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636 with VM44 Carb
Topic Started: Jun 22 2016, 08:49 AM (571 Views)
1969Jackal
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Knight
[ *  * ]
Does anyone run a rotax 636 with a VM44 carb on here and manage to get it to run cleanly throughout the rev range.
The problem I have with my set up is at about 3/4 throttle opening the bike starts to splutter, which suggests to me that the needle and needle jet are running on the rich side. If I pin the throttle wide open when it's stuttering so that it is running on the main jet, it picks it's heals up and runs cleanly.
To verify that the jet needle is running rich I dropped the clip down a notch on the needle to make the mixture richer. As expected the stuttering got worse. Lowering the needle to lean the mixture off didn't seem to improve things so I've stuck with the middle groove for the needle at the moment.
Taking the air filter off did make a slight improvement but the stutter is still there.
The needle jet is z-0 and the needle jet is 7dh5 which I understand to be the leanest of the available jets and needles.
Air mixture screw is out 2 turns.
Any suggestions anyone, as it's getting a bit of a pain having to ride wide open throttle all the time.
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stevo604
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God
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1969Jackal
Jun 22 2016, 08:49 AM
Does anyone run a rotax 636 with a VM44 carb on here and manage to get it to run cleanly throughout the rev range.
The problem I have with my set up is at about 3/4 throttle opening the bike starts to splutter, which suggests to me that the needle and needle jet are running on the rich side. If I pin the throttle wide open when it's stuttering so that it is running on the main jet, it picks it's heals up and runs cleanly.
To verify that the jet needle is running rich I dropped the clip down a notch on the needle to make the mixture richer. As expected the stuttering got worse. Lowering the needle to lean the mixture off didn't seem to improve things so I've stuck with the middle groove for the needle at the moment.
Taking the air filter off did make a slight improvement but the stutter is still there.
The needle jet is z-0 and the needle jet is 7dh5 which I understand to be the leanest of the available jets and needles.
Air mixture screw is out 2 turns.
Any suggestions anyone, as it's getting a bit of a pain having to ride wide open throttle all the time.
hi mate their is a thread on here from forum members that are running the vm44 carb and their setup and finding
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stevo604
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God
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stevo604
Jun 22 2016, 09:01 AM
1969Jackal
Jun 22 2016, 08:49 AM
Does anyone run a rotax 636 with a VM44 carb on here and manage to get it to run cleanly throughout the rev range.
The problem I have with my set up is at about 3/4 throttle opening the bike starts to splutter, which suggests to me that the needle and needle jet are running on the rich side. If I pin the throttle wide open when it's stuttering so that it is running on the main jet, it picks it's heals up and runs cleanly.
To verify that the jet needle is running rich I dropped the clip down a notch on the needle to make the mixture richer. As expected the stuttering got worse. Lowering the needle to lean the mixture off didn't seem to improve things so I've stuck with the middle groove for the needle at the moment.
Taking the air filter off did make a slight improvement but the stutter is still there.
The needle jet is z-0 and the needle jet is 7dh5 which I understand to be the leanest of the available jets and needles.
Air mixture screw is out 2 turns.
Any suggestions anyone, as it's getting a bit of a pain having to ride wide open throttle all the time.
hi mate their is a thread on here from forum members that are running the vm44 carb and their setup and finding
RB636 set upThe go to setup is
300 Main
45 Pilot
Z-0 Needle Jet
7DH5 Needle
3.0 Slide
1.5 Air jet
3.5 Float valve
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1969Jackal
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Knight
[ *  * ]
stevo604
Jun 22 2016, 09:01 AM
1969Jackal
Jun 22 2016, 08:49 AM
Does anyone run a rotax 636 with a VM44 carb on here and manage to get it to run cleanly throughout the rev range.
The problem I have with my set up is at about 3/4 throttle opening the bike starts to splutter, which suggests to me that the needle and needle jet are running on the rich side. If I pin the throttle wide open when it's stuttering so that it is running on the main jet, it picks it's heals up and runs cleanly.
To verify that the jet needle is running rich I dropped the clip down a notch on the needle to make the mixture richer. As expected the stuttering got worse. Lowering the needle to lean the mixture off didn't seem to improve things so I've stuck with the middle groove for the needle at the moment.
Taking the air filter off did make a slight improvement but the stutter is still there.
The needle jet is z-0 and the needle jet is 7dh5 which I understand to be the leanest of the available jets and needles.
Air mixture screw is out 2 turns.
Any suggestions anyone, as it's getting a bit of a pain having to ride wide open throttle all the time.
hi mate their is a thread on here from forum members that are running the vm44 carb and their setup and finding
I think I've come across it and used the recommendations to set the carb up initially.
I'm using the following set up:
300 main jet.
45 pilot
Z-0 needle jet
7DH5 Needle
3.0 slide
2 turns out for air mixture screw
needle clip in middle groove.

