Running to lean

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Jul 23, 2005
I have rebuilt the carbs and bike runs but plugs are way to lean. I've set the carbs up to factory specs. I am useing NGK bp7es, are these wrong? The bike does sound like it has been pump up a little bit. Load and lumpy. Will doing all this testing without the air filter make run lean? I raise the needle, no big difference. I've adjusted and adjusted still the same. I guess i will go to bigger mains. Any suggestions?
I just read you other posts and realized you are probably talking about your 850.

NGK BP7ES are correct.

Yes the air cleaner makes a diff.
850's come with 3 1/2 slides. I run the needle in the raised position, with 260 mains with the ham can.
If you have the black box, it's leaner.

The 850 will also run nicely with #3 slides, needle in the center position, 260 mains.

Make sure the needle is matched with the spray tube. The cutaway spray tube needs the 4 ring needle, the straight spray tube needs the 2 ring needle. These rings are above the clip grooves.
Thanks for the help Michael. This ones my 71 750. I't been driving me nuts and i've worked with the settings every which way, and still lean. I assumed since everything was new or rebuilt with the carbs i had the basics stuff finished. Then at 12:30 last night while staring at my bike, i realized i had assembled the heat insulators & carb to manifold gaskets opposite of were they belong. I had actually installed the heat insulator between the carb and manifold and used the paper gasket between the head and manifold. Total brain fade! I still can't believe i assembled it that way. I am going to change it when i get home tonight. That has to be it.(big intake leak) Just goes to prove not to look to deep until you are sure you have correctly done the intial setup. Your tips on the 850 will be helpful when i begin to tackle my 74 850 in a week or two. That black air box was the first thing to go. Thanks Mike
Aw jeepers, plug in the diagnostic module and
read out the fualts and the actions required : )

Just reversing the gasket order should not make an
air leak and generallly the more throttle opeing the
less it leans plus carbs and gasket tend to get sucked
to head. Didn't notice much plug chop leaning till
my gasket fell flat out hanging and then only notice
was mild off throttle surge and dying at idle.

If gaskets don't do it for you, check choke cable
top entery holes plugs, if not using choke set up.
A low float level of course. Too often bowl gasket
can hang / restrict it. They do distorrt between wet
dry seasons. Fuel taps get cloggy too for a long shot.

MIght review plug chop conditions that could skew your
view of mixture.

I'll put everything in order and do some shut off plug testing. It was really popping at idle and when i let off the gas. Do i have to run a gasket from carb to manifolds or is the o-ring on carbs it, as parts book shows? My headers are turning blue, This thing is running to hot(lean) Could timing be off, Would that cause these problems if to advanced?
Herbert, I have had similar backfiring issues with my 1970 750 Commando. One of the things I found was that brand-new cables for the choke slides were the wrong length allowing the slides to protrude into the venturi even though the cable actuator was fully opened.
Additionally I found with a Boyer ignition that the static timing, set up with the red dot of the rotor visible through the viewing window on the trigger mechanism at the points side was way out and incorrect timing will also cause this backfiring. Dave M
I'm running a lucas rita that came with the bike. I'll dig out the timing light and check it. I also am not running the choke and have it plugged off. I tore down carbs last night set everything back to factory spec. I'm going to reassemble tonight as long as all i need are the heat insulators and the o-rings. Am i suppose to run paper gaskets between head or carbs? Or can any one suggest any gasket info. My nearest shop is over 30mi away
Herbert M. said:
Am i suppose to run paper gaskets between head or carbs? Or can any one suggest any gasket info. My nearest shop is over 30mi away

No paper.
Head - heat insulator - manifold - carb w/0-ring.

I had lean problem after rebuilding carbs once. I had gotten some 'Parts Soak' on the o-ring, it had swelled not sealing correctly. Blued the h%&^ out of the pipe. New o-rings cured it.

Another tip, do NOT tighten the carb to manifold without the carb top installed. Easy to warp the carb body. (Speaking from experience)
Install manifold to carb with top on, then install assembly to head.
You only need the o ring and not any paper gaskets between the carbs and the manifold, however while you have the carbs off put a straight-edge across the mounting holes to make sure the flange is not bowed. Also did you replace the small rubber O-rings on the idle mixture screws when you reassembled the carb?
I rebuilt the carbs using all new parts except main jets.Used new o-rings and gaskets. I even trim the float gaskets very carefully for float clearance. I will tryt he straight edge check when i reassemble tonight. Thanks for the order of assembly mike very helpful. I'll post tomorrow with the results.
Also, make sure you keep the slide in the carb it was made for. They were individually hand lapped by Amal to fit each other and are NOT interchangeable.

I found this out the hard way when I was working at N-V. We had a hack 650SS that was used for running errands (and by me for my daily commute for a while). We did a brief research program with a larger single carb installation - don't remember the results. I think it was when we were researching a customer complaint about poor fuel mileage. He was only getting 34 mpg (UK Gallon). Turned out he was hauling a double-adult sidecar with his wife and two younger kids in the chair and a teenager on the back of the bike! I reckoned 34 mpg for 5 people with only a 650 motor wasn't too bad.

When I re-installed the original twin-carb set-up, I never thought about matching slides and bodies.

All was well until I used full throttle in second gear to pass a slow-moving car, then cut back in behind a double-decker bus. When I rolled the throttle off, it stuck wide open! It was a good job that particular bike had magneto ingintion with a kill button right next to the twist-grip! I found that the brakes wouldn't hold a 650SS at full song.
Should that read: Running Too Lean.

Sir Roy (Pedantic of Tunbridge Wells),

I believe that is a question therefore should have a question mark at the end of the sentence to be correctly pedantic?
Sir Roy, Your'e right, It should have a question mark. I guess all that wet weather you have over there gives you more time to work on your grammer. Thanks
Herbert M

I (L.A.B.) was actually correcting Sir Roy!

But you are probably right about the weather!
the question in question was in fact a rhetorical one, hence the requirement for a question mark is debatable, said grammatical device was ommited in order to avoid any unnecessary confusion, ponderance or deliberation in ascertaining a suitable literal answer.
I thank you for your concern over the level of precipitation upon these shores, and can reassure you that recent rainfall is well below the seasonal average.
With regard to the correspondents question over the 'lean' of his engine, It is my understanding that all Commando engines do have a foreward angular displacement from the vertical.
Yours faithfully
St.John Ponsonby-Smythe, Col. (ret'd.)
That certainly puts a new slant on things sir Roy.

But a question without a question mark is a statement.


Posessive case therefore -correspondent's.
Both of you....

Should we get out the swords? There is surely a field in he area that you can both use to settle this. :wink:

By the time you finish ripping each other up...all of us that are too lazy to use "Word" to correct all the typoes we make, will be indeed too lazy to even push the "Submit" and comment on all this.

Afta awl, wee donts wanda git carreed awae heer, ore?

:wink: :wink: :wink:

Greetings on a lousy Fall day from Never Never Land.............
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