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woodly1069 11-15-2012 03:26 PM

Dead airhead, what now?
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:cry

Big Bamboo 11-15-2012 03:32 PM

It was probably just unburnt fuel igniting in the muffler. How long was it idling? Try leaning the mixture some.

spo123 11-15-2012 03:36 PM

some thoughts...
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.


woodly1069 11-15-2012 03:38 PM

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

disston 11-15-2012 03:43 PM

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.

woodly1069 11-15-2012 03:53 PM

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!

disston 11-15-2012 04:06 PM

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? :lol3

woodly1069 11-15-2012 04:09 PM

Yep, the breeze out of the plug hole is a Kentucky compression test! Or at least tells me I have something:D

pbarmy 11-15-2012 05:21 PM


limeymike 11-15-2012 05:37 PM

Always go back and reverse the last thing you did before the problem occurred.

batoutoflahonda 11-16-2012 01:25 AM

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.:D. 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.

Rob Farmer 11-16-2012 03:51 AM

That's a classic points problem if ever I heard one.

Bill Harris 11-16-2012 05:35 AM

I'll agree with y'all. Start with the points. And check the carbs.


squish 11-16-2012 02:00 PM

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.

woodly1069 11-16-2012 03:39 PM

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!

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