R100/7 high speed hesitation

[blackwater]

High Speed Hesitation

I have a 1978 R100/7 (dual plugged, electronic ignition) that has recently been giving me problems at high speed. I have owned the bike for 6 months, and this problem popped up in the past 2 weeks.

The bike starts and runs fine while warming up on the road. It accelerates through all the gears with no issues. However, when I get it out on the expressway when the bike is warm, it will not maintain any speed over about 65 mph due to pretty serious hesitation (feels like its dropping a cylinder). If I back off the throttle for a few seconds, then give it throttle, it will then re-accelerate back up to 60-70 mph, but it will then begin to hesitate after about 5-10 seconds of sustained speed. As long as I stay on the back roads and keep the speed below 55mph or so, it will cruise and even accelerate hard with no issues. It seems that sustained high throttle is the point at which the hesitation kicks in.

I am relatively new to motorcycle maintenance, but here is what I have done to check the bike:
1) I have checked the plug wire connections, the wires are pretty new (previous owner) and the connections look good. I applied dielectric grease to them also.
2) I don’t think it is any kind of clutch slippage because it pulls under load in all the gears with no issue and there is no revving of the motor when the bike hesitates.

3) My limited troubleshooting led me to look at fuel starvation under sustained load, but I haven’t figured out the cause yet. I removed the float bowls and checked the fuel flow and operation of the floats. Both seem to be ok. The bike currently has small red briggs and Stratton screens on the fuel lines, they are both clean. I haven’t pulled the petcocks, but the inside of the tank looks clean. (One note, I can only get 10% ethanol gas here in VA right now)

I bought the napa fuel filters that snowbum recommends, but haven’t installed them yet. I have not pulled the carbs yet to check the jets for any kind of blockage. I don’t have any experience with motorcycle carbs and was trying to pinpoint the easy stuff first. Pulling the carbs and cleaning the jets appears to be my next step. The previous owner said he rebuilt the carbs last summer (I believe he did, he was a pretty serious bmw guy). The insides of the float bowls are clean, the floats are clean, and the bowl gaskets look pretty new. The only other related issue I have had was an overflowing left carb, I pulled the bowl and moved the float up and down which solved the problem (I was careful not to bend the float). I think that float was sticking.

Hope that is enough info, I would appreciate any tips y’all might have. I love the bike, just have a lot to learn about wrenching on it.

[20774]

What type of electronic ignition? I have a Dyna III on my R100/7 and with the first one (after many years of use) it would miss a beat ever so often. Certainly, not like what you're talking about. At some point, I had hard failure and was only running on one cylinder. I troubleshot it down to one of the pickups was bad. I bought another and have been running fine ever since. After the fact, I've come to find out that it's possible I may have been having a grounding issue. The Dyna plate is supposed to ground to the engine and there may have been corrosion beginning. I'll keep that in mind for next time...hopefully there won't be one!

The other thing...when you get this hesitation, try cracking open the fuel cap. If the problem goes away, then you have a venting problem and/or a fuel flow problem. You might want to find out what the main jet number is and be sure it the right size. Ultimately, you might want to be sure of all jet sizes and needle settings so you know where you're sitting and see if they're appropriate.

You could also have holed diaphragms. If they have holes in them, the vacuum can't pull the slides up for the commanded speeds and the performance suffers.

[108625]

Can you reproduce this hesitation with the bike warmed up and stationary on it's centerstand? It would be safer than cracking open the fuel cap while at speed; lest you do your "Colin Edward's flaming Aprilia Cube" impression.

Does it happen with the petcocks set on reserve? The filter screens on the petcock inlets that can gum up with or become clogged with crud, even if the tank looks clean.

[JETHRIDGE]

Try opening both fuel valves from the tank. I had this problem pulling a heavy load and opened the other valve and problem solved.(R100RT)

[blackwater]

Thanks for the tips. Here are the answers to the above questions:

1) I have a Dyna III ignition. I have not attempted to troubleshoot it, and really wouldn’t know where to start. I can pull the tank and check its connections though.

2) I tried cracking the fuel cap this weekend while riding. The problem still occurred.

3) I have not taken a look at the carb jets, needle valves or diaphragms. I am thinking I will have to do so.

4) I have not tried to recreate the condition on the center stand. I would think I would need to run it at least 3-4k rpm or so for 30-40 seconds to get it to recreate the hesitation or stumble. Still not sure if that will create the amount of sustained load necessary to create the stumble. What should I be looking for if I can recreate it?

