Bad Low End

brasco18 · Posted: June 4, 2007, 3:41 pm Post subject: Bad Low End

I don't know if it's normal or not, but when in first gear, just starting out, if I fully engage the clutch at lower than 2000rpms, the whole bike shudders. Is this normal? This actually happens for any gear, despite how fast I'm going; whenever I get below 2000rpms the bike doesn't like it. Any ideas?

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

The whole bike shudders? Or are you just lugging out your motor? Do you have highway gears? What size sprockets are you running? Are your motor mounts good n tite?

brasco18 · Posted: June 4, 2007, 4:01 pm Post subject: Re: Bad Low End

The gears for the bike are stock '75. Same goes for the sprocket size. I have not checked the motor mounts. I will do that. I bring up the point because I just recently hopped on my co-worker's sportster and noticed that once you got the bike moving you drive way slow and the motor just slowed down. Mine does too but when you try to speed back up the bike shudders. It reminds me of before I put new swing arm bushings in.

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

Well, the xs does have a bit of driveline snatch to it. Must be all the torque! Definitely check those mounts. If any are loose, and a xs will loosen bolts, then that might be the source of your trouble. Crank em down good and then see what happens.

xsjohn · Posted: June 4, 2007, 4:34 pm Post subject: Re: Bad Low End

brasco18....correct carb setting help on the lower rpm's........

John

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

In my somewhat degenerated opinion, they should've kept 4th gear boogie style like it is and dumped 5th for serious cruising. When MamaYama builds the XS1350 twin hopefully it'll have six speeds so we won't have carry two sprockets around everywhere we go.

I don't have screaming moped syndrome do I? I just like having that uptown power. You can run down almost anyone off the line with the light bike and combined torque. It's the ultimate streetfighter really. That and the xr750. Champions of torque. I'm not in need of horsepower, although I'll take what I can get. Thats for you highway guys. Crazy people ride there. Monolithic suvs womanned by the little soccer moms from pasadena.

xsjohn · Posted: June 4, 2007, 10:48 pm Post subject: Re: Bad Low End

Crazys roam the local streets everywhere...Me I just like to dodge deer at 3000 rpm....Getting old I guess....No such thing as an old fool because you don't get old by being a fool.........(quote) Richard Prior.....He dead ....

John

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

R U kidding......florida is the land of the old fool. Did some deer dodging back in minnesota and the old days. Not to many of those around here. If I hit a gator I'll finish him off and built a custom seat from him.

xsjohn · Posted: June 4, 2007, 11:09 pm Post subject: Re: Bad Low End

Better off eating the gators--their seats have been proven to cause jock itch....

innesota hell I'm in North carolina where there is a turn around every turn....Full of those fricken deer and they taste like s....

John

jayel · Posted: June 5, 2007, 6:32 am Post subject: Re: Bad Low End

could be clutch dampener springs are weak but below 2 grand your lugging the engine, it needs a few rpms to pull the load

brasco18 · Posted: June 5, 2007, 9:41 am Post subject: Re: Bad Low End

I tightened the motor mounts, at least the ones I could get to. I will see if that helps.

xsjohn · Posted: June 5, 2007, 3:14 pm Post subject: Re: Bad Low End

Read somewhere that the chainwheel is pressed on the cam and is not hard to press off.....Wonder if the chainwheel could be repositioned slightly on the cam for better low end torque....Anyone out there know about this sort of thing.......

John

grizld1 · Posted: June 6, 2007, 3:25 am Post subject: Re: Bad Low End

Degreeing the cam by moving the sprocket is a very basic procedure. In the 447 engine, moving the sprocket .009" as measured at the cam boss that it seats against yields approximately 1 degree of valve timing change. Lay a feeler gauge against one side of the timing mark notch, scribe the sprocket, and move the scribed line to the corner of the notch. With the notch vertical, lay the gauge against the left side of the notch and scribe if you wish to retard the timing, and against the right side if you wish to advance it. I'd advise you to scribe the sprocket yourself and take the job to a professional machine shop. It's difficult to keep the sprocket from moving out of position as it's pressed back on, and it's easy to break the cam boss; one of the engines in my salvage stash yielded a cam that was in very nice shape, except for the chunk broken out of the boss where a local mechanic with a press had taken a go at it.

