'78 R100/7 -- idles great, coughing when warmed up; Mikuni VM32 carbs

[YipcanjoDotCom]

So, I'm at a bit of a loss here. My '78 R100 has been gone through and gone over for the better part of this year. New wiring, Motogadget brain, new ignition coils, and (of course) new Mikuni VM32 carbs. It's a long story that's probably too sad for a forum like this

Anyhow, I've had help getting my Mikuni carbs dialed in with the right parts and decent settings. Brand-new VM32 carbs left and right. 3.0 slides, 40 pilot jet, 6DH3 jet needle (on the middle clip), and a 159 P-0 needle jet. I'm in the Seattle area, which is part of why those parts/settings were chosen. I had very good support from Bill @ Rocky Point Cycle getting to where I'm at.

All that said, the bike is now starting great, it idles perfectly (~950 rpm or so), and initially runs down the road with no issues. Unfortunately, after about 10 minutes or so -- I guess when fully-warmed -- the bike starts to cough under acceleration. It's similar to how a bike runs when you're running out of gas. At a stoplight (idling) it runs great again. The behavior sounds "starved of fuel" to me, but according to the Mikuni Pocket Tuner guide, it's more representative of a TOO RICH condition. My research also shows that it's likely in the "needle jet" or "jet needle" portion of the carb, if it's actually carb-related at all. I just don't know if I keep throwing parts at it to figure it out, or look elsewhere for the cause.

Thoughts or advice? Should I be looking at timing or valve clearances?

Thank you in advance.

Sincerely,

Stymied in Seattle

 

[20774]

Stymied -

Welcome to the forum! I'm not familiar with Mikunis, but if you've had help from Rocky Point, I would think you're in good hands there. Personally, I'd opt for an idle RPM more around 1100...you get better oil flow/splash for the lower end.

Valve clearance issues usually show up more at idle, not at speed. Is the air filter new? Clogged air filter could cause this.

Since I don't know those carbs, my thoughts go to the starved for fuel aspect but wonder about your gas tank. Is it possible that the tank is not venting and thus allowing fuel into the bowls? You can do one of two things. Easiest is to crack the gas cap safely when it starts to shows the symptoms. If the problem corrects itself in a few seconds, that's your answer. Another thing you could do is find a spot where you can shut the bike down safely. When it acts up, close the petcocks and shut off the ignition...coast to a safe stop. Drop the float bowls. What's the gas situation in the bowls? If they're dry, that clearly focuses on fuel deliver to the bowls. If the bowls have the right amount of fuel in them, then it's back to the carbs...somewhere.

Not sure what a "motogadget brain" is?

 

[1074]

The advice from Rocky point is usually pretty good but it is still ball park in nature. Each engine is a little different and even with the stock Bings, some variation can be noticed and changes made to improve performance.
I've known several people who have spent up to a year getting the Mikunis working correctly, changing jets, needles and needle positions to dial it in. And remember, they have to work the best when the engine is at operating temp. This includes everything from idle to WFO. You can get lucky and get it all correct on the first try but the odds are against it.
So. My answer for having the carbs work at their best is to find someone who has a dyno, knows how to use it, can read and have a good collection of Mikuni needles and jets, and has an exhaust gas analyzer that he/she also knows how to use. It'll cost some money but you should be quite happy with the results and the minimum down time it takes to do the job right. Now, you've read it on the internet so it has to be true.

 

[beemerphile]

I finally got Mikunis working well on my R75 and they have been solid for several years now. I would recommend that you just rule out spark advance by putting a timing light on it and making sure it advances properly before going further down the carb tuning dirt road.

 

[YipcanjoDotCom]

Personally, I'd opt for an idle RPM more around 1100...you get better oil flow/splash for the lower end.

Clogged air filter could cause this.

Easiest is to crack the gas cap safely when it starts to shows the symptoms.

Thanks. I'll get the bike warmed up and adjust the idle a bit higher. My air (pod) filters are clean. Also, I completely spaced about the gas cap. I'll try that too.

My answer for having the carbs work at their best is to find someone who has a dyno, knows how to use it, can read and have a good collection of Mikuni needles and jets, and has an exhaust gas analyzer that he/she also knows how to use.

I actually contacted a local dyno place today. I'm hoping to get the bike to them this week -- even if I can fix the problem myself somehow. I'd love to have it dialed in.

I would recommend that you just rule out spark advance by putting a timing light on it and making sure it advances properly before going further down the carb tuning dirt road.

I wish I had a timing light to do this. Actually, I wish I knew how to check timing at all!

