Hi,

My 1980 FXS (recently bought in the USA ) suddenly stopped running as I was stopping at a crossroads.
I had checked:
. sparkplugs OK, although I find the sparks a little weak compared with these of my other bike (BMW)
. ignition point OK, the current gap is 0.4 mm , is it correct ?

I have dismantled the - dual Mikuni - carb bowls.
They are clean and there is fuel in them but I am not sure the fuel is injected in the cylinders for I don't find any on the sparkplugs.

Neither does the bike smells gas at all even after many starting attempts.

I have tried to start the bike by sparying "Start Pilote" in the carbs but it does not work.

Could the problem come from the condenser or the coil, how do you check them without dismantling (if possible) ?

I am attaching a picture of the Mikuni carbs.

Any ideas ?

Regards

Bernard from Paris - France

 

Make sure you have good gas flow from your tank to the carbs.
My experience with a bad condenser is it will at least pop and run but very rough so as to make it unrideable. A bad coil will definately make it quit running but if you have spark it's probably OK. You can do an electrical check on it using a volt/ohm meter. Measuring between the top and bottom terminals of the coil (with wires disconnected) you should have 4.7 - 5.7 ohms. Measuring from either top or bottom terminal to one of the spark plug terminals should be 16K - 20K ohms.
Your point gap is .018". I dont know how many mm that is. A good rule of thumb in an emergency is the width of a match book cover!
I know nothing about Mikuni carbs so I cant help there..
Good luck!!!
jim
I just thought of someone who may be able to help with your Mikuni setup. The first poster on this thread link has a similar setup. PM him and he may be able to help!
https://www.hdforums.com/forum/shove...ce-myself.html

 

Well .4mm is about the same as .016 so you need to increase the point gap.

Usually coils will give you some notice of "I am going to fail" with intermittent miss prior to crashing. If you have an extra it is an easy switch to determine if the coil is the problem.

If your condenser say "Hecho de Mexico" you need to change it out. Not sure who is making them but people have had problems with Accel components as well (coil and condenser). I still have a couple of Delco-Remy condensers that I keep for my pans.

As far as gas, pull the feed line to the carbs and open the petcock; you have gas coming out it is not a problem withe the petcock or tanks. If no gas is flowing you could have a problem with the petcock or tanks being filled with $hit and blocking the flow (vent line, gas caps or sludge in tanks). Do you have a fuel filter installed? When was the last time you cleaned it?

You would think that with dual carbs at least one would fire but give you a hard miss, so I am not thinking a carb problem.

K...just saw in you pic that you have a fuel filter. Disconnect before the fuel filter and turn on petcock. No gas...probelm is in the tank. Disconnect after the filter and turn on petcock. No gas...probelm is in the filter.

 

Hi,

The fuel filter is OK and the gas does arrive at the carbs but it does not seem to get to the cylinders.
I have sprayed "Start Pilote" (strong gas substitute) in the cabrs but nothing happened.
I have also put new sparkplugs, new condenser, and reset the points gap (0.40mm)
I could only make the engine run for a few seconds by pouring a small amount of gas directly in the cylinder.
Strangely enough, the bike does not smell gas at all even if you try to start it for a long time with full throtle, which is the case with all my other bikes fitted with carbs.

 

It was running, and all of a sudden, at a crossroads it died and would not start. You didn't say what it was running like up to the point at which it died. Sounds electrical to me.
I would start at the kill switch on the handlebar (if it still has one) and work my way through the electrical breakers to the points and coils. You should have "power in" and "Power out" at each point of the inspection.
One more thing though. Check to see if your gas cap has caused a vapor lock in the tank. Just loosen it up, give it a minute and see if the bike starts.

 

So checked the line from the tank to the filter and from the filter to the carb. At some point the line separates to feed each carb. Check that next.

If that check s out then...

At the bottom of the float bowl there is a main jet plug and washer. If there is gas coming into the float bowls gas will come out of each hole.

If that checks out then you should probably check out your float levels. That will require you to remove the carbs from the bike because you have to flip them upside down to measure correctly.

And I am not sure how old the carbs are but my Mikuni book is from 1975 so I will throw this out FWIW...

There were two types of floats used on the older Mikuni carbs...

an independent type that was made with a non-corrosive plastic compound. These were standard in 24-44mm spigot and flange type carbs. Each of the two floats rise and fall independently. This type of float system maintains a constant fuel level in the float bowl during accelration and deceleration.

a twin float type that was a brass composition. These were standard in 18-24mm flange and clamp on carbs. The floats do not act independently.

What I am thinking is that if you twist the throttle and get no gas (if you have the air cleaner off you should be able to see gas being squirted into the cylinders) then either the jets are fowled or you have a problem with the floats. I know that the old Linkert brass floats are/were/still are $****. They tend to leak at the seams, fill with gas and sit on the bottom of the bowl. If that is occurring you will not get gas to the cylinders.

Another thought...you are sure that the throttle is engaging each throttle valve slide when you twist the grip?

 

Hi,

Thanks very much for kind assistance and the time you are spending trying to help me.
I will check and work on all this tomorrow (it's almost 10pm here in Paris) and I will keep you informed.


Regards

Bernard

 

Thanks, best of luck because that is way too nice to be sitting, especially with riding season approaching.

 

Hi,

I have double checked everything and the bike still won't start
even when I put (brand new) gas directly into the carbs or even into the cylinders.
I now suspect two last possibilities:
. a weak spark at the sparkplugs due to a weak (32year old) coil
- I will try with a new coil tomorrow
. stuck amission valve(s) that stop the gas from getting into the
cylinder(s) although I find this very unlikely on a 4500 original miles bike always driven very carefully...

Regards

Bernard

 

HI,

I have put a brand new coil and although the spark seems to bea little better at the sparkplugs the bike still won't start
(It actually did start very briefly after I put some gas directly in the cylinders through the carbs).

I now suspect a fuel admission problem again,
since the gas that's in the bowls does not seem to get up and be sent to any of the cylinders.

As I had already dismantled the Mikuni carbs in the first place,
found nothing strange, and cleaned every orifice with compressed air.
And as I have put new sparkplugs, new coil, new condenser,

I don't know what else to do or where to look...

Any suggestion welcome !

Bernard