Flat throtle response and surging below 50f

[mrclubike]

After replacing the needles and needle jets the bike runs great above 60 deg F
But when the temps get closer to and below 50 deg
It starts getting  a flat throttle response between 4000 and 5000 rpm and the colder it gets  the worse it gets
Has good power up hills and and taking off from a stop

The carbs are freshly rebuilt with the factory sized jetting new diaphragms  all gaskets and Orings

valve lash set and I have no valve recession problems
 
The problems I had with the timing and spark knock  is all sorted out
(Went back to the stock ignition system and decoked  the heads )

Is this normal for the colder temps and do I need to do some jetting change for the winter

I have been looking at the VM 32 Mikunis and they do sound good.   

If I just run under 4000 rpm it runs fine

[Justin B.]

Maybe set the needles a notch higher?

[georgesgiralt]

Hello !
I've heard that some carb float are out of spec and seize in the bowl when the temperature is cold. So you may have a look at this part... if they're new.
I have a similar problem with my bike. She refuse to run above 4500 RPM in winter and this is intermittent. I checked everything I could including compression to no avail. So I plan, this winter, to look at the floats if the same problem arose !

[Barry]

Raising the needles a notch would probably do the trick between 4-5000 rpm but it might be too big a change lower down.  Raising the needles one notch results in a jet flow area increase of 44% at initial lift reducing to 5% increase at maximum needle lift. I'm thinking that's too little where you need it and too much where you don't.  I'd be inclined to try raising the fuel level a touch whether that be because the floats are sticking or they are just set too low. My last new floats were so light I had to set them slightly above parallel to get the right fuel level

[Bob_Roller]

Does the engine finally go through the flat spot in power

Have you added an inline filter ??

You should not have to change jet size, etc.. for colder temps .

I run the same jets, needles, needle setting for temps from 120 F, 49 C, to 20 F, -7 C witout any problems .

But the engine has more power in the colder temps compared to the furnaces of summer !!!!

According to the temperature/altitude conversion chart that is in the Bing manual, during the typical summer day here the 'density ' altitude in Phoenix is around 6,000 feet, a bit under 2,000 meters .

I had a similar experience that Barry described with new floats .

When I got my '82 LS, I replaced all the comsumable parts in both carbs, before taking the bike for a ride .

I developed a problem that  after I got on a urban expressway with a 65 mph speed limit, after 2 miles, the bike would start losing power .

I rechecked the float level and it was parallel to the carb body, no amount of float adjustment helped, I put the original floats back in and it was an instant cure . :-?

[mrclubike]

Thanks all for the help

yes it does pull thru the flat spot and runs good above 5000 rpm

The floats are 2 years old
And yes I have an in line filter
I have cleaned the bowls and gone straight out to drive it with same results 

I have put bigger needle jets in but then it starts  going rich at 3/4 and above throttle and the fuel mileage goes down

I have a set of 148 main jets coming that I can throw in to see what that does
i am running the stock 145's now

I may also chuck up the needles in a drill  and thin down the lower part of the needles with some emery cloth a bit to richen it
But I am not exactly  sure just where to do it at 
I was thinking taking .001 to .0015 of an Inch (.025 mm to .04 mm) from about 1/2 to  3/4 down the needle
Does this sound about the correct area of the needle in question

[Barry]

If it pulls through the flat spot then you would think the mains are big enough.

So that leaves the needles and how much to reduce the diameter and where on the needle. I have a spread sheet which calculates the needle jet flow area for each 1mm increase in lift of the needle. It's simple enough in principle, just calculate the jet area and subtract the area of the needle. That turned out to be relatively easy as rather than being a complex shape the needle is a simple truncated cone.  If you are familiar with Excel you should be able to figure out how it works and substitute your own proposed dimensions. The tricky bit will be knowing what needle lift is applicable to your flat spot.

XLS file are not permitted as attachments so I'm going to try and fool it with a different file extension. You will have to rename the file extension to xls to open the file with Excel.

[mrclubike]

  The tricky bit will be knowing what needle lift is applicable to your flat spot.

Yes knowing where to reduce the needle is the hard part

I was thinking of removing the intake elbows and running it down the road and then reach down with the phone and snap a picture inside the carb
that may give me a rough idea where I need to be

By the way I rebuilt the carbs  after It got cold and started having this problem (including the diaphragms)

I tried opening the file and it comes up with just  comes up as little squares

[wilcom]

tried opening the file and it comes up with just  comes up as little squares
                   

I saved the file and then opened it with "Open Office Calc"

[Barry]

Or change the file extension fro .txt to .xls

[mrclubike]

I will forward it to my work computer an see if i can open it there

[Bob_Roller]

Did you get Bing diaphragms, or pattern, non OEM parts ??

[mrclubike]

Did you get Bing diaphragms, or pattern, non OEM parts ??

Genuine BMW kits
I have new springs coming 
I will install those first and see what happens

[mrclubike]

After reading Georges thread on carb (jet) experimenting
I think I am going the wrong way the main jet
I was going to try a larger main jet but I think that will just cause my high end to get over rich like some one said

I think I  need to drop my main jet  down in size and increase my needle jet  size

I have ran a 2,68 needle jet and  that seemed to help the 4k to 5k  band but my full throttle band got too rich.
So if I install a smaller main with a larger needle jet    shouldn't that lean my full throttle band back down

Opions  welcomed   

[georgesgiralt]

Hello !
I follow http://www.factorypro.com/ procedure for low RPM engines to dial in settings of the carbs.
So IMHO, you should try to find the main jet first : find the main jet that gives you the most power when opening full at 5000 RPM. Do not touch anything else before doing that.
Once this main jet properly set-up, find the needle proper position : open full throttle at around 2500 RPM and see what gives. Change needle position to find the correct response. If no position  is perfect, then try another needle jet.
In my experience, changing the needle jet only change the fuel mileage to the worse when you go bigger. But YMMV ;-)
Hope this helps
P.S. : The procedure given in the Factory Pro web site is very good. But they suppressed the direct link in order for you to navigate and see their site... (go to support then to Motorcycle tuning tech and next CV carb then low RPM engine.... )