Carb Issues

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Carb Issues

Post by arzueck » Sun May 07, 2017 7:32 pm

Hey guys,

New here. Just came into owning a '78 R100/7. Was my Grandpa's bike for a long time and now I've inherited it. Looking forward to working out its kinks and getting it on the road - maybe someday handing it down another generation. It got new paint, seat, etc. in 2001 or so, and I've found a receipt for a carb rebuild around that time, but I don't have any record of any deeper engine work. 50,000 miles, but since 2001 it has had less than 500 miles on it and has been sitting for several years unused.

I've replaced all the fluids, put new spark plugs in (old ones were black and wet, looked really bad), and done a few other small things. The sticking point I'm dealing with is with the carbs:

When isolated, the left cylinder runs okay. It needs the choke until warmed up, then runs without the choke as long as I keep some throttle on it. Without throttle, it tends to die.

The right cylinder, however, doesn't want to run at all. When isolated, it coughs and barely starts. And when idling the bike with both spark plugs in, it barely feels warm by the time the left is hot. I've taken this carb apart and replaced all the O-rings and cleaned out the orifices - it was a mess in there. The left one was much cleaner for whatever reason but probably wants to be cleaned out also.

I have the mix screws at the base adjustment (one turn out, I think) because I can't really seem to get to a point where it is warm and idling on its own - thus I can't get a true adjustment.

So, any thoughts? My next step is new floats. The ones in there certainly don't look white and pristine...

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Re: Carb Issues

Post by SteveD » Mon May 08, 2017 10:29 am

New floats might help. Fuel flows ok? How much fuel in the bowls if you carefully remove them? Are they equal?
Have you cleared the small jet in the corner of the R float bowl? Starting is difficult if it's blocked.
Cheers, Steve
Victoria, S.E.Oz.

1982 R100RSR100RS supergallery.
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Re: Carb Issues

Post by Seth » Tue May 09, 2017 12:18 am

I find this to be the best carb cleaner

Gumout etc doesn't come close.

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Re: Carb Issues

Post by bluesman » Tue May 09, 2017 6:31 am

Don't assume it's a carb issue. I had a similar problem with 650 Yamaha. I had the carbs apart many times, only to find I had no compression on that side. Bent valve . Remember, the basics spark, fuel, compression.

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Re: Carb Issues

Post by jagarra » Tue May 09, 2017 10:51 am

Still sounds like you have some blockage in some small orifice on the low speed circuit. You may need some aerosol type cleaner, like a carb and choke cleaner or the other recommended item and use a wire ( some have tried fishing filament) to push through the small openings in the body. Make sure every jet is removed and all passages blown through with cleaner and air pressure if you have a compressor. Sitting and fuel evaporation clogs them.
Just went through process of setting floats, allow bowls to fill, closed petcocks and removed bowls and measure fuel height with vernier caliper, used 19-20 mm as a good starting point. With all the sitting, new floats would still be on list. Fuel lines also deteriorate over time, internal wall issues there causing flow problems.
Pull the coil wires at the coils, check for corrosion at the ends, both on the wire and inside where the spark wire attaches, great spot to find some green stuff.
1974 R90/6 built 9/73
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Re: Carb Issues

Post by arzueck » Wed May 10, 2017 11:18 pm

Thanks for all the replies.

When I cleaned out the right carb on the bench I used B12 Chemtool. First time taking a carb apart but I feel pretty sure that it is clean throughout. I have compressed air in the garage.

I guess its time to consider that spark or compression could be the issue, as bluesman said.

The left bowl is roughly 2/3 full when I pull it off; the right is maybe 1/2. I'll try to adjust the floats tomorrow...

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Re: Carb Issues

Post by Rob » Thu May 11, 2017 2:20 am

If you took the enrichment circuit apart, be aware that the disc needs to go in correctly for it to work.
And if memory serves, their orientation is mirrored each other on the two carbs?

I believe one of our contributors from England has a page with pictures. Rob Frankham? It looks like Rob's site is down.

Anyway, if you have not started already, take LOTS of pictures as you take things apart, so you have a reference putting them back.


From Largiader's website:
The choke is very important. Disassemble the choke assembly so you have the aluminum disc by itself and you will see a series of four small holes in an arc, with decreasing diameters. On bikes with a two-position detent on the cold-start lever only two of the holes are drilled through, but on older bikes three are. Each of these holes must be clear; they give you the increasing fuel enrichment as the disc is turned. When reassembling, make sure the gasket is flat and that the metal surfaces are clean and dry. It is almost certain that you will have to replace these gaskets because the old ones have sucked inwards.*

*emphasis mine
Rob V

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Re: Carb Issues

Post by barryh » Thu May 11, 2017 1:16 pm

It's very easy to be sure the enrichers are correctly assembled. Just make sure the pear shaped slot is at the top of the disc and the narrow end points towards the cylinder head. Visualise how the enrichers work with the pear shaped slot allowing no air in the off position and progressively more air into the outlet port to match the increasing size of the 4 fuel holes. Once you understand how they work you can't get it wrong.
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Re: Carb Issues

Post by Rob » Thu May 11, 2017 2:46 pm

Rob V

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Re: Carb Issues

Post by Rob » Thu May 11, 2017 2:55 pm

arzueck wrote:
Wed May 10, 2017 11:18 pm
Thanks for all the replies.
BTW, arzueck, if this bike is original, then you should have the tool kit and the owner's manual.

Both are amongst the finest of any production machine in the industrialized world. You have pretty much everything you need to do basic maintenance, plus a whole lot more.
And it's a good thing, because you will need them! :lol:

No, just kidding, it is a fine machine, but compared to fuel injection, computer controlled ignition and real brakes, it is a different world - which is probably why you are here! Stepping through the looking glass, so to speak. :) Have fun!
Rob V

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