17 replies to this topic

[raygrilli]

Hi All,

I hope collective wisdom and experience can help me sort out a fuelling issue I have with my 2011 2.9.

As received from the previous owner the engine was often 'bogging' quite badly, and on a number of occasions would hunt when engine speed came back to idle. A few fellow riders suggested to'clean it out' by revving the guts out of it for a few secconds before starting a section, and this did seem to help, but in longer sections the bogging would start to re-occur.

So, I checked the carb setting were as per Splatshops' recomendations :- https://www.splatsho...rco-preparation, and it seems that the 2.5mm restrictor in the fuel inlet to the carb was missing and also the float height was at 21mm whereas it should have been 18.5mm with the 250 jet that is fitted.
After taking the bike out again nothing seemed to have changed, so I had a fiddle with the needle height to see if it would make a difference.
Splatshop spec for needle height says middle groove on needle with washer placed nearer to nut. This is what I had as received. I had a go at lifting the needle to see if richening would help, and it made the bogging worse. Dropping the needle seems to be the best improvement I can make, and currently I have the circlip in the top groove with the washer between the circlip and the main slide itself, which seems to be the best I can get it. The fuelling is still not right though and the bike is still sluggish from low engine speed compared to a 2.5 of the same year (although this was Luke Walker's bike and he told me he'd changed the carb from the original to make it run right!). The hunting at idle is no longer happening. Also, with the needle set at any height the engine is fine for medium to high speed running.

Can anybody shed any light on how to improve the fuelling from this current condition? I'm not that familiar with fiddling with carbs and not certain what to try. I 'think' that changing the pilot jet would probably be the next thing to do, and perhaps fitting a smaller one to lean out the low speed running and perhaps lift the needle back up again, but not really sure.

I've spoken with Chris at Splatshop, who has been very helpful, but he's on hol this week and i'd like to get this sorted sooner rather than later....

Any knowledge/thoughts/experience would be greatly appreciated.

Cheers,

Ray

Edited by raygrilli, 30 January 2012 - 03:25 PM.

[scraggydog64]

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[scraggydog64]

I had the same problem albeit with a 250 Rev 3. I rang BETA Uk to purchase a smaller pilot jet. The gent I spoke to asked what ratio of pre mix I was using, which at the time was 65:1 using fully synthetic oil.

He told me to try running 80:1 before altering the jetting. He was spot on, the bike ran loads cleaner and there was no need to "clear it out" before riding a section.

Regards Michael

[dadof2]

Strange one that scraggydog64. If you wanted the mixture leaning out you would have put more oil in your fuel - not less.
% oi in fuel has very little to do with whether the bike runs clean or not - its the amount of petrol : air that matters.
Mark your throttle and twistgrip with tape and pen marks or liquid paper. Mark it at 1/16th open, 1/8th open, 1/4 open, 1/2 open & 3/4 open. Find as good steady long hill as you can - you want the engine to work hard at the required throttle openings for as long as possible, 1/2 mi;le will do, 2 miles a lot better, you can use the brakes a bit but dont overheat them.
You need a cleanish used plug for the test. Warm the bike up then climb the hill in whatever gear makes it pull steadily at the martked throttle opening. As soon as you get to the top close the throttle and press the kill switch.
Remove the spark plug. It should be pale chocolate brown - darker means rich / too much petrol, pale or greay means weak / too little petrol.
Slow running jet / air scrw has most effect up to 1/4 throttle, needle position most efect 1/4 to 1/2 or 3/4 throttle and main jet from 1/2 or 3/4 to full throttle.
Better still buy http://www.amazon.co...l/dp/1859606199 and follow that.
Cheers

[raygrilli]

Cheers for the info so far but I think i really need to understand what to do with the pilot jet sizing to fix the problem.

I'm running 80:1 mix and this was fine in my 2002 GasGas, so fairly certain it's not that.

Also, I can't run the bike up the road as it's not road-going!

