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2006 Honda Shadow Spirit VT750DC
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Discussion Starter · #1 ·
06 spirit 750.

When I tore my carb apart last week, I had a hose that seemingly came from nowhere. No clamps or clamp marks on the hose. I pretty much thought it was useless, but I think I found where it goes. I am just asking to make sure.

On each carb there is a small port near where the choke screws in. What is this port for? I believe my mystery hose connects both of these ports together based on the various pictures I took during removal, but like I said I want to double check before I do anything. The curvature of the hose also leads me to believe it goes here.

I never paid attention to it because all the instructions I followed mentioned nothing about this.

A few pictures of the hose and port.
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1983 Honda vt750 Shadow
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Put a piece of hose on it and blow into it to see if it is open inside.
It may be a port for a different bike- maybe a Calif emission port?
 
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Discussion Starter · #3 ·
I'll try that tomorrow, but that's what I'm starting to think. The first picture I took of the carb on my bench, I can see the hose in question just hanging there and as far as I know I never touched it.

A lot of videos I am looking at don't even have a hose anywhere near there also lol.
 

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If you are talking about the hose being touched in the first picture, it doesn't appear to be connected to the carb at all in your pictures, or anything at all for that matter.

Could be a breather line from the crankcase breather to the air box? Such a hose can do its job with no clamps.

Your situation demonstrates why it's important to document in DETAIL. I used to draw little pictures and stick them in the ziplock bags with the parts, but now I take lots of digital pictures as I disassemble. They just sit on a thumbnail drive to get deleted later, but sometimes are very valuable.

The latest and greatest method is to have a couple of video cameras pointed at the task at hand. Then you can play back the video later and see just what you did when you were thinking about something else. There's always that one part that comes out attached to something else, drops off quietly when you aren't looking, and rolls away.
 

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Discussion Starter · #5 ·
Trust me, anything I physically disconnect or remove gets a picture taken. That's why I have no idea about this because I don't believe I even touched the hose during disassembly.


Edit: And yes the hose in the first picture is the one in question. Once I got the carb out and on the bench I took a picture and the hose was just hanging there. I'll find and post that picture here in a second.
 

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Discussion Starter · #6 ·
This picture is literally carburetor out of the bike, put on the bench and a picture taken. I believe the mystery hose is on the LH side between the choke cable and under the throttle cable wheel.


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Discussion Starter · #8 ·
I will double check tomorrow. Didn't get to the bike today cuz it was pretty humid and crappy to be hanging out in a garage for a while.

I do believe the hose in the back of the air cleaner box (the one that has hoses going to the carbs) is where it is supposed to be. I know for a fact that hose either was disconnected already, or became disconnected when I started everything because I took a picture of it. I also have not reinstalled any of the air cleaner stuff because I cleaned them yesterday and had to apply some sealant to part of it.

I do appreciate the suggestions though. I will go over the steps I followed and see if I missed anything.
 

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1999 Shadow 750 ACE VT750CD3 Modified
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Go here to get the Service Manual:
Look at the General Information - Wire and Hose Routing and also the Fuel System section.
California versions have different routing and gismos so check them out if it started out as a California spec. bike or got the carbs from one.
49 state bikes have the bowl vents run to a T and a hose that just hangs down to open air and AFAIK no open ports by the mixture screw, are they drilled through or blank inside.
 

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Could it be the tank vent hose and just got pulled off with the rest?

The nipple in the 3rd pic looks like one of the float bowl drains
 

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Discussion Starter · #11 ·
It's not the tank vent I know I hooked that up lol.

I plan on taking the carb back off tomorrow anyways, but I will take pictures again with carb removed and post. Maybe that might help.
 

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Discussion Starter · #12 ·
I pulled the carbs to double check a few things, and I connected the hose between the two ports near where my choke cable threads in. I hooked the hose up to each port and blew into them, and they were open, so I figured why not. I had also seen a TJ brutal customs video where he was talking about removing the EVAP system and had plugs on what looked like these ports. They go to the throttle plate, which I will include a picture of.

I was able to get the bike started much easier, still with choke, and eventually have it running with the choke off. Then I could start it with no choke, so I am heavily leaning towards these ports needing the hose or plugged for whatever reason. I cannot find anything that really says what they are, but I am guessing with them not plugged, more air is getting into the carb which isn't supposed to be.

