r100 carb issue?

[Joe S.]

At least I think its a carb issue.

A friend has a 1979 R100T which he wants to sell and I'm enamored of.

It seems to warm up slowly and has a horrible amount of low speed surging and jerking until it does get warm, not like any boxer I've ever seen.

HE has repaced about everything in the carbs diaphraghms, seals and so on that you can get to without removing them, about everything but the low speed jets from what he says.i.

Even when hot it still stumbles at low speed, just not as badly.

He states the points are set right, the tank was cleaned out and the engine sounds very good. Interestingly, it idles near perfect.

He is a loong time friend and i don't doubt anything he has told me so that is not an issue, but i do want to be carfeful what i get into.

For a lot of reasons I really like this bike but would appreciate any thoughts some of you might have.

[szurszewski]

I am not smart enough to do much carb work in person, let alone guessing via the internet, but have you considered taking the bike to a good airhead mechanic? If you were seriously considering a bike owned by an unknown party, I would think that would be a great idea, and in this case, finding out what's going on/what needs to be fixed could help maintain your friendship. In any case, even if you find out it's something you don't want to take on, your friend might have an easier time selling the bike if he actually knew was what causing the issues.

josh

[lkchris]

First thing to do is verify correct valve adjustment and test engine compression.

 

Then verify condition and tight connection of rubber "manifolds" between carbs and cylinder heads.

 

When these are OK, then you can move on to carbs.

[elkroeger]

Could be the carbs, could be a lot of things... Sounds like its just an adjustment, rather than something serious. But what do I know? I'm way up here in seattle. Either way, keeping a "mature" cycle like that going can be a rewarding experience.

 

Are those Bings? I had a 78 R100/7. The carbs are easy to rebuild. get the kit from bingcarburetor.com. It's been a while, but as I recall, the idle and main jets were two separate components. That would explain how it idles great, but sputters when you open it up. If it's the carbs, I'd put money on some clogging or maladjustment in the main jets. When was the last time they were rebuilt?

 

Boxerworks.org used to have a great forum, but I can't figure out where it went.

 

there's also the airheads.

 

http://www.airheads.org/

 

 

[Michaelr11]

Did he replace the needle jets and the needles? Do you have (does your friend?) the Bing Carb book. The book lists all of the settings for different year/model bikes and carbs. The book is cheap, like $10 or $12. Make sure that the right needle jet was installed, if he replaced them. Perhaps you should take them out and spray them with carb cleaner to make sure there is nothing blocking them. The needle jet is what takes you off idle until you open the throttle so far you are on the main jet. Sounds like that's where you are encountering the issues.

[Joe S.]

These all sound like great ideas. I know he ordered carb rebut kits but stopped short of ttaking the carbs off. This is why he suspected the low speed circuit, apparently you have to removenthem to get at it?

They are in fact Bing . I'm not sure if he did a compression test or not but it's a good thought.

I tend to believe it's relatively minor, the rest of the bike just looks so good. I know my friend is telling me all he knows, it's just that there is not a lot of known history on this bike as it was a trade in at a Honda dealer.

Very much appreciate the ideas so far.

[lkchris]

These all sound like great ideas. I know he ordered carb rebut kits but stopped short of ttaking the carbs off. This is why he suspected the low speed circuit, apparently you have to removenthem to get at it?

 

Only things requiring carb removal are o-rings on throttle shafts. These require replacement approx every 20 years.

 

Of course to me "removal" means disconnecting the throttle cables, which is the only thing that really creates any extra work.

[taylor1]

AS elkroeger mentioned try : boxerworks.com/forum2 I go to their site quite often just to read and ask the occasional question. They are much like the folk here, very knowledgeable and willing to help you through it. You may also want to google Snowbum and Duane Ausherman. They are both airhead guru,s with excellent web sites that cover every nut and bolt on the airheads

[rich t]

Since you're having rough idle, I doubt the problem is in the needles. Look for the issue in your idle passages. I also would not bother with a mechanic; these are some very simple carbs.

 

Make sure your enricher is operating correctly. I hade a loose nut on mine at one time, and the 'choke' actually was stuck open. Run the choke back and forth and validate the circuits are moving. It is also possible the enricher circuit could be put together backward (blame it on the previous owner).

 

I'ts been a couple years since i've been into the bings, but if i remember correctly, there may be a fuel passage in the float bowl that could be plugged. If you take the float bowls off, you will should see one corner of the bowl with a round casting and a tiny hole in the bottom of the bowl.

 

I would also take the carbs off the bike. It's a little work to make the job a lot easier. If you take things apart carefully, you can get away with not having to buy a 'rebuild' kit. The bings on my old airhead had a split barrel that allowed the removal of the throttle cables without having to mess with throttle settings, so i didn't worry about throttles being out of sync.

 

Check the plugs and pipes after idling, to be sure the bike is getting fuel (google reading spark plugs if needed). The pipes will be too hot to touch after just a few seconds of running.

 

if this bike is a 79, it may be setup for higher compression, and might not like 87 octane. One last item to try is to use higher octane fuel. I'm putting my money on the idle passages though.