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Post by wideload on Jun 4, 2014 22:21:53 GMT -5
Last fall, my stock 76 was not running quite right. So during the winter, I decided to give it the once over. It received the typical mechanical refurbs. Cam chain tension was correct, as were valve clearances. It has new plugs, points, condensers and the advance unit is operating correctly. Additionally, I went through the dirty fuel system - cleaned the tank, petcocks, carbs, gas & air filters.
The bike does not idle consistently and it surges a bit at 65 MPH. At all other RPMs the bike runs well. About 75% of the time, idle speed will decrease at stop lights till it stalls, other times it will hold steady at 1200 to 1500. I do not notice surging till riding at a steady speed of about 60 or 65 MPH, and this symptom is intermittant as well. Both plugs have a similar appearance - they look like the mix is just about right perhaps a tiny bit rich. I sprayed carb cleaner at the carb holder area while idling and did not notice a change in engine speed. Symptoms seem fuel related correct? I’m stumped regarding where to begin looking for the problem.
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Post by grizld1 on Jun 5, 2014 11:01:17 GMT -5
Erratic idle with rpm drop and stalling at stops often indicates float valve (needle and seat) sticking intermittently; check float level as well, and bench test. Re. the 65 mph surge, also check the condition of your diaphragms and needle jet O-rings. What happens when you spray the throttle shaft ends?
Don't take ignition for granted. Does advance timing hold steady under the strobe? Also, how old are plug wires and caps?
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Post by wideload on Jun 5, 2014 12:31:57 GMT -5
The float levels did not require adjustment even though I changed out the float valves and seats. I did experience unexplained gasoline leaks out the overflow tubes that I have not seen recently. If I still have em, I guess I will be putting the old valves and seats back in to the carbs. A visual inspection of the diaphragms against sunlight did not reveal visible holes; the slides move down very slowly into the carb when blocking off the vent with my thumbs so I think they are ok. Any other test you recommend? I didn't remove the needle jet to check the O-rings. Is a visual inspection after pulling out the jet acceptable? Before repeating disassembly, I will check the throttle shaft ends with carb spray.
I'll use the strobe to look at the advance, this time under hot and cold running temps. Plug wires and caps are new, but Ill check resistance too.
Thanks for the list of things to investigate.
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Post by grizld1 on Jun 5, 2014 17:33:58 GMT -5
Those "overflow tubes" are the float chamber vent lines, and their main function is to maintain atmospheric pressure in the float chamber. Whenever you see fuel in them there's only one explanation: the fuel level is too high, either because the float valve isn't doing what it should or because the float level is set too low. I'd deal with the float valves first if I were doing the job, then pull the needle jets if that doesn't clear things up. Best practice is to change out the NJ O-rings whenever the jets have been removed; they're probably 38 year old originals. Chinese repop float valves are usually trouble. Genuine Mikuni valves cost more, but they can be trusted.
I hope getting the float valves right clears things up for you, but if not, remember--gremlins can hide, but they can't run. You'll find the rascals.
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Post by wideload on Jun 17, 2014 21:00:53 GMT -5
Well, I removed the new brand-X float valves and replaced them with the original Mikuni valves. Engine starts on the first kick, idle speed is steady and no more stalling out at stop lights. Unfortunately, the engine is running very rough beginning at partial throttle opening - 3K RPM (The original problem I was chasing was moderate surging at 60 MPH!). It doesn't accelerate very fast when I open the throttle, and both cylinders seem to be affected. It feels like it is starved for fuel - flipping the enrichment lever on makes it run a little better. Correct me if I am wrong, but I am still thinking this is a fuel problem. This is the third XS carb set I cleaned & adjusted folowing the carb guide, but the first time it didnt run great after reinstallation. Maybe the float level is set too high. I was anxious to get the bike running so I did not order new needle jet O-rings. Hard to believe o-ring replacement will cure this severe symptom, but I am gonna order them now and pull the carbs once more. While I'm in the carbs once again, are there other things I should be looking for? Rechecked the timing and advance at cold and hot temps - the advance is smooth. Resistance checked out ok on the plug wires/caps.
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Post by grizld1 on Jun 19, 2014 10:32:18 GMT -5
You're probably right in diagnosing fuel starvation due to float level, since the misbehavior at 3K started after you reinstalled the good float valves.
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Post by wideload on Jun 29, 2014 20:26:04 GMT -5
I raised the fuel level and installed new needle jet o-rings. The engine runs fine at all RPMs.
I learned at least two things: 1) Use real Mikuni parts. 2). Use a sight tube to measure the actual fuel level in the float bowl. Setting the floats to the specified height did not equate to correct fuel level. Before removing the carbs this last time, I found the fuel level was too low, even though the floats were set at 24 mm. I lowered the floats to the bottom end of the specification range, then confirmed the correct fuel level while on the bench using the sight tube.
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