Dolmar 7900 Carb upgrade?

[Poleman]

Shep,

Your modding a C1Q??? I've looked at those as I have 2 5105's & a 510. Played with one of these but haven't put it on yet to see what happens.
You'll have to keep us posted on this!!! A lot of saws use this carb.

Wow...Zama C3M, Walbro HD and now C1Q mod's!!!! AWSOME!!!!!!!

 

[Terry Syd]

Thanks Terry for showing Hd12. They are on for later today & tomorrow afternoon. Want to try and get one done for Mon as I have help on Mon.
Shep

Great stuff! Looking forward to what you come up with for jetting on the torque port. You might want to do two carbs and increment them up together. Say, one with .30mm and the other with .35mm - find out which works best, then mod the 'loser'. That way you always have an objective reference point to refer to. If you just mod one carb you end up with subjective evaluations that may or may not be correct based upon your memory of the previous mod.

If you don't like the torque port, just fill it in with GB Weld. I modded the HD-5 just to demonstrate the concept, however, I'm going to fill in the torque port and mod the bleed holes to run on a 64cc saw (duplicating the 460 carb). I figure a lot of guys with the 039-390 series would like a cheap and easy mod of their existing carb.

 

[VINIFIREWOOD]

Not to derail this thread away from the Zama and the 7900
But
With all the discussion about the 660 and how much power can be had by a simple switch to a dual port muffler. A guy has to wonder about the carb on those also.

 

[Terry Syd]

If the stock carb is an 'emissions' carb, then there should be a fair bit of power waiting to be released. Take a look in the carb and see if it has a low speed jet installed.

 

[Derf]

I've read this whole thread and I'm interested in learning some more about carb porting.

For me specifically, I'm interested in seeing gains made with the HD12 since it has the horizontal (non-offset) bolt pattern that fits my 372xp (among other saws). My saw is a ported 75cc jug, and I'm going to be doing a muffler mod to help with the breathing. But after that, I'd really like to dial in the breathing through the carb.

As SawTroll eloquently put it in a post somewhere, 'the manufacturers are all playing games with us somewhat, on where to detune a saw to meet EPA requirements: either on the carb, the exhaust, or the porting.' I know that the 372XP x-torq came with a different carb than the 372XP pre-xtorq.

For the 372xp
The air filter boot to carb has a 23mm ID.
The rubber boot between the carb and the jug has a 22mm ID.

-------- Pre-Xtorq -------------- Xtorq
Carb : Walbro HD12 ---------- Tillotson RWJ-4
Carb intake : 22mm ------------ 22.5mm
Carb venturi : 17.46mm --------19.1mm
Carb exhaust : 20.5mm--------- 22.5mm

Quite a few people have explored putting the Xtorq carb onto the pre Xtorq saw for better flow.
It seems clear that these saws, especially ported, can benefit from some additional flow through the carb. I'm going to do a little experiment to see if I can bore out the HD12 to close to RWJ-4 dimensions, and would love some feedback from the crew here when it comes to tweaking the jets, and perhaps adding a torq-port.

I'd rather put the effort in to modify the HD12 than adopt the RWJ-4, because 1) even though it is possible with alternative mounting pieces from other saws to accommodate the offset bolt pattern, you run into some fitment issues with the HD filter and the choke lever sticking out a little far, and 2) I have access to a machine shop.

 

[Terry Syd]

You may want to look at the 460 carb as it is drilled for a 76cc engine. All you have to do is drill out the low speed jet to get it in the ballpark for jetting. You will have to swap the shafts - and the impulse line goes to the lid, so you may have to run a longer impulse line or drill and GB Weld a pipe in the side of the carb for the impulse line.

I bought a cheap Chinese replica for around $20 on Ebay. I'm running it on a 64cc engine (Stihl 029/390). That's an easy swap for guys that want to upgrade their 64cc Stihls. Excellent idle and throttle response and good torque down low.

 

[Derf]

You may want to look at the 460 carb as it is drilled for a 76cc engine. All you have to do is drill out the low speed jet to get it in the ballpark for jetting. You will have to swap the shafts - and the impulse line goes to the lid, so you may have to run a longer impulse line or drill and GB Weld a pipe in the side of the carb for the impulse line.

I bought a cheap Chinese replica for around $20 on Ebay. I'm running it on a 64cc engine (Stihl 029/390). That's an easy swap for guys that want to upgrade their 64cc Stihls. Excellent idle and throttle response and good torque down low.

