Boring Out a Carb

[Chris-PA]

I've been toying with trying this for some time, just to see how it went. By now I've accumulated a few Walbro WT391 carbs that are used on various 42cc Poulan, PoulanPro and Craftsman models with A/V - like the PP260.



These are 28/64" (11.1mm) venturi carbs. They are very simple units without the brass H outlet/check valve assembly, rather the H outlet is just a hole at the edge of the venturi and the check valve is in a fitting that supplies both the H and L needles.




I figured they'd be easier to mod, and that the absence of the brass H outlet/check valve would help the flow. Plus I can mount it on the squish modded Poulan for a test.

I printed a mount for the lathe at work. It was a bit sloppy and I had to center it carefully, but this would have been true of most any mount I made:



Then I bored it out until the venturi ID was even with some other cast features of the carb. I'm sure you could go further but I didn't want to get into internal passages. It ended up right at 14mm. This left me with a quite wide venturi, so I flipped the carb on the mount a tried to cut the taper back. It didn't come out that smooth, but the factory is pretty sloppy there too.

Then I found a deep socket of about the right diameter, wrapped some fine sandpaper around it and chucked it in the drill press. I did a little smoothing of the edges with a diamond bit on the dremel. Here is how it came out:




I ground down the extra screw length after reinstalling the choke and throttle plates - I could have thinned the shafts and ground the screw heads too, but wanted to be able to disassemble it again.



Last night I put it into the saw, swapping out the WT657 I had in there. That is a 34/64" (13.5mm) with a brass outlet. This carb is not that much bigger, but it has fewer obstructions. It fired up and ran on L, but I had to open the H all the way (well past the needle tip) to get it to run on the H, and it still was not 4-stroking. I would have liked to add a small hole in the welch plug as a fixed additional jet, but that would have bypassed the check valve on this carb. I could have raised the metering lever, but that would enrich the L too and the needles would have needed to move in further than normal. So I decided to drill out the H seat. I'm not sure how big it was originally as I had no bits that small, but looking at the marks on the needle taper I guess somewhere around 0.035". I drilled it to 0.042" on a WAG:



This morning I tried it again, and it seems to be spot on - the initial H adjustment seems good at 1-3/4 turns, and I'll tweak it from there once I get it into wood.

The rain we weren't supposed to have has stopped now, so once it dries out I'll try to get a video and check how it runs and what rpm it's running at.

 

[Chris-PA]

Well, that was an interesting experiment. I didn't like it at all. It sure did piss rev nicely and sounded like it would be a real demon, but it felt gutless. I could not push it.

So I put the WT657 back on and tried again. The video is really crap - I didn't have a large log and the rounds I was cutting were moving, and I need to look at that chain too. The first 2 cuts are the bored out WT591 and the second 2 are the WT657. On the video it seemed like the bored out carb was more consistent in revs, but what's happening is that I'm controlling the pressure to keep it from bogging. The 2nd cut is about a second slower than the 3rd, even though it's holding a few hundred rpm higher.

 

[Chris-PA]

Looking at 2nd and 3rd cuts in the video again, I measured a 2sec section towards the end of each cut where the log stopped moving around so much and the saw was loaded more consistently. Here I got around 9500rpm for the bored out WT591 (from 30 to 33sec), and 10740rpm for the WT657 (from 50 to 52sec). There was more rpm variation on the bored out carb too. That's what I was feeling - under a good load it just caved in.

I suspect that at lower air flow the larger venturi is falling below it's range and just isn't supplying enough fuel. That's not to say you couldn't make a 14mm carb that would work here, but just boring one out isn't a sure way to get there and it doesn't give an optimum venturi shape. This carb might work better on a larger displacement saw.

 

[Big_Wood]

i have played around alot with boring carbs. i have come to the realization that it is just a complete waste of time. if i want a larger carb i measure the intake where the carb mounts and find a carb matching that as close as possible with a larger or same size venturi. that's the only way i see gains. like the 390 husky. bore out a 118 and it turns into a turd that does RPM but has **** torque. install an RWJ4 and you see a difference because the throttle bore is close to matched to the 390 intake along with the larger venuri. i don't mod carbs anymore. leave them as is and fit them to a saw that has an intake same size as the throttle bore. good enough.

 

[Chris-PA]

i have played around alot with boring carbs. i have come to the realization that it is just a complete waste of time. if i want a larger carb i measure the intake where the carb mounts and find a carb matching that as close as possible with a larger or same size venturi. that's the only way i see gains. like the 390 husky. bore out a 118 and it turns into a turd that does RPM but has **** torque. install an RWJ4 and you see a difference because the throttle bore is close to matched to the 390 intake along with the larger venuri. i don't mod carbs anymore. leave them as is and fit them to a saw that has an intake same size as the throttle bore. good enough.

I'm not surprised. A venturi can't have an infinite range, and at some point it's just not got enough pressure drop to pull enough fuel. That's made worse if the venturi profile and fuel outlet location are not optimal. I was playing with this WT since this is about as large as they get.

