Happy Spring,
After working on my 81 R100RT 68k this winter, I took it for it's first ride this year. The bike makes, for lack of correct term, a burbling exhaust noise when not under load such as slowing down or riding downhill. It's not a loud back firing noise, but consistent burbling noise. I worked on the exhaust system replacing clamps where mufflers meet head pipe, and replaced cross over pipe that had rusted pin holes. Checked valve clearances, checked timing, all hose clamps to and from carbs. Everything to specs and tight. Bike starts right up and has good power through acceleration in all gears. Put fresh gasoline in, Sunoco 93.
Is there something I may have over looked?
redhorse
2 things can cause that....
• Air leak at the cross-over tube. There is plenty of un-burned fuel inside the exhaust. On decel, air gets sucked in through cracks and the fuel burns inside the pipes. When you assemble these exhausts you want to install that cross-over pipe with a coating of silver silicone sealer. Sometimes you can successfully add that after the fact from the outside without disassembly. (Look for small exhaust leaks marked by small black marks indicating a hole.)
• Extra-lean on the slow speed jets caused by slight Ethanol clogging. Add 1oz of Star-Tron fuel treatment at each fill-up and see if things don't improve in the next 2 months.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Wobbly,
Thanks, I remember reading in another thread, you mentioned the silver silicone. I did that when I put the mufflers back on the header pipe, but I believe I didn't do that when I replaced the cross over pipes. I will check them and apply the silicone. I'll add the star-tron additive and see how that works. Will balancing the carbs correct the low speed issue?
Will check back in a week or so to let you know. Snow in the forecast-aargh. If it's not on thing keeping you inside it's another.
be safe, stay healthy.
Balancing the carbs is always a good thing. Especially if it's been 5K miles or more since last done. When you're doing that, be sure and twist the idle mixture screw in and out before returning it to its normal 1-1/2 turns out. That helps dislodge build-up too.
I simply recommend the StarTron because the "blockage" is probably no more than a thin varnish-like coating on the jets. You notice it at low speed due to the burble, but if it's there... then it's on everything. And riding around with a cleaner sure beats sitting at home with a bucket and a toothbrush !! 😆
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Finally got some decent riding weather. After applying the silver high temp ATV silicone sealant to the cross over pipes and re-tightening all clamps the burble noise has greatly subsided, although still there. I also replaced the spark plugs with a one step up in heat range, Bosch W6DC.
I can live with the noise and it's not offensive. I just wondering if there could be any long time effects?
redhorse
I also replaced the spark plugs with a one step up in heat range, Bosch W6DC.
Thoughts.....
• What was the purpose in changing heat ranges ? You should be running the heat range recommended by the spark plug manufacturer, unless you've lost a significant amount of compression or burning lots of oil, and simply need the plug to make up for a mechanical fault.
• Was this "one step change" to a hotter or colder plug ? For some manufacturers going to a higher number is "colder", of others it's "hotter". There's not one common numbering system for spark plugs and it can get very confusing.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
My reasoning for going up "one step" in heat range was because if there are un-burnt gases then, in my thinking, a hotter plug might give a more complete burn in the cylinder and not in the exhaust pipes. Bosch nomenclature is the higher the number the hotter the plug, went from a 5 to a 6 non-resistor plug. The opposite is true with NGK, higher number, colder plug. I've attached a web site for spark plug manufacturers, models and their heat ranges.
Thanks for the feed back
redhorse
There is plenty of un-burned fuel inside the exhaust. On decel, air gets sucked in through cracks and the fuel burns inside the pipes.
That's not correct thinking. There is always unburned fuel in the exhaust, that's why modern vehicles have catalytic converters to help burn it. On decel there's even more because air is moving through the carb (picking up fuel) and then through the engine where it is not burned.
Additionally, to get the flow dynamics correct, both valves are open at the same time. Air in the air filter rushes through the carb and into the engine because the hot exhaust is at a lower pressure. A lot of that rushing air contains fuel that's going right into the exhaust. Next time you adjust the valves, run the engine to TDC. One cylinder will have both rocker arms depressed. The rocker arms can't be tight unless the valves are open.
I've had assembled engines on the bench that at TDC you could see straight through the combustion chamber.... the valves had that much lift.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
I may have solved my problem. This past winter i completed a push rod tube seal replacement. After pulling the heads i could see the pistons (attached photo) and found this incredible amount of carbon build up. After having the cylinders honed and valves reseated by a reputable machine shop, i cleaned the pistons, reassembled the cylinders and heads along with new PRT seals and rings, following Oak's top end manual. Took the bike for a break-in ride and the burble was gone. I'm guess that the heavy carbon build up was causing some sort of pre-detonation? I've completed the break-in period for the rings and the bike is running smoothly and still no burble.
redhorse
81 R100RT
And so which set of spark plugs are in the engine now ?
