Tillotson HS idle/transition ports

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ballisticdoughnut

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So, question for anyone who’s played around with the idle and transition port on HS series carbs. Just put together a husky 61/272XP hybrid. No porting (thinking about it), did some muffler work, other than that it’s essentially a stock saw. Got a HS254 carb on it at the moment. Something I noticed is the saw has a bit of a flat spot when going from idle to WOT. It’s subtle but definitely noticeable. If I open up the high speed a bit it goes away but then becomes too rich. I’m gonna play around with it some more, I’ve got a HS260 I could swap as well. I’m wondering if the idle and/or transition port mite need to be enlarged slightly? I can dial in the high and low to where the saw idles proper and has a nice four stroke around 12.5K but I get that flat spot. Thoughts?
 
So, question for anyone who’s played around with the idle and transition port on HS series carbs. Just put together a husky 61/272XP hybrid. No porting (thinking about it), did some muffler work, other than that it’s essentially a stock saw. Got a HS254 carb on it at the moment. Something I noticed is the saw has a bit of a flat spot when going from idle to WOT. It’s subtle but definitely noticeable. If I open up the high speed a bit it goes away but then becomes too rich. I’m gonna play around with it some more, I’ve got a HS260 I could swap as well. I’m wondering if the idle and/or transition port mite need to be enlarged slightly? I can dial in the high and low to where the saw idles proper and has a nice four stroke around 12.5K but I get that flat spot. Thoughts?
Have you verified the operation of the idle and transfer ports by seeing if carb cleaner will spray out of the holes? If you can't be sure, might have to remove the welch plug covering them to see what's going on, that carb could have 2 transfer ports behind the idle one.
 
Yes the idle and transition port are clear and functioning as they should. I’m thinking of swapping the HS254 for a HS224. I’ve got a 266/268XP hybrid that does the same thing. Same carb in fact. Best way I can describe it is like a carbureted V8 that’s a little lean on the accelerator pump.
 
Use a tag wire to probe those ports first. I have done this to quite a few carburetors and it has worked. Just a little restriction will cause it.
Good point. I’m gonna try as many non invasive things as I can first. I don’t wanna make any permanent modifications unless I absolutely have to. I may end up putting a WJ125 on it, they’ve got two transition ports where the HS series carbs have one. Not too thrilled about dropping a Benjamin on a new carb but if that’s what it takes, so be it.
 
Good point. I’m gonna try as many non invasive things as I can first. I don’t wanna make any permanent modifications unless I absolutely have to. I may end up putting a WJ125 on it, they’ve got two transition ports where the HS series carbs have one. Not too thrilled about dropping a Benjamin on a new carb but if that’s what it takes, so be it.
Poking a tiny wire down the ports is not invasive and can be done without taking the carb apart
 
This is only to clean the ports from the throttle bore up without taking the carb apart. Those ports are progressively adding fuel as the throttle plate opens. Any restriction will cause a bog. I am talking only opening back to original.
 
This is only to clean the ports from the throttle bore up without taking the carb apart. Those ports are progressively adding fuel as the throttle plate opens. Any restriction will cause a bog. I am talking only opening back to original.
Yep, I’m with you on that 100% gonna give it a shot today. Carb was cleaned in my ultrasonic cleaner but sometimes a little hands on work is needed. Im hoping this is one of those situations.
 
Make sure the notch in the throttle butterfly is oriented correctly…in front of the transition holes. If so, make the notch in the throttle butterfly a little bit bigger. The slower moving air should force you to open the low speed jet a little bit. Then, when you crack the throttle, it will get more fuel.
 
Make sure the notch in the throttle butterfly is oriented correctly…in front of the transition holes. If so, make the notch in the throttle butterfly a little bit bigger. The slower moving air should force you to open the low speed jet a little bit. Then, when you crack the throttle, it will get more fuel.
If it doesn’t work, you can always solder the cutout back smaller and file it however you want
 
You mention opening up the H screw removes the bog. It’s not the right way to do it.

The 2 progressive idle drillings are fed off the L speed circuit and those are responsible for removing that flat spot between idle and the main jet. Richen that instead. If the drillings are clear and the carb is the correct size, there shouldn’t be an issue.

This is where accelerator pumps really shine. You can tune the L for idle fuel supply and lean it for best acceleration without the concern of a bog, the pump covers transitional fuel supply (along with the progressive drillings) and then the high screw for full throttle.

Edit to add:

What I have found on occasion, richening the L too much can interfere with the H setting and actually limit how lean you can tune it.

