carb adjustments

[pdx_r100s]

I have been fiddling with my carbs for a while and can't quite get things dialed in. I've read a LOT of info online. Wondering if folks on here have some knowledge to share.

My bike is a 78 r100s, 40mm bing carbs, dual plug setup. I installed a new electronic ignition/coils (enduralast), set timing, set valves, totally stripped down carbs, cleaned, and rebuilt with new brass and orings, installed new throttle/choke cables, balanced carbs.

The three things that I can't get quite right:

1) it is hard to start cold--often takes a lot of cranking. But then it is even harder to start when it is hot--sometimes I have to try and then let it sit a minute and then try again.

2) there is a LOT of popping on deceleration. The bike came to me with stain-tune exhaust. I've heard it is normal for some deceleration popping, but what I have is pretty annoying. It isn't backfiring, just a lot of audible pops. leaning out the air screws doesn't seem to help with this. I would really like to reduce this symptom, if possible.

I noticed that the PO had slightly larger than stock need jets (2.66 vs. 2.64). I've read that sometimes a larger jet is used with staintune exhausts. I tried going back to the 2.64 and it didn't improve things. Currently I have the needle and the fuel screws set per the clymer manual (2nd position on needle clip and ~.5 turns out on screw). Also didn't help the bike run better or reduce popping.

3) I cannot get the chokes adjusted so that at full close the lever arm at the choke body hits the end stop, and at full open it hits the other end stop. Snowbum's article emphasizes that they must be adjusted for this, but I do not see any way to adjust the cable so that it has this range of travel. I even took apart the lever mechanism where it attaches to the engine, lubed it, put on new cables, etc. What am I missing?

[gspd]

I'll approach this as if I had just purchased this bike and have no clue of its repair and maintenance history

1) it is hard to start cold--often takes a lot of cranking. But then it is even harder to start when it is hot--sometimes I have to try and then let it sit a minute and then try again.

First thing I would do is check the compression, with the carbs removed (or butterflies and slides held open with a screwdriver).
Disable the ignition before you do this.
If it's over 100psi it should start and run OK.
Depending on your compression ratio and engine wear, 120psi to 160psi is ideal.
Just for the record, 'harder to start when hot' is generally an ignition concern.

2) there is a LOT of popping on deceleration. The bike came to me with stain-tune exhaust. I've heard it is normal for some deceleration popping, but what I have is pretty annoying. It isn't backfiring, just a lot of audible pops. leaning out the air screws doesn't seem to help with this. I would really like to reduce this symptom, if possible.

A certain amount of 'popping' on decel is normal and often incurable with free(er) flowing exhaust systems.
If you block the exhaust at idle, like with a gloved hand at each muffler, the bike should stall.
If it keeps on running you have an exhaust leak which will exacerbate the 'popping'.
Your carbs don't have 'air' screws, they are 'fuel' screws.
In is lean, out is rich.
Richer will normally 'pop' less than leaner, but not always.
Try 1.5 to 2 turns out, see if that helps.

I noticed that the PO had slightly larger than stock need jets (2.66 vs. 2.64). I've read that sometimes a larger jet is used with staintune exhausts. I tried going back to the 2.64 and it didn't improve things. Currently I have the needle and the fuel screws set per the clymer manual (2nd position on needle clip and ~.5 turns out on screw). Also didn't help the bike run better or reduce popping.

2.66 vs. 2.64 won't make a difference for starting and low speed running.
There will be a slight difference at higher speeds and at WOT (wide open throttle), but you probably won't notice it without a dynamometer or exhaust gas analyzer.
I run 2.66's in mine.

3) I cannot get the chokes adjusted so that at full close the lever arm at the choke body hits the end stop, and at full open it hits the other end stop. Snowbum's article emphasizes that they must be adjusted for this, but I do not see any way to adjust the cable so that it has this range of travel. I even took apart the lever mechanism where it attaches to the engine, lubed it, put on new cables, etc.