I can't completely get my head around the idle mixture adjustment either - does unscrewing the mixture screw (not the slide stop!) (i.e. turning clockwise) incease the amount of fuel, or is it an air circuit that by unscrewing increases the amount of air? I know this has only a very small effect on low speed running - 0 to 1/8 throttle, but it does have an effect, and if I understand what the adjustment controls it might help me understand how to fix the problem!

Ta,

Ray

[paul w]

its really difficult to diagnose carburation issues like this but i think your looking in the wrong area.
If the majority of bikes the same as yours run on the "base settings" then so should yours.
its bite the bullet time.
First check the ignition timing, doubtfull but its easy to do.
before going any further, re post and ask if there is some one willing to let you borrow a carb ( 10 mins anywhere should show up if there is an issue)
Next remove the reed block, are the reeds sitting down? if not try turning them over to see if that makes a difference before replacing.
make sure that when you replace the reed block it is in good condition and that its mating surface if clean and sealed (smear of silicon)
Failing that, and if it was my bike, i would strip down the top end and inspect the piston, long shot i know but youve got to start somwhere. while the top is off check for big end and main bearing play. if ok i would still REPLACE the crank oils seals.
new base gasket and rebuild the top end.

its a 2 stroke, thats covered the basics, its not a complicated engine just annoying.

let us know how you get on.

[naichuff]

How is the exhuast system if choked may give the same effect by not letting the gasses out freely

[copemech]

raygrilli, on 30 January 2012 - 02:46 PM, said:

Hi All,

I hope collective wisdom and experience can help me sort out a fuelling issue I have with my 2011 2.9.

As received from the previous owner the engine was often 'bogging' quite badly, and on a number of occasions would hunt when engine speed came back to idle. A few fellow riders suggested to'clean it out' by revving the guts out of it for a few secconds before starting a section, and this did seem to help, but in longer sections the bogging would start to re-occur.

So, I checked the carb setting were as per Splatshops' recomendations :- https://www.splatsho...rco-preparation, and it seems that the 2.5mm restrictor in the fuel inlet to the carb was missing and also the float height was at 21mm whereas it should have been 18.5mm with the 250 jet that is fitted.
After taking the bike out again nothing seemed to have changed, so I had a fiddle with the needle height to see if it would make a difference.
Splatshop spec for needle height says middle groove on needle with washer placed nearer to nut. This is what I had as received. I had a go at lifting the needle to see if richening would help, and it made the bogging worse. Dropping the needle seems to be the best improvement I can make, and currently I have the circlip in the top groove with the washer between the circlip and the main slide itself, which seems to be the best I can get it. The fuelling is still not right though and the bike is still sluggish from low engine speed compared to a 2.5 of the same year (although this was Luke Walker's bike and he told me he'd changed the carb from the original to make it run right!). The hunting at idle is no longer happening. Also, with the needle set at any height the engine is fine for medium to high speed running.

Can anybody shed any light on how to improve the fuelling from this current condition? I'm not that familiar with fiddling with carbs and not certain what to try. I 'think' that changing the pilot jet would probably be the next thing to do, and perhaps fitting a smaller one to lean out the low speed running and perhaps lift the needle back up again, but not really sure.

I've spoken with Chris at Splatshop, who has been very helpful, but he's on hol this week and i'd like to get this sorted sooner rather than later....

Any knowledge/thoughts/experience would be greatly appreciated.

Cheers,

Ray

I have not owned the VHST carb, yet hopefully I can give you some insight. From your description of bogging and hunting idle, it would suggest that you are running out of fuel or running lean.

Dirt and/or water can do this. I suggest firstly you start over with the carb, plus insuring you fuel tank vent line and check valve are totally clean and unrestricted(not pinched by mudguard).

Remove the carb carefully and break it over a clear glass or plastic bowl. Inspect what comes out closely looking for water accumulation in the bottom or grit as well. It only takes a couple drops of water to do this.

