Unable to get above 65mph

[marcmax]

I recently bought a 1982 LS that the po said would not go over 65mph. Sure enough I took it out on the interstate and it just barely makes it to 65. It starts fine, 1st hit on the starter with no choke, idles about 400-500 until warm and them idles about 800. Runs great around town, pulls away from a stop strongly, never stumbles. Runs up through the gears and the rpm's seem consistent with where they should be given the gear/speed combination. It seems perfectly happy running at 50-55. Once I shift into 5th it just has nothing and refuses to pull. At 60mph it is turning about 4500 rpm. After about 10 minutes it will creep up to about 65mph but that is it.

Bike has 9700 original miles. The po took kit down to the frame and replaced anything questionable while rebuilding it. He did say that he did not rebuild the carbs as they were clean and not sticking when he started the rebuild.

Any ideas on where to start looking?

[Barry]

Split diaphragms in the carbs is a strong possibility and easy to check.

[Bob_Roller]

I would start looking at the fuel system .

First there should be a small mesh screen type filter, where the fuel line threads into the fuel tap .

If you don't have a filter there, check the screen that should be over the pick-up tubes on the fuel tap in the tank .

Another low possibility, is that the fuel cap, which vents the fuel tank might be clogged, try loosening the cap a bit, and see if it helps at all .

[marcmax]

Thanks Bob. I put a new screen on the fuel tank end of the fuel tap when I got the bike. I have also tried two different fuel caps from different bikes so I had ruled out the fuel delivery until your comment made me take a second look. The po installed a small, see through in line filter and I am now wondering if that is restricting the fuel flow. I have enough oem BMW fuel line as well as polyurethane fuel line to replace it all so that will be my after dinner project.

[bruce_launceston]

I'm with Barry, check the diaphrams.
It happened to my bike and it was months before another R65 rider told me that his had done the same and it had a split diaphram. I checked mine there and then and it had a hole in one diaphram, he gave me his spare and my bike ran like new again.

In the meantime I was chasing my tail checking compression and fiddling with exhaust systems etc etc trying to find out why it wasn't running as it should.

[nhmaf]

My money is on the carbs at the moment - either the diaphragm is cracked or is not properly sitting in place with the little tab on the inside edge where it belongs - either way, you won't get the necessary vacuum.   Or, it could be something else is amiss with the carb jets and air/fuel adjustments as well - if you can start it with no choke and the air temperature is below 80 degrees, then it is definitely setup too rich.   A too rich condition could also cause it to 'bog' at higher RPMs, such as when going above 60 MPH in top gear.   Will the engine pull strongly to 6K RPM in the lower gears (where you are less likely to get a speeding ticket)?

[montmil]

It's the carbs. They need an overhaul including a complete disassembly and cleaning, new diaphragms, inspect all jets, perhaps new floats. I had a similar issue with my '81 R65. After the rebuild on the carbs, it's a rocket.   Monte

[marcmax]

Thank you one and all for the advice. I have the luxury of a second set of carbs on another R65 in the garage so I'll pull them and rebuild them and swap the pair. I guess my weekend has been planned for me.

[Bengt_Phorqs]

My vote is diaphragms also.  A quick way to check is to warm up the bike, then lift the throttle cable on each carb separately.  It it speeds up then drops off, or doesn't increase rpm at all, there is most likely a hole in the diaphragm.  That's my 2 pesos.

[R65_Mark]

I’ll join the crowd and give another vote for the diaphragms as the prime suspects, but check ignition timing as well, particularly that you are getting spark advance as revs rise – simple to do with a timing light.  When rebuilding the ‘carbs be aware that the jets are very fine.  If there is any chance of fuel “varnish” build up use a lot of “carb cleaner” and some kind of bristle to agitate the cleaner in the jets.  Mark

[ambrose78]

Just wondering - have you checked the tank breather pipe?

[marcmax]

This forum is the best and Barry gets kudo's for being first. I pulled the carbs this morning and not one but both of the diaphragms were bad. One had a hole as big as my finger through it. I had a rebuild kit in the garage for my other basket case so I replaced both diaphragms, replaced the carbs and headed out on the interstate only to scare the $#!t out of myself. This is a 1982 with an 80mph speedo and the needle was buried and I was still accelerating. Yippee. Life is good!

[Mike V]

All good suggestions, let me add to your list of to-do's by suggesting you do a quick and easy fuel flow check below the in-line filters with the gas cap secured. Also make sure the mechanics of your throttle cables are operating correctly and completely opening your carb throttle mechanisms to full position (visual inpection). I would then check your timing, if you're getting full advance go to your carbs. Are you experiencing any strange audible tones at full throttle such as a deep hollow sound? Keep us informed on your progress...

[nhmaf]

We are glad to help - with the collective (and growing!) brain-power of this group, there aren't too many things that cannot be solved.  We do get distracted from time to time, though..