Wonder if I can get some advice here? The other day I went out and started up the old r75 same as always, she irked right up and seemed to be ok. Here is the issue, I had it a little richer than normal because i had been tinkering with carb settings over the last week or so but she still started right up and seemed ok, at first. As I at there in the driveway with the choke on and engine warming I burped the throttle a few times and then BOOM, a big backfire! Oops! It sputtered and died and I have not been able to get her running again. I have had the carbs off a few times looking for dirt or something but find none, I seem to have spark at the plugs but it just spins over and backfires some times. Any thoughts? It would always fire right up before, maybe turn over once and catch and idle just fine. When I pulled the plugs looking for spark I did take a flashlight and look into each cylinder, it seems that the valves are all moving ok as well as the pistons, it blows my hair back as I look into the plug hole...hummmm
It was probably just unburnt fuel igniting in the muffler. How long was it idling? Try leaning the mixture some.
Possibly the points, if that is what the bike has.....have closed up some. Check ALL electrical connections......clean and grease. Could be CARB troubles....Perhaps you adjusted them a little TOO much....Maybe BACK off the settings that you changed, BACK to where they were before. New or clean and gap the SPARK PLUGS. Backfiring is usually TIMING and or LEAN condition. Good Luck and please post back. spo
Checked one when I had some time last night, hope to get to the other tomorrow, what could have happened to them? Rupture? They were ok when I went through the carbs this spring. Guess I will have to pull the valve covers and timing cover tomorrow evening as well...although I don't really know what to look for now
In your profile it says you have a 1973 R75/5, I suppose this is the bike in question? Do you have a booster or any other modification to the ignition? I would start with ignition. You have points and they can be loose. Check the point gap and then check timing. I do not suspect the carbs at all. A little rich on the mixture screw does not blow up engines. It makes the plugs black and that is usually all it does. Many engines today are being run rich because of the crap we have for gas. But while we are talking about carbs? The ones that your bike was built with are 64/32/9 & 10. If these have not been modified the mixture starting point for them is 0.75 (3/4) turn out.
Yep diston, same bike we have talked about before, stock everything. The plugs were black when I pulled them as I had the mixture screws almost one full turn out. Definitely richer than normal, usually the bike likes things closer the the half out point and it runs pretty good, more and she gets a little too fat and muddy. Anyway, the timing was next and that's what I will tackle tomorrow. Dang thing is, I just checked the timing and points about 100 miles ago and all was well...I will let you know what I find out! Thanks as usual!
Every time everybody else is saying carburetor, I almost always start with the ignition. We have lost touch with these things in our joint consciousness. The fact is you could have adjusted the points yesterday and they are still suspect today. Often times the screw holding the adjustment is not tight enough and it slips. Or grease was not put on the rubbing block and they wear prematurely. If you don't have a broken valve head bouncing around inside the cylinder then I think it's ignition. Many things will cause poor running and then eventually not running. But the ignition can go all at once more often than a lot of other things. I never heard of the wind blowing my hair type compression test?
Yep, the breeze out of the plug hole is a Kentucky compression test! Or at least tells me I have something
I'll take this at face value and say you had an backfire and it came through the airbox. If this was the case then I would look at the carbs and see if any seals were blown out or if the diaphragms were ruptured. I would also check the floats and see if there is any crud. Turn the gas off and take one of the bowls off. Turn the gas back on and see if you have flow. Then gently push up on the float to see when the gas stops. Do the same for the other side. But avoid a mess and put the other bowl back on.. The top of the floats should be about even with the bowl mating surface of the carb. Ruff estimate. I've had this happen too with miss adjusted valves, jumped timing and bad ignition.. But since it was running fine and you were fiddling with carbs I would start there. If you do the above and all seems well, I would re-do the carb adjustments, and check to see if the cable runs are correct if you had the tank off.
Another vote for looking at the points. A few things to add, replace your plugs and make sure the HT leads and coil are also in decent shape. Make sure the battery has a nice full charge. Double check all the grounds, especially the ones in the ignition and starting wiring. These bikes bounce and vibrate and stuff can get loose. Get a weak spark slightly foul the plug up and stuff spirals quick.