It’s best to set the L speed screw as lean as you can ensuring it still starts easily, transitions to WOT smoothly and returns well too.
 
You mention opening up the H screw removes the bog. It’s not the right way to do it.

The 2 progressive idle drillings are fed off the L speed circuit and those are responsible for removing that flat spot between idle and the main jet. Richen that instead. If the drillings are clear and the carb is the correct size, there shouldn’t be an issue.

This is where accelerator pumps really shine. You can tune the L for idle fuel supply and lean it for best acceleration without the concern of a bog, the pump covers transitional fuel supply (along with the progressive drillings) and then the high screw for full throttle.

Edit to add:

What I have found on occasion, richening the L too much can interfere with the H setting and actually limit how lean you can tune it.

It’s best to set the L speed screw as lean as you can ensuring it still starts easily, transitions to WOT smoothly and returns well too.
The problem I’m having is even with the low speed tuned properly I’m getting a off idle/transition to high speed bog. With the low speed set to give max idle speed and adjusted to proper rpm with the T/LA screw, and high speed tuned to around 12.5K I’m getting that bog. It’s got a HS254 on it now, IPL calls for a HS260 (I have one on another saw) which has since been replaced by the Walbro WJ125. Only difference I can see between the 254 and 260 is the 260 has a bigger high speed nozzle. Venturi size appears to be very close if not the same. I’m gonna play around with it more, will report back with my findings. Was gonna run it Sunday but more important things took precedence.
 
The problem I’m having is even with the low speed tuned properly I’m getting an off idle/transition to high speed bog. With the low speed set to give max idle speed and adjusted to proper rpm with the T/LA screw, and high speed tuned to around 12.5K I’m getting that bog. It’s got a HS254 on it now, IPL calls for a HS260 (I have one on another saw) which has since been replaced by the Walbro WJ125. Only difference I can see between the 254 and 260 is the 260 has a bigger high speed nozzle. Venturi size appears to be very close if not the same. I’m gonna play around with it more, will report back with my findings. Was gonna run it Sunday but more important things took precedence.
That’s not L speed tuned properly, not sure who told you that! You will get a bog if you are tuning L for max rpm.. it should be on the rich side of max rpm.. open the L 1/4 of a turn and try again

Here this may help
 
How much did you open it? Upload a video of the saw running.
I’ll have some time to play with it tomorrow. I should also point out I work on power equipment for a living, I’m well versed in the nuances of adjusting a carb. This one is behaving strangely which is why I’m wondering if there’s something going on with the transition ports. One of them could very well be blocked or partially blocked. I’m gonna pull the carb and remove the welch plug. Only way to 100% know for sure.
 
I’ll have some time to play with it tomorrow. I should also point out I work on power equipment for a living, I’m well versed in the nuances of adjusting a carb. This one is behaving strangely which is why I’m wondering if there’s something going on with the transition ports. One of them could very well be blocked or partially blocked. I’m gonna pull the carb and remove the welch plug. Only way to 100% know for sure.
You can’t be well versed if you think tuning a carb involves bringing the L to the fastest rpm and calling that tuned..

You aren’t listening. I’m out, good luck with the repair.
 
You can’t be well versed if you think tuning a carb involves bringing the L to the fastest rpm and calling that tuned..

You aren’t listening. I’m out, good luck with the repair.
No you’ve simply misunderstood what I’m saying. Your attitude is 100% not necessary. It’s just a chainsaw not a piano I’ll get it sorted out. Not worried about it in the slightest.
 
No you’ve simply misunderstood what I’M saying. Your attitude is 100% not necessary. It’s just a chainsaw not a piano I’ll get it sorted out. Not worried about in the slightest.
no I have understood very well, it’s not an attitude, I’m being factual:

“I’m getting an off idle/transition to high speed bog. With the low speed set to give max idle speed and adjusted to proper rpm with the T/LA screw,”

Firstly this isn’t the way to tune any chainsaw carb… it’s completely wrong.

You then try and follow it up with

“I’m well versed n the nuances of tuning a carb”


But you’ve proved you’re not at all. Your process and procedure is completely wrong. You don’t
find fastest RPM on L and call it good, you don’t use the butterfly to set the final rpm, you need the L to do that to set the La between the idle drilling and first progressive drilling. It couldn’t be more wrong.

I’ll tell you what is annoying me, that people like
you come to a forum, get excellent advice from members, yet you decide to still do what you think or your version of the advise given and then back it up with that you have experience. What annoys me is you waste peoples time.

You certainly could have a progressive idle blockage, but until you learn to tune a carb properly, blockage or no blockage it ain’t gunna run right!
 

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