Walbro carb swap, venturi and other questions. Makita Dolmar DCS401

[joelbert]

I have a Makita/Dolmar DCS 401 with a bad carb. I'd rather not buy a new carb for it, and it isn't a common enough saw to see used carbs on ebay. The stock carb is a Walbro WT-174, superseded to a WT-390, and then to a WT-506. It is a 12.7mm venturi. Looking at part numbers and IPLs on walbro's site it looks like the WT-174 shares a lot of the internal components with the WT-286 which is a more common carb (Stihl 021,023,025) with a 13.5mm venturi. Can I get my hands on a WT286, and swap out a few parts from the WT174 and be good to go?

The Makita has a separate impulse line going to a fitting on the carb fuel pump cover, so I figured I could swap the fuel pump cover, to get the fuel pump working. I can swap the throttle and choke shafts, so I have the right pieces to work with the Makita's linkages.

Would this work? The WT286 has a bigger bore than the WT174 (17.44mm throttle bore on the wt286 vs 15.84mm for the wt174), do I need to open up the mounting block for the carb, round the edge a little for the extra 1.6mm throat? Do I need to do a muffler mod to let the saw breathe better with the bigger venturi? Will a bigger carb etc reduce the life/dependability of the saw?

 

[nmurph]

I don't know about the parts swapping, but on a saw that small, that much of a change in the venture without changing anything inside the motor (to increase air/fuel demand) may make the motor sluggish. Just a thought....might not hold much validity.:msp_rolleyes:

 

[dozerdan]

I have a Makita/Dolmar DCS 401 with a bad carb. I'd rather not buy a new carb for it, and it isn't a common enough saw to see used carbs on ebay. The stock carb is a Walbro WT-174, superseded to a WT-390, and then to a WT-506. It is a 12.7mm venturi. Looking at part numbers and IPLs on walbro's site it looks like the WT-174 shares a lot of the internal components with the WT-286 which is a more common carb (Stihl 021,023,025) with a 13.5mm venturi. Can I get my hands on a WT286, and swap out a few parts from the WT174 and be good to go?

The Makita has a separate impulse line going to a fitting on the carb fuel pump cover, so I figured I could swap the fuel pump cover, to get the fuel pump working. I can swap the throttle and choke shafts, so I have the right pieces to work with the Makita's linkages.

Would this work? The WT286 has a bigger bore than the WT174 (17.44mm throttle bore on the wt286 vs 15.84mm for the wt174), do I need to open up the mounting block for the carb, round the edge a little for the extra 1.6mm throat? Do I need to do a muffler mod to let the saw breathe better with the bigger venturi? Will a bigger carb etc reduce the life/dependability of the saw?

You may have to trim a little off of the intake block but everything else should bolt on. I change shafts and covers a lot on the Wt's when I want a larger bore carb.
I would open the muffler outlet.
Dependability wont be an issue.

Later
Dan

 

[joelbert]

I don't know about the parts swapping, but on a saw that small, that much of a change in the venture without changing anything inside the motor (to increase air/fuel demand) may make the motor sluggish. Just a thought....might not hold much validity.:msp_rolleyes:

The 021,023, and 025 all use the same carb. The 021 is 35.2cc the 023 is 40.2cc, and the 025 is 45.2cc, so the Makita at 39cc is right in there, but I think the Makita is supposed to top out at 9,000 rpm. The 021 is supposed to max out at 12,500 and 023 and 025 at 13,500 rpm.

 

[joelbert]

You may have to trim a little off of the intake block but everything else should bolt on. I change shafts and covers a lot on the Wt's when I want a larger bore carb.
I would open the muffler outlet.
Dependability wont be an issue.

Later
Dan

Thanks Dan.

Will opening up the muffler and putting in a bigger carb give me more torque, or more rpms? Makita specs it as a 9,000 rpm saw, should I tune it to this with the new carb or can I go higher?

Sorry for the newbie questions, but I am generally happy with my saws running as designed, and have not gotten into 'improving' them. This might open a whole new can of worms.

 

[sugarbush]

The 021,023, and 025 all use the same carb. The 021 is 35.2cc the 023 is 40.2cc, and the 025 is 45.2cc, so the Makita at 39cc is right in there, but I think the Makita is supposed to top out at 9,000 rpm. The 021 is supposed to max out at 12,500 and 023 and 025 at 13,500 rpm.

I believe the Makita is 12,000.

 

[joelbert]

I believe the Makita is 12,000.

I got the 9,000 rpm figure off of Bailey's page here There is a pdf of the owner's manual there, and it lists the same 9,000 rpm.

 

[nmurph]

That's the peak power are pee m

 

[sugarbush]

I got the 9,000 rpm figure off of Bailey's page here There is a pdf of the owner's manual there, and it lists the same 9,000 rpm.

9,000 load rpm, 12,000 unloaded ( top rpm's)

 

[ft. churchill]

Thanks Dan.

Will opening up the muffler and putting in a bigger carb give me more torque, or more rpms? Makita specs it as a 9,000 rpm saw, should I tune it to this with the new carb or can I go higher?

Sorry for the newbie questions, but I am generally happy with my saws running as designed, and have not gotten into 'improving' them. This might open a whole new can of worms.

Once you do a muffler mod you'll be hooked. It happened to me.

 

[joelbert]

Thanks Nmurph, and Sugarbush.

9,000 seemed pretty low to me. This thread also had several people talking about higher rpms out of the Makita.

Guess I have to go get a carb, and try this experiment.

