel in underground tanks. That means the temperature of the fuel is that of the ground (probably about 70 degrees in Florida) and will not fluctuate greatly from day to day. Once the gas is pumped into the bike's tank it will expand quickly as it absorbs the heat of the tank and surrounding air/sun. If not properly vented, the expanding gas could quickly pressurize the tank.


I don't know the effects of a pressurized VN750 gas tank, but niteriders post suggest that it is similar to POOGS. This might explain why it happens just after leaving the gas station, but does not explain why the event only happens during hot weather. Not since you have removed the diaphrams from the gas cap vent. Had they still been installed, then it might answer it (there are two air paths within the gas cap vent controlled by those diaphrams). However, if opening the gas cap solves the problem, then perhaps the gas cap vent or vent line is still not functioning properly or blocked.
 

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3,027 Posts
If the gas tank vent is the problem, opening the cap will solve it. Then you will know for sure. If it doesn't, thats not it. As for that hose that plugs into the right side air cleaner housing, it is nothing bit a carb float bowl vent. There are 2 of them, one from each carb, connected to a T fitting, then the one line is stuck in the back of the filter housing. However, mine is no longer hooked up this way. While removing all the emissions crap, I decided to clean things up as much as possible, I now have a separate several length hose on each carb, with the ends open, no T fitting, and no hose that plugs into the air filter housing. The bike runs fine. In fact, the only reason I can see for even needing hoses on the carb vents is to help keep dirt out of the carbs. On my dirt bike, I have a filter on that vent hose. On the Vulcan, I occasionally spray a little WD40 in the hoses, to catch any dirt that might get in there. These hoses only go to the float bowl, dirt cannot be sucked into the engine through them. Jerry.

If the tank vent is not it, I would suspect an air leak, insufficient supply of fuel to the carbs, or a problem in the carbs (or maybe just one) themselves. When I first bought my new '93, I had a similar problem. I finally traced it to the rear carb. Sometimes it would run way to rich, sometimes the rear cylinder would cut out completely. I removed the carbs, took the rear one apart, and found a piece of flashing on the float was rubbing against the float bowl, sometimes causing it to stick open, sometimes closed. After sanding it down and putting it back together, it ran fine ever after. Jerry.
 

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Thanks for all these helpful suggestions, but can anyone answer why, if its vent/vacuum hose related, does this problem ONLY occur when its really HOT outside???? I ride early mornings with no issues whatsoever, but in the afternoon it happens everyday for most of my ride home, which is all back roads at 60+ mph.

So please, can anyone shed some light on why a mis-routed vent hose would only be affected during HOT weather?
I'm really interested to see if anyone can answer this question. I've experienced the exact same symtoms when my bike is at operating temperature but only in hot weather, say 95 degrees or more. I've checked everything mentioned in this thread and then some.

It's not: the bowl vents or hose, floats, tank vent, tank pressure, spark issue, fuel flow, vacuum leak, air leak, jetting, lack of seafoam (amazingly!) or cables but there's a gremlin in there somewhere!

Could it be vapor locking? If so, how do you fix that? There's no other way to route the fuel lines.

The carbs have been removed and thoroughly cleaned. Like CDUB says, the bike runs perfectly until a it gets extra hot outside.

Put your thinking caps on people!
 

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'93 VN 750
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23 Posts
Discussion Starter #16
Well, here's an update on my situation now...
I spent some more time cleaning my tank vent line going thru the tank (not the rubber tube) with pipe cleaners and carb cleaner spray. There was still some dirt that came out on the pipe cleaners that spray alone wasn't getting. I've also changed the spark plugs. I've ridden over 200 miles and the "mystery" problem hasn't happened nearly as bad as before. I'm only getting slight hesitation from time to time. I keep a key in the tank and pop it open at the first sign of trouble ....and keep on ridin' !!!
 

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'93 VN 750
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23 Posts
Discussion Starter #17
Oh, btw IronMan750, I got a real good laugh at your list of "modifications" :D
Sounds like a sweet ride!!
 

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Well, here's an update on my situation now...
I spent some more time cleaning my tank vent line going thru the tank (not the rubber tube)[ with pipe cleaners and carb cleaner spray. There was still some dirt that came out on the pipe cleaners that spray alone wasn't getting. I've also changed the spark plugs. I've ridden over 200 miles and the "mystery" problem hasn't happened nearly as bad as before. I'm only getting slight hesitation from time to time. I keep a key in the tank and pop it open at the first sign of trouble ....and keep on ridin' !!!
If you still have the problem and opening the gas cap while riding solve it, then you still have blockage or a failed check valve within the gas cap. The entire air path from tank to vent tube exit must be free and functioning properly to prevent POOGS. You need to complete the cleaning process by flushing the rubber vent line and disassembling and cleaning the gas cap vent. Full instructions below;

I just went through this on my ’06 (non-CA model), exact same situation too (stall at ½ tank gas). Try this (again '06 non-CA model, other models and CA emissions may differ):

Step 1: Clean the gas cap vent tube:

1. Place a piece of paper in front of the REAR tire
2. Open the gas cap
3. Spray carb cleaner into the vent tube located on the tank, near the base of the gas cap hinge. Use the straw attachment for the spray can.
4. Observe the drips onto the piece of paper. Continue spraying until it runs clear.


