Weird running 1979 R65

[Kookaburra]

Thanks Montmil and Barry. Needles and seats were purchased from Motobins and I was careful to ensure correct specification and to fit them to the right notch. There was visible wear on the old needles and the seats and a massive amount of fine crud blocking up the seats. With new needles and seats I would expect the bike to run leaner at unchanged mixture screw settings. It is all quite confusing. Bike has always and still requires limited choke to start and run indicating a rich mix. Just to further complicate the evidence, the left plug is the perfect colour and the right plug is sooty.

I will lean up both idle mixture screws about an 8th as the first experiment and see what happens.

[Mender]

Mender,

Your symptoms at 6000 RPM sound like the classic roll off test for selection of the correct main jet. When the throttle is rolled off a touch at 6000 RPM it momentarily richens the mixture. If the engine runs rough when you do this that usually indicates the carb is set up too rich. If the engine pulls better when you roll off then it's set up too weak.

Thanks Barry, sorry for the delay in replying needed to check and clean!

After much cleaning and checking float levels it is still behaving exactly as your description re the roll off test. does this mean I may have too large main jets? or can I lower the needle as per the old Brit Amals?

[Barry]

Got to be worth trying a change in needle position first - it's free !

These carbs run on the needle anyway except for close to full throttle.

[georgesgiralt]

Barry,
May i respectfully disagree ?
These carbs have a procedure to be properly set up.
First install the correct main jet giving the strongest pull at full throttle and no problem rolling off.
Then play with the needle position and jet size.
After that adjust idle jet size if needed as it will contribute (a few percent but contribute) to mixture richness going off from idle and even at full throttle.
Of course, any of this has to be done on a sound engine : compression good, valve lash properly set, ignition set properly and perfectly timed.
And most of all clean and leak proof carburetors...

[Barry]

I agree that's the correct methodology Georges but if he hasn't got different sizes of main jet to hand changing the needle position is worth a try although at full lift one needle position only makes approx. 5% difference which might not be enough given that one main jet size is more like 10%.

[nhmaf]

Does the problem under throttle rolloff appear with the bike just sitting on the sidestand versus actually running on the road?   Under road conditions, the decel of the bike causes the float bowls' fuel level to pitch forward, momentarily enrichening things by a significant amount.
Doing this with the bike on the centerstand of course doesn't create this effect - the evidence may help point in the right direction.

[Mender]

The float levels are correct, and are unaffected by the very slight roll off at 6000rpm in any gear. I suspect that after 75000 miles the needles are worn so I will drop them a notch or two to see what happens. It has been getting progressively better as I rack up more miles since replacing the push rod rubbers.
I reckon it is running a bit rich but that in my opinion is more desireable than too lean.
I'll update as soon as I've tried it. Thanks guys.
  

[montmil]

As any Airhead & Bing fettler knows, those needles are not rigidly fitted into the slide body. They wiggle a bit and this does permit them to "center" in the jet opening. Eventually, that continual wiggle of a steel needle wears the brass jet.

These small bits are economical enough to replace if you suspect a concern in that part of the carburetor. After a full carb rebuild on my '81 R65, I found the mid-range engine ops to be much improved with a one-notch needle clip change. Sure, there could have been other factors involved but tinkering with the needles is easy enough to try.

[Barry]

For what it's worth dropping the needles one notch will compensate for approx. 0.004" of wear on the needle although I guess the needles wouldn't wear evenly.

Needle jets are reckoned to wear more than the needles and a 2.66 needle jet only has to wear by 0.0008" for it to be the same as the next size up 2.68 jet.  That's less than 1 thou wear so it's no wonder that needle jets are recommended to be replaced every 30,000 miles or so.

[Kookaburra]

Feel I should report back to you on the results of a series of small experiments to try and eliminate the trailing throttle popping on the 1985 R65 after replacing the needles and jets.
1. Check for air intake and manifold leaks
2. reset float bowl levels to precisely 24mm
3. reset idle mixture screws to a leaner optimum
4. adjust warmed up tickover to around 1100
5 check and adjust carburettor balance.

