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1978 r100rs won't start

J

jsey

New member
Hi all,
just tried to start my 1978 R100RS and won't start. Battery fully charged and cranking, spark plugs sparking, float bowls are filling up thus gas lines are clear. Has 40 MM Bing Diaphram carbs. Took the carbs off and found that the pilot jets where clogged so cleaned them with carb cleaner as well as all of the circuits and put them back on. Still nothing. The plugs are dry as a bone. Obviously not getting gas. What am i missing? Help!!
 
Do you usually use the choke system? I don't think my /7 will start without the choke.
 
Spark

So, first, check the valve adjustment probably not the reason but it is the first thing to tune up.

Second, just because you have spark, doesn't mean you have it at the right time. Check your point gap and timing.

Last thing to check is the carbs. Good luck, St.
 
STEVENRANKIN said:
So, first, check the valve adjustment probably not the reason but it is the first thing to tune up.

Second, just because you have spark, doesn't mean you have it at the right time. Check your point gap and timing.

Last thing to check is the carbs. Good luck, St.
Click to expand...

Sounds like a good approach. Could old diaphragms effect starting?
 
jsey said:
Could old diaphragms effect starting?
Click to expand...

Cracked or holed diaphragms probably could, or at least affect the way it runs. The purpose of the diaphragms are to raise the slides in the carb as air rushes through underneath the slides. The idea is that this rushing air lowers the air pressure in the throat of the carb and there is a tube that connects the throat to the area above the carb. So if the pressure is lower underneath the slide, the pressure has to equalize, so the slides have to move up.
 
20774 said:
Cracked or holed diaphragms probably could, or at least affect the way it runs. The purpose of the diaphragms are to raise the slides in the carb as air rushes through underneath the slides. The idea is that this rushing air lowers the air pressure in the throat of the carb and there is a tube that connects the throat to the area above the carb. So if the pressure is lower underneath the slide, the pressure has to equalize, so the slides have to move up.
Click to expand...

I’ll pop the lids in the morning and see what they look like after checking the valves and timing. The last time I ran it (Spring 2023) it ran fine soI kinda doubt it’s the timing or valves. Running a dyna electronic ignition
 
diaphragms

LOL, my friend just had a bike in the shop the past few days with two blown diaphragms. We are trying to figure out why both went at the same time? Anyway the bike would start and run but not ride very well at all. Great idle, crap pull.

So Dyna system, means no points, LOL, since I hate points that is good. I will figure for now since no points to get out of adjustment, your timing most likely will not have changed.

If the bike ran before storage, I am going to assume the valves were adjusted close enough. They sure won't change adjustment with the bike sitting.

That leaves carbs. Was the bike stored with gas in it? If yes, then gunk may be plugging up the carbs. Also, are the chokes working properly?

Where are you located? St.
 
Did you check air filter blockage? I've had few bikes, mine include that had mice invasion and on on guys mice nest and a snake skin, his had sat for only 3 months, just a thought.
 
If the Dyna III system "points" plate that was installed in place of the original points plate develops corrosion between the plate and the engine case, the necessary grounding needed would be lost. Things tend to not work when that happens.
 
Kurt, would he still have spark if the grounding was lost, I've not diagnosed his type of ignition? He said they were sparking in the 1st post. Dry plugs with sparks, I'm guessing valve adjustment or no air?
 
Good point, but it might throw off the timing somehow...it was something pointed out long time ago after I acquired my Dyna III.
 
Got it, creates a possible timing issue or too weak a spark maybe. I went from points to the crank style from EME which lets me keep the points just in case. Thanks for the info.
 
kinterridge said:
Did you check air filter blockage? I've had few bikes, mine include that had mice invasion and on on guys mice nest and a snake skin, his had sat for only 3 months, just a thought.
Click to expand...

+1 on checking the air box. They are enticing for mice.
 
ksdcr said:
+1 on checking the air box. They are enticing for mice.
Click to expand...

Ok I'm back. Sorry for the long departure. I got into the 40 mm Bings and have subsequently learned about the CV nature of them. They are the 94/40/105 and 94/40/106 model. Diaphragms were not torn but were thin. More importantly the five O rings ( butterfly valve arm, pilot jet, idle mixture screw, choke, and main jet holder) were all in very deteriorated condition, especially the pilot jet. I suspect that I had some serious vacuum leakage. I've now rebuilt both carbs and offered them a thorough cleaning. Peened butterfly valve screws was also a learning curve relative to their removal. Everything is back on and will try firing it in the morning. I once did have mice live in my airbox one winter but I subsequently installed a very close knit stainless wire mesh in the rubber airbox inlet so they can't chew through the OEM plastic grille and get in. It was clear. The only odd thing about these carbs is the the idle mixture screw is located inside the float bowl making adjustment during running impossible. Makes no sense to me
 
jsey said:
The only odd thing about these carbs is the the idle mixture screw is located inside the float bowl making adjustment during running impossible. Makes no sense to me
Click to expand...

Yeah, makes no sense. Never heard such a thing. Maybe the item you're looking at isn't really a mixture screw??? It has to be on the outside.
 
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