R65LS 1982 hesitation @ & around 4500 rpm

[jgp1854]

Freshly rebuilt carbs by BING, fresh valve adjustment, carb sync, air filter, plugs.  The LS starts, runs, idles fine, however, at around 4500 rpm minor hesitation.....

Any idea what could be the fix?  

Would a different jet(s) mitigate the issue?  If so, which jet(s) and what size should I look at?

Any other suggestions?

thanks

jgp

[Barry]

Not a very obvious one this.

First step is to determine is it ignition or fuel and if fuel is it caused by weak or rich mixture.

If Ignition you might see some erratic behavior of the tacho.

If fuel, rich would be unusual and it would have to be along way out to miss. Test for weak mixture by seeing if it goes away after applying some choke.

If it wasn't for the Bing rebuild You might suspect diaphragms holed.

[jgp1854]

Barry,

thanks for your response, with respect to ignition, when the engine rpm reach the 4500 rpm range, the tach "stalls" or "slows", it isn't like it is a steady climb up and thru the rpm range.  If I were to focus on ignition, where would you suggest?

I will test the weak fuel suggestion today using the choke.

thanks

jgp

[Barry]

If I were to focus on ignition, where would you suggest?

One test for marginal ignition is to close up the plug gaps a little to see if that helps but with new plugs you probably don't want to do that and in any case electronic ignition systems should not have a lack of spark energy unless there is a fault. The known issue is overheating of the electronic module under the tank. Perhaps those who have experienced total ignition failure due to overheating could say if it was preceded by any intermittent issues.

[Cano]

With the stock US jetting, my 82 LS had a bit of a flat spot, but if I recall correctly I think it was at about 5000-5500 RPM.  Bike would not cut out, stumble or anything, but just did not "pull" well in that range.  Went to slightly bigger main and needle jets (I think it was to the Euro ones per Haynes) and that cured my issue.  Plugs are now more of a light tan color as opposed to an almost glazed look.

[Barry]

It's true that US jetting was weaker than European jetting in order to meet tighter emission standards and then on top of that modern oxygenated fuel runs a bit weaker still. You'd think though that the Bing agency would be aware of these things. Haynes do list both sets of jetting.  

[steve hawkins]

I had this problem when I mover away from the standard settings on my 1979 Cafe Racer.  Different silencers and different air filters, post 81 heads, etc.  Flat top carbs.

I felt the need to go to larger jets but it did not solve the hesitation I was getting at similar RPMs.  I returned to standard jetting but actually ended up dropping the needles - i.e. moving the circlips up and making it leaner.  Cured my issue.

Now my issue might not be the same as yours, but I would play what you have first and don't make the mistake of assuming a stumble at high RPM is due to lean mixture, it might be due to too much fuel.  YOUR BIKE STANDARD?

Alternatively book yourself into a Rolling Road.  It might cost you less in time and money.  (Depends how much you value your time.)

Rev. Light.

[jgp1854]

Steve,

My LS is totally stock.

jim

[jgp1854]

Barry,

I don't have the HAYNES, what are specs for the Euro jets?

thanks

Jim

[Barry]

Jim

According to Haynes your US spec carbs should be 64/32/325 & 326.  Jetting differences between UK and US:-



Main jet 148 UK and 145 US

Needle jet 2.64 UK  and 2.66 US

Needle Position 4th UK and 3rd US

Idle jet 45 UK and 40 US

Pilot Mixture Screw 1/2 UK and 3/4 US


Comment :- Not all of the differences are entirely obvious and easily compared because of the odd needle jet/needle position combination.

US Main jet is slightly weaker.

US Idle jet is weaker and therefore the mixture screw base setting is further out ( richer) which is to be expected although these are only base settings to tune from.

The US needle jet is actually richer than the UK but the needle position is weaker making direct comparison messy.  

Steve makes a good point about not making assumptions. In particular the tuned air intake system with snorkels could have strange effects at certain revs including making the mixture richer. This may not be directly applicable but still worth a read. http://bmwmotorcycletech.info/R80-fuel-mileage-problems.htm

[jgp1854]

Barry,

I took the LS out today, here in Michigan temps were in the upper 50's* low 60's*.  Got the LS warmed up, and ran it up to the zone/4500rpm's and in 5th gear.  This is the point where things start not pulling as one would think it should.  I activated the choke to the "first" position, in doing so, I clearly felt an positive increase in the pulling (rpm climb).  So I strongly suspect the LS is not getting the fuel it would like, correct?  

Having said that, what would you suggest I do first?  

thanks

Jim

[steve hawkins]

You could try by pulling the needle up one notch.  I.e moving the circlip down....

One change at a time and note your findings.

Cheers

Rev. light

[Bob_Roller]

I'll go out on a limb here, did you have this problem before BING rebuilt the carbs ?

If you can post the carb numbers that you have, it's located on a vertical flange at the forward part of the carb .

Should read something like : 64/32/335 .

I've got a manual from BING that shows what parts were in the carbs from the factory .

Just maybe, wrong parts were installed, probably not, but anything is possible .

[Barry]

I activated the choke to the "first" position, in doing so, I clearly felt an positive increase in the pulling (rpm climb).So I strongly suspect the LS is not getting the fuel it would like, correct?
 

Yes correct. And as steve suggests raising the needle a notch will richen it up significantly. It will be interesting to see what position Bing had set the needles.  If they were in position 3 already then that is the next to richest position. One notch is a big change and I'd be a bit surprised if it needs position 4 but you have to try it as it's the easiest and quickest 1st option. Our needles don't have slots but the diagram below clarifies which direction to go in. To move our needles you twist them through 1/4 of a turn and pull or push them into the next position.

It's a good idea to measure the exposed needle length before making any changes so that you know where you started from. Measurements should be something like this.

Needle Position            Needle Length
         1                           42.37
         2                           40.57
         3                           38.79
         4                           37.10


 If it works but you think it's now too rich you might also consider the slightly larger main jet instead which may work while giving a smaller increase in richness.

[Luca]

This might be a long shot (but if everything else checks out)...

Checkfor an air leak.  Quick and simple diagnostic.  Put a smoking cigaretter or incense stick near the carbs boots and whack the throttle, looking for a disturbance.   Or spray some WD around the boots and listen for an idle change.

My reason for thinking that this maybe is a possiblity is that you could have your idle jet tuned to run on a carb that's getting more air than it should, and as you approach higher rpm's, where the idle circuit is less important, you start running lean.  However, unless both carbs are leaking, you would probably notice some vibration.

A few other things:

It's also a good idea to check your carb slides for smooth action, and make sure that your petcock screen and tubes are clean.

Make sure BING put the proper slide springs in your carburetors (hopefully somebody knows how to identify them, because I don't).  Perhaps you could try revving the bike without them in the driveway.  They can be removed easily without taking the carbs off the bike.  If they are too stiff, they will not rise enough as you open the throttle.

Try getting your bike up to its choking point while riding on an open road, kill the engine and pull the clutch simultaneously, and then read the plugs.  This will give you a good indication of the air/fuel mixture at your problematic RPM.