missing at low revs/flat spot

[kidda]

Hiyya folks,got an issue with it seems as though it is missing at low revs,it runs sweet when out on run but at low revs seems to miss and have a flat spot when you start opening the throttle?24/24 carb,125 main jet,177 kit on a PX125E. Thinking dropping the main jet a size and adjust the timing from 19 degrees see what happens?engine just been fully rebuilt by me so I'm running it in,done about 200 mile,on 1st start up seemed to have a flat spot but wasn't to bad and ran ok,but today done about 20 mile then stopped let it cool down,kicked her back in it wasn't enjoying ticking over and had to keep the revs up to stop it dying?going have a mess tomorrow see if I can see anything,any ideas?

[pxguru]

Firstly its nothing to do with the timing or main jet! What pilot jet is fitted?
That flat spot means its running weak at low revs. Don't cruise around at low throttle until its fixed, or it might heat seize.
This is what Sime has been working on with his, so he might feel like helping you with what he's learned so far!

[kidda]

I have been reading sime's threads with great interest,not sure what pilot jet is in I'll have a look later it was a standard 24/24E carb except I upjetted the main to a 125. This is what I couldn't understand because when running with throttle open was really nice plenty of power and smooth.

[L12ARD]

On my 125 , I added a SIP road 2.0 and a p200 filter (pre-drilled air filter) and just went up to a 124 main jet , it was also bogging down and hesitating at idle to low revs ...

I upped the Pilot (idle) jet from (44/140 standard) to 48/140 and it cured it instantly , no more flat spot at low revs ...

Not sure if the above helps wit your kit but sure sorted mine...

Steve 

[kidda]

Nice one Steve and pxguru,I'll have a look later see what's in and have a look at my spare carbs see what's in them,I've a 20/20 and 26/26 knocking about in the shed,cheers fellas

[sime66]

I'm not sure what to write because pxguru has already given you the answer, and the detail is in my thread, which you're reading, but I'll have a bash anyway.

This is not to do with your Main jet or timing, nor is it time to start playing with either of those anyway; they're a long way off. Your timing is probably going to be OK where it is (I'm not clear on what this 'mazz crank and ported' refers to exactly in your description with the film, or what your aims are, but it isn't important now anyway), and your MJ is probably rich, which is good for now.
I'm working through pxguru's running-in and setting-up crib sheet; you are at #2:

1. Get it running acceptably on some jetting that you guessed or copied.
2. Adjust the mixture screw for ideal running.
3. Fit a pilot jet with the small number so the mixture screw ends up 1.5 (or so) turns out.
4. Adjust the progression with the big number on the pilot jet (while keeping the now set small number the same(ish))
5. Adjust the MJ by feel
6. Adjust MJ by plug chops
7. Ride as fast as you like
8. keep going round in circles till its done but Pilot/progression/MJ/Atomiser is the sequence...

Mine was spluttery with a flat spot at low revs and not wanting to idle until this point too. You can quite easily sort the idling out with a tweak of Mix and Idle screws, just to get it ticking over, so you can go on to the next bit:

Your low revs flat spot is because of your Pilot jet. You've done 200 miles rough-running, so that's good. Now it's time to tighten everything up and turn your attention to #2, then #3, then #4. Have a look at what Pilot you have, and see where it is on the table in my thread – actually it's no bother to repost it, but I haven't got a blank one in my Flikr, so here's my marked-up one:



Think of the small number as fuel (like a MJ number) and the big number as air (like an air corrector), and for now, the lean-rich scale on the left.

You've got it idling now already (at #2), so this is the way to adjust the jet (or decide if it's too big):

When adjusting a new jet, the mixture screw should start at 4 turns out, with a slow tickover. As its wound in it will speed up. If it goes too fast, drop the tickover and continue to wind in the mixture until its at its fastest, then slow the idle to normal. If the revs dont pick up until the mixture screw is too far in then the pilot is too big.

You're after a slow idle with the mix screw about 1½ turns out, and a responsive, crisp, smooth throttle response to a quick rev and a quick return to idle without popping or spluttering – that's what all those films on my thread are for at this stage.

Once you have #2) & #3) sorted, it's time to go on to testing the Pilot jet for the Progression, #4 (your low revs riding); with plug chopping with a warm engine, on at least a mile run with slide open 2mm. This is where I'm at, and to do it properly is time consuming, and repetitive, but it is important. If I write too much I'm only repeating what's been said to me in my thread, and you're reading that anyway. It is tempting to skip all this, but I'm convinced that a bit of faffing about will prevent my invested time and money turning into a seized-up mess, and the work I've put into the build is worth the bit of extra time and effort now. A final quote from our Sponsor:

The penalty of getting the carb wrong is so painful on a two stroke. One little set up mistake and sometime in the future if its in one particular throttle position for too long its going to bite you!

So, I'd say;
Tighten everything, check your Pilot jet and see where it is on the table above, set your mix and idle, set-up your Pilot jet (small number), change it if you can't set it up, when you've got a jet you can set up and a god rev-idle-rev, and it runs smooth through all the rev-range without flat spots, spluttering etc., it's Progression plug shops (big number).

It' not easy writing out a procedure for something I'm just getting straight in my head for the first time, but I think that covers the important points. Best thing to do is to make a start and report back with what you find and take it from there. Hope it helps, and hope you haven't been put off, and decide to just swap it and ride.

[kidda]

Nice one sime I have been reading all your posts with great interest really good informative reading,I'll report back later when I finish work and get chance have a play with it,cheers again fellas for all your help

[kidda]

Pilot jet that was in was a 50/160 and looked bent slightly(maybe my eyes)???swapped for a 55/160 and one turn out on the mixture screw and just been for a spin,throttle seems responsive and picks up well. I've got a 52/140 I can try but going run at this for now and keep an eye on the plug but seems alls well at the moment fingers crossed

[pxguru]

Good result but I think from what you are saying that the 52/140 would be closer to correct. At only one turn out the pilot jet is too big and this means too much fuel will be getting in at low progression throttle. Won't do any harm and if it feels fine, without spluttering then should be ok but you will notice it drinks more fuel. Make sure you re-check your main jet at WOT. Changing the pilot jet means that it is now not the same as before at WOT and if it was correct before it will need to be altered.

[braindrain]

Does 4 turns out applies to spaco carb aswell? Because different thread type than dellorto's one.

"When adjusting a new jet, the mixture screw should start at 4 turns out, with a slow tickover."

[sime66]

Yes, that's for a Spaco carb; it's just the start point though; you need to follow the whole thing, not just the 4 turns out snippet - it's all there, above.