5) I always run both petcocks open, I will try to run them both on reserve to see if that eliminates the problem. Didn’t think about that to trouble shoot the petcocks.

I am open to other tips. Will probably be this weekend before I can take a look at the more complicated stuff. Thanks!

[20774]

The connections of importance, at least the ones I was referring to, were under the front cover, not the tank. That is where the sensor plate mounts and where the pickups are. Wouldn't hurt to check all connections...

What is your fuel flow rate? I was curious and performed some tests using a graduated cylinder and ran the fuel for small periods of time, like 3, 5, 10 seconds. I also ran various combinations of petcock on/off...I think I did all mine with a reasonably full tank. A number that sticks in my head is 350cc per minute. I have info at home in my spreadsheet.

Update: checked my spreadsheet and the number I'd heard, a minimum of 350cc per minute per carb, was correct...got that from Tom Cutter. My flow measurements showed I was getting close to 600cc/min...with a full tank.

[jforgo]

Since the problem just popped up, I am initially suspecting something with the main jet/needle. Like crud on the end of the needle / in the main jet, causing a lean condition. The alcohol leaches black goo from your o-rings/fuel lines. Then it gets in your carbs.

Of course, a no longer intact diaphragm could mimic the above condition, as well.

Related alcohol problem is water buildup. Have you tried draining tank completely, refilling with fresh fuel? This would also drain any particles in your tank.

I had a more severe version of this, with full cutout at times. There is a little internal filter inside your petcock - mine was plugged with particles. Take it apart and look.

You can get the full rebuild kit for the carbs from EUBMW for about $30 - if you don't need it, stick it on the shelf. You will need it sooner or later.

[jdmetzger]

It definitely sounds like a fuel starvation issue, to me. Trying to recreate on the centerstand may not work as you're not pushing against wind, road resistance, carrying weight, turning the transmission/rear wheel, etc.

For petcock flow, you could just remove the fuel line from the carb and run it into a container to see if they both flow about equally and seems "fast enough". As others have mentioned, I've seen the petcock screens with crud (if it's orange - a little tank lining material), which can be an issue. Running on reserve will let you know if it's one pickup tube clogged, (you'll be pulling from the shorter straws). If you don't get any joy there, it will be time to check the carbs. If the guy did rebuild them, they sure shouldn't be giving any problems, and usually that stuff doesn't creep up suddenly.

[manicmechanic]

Have you recently changed where you buy gas? You might also check for some water in the fuel. Has the gas station recently started running with alcohol in it? And as a last resort, check the carb diaphragms.

[19991100RT]

Really sorry to read about your problem. I have a feeling you may be running too lean. Are you getting any pinging ? Pre ignition ? If it were my ride, AND if you are getting pinging or pre ignition, I don't think I would chance riding her. I'd have it towed into a shop and have the fuel system checked out. I know it is running well at idle and at lower speeds but, you may be being lulled into a faults sense of security. You surely don't want a hole in you pistons.
I hope you get it straightened out. All the best !

[BMWRich58]

Yes definately check the diaphrams in the carbs. Even a small tear/hole will give problems.
And remember,the tops of the carb's where the "silver plug" is,have been known to leak (lose vaccum)as they wear with age.
Check the area circled in Red.
http://img.photobucket.com/albums/v648/BMWRich58/colored.jpg

[sgborgstrom]

Sounds familiar, two possibilities come to mind:

1) Dirty air filter, won't flow enough air at speed
2) One of the spark plug caps is going bad and manifesting it's weakness under load

BTDT on both of these...

[DennisDarrow]

+1 on the diaghrams. Easy to replace and not to expensive. Be careful when removing those screws in the carb dome cause the heads are easy to strip out. While you have it torn down you can easily replace the O rings on the jets. If the motor was running fine before this happened leave your needles as they are and the same jets should be just fine also.
You might do a bit of searching about those screws I mentioned because there are several kinds over the years and some folks have found various tricks in dealing with them. I think Bing sells some Allen head ones that will help in the future.
Vetch sells carb rebuild kits to include the O rings and diaghrams at a very fair price..........Good Luck.......Dennis

[crazydrummerdude]

I've read about 1/20th of this thread, but I will say carb diaphragms.

My R75 would start and idle fine, would run up through the gears ok, but at about 65, it would completely die out. I would give it gas and it would just gllllluuuuugggg. I'd let off and coast back down to 50 or so, and it would be fine. Then, try to take it back up to speed, and glllluuugggg at about 65.