OK you guys saved the day once again! SHE IS ALIVE! Looking at the points after work showed that the gap was a little off so I decided to swap them and condenser for the items I bought some time back. I really didn't like the thought of getting the old girl some distance from the house and having an issue on the side of the road. Anyway, the replacements fixed me up! The only thing I really saw was this original part... Says it was made in West Germany on the back so it was an older part, maybe original, anyway she is running and I thank all that replied! Hope someone else will be helped by this as well! LOVE ADV!
It looks like the condenser was compromised for what ever reason. No condenser, no spark. And changing the points was probably a good idea. It is really hard to gap a used set of points, sometimes impossible. It is also considered an improvement of the system to add a Booster because the points will last for a long long time with a Booster and the condenser is not used with a Booster. If there should be a problem the Booster can be by passed and the ignition returned to OEM in a few minutes (unless like me you have bar backs which adds a bunch of time to the procedure, I know, recently had to do this in a shopping mall parking lot) There are other electronic ignition systems that can be used on Airheads. Some have other added advantages but the Booster is the one I prefer (I have had a Dyna III). But I will be running the OEM system for awhile, till I get a new job.
yeah, I too prefer the points even though I have had this issue, and I prefer to keep things stock. I just chalk it up to the bike having sat for say 20 plus years and I think I should be good to go now. The idea that I could do this on the road side in thirty minutes is a big plus for me and now I know the symptoms. By the way, the condenser tab was in place and still touching the contact inside because of the metal bracket that supports the rubber boot that the points wire rides in. I was still getting spark but not nearly fat enough. I thought when I was checking things over and messing with the carbs so much that the spark from the plug looked a little anemic but I thought it was just me. Because I am the one that got the old girl running in the first place, one of the first things I did when I got her home was to get a new battery and turn her over a few times with the plugs laying on the jugs so I could check for spark and look into the plug hole for any movement and do my Kentucky compression test! I really think that the time I spend in the garage "looking" at my equipment helps to troubleshoot and repair down the road because I kinda know what to look for...even though I had to ask you guys this time. The bike seems to run better than it ever did and I may think about adding the booster in the future. As for the parts, they were ordered from BMW in August of this year and I did get the crappy points set from China, at least that's what it said on the box, the points, who knows, no markings. I will reorder some of the good ones next week! This place is an invaluable tool for old wrenches and quite the community that I refuse to live without! Good day gentlemen, I am going to ride my airhead!
Per Dyna's instructions, the stock condenser is still to be used with their Booster. It serves as a connection poibt between the engine wiring harness and the points wire. Used with a Booster, the condenser leads an easier life since it doesn't have to absorb the voltage spikes from the coil primaries. Good idea to replace it when the Booster is installed. It does make the points last indefinitely, though the rubbing block wears with use and you have to periodically adjust the point gap and refresh the point lubricant. As the rubbling block wears the ignition timing drifts "retarded" and this can be seen with a timing light. And if you do have to revert back to points-only all that needs to be done is to switch two wires under the tank. The BIG advantage of the Booster besides point life is that the "dwell" (which determines the magnetic saturation of the coils before they fire) is determined electronically by the Booster instead of mechanically by the points and point cam, so that variable is eliminated. --Bill
I think I had my booster hooked up wrong then. I'm pretty sure I didn't have the condenser in the circuit. I'm really good at following instructions tho and this is not a kind of mistake I'm likely to make. Not likely doesn't mean I think I never make a mistake. Could be the reason the Booster quit? It was probably 20 years old and could of died of natural causes. I'll double check the directions later and the next time I get one of those. Since I'm so poor lately maybe I'll do one of the homemade types. EDIT; I just did that. I checked the directions from Dyna Tek. They are ambiguous. They say leave the condenser in place but they say to hook white wire to points. I'll try to remember what you said Bill. I had mine hooked up with the white wire to the points but neither wire to the condeser.
Hmmm. I was wrong-- the Dyna instructions are pretty ambiguous and my "online" copy doesn't say anything about the condenser. I *don't think* that leaving the condenser in the points circuit would make any difference one way or the other. Let me check with mu Local Expert on that next week. Didn't mean to stir things up... --Bill
