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Low Speed Bucking

Moderators: Curt!, Pa

Post Fri Jul 21, 2006 5:56 pm

Posts: 530
Location: Ogden, Utah, USA
OK now I get to be the dummy. The ULH is up and running. Easy start but needs some warmup. 500 miles and it is getting a little bit free on the spin. Seems to run good, smooth and tourqey. A little slow when compared to some of the postings on this site. I can run about 65 on the level, some faster if the slope is down. 55-60 if the grade is up. Leak test is good. No leaks at 15 PSI. Stock linkert good idle and very smooth transition from low speed under load and strong acceleration as much as the motor makes anyway. 23 tooth counter shaft sprocket and stock back wheel. Close ratio gear set with a nice jump to 4th. Quiet under trailing throttle but if the bike is just holding a steady speed, around 20 mph it will buck and snatch at the drive chain. 2nd gear is too low and 3rd is too high. Feels just like a low speed miss. Turn up the throttle and it pulls very nice. Shut the throttle and it will slow to idle speed with out a fuss. Just off idle is where it gets rough. Have I put something in the carb wrong? I used the factory 40=47 book cause that is what I have. Followed the instructions as best as I could. Replaced the throttle bushings and hand lapped to the same shaft. Set the float up like the picture and no leaks, no overflow. 4 and 1/4th turns out on the low speed needle. 1 and 1/8th turns on the hi speed needle. Rechecked the float level and right where it is supposed to be. It still gets very warm on the way home from work, but it has been over 100 degrees F here for two weeks. The back head will get the blue jeans warm if you have to sit at a light or in stalled traffic. Timing is set at the back of the timing plug hole and has stayed just where I set it. Plugs look OK but ??? Who can tell with the new stuff we got for gas. Ut has no ethanol and I have run the mid range stuff at the pump. About 30 to 40 miles per gallon. The butterfly has an offset on the edge and I put it back so it fit with the small angle where it meets the venturi. Nice fit no scars no grooves so the carb has had limited wear and tear. Stock timer wth chevy points installed. Seems to be OK. No shorting at the cap to cover and everything else is tucked in real nice. Oh ya we are about 5000 to 9500 feet in elevation and it works over that range with no fuss. On the knuckle I have to be careful to not run under idle speed or it will do the same. But it is set up a bit different. Any way it is all I have to compare to. New valves, guides, springs and 1 lifter block. New rings and 1 nipple in the front cyl. Cotten helped a lot in fitting that damn nipple back into this thing THANKS. Cyls have some relif work on them. No way to tell if stock or not. Looks like factory work or the effort of a good craftsman. No pop up and aluminum heads with big plug threads. Maybe original may be early aftermarket. No relief work on the heads. Just valve pockets. Plugs are centered not offset to the intake valve. Any thing obvious I have done wrong?
Steve H

Post Fri Jul 21, 2006 9:42 pm

Posts: 3134
Location: Central Illinois, USA

You may have done nothing wrong.
These tired old girls develop complaints on their own.
And just to make it more perplexing: the more you tighten up loose ends, the more minor things become major distractions.
With all due respect, (Geez I hope I don't get censored again) these machines are quite female in nature. If you are married, you understand.

Although there are a great many possibilities, I shall only address what I study most: the carb.

Did you hold the freshly assembled carb up to the light and inspect for daylight around the venturi? Did it stay put at a light press fit, or did it drop though like a rock?


Post Sat Jul 22, 2006 7:04 am

Light throttle surge is frequently too lean or too much spark.

Is this fixed spark advance?

"Plugs are centered not offset to the intake valve"
That's not a stock head.

"Close ratio gear set with a nice jump to 4th"

Stock original flywheels?

What sprockets?

Post Sat Jul 22, 2006 10:09 am

Posts: 530
Location: Ogden, Utah, USA
Cotten: A light press fit for the venturi, and a light leak test with the bifocals installed. Female ref to these old bikes is most appropo. With my history, 4 daughters and 20 plus years on the 3rd marriage I have to figure 1, Slow leaner. Or 2, Not smart. Oh well life has been interesting. Both of my bikes have distinct "personalities" that were expected and welcomed. If I wanted mechanical perfection I would still have a Honda or other Japanese Motorcycle. I worked for Honda and Yamaha for several years to support my racing habit. Pushed european bikes cause they were faster for me. I still love the way my old Bultaco would drift thru the sweeper on the local track. :D

