Slight Surge At 70 MPH

[bdholsin]

I took my '83 Heritage Special on the highway for the first time the other day. I hit 70 MPH with no problem (yay!). I did notice that cruising at a constant 70 MPH I'm sensing a slight surge...almost as if the wind is pushing and pulling at me ever so slightly. Mind you, it is ever so slight but it is there.

I'm running stock exhaust, stock BS34's and pod filters.

I'm wondering if it is leaning out a bit at that speed. My tach is showing 5,000 RPM. I am soliciting an opinion on if I should or shouldn't step the main jet up one size. I have one on hand but wonder if it's worth the effort of removing the carbs to swap that jet.

Thanks.

 

[5twins]

If your jetting is stock then, yes, I would go up one size. You don't need to pull the carbs, just drop the float bowls and change the jet using stubby screwdrivers. Another option might be shimming your needles with a washer or 2 - but I would try the main jet swap 1st.

 

[bdholsin]

Thanks 5twins. Your opinion backs my suspicion. Much appreciated.

 

[Classic]

Is the main jet even open when using that little throttle?

I'm dealing with exactly the same issue on cold days and upping the main jet didn't do anything. I'm guessing the idle jet is the one to change since at 70mph the throttle is only open a tiny amount.

 

[bdholsin]

Is the main jet even open when using that little throttle?

I'm dealing with exactly the same issue on cold days and upping the main jet didn't do anything. I'm guessing the idle jet is the one to change since at 70mph the throttle is only open a tiny amount.

???? I'm turning 5,000 RPM at 70 MPH (although that's going to change by next week when I put my 30 tooth rear sprocket on). I'd say the throttle is better than 3/4 open. Not trying to be a smart ass here but are you thinking of this clearly? Granted, I have throttle left but it isn't a whole heck of a lot.

 

[bdholsin]

Just had a thought...there was a chart floating around somewhere that detailed how the jets overlap. I can't seem to put my mouse on it right now but I'll do a little searching to see if I can find it.

 

[Sundie]

@BD:

The problem is that your throttle (butterflies) and slide (main jet) have to real connection. How much the slide opens is dictated by air pressure differences. It's possible that with the butterflies open to 3/4, that the slide could be only 1/4 to 1/2 up, if you're just cruising along. Crack the throttle fully, the engine starts *really* sucking air, and the slide goes up.

The slide really doesn't open fully until 6k rpm with open butterflies.

 

[xjwmx]

^That's an interesting thought. WIth a different jet, the needle might be in a different place at 70 so you don't get what you expect.

 

[Sundie]

Well, no, the slide will still be in the same place, regardless of jet. There's just a bit more space around the needle.

To be honest, at 5k, i guarantee the cdn needle/needle jet setup would fix his issues. That's where it works since they adjusted the US needle for leaner cruising for EPA.

 

[xjwmx]

Well, no, the slide will still be in the same place, regardless of jet.

I don't think that's necessarily true. For example - if horsepower increased, the throttle plate would be open less, and therefore the slide would be lower. Would it not be?

 

[Classic]

???? I'm turning 5,000 RPM at 70 MPH (although that's going to change by next week when I put my 30 tooth rear sprocket on). I'd say the throttle is better than 3/4 open. Not trying to be a smart ass here but are you thinking of this clearly? Granted, I have throttle left but it isn't a whole heck of a lot.

I am recalling correctly. I need close to 3/4 throttle to maintain 105mph... 17/34 gearing

But if you are at 3/4 throttle i'd have to agree that the main jet needs a bump.

 

[bdholsin]

Just stepped back into this discussion. I had forgotten about the needle and how it reacts to vacuum.

I'll certainly report back on my results after I swap out the mains to see what happens. Stay tuned.

Thanks for all the input.

 

[nj1639]

Well, no, the slide will still be in the same place, regardless of jet. There's just a bit more space around the needle.