Step 2: Clean the gas cap vent mechanism

1. Open the gas cap and remove the key.
2. Cover the tank inlet tube with tape or a piece of paper to prevent dropping things in the tank. I used a large PostIt note, which worked well.
3. Remove the two brass screws that hold the latch mechanism on the bottom of the gas cap.
4. Remove the latch mechanism. Note: there is a loose component in the base that covers the latch spring. Be careful not to drop it into the tank during removal. Inspect the latch, clean as necessary (chances are this is NOT where the problem is), then set aside.
5. Careful not to loose the 5 small springs mounted on posts under the latch mechanism. You can leave them where they are or remove them for safe keeping.
6. Before proceeding remove the small collar from the male latch connector (look for the round key cylinder). The collar acts like a bearing when opening and closing the latch using the key. It is loose and easily lost. Slip it off and put it aside for safe keeping.
7. Remove the two short screws at the base of the gas cap vent, but DO NOT yank the vent off yet.
8. The vent gasket is glued to the cap (at least it was on mine) and the vent assembly might be stuck to the gasket. GENTLY, wiggle the vent back and forth to loosen it and prevent damage to the gasket.
9. With the vent assembly removed, inspect the gasket for tears or bulges. Spray with carb cleaner (just a little) and wipe with a clean cloth. Inspect the cloth for signs of debris.
10. Now for the root of the problem; Remove the small round plastic check valve/liquid separator located on the underside of the vent assembly. GENTLY ply it out, careful not to damage the silicone valve covering the base of the check valve. Remove the gasket as well and place it aside. Inspect and clean the check valve, ESPECIALLY the tiny 90 degree elbow which leads to the vent hole. NOTE: there is another silicone valve cover permanently mounted in the gas cap vent itself (part of the pressure release part of the valve). DO NOT attempt to remove it or you will damage it. Just make sure it is clean and functioning.
11. Now check the vent hole. Hold the vent assembly up to the light and look up from where the plastic check valve was mounted. See any light where that tiny 90 degree elbow fits? Follow the vent channels on the opposite side of the vent assembly. Any blockage? Unplug the holes and clean the channels.
12. Reassemble.


Step 3: Repeat Step 1 frequently (at least once a year) to prevent this from happening again.

UPDATE 5/5/2010: My Phantom has been uncloaked! I took a closer look at where/how the rubber vent tube exits in front of the rear tire. I found that the tube had been (and still is) rubbing against the rear tire. The 'black fluffy carbon' blocking my vent was probably rubber shavings caused by the vent tube rubbing against the tire. I've temporarily rerouted the vent tube to exit along side the coolant overflow tube to eliminate the vent tube/tire rubbing. I still plan to flush the vent once a year to reduce the possibility of dirt and dust reaching and blocking the actual gas cap vent.​
 

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Hey Chris,
That was a nicely composed reply Dude! I went through my gas cap (out on the road) and of course I lost the little collar that you mentioned. It flew right off and landed another demention! The cap still works but not as smoothly as before. With your detailed proceedure, I'll go through my cap again but alot closer this time. Hopefully I can exterminate that Gremlin.

Oh, CDUB....I'm glad you liked my sig desctription! It's just part of my weird sense of humor I guess. Your "Sounds like a sweet ride" response made me laugh too! The way I see it, if you're not selling it..........BTW, was it really hot outside on your trip?
 

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'93 VN 750
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23 Posts
Discussion Starter #20
Oh, CDUB....I'm glad you liked my sig desctription! It's just part of my weird sense of humor I guess. Your "Sounds like a sweet ride" response made me laugh too! The way I see it, if you're not selling it..........BTW, was it really hot outside on your trip?
To answer your question, YES! It's been hot out. I've been riding in 95+ degree weather (heat index of 105+) here lately. Even our "cool" mornings have been 80-85.

So, I've taken the advice others have mentioned, if popping the gas cap seems to help then I would really focus on cleaning those lines and taking the cap apart for a real good cleaning. Even try to push wires through the holes to open them up larger (which is what I did). :smiley_th
 
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