1991 R100 high speed sputtering out...

[MotoD]

Hey guys, I am finishing up a 1991 R100 and hung up on the final part..This bike is putting me through the ringer here. I've tuned a good handful of these awesome machines and this one isn't wanting to play nice.

Heres the deal...it runs beautifully up until high speeds. At 60-65mph it starts to cut out/sputter until I let off the throttle a bit and drop speed then it is back to normal.

Here is whats been done:

Full carb rebuild, new bing carb kit and diaphragms. DNA pod filters, mac 2-1 exhaust. Fuel level set to 24mm, jet needle clip set on lowest point. I tried a 145, 150 and 160 main jet. No difference really, actually the 145 runs the best. Plugs after a good hot run are pretty white still which would indicate a lean condition. Also has slight back fire at times when off throttle at speed.

I could get it up to 75mph and thought it was an ignition issue. It originally had an Accel two pole 3 ohm coil on it. I thought maybe it was doing some weird stuff so installed a new 3 ohm euromotoelectronic coil and new NGK wires.

Now I can only get it up to around 60-65. Ran better with all the old stuff. At first, I checked the float level and it was LOW like 12mm low. Raised it to 24mm as I have the 32mm carbs. Helped slightly but still cuts out. I pulled the inline filters, new fuel hose clean and clear tank. Carb sync good to go as well.

I tried opening the cap when the issue is present and no change. I really think it's still a fuel issue but running out of ideas now. I was surprised the 160 main jet made it run worse before replacing any of the parts it would ramp up to 75mph no issue then like others have mentioned it just sputters out, like the fuel is cut. I need to double check valve clearances but they where set to spec roughly 125 miles ago. I drilled a small hole in the bottom of the stock gas cap to and no change. Appears to be venting normally.

The bike starts, idles and runs absolutely flawless besides this damn issue. I did order some more thermal paste for the ICM. It appears to be lacking a healthy amount there. I also do have a full motogadget setup, stock ignition otherwise and I did see a thunder child diode board last time I was in there.

Tomorrow I may toss the old accell coil back in and just see what happens...
Thanks for anyone who has some comments/info. You all ROCK!

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[gspd]

Try it without the air filters and see what happens.

also...

You didn't mention ignition timing.
Regardless of where it's timed now,
turn your ignition bean can a few degrees one way and go for a short test ride.
If that doesn't work, turn it a few degrees the other way, and go for another test ride.

Report back.

[Rob Frankham]

Try returning everything you can to stock. This includes the pipes which may well be what is causing the problem. This will establish whether the mods you have made are causing the issues. If the engine runs properly in stock configuration, then it's something in the mods you've made, Try putting them back one at a time until the issue resurfaces. If engine still runs badly, then you know it's something in the stock parts you have retained.

I would re-iterate, try it with a stock exhaust. Changing the exhaust configuration can have severe and unpredictable effects on engine running. It's also a good idea to re-instate the fuel balance pipe if, as I suspect, it has been removed.

Also, if possible, try another bean can... the hall effect sensor, if it's getting beyond it's sell by date, can be affected by vibration... the same with the ignition unit.

So many things can cause this type of symptom... but the first thing is to establish whether it's anything you've changed...

Rob

[SteveD]

Heres the deal...it runs beautifully up until high speeds. At 60-65mph it starts to cut out/sputter until I let off the throttle a bit and drop speed then it is back to normal.

Diaphragms. I know they're new, but check they're correctly seated.

[daz]

I'm with Rob and Gspd. I think the problem is either with the air cleaners or the exhaust. Or the fenders.

[Tom H Ca]

I had something like this. Would act like it was out of gas or cutting out at about 70mph. Went through the ignition system and still had the issue.

I think I have solved the issue. I removed my VW style fuel filters and I think the problem has gone away. Need to ride it more to be sure.

Tom

[MotoD]

Thanks all for the info:

So here is what I did. Pulled the pod filters and played with the bean can timing a bit with no difference.

I pulled the chokes up a bit and could actually get it up to 78, it still hiccuped but not nearly as bad. Hence richening the mixture a tad.

Here is what I noticed though, when I pull the left choke up the bike wants to try and die...no shock there
but when I pull the right side choke nothing...keeps running normally. So I am wondering if at high speeds with higher fuel flow if the right side carb is giving issues, sputtering and cutting out essentially killing power. So I will pull the right carby later and go back through it. These carbs do have individual pulls installed.

Besides this like I said the bike pulls incredibly hard and runs fantastically so don't think it is the exhaust...maybe like @daz said its the loss of fenders

[gspd]

Heres the deal...it runs beautifully up until high speeds. At 60-65mph it starts to cut out/sputter until I let off the throttle a bit and drop speed then it is back to normal.

moto D -
High-speeds is a subjective term.
I have no doubt that those tires could make 70mph feel super-scary fast but just for the record, 70 mph is not even close to 'high speed' for a r100; it's just a very comfortable partial throttle cruising speed.
Any year/model r100 worth it's salt should do somewhere around 40mph in first gear, 60mph in second gear, 80mph in 3rd gear, 100mph in 4th gear and well over 100mph in 5th.

I guess the first thing to do is sort your choke issue.

What have you done with the crankcase breather?

Does the bike pull hard to 7000rpm in the lower gears?

[MotoD]

Thanks mate, these tires are actually quite decent for road use. They are Heidenau K60 Scouts. 100+mph speed rated.

I agree, High speed is subjective. Max speed around my area is roughly 50mph. Even the highway is 60mph but when heading south to Seattle it’s near impossible to comfortably cruise at 70mph which is what the freeway turns into.

It can definitely get up over 7k rpm’s in the lower gears. I’m getting most my issues in 4th/5th gear.

Crankcase breather has a filter on it. The new engine cover has a 3D printed air opening to allow the filter to vent to open air. I didn’t want it fully enclosed.

I’ll take my right carb apart and see what’s up!

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[Rob Frankham]

I think it's a mistake to think that tyhe ability of the engine to rev in lower gears is an indicator to what is happening. An airhead will rev to the red line with about a third throttle in first. What affects the output power of the engine is the carburettor setting and sustaining 7,000 rpm in top gear requires a much greater carb opening then that required to sustain it in a lower gear. With a CV carb, the throttle opening (i.e. the lift of the piston) isn't even dependant on the butterfly setting (i.e. the amount of twist on the wrist). All it suggests (and that isn't for certain) is that the problem is more likely to be fuelling or gas flow than electric.

On the exhaust... don't discount the non standard exhaust. Exhaust tuning is used to tailor the engine response at various revs and throttle openings. An exhaust set up that improves torque at low engine revs is very likely to cause issues at high revs and throttle openings. I'm not saying that's what it is, just that it's something that needs to be eliminated.

The relationship between engine revs, throttle opening and load as subtle and complex. It is very unlikely that something that improves performance in one area of the load/power/rev diagram won't detract from the performance elsewhere...

Clearly your right carb has an issue and that's the first port of call. Hope it solves the problem...

Rob