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Still running the Bosch W6DC, warmer than "recommended W5DC". I'd like to put a few more miles to see how gas mileage is and check the plug color. Tuned carbs with a Twin Max balancer, power is consistent through the full power range and gears. I also changed the carb jetting from 2.66 to 2.68 needle jet and 160 to 170 main jet. Left the notch setting at 2 for the jet needle.
170 mains are quite large for a US-spec R100.
Whenever I have had deceleration popping or burbling, after first ruling out exhaust leaks I have always found that opening the mixture screws 1/4 turn from fastest ide to cure things.
After working on my 81 R100RT 68k this winter, I took it for it's first ride this year. The bike makes, for lack of correct term, a burbling exhaust noise when not under load, such as slowing down or riding downhill. It's not a loud back firing noise, but consistent burbling noise.
• Pre-ignition (aka spark knock & pinging) only happens on acceleration or when the engine is under load, such as going up hill. It can be helped by removing carbon build-up, but again only when the engine is under load.
• The carbon on top of the pistons is proabably a mixture of fuel additives from the 70's/80's, and burned-on engine oil from decades of riding. Removing these deposits is always a good thing because it helps the combustion chamber perform as designed. I do not condone taking the engine apart solely to do this work, but its removal while the engine is apart for other reasons is good.
• 170 mains IS quite large. I would use 165 as a max, or even go back to 160. But again... the MOST difference is to be had on the Needle and Needle Jet because that's where 99% of all riding takes place.
• I believe the burble is gone simply because you possibly did a better job of sealing the cross-over tube while the complete exhaust system was removed. Or possibly because other minor (but significant) changes were made during this work. I believe you have made finding the true solution very difficult because you are changing multiple things (jets, plug heat range, carb balancing, engine disassembly) in a "shotgun" approach. IMHO, that is not the proper way to approach a 'tuning issue' at all. However, if you are happy, then I am happy.
All the best.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
This post appears to be just what I needed to point me in the direction of being able to rid my '77 RS of the annoying exhaust burble on deceleration. I have identified my exhaust cross tube as the most likely culprit, as is was quite loose on the left side even after tightening. Based on the fact that it was still loose after tightening, I am wondering if I I should bite the bullet and replace the cross tube with a new one? Also, earlier in this thread there was a mention of using "silver silicone sealer" to help seal a leaking cross tube. I really don't know what this is. I asked some guys at a couple of local parts stores and no luck there either. Even a search of the interweb proved unhelpful. Any advice is certainly appreciated.
Thanks
This post appears to be just what I needed to point me in the direction of being able to rid my '77 RS of the annoying exhaust burble on deceleration. I have identified my exhaust cross tube as the most likely culprit, as is was quite loose on the left side even after tightening.
I'm happy you were able to find this thread. I've been working hard to change the post titles to actually reflect the contents of the discussion in hopes of aiding future readers like yourself.
Based on the fact that it was still loose after tightening, I am wondering if I I should bite the bullet and replace the cross tube with a new one?
What you need to realize is that as air enters and ignites the fuel inside the pipe, that a tiny amount of "blow torch" effect takes place, and over many years the crossover pipe fits looser and looser. So, based on your year model, I'd order a new pipe to have handy the next time the exhaust system comes off.
Also, earlier in this thread there was a mention of using "silver silicone sealer" to help seal a leaking cross tube. I really don't know what this is. I asked some guys at a couple of local parts stores and no luck there either.
You know, I read and re-read every post trying to be as precise as I can possibly be. Sometimes you might see my posts with 6 or 8 revisions. That's just me trying to make details clear. But I guess in small ways we are all trapped by the English language ! So please allow me to back-up one more time and correct myself.
"Silver" (beside being one of the few words in the English language with nothing to rhyme with it) is both an elemental metal and a color, presumably named after that metal. Your standard, automotive silicone sealers have become so popular that they are now offered in an array of colors including: blue, black, white, clear, beige, red and.... silver. Which in my research I find that the makers actually refer to as "gray". My sincere apologies.
What I'm suggesting is that while any old silicone gasket maker will seal the exhaust cross-over pipe and do a great job, that by using the gray colored sealer, it will be easier to hide on the chrome pipes of your otherwise pristine Airhead. As opposed to choosing (for instance) the garish red colored sealer which might stand out so much as you keep you from winning first place in the "Best Looking" category at the next Airhead rally. We wouldn't want that to happen !!
This is what you need...
In the larger caulk gun tube sizes, I think that GE refers to the color as "Aluminum" because in that size, its main use is sealing aluminum gutters. So silver, gray, aluminum... trapped again by the English language.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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