Struggling at 70mph

[Firenze]

Just had a few days touring in Normandy ,first time out on the '78 R80/7 ( thanks for the advice on my brakes) and had a great time on the country lanes,going around the D Day sites etc. Good starting and tick-over,quiet motor which pulls well,only issues were at about 70mph it stutters (surges) a bit and just has nothing left to offer, Also after pulling over after a good run up the main carriageway it was blowing whiteish smoke from only the left cylinder pretty badly. This stopped after resuming a slower pace. I have never had the heads off this one but I am thinking piston rings , worn bore ,valves and guides or all three .I but why is it ok most of the time ,Also I had good compression on both cylinders last time I checked . Anything else I should be looking for ?
I want to re check ignition timing this week with a timing light but apparently cant adjust the back plate while running with a timing light as the advance and retard unit is in the way on the screws. I have been told It is a case of trial and error , is there any way to adjust with the motor running? Just to say I am loving this bike, after years without a bike or with my last one a K100rs this one even sounds right !

[Jean]

On my 78 R100s:
Stuttering and surging suggests you need to change the carb diaphragms.
Are you still running the old coils? I also had problems around 70mph until I changed them...
I don't know what you've done so far on this bike other than the brakes so just telling what worked for me.
As your bike is also a '78, it might be the trick if you've not already don these things.
I didn't SEE anything wrong with my carb diaphragms but when I put new ones in, there was a huge difference in acceleration. New coils improved gas milage and better running at speeds over 70, too! Not to mention easier starting.
By all means, check compression again just to be sure something didn't suddenly happen. What do the spark plugs tell you? If serious oil in left side, the plug ought to show it. (Are you using the side stand? Left side will smoke a bit if you don't stop engine first before leaning it over.)

[Jean]

OK, found your post about rattling brakes.
I will suppose your bike has been sitting around a while (?) so another thing came to mind about the poor running at highway speeds.
If you have not cleaned your carbs, there might be gummy deposits in the jets where you cannot see easily. Assuming you have BINGS, the main jet is an assembly and up inside the parts that hold the nozzle is a place where gum accomulates. You have to undo all the main-jet parts to clean it out. (This is ALSO a problem that gave me a hard time solving, until I discovered the solution.)
Compressed air and solvent soaking will NOT do the job, you have to worry the stuff away. Toothpicks and tiny COPPER wires do the job.

[chasbmw]

Diaphram problems usually show themselves at lower speeds.

A refusal to go much over 70mph, are likely to be fuel problems, either rate of flow from the tank (clean filters and screens) or more likely in my experience to be floats adjusted too low.

[Major Softie]

Dying coils can do this as well.

I'd inspect the diaphragms first. If you find any suspect spots that might be a pinhole leak you can use a little silicone seal and, if it fixes it, you know for sure that's the problem. If the problem is only on one side, you can very quickly determine if it's electrical or carb by switching components from side to side.

[Firenze]

Thanks for the replies, to answer a few points, Yes! it has indeed been sitting around for a few years, and after owning a K100 I Never use the side stand if I can help it ,in fact on this one I have it permanently tied up to prevent the risk of smoking on start up. Yes! it is still running with the old coils on it , No I have not looked at the diaphrams or had the carbs opened up yet except having a look in the bowls which looked clean.. . I haven't changed anything on this one except replacing the contact breakers (points) and condenser couple of years ago. Plenty to check out , I will start with the floats and diaphrams and fuel supply and work on from there . off to make a start right now! thanks again .

[chasbmw]

You do need a ful carb rebuild as soon as possible, change all the gaskets and the diaphram, change all the O rings except perhaps the butterfly spindle ones. I would also throw in replacement needle jets, as they are a wear item. Remember to check the float level as well.
Blow through all the internal passageways with carb cleaner.

[Duane Ausherman]

As Jean said, do not expect compressed air to do the job. It may work, but probably not. Very soon in my repair shop I learned to never waste the time. A wire does it for sure.

[Jean]

Sitting for a few years means: Head straight for the carbs. Pull ONE and undo the main jet parts. I bet you'll find gunk in there. If you don't have high milage, I think the actual parts will be OK. Probably the O-rings should be replaced if you can get a rebuilding kit that includes ALL the correct parts. Last kit I bought had wrong size O-rings for the air screw.
A piece of lamp cord or wire from an auto wiring harness will yield some fine copper strands that are nice for carb cleaning. Don't use steel, it will either score the tiny carb bores or enlarge the holes.
I use pipe cleaners with the end folded over to clean the larger voids.
What carbs do you have? I think R/80 is a "32".

[Rev Light]

Check your float bowls for water. If you have water in them, drain and flush your fuel tank.

I had this issue on my R100 when I first got it.

You can see the water clearly. It don't take much.

Cheers

Rev. Light.