Retarding valve timing with the stock cam just a little moves peak power toward the higher rpm range. You might try 4 or 5 degrees of advance and see what it does for you.

Not all performance cams are "radical;" the Shell #1, for example, is a torque cam, designed to optimize the low to middle rpm range. True, with the wider valve lash (.006" I, .012" E) you'll get more noise from the tappets, but if low mechanical noise is a priority there are much better picks out there than the XS650 engine.

xsjohn · Posted: June 6, 2007, 8:17 am Post subject: Re: Bad Low End

grizld1....Great......I am geared tall 18/29 and weigh nothing 139 lbs....I seldom if ever turn over 4500 and everything is stock except for carb tuning.....Sounds like 4-5 degree advance would pep it a bit on the lower side...Not looking for anything radical.....A noisy cam would make me crazy.....Thanks for taking the time to explain this to me.........

Have you experienced moving a stock cam with any noticeable success....

John Underwood

grizld1 · Posted: June 6, 2007, 11:27 am Post subject: Re: Bad Low End

Retarding the action a touch, yes. Never tried going the other way. You might play with a degree wheel, good TDC locator and dial indicator some; the cam timing (and ignition timing marks, too) are often a little off, and you might be surprised what happens just correcting the mass-production error here and there. I don't think there's much tweaking you can do that will yield a lot of down-low grunt at that gearing, though; can't eat your cake and have it. The only thing I know of that'll help you much is more displacement. Stock liners can be bored to take Shell's 77.5 mm JE pistons, which bump displacement to 700 cc. They're very light for forged pistons, with a total assembly (piston, pin and rings) weighing only 0.6 oz. more than stock per side, and they're domed to yield approximately stock compression--Shell says they'll operate fine on 87 octane. I can tell you from experience that a pair of well-balanced 77.5's in a well-tuned engine can run very smoothly.

xsjohn · Posted: June 6, 2007, 4:20 pm Post subject: Re: Bad Low End

Actually this thing withmy skinny ass pulls better that any of my friends 77 and 79 xs's geared stock...Neither can stay up with me..... Just thought since I never rev this thing I could get a small chunk of better torque starting from 2000 isntead of 2500...Maybe Im just dreaming but I have 4 cams and if one got trashed so what.....These things really have nothing in the teens.....Move something off the top that I never use and move it downward......Am I thinking dumb.....60 mph 3500 50mph 3000 70 mph 3900 plenty of umph now on the express even at these mid rpms......

Also wonder if advanced valve timing would make it run smoother at lower rpms..I may turn out rougher sense it would be performing better ( in theory)..Seems the race cars run rough at low rpms with retarded cams..I would be going the other way....Second guessing never works though...Just have to try it.......

John

grizld1 · Posted: June 6, 2007, 5:18 pm Post subject: Re: Bad Low End

John, I'd be seriously concerned about putting much load to these motors at extreme low rpms (teens). Lugging will tear up the big end just as quick as wringing 'em out. But it looks like a worthwhile experiment. If it doesn't work out you've just invested a little time, but you'll have made some discoveries along the way. Good luck!

xsjohn · Posted: June 6, 2007, 7:22 pm Post subject: Re: Bad Low End

CB900 honda keihin carb problem......My friend has a pristine CB900 1980...THe carbs were crudded when he got it and it would run with the choke out but would stall if pushed in....We cleared the carbs finally and re-adjusted 3 floats that were to high and set them to the one that measured about 15.5mm the book says 15.1 +-1mm.