Here is a picture of the ports in question, and the hole the air was coming out of. I appreciate everyone's input! Thank you!

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Manifold vacuum port
Vacuum sources for California emissions evap diaphragm valves , I see photos of the carb body in the Service manual both with and without that hose fitting.
IIRC that is the same place the air-cut valve gets vacuum in the carb body.
If it isn't a California emissions bike I'd just cap the fittings and call it good.
 

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Manifold vacuum port
Vacuum sources for California emissions evap diaphragm valves , I see photos of the carb body in the Service manual both with and without that hose fitting.
IIRC that is the same place the air-cut valve gets vacuum in the carb body.
If it isn't a California emissions bike I'd just cap the fittings and call it good.
Good call gentlemen!

Here's a write-up of somebody who wanted to remove the CA emissions stuff from their '98 750 ACE. California Emissions Removal – VT750DC.com There's a little discussion and some diagrams there. Honda calls the vacuum hose that powers the valve, #10, and it doesn't show if it is sourced from one carb or teed to both. My first instinct would be that crossing over the intake ends of the carbs of dual carbs would be a bad thing. Only one carb is sucking at a time, wouldn't mixture be motivated to go first one way, then the other through a crossover hose?

Unfortunately the image at the link that shows which ports the author capped is missing.

My VLX 600 is a CA bike, and I think that there is a fault in the vent emissions collection system that is causing the bike to run rich, but I'll investigate that further when I get some spare time to pull the tank and fool with it. I can look at how my 600 is plumbed at that time.
 

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The evap system is to keep hydrocarbon fumes out of the atmosphere.
So the gas tank and the carbs are vented into the charcoal canister, that stores the fumes, and then when the engine is running, valves open to draw the fumes back into the intake to be burned off.
It is OUTSIDE all the engine mixtures sytems.
So doesn't cause engine problems unless a vac hose leaks , the carbs overflow, a control valve goes bad, etc.
But it can be complicated to find which valve is defective also. The manual shows tests on them.
 

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So the gas tank and the carbs are vented into the charcoal canister, that stores the fumes, and then when the engine is running, valves open to draw the fumes back into the intake to be burned off.
It is OUTSIDE all the engine mixtures systems.
It appears to me that if the control valve diaphram is damaged, the intake manifold vacuum line could be sucking from the bowl vent line. With the vibration inherent in a moto, I would think this would include some amount of raw fuel as well as vapor direct to the intake port.

Other failure modes could have this same intake port sucking from the tank vent line, or sucking direct from the atmosphere if the purge valve is defective. I would think any of these faults would affect mixture, same as running with the choke on, or with a vacuum leak.

I'll investigate more and report once I get to the bottom of the issue. All I'm sure of at this point is that after clearing the usual suspects, and re-jetting to stock, my CA VLX continues to run noticeably rich and fuel economy is bad, high 30s for a machine that should be yielding almost 50 mpg.
 

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Discussion Starter · #20 ·
Good call gentlemen!

Here's a write-up of somebody who wanted to remove the CA emissions stuff from their '98 750 ACE. California Emissions Removal – VT750DC.com There's a little discussion and some diagrams there. Honda calls the vacuum hose that powers the valve, #10, and it doesn't show if it is sourced from one carb or teed to both. My first instinct would be that crossing over the intake ends of the carbs of dual carbs would be a bad thing. Only one carb is sucking at a time, wouldn't mixture be motivated to go first one way, then the other through a crossover hose?

Unfortunately the image at the link that shows which ports the author capped is missing.

My VLX 600 is a CA bike, and I think that there is a fault in the vent emissions collection system that is causing the bike to run rich, but I'll investigate that further when I get some spare time to pull the tank and fool with it. I can look at how my 600 is plumbed at that time.
Third or fourth picture down, the ports in question are pointed out and it says to cap them. The picture is clear as day. Makes me wonder if this hose was originally there or not still. I think I like the idea of not having them connected as mentioned in an above post. I received some caps in my rebuild kit. I will remove the hose and put caps on tomorrow and see what that does. Excellent link, thank you!
 
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