Carbs - how often should you rebuild?

[paulfinney2]

On an '86 R80RT, carbs were rebuilt with new floats at 37,000, six years ago.
Now has 76,000, and I will be replacing jets and needles. Haven't opened up carbs yet, or ordered parts. Should I expect to need new gaskets and floats?

[sumran]

Before the experts start weighing in, a few more questions. I will also be rebuilding soon. How often do the various hard parts need to be replaced (needles, jets. seats, etc.)? I have seen various kits, some with these parts and some without.

[bikerfish1100]

are you seeing a degradation of performance that can't be addressed thru an adjustment?
my '78 RS had 95,000mi on it when I traded it for a K75 (ah, fuel injection:heart ). the only carb parts I'd replaced to that point were the diaphragms.

[osbornk]

If it ain't broke, leave it alone. If it runs OK, you aren't gaining anything by working on them and you are risking messing something up.

[sgborgstrom]

Well, there's running "OK" and running "sweet".

The current rebuild has 39,000 road miles on it. that works out to what, about 1000 gallons of fuel through the system? How many heating cycles does that work out to?

bikerfish1100 has a point, as long as the carbs are responding to normal adjustment there probably isn't anything too wrong with them as they are. The problem is that the small leaks that develop in the system can be slow to make themselves apparent; slow enough you don't really notice they are there until something goes sideways enough to require major surgery.

Your carbs are probably OK depending on how complete the last rebuild was. Obviously something made you think about doing it again, so why not do it again? It's a fun project and not all that difficult.

[Isamemon]

so many variables
such as the extra miles... in those miles was the gas clean or were you on the back roads to alaska pulling gas from a truck

when was the last time the carb bottoms were off, might be full of yuck if it has been 30k plus miles

if its running good now, and you want a fun project, wait until winter and add them to your list of winter projects

stick with stock bottom gaskets, dont get the neopreme IMHO

[twintoaster2]

I haven't worked on carbs in a long time, love fuel injection. If it has been 39,000 miles I would rebuild them. Make sure you check the throttle shafts for play. Some shafts wear to the point that you have to have bushings installed. Plus there is the junk that collects at the bottom of the bowls. Better to clean them in the garage then on the side of the road. JT

[paulfinney2]

My gas mileage has dropped some, and has been inconsistant. When I rebuilt 37,000 ago, I just did gaskets and seals and the floats, but not the jets or needles. A knowledgable friend sugggested they will be showing signs of wear at 76,000, and should be replaced. So, I figure I might as well go ahead and do a complete rebuild, if the other parts need renewing.

[kstoo]

I just rebuilt the Bings on my '80 R100 just because I needed a point of reference. There are 82k miles on this bike and only about 3-4k are mine since almost a year ago. I don't have a history on her and she wasn't speaking clearly so this is one of the jobs that I did lately as exploratory surgery. Now that I know where to start counting from I would just check the floats for for discoloration and check the diaphragms for hardening, maybe once a year? Bear in mind that there are a couple of delicate things that can break expensively and needlessly while you are in there so do you really need to be in there? IMHO

[flash412]

How long is a piece of string?

[sumran]

How long is a piece of string?

Are you suggesting we should go through the carbs from one end to the other?:uhoh

[flash412]

Are you suggesting we should go through the carbs from one end to the other?:uhohNo, I am suggesting that the question makes no sense. Carbs tend to work until they don't. They need work when they need work. "How often" is a question that may be answered in years, based on dry rot of rubber components, or may be miles, based on wear of metal and/or rubber bits. Then again, it will depend on how it is ridden, the fuel run through the carbs and/or how the bike is/was stored when parked.

If you bother taking the carbs apart, replace all the o-rings just because you're there. Replace the diaphragms if they need it and leave them alone if they don't. They're easy enough to replace by the side of the road if necessary. LOOK at the needle jet and replace it if it is no longer round. Replace the float needle if it is no longer conical on the end.

Why are you replacing the jets and needles anyway? Unless they have suffered a mechanical insult from a ham-fisted disassembly process, none the other jets will ever need to be replaced. Fuel does not dissolve or wear away bronze or steel. Anyway, the steel needles may wear the bronze needle jets. The jets do not wear the needles.

IF they need work, take 'em apart and replace the o-rings and spray some carb cleaner through all the orifi and reassemble them. They'll probably work just fine after that. (You would probably benefit more by throwing away the metal-clad Beru spark plug caps and replacing them with sealed, plastic, NGK ones before you go to the trouble of rebuilding the carbs.)

[bikerfish1100]

:clap over and out.

[sumran]

Like I was saying, I didn't want to rebuild those carbs anyway. Think I'll just give the float bowl a quick cleaning ad ride the motorcylce.:D

[paulfinney2]

Most recent was a 2000 mi trip with mpg in low 40's. Then today, 265 mi, with 39 mpg. Probably will do a tune up, and end up checking the carb jets. Another Beemer head suggested things wear with age and use. Worth checking.

[sumran]

My mileage is consistently in the low 30's, which is what is making me look at the carbs. Everything I have read suggests it should be at least the high 30's or the low to mid 40's.