Panic: Close ratio gear set with the tall 1st and the close 3rd. Countershaft is 23 teeth with a stock 51 tooth back sprocket. Primary is a belt drive same as I have on the knuck. I think the primary ratio is the same as the stock chain set for a big twin. At least when I did the first chage over on the knuck I felt no difference except for the smoother and quieter operation. I never did think to count the teeth to be sure. My bad. Where is the stock location for the spark plug? Over the intake valve? The contact breaker (distributer) is moveable stock unit but I keep it in 1 place. Have had no problem kick starting with it advanced like I would on the knuck. I will try setting the timing back a little to see if a good compromise can be found.
Steve H

Post Sat Jul 22, 2006 10:29 am

Posts: 530
Location: Ogden, Utah, USA
Panic: I forgot the rest of the response. Stock flywheels, Heavy and wide on the drive side and standard looking on the pinion side. I think the gear set in the trans is Andrews. At lest that is what I was told. I have not pulled the top off to see inside cause it is working and not leaking. Smooth and quiet, shifts vey nice with typical long movement same as all the old 4 speeds I have ridden. I did replace the bent 1 piece kicker with a repop 1 piece but that was a straight forward part change. This morning I added a 8 click rich adjustment to the Hi speed and it helped the top end RPM some so maybe I have been a bit lean and that would explain some of the hot rides home. Time will tell. Motor is starting to feel very good during the break in and is a little freeer on acceleration. This is the time for me to stay patient and let the break in continue. I did measure the Rod bearing clearance just like the book said, side shake at the top of the rod. Very small 0.006 inch. Tight bottom so a little care is in order.End play on the crank was not something I could do with out pulling the Primary off. So I don't know what that is. Maybe I will pull it off this winter, but maybe not. I have learned that old tip,"if it aint broke, don't fix it" the hard way. :)
Steve H

Post Sun Jul 23, 2006 5:48 am

Posts: 53
Location: London, England
the timing mark should be just forward of centre in the timing hole. I think you have timed it the same way as your Knucklehead. Hope this helps.

Post Sun Jul 23, 2006 8:49 am

Posts: 197
Location: Bucktown, PA
This may be a dumb question, because you sound like you really know what you're doing, but...
Did you set your jets to some kind of spec., or did you actually adjust them with the bike running (which is the only way to get accurate adjustments)?
Did you adjust the low speed after the engine was fully warm?
When I'm faced with problems that could be mixture related, I reach down and give the choke a click and see if the problem gets better or worse.
I've never felt the need to adjust a low speed needle on the fly, but you might want to get the bike in that trouble spot and see if you can adjust it out. If you can, then see how the bike idles with the new adjustment.

Post Sun Jul 23, 2006 9:18 am

Posts: 3134
Location: Central Illinois, USA

Can you clarify:
>>The butterfly has an offset on the edge and I put it back so it fit with the small angle where it meets the venturi.<<

Apparently you refer to a flat chamfer of the edges of one or both sides, at 9 o'clock as you look at it installed? (9/X discs are an example.) The bevel should close upon the idle bleed holes on "idle side" of the bore. The venturi sits deeper than the disc's arc.

They often run fine with the discs in backwards. Sometimes even the venturi, too!


Post Tue Jul 25, 2006 4:31 pm

Posts: 530
Location: Ogden, Utah, USA
Ron, Charlie, and Cotten: Thanks for the help. Yes I timed it like the Knuck, yep I did. I moved the timing back and smoothed it out some. Cotten, I put the disk in as you describe, bevel covers the holes and fits smoothly with the insert. It was backward on dissambly and the sharp interface seemed wrong. The book is actuly clear on this item and was simple to follow. I to have seen venturies in backwards and running, go figure. What made the most difference was two brand new spark plugs. I found some at a local Checker auto parts. Redid the low speed needle to 2 and 3/4 turns and the Hi speed close to 2. Made 70 mph on the way home and it is still HOT here. Third week at 103 plus. New plugs? Oh well, ya just have to figure. The surge is very small, noticeable cause I get stuck in traffic on base at the end of the day. The wife has her MC endorsement and is helping with the break in. With a Glide front and some risers it looks like an old Pan, from a distance. Turned a bunch of heads coming home and that was fun to. Panic, Thanks for the heads up on the timing. :D
Steve H

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