I opened the tops of the carbs, and there was about a 2mm tear in one of the diaphragms. Replaced both, and I can cruise and whatever speed I want to now. Man what a feeling!

[boxermaf]

I don't think that you mentioned whether or not you're running a stock air cleaner housing or those air pod things... while my first thought is that it is time to check the diaphragm and clean the jets and needles in the carbs, running out of steam around that road speed level can also often happen with those filter pods things - they actually don't flow enough air at higher road speeds and you end up with an overly rich fuel mixture, which can cause stumbling, etc. It is a bit of a tradoff trying to get the air/fuel mix right with those pod filters on bings on airheads - you're either running lean on the bottom end or rich on the top end.
The stock air filter system still seems to work better, IMHO.

[97077]

My vote goes for the diaphragms. Good luck.:brow Kevin

[EXR911]

Thanks for the tips. Here are the answers to the above questions:

1) I have a Dyna III ignition. I have not attempted to troubleshoot it, and really wouldn’t know where to start. I can pull the tank and check its connections though.

2) I tried cracking the fuel cap this weekend while riding. The problem still occurred.

3) I have not taken a look at the carb jets, needle valves or diaphragms. I am thinking I will have to do so.

4) I have not tried to recreate the condition on the center stand. I would think I would need to run it at least 3-4k rpm or so for 30-40 seconds to get it to recreate the hesitation or stumble. Still not sure if that will create the amount of sustained load necessary to create the stumble. What should I be looking for if I can recreate it?

5) I always run both petcocks open, I will try to run them both on reserve to see if that eliminates the problem. Didn’t think about that to trouble shoot the petcocks.

I am open to other tips. Will probably be this weekend before I can take a look at the more complicated stuff. Thanks!

Are the hoses from the carbs to the heads leaking (sucking air on high flow/load)? I had this problem and the hoses were as hard as wood. A new pair and careful clamp tightening and the problem disappeared.

PT9766

[blackwater]

Great stuff, thanks for all your replies.

A couple quick answers:

1) stock air cleaner housing and no evidence of pinging, pre-ignition etc.

2) I do buy gas at a couple different spots around town, always buy 93 octane. All the gas I have been able to find in my town has 10% ethanol. I have tried to find plain gas, but no luck so far.

I plan to start at the top of the fuel/ air induction system and work my way down:

1) Check fuel flow rate, Drain the gas, check the petcocks.
2) Check the air filter
3) Check carb jets and diaphragms, check air hoses into and out of carbs. Also check top of carb for leaking.
4) If that doesn’t work, new plug wires and start looking at the electrical system

Thanks for all your help. May not be able to get through all of this over the weekend, but I will let y’all know when I solve the problem (or create a bigger one)

[Twinfan]

Have you checked your valve clearance lately? Tight valves will cause this problem every time.

George

[Easy]

The same thing was happening to a friend’s bike. It turned out to be the spark plug wires.

Easy :german

[GregL]

This is exactly what my 81 R100 did after about 1000 miles on a set of Dyna dual-plug coils. I went back to the stock coils and single plugs and never looked back.

When a big twin drops a cylinder it is far more radical feeling than most fuel problems, as you are instantly losing MORE than half the available power... the running cylinder has to pull the dead one through compression strokes.

[blackwater]

Thanks for all your replies.

I think I fixed the issue, but haven't been able to ride the bike enough to know for sure.

I checked my fuel flow rate from both petcocks, was getting over 400 ml of fuel per minute (on reserve or normal) so I ruled out that possibility.

I then broke down the carbs checked the main jets and diaphragms on each carb. Jets were clean, needles were clean, and the diaphragms were practically brand new (previous owner said he rebuilt the carbs last summer) I did the left one first, went out and rode the bike, problem persisted. Then did the same to the right one, went out and rode the bike, no hesitation. Put about 15 miles on the bike and couldn't get the hesitation to occur again. Rode at high rpm in order to try to create fuel starvation or loss of vacuum on the slides, the bike kept on trucking. I am not convinced that this fixed the problem, but the weather and my work commitments have kept me from testing the bike more.

I am going to try to get on the bike again this week to see how she runs. I know I need to sync the carbs as well. My throttle cables are pretty well synced from checking them. I have never synced carbs, but I have read all the posts about it on here and snowbums site, so I will give it a shot. If anybody wants to make a youtube video of them doing it next time they set it up, you will have the undying gratitude of Airhead novices around the world.