R75/5

[vtmando]

Just found and purchased a 73 R75/5 in great shape. Owner said they did a tune up but it runs rough and once warm it will not idle. I pulled the plugs and see that it's running very rich. I don't want to mess with the carb's just yet.

Any advice on where to begin getting this straightened out?

[Bob_Roller]

Sounds like the carbs need to be gone through .
The foam floats tend to absorb fuel over time and don't float like they should .
Check the float level, float needle rubber tips for distortion from sitting on the seat .
Make sure the jets and passages are clear, the idle jet is prone to getting clogged and it is difficult to tell whether they are open or not .
The carbs are not that difficult to work on .
A carb synchronization should be performed as well, after working on the carbs .

[Einar]

Congrats on the purchase! 73 R75/5 is one of my dream bikes.

Have a look at the post "Advise on carburetor rebuild" to give you a few ideas, though I expect you don't have Bing carburetors on yours.

My bike was running rich when I bought it as well, a very simple fuel mix adjust can go a long way. Here's how I do it:

1. Fire up the bike and get it idling at a constant RPM. (~1500)
2. While idling; locate the fuel mix screw on one of the carburetors and slowly turn it either in or out. Listen for increase/decrease in RPM. Find the spot where you are at max RPM
3. Do the same for the 2nd carbu.
4. Adjust the idle screws to run between 900-1100 RPM on a warm engine. Try to make sure that you have equalized the contribution from each cylinders as much as possible.

** If you find that the max RPM is when the screw is all the way in, then you will need to open up the carbu to properly sort out the mix issue. **

Check out this link from Chris Harris, it's pretty good.
https://www.youtube.com/watch?v=a_HyX1qLPbQ

If I remember correctly Chris Harris disconnects the HT lead from the spark plug hence running only 1 cylinder at the time. This is probably a more accurate way of doing it, however I had positive response on the spark plugs doing it with both cylinders running as well. They now run on a dry brown-ish colour, which I have been told is acceptable.

[jjwithers]

Don't do the shorted spark plug method if your bike has an electronic ignition.  It may fry it.  It only works if you have points still.  

[Einar]

@jjwithers: thank you for that very important piece of information!

[montmil]

Carb adjustments and fine tuning require the engine to be at normal operating temperatures. To warm the engine properly requires a ride of 20 minutes minimum. More depending on ambient temperatures. Attempting adjustments soon after engine start will produce potential unwanted results after the engine has been properly warmed.

[vtmando]

Thanks for the advice everyone.
I've continued with my work on the bike. I've made some slight adjustment to the carbs but with more testing I've found that I have a compression difference between heads. Roughly 100psi on left and close to  :-/120 on right...most of what I've read suggest no way to adjust carbs with a compression problem.
Question...is this amount of difference significant enough to cause trouble?

[Barry]

Assuming you did the compression test either with the carbs off or with the slides raised and the throttle open then then readings are not good. Anything below 120psi is considered poor and the 20 psi difference is a little too much.

Unless I missed it valve clearances didn't get mentioned. A closed up exhaust valve could produce your symptoms.

[Bob_W]

After adjusting valves, checking float levels but before anything else check air filter. Critters have been known to reduce airflow with various objects.
Carb adjustment screw sets mixture at idle and has minimal effect at operating speeds.
Not only must carbs be removed but engine must be at operating temps to check compression.
Bob

[montmil]

... Not only must carbs be removed but engine must be at operating temps to check compression.
Bob

The point being that the air flow into the cylinder's intake tract must be unobstructed.

Twisting the throttle grip with CV carbs will open the carb's butterfly disc but will not lift the slides. As such, you'll get a false compression reading. Should your 75 slash 5 be fitted with different carbs, you may be able to lift the slides via the twist grip. Confirm prior to setting up your test area.

As mentioned, you'll also want the spark plugs removed but they must be grounded to the cylinder or head if you have an electronic ignition system. A simple woodworking clamp -those with rubber caps on the feet- works just fine to secure both plugs to the jugs.

[Bob_Roller]

How accurate is you compression gauge