Remove the pi;ot jet and the mix screw from the carb. Locate a fine wire that will actually pass through the pilot jet(one on side of baffle ring)as to insure there is no clear crystal sand bit init. Blow through passages using compereesd air if possible or brake cleaner or carby spray. You should have flow from the screw orfave up through the little holes in the bore of the carb.

Ck main jet as well. Then put things back stock. 33 pilot should be ok.

On your mix screw, it is on the engine side of things I believe, making it a fuel trim screw actually. So out is more fuel and in is less. I am not sure on that carb, but on the PHBL(similar) 2-3 turns out is common, possibly a bit more.

You should be able to eat off the carb when done. I am qurious just what comes out the bowl under close inspection!

Try those things, hope it helps! They will not tolerate any crap!

[raygrilli]

Thanks for the info.

I have had the carb apart and cleaned it out thoroughly with compressed air when I checked the jets were the right ones. Cleaning it out didn't seem to make any difference, althoug I will do it again paying particular attention to the pilot drillings.

The issue is, to be honest, a pretty subtle one now that I've dropped the needle. The engine is just lacking urgency at low engine speeds.

I suppose there is a chance that there is an air leak between the carb and the crankcase - I'll spray some carb cleaner around there when I've got the engine running to see if it makes a noticeable difference to how the engine runs. Maybe it's an issue with the reeds??? Although I don't know much about the effect of reed problems on engine running.

Thing is, I've dropped the needle to make the engine run leaner which has made a marked improvement to the way the engine runs at low speeds with small throttle openings, this is why I think that I may need to go down a size on the pilot jet, although no-one has yet suggested that this may be a solution to the problem????? But, Splatshop only list a 32 pilot as the smallest available and it's got a 33 in at the moment, would going down 1 size make a discernable difference?

Hmm...

[copemech]

raygrilli, on 01 February 2012 - 08:21 PM, said:

Thanks for the info.

I have had the carb apart and cleaned it out thoroughly with compressed air when I checked the jets were the right ones. Cleaning it out didn't seem to make any difference, althoug I will do it again paying particular attention to the pilot drillings.

The issue is, to be honest, a pretty subtle one now that I've dropped the needle. The engine is just lacking urgency at low engine speeds.

I suppose there is a chance that there is an air leak between the carb and the crankcase - I'll spray some carb cleaner around there when I've got the engine running to see if it makes a noticeable difference to how the engine runs. Maybe it's an issue with the reeds??? Although I don't know much about the effect of reed problems on engine running.

Thing is, I've dropped the needle to make the engine run leaner which has made a marked improvement to the way the engine runs at low speeds with small throttle openings, this is why I think that I may need to go down a size on the pilot jet, although no-one has yet suggested that this may be a solution to the problem????? But, Splatshop only list a 32 pilot as the smallest available and it's got a 33 in at the moment, would going down 1 size make a discernable difference?

Hmm...

The bikes tend to run blubbery with too large a pilot, yet not bog when the throttle is cracked. Reed problems not common on the stock ones, but you could look at them, only four screws more.

That carb being similar yet different than the PHBL, I would only chance to say that NO, one step on the pilot is not going to make a ton of difference in the running.

Doubt I could run a 33 here on our fuel, as mine was up to a 38 on the PHBL prior to removal. A 33 would bog like that!

Chris says they will run on the 33 so who am I to doubt it, yet I do know through the threads that several in the UK are now up to the 35-36 range on the PHBL carbs. Just how that might affect the VHST, I am not sure. All has to do with the fuel.

Ck the rest still, as it is critical and can nip you in the rear. Better men than me have been bitten by this and you must not overlook anything.


Cheers,

[nigel dabster]

The problem is the pump.
Might be worth trying to fill up and by pass, dellortos always ran clean pre pump on shercos.

The kehin is much better once set.

[curt531]

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[curt531]

Hi Ray, your having all the same problems that I had with my sherco, never did get the carb to work right, only thing that did worked was to get a Gas Gas no help to you at all, sorry

[raygrilli]

An update.....