Thanks for the advice, Terry.
I'm looking at this page from Walbro and they show four different carbs that went on the MS460.
HD-8-318 (15.08mm venturi)
HD-9A (17.46mm venturi)
HD-16-318 (17.46mm venturi)
HD-24A (NLA)

I'm not that familiar with the history of Stihl saws very much, but I assume these changes had to do with EPA regulations changing. Which carb were you suggesting, and why is that better than modifying the HD12 if I have to make changes including JB welding an impulse pipe to the carb?

 

[Terry Syd]

My carb is the 17.46mm venturi. I liked the 460 carb as it has a cleaner venturi than the earlier Walbros. The venturi is offset to the side and will likely flow better than the earlier ones. You are looking for more flow, so that seems like a better carb to start with.

The idle and throttle response should be spot on for your engine. The engine might like a bit more fuel flow on the low side for better low end torque. You can try a torque port and see if you pick up some more low end.

Although it wasn't necessary to block off the extra high speed jet, since I only had about 1/8 adjustment on the high speed needle, I blocked it off to make sure I could get more adjustment if I needed it. I ended up with about 2 turns out. I cut at 2,000-2,500 feet of altitude and I won't be cutting any higher, so I should be fine. You may have to do the same mod if you put in a torque port. The torque port will increase the fuel flow and you might not be able to lean out the high speed needle enough.

 

[Derf]

My carb is the 17.46mm venturi. I liked the 460 carb as it has a cleaner venturi than the earlier Walbros. The venturi is offset to the side and will likely flow better than the earlier ones. You are looking for more flow, so that seems like a better carb to start with.

The idle and throttle response should be spot on for your engine. The engine might like a bit more fuel flow on the low side for better low end torque. You can try a torque port and see if you pick up some more low end.

Although it wasn't necessary to block off the extra high speed jet, since I only had about 1/8 adjustment on the high speed needle, I blocked it off to make sure I could get more adjustment if I needed it. I ended up with about 2 turns out. I cut at 2,000-2,500 feet of altitude and I won't be cutting any higher, so I should be fine. You may have to do the same mod if you put in a torque port. The torque port will increase the fuel flow and you might not be able to lean out the high speed needle enough.

If I can just clarify, when you say the "venturi is offset to the side" I assume you know the venturi is the smaller diameter of the barrel designed to reduce volume and increase flow rate. I assume you meant the main jet is offset in the venturi, and you are implying that it therefore allows better air flow through the venturi because there is less turbulence or because it blocks less air?

As for offering more flow for my saw, do you mean it offers more flow compared to the HD12? I'm not sure I would conclude that the offset main jet creates a significant difference in flow since they both have the same venturi diameter. I wish I had a flow bench to measure though.

 

[Terry Syd]

Yeah, I meant that the main discharge nozzle was offset to the side. I think I discussed it in the thread why there would be less turbulence past the nozzle.

It would be great to have not only a flow bench handy, but a dyno to see what difference each carb and jetting made. I expect that some of differences would be easier to pick and then where the next mod should made. Maybe one day Chadiman will have some time to run a few different carbs on the same saw to see what the different powerbands look like.

 

[hwrdpromac7900]

Has anyone actually figured out how much more power you're going to get from the modded carb? Any videos?

 

[Derf]

I don't know if anyone is really making calculations. If I was to hazard a (wild) guess:
+20% from porting the cylinder (better flow, higher compression = more HP)
+5% porting the muffler, and +8% if porting the muffler and the jug is already ported
+5% from porting the carb, and +7% if the muffler and jug are already ported

 

[Terry Syd]

Has anyone actually figured out how much more power you're going to get from the modded carb? Any videos?

If we're discussing just changing the jetting in the carb, then you won't see any increase in the peak horsepower. However, there is increased torque when the torque peak was formerly running lean. The resistance to bogging is also increased.

Rich discussed the effect in his posts where he could now stop the saw in the cut, pull the trigger and the saw would take right off.

 

[Derf]

I agree with Terry that there is more torque, but since HP is the relation of torque to RPM, if the RPM is the same, and there is more torque, it stands to reason that there is more HP, no?

OK, I just bought into this carb-modding-voodoo-magic-hocus-pocus. Got a bunch of (old) toys coming from the 'bay.
Going to be making a bunch of measurements, modifications, and tests.

Also, Terry. I have an idea that a centered or offset main-jet may not be a make-or-break feature. If I lower the main jet in the bore so its opening is closer to the venturi wall, I think I will put the jet opening in a higher air velocity stream and have i) better laminar flow, ii) better vaporization/emusion of fuel.

Are these jets just press fit into the body? Have you ever removed one? It looks like there is a slot for a screwdriver in the back side, but I see no threads. [Speaking about a Walbro I'm looking at at the moment]

 

[Terry Syd]

I agree with Terry that there is more torque, but since HP is the relation of torque to RPM, if the RPM is the same, and there is more torque, it stands to reason that there is more HP, no?