 

[Big_Wood]

IMO it's about carb design. leave a carb as is as the throttle bore is matched to the venturi for that particular carb. as long as you don't mount a carb with a way bigger throttle bore then the intake boot that is. the stock tilly 296 and WJ118 on the 390 has quite a bit smaller carb throat then the intake boot where they meet. this to me is a restriction but the answer isn't to bore the carb venturi. the answer is to find another carb with the same size carb throat as the intake boot. the 69 and 39 are pretty damn close to the intake boot 69 being 19mm venturi and 39 being 18.25mm venturi. both work well on the 390 but bore a 296 or 118 to either of those sizes and it does nothing for performance.

 

[Big_Wood]

so in other words, i agree with you lol

 

[Chris-PA]

IMO it's about carb design. leave a carb as is as the throttle bore is matched to the venturi for that particular carb

I think it is a range, as with the WT's mostly all have the same throttle plate/bore size yet come with a variety of venturi sizes. However, the max venturi size for the WT is only 34/64 (13.4mm), at least as far as I know. If you hog it beyond that like I did here, there is not enough difference between the bore and venturi diameter, so you get less of a pressure drop. And that's made worse by non-optimal shapes as I said. I kind of expected that, which is why I have not tried it before.

For a 42cc saw a larger carb isn't necessary anyway, but there are 50cc and up saws that used WT/C1Q-size carbs and fitting a larger carb is a PITA. My solution to that is not to bother with those saws.

So I agree with you as well, and won't be trying this again.

 

[Big_Wood]

I think it is a range, as with the WT's mostly all have the same throttle plate/bore size yet come with a variety of venturi sizes. However, the max venturi size for the WT is only 34/64 (13.4mm), at least as far as I know. If you hog it beyond that like I did here, there is not enough difference between the bore and venturi diameter, so you get less of a pressure drop. And that's made worse by non-optimal shapes as I said. I kind of expected that, which is why I have not tried it before.

For a 42cc saw a larger carb isn't necessary anyway, but there are 50cc and up saws that used WT/C1Q-size carbs and fitting a larger carb is a PITA. My solution to that is not to bother with those saws.

So I agree with you as well, and won't be trying this again.

You will find certain carbs of the same with different venturi's will behave differently Bored the same. Depending on how the carb is designed the pressure drop affects the metering chamber differently. For example, a carb with a fix main high speed bypass behaves differently then the same carb without one after being bored larger. Another example is carbs with a low speed circuit that feeds off the high speed circuit will behave differently then ones with their own independent low speed circuit. All in all, not worth messing with them. Changing things on them can severely change running behaviour for the negative. Like the HD12 and HD6. At a glance the only thing different about them is the HD6 has it's own independent fuel pick up for the low speed in the metering chamber BUT don't try add that fuel pick up to an HD12 cause you will have ruined your carb. Don't ask me how I know lol whenever I just fit a larger carb I have great results, boring never gets me anywhere. Took a lot of good carbs before I decided that too.

 

[Justsaws]

Print yourself out a velocity stack short (1/4"-1/2", play around), matched bore, no taper, no bevels, absolutely no rounded edges, as thin a wall as possible.

It might help to grind the area where the choke is located to the venturi as flat as possible, light touch with the grinder, really not looking to make the venturi shorter.

 

[CR888]

Why not try fitting an air divider in or before the venturi to up the air speed and improve signal?

 

[Backyard Lumberjack]

I think it is a range, as with the WT's mostly all have the same throttle plate/bore size yet come with a variety of venturi sizes. However, the max venturi size for the WT is only 34/64 (13.4mm), at least as far as I know. If you hog it beyond that like I did here, there is not enough difference between the bore and venturi diameter, so you get less of a pressure drop. And that's made worse by non-optimal shapes as I said. I kind of expected that, which is why I have not tried it before.

For a 42cc saw a larger carb isn't necessary anyway, but there are 50cc and up saws that used WT/C1Q-size carbs and fitting a larger carb is a PITA. My solution to that is not to bother with those saws.

So I agree with you as well, and won't be trying this again.

> and won't be trying this again

with a lot of air metering device modifications experience under my belt... I found your carb boring and mods thread to be quite awesome! imo, great fotos, good application of principles to practical application, good testing and very good documentation and reporting. additionally, imo... very interesting!

I plan on some carb mods for my 044 project once I get the AM piston whittled down closer to OE weight... my approach will be to just blend in the numerous sharp mill edges and casting flash noted in my carb, etc. I liked your throttle plate screw mods, as well... also your efforts with the H jet.

excellent tech thread!

 

[blsnelling]

I've had very good luck boring carbs for 372s and 390s. However, I now simply install a RWJ4.

 

[Chris-PA]

Print yourself out a velocity stack short (1/4"-1/2", play around), matched bore, no taper, no bevels, absolutely no rounded edges, as thin a wall as possible.

