Hesitation on initial throttle

[shortman1166]

When I sit at a light for a while and then it turns green the scooter seems to hesitate as if it is getting too much gas but then runs fine once moving. If I only stop for a second or two it does not do it. The only way I have been able to get it to stop is to have my idle screw turned down to the point where it slows the rpms down at idle 1-1/4 turn with a #30 pilot jet on a 72cc bbk. If I open the screw more at idle the rpms rise and seems to be a better idle but it hesitates with the throttle again.  If I lean out the jet needle it seems to chop at varying throttle. So the way it is set now it runs great once going but the idle is semi rough and I am assuming lean, but the hesitation seems to be gone when leaving lights.  What could be off or is it just in my head?

[pinkscoot]

Have you checked your valves? Does it happen when the engine is cold?

[shortman1166]

No it does not seem to happen when cold and it's better now but that just seems like an awfully small jet and screw setting for idle, and the fact that it idles better with more gas... The valves are also set correctly I just checked.

[ratdog]

My first thought was that your getting a lean surge ( not rich). If I’m right, a larger polit jet would help. I might also try raseingbthe needle a bit. Have you pulled your plug to see what it looks like? I’m just wondering if the bike is running lean on the main as well. Love to have you take it for a long run as much as possible at wot and with as little idleing as possible then pull the plug and post a pic. As close up as possible so we can see “the white part” of the plug

[fhol]

Hi! I'm having a similar problem on my stock Peugeot Kisbee '16. It is running absolutely great (top speed 50 km/h when it hits rev limiter) except when i give full throttle after idling at a stop sign. It is like the engine dies when i do that. Giving it small push with my legs and a bit less throttle makes it come to life again. It is especially problematic when i'm stopping before going up a hill. It doesn't happen when on the stand and rear wheel is in the air - only when i sit on it and need the power from the engine.

I have tried adjusting the air/fuel mixture by turning screw 0,5 to 1 turns (testing both directions), but it doesn't help much. Scooter has gone only 2600km since new. Engine oil & gear oil changed recently. Also cleaned air filter recently with compressed air. Same problem before that though, since i bought it (2400km).

Here is what the spark plug looks like:



A pro tip or two would be much appreciated!

[fhol]

shortman1166: I'm sorry for hijacking your thread by the way, but maybe we can both learn from our separate similar issues?

[fhol]

Managed to remove the air/fuel mixture screw on carb, then sawed a small groove in it so that the 22cm long fine-tip screwdriver i bought, fits snug into the groove.

It was very difficult to get the screw out of the carb in the first place. Ended up using a somewhat larger screwdriver for that. Had to be very careful because spring + shim + o-ring sitting loose on the screw. The screw head is shaped like the letter "D" when seen from above. It was also covered in a blue, rubbery gasket-like substance. Now it should be much easier to adjust mixture without special tools, going through the small hole from the side of the Peugeot Kisbee, where passenger foot-thingy is placed. That hole i will make a little larger with the christmas tree drill i bought.



My plan now is to try fine-tuning mixture. If that does not solve "hesitation" problem, i'll move the clip on the needle down one step, to give it more fuel. I suspect it is running somewhat lean but i'm unsure.

[pinkscoot]

"D" bits aren't very expensive and come with a flexible shaft driver.

[dexameth]

My guess is that everything is set up correctly when you have the idle screw adjusted for the "higher rpms". The hesitation is a lean-out when the butterfly opens and lifts the slide/needle. The hesitation goes away once you are feeding more fuel into the venturi at higher rpms and it runs okay. I would move the needle clip DOWN one notch to allow a slight bit more rich mixture where you want it.

[fhol]

Aug 30, 2018 9:30:04 GMT -5 pinkscoot said:

"D" bits aren't very expensive and come with a flexible shaft driver.

Oh! I have never seen "D" screws before but it is my first scooter tho. MC-shop where previous owner bought it new, told me "it's a special tool provided by Peugeot, that we use". So i figured, lets use a saw!

Anyways, adjusted the clip on the throttle needle. Problem is, it was already at the downmost position! There are three pos to choose from, so i put it in the middle just to see... and it did in fact make it slightly better. Is it running too rich? Strange.

Before:
- Clip at bottom (max rich)
- Mixture screw 2 revs out (rich)

After:
- Clip at middle (leaner)
- Screw 1 rev out (leaner)
= much better but still small tendency to stall on full throttle from standstill

Btw, screw is on the cylinder and not air filter side of carb, so i guess screwing it in will make mixture leaner, right?

Gonna continue tuning tomorrow and se if i can get it to run perfectly. I'm a little worried though as my spark plug was very light brown "before". Don't want to run it too lean. Hmmm... why are my findings the opposite of what i (and some of you guys) expected?

[fhol]

One more thing, spark plug electrode gap?
Workshop service manual (downloaded pdf) says 0.6 - 0.7mm.
Service manual that came with bike says 0.7 - 0.8mm.

I used stacked shims tool and set 0.7mm gap on new plug that i fitted yesterday. The previous pic shows the old plug. Also measured shims beforehand with calipers to verify.

What gap would you recommend? Could it be related to "hesitate" problem?

[fhol]

Nah i'm about to give up on this. Today i tested throttle clip in top most position = nogo, it barely ran no matter mixture screw pos. Also removed intake manifold and noticed the metalwork was quite rough in some places along with a small rubber thingy hanging down in the intake. I fixed all that, made it smooth and hoped... but it made no difference at all.

I have also adjusted spark plug gap to 0.8mm = no difference. Best setup so far is
clip in middle pos, air/fuel mixture screw somewhere between 1 and 2 full turns out.