THe bike idols perfect but wont take the throttle with the air box off unless you cover part of the intakes with your hands.....Went from 105 to 110 main jets and put it together with the stock air boxes.....

Now it wont take the throttle above 3500-4000 rpm and appears lean to me.....

Ran good on the hi end before we set the floats with the choke out. ......There is really no more room to raise the floats because now if you depress them the float will actually barely touch the carb body.....

I know nothing about keihin carbs but would want to put huge MJ's in it as bad as it runs on the mid to high side....But it ran for 20000 miles with the 105 MJ's.....

Anyone know any thing about these keihin carbs......Please help before I hose my friend's bike an become a statistic.....

John

jayel · Posted: June 6, 2007, 7:59 pm Post subject: Re: Bad Low End

sounds like plugged idle, pilots, john here's a keihin carb pic as you can see the low speed jets are doing the most until 1/2 throttle

xsjohn · Posted: June 6, 2007, 9:04 pm Post subject: Re: Bad Low End

Jayel....Idols perfect...and runs along slow but will not go over 60mph even with the choke out..Sounds very thin lioke you were in the mountians with lean carbs..If the floats were lean it seems that pulling out the choke would richen it to let you know the floats were hosed....Won't rev with the air box off without hands over the intakes acting like a choke....

Runs along at low rpm but will not go over 60 mph even with throttle wide open...Again pulling the choke at this point should make some difference if the floats were lean..Does nothing....Nice pull butterfly type choke.....

When we first started the bike it before cleaning the carbs and dicking with the floats it would not Idol without the choke pulled a lot but ran like a scalded dog on the high end but had to be choked....Tripple cleaned the carbs and can find no crud in them...Cleaned tank and have a fuel filter....Haven't tried removing gas cap....

Thought we cleaned the carbs extra well and did lean a couple MM's 3 of the 4 floats to match the one that measured ok.......

John

jayel · Posted: June 7, 2007, 12:17 am Post subject: Re: Bad Low End

could the carb are badly out of sync, if you don't have a four pipe mamometer i think you balance them 1-3 2-3 1-4 something like that, you get a couple set and compare to the others, pain in the a** two to many carbs, does it have reed valves in the intake ports like the old 750's? make sure they aren't hammered

xsjohn · Posted: June 7, 2007, 12:31 am Post subject: Re: Bad Low End

Carbs sync good no reed valves.....Acts like it's starving......Idols and runs lower rpm perfect just won't go when you open the throttle....Like the MJ's were 20's or something....keihin cv carbs......

John

Xumi · Posted: June 7, 2007, 1:18 am Post subject: Re: Bad Low End

xsjohn wrote:

Carbs sync good no reed valves.....Acts like it's starving......Idols and runs lower rpm perfect just won't go when you open the throttle....Like the MJ's were 20's or something....keihin cv carbs......

John

I'm going to be sounding like captain obvious here...

Slides moving ok? Diaphragms pinholed?
Have you checked float height using the clear tube method?
You mentioned it ran great before and you lowered the floats in 3 of the 4 carbs.. could your book setting be off?

Are the air filters supposed to be oiled and are running dry?
Check for exhaust leaks too.. That'll thin it out...

xsjohn · Posted: June 7, 2007, 1:18 pm Post subject: Re: Bad Low End

Xumi----The three foats I discussed lowering were so high that the float would touch the upper body of the carb long before the needle spring was depressed so we set them to the one that didn't touch.....Maybe we just have to try a bigger MJ and maybe different needles. Don't want to raise the floats......Thanks for the suggestions and will try clear tube....These are cv but dont have diaphrams and all four are moving correctly........These 80 bikes are hard to deal with....The needles are locked so you can't easily dick with them without doing something that is hard to back out of.....

First and least expensive is trying larger mj although we went from 105 to 110. Then maybe some different needles....Then again taking these apart maybe we can find something.....Thanks

John

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