It might help to grind the area where the choke is located to the venturi as flat as possible, light touch with the grinder, really not looking to make the venturi shorter.

In general a velocity stack is (supposed to be) a form of intake resonance tuning, which isn't what I was trying for here. It would take some analysis to work out the proper lengths, etc. The point of the tapered cone of the velocity stack is to try to achieve a tuned intake without such a long tube.

Why not try fitting an air divider in or before the venturi to up the air speed and improve signal?

The only way to do that would be to use some sort of deflector to send more of the air that flows into the carb past the fuel outlet, which would of necessity add air resistance/turbulence. Given that this was only marginally larger than the WT657 (14mm vs. 13.5mm), and some of the benefit I hoped to see came from using a carb without the brass fuel outlet restriction, I think it would be defeating any benefit of enlarging the venturi.

> and won't be trying this again

with a lot of air metering device modifications under my belt... I found your carb boring and mods thread to be quite awesome! imo, great fotos, good application of principles to practical application, good testing and very good documentation and reporting. additionally, imo... very interesting!

I plan on some carb mods for my 044 project once I get the AM piston whittled down closer to OE weight... my approach will be to just blend in the numerous sharp mill edges and casting flash in note in my carb, etc. I liked your throttle plate screw mods, as well... also your efforts with the H jet.

excellent tech thread!

I'm glad you liked it! I suspect that one of the reasons this carb mod did not work well for me is due to the fuel outlet placed on the edge of the venturi. I chose that carb because it would have less air flow restriction, and because it would be a lot easier to work on without having to remove the brass outlet. However, with the outlet port on the surface of the venturi it is located in slower moving boundary layer air, so it gets a less of a pressure drop. That matches what I found, where at lower rpm/air flow it did not appear to be supplying enough fuel.

So if I had started with a carb that had a brass fuel outlet further away from the venturi wall it might work better at lower air velocities - but then I'd need to enlarge it more to make up for the restriction, and I would be getting even closer to the throttle plate bore diameter. Kinda diminishing returns. I think there's a reason the manufactures didn't make these with bigger venturis.

 

[Chris-PA]

I've had very good luck boring carbs for 372s and 390s. However, I now simply install a RWJ4.

I'm not familiar with those, but I'm sure under some circumstances it can work. It's problematic when the venturi gets too close in diameter to the throttle bore.

 

[blsnelling]

I think you may have something else going on here. I took the 390 carb, both Walbro and Tillotson, out to nearly .750"and they run perfectly. You have a lot more venturi left here than I did.

 

[Chris-PA]

I think you may have something else going on here. I took the 390 carb, both Walbro and Tillotson, out to nearly .750"and they run perfectly. You have a lot more venturi left here than I did.

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.

 

[Justsaws]

In general a velocity stack is (supposed to be) a form of intake resonance tuning, which isn't what I was trying for here. It would take some analysis to work out the proper lengths, etc. The point of the tapered cone of the velocity stack is to try to achieve a tuned intake without such a long tube.

It can also be used to increase the velocity of air through the venturi. Not looking for a tuned intake pipe, rather a velocity stack. It is the easy way to correct of of the problems making the distance from the leading edge of the venturi to where the liquid is shorter which you did when you hogged out the venturi, not to mention helping to overcome the misshaped liquid pickups from sanding.

I think you got the expected results, and there is still lots of room to make them better.

 

[Chris-PA]

It can also be used to increase the velocity of air through the venturi. Not looking for a tuned intake pipe, rather a velocity stack. It is the easy way to correct of of the problems making the distance from the leading edge of the venturi to where the liquid is shorter which you did when you hogged out the venturi, not to mention helping to overcome the misshaped liquid pickups from sanding.

I think you got the expected results, and there is still lots of room to make them better.

Actually if you look at the picture of the carb before I bored it you can see that the fuel outlet port used to be just in front of the venturi minimum diameter (on it's face), and after boring it moved further back from the lip. Unfortunately it's now sort of behind that cast boss, which probably acts to deflect the air flow away from it. Also the outlet port is nice and round after fine sanding as it originally was. I did wreck one while working on it so maybe I'll try grinding off that boss - I suspect it's hollow and I'll break through but I'm not sure.

As for velocity stacks, check out the Wikipedia page as it has a half decent write up. There may be a slight advantage to smoothing the entrance, but it's probably quite small especially compared to having that choke plate hanging there. Beyond that the point of a velocity stack is to act as a resonance tuned pipe. Because the funnel is smooth with no abrupt transitions to reflect pressure pulses it can effectively act as a longer pipe (same volume as a longer fixed diameter pipe). That stuff works much better on a fuel injected engines as with a carb the venturi is always there as a restriction, so you'll end up with pressure pulses reflecting off the venturi rather than the port where you want them. To resonance tune a carburated engine you want the runner length after the carb, but on a saw that is impractical. On a race saw with the choke plate removed and everything else optimized maybe a velocity stack will give you that last nth bit of advantage.

 

[Chainsaw Jim]

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