I think I'm close to having sorted the problem. Dropping the needle to lean the misture overall has made a massive improvement over the original setting of the needle in the middle circlip position.
The final fix seems to be running the idle mixture very lean - only 1.5 turns out on the idle mixture screw. This has now corrected the poor low speed running, but I do now have a little bit of hunting on overrun as the engine comes back to idle.
I think the final final fix will be to fit the only smaller pilot jet available, which is a 32 instead of the existing 33 jet, as this will hopefully allow me to run a little richer at idle so that the slight hunting is corrected, whilst allowing the mixture at small throttle openings to remain similar to the mixture with the current pilot jet/idle mixture screw settings.

Reckon I'll hang on before buying a new Gas Gas with leaky brakes!!

Ray

[copemech]

raygrilli, on 19 February 2012 - 10:34 PM, said:

An update.....

I think I'm close to having sorted the problem. Dropping the needle to lean the misture overall has made a massive improvement over the original setting of the needle in the middle circlip position.
The final fix seems to be running the idle mixture very lean - only 1.5 turns out on the idle mixture screw. This has now corrected the poor low speed running, but I do now have a little bit of hunting on overrun as the engine comes back to idle.
I think the final final fix will be to fit the only smaller pilot jet available, which is a 32 instead of the existing 33 jet, as this will hopefully allow me to run a little richer at idle so that the slight hunting is corrected, whilst allowing the mixture at small throttle openings to remain similar to the mixture with the current pilot jet/idle mixture screw settings.

Reckon I'll hang on before buying a new Gas Gas with leaky brakes!!

Ray

In my reconing you are reversed, and need to go up a size, yet see how you get on.

I run the big pilot, with a lean idle screw, which richens the transition circuit at low throttle openings prior to transition to the needle. Now if your idle hunting is slight, but things are otherwise good, this may be the best you get, as this carb is not all that great a mixer in the first place as compared to the kiehin, yet when right they work fine, at least the phbl, the vhst is similar but different and the float level sensitivity seems to be e crutial point.

Let us know how you get on.

[raygrilli]

Made an interesting discovery after talking to Chris at Splatshop. The reeds in the engine are twin reeds, so as far as I can make out this means that they are probably Boyesen reeds and not the original ones (which would be a single reed petal on each side instead of the two reed petals per side that i have).
Digging around the forum has enlightened me to the fact that Boyesen Reeds tend to cause the engine to run richer than with std reeds. Seems I've got my diagnosis right all along in thinking it's been running rich. So I've just received a U33 pilot jet from Splatshop which I will try in place of the existing S33 pilot jet. Apparently the U jets have larger air holes than the S jets so therefore give a leaner mixture for the same fuel orifice size.

I'll fit it and let you know how it runs....

[copemech]

raygrilli, on 23 February 2012 - 08:20 PM, said:

Made an interesting discovery after talking to Chris at Splatshop. The reeds in the engine are twin reeds, so as far as I can make out this means that they are probably Boyesen reeds and not the original ones (which would be a single reed petal on each side instead of the two reed petals per side that i have).
Digging around the forum has enlightened me to the fact that Boyesen Reeds tend to cause the engine to run richer than with std reeds. Seems I've got my diagnosis right all along in thinking it's been running rich. So I've just received a U33 pilot jet from Splatshop which I will try in place of the existing S33 pilot jet. Apparently the U jets have larger air holes than the S jets so therefore give a leaner mixture for the same fuel orifice size.

I'll fit it and let you know how it runs....

Well there you go, hopefully you are onto something. I would also inspect to insure the stock reed plates stops were removed(large metal plates)to go with the boyesen dual reed install.


I think I have a set of stock reeds and plates should you decide to go back to square one. Boyeseens seem a bit smoother off the bottom though. A lot depends on your style and prefs.

Keep us posted!