Also, Terry. I have an idea that a centered or offset main-jet may not be a make-or-break feature. If I lower the main jet in the bore so its opening is closer to the venturi wall, I think I will put the jet opening in a higher air velocity stream and have i) better laminar flow, ii) better vaporization/emusion of fuel.

Are these jets just press fit into the body? Have you ever removed one? It looks like there is a slot for a screwdriver in the back side, but I see no threads. [Speaking about a Walbro I'm looking at at the moment]

There won't be anymore torque at peak horsepower (because you are already tuning the high speed needle to get the most power at that RPM), however as you move down the powerband towards peak torque, many of the carbs are going unto the lean side of the mixture curve, that's where you can fatten it up a bit and pick up more torque. Look back on the various posts in the thread to see some of the fuel mixture charts I referred to.

The idea of lowering the main jet may be a good one, I did that on a Zama to clean up the venturi. [There is also a diagram of laminar flow (I think it was in the Chinese carburettor thread) as it goes through the venturi.] Yes, the main discharge nozzle is a press fit and you can reposition it. On one of the Zamas I had to modify the discharge nozzle a bit to make sure it would still pick up fuel from the carb passages. Just measure where the entry hole is on the nozzle and make sure it can get fuel.

Good luck, sounds like you're keen - we need more blokes like you and Rich that want to give the carb mods a go. If you get that HD-12 cranking out more power, you're gonna get the attention of a few folks.

 

[Derf]

Good luck, sounds like you're keen - we need more blokes like you and Rich that want to give the carb mods a go. If you get that HD-12 cranking out more power, you're gonna get the attention of a few folks.

Thanks for the encouragement, Terry. I downloaded a Walbro service manual and have read through it twice so far. I'm sure I know what I need to do to tear down the carb. If I can get the HD12 making more HP, I'll be happy if people sit up and take notice.

 

[bandres34]

First I want to say thanks to everyone for all of the great information on this site. I have to say I have caught the CAD bug. About a year ago I did a MM on my MS390 thanks to info from this site and now I have purchased a HD Makita 6421. I am in the process of putting a 79CC top end on it so I am very interested in this thread. I was wondering if anyone that has modded their jets for the 79cc top end has any feedback on the 30/35 or 30/30 orifice sizes before I go boring holes in what I used to think was a perfectly adequate carb!.

 

[fordf150]

First I want to say thanks to everyone for all of the great information on this site. I have to say I have caught the CAD bug. About a year ago I did a MM on my MS390 thanks to info from this site and now I have purchased a HD Makita 6421. I am in the process of putting a 79CC top end on it so I am very interested in this thread. I was wondering if anyone that has modded their jets for the 79cc top end has any feedback on the 30/35 or 30/30 orifice sizes before I go boring holes in what I used to think was a perfectly adequate carb!.

Contact @Poleman for some guidance. He has taken these carbs allot further than what is in this thread. I have one of his old trial carbs but have yet to really test it.

 

[460cixy]

Gidday all I have a zama terry modded for me and was ment to get out this weekend and give it a good work out but just never got there. So watch this space and ile let you know how it goes on a stock 372xp

 

[Terry Syd]

I hope you have a chance to run it with a stock carb, back to back, so you can make a comparison.

Here's a carb tuning story - A couple a years ago I rebuild a mate's 365 to 77cc with a modded carb. He was impressed with my saw, so I reworked his saw. Well, his brother came by and was saying the saw was running like crap.

I tried firing it up and it wouldn't idle and couldn't rev out. The saw was filthy, so before I started tuning I thought I'd check the air filter - freaking absolutely covered, like two years worth of dust. I blew down the saw and cleaned the air filter, then tried running it again. It still wouldn't idle, so I checked the idle screw - it was more than 1 1/2 turns out. The carb was tuned for the idle screw to be 1/2 turn out. Then I realised the throttle plate was all the way closed, so I dialled in the idle.

Once the saw was idling, I tried revving it out - still wouldn't go above 11,000 rpm. Screwed in the High speed needle to 13,400 and the saw was alive again.

When he came by to pick up the saw I asked him about why he was running the High speed needle so rich. He said that was the way he always tuned a saw - he liked the saw to four-stroke when he lightened the load on the bar. I said "don't you like cutting smaller logs at 12,000?" His reply was, "that sounds like it is revving too much"

So, after doing all the work to blend in the two fuel curves, he decides to tune the saw his way - whether it works better or not.

Sheesh, I'll bet he's back in another two years....