Topic: Throttle Opener Problem

[clintcd]

Hi,
I have a problem with the throttle opener on my 1977 CB750A. After the engine has warmed up if I select "L" gear with the bike in a stationary position the throttle opener activates and the engine begins to rev very hard. To stop it I have to either kill the engine or change back to neutral. Before the engine warms up this does not happen. Can anyone suggest how to troubleshoot this. I suspect that there must be something wrong with the speed sensor bit I am guessing. Is there any test for this.

Thank you.

[raymond10078]

Sounds like nothing more than an adjustment problem.

The throttle opener is supposed to work at a stop - and up to 20 MPH or so, then it shuts off.  It's function is to open the throttle just a bit so the engine doesn't lug/bog/die when the transmission is in gear and the bike is stopped.

Look in the service manual - chapter 2 - and then adjust the correct screw with the bike in gear and the brakes applied; or uncrew it some (with the bike off, or in neutral), and do a "guess and check".

I don't quite understand the only when hot statement - since when the bike is on the choke, the idle speed is raised slightly there, too.  Regardless, give the adjustment a try.

[salukispeed]

I 'm with kandratech. It sounds like the adjustment screw is in to far and when you put it in drive or lo it is too much. the screw comes with a plastic cover over it and is accesed from the right side and the screwhead is facing  towards the top of the aircleaner. It is a little tricky to get the screw driver on it. Sometimes raising the back of the tank helps. The book has a decient picture. Are you trying it on the center stand or on the ground. If I run mine on the stand it revs pretty fast and the rear tire spins  to fast for my comfort. It should be adjusted so you get close to the same rpm both in and out of gear with the wheel stopped with the brake to just compensate for the added load of the converter.

[clintcd]

Hi

Thank you for the information, this was very useful, I now have the throttle opener under control but I still have a problem with it not activating until the bike is warm. I now suspect that the contacts in the speed warning unit may be at fault so I have ordered a replacement on eBay. Hopefully this will solve the problem.

Many thanks

[clintcd]

I have set the various components up on the bench and tested them, the change relay is fine but I has a problem with the speed warning unit.

I bought a replacement speed warning unit on Ebay and this sorted out my problem. I then bench tested the old one with a change relay and a power supply. When cold the unit would not work but when I put it under a lamp for a few minutes it would work fine until I let it cool down again. I then decided to investigate the contents of the unit and was surprised to find a small circuit buard with a transistor, capacitors resistors etc.. I am now fairly certain that the problem with my unit was a "dry" solder joint which would explain the dependance on temperature but the real surprise was to find how sophisitcated the warning unit is internally, it is not a simple relay as indicated in the drawings.

I think the reason for the transistor circuit might be to prenent unwanted operation of the throttle opener if the speed is going up and down at about the threshold. If anyone has an accurate description of the speed warning unit circuit I would love to hear from you,

[raymond10078]

Here's what I believe.

When I took apart my speedometer, there is a type of sensor - maybe hall effect - don't know really what kind it is - but it is clearly not a switch.  My point is that the output of this sensor is routed to the speed warning unit - which using the electronics - changes an output used by other parts on the bike - eventually turning on/off the solenoid.

I am disappointed that the manual is misleading for this component . . . .

[raymond10078]

Oh - forgot to add . . . .

The 76 CB750A doesn't have a speed warning unit.

Once again - what I believe - the speed warning unit "shuts off" the idle speed increase solenoid above 17 mph or so.  What is gained?  I only have two guesses - more engine braking; and possibly something to do with emissions.

I personally think it has to do with engine braking.  In reading several of the magazine reviews of the first CB750A's, it appears that Honda made a number of changes for the 77 model.  I recall reading about how some didn't like the lack of engine braking.  But - as I said, it is just a guess.

I had posted a question on this forum some time back, asking if anybody had ridden a 76 and then a 77/78 - asking about engine braking.  I got no answers.

If my speed warning unit decided to die on me, I'd probably just bypass it - converting the system to a 76 type.

[salukispeed]

I will look at my book and bike later to better understand this system but I thought the speed warning unit was in the spedometer and this was where the signal came from to activate the change relay and then the vacuum solenoid. Is the speed warning unit replaceable  seperate from the spedometer? As I am starting to occasionally have the same issue until the bike warms up.
Thanks. 

[raymond10078]

The speed-warning unit is located under the tank.

Here's a pic of one.

[salukispeed]

Thanks. I really need to look more into the system and better understand the set up. The whole thing is triggered by the sensor/switch in the spedometer at 17-20 mph. I have put almost 500 miles on the old beast this week with no issues besides the lack of the throttle opener when cold.

 It sure runs and drives good and I like riding the ol girl but also wish it was a little better looking. Looks better as the day turns to evening and night. A nice front and rear fender would make all the difference but seem to be hard to find for reasonable price. I may look at getting them re chromed. Does anyone know if a 7 inch Hella replacment lamp with a standard H4 bulb will fit in the headlight housing. It fits in my 74 K4 but the housing does not look as deep on the 77A

[salukispeed]

I finally got around to working on the speed warning problem last night. Mine also worked when warm sometimes but rarely if cold. I removed the unit from under the fuel tank and pealed the aluminum housing from the unit only to find it completely filled with potting epoxy. Definitely not an easy fix. I was able to carefully  chip away the epoxy and get the circuit board exposed. I hooked it back up and ran the bike while poking and wiggling the board  until it stopped working and I also found two very shabby solder joints. I cleaned and re-soldered the joints and it seems to work properly. I re-potted it in 5 minute epoxy and went for a ride. So far so good. Thank you for the bad solder joint  tip and the rest of the input from all. I will test it New years day on a short ride to bring in the new year.
Have a safe and happy new year to all