Boring Out a Carb

[Backyard Lumberjack]

I don't have time to look up those carbs, but there could be several differences:

1. The carb size compared to the displacement.
2. The discharge port on the edge of the venturi.
3. The size of the carb bore relative to the intake and filter mount. There must be some reduction of diameter to make an effective venturi, and here the outlet blends smoothly into the intake port. If it opened up a lot at the outlet the entire carb body may act as the venturi.

Again, it's not like it didn't work at all, rather the venturi just didn't work quite well enough at lower air flow, leaving a hole. If the choke plate were not in the way I might try to make a booster venturi like is used in many other carbs.

>If the choke plate were not in the way I might try to make a booster venturi like is used in many other carbs.

I see where some on modded saws do not run a choke plate... and to start cold use thumb etc over venture to richen it air/fuel ratio for start. should bump the cfm, but also lean it out, too.

 

[Chris-PA]

I couldn't help myself and decided to see if that lump could be eliminated. I broke through on the junk carb but found I could take a lot of it off:




I dunno if/when I'll bolt it up and try it again, but I'm sure I'll be tempted eventually.

>If the choke plate were not in the way I might try to make a booster venturi like is used in many other carbs.

I see where some on modded saws do not run a choke plate... and to start cold use thumb etc over venture to richen it air/fuel ratio for start. should bump the cfm, but also lean it out, too.

Well yeah, but I use my saws and want it to remain practical.

Find a carb from a larger saw with a smaller venturi and you should notice a gain.

I'm afraid I'm not catching on to what you meant. This was a 42cc and the carb started off as 28/64". I'm running a 34/64", and this one is now enlarged to 35/64". I'm not aware of any saws larger than 42cc that use a carb smaller than 28/64", and I don't see why using one would help?

Most WTs have a 5/8" (16mm) throttle bore, except for the really small ones. So regardless of where you start you'll end up at about the same ratio of venturi to throttle bore anyway.

 

[blsnelling]

That's the spirit! Never give up

 

[Chainsaw Jim]

The venturi isn't there to restrict flow to begin with. It helps build velocity at the point where the fuel makes contact with the air flow so it flows with the air instead of flowing like a river down the side of the carb barrel. You can poke the main nozzle further out into the barrel to get the fuel into the stream better.
If you look at the difference between the 394xp carb and the ms660 you'll see a larger venturi in the 660, but the 394 carb is better.

 

[Chris-PA]

The venturi isn't there to restrict flow to begin with. It helps build velocity at the point where the fuel makes contact with the air flow so it flows with the air instead of flowing like a river down the side of the carb barrel. You can poke the main nozzle further out into the barrel to get the fuel into the stream better.
If you look at the difference between the 394xp carb and the ms660 you'll see a larger venturi in the 660, but the 394 carb is better.

The venturi is there to force the air to speed up, and the increased air velocity causes a pressure drop at the outlet port which is placed there. If there is no reduction in area then it's just a tube not a venturi. It is somewhat restrictive but that's unavoidable. The larger you go the less the restriction, but also the smaller the pressure drop and the minimum air flow needed to pull fuel goes up.

After the fuel is pulled out of the outlet it's a vapor, not a liquid (liquid fuel won't burn much), although it can condense out on cool surfaces afterwards.

This carb has no separate main nozzle, it's just a hole on the wall of the venturi.

 

[Justsaws]

Never mind, stay satisfied not making things work, cause you read it on Wikipedia.

 

[Backyard Lumberjack]

BL... I see where some on modded saws do not run a choke plate... and to start cold use thumb etc over venture to richen it air/fuel ratio for start. should bump the cfm, but also lean it out, too.

C-PA... Well yeah, but I use my saws and want it to remain practical.

good point! I have asked myself would I do one up so that I had to be the choke blade? lol... conclusion was I could live with it. a specialty modded saw. fuel, oil, remove cover, add choke or be choke... pop, me off or choke stuff out... and pull to start and set down to let idle... once warmed... replace cover, filter... tweak jetting as required... and go have fun. not as convenient as an off the shelf stocker... but for me, I could live with it... guess my thinking suggests small price to pay as once warmed will fire right up... no choke required!

 

[Chainsaw Jim]

The venturi is there to force the air to speed up, and the increased air velocity causes a pressure drop at the outlet port which is placed there. If there is no reduction in area then it's just a tube not a venturi. It is somewhat restrictive but that's unavoidable. The larger you go the less the restriction, but also the smaller the pressure drop and the minimum air flow needed to pull fuel goes up.

After the fuel is pulled out of the outlet it's a vapor, not a liquid (liquid fuel won't burn much), although it can condense out on cool surfaces afterwards.

This carb has no separate main nozzle, it's just a hole on the wall of the venturi.

Is there any air flow restriction at the filter or the air inlet on the top cover?