Be sure that your 'choke' mechanisms and their levers are all in their proper positions.
They MUST swing from fully 'on' to fully 'off' when actuating the lever.
Left and right 'choke' parts are not interchangeable.
Be sure the 'boomerang' arm position and orientation is correct with the dot on the shaft.
I've seen mistakes here many, many, many times over the years.
Improper assembly can cause all the issues you've described.

What am I missing?

As a last check, if all else fails... in a dark area, lay your 4 spark plugs (still connected) beside each other on a cylinder, assuring that their bodies are well grounded, and crank the engine over. You should see a bright blue spark at each plug. Orange or yellow won't cut it.
Oh, and just for the hell of it, be sure that your carb spigots are tightly screwed into the heads so there are no intake leaks.

[SteveD]

I expect gspd has that well covered.

A couple of things to also consider.
1. Are the jets in the bottom of the float bowl clear. They're part of the starting circuit.
2. Did you remove/replace butterflies and their o-rings? This is very fiddly and easy to not quite get it right.

I think I'd start at the choke setup, especially if the bike was running well beforehand. It's clearly not working as it should.
The EI is the next "new thing". I had a Guzzi LM that wouldn't start when hot with an aftermarket EI.

Popping with Staintunes. Not usually an issue ( I have a set), but yes, back out the mixture screw a bit at a time. GSPD mentioned this last...a place that can loosen but is a bit out of sight, is the carb head spiggot, where the big rubber hose attaches. Check them both for tightness.

[pdx_r100s]

Thanks to you both for this information.

I had it backwards about the fuel screws--I had read in a post somewhere that leaning it out would reduce the popping. good to know.

the compression is good--150 PSI in both cyls.

I'm most interested in what you (GSPD) said about the issues with the choke. It clearly isn't right, because no adjustments that I can make where the barrel adjustor is at the carb can make it so that the lever travels the full arc from stop to stop. Could you elaborate on this comment?

Be sure the 'boomerang' arm position and orientation is correct with the dot on the shaft.

I took the mechanism apart and applied some grease, and it seemed like I had the teeth inside of the mechanism aligned to that they would engage with the teeth on the arm/lever at the earliest possible point, but perhaps I assembled it incorrectly. But it was also not working correctly before I took it apart.

also,

a place that can loosen but is a bit out of sight, is the carb head spiggot, where the big rubber hose attaches.

is this referring to the short rubber boot connecting the carb to the cyl, held on by two hose clamps? I've checked these for tightness.

If there is a small kink in the throttle cable just above where it connects to the carb, will this affect performance? One side is perfectly straight, but one side got a little kinked when I had the carb off the bike but still hanging from the cable.

the comment about the ignition also concerns me. I had read good reviews of this brand and got it from a reputable BMW parts website--how do I check if the ignition is a problem with hot starting? I'm probably not describing the issue well: when the bike is totally hot after its been running a while, it starts fine, one press of starter. The issue is if I stop it, run into a store, and then come back out--so perhaps it is "warm" starting. Then it can really be difficult to get started. I wonder if my technique is bad (I usually try throttle closed, and then if that doesn't work, I try again with the throttle partly open).

thanks again for your help.

[gspd]

A couple of things to also consider.
1. Are the jets in the bottom of the float bowl clear. They're part of the starting circuit.
2. Did you remove/replace butterflies and their o-rings? This is very fiddly and easy to not quite get it right.
I think I'd start at the choke setup, especially if the bike was running well beforehand. It's clearly not working as it should.

Both very good points to check (or re-check) Steve, however...
1- the jet in the bottom of the float bowl being blocked will impede cold starting (choke on) but doesn't matter for hot starting (choke off). It won't affect the actual running after initial startup.
2- The butterfly(s) being improperly positioned (or even installed with the taper backwards) will prevent lowering the idle speed to acceptable (1000 rpms) speeds, but it will have virtually no effect on actual running or hot starting. It could affect cold starting but probably only in weather colder than you'd want to actually ride in.

The EI is the next "new thing". I had a Guzzi LM that wouldn't start when hot with an aftermarket EI.