Result:  After 200kms, both spark plugs looking good and no popping under trailing throttle. Thanks all for your comments and observations. Memo to self. Now leave it alone.

[georgesgiralt]

Hi !
I would be delighted to read how you managed to :
"2. reset float bowl levels to precisely 24mm"
Because I can't do it myself and my left boot ask for it.  
Thanks in advance !

[Kookaburra]

Ok George here goes-
1. Take a narrow flat stick (trimmed emery board stolen from swmbo and also useful for cleaning electrical contacts) and mark off at exactly 24mm.
2. Remove left hand float bowl, empty (back into tank if no water globules) and cleanout any crap and circular central well of the floatbowl.
3. Refit the float bowl and turn on petrol.
4. Turn off petrol when translucent inline filter indicates no further petrol flow.
5. Carefully remove float bowl (no spillage) and place on a level surface.
6. Place measuring stick upright in central well and check petrol level against 24mm mark. (Leaning stick backwards and forwards can indicate how far off the petrol level is and compensate for the miniscus curve.)
7.1 If petrol level is above 24mm, open up the hinge angle on the float body by inserting a screw driver and gently twisting. (ie needle valve will close sooner)
7.2 If petrol level is below 24mm close the hinge angle on the float body by inserting fine long nose pliers and gently squeezing.(ie needle valve will close later)
8. Empty float bowl and refit. Repeat steps 3-7 as needed.
9. Repeat steps 2-9 on the right hand float bowl.

The same measurement process can be undertaken after returning from a ride. Close off the petrol, put the the bike on the centre stand, whip off a float bowl and check the level with your custom measuring stick.

It is probable that the above method (adapted from guru Snowbum a.k.a, Rob Fleischer) only really works with relatively new floats (with the same standard flotation values, ie not variable sinkers), good needles, springs and unworn float pins. I think several doctoral theses have been written on this stuff.

[georgesgiralt]

So you do the same as I do.
But, when you turn the petcock off, there is still some petrol in the fuel line and when you remove the bowl, how do you prevent this to flow into the bowl ?
I am unable to do this, especially on right hand Bing, because the tubing is very long and hold a lot of petrol. So my measuring is always false.
This is why I asked.

[Barry]

I agree Georges that the fuel in the lines will potentially cause an error. I don't think there is a perfect solution other than pulling the fuel hose off the carb before removing the bowl but that's something I wouldn't want to do often.


How big is the error ?

I calculated the volume of the fuel hose at 9.6cc per carb. which is an average because it won't be the same for each carb.

Th float bowls contain 46cc or 2.1 cc/mm of height.

9.6 cc is therefore potentially going to cause an error of over 4 mm in fuel height which frankly is hopelessly inaccurate.


I know this is far from perfect but in an attempt to reduce the error one thing I have done in the past is to shut off the fuel tap some short distance from home in order to use up the fuel in the hose. I calculated it as approx. 0.3 miles and considered it better to aim for using most of the fuel rather than all of it because there is no chance it will use the fuel evenly for both carbs.

Because you can't easily infer operational fuel levels accurately using the remove the bowl method, it becomes even more important that the floats weigh the same as each other, so that when set parallel, at least they will produce the same operational fuel levels.

In the end the only real way to do it accurately is to modify the bowl to fit a sight tube.  It's been on my to do list now for several years !

[georgesgiralt]

"In the end though the only real way to do it accurately is to modify the bowl to fit a sight tube.  It's been on my to do list now for several years. "
That's a good idea !
I own a badly rusted bowl with the overflow tube broken. It makes a good candidate for modification !
If I remove the right bowl with the petcock closed, the bowl is at the overflowing limit... Maybe my line is too long or I start with the level too high ?