I know the Stihl service manuals give you a max rpm to tune it to with the tach. Any chance of finding a Makita DCS 401 service manual? I know, I know, I'll go post this in the manual beg thread.
[edit] okay, got the service manual from the beg for manuals thread. Thanks Germanoak.

 

[sugarbush]

Thanks Nmurph, and Sugarbush.

9,000 seemed pretty low to me. This thread also had several people talking about higher rpms out of the Makita.

Guess I have to go get a carb, and try this experiment.

I know the Stihl service manuals give you a max rpm to tune it to with the tach. Any chance of finding a Makita DCS 401 service manual? I know, I know, I'll go post this in the manual beg thread.

with a tac, L 2600 - H 12,000, but if you change carb you'll have to set by ear. will be interesting to see if a larger carb gives better performance.

 

[joelbert]

with a tac, L 2600 - H 12,000, but if you change carb you'll have to set by ear. will be interesting to see if a larger carb gives better performance.

By ear? I thought the top spec'd rpm was supposed to be some kind of manufacturer's suggested mechanical limit, past which you risked doom (bottom end failure or piston/cylinder scoring). Any guidelines for tuning it by ear. Make it scream like it is going to explode, and then back off slightly? Make it scream like it is going to explode, and then put in earplugs and go further?

 

[weedkilla]

Just bumping on old thread - hoping OP will chime in and let us know his results. Cheers!

 

[nmurph]

Just bumping on old thread - hoping OP will chime in and let us know his results. Cheers!

Evening...I'm interested too.

 

[weedkilla]

Evening...I'm interested too.

Good evening! Just that time of night when it's too cold in the shed and it's time to go inside. First frost of the year this morning, bye bye mozzies!

 

[nmurph]

I'm on the opposite schedule...just woke up and the skeeters are just starting to be a bother.

 

[Chris-PA]

I had not sen this thread - I have a 40cc Husky (Poulan) 142e that came stock with a 34/64" (13.5mm) venturi, and it runs great. So I don't think it's too much carb, but of course any change will require checking the tuning, and will negate the factory WOT rpm setting.

In looking at swaps with the WT carbs I've often been foiled by reverse direction choke plates. The choke plate part of the casting is made two ways, and while you can swap shafts it's tough do deal with the choke wanting to operate the opposite direction.

 

[weedkilla]

I believe that this saw may be more sensitive to venturi size, the inlet tract is very restrictive. It narrows to 13.5mm at one point, I'm curious to see how well it works when the venturi isn't the narrowest point by a decent margin.

 

[joelbert]

I haven't used the saw a whole lot since I made the carb swap. I do most of my cutting clearing hiking trails, and we were winding down from Sandy cleanup when I did this swap. This past winter was not so bad in terms of blowdowns here. I think Sandy cleared out the woods of anything that was even thinking of falling over.

I have only been out with the saw twice since the swap. Once last fall, and the saw worked fine, but seemed to be running a bit hot. The second time I was out with the saw was on Wednesday, and I had a bad day with the saw.

The first couple of cuts of the day it worked fine, but it took a bunch of pulls to get it going in the morning. After the first few cuts I noticed it was smoking from the muffler, and remembered that the last time I was out it seemed to be getting the muffler too hot, and was starting to melt the plastic housing next to muffler. I was concerned, so I tweaked the mixture a hare richer. From there it was a bugger getting it to run right. After the next re-fuel I couldn't get it started. After pulling about 20-30 times I figured it was probably good and flooded. I grabbed the scrench to pull the spark plug, and was promptly reminded that the Makita uses a baby spark plug. When I first got the saw I thought I should get a scrench the right size for this saw for field use, but I forgot, so here I was a few miles out in the woods, and no way to pull the spark plug. It was past lunch so we stopped, ate, waited a little bit and tried it again. After a bunch of pulling and tweaking, and probably some cursing, it got running again, so I could finish that tree. On the next couple of trees I got it running a little better (closer to the way it was tuned when I started the day).

Then the last cut of the day was an awkward cut. Aside from the bad angle/access to the cut, the saw was stalling at idle sometimes, so there were a few restarts. Then the bar started to get pinched by the tree, I think there was some torsional bind. Managed to pull the saw out and started another cut and got pinched again. Managed to pull the saw out and when I tried again the chain came off the bar. I went to reinstall it and I found a couple of the drive links had some damage that wouldn't let them fit in the bar. I grabbed a new chain, and that went on the bar fine, but when I went to adjust it the adjuster screw was not sitting right so it would not adjust the bar properly. Moved the adjuster to the right spot adjusted the chain tension, and found I couldn't pull the chain around by hand. It would go a couple inches and stop. After a bit of playing with it I decided that the sprocket has a problem in the hole/slot for one of the drive links. Thoroughly frustrated had to give up and hike out.

On the hike out my swamper pointedly pointed out that we never walk away from tree in defeat when I bring the Stihl. He was campaigning pretty hard that I don't need to get rid of the Makita, but that I certainly shouldn't bring it out for trail work ever again. I suggested he should carry the saw if he thinks we should always bring the heavier saw. I'll got out with another swamper next time I use the Makita.

All in all I think the Makita will be okay when I work out the fine adjustment of the carb. I am uneasy about a carb with the L jet 5/8 of a turn out, and the H jet 3/4 of a turn out, gets me worried that I am too lean. The sprocket bothers me. I think my pulling and pivoting the saw to get it out of the pinch may have messed it up. I may need to get used to the little Makita being more 'delicate' than the 026 I am used to carrying.

Sorry for the long response, bad day with the saw and needed to vent.