I was just wondering if anyone had similar problems with this set up and what they have done to overcome it.
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Ant P
Viscount
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Im running a dellorto pump phm 40 and having similar probs. Great idle and great if i wheelie the nuts off it everywhere but get a misfire type effect on 1/4 throttle. Have to have needle on 2nd clip from the top just to tame it. Im waiting on an exhaust mod at a shop then fitting a trumpet and will get on a rolling road to sort. Roadster reccomended a place in Orpington depending where you are maybe give them a shout. HM Racing.
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1969Jackal
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Knight
[ *  * ]
Ant P
Jun 23 2016, 06:25 AM
Im running a dellorto pump phm 40 and having similar probs. Great idle and great if i wheelie the nuts off it everywhere but get a misfire type effect on 1/4 throttle. Have to have needle on 2nd clip from the top just to tame it. Im waiting on an exhaust mod at a shop then fitting a trumpet and will get on a rolling road to sort. Roadster reccomended a place in Orpington depending where you are maybe give them a shout. HM Racing.
I'm using the old 'seat of the pants' dyno at the moment but I think I've come to the limit of my tuning ability.....unless anyone can offer any further guidance on what to twiddle.
It's looking like I might have to bite the bullet and put it on a proper dyno to get it set up correctly, but I've never used one before so I'm not too sure what to expect. Do they just strap it on and advise where it is running rich/weak in the rev range, or will they re-jet and tweak the carb as well.
Has anyone any experience of Race HQ in Leeds?
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RB636
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Potato Farmer
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had two bikes with vm44s, both ran great.
I don't know what else has been done to your bike.

Not doubting what you know but here's what I know.


First of all, 3/4 throttle is already on the main jet, and coming off of the needle jet.


I would try a smaller main, and then you can play with the needle.

Lastly,

Those settings I posted worked with Headwork, an open air box, decent filter, and a free flowing exhaust. Never tried it with the stock r30 exhaust, scorpion or similar custom job is what you want.

Without supporting mods, it probably is a bit too rich.
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1969Jackal
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Knight
[ *  * ]
RB636
Jun 23 2016, 12:14 PM
had two bikes with vm44s, both ran great.
I don't know what else has been done to your bike.

Not doubting what you know but here's what I know.


First of all, 3/4 throttle is already on the main jet, and coming off of the needle jet.


I would try a smaller main, and then you can play with the needle.

Lastly,

Those settings I posted worked with Headwork, an open air box, decent filter, and a free flowing exhaust. Never tried it with the stock r30 exhaust, scorpion or similar custom job is what you want.

Without supporting mods, it probably is a bit too rich.
it's not a standard engine in this, apart from the 636 conversion, its got a 860 cam, k&n air filter, open air box, ported head, and a free flowing exhaust.
I'm thinking its the needle jet or needle because when you pin the throttle open during the stutter it pulls strong and takes off with no more stuttering. It might be just before 3/4 throttle when it happens, its got a quick action throttle on it and things are getting a bit frantic when I'm going from stutter to wide open throttle to see exactly where the throttle is !!!
Do you think going smaller on the main will have any effect on the needle fuelling? I suppose there is a bit of a transition from needle to main, but I would have expected it to be a bit flat at full throttle if the main was too big, whereas it pulls really well at that point.
I've tried a clip position either side of the central groove, but not real improvements either way, if anything it got worse the richer it got. I could try the very top groove on the needle to try and lean it off further. Have you any experience/success with different needle positions?
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RB636
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Potato Farmer
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Yes, winter I go a notch richer, but I don't use it in winter these days. Try a smaller main and see what happens.
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