A dip or slight hesitation in power

[Heimir Bardason]

Dear Friends of the Board.

Is it normal for these old bikes to have a slight hesitation in power / acceleration from 2.500 to 3.000 rpm ?
Mine does so, it has more or less the same pull from idle ( 1.200 ) to around 3.000 and then it´s a bit like it "comes on the cam" and accelerates with more authority.
It´s a 1978 model 38 mm carbs, pod filters and brit style mufflers ( free flowing).
Head has been worked mostly to take out "errors" or ugly spots. Carb has been jetted for this free flowing system.
Pamco electric.ignition , otherwise standard.

Thanks.
Heimir Bardason.

 

[5twins]

My '78 has pretty much the same mods and doesn't act like that. Maybe you're not quite done with the tuning. These carbs usually require that you change the mains, pilots, and adjust the needle. What pods do you have and what jet sizes are you running?

 

[peanut]

Dear Friends of the Board.

Is it normal for these old bikes to have a slight hesitation in power / acceleration from 2.500 to 3.000 rpm ?
.

definitely not
Your bike is the same year as mine with the same carbs, filters and exhaust.
What jets do you have fitted to yours ?
It would be very helpful to hear a sound track of the throttle blip at 2500rpm.

sorry 5T I have duplicated your last post

 

[retiredgentleman]

I'm assuming you don't have the E-advance option. Your mechanical advance (ATU) with the fly-weights may be sticking. These engines will accelerate to around 2500 rpm with no advance at all, but will then hesitate. Perhaps your advance at 3000 rpm, goes quickly to full advance, which will seem like a real burst of power. Have you ever taken the advance rod out,cleaned and lubed it? Do the fly-weights move in and out freely? Also if the little springs on the weights are too stiff it will hold and delay the outward motion of the weights.

 

[grizld1]

If your Pamco ignition uses the OE mechanical advance unit, one (or both) of two defects there could be causing poor low rpm power followed by a surge. First, if bob weight tips are worn, the interval between full retard and full advance will be extended, and low rpm timing will have to be set closer to TDC than spec in order to keep advanced timing out of a range that will hole a piston. This will result in what you've described. The same thing will happen if bob weight springs are too tight. There are several threads on cures for this condition, and a few searches will locate them. Of course if you're using a Pamco ignition with electronic advance, none of the above applies.

Edit: Finished the post and there ya were, RG--great minds run in the same circles! OP, there's a third mech. advance defect for you to think about!

 

[retiredgentleman]

Yeah griz, I guess we were just typing at the same time.
My thoughts are that the mechanical ATU did its job for 20 or 30 years, but as they age they just wear out and lose reliability. I was really happy when Pamcopete brought out the E-advancer. I was tired of playing with the ATU. The E-advancer makes these bikes run beautifully

 

[XSLeo]

I was thinking the pods may be the problem. They may restrict air flow into the ports on the carb bell. Slide lift issues. This can cause off idle flat spots. Also if they are a short pleated element pods such as K&N's they can also cause an off idle flat spot. The shorter and more tapered the more the effect.
Leo

 

[heimirb]

Thank you all for responding, Gentlemen!

The jetting on the 38 carbs has been changed from stock and is;
Main #140 (standard #135) // Jet needle 4M1 (standard 502-3) // Needle jet Z-8 (standard Z-2) Other jets are standard.
The jetting seems spot-on when I ride.

The pods are 73 mm in length and taper down.
My mecanical advancer is lubed and moves freely.
The bob weight tips are fairly new but there is a noticable free-play in them. Came so standard.

One spring broke a while ago where it is mounted so I bent a loop in it at the end ( so it is one coil shorter) and connected it. I can hardly feel any difference in the tension. Would it make such a difference ?
Would it be worth it to take the pods off and see what happens ?

Regards,
Heimir Bardason.

 

[5twins]

It would be more worth it to tell us what kind of pods you have - foam or pleated K&N style? This is very important with these CV carbs. The pleated K&N type don't work right. You will never get the best state of tune with them.

But ..... more really big red flags go up after seeing your jetting changes. If you put a '76-'77 4M1 needle in a '78-'79 slide, that just won't work. It doesn't matter that you changed the needle jet, it's a needle length thing. There's a 5mm difference. That's huge as far as needles go. You didn't read the "Carb Guide" did you ..... 'nough said I guess .....

 

[heimirb]

Thank you for your above reply 5twins.

I have pleated K&N style, short (7.2 cm) but deep.

Yes, I am guilty of not reading the "Carb Guide"!! Best to make a cup og coffee and start reading.......

I bought the bike 5 years ago with the modifications and jetting. I have not altered the jetting since then as the jetting seems OK.
Clean and crisp and revs way up.
It´s just this dip or hesitation ( not stumbling) between 2500 and 3000 rpm that I would like to get better.
It´s undependent of throttle position be it small or full opening of throttle, the dip is always there.
You have pointed out that this is about where the mecanical advancer kicks in so I think I will focus on that to start with.
Then I will look at the carb.

5twins; What needle would you put in instead of the 76-77 4M1 ?