Thanks for your advice so far! I'm going to test some of these items later today and see what comes of it.

 

[YipcanjoDotCom]

FYI, here are some photos of my bike:

https://imgur.com/a/zIpin

I've tried to post a photo directly to this thread, but I'm thwarted at every turn. Perhaps because I'm a new member.

 

[69zeff65]

Pictures tell many tales

The air box is eliminated and you have a modified exhaust on the bike. You might be in for some fiddling with those modifications in play, a stock exhaust and air box would almost be a plug & play situation but the mods may be a game changer on an easy tune for the bike. Bike looks good so it deserves to run good too so keep at it.

Good Luck.

 

[ccolwell]

How old are the carbs? One thing that would cause this is a sticky choke, i.e., the choke remains on when you turn it off. As the Mikunis I've had have starting circuits, levers that you push down to start, I'd look there and replace the orings, gaskets, or whatever seals that circuit. If it employs needle valves, replace those -- I had one of those go bad on a set of Keihin carbs (on a Honda).

 

[YipcanjoDotCom]

The air box is eliminated and you have a modified exhaust on the bike. You might be in for some fiddling with those modifications in play, a stock exhaust and air box would almost be a plug & play situation but the mods may be a game changer on an easy tune for the bike. Bike looks good so it deserves to run good too so keep at it.

Thank you for the kind words about my bike Yeah, the pod filters + exhaust definitely aren't making the job any easier. Still, I kinda wish I hadn't taken advice to switch to the Mikunis. They haven't exactly wowed me.

How old are the carbs? One thing that would cause this is a sticky choke, i.e., the choke remains on when you turn it off. As the Mikunis I've had have starting circuits, levers that you push down to start, I'd look there and replace the orings, gaskets, or whatever seals that circuit. If it employs needle valves, replace those -- I had one of those go bad on a set of Keihin carbs (on a Honda).

The carbs are brand new from earlier this year. They probably have less than 250 miles on them at this point. THAT is the kind of fun I've had with this whole project...

 

[pauls1150]

Hey Sty (Yip?) - Yes you absolutely must verify that the timing (and the points' gap if it still has points) and the valve clearances are correct - poor setup here will surely confound any attempt to set the carbs. The timing and points, and the valve lash, are known standards, while the carbs are the "issue to sort out".

There are threads in this forum - also look at Snowbum's site http://bmwmotorcycletech.info/ - that explain the timing setup, and set the valves just a bit loose until you get things squared away.

Is this a 32 millimeter carb (throat size)? Maybe that's too small...?

 

[1074]

Hey Sty (Yip?) - Yes you absolutely must verify that the timing (and the points' gap if it still has points) and the valve clearances are correct - poor setup here will surely confound any attempt to set the carbs. The timing and points, and the valve lash, are known standards, while the carbs are the "issue to sort out".

There are threads in this forum - also look at Snowbum's site http://bmwmotorcycletech.info/ - that explain the timing setup, and set the valves just a bit loose until you get things squared away.

Is this a 32 millimeter carb (throat size)? Maybe that's too small...?

R100s came with 32mm as well as 40mm carbs and work just fine overall with either size because they came with different corresponding valve sizes to match the carb size.

 

[pauls1150]

Thanks, Bruce, I didn't know that.
Beside the obvious difference, I was also wondering if the "small" throat might be the restricting point of fuel flow, seeing as how the intake and exhaust have been freed up.
Along the same line, I've heard a few times how those pod filters sometimes mess up the intake, due to a lack of air control - note how so many bikes' designers, especially performance types, take great pains to have a controlled plenum on the intake.
Also note the open ports on the carbs (Yip's fourth picture) - one is probably the fuel overflow, the other may be a vacuum port? If it's vacuum, certainly it needs a cap; the wide open overflow port may be having fuel sucked out by external turbulence, so get a hose on it.

One more possibility, along with the ignition check: perhaps the ignition timing is not advancing fully? This "should" show up earlier than half throttle, though, and wouldn't be temperature sensitive.

re Kurt's post - I've seen several cases where tight valves inhibit higher-rpm running, and this will get worse as temperature rises. A vacuum in the gas tank (blocked port in the cap) is also well worth the time to investigate.

 

[1074]

A Ha! The pictures tell the story. Whenever you modify the intake and exhaust you enter a whole bunch of carb problems. Starting with the original exhaust and intake will make it a lot easier because many have made the switch to Mikunis and have the experience to get the jetting and needle size and position correct. Changing exhaust and intake creates a real can of worms.
Back to the recommendation for the dyno time!