Ignition issues are a possibility but it's better to rule out all the other usual suspects first, especially since the carbs have been recently worked on.
Both 'chokes' have to move from fully 'on' to fully 'off ' and pdx says they're not.
This is the primary issue to resolve.
This will definitely mess up both hot and cold starting.

[gspd]

I'm most interested in what you (GSPD) said about the issues with the choke. It clearly isn't right, because no adjustments that I can make where the barrel adjustor is at the carb can make it so that the lever travels the full arc from stop to stop.

This has to be rectified or it will never start properly.
Are you aware of the DOT on the choke shaft, and where it has to be? (snowbum pic below)
Is the arm the right way around?
Do you get full travel at the choke when moving it manually with the cable disconnected?
Maybe post some pics.

a place that can loosen but is a bit out of sight, is the carb head spiggot, where the big rubber hose attaches.
is this referring to the short rubber boot connecting the carb to the cyl, held on by two hose clamps? I've checked these for tightness.

Just try turning the carb to be certain that the metal spigot inside the rubber tube is not loose in the head.

If there is a small kink in the throttle cable just above where it connects to the carb, will this affect performance? One side is perfectly straight, but one side got a little kinked when I had the carb off the bike but still hanging from the cable.

Unless it's seriously kinked and impeding movement that's the least of your worries right now.

I wonder if my technique is bad (I usually try throttle closed, and then if that doesn't work, I try again with the throttle partly open).

Your 'technique' is not the problem.

[SteveD]

Both 'chokes' have to move from fully 'on' to fully 'off ' and pdx says they're not.
This is the primary issue to resolve.
This will definitely mess up both hot and cold starting.[/color]

Yep, I mentioned that's where I'd start too
The rest were just thoughts that came to mind...

[Rob Frankham]

Is tyhis the version of the choke control where the cable is attached at the carb with a screw on nipple? If so, the cable can be adjusted by loosening the nut and adjusting the position of the nipple on the cable.

Rob

[pdx_r100s]

So much good information! I'll try to respond to everything so you can understand where I'm at...

I did double check the position of the butterflies, just two days ago I made sure that they were installed correctly per snowbum's detailed instructions. I had put in new butterflies, shafts, screws, shaft o-ring, etc. when I rebuilt the carbs. They appear to be aligned correctly.

The dot on the chokes is correctly oriented in relation to the "boomerang" arm.

Yes, the choke arm moves fully if I move it manually. As Rob comments, mine looks slightly different from the one in the snowbum picture--(i.e., it isn't a boomerang shape because no return spring). It attaches to the choke cable with a metal nipple. I've tried to adjust this, but I can only make it so that it its one of the stops--it just doesn't seem that the arc of travel available on the choke lever (mounted on side of engine) allows the cable the full travel needed to fully swing the choke levers from stop to stop. This is what has puzzled me--I've spent a lot of time fiddling with it!

One other thing to note is that I'm still having these issues when I manually push the chokes open to the full stop, using my finger, before starting, and then manually closing them after it is warmed up. But let me adjust the fuel screws a bit as you suggested above to see how that changes things.

[gspd]

Sounds like your choke actuating lever travel is somehow being limited or blocked by something.
With the cables disconnected at the carbs are you getting the same amount of lever travel as shown in this video?
https://www.youtube.com/watch?v=l8rGcg_0UBo
oh, and just for the record, you should turn the choke off as soon as the bike starts (just hold the throttle open a bit so it doesn't stall).
Letting it warm up with the choke on is a very bad habit; it causes tons of unnecessary (and detrimental) carbon build up over time.
Best to ride away almost immediately after startup and just take it easy for the first mile or two until it warms up.

Once at operating temperature, fully screwing in the mixture adjustment on either side should cause that cylinder to stall.
If it doesn't, you have a blocked idle jet or passage.

The info in the link below should be mandatory reading for all BING owners, maybe some of it will help.
https://forum.boxerworks.com/viewtopic. ... fo#p130515