High idle R75/7

[Rohls]

So I almost have brought my stock R75/7 up to running condition again; rebuilt the carbs (not the diaphragms though...cleaned all the passages, new gaskets and o-rings)set the valves, installed Dyna ignition, fluids, cables are slack and pretty much adjusted at the moment...etc...

Cranked her over, and I can't seem to get her to idle correctly no matter what I try...the idle is really high, like 2500rpm...so i don't leave it on too long in between tinkering.

Some questions:

What groove should the needle be at? (Clymer manual says 3rd from top...) I moved it up or down and it didn't seem to do much.

How can I really tell if my diaphragms are bad? They are not ripped or torn and still are flexible.

What is the float height? I couldn't find a real answer in the manual...unless I skipped over something.

Any glaring things I may be overlooking?

Could this be a vacuum leak? If so, how can I discover the problem area? Carb cleaner?

Any help is much appreciated.

Regards,

Nick

[SteveD]

You don't say if the idle has been adjusted after a decent warm up ride, eg 20 minutes/miles?
There has been a little discussion over the years re: the high idle. Sticking advance gets a mention. Duane has a page I believe.

How can I really tell if my diaphragms are bad? They are not ripped or torn and still are flexible.
Sound ok. Other than removal and stretch to look for holes, I'm unsure. Holed diaphragms usually cause problems at higher rpm, eg trying to go faster at 130kph, but can't.

What is the float height? I couldn't find a real answer in the manual...unless I skipped over something.


Any glaring things I may be overlooking?
The warm up?

Could this be a vacuum leak? If so, how can I discover the problem area? Carb cleaner?
Usual culprit...inlet stub loose at the head.

[vanzen]

If this were the usual "won't return to idle after warmed" problem then SteveD's post would apply.
If the MC cannot be tuned to idle correctly from the get-go ... another problem is in effect.

Assuming correct ignition tuning and valve adjustment ...

Any and all of the usual idle system maladies are suspect:
fuel delivery issues from tank to petcock to carb,
incorrectly sized or blocked idle jets,
blocked idle circuit passages in the carb body,
malfunctioning or incorrectly installed enrichener circuits,
and /or air (vacuum) leaks anywhere from head to carb including the throttle shaft.

Needle position will not affect idle tuning,
but rather would be noticed during transition to the mid-circuit.

Easy float check:
Remove a float bowl, hold the float up with a finger, open petcock,
with your finger, lower the float until gas dribbles out -
at that place where gas begins to flow,
the float should be parallel to the machined surface where the float bowl attaches.

[Rohls]

Thanks guys.

So i can't really warm it up. The bike has been sitting for 5 years and I'm just getting 'er up and running again. Fuel delivery to the carb seems okay; petcocks function, though what was left of the gas may be a little stale ( I added 1 gallon of fresh fuel).

Another thing I forgot was that the bike will only really start on full choke...

I did rebuild the carbs; new float needle, gaskets, orings...cleaned out the jets. Maybe it's worth another cleaning?

This is my first time with electronic ignition, but it seems to ignite alright...could this be the culprit? The static timing is dead on according to the instructions.

Valves are set...

Does float height really play that big of a role? (One float needle was bad; carb leaking...both were replaced)

Should I set the needle to the 3rd notch down since it may not control this circuit?

Any leak down test I could preform on the carbs?

many thanks,

Nick

[robert]

If the throttle butterflies were removed during carb rebuild, you may want to look to see if they seat properly.

[Rohls]

Thanks Robert, I'll do that. I thought they set alright, but not 100%.

[Garnet]

Did you make sure the butterflies went back in the correct way around and the right way up? I think they have a dot on the bottom of the engine side, but you should look that up to make sure. It is a very fussy job makeing sure that they are seated properly on the shafts. Hold the carb up to a bright light and make sure no light is visable around the closed butterflies. A tiny bit of light will let a lot of air through, which will make it hard to start cold as well as idle high.

[Major Softie]

And, if it's not that, then odds are there's air getting in some other way.

[vanzen]

So i can't really warm it up. The bike has been sitting for 5 years and I'm just getting 'er up and running again. Fuel delivery to the carb seems okay; petcocks function, though what was left of the gas may be a little stale ( I added 1 gallon of fresh fuel).

Start with 100% fresh fuel.

Another thing I forgot was that the bike will only really start on full choke...

My first thoughts would be that either the idle circuit is not delivering fuel or air is leaking into the system.
A popular place to leak air on an old airhead that has been sitting
will be the rubber tube that connects the carb & intake stub.

I did rebuild the carbs; new float needle, gaskets, orings...cleaned out the jets. Maybe it's worth another cleaning?

Absolutely yes.

This is my first time with electronic ignition, but it seems to ignite alright...could this be the culprit? The static timing is dead on according to the instructions.

Easy enough to check the plugs and verify IF they are firing, eh ?
If static timing is correct the MC should idle. If the advance is stuck, worn, or malfunctioning,
the bike will idle fine but then run poorly as throttle is applied and engine speed is raised @ 3k RPM.

Does float height really play that big of a role?

Float height helps determine idle mixture.

Should I set the needle to the 3rd notch down since it may not control this circuit?

The MC should idle regardless – adjust transition when you get there ... and you ain't there yet !

Any leak down test I could preform on the carbs?

A "leak-down test" is used to assess engine condition ...
A common method to check for carb and intake plumbing air / vacuum leaks
will be to spray WD-40 around any suspected leak (like those rubber tubes and the throttle shafts)
and then listen for a change in running, i.e. RPM.
Note: WD-40 is extremely flammable !

[Jean]

Diaphragms: Should not make any difference at idle. The BING book says to change them every 2 years...I'm changing mine about every 5 years (or so) since I found they do not respond as well when they get old.
Some time ago we had some spirited discussions about diaphragm life but in practice, they DO need changing a lot sooner than the time it takes for holes to show up.
You'll know you passed that time when you operate the throttle to pass someone on a 2-lane road and not much happens, and ALWAYS when you spot an oncoming vehicle...

My first guess is the butterflys aren't closing all the way. Another is "Did you do the chokes one at a time, paying careful attention to the arrangement of the parts?"