Kind XS regards.
Heimir Bardason

 

[5twins]

The needle you need will depend on the slide type you have. The older slides that used the 4M1 needle retained the needle in the slide with a large plastic disc held down by the slide return spring .....

For the '78-'79 BS38 carb set, the slide was redesigned. It retains the needle with an e-clip. There is also a small spring under the needle e-clip. The big difference with this design is that the needle sits higher in the slide. A longer needle is required so it hangs out the bottom the same amount as the older slide/needle combos .....

The 4M1 needle is 45mm long from it's top clip slot to the tip. The '78-'79 5O2 needle is 50mm long. You have to use the needle length that goes with the slide type.

You say the bike is a '78, but that doesn't necessarily mean the carb set is. They are easily swapped. I think the first thing you need to do is positively I.D. the year of your carb set. Jetting (and re-jetting for mods) is based on the year of the carb set, not the year of the bike.

 

[Heimir Bardason]

Thank you for your pictures, write up and help, 5twins!
Explains a lot for me.

">>You say the bike is a '78, but that doesn't necessarily mean the carb set is.<<"
You are correct, 5twins. The carbs on my XS is the upper one so I do have the 76-77 38mm carbs. The 4M1 would then be OK I presume ?
So the jetting changes are a bigger main jet #140 (standard #135) and Needle jet Z-8 (standard Z-2).

I will change the pods to foam and order new springs for the mechanical advancer.

Kind regards,
Heimir Bardason

 

[5twins]

Yes, the 4M1 needle would be correct for your slide type but so far, all we know for sure is the slide is the older style. That doesn't necessarily mean the carb set is older. You could have a '78-'79 carb set that just had the slides changed. Those newer design '78-'79 slides are very hard to get so it's very possible someone fit older ones because they couldn't find the correct ones. There are a few more things you can check to help positively I.D. the year of the whole carb set.

Start by looking at your float bowls. '76-'77 bowls will have an overflow pipe in them and a brass drain nipple for it on the bottom .....



'78-'79 bowls don't have those items .....

To I.D. the main bodies, look at the intake bells. On the '78-'79 carbs, the float chamber vents were moved from the outside to the inside. There will be 2 more round holes on the intake bells at 3 and 9 o'clock .....

The style of mix screw also differs. The '76-'77 mix screw will have a long tapered point .....



The '78-'79 mix screw has a stepped point and also uses an o-ring .....

 

[5twins]

Now let's discuss your jetting a little. As I mentioned, you jet these (and re-jet for mods) based on the year of the carb set, not the year of the bike. But, in your case, you will most likely need to tailor your jetting around those 4M1 needles and Z-8 needle jets regardless of the carb set year. You'll have to jet to match (and work with) those 2 components. To start with, I think the 140 mains are probably too big. The 4M1 needle/Z-8 needle jet combo was originally run with a much smaller 122.5 main. For mods, these 650s usually require 1 up to maybe 3 or 4 sizes larger on the mains at most. You're 7 sizes up. I have a '76-'77 carb set that I dialed in and run occasionally on my bike. The biggest main I can use is a 132.5.

Stock jetting for the '76-'77 carb set was a 25 pilot, 122.5 main, and the 4M1 needle/Z-8 needle jet combo. On my set, I'm up 1 on the pilot to a 27.5, 4 on the mains to a 132.5, and the needles are set one step leaner. Usually, anything more than a size or 2 increase on the mains requires you to lean the needle a step. A larger pilot is usually needed once you do lean the needle.

So, your "lag" problem could be related to the advance but it could also be a wrong type pods/wrong jetting thing. Or it could be any combination of them.

 

[grizld1]

Heimir, I suggest that you follow the old Troubleshooting Rule of Thumb: Ignition First! To determine if ATU defect is responsible for your flat spot, connect a timing light, and ensure that the ignition can be accurately set full retard at idle speed of 1200 rpm without excessive advance resulting when you rev the motor to full advance. To find out if spring tension is delaying advance action, wrap a rubber band, etc. around the end of the throttle grip to hold its position, set the throttle at 2500 rpm, and check with the timing light to see if the ignition has reached full advance. If it has not, lighten spring tension by giving the modified spring a little stretch and try again. You need to find the "sweet spot" where the springs allow the ignition to advance freely and retard promptly. When all's well there, proceed to carbs, filters, etc. if there's still a problem.

+1 re. electronic advance; it prevents a lot of grief.

 

[heimirb]

Result; The problem of this stutting or hesitation of power seems to be the spring tension in the mechanical advancer ( ATU) like retiredgentlemen suggested. Not enough tension in the springs. Bought a pair which "looked good" in a local hardware store and now the bike is much better. It looks like the springs frome Mike´s are way too soft. It´s funny how you can actually tune the caracter of the ATU with the spring tension.

Thank you kindly 5 Twins for taking the time to assist me with putting in photos of the carb, grizld1 for your input and all of you who took the time to respond. It really is great to have this wealth og know how for the XS bike !!

Kind regards,
Heimir Bardason

 

[5twins]

Original advance springs are N.L.A. from Yamaha but if you watch eBay, they usually come around for a good price.