[crazydrummerdude]

are you seeing a degradation of performance that can't be addressed thru an adjustment?
my '78 RS had 95,000mi on it when I traded it for a K75 (ah, fuel injection:heart ). the only carb parts I'd replaced to that point were the diaphragms.

How did you do that?! Every time I attempt to get to the diaphragms, the little screws on top strip out.

[sumran]

How did you do that?! Every time I attempt to get to the diaphragms, the little screws on top strip out.

The factory screws require a unusual screwdriver, a reed and prince. If you use a phillips head the screw heads will usually be ruined.

http://www.tpub.com/content/aviation/14001/css/14001_39.htm

Some general info comparing the two is available at this site.

[jdmetzger]

How did you do that?! Every time I attempt to get to the diaphragms, the little screws on top strip out.

Aside from the other mention of a "prince and reed" screw (some actually ARE phillips), once you get them out, make sure to coat them with anti-seize before re-installation. Mine were pretty stuck and I used some PB-blaster and heat (small butane torch) and a good phillips screwdriver, until they came out. It wasn't TOO bad, actually. Just make sure the screwdriver fits well.

Regarding carb rebuilds: I don't agree that carbs just "work until they don't". You can get some pretty major degradation in performance before they give up the ghost. My R100GSD with 60000 miles on it was getting 26mpg when I got it. Even though it ran "well", I consider that broken. They ended up being filled with black sludge, the floats didn't float very well, and the idle jets were gummed up. Also, the gasket around the choke was not sealing properly and some of the O-rings were dried out and damaged. Just because "it runs" doesn't mean it's actually working right.

[kstoo]

I did not know that about the difference between Prince&Reed and phillips.
Glen at Thoroughbred makes sure that I get stainless steel phillips replacements with my rebuild kit. I have not had a problem getting diaphragm lids off lately anyway.
When I first got my 1980 R100(T) I was surprised to find that I got low 30 mpg as compared to low 40 mpg on the 1988 R100RT. I chalked it up to the fact that the 1980 has 40mm carbs, more HP and 32/11 final drive. I was shocked when I put it back on the road a couple of weeks ago to find that my 1st trip out was 46 mpg which settled down to a 43 mpg average since. The work done included carb rebuild which was mildly necessary but also major tune-up and fixing some very inefficient things like wheel bearings, shaft drive spring and rebuilding the final drive. OK finally here is my point: now that the weather has turned slightly cooler (it was 39° this AM) R100 is cranky. Lots of stalling when I start up from a red light and a 'surging' feeling when cruising at 3500 - 4000 rpm (5th). When I roll on from that surging condition there is a slight hesitation and then every thing clears up as I accelerate. My point? Oh, I think that the low - mid 40 mpg and cold weather behavior indicate that I may have some things wrong with my carb settings. I have some doubt as to if the needle position was correct as stated in Clymers (surging when cruising at speed) and that my idle jet adustment needs to be richer compensated by more idle speed screw. Are the float bowl level settings too low? I have to revisit these to see if I can get rid of the 'crankiness'; Gilera and I usually get along better than that. What is your opinion on the cold weather behavior that I am getting? Am I on the right track?

I have one other question in mind about the crankiness that concerns the spark plugs that were replaced. I will ask that in another thread to stay on topic here, sort of.

[kstoo]

am I off topic?
Concerning the previous post, I have some more confusion on this bike which for some reason does not follow the specifications given for a 1980 R100. The idle mixture screw is supposed to be 1/2 turn out but I couldn't get it to work until I resorted to the 1979 R100T specification of 1.25 turns. When I discovered that I neglected to use the 1979 specification for the needle position (3 clicks) instead of the 1980 (2 clicks).
I just took the tops off and readjusted the needle to 3 clicks and took a short test drive without warming the bike up. Hmm, seems to be better. I will know for sure tomorrow AM, 42°

[flash412]

Regarding carb rebuilds: I don't agree that carbs just "work until they don't". You can get some pretty major degradation in performance before they give up the ghost. My R100GSD with 60000 miles on it was getting 26mpg when I got it. Even though it ran "well", I consider that broken. They ended up being filled with black sludge, the floats didn't float very well, and the idle jets were gummed up. Also, the gasket around the choke was not sealing properly and some of the O-rings were dried out and damaged. Just because "it runs" doesn't mean it's actually working right.Like I said, they work until they don't. YOUR definition of "not working" was crappy mileage. How does one define "crappy mileage" in terms of miles or time since the last rebuild? Your bike still ran. But by your definition, your carbs were no longer working properly. Your definition had nothing at all to do with the miles or age on the bike. Your defintion was "unacceptable fuel economy." I.e. the carbs no longer work (to your satisfaction).

Q.E.D.

[crazydrummerdude]

The factory screws require a unusual screwdriver, a reed and prince. If you use a phillips head the screw heads will usually be ruined.

http://www.tpub.com/content/aviation/14001/css/14001_39.htm

Some general info comparing the two is available at this site.

Weird... all my screws are flathead.

Yeah, I know the difference between the two kinds, but for my case, my options are Strip & Cuss or Strip-a-lot & Cuss-up-a-storm.

[flash412]

I always use an impact driver to remove the tops. I seat it with a hammer blow (without any torque) and THEN hit it again for torque. Never had a problem; flat head screws or philips.