Starting to think its something else, like the rev limiter kicking in too early but only on standstill. Doesn't make sense though, because 3/4 throttle and it comes to life again (and revs higher). Though it _feels_ almost like when i run 50 km/h and rev limiter activates. Engine sound is a bit different comparing the two tho. On standstill, full throttle, it "bluddrar" as we say in Sweden. Bluddr-bludd-bluddr, not running "clean".

Think i'm gonna test 0.6mm spark plug gap tomorrow (0.7 and 0.8 tested now). Apart from the standstill problem, when above 5-10km/h, it runs absolutely great, even if rolling slowly min -> max throttle.

It would be interesting to deactivate rev limiter just to see if it is related to the problem or not... but i dunno how to do that, and don't want to deactivate it permanently because of potential insurance problem if i'm in an accident.

What to do?

[ratdog]

Your spark plug looks dead lean to me. I would suggest you go to a larger Jain jet. Keep going larger until that plug insulator is a light tan. I would also try a larger polit jet. Can’t tell you what size without knowing what is in it.

The needle in the slide helps control the mixture from about 1/8 to about 1/4 throttle. The farther the needle is into the carb, the leaner it gets. As you are running lean, you want the needle as far out of the carb as possible. I would call that the “bottom” position. Where as much of the blunt end of the needle is above the clip as possible.

Screwing with your mixture only effects how the bike will idle. You have basicly two things that effect your idle. The throttle stop which is where your cable from the twist grip attaches to the carb, and the mixture screw which should be on the other side. To set the mixture, first take the bike out and ride it around so it is warmed up. Next while the bike is idling, turn the throttle stop screw until you get to the lowest RPM the bike will still run at. Now adjust the mixture screw until the bike is running the fastest. That my still be to low a idle speed so you might have to adjust The throttle stops screw to get it at a comfortable RPM. If you don’t adjust the throttle stop before you adjust the mixture screw, you my find the mixture screw has no apparent effect. Hope I explained that well enough. At any rate, the mixture screw has no effect once you are above about 2000 rpm, so setting it dose little to effect the drivability of the scoot



The gap on your plug plug is not causing your problem.  The issue is the air fuel mixture.  In general, a wide plug gab helps most at low RPM.  The coil in my bike will run a very wide gap and still give a nice fat spark.  I run .035” , which would be just a bit under ,9.  Now your coil may not like that big of a gap.  I would take the plug out, turn the light down a little in the room, thentoch the metal of the plug to some clean metal on the bike, and crank the electric starter.  If your getting a good healthy spark at .85mm leave it there.  If it’s a weak spark that some times misses, go to a tighter gap.

[fhol]

Thanks ratdog! That was very well explained! Some of those things i already knew but some i didn't.

Before i read your post, i have found a new issue that i have now resolved. Posting again in a few minutes with some pics.

[fhol]

EDIT #2: I have flipped it back to the "wave" pattern once again. It looks like cr** but... In the end, I found it very hard to install it the other way and to get a good seal around the edge prior to mounting the lid. Therefore, please skip my BS below in this post.

I found out that the diaphragm was mounted upside down, probably from factory or when shop did the 500km first service. I have myself never touched it, or a carb of this type before. It kinda looked funny... with that "wave"-pattern. Then in workshop manual there was an illustration that gave me an 'aha'-moment!

Before:

EDIT: The above pic shows the "mushroom" shape i talk about later on in this thread. If you in the above config pull up on it into a "bowl" shape, then you see that rubber aligning thingy points straight up, which tells me it is installed upside-down onto the throttle itself. If you don't do that, and just install it into the carb this "mushroom" way, the wave pattern will appear. That pattern contains more wrinkles on diaphragm, and puts it into excessive strain and lowering lifetime, i would suspect.

After removing it from throttle and mounting it upside-down (correct way):

EDIT: The above pic show the "bowl" shape i talk about later in this thread. Look carefully at the outer edge and imagining flipping diaphragm upside-down. Then the integrated "o-ring" facing up on the edge would point down instead. The "bowl" shape is how you are supposed to lower it down into the carb, i think.

In the last pic, it was hard to get it into its groove all the way around before mounting the lid. I did my best and mounted lid... but i'm not 100% sure that it is in the groove now when lid is on. What would happen if it is not? Same as with a crack in it?

After this, i did a test-run. Unfortunately, it made no difference on the original "hesitate" problem.

Then i pulled plug and adjusted gap to 0.6mm from 0.8mm. New test-run. Maybe a little better. After that, i figured lets try throttle clip in downmost position again to give it more fuel. Diaphragm fix and plug gap might have changed something so wanted to try that again. And it did in fact get BETTER now!

So now i run:
0.6mm plug gap
clip in downmost pos (max fuel)
mixture screw 1 and 3/4 turns out

It still "hesitates" just a little for 0,5-1 sec or so, but this is the best setup so far. I don't have to release throttle from max now, it stops hesitating by itself now.

ratdog : Next thing to try is put slightly larger jets. With the diaphragm, needle and gap fixes today, it looks like the original too lean theory is back on the table.

Maybe also do the "dark room" spark test. Wider gap should help ignite the fuel better, provided you always get a good spark, as ratdog pointed out.

I can live with how it runs now even though it hesitates slightly. Therefore i'll put this stuff on hold for a while. Gonna pull plug again sometime in the future and see how it looks. If it runs a little lean, then i'll put larger jets in carb.

By the way, here is a pic of the rough edges and not so smooth surface inside the intake manifold, that i fixed earlier.