High idle revs

[Barry]

Apologies if this has already been covered but you will get low readings unless you do the compression test on a hot engine with the carbs off or throttles open and Slides manually lifted ?

[79beem]

Engine warmed up, carbs on, throttle closed, just kicked over until i got the highest reading. So going back to my last posted questions?  

[nhmaf]

79beemer - if you did not perform a compression test in the manner that Barry indicated, then your numbers are invalid.  Nothing useful can be gleaned from them unless/until the test is performed with (ideally) carbs off completely, or at least throttles held wide open on a fully warmed up engine.

You're asking the equivalent of guessing how fast you can run the 100 meter dash, based on a single timing of how well you ran the 400 meter run with duct tape over your nose and mouth with nothing but a soda straw to breathe through..  make sense?

[Barry]

You're asking the equivalent of guessing how fast you can run the 100 meter dash, based on a single timing of how well you ran the 400 meter run with duct tape over your nose and mouth with nothing but a soda straw to breathe through..make sense?  

Wonderful analogy !


79 beemer, You asked the engine to compress what it could breathe through a straw ... the amount of air available to it at idle.

If you let it breath I'm sure you'll get good compression figures.

[79beem]

 Sorry Barry,read your reply at 4am .

So the the clymer appears to contain errors in it procedures?

I didnt read anywhere to take the off carbs or to open the throttle,slides up.

Cheers for your patience

[montmil]

...So the the clymer appears to contain errors in it procedures?

Multiple. Frequent. Contradictory.

I have the Clymer and Haynes manuals plus the original BMW R65 owners handbook -available in our FAQs section- plus the best resource for the R65... R65 dot org.

The commercial manuals often assume a certain level of mechanical knowledge. And you know what happens when you assume something.

Not to worry. Nothing damaged and you may end up saving a few shekels. [smiley=thumbsup.gif] Press on!

Monte

[Barry]

OK forget what Clymers says lets be clear about your compression test.

Your compression results should not be a cause for concern yet and you may not need new rings.

When carrying out a compression test it is always required to have an unobstructed inlet because the engine needs a good supply of air at atmospheric pressure to compress. With the throttle closed at idle there is only a tiny amount of air able to get through so you are trying to compress a partial vacuum and a low result from your compression test is inevitable. To carry out a compression test with conventional slide carbs it's acceptable to just fully open the throttle which lifts the slides clear. With a CV carb that won't work because there is not enough air velocity to lift the piston so you must either lift them manually somehow or remove the carbs to leave an open inlet. Frankly if you  leave the cables connected it's probably easier to remove the carbs from the inlet rubbers and hang then to one side a little. A 2 min job. If you do another compression test like this with open inlets you will get much higher figures that can be properly compared with the specs.

As for Clymers I don't have one so I don't know how good or bad it is. Manuals tend to get a bad name but I find The Haynes quite good for the most part and in this case it describes a compression test correctly including making the distinction between slide and CV carbs.

[Milo_357]

... I HAD also made minor adjustments to both carburetors (tightening the main jet to 1.25 turns out) & the idle adjust just before leaving in an attempt to reduce the idle which was running around 3000 rpm when warm.  My gas mirage also dropped about 10 mpg.

Tightening it out? The main jet is not adjustable by loosening or tightening. Variances in the main jet are solely the responsibility of the jet's orifice. If you have loosened the main jets in the Bings, go tighten them back up immediately.

... I read Snowbums tech paper on adjusting carbs.

Really?
[/quote]

Don't be Tom now, Monte!  

Ok, so if it's not the main jet, the large center screw on the base of the carburetor.  Oh, and tighten out is "dumb ass" for loosen or unscrew.  

Seriously though, between the various adjustments (idle adjust, choke cable adjust,  ect.) is there a "baseline" where to start when balancing your carbs?

[79beem]

Proper procedure = excellant results

Did the proper check last Dec, just following up now.

Thanks chaps.

[Danie]

I also experience high idling revs on my R65  -  but only when the engine gets quite hot - i.e after a ride of about 10 - 15 kms.

During start up, or when the engine is still cold, the engine runs at low revs (about 700 Rpm).

Why is the engine running at higher Rpm when the engine gets hot ?

Thanks.

Danie

[Bob_Roller]

There a  couple of possibilities .

Valve clearances have closed up, performing a carb synchronization with the engine not up to operating temperature .

Advance mechanism in the 'bean can' hanging up and not returning when the engine speed is reduced .

Intake system leaks between the carb and cylinder head .

[Barry]

Why is the engine running at higher Rpm when the engine gets hot ?

It's normal for an engine to idle slower until fully warmed up because the engine just doesn't run efficiently until then. Besides less than perfect fuel atomisation producing less power it takes that long for the oil to reach proper operating temperature and for oil drag to reduce.
 
There are a couple of reasons for run away idle speed but 9 times out of 10 it's carb settings and a failure to set the idle speed with the engine fully warmed up.

Going off on a bit of a tangent the effect of valve clearances on idle speed is interesting. We all know that valve clearances should be set before setting idle speed but why ?  There is the possibility of closed up clearances which will make an engine not want to idle properly at all due to loss of compression. The actual gap also has an effect on idle speed because the valve timing or more particularly the valve overlap when both inlet and exhaust valves are open at the same time is being varied slightly and the engine is very sensitive to this at slow speeds. Opening up the valve clearances reduces overlap and makes the engine run slightly more efficiently at idle so the idle speed goes up. This is not just armchair theory opening up the clearances by 2 or 3 thou will actually produce quite a noticeable effect. All of which is to say you must set the valve clearances before adjusting the carbs.

 

[tvrla]

It's normally just tune-up stuff as already mentioned. I'll bet if you adjusted the carbs after a ride when it's idling high, you'd fix it right there!

[Danie]

Thanks for all comments. The engine usually runs at about 700 Rpm when cold, and at about 1300 Rpm when warmed up.

I will check the valve clearances, and then have a look at the carb settings.

[Barry]

The engine usually runs at about 700 Rpm when cold,

We usually say if an engine will idle stone cold with the choke off then that's a good indicator that the idle is set too high and possibly too rich. All depends on ambient temperature I suppose and no doubt it's a touch warmer where you are. Mine won't idle at all just after starting. It manages 800RPM after 3 miles and 1050 RPM when fully warm.