Topic: Missing the Vacuum Solenoid and Checkvalve

[saavejo]

Hello Hondamatic 750a owners! I have not posted in a while but I have been riding my bike with no issues.  Thanks for all your help!

I own a 1978 and I had to remove the fuel tank this weekend and to my surprise I found out that my bike is missing the vacuum valve solenoid and the check valve that is supposed to be mounted to the carbs.  The bike runs great and idles fine.  The vacuum hoses current configuration w/o the solenoid and check valve  is as follows:

Hose connected to the carb is plugged
Hose connected to the Vacuum advanced is open to the outside air
Hose connected to the air cleaner is plugged

The service manual indicate that the normal operation of the solenoid valve is air flow from a to c which is equivalent to airflow from the air cleaner filter to the vacuum advanced valve.  And is my understanding that airflow vacuum to the carb will open when speed is above 20km. Why is this needed?  The vacuum airflow to the carb is plugged and I have not noticed any issues!

I am getting about 40 miles per gallon and the bike is running great and it idles fine on gear.  They only thing I noticed that after the engine is warm and I put the bike on neutral gear the idle speed is high.  These parts are no longer available new...some use ones on ebay but the condition is unknown.

Any ideas and/or suggestions?  Thanks

[raymond10078]

The manual isn't that clear (or is just plain wrong) - but the solenoid should operate as follows (referencing page 13-8 of the Shop Manual).

Solenoid OFF: A - B closed; B - C closed; A - C open.
Solenoid ON: A - B open; B - C closed; A - C closed.

On the 77/78 A bikes, the solenoid is energized when the bike is going below 20 km/h.  The 76 A bikes - the solenoid is energized anytime the bike is in gear.

The purpose of this system is to prevent exactly what you are experiencing - that is - to provide a stable idle regardless of the gear position (neutral, low or high).  On your bike, the idle has been set high - so that when the bike is put into gear, the idle remains high enough to keep running.  The downside is that a "lurch" that happens when putting the bike in gear.  The system allows a correctly tuned bike to be put into gear without much (or any) lurch at all.

In the end, you don't really need this system, it is a convenience feature.

[saavejo]

Thanks for your excellent technical reply!  That explains why when I first starts the bike and shift from neutral to 1 or 2 it jerks. When shifting from 1 to 2 when riding its fine or less noticeable.  I can go ahead and hook up the vacuum lines in either configuration, energized or de-energized)What you think is the best configuration for not having the solenoid?  What are the pros and cons for each configuration?  Thank you for your analysis and recommendations, I appreciate it!
Thank you,
Jose

[raymond10078]

If you want to get rid of that lurch, then buy a solenoid.  They are often on ebay used.  Don't forget the check valve either.

Before you start buying parts - make sure that the diaphragm is still is good shape.  With a hose, you should be able to suck on it and see if it will hold.  If you have a vacuum set - all the better.

I've corresponded with another owner who sets the idle in neutral, and just gives it a little gas to keep it running in gear.

To each his own!

[saavejo]

Thanks for the quick response.  I found 4 solenoids/checkvalves on ebay however they are attached to the carbs and the ebayer won't separate from carbs!  I will check the diaphragm and continue to look for solenoid and checkvalve.  I reviewed the manual chapter 13 and would like to discuss it with you to make sure I understand the vacuum flow;

The vacuum is pulled from the aircleaner (c)
When solenoid de-energized vacuum is pulled from air cleaner to diaphragm (what does the diaphragm do at this point?)
When solenoid is energized vacuum is pulled from the aircleaner to the carb (what does the vacuum at the carb effect is at this point?

I can probably find an automotive check valve but I am not sure about the solenoid.

Thank you very much!

[raymond10078]

The vacuum is pulled from one of the carbs. I recall it is from carb number 2.

Look at pages 2-12 and 2-13 from the Shop Manual.  It should become clear after that.

The line to the air cleaner is a vent line.  When the solenoid is turned off, clean air from the air cleaner gets sucked into the diaphragm.  When the solenoid is turned on, that "clean air" gets pulled into the carb (that's why the air needs to be clean).

[saavejo]

Hi Sam,
Thanks for the explanation! Without the solenoid what will be the best way The vacuum lines? The current config of my bike is as follow?

Carb #2 vacuum hose is Capped (closed) not connected
Diaphragm  vacuum hose is open to ambient air, not connected
Air Cleaner vacuum hose open to ambient air, not connected

I want to try to improve the system even that I am missing the solenoid valve and check valve.
What do you think is the best approach given these circumstances?  Leave the hoses as they are or: (1) Connect the Diaphragm to the aircleaner which is equivalent to a de-energize solenoid or (2) Connect Diaphragm to carb with check valve which is equivalent to an energized solenoid.  With options (1) or (2), what will be the correct or best position (i.e. N, 1, 2) to set the iddle?

Thank you very much for your expertise!
Jose

[saavejo]

I am sorry I mistake your name.  Thank you KandrTech!

[raymond10078]

Without a solenoid, your existing setup - open tubes and all, is the way I'd set it up.

See page 13-2 of the Shop Manual - it shows which pipe goes where.  Your number two carb has a port - that is capped somewhere.

In summary, what you have is working for you now.  Unless you get a solenoid/check valve and wire it up - I wouldn't change a thing.

[saavejo]

KandrTech,

Right know my configuration is equivalent to a de-energized solenoid and I get the lurch when shifting from neutral to a gear and when shifting from a gear to neutral the idle goes higher in neutral.  I assume if I open the carb input to the diaphragm which is equivalent to an energized solenoid then I will have to give it a little throtle when stoped in gear to keep the iddle up and kepp the bike from stalling.  I will try both configurations and use the best until I find a solenoid.  Any body that has a spare solenoid valve that would like to sell it or trade for other parts?  Thank you and best regards!

[raymond10078]

Yes, I understand your current configuration.  I assume that someone has adjusted the normal (i.e., "not in gear") idle stop higher than it should be.  If you apply constant vacuum to the diaphragm, it won't change how the bike behaves.

Normally, the normal idle adjustment controls the in neutral idle speed.  This is adjusted by means of the large idle screw mounted on/around carb 4 (the right carb when sitting on the bike).  This screw can be reached when riding - I've made adjustments on the fly myself.  Anyway - getting back to the explanation - when the bike is put in gear, then the diaphragm has vacuum applied - to open the throttle more than the in neutral setting.

If your goal is to reduce the "lurch", then after the bike is warmed up, adjust the idle screw until you get a slower idle.  If you don't have a tach of some kind, you'll likely just have to adjust it up/down until you find a setting that you like.  Too low of an idle speed will cause primary chain slapping - which is pretty noisy - and can sound like something is wrong.

Once you adjust the in neutral idle speed, you will have to give the bike a little gas while in gear - as you describe.

[saavejo]

KandrTech,

Great explanation, Thanks for the guidance, I appreciatte it!  I will try this weekend and will post the results.  It is a litte cold in FLorida righ now but it should warm up a little.  Best regards 

[xntrik]

I remember reading SOMEWHERE years ago -when I first bought my bike and mistakingly thought that solenoid was missing- that a "fix" back in the day was to use some type of solenoid off of an old dodge/Plymouth K-car.....


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[xntrik]

10 seconds on google brings you right back home... Here it is http://archives.sohc4.net/index.php?topic=8646.0


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[xntrik]

Link he refers to having info on solenoids in that post was moved to...... http://minimopar.net/solenoids.html        Voila


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