To be honest, at 5k, i guarantee the cdn needle/needle jet setup would fix his issues. That's where it works since they adjusted the US needle for leaner cruising for EPA.

If I didn't have xsjohns needles I'd go with the cdn setup.
BD, looking forward to your report on the 30 tooth.

 

[bdholsin]

I ordered the 30 tooth today. Give it a few days to get here and give me a day during the weekend to put it on. I should have some sort of preliminary comments early next week (weather permitting). I'm actually quite excited. I'm hoping it reduces some of the vibrations at highway speeds. After about 10 miles on the highway at 70 MPH my balls were vibrating like an electric toothbrush.

 

[bdholsin]

Just finished swapping out the mains and stepping up to 135. It's late and I had to help put the boys to bed. Didn't get a chance to take it for a spin. Should be able to make an update late tomorrow morning.

 

[5twins]

Many have reported that their totally stock BS34 equipped bikes run better with one size up on the mains. Since you changed filters, I think the main jet increase is called for. The mains can influence the entire operating range. Just one size change probably isn't going to have much affect down at idle but it should influence your upper midrange and help with that surging. Sure, the Canadian needle jets and needles would probably help but you're talking $40 as compared to about $7 for a pair of main jets.

 

[bdholsin]

Too much rain last night and today. Not going to be able to ride and evaluate the larger main jets.

Took the opportunity last night while swapping the mains to switch from phillips head screws to allen head screws to secure the float bowls. Almost managed to pull it off with the carbs in place. One screw just wouldn't come out so I had to pull them. While struggling with that last one my wife watched and just shook her head. I imagine that I looked like a monkey with a math problem. No matter what angle I took and what tools I was using it just wouldn't come out.

 

[Sundie]

If I didn't have xsjohns needles I'd go with the cdn setup.
BD, looking forward to your report on the 30 tooth.

Just FYI - if you have CDN needle jets, do *not* go with John's needles. They will wash your cylinders in under 60 seconds, and clog your plugs.

 

[bdholsin]

My needles are stock...so far as I know. What I've gathered about the PO based on what I've seen on the bike, I seriously doubt he would have had the know-how to do anything with the carbs.

 

[bdholsin]

The weather was nice today and I took her out to test the larger mains. The hesitation/surge seems to be less pronounced. Almost to the point that I can't detect it. Now I have three choices:
1. Leave it and be happy. Going to check my mileage and see what the end result is.
a. Based on Sundie's comments I may not be hitting the range to open the needles and really benefit from the larger main. Like I said earlier, it seems to be better but I can't be 100% sure right now.
b. Still thinking on this.
2. Step up the mid range since I may not be working in the main range. If I step up the mid range jet I may take the main back down to stock since I may not be benefiting from the size 135.
3. Step up another size on the main (this is just idle thought and probably won't happen).
4. Ok, discuss and lay out your opinions.



We all love our XS650's and thoroughly enjoy them. We adopt/rescue them from unfavorable conditions or premature death. We give them nice homes and (hopefully) a warm/dry garage to reside. New parts, TLC and fluids.

So tell me....WHY THE HELL DID MY BIKE DECIDE TO TURN ON ME AND TRY TO KILL ME? I decided to try and get into the mains and hit WOT on a nice straight road. Nobody was around so I was pretty safe. When I bottomed out the throttle it stuck in the full open position. The cable seemed to get caught and wouldn't return the butterflies to closed. This lasted about 1 or 2 seconds. Enough to pucker my asshole as I mentally flew through a number of scenarios. First was to just hit the kill switch. Second was to turn off the key. Third was to chop the throttle to loosen the cable...that worked!!! Whew. I had plenty of road to work out the problem and I was just getting to 5th gear when it happened. Got a little hairy there for a few heartbeats. Why I decided to chop the throttle instead of cutting the engine is beyond me. Just seemed right at the time.

So I learned that WOT is a no-no.

Just passing that along.