 

[shorthunter]

After the fuel is pulled out of the outlet it's a vapor, not a liquid (liquid fuel won't burn much), although it can condense out on cool surfaces afterwards.

Great thread, it's nice to see guys trying different things and sharing their results. I have a question about the statement above.

Wouldn't the fuel be in a liquid state until it has been exposed to the warm crankcase?

 

[Chris-PA]

Never mind, stay satisfied not making things work, cause you read it on Wikipedia.

Sorry, I didn't mean to offend. I've been tuning, modifying and studying carbs for over 37 years. I modified the air horn/top cover on the Autolite 4100 on my Mustang about the time I got my license. Velocity stacks are not new concepts to me, and the Wikipedia reference was a convenient way to describe what they are for without me having to type it out myself. I don't buy into the idea, especially when you need an air filter. Even on modified engines in SCCA nobody bothered with them. The US auto manufacturers never did either, and did their intake tuning between the carb and the intake port.

Is there any air flow restriction at the filter or the air inlet on the top cover?

Not much going into the air box, and the filter is a big foam block. There is a spacer to keep the foam from blocking the inlet:

Still, the problem was not one of getting enough air flow, as it revved really well no load. The problem was no guts under load at lower air flow, which meant the modified venturi wasn't working well at lower air flow. That also means it was leaning out under load, which is partly why I'm hesitant to try it again.

Great thread, it's nice to see guys trying different things and sharing their results. I have a question about the statement above.

Wouldn't the fuel be in a liquid state until it has been exposed to the warm crankcase?

It's not supposed to be. The fuel is pulled out of the outlet port/nozzle into a low pressure fast moving air stream and is supposed to be vaporized. Carbs are nominally better at that than fuel injection, which squirts liquid fuel out in fine droplets which are then supposed to turn to vapor. Some carbs are better at that than others, and likely this one with the outlet port on the venturi wall is not so good.

With a 2 stroke the fuel/air mix goes into the case with lots of surface area and spinning things. Some of it does drop out and stick to the surfaces, which is how it gets lubricated, but it's not great for complete burning or efficiency.

 

[Chainsaw Jim]

Not much going into the air box, and the filter is a big foam block. There is a spacer to keep the foam from blocking the inlet:

Lack of air flow will still allow the saw to piss rev without a hitch. Sometimes you'll notice bogging down, but usually it'll just act as a limiter when under load.
Run one cut with the filter cover off and you'll find out if that's the case.
With the flow you're using on these Poulan saws...if it were me I'd eliminate the entire filter base and go with a green weenie. Of course you'll have to chop the top cover up so it has room.

 

[Chris-PA]

Lack of air flow will still allow the saw to piss rev without a hitch. Sometimes you'll notice bogging down, but usually it'll just act as a limiter when under load.
Run one cut with the filter cover off and you'll find out if that's the case.
With the flow you're using on these Poulan saws...if it were me I'd eliminate the entire filter base and go with a green weenie. Of course you'll have to chop the top cover up so it has room.

OK, but this is a comparison of two very similar WT carbs on a known saw. Filters, case, everything else is identical. The slightly larger carb with less restriction is the one that bogs under load. It seems clear it isn't a problem of too much restriction. The problem is keeping the larger modified venturi working as the air flow drops under increased load.

 

[Grande Dog]

Howdy,
Had a little experience on this subject. We used to "hot rod" 181's, and 2100's with HS carbs. On the intake side besides the velocity stacks and intake manifolds, we modified the HS carbs. We pushed the main nozzles out and taper reamed the carbs. This also meant we had to drill the throttle shaft holes out to accept the larger throttle shaft, and flap. There is some hp to be had there but, in testing the taper was critical.
Regards
Gregg

 

[Chainsaw Jim]

OK, but this is a comparison of two very similar WT carbs on a known saw. Filters, case, everything else is identical. The slightly larger carb with less restriction is the one that bogs under load. It seems clear it isn't a problem of too much restriction. The problem is keeping the larger modified venturi working as the air flow drops under increased load.

Like I said...you need to run it without the cover to see. If the bigger carb needs more air and the cover doesn't allow it then you'll figure that out real quick. You may also need to open up the muffler more to get this flow happening.

 

[Chris-PA]

Howdy,
Had a little experience on this subject. We used to "hot rod" 181's, and 2100's with HS carbs. On the intake side besides the velocity stacks and intake manifolds, we modified the HS carbs. We pushed the main nozzles out and taper reamed the carbs. This also meant we had to drill the throttle shaft holes out to accept the larger throttle shaft, and flap. There is some hp to be had there but, in testing the taper was critical.
Regards
Gregg

When I was out grinding off that lump yesterday I was looking at various carbs and the shapes of the venturi and the trailing taper. This is what I was trying to say initially in regard to the shapes and the outlet location. I know those shapes are important, but getting that right will be part theory and part empirical testing and I don't have the time, parts or proper equipment to do that. It looks like if I had a 16mm diameter, 14deg tapered ream I could get fairly close to what Walbro is using. And it would be better to start with a carb that had a removable outlet nozzle.

So basically, just boring it out straight will never really be optimal. Also, I'm more than happy with how this saw runs anyway and was only using it as a test bed, but I'm not thrilled with risking it trying out carbs that may be leaning out under load.

 

[Chris-PA]

Given that it's another rainy weekend I decided to run out to HF and pick up this bit:


The center bit is 3/4", so it needed to be cut down. First I marked it and scored it with a cutoff wheel:


And then ground off the wide part:

I chucked it in a drill press, but it still cut extremely slowly. Eventually I got a decent taper and could not get it to cut any further. These are some not too well focused pictures - the outlet is now on a narrow flat on the edge of the venturi minimum diameter:





I have a bunch of dead ash trees that need to come down as soon as the weather gives me a break, so I intend to try it again. I'll get a better secured log and do an A to B comparison.

 

[Chris-PA]

It's like eating chips - just one more! I had a WT624 laying around from an old non-A/V Poulan. It is similar to the WT391 in that it has no separate brass outlet nozzle, but it has a reversed choke. It also has an accelerator pump, which I prefer. So I decided to try boring that out too. It went much easier - there seems to be some variability in alloy that makes cutting some easier than others.

Since the choke is reversed the shape of the casting features near the outlet is different. Here it is bored to 14mm with the tapered bit:




Then I had to do a mashup with the throttle and choke shafts. The choke shaft is from the WT624, which means it has no auto shutoff, just detents - that's OK with me because it helps with my endless battle with vapor lock. But it also means there is no fast idle, which I don't like - it's just the way the WT624 is. I think the throttle linkage on the non-A/V saws props the throttle open when the choke is on, but this won't have that.

The throttle linkage is from the WT391, but I had to grind a flat in it for the accelerator pump. The focus is not too good here:



So it looks like it might work too, but the lack of a fast idle might be a deal breaker. I may try it if the bored WT391 still doesn't work as I like.

 

[Chris-PA]

I had time to drop and ash today, and did a better comparison video with a log that wasn't moving around:



The first two cuts were with the unmodified 13.4mm WT657. Second 2 cuts WT591 bored to 14mm and an increased taper behind the venturi as shown above. (order swapped from first test). I have to say the bored out carb now works well, and I think I will leave it on. It doesn't fall on it's face when you lean on it now.

On the other hand I can't say it works any better. I'd say the cuts might be 1sec faster with the bored carb, but nothing says the tuning was identical either so it's well within the margin of error. The saw was running around 10krpm in all cuts.

The only caveat is that this was a pretty big round for a 42cc saw with a 19" bar, so maybe with smaller wood there might be some additional rpm. If the loading is keeping it from revving and the smaller carb already flows sufficiently then a bigger carb is of no help.

 

[Rx7man]

I've never fiddled with Walbro or Zama carbs and re-boring them, but I've done a few Tilly's with good results, and a few I buggered up too. On my Husky 65's, I got great results boring them from 9/16th to 11/16th (3/4 is too much!), and they had great power and didn't die when you lugged them... Engine stall speed usually went up significantly.

The thing is the WHOLE system has to flow better when you're putting a bored carb on... I remember a rather embarrassing video I made of one of my Huskys, it sounded fantastic unloaded but totally failed when cutting... Most of this was from the larger carb and very aggressive intake timing, but not having the transfer port flow to match... Once I reworked the transfers everything started to work together once again and the saw now runs great.
It's no use putting a 750 double pumper on a stock 350.. you need the heads, headers and cam to provide the carb with the velocity it needs before it'll work

 

[Chris-PA]

I've never fiddled with Walbro or Zama carbs and re-boring them, but I've done a few Tilly's with good results, and a few I buggered up too. On my Husky 65's, I got great results boring them from 9/16th to 11/16th (3/4 is too much!), and they had great power and didn't die when you lugged them... Engine stall speed usually went up significantly.

The thing is the WHOLE system has to flow better when you're putting a bored carb on... I remember a rather embarrassing video I made of one of my Huskys, it sounded fantastic unloaded but totally failed when cutting... Most of this was from the larger carb and very aggressive intake timing, but not having the transfer port flow to match... Once I reworked the transfers everything started to work together once again and the saw now runs great.
It's no use putting a 750 double pumper on a stock 350.. you need the heads, headers and cam to provide the carb with the velocity it needs before it'll work

I realize the cuts were kind of slow in that video, but it's only 42cc and that's not a pine racing cant. I'll have to try it with a short bar in smaller wood to see how it runs. I suspect it will run nicely. This carb is not that much bigger, but without the brass outlet nozzle there is less restriction - the bored carb actually felt better but there was no evidence of significant improvement on that log.