Stroking a Dolmar 7900

[Fruecrue]

Looked at it again and pulled the limiters.
About 2-1/2 turns each on the screws.
Throttle plate is good, no air leaks.
I can kill the saw by placing my thumb across the intake elbow.
Idles in the mid threes and I have full adjustment on the H screw.
May end up soldering the throttle plate to reduce airflow at idle.

 

[huskihl]

Looked at it again and pulled the limiters.
About 2-1/2 turns each on the screws.
Throttle plate is good, no air leaks.
I can kill the saw by placing my thumb across the intake elbow.
Idles in the mid threes and I have full adjustment on the H screw.
May end up soldering the throttle plate to reduce airflow at idle.

I dont think that's it. I've done it a few times and never had an issue. The butterfly might be slightly crooked. Or backwards where the tapered edges aren't sealing to the bore

 

[Fruecrue]

I dont think that's it. I've done it a few times and never had an issue. The butterfly might be slightly crooked. Or backwards where the tapered edges aren't sealing to the bore

I looked Kev, sealed up tight around the plate.
Held it up to the light.
Throttle plate has two holes in it. One the size I’m used to seeing and one twice that size.

Edit: I should say notches, not holes.

 

[huskihl]

I looked Kev, sealed up tight around the plate.
Held it up to the light.
Throttle plate has two holes in it. One the size I’m used to seeing and one twice that size.

Edit: I should say notches, not holes.

Hmm. Might be.

Timing advanced a bunch? I've seen that do it on different saws

 

[Fruecrue]

Hmm. Might be.

Timing advanced a bunch? I've seen that do it on different saws

It is, and port durations are high which I believe plays a part.

 

[Deets066]

I just thought all RWJ’s were like that.
It’s not uncommon to see them runnin at 4 turns out.

 

[Fruecrue]

I just thought all RWJ’s were like that.
It’s not uncommon to see them runnin at 4 turns out.

First one I’ve ran.

 

[Deets066]

First one I’ve ran.

I’ve ran em on a 681, 665/7910, and 346.
They were all at least 3 turns out, I think @Hedgerow has one at 4 turns

 

[Fruecrue]

I’ve ran em on a 681, 665/7910, and 346.
They were all at least 3 turns out, I think @Hedgerow has one at 4 turns

Good to know.

 

[huskyboy]

Rwj are at less than 2 turns on the 372XT strato saw they were designed for with that split bore, but on the two non strato saws I tried them on they were a lot more turns out on the jets. The venturi might be 19mm all together but only half of it flows fuel. Pulling the divider only makes it worse because it’ll have less velocity/pressure through the “original” venturi and pull less fuel. It should flow a lot of total air volume through it though in theory. Maybe screwing around with the jetting would fix that? That’s beyond my knowledge level though.

 

[Lightning Performance]

I have a question about the max rpm of the saws theses carbs were pirated from. Seems to me the shorter strokes will have richer jetting because of less vacuum signal overall and choked up more on certain mufflers. The question is will the H setting at a high rpm still be able to cover up the lean condition in the cut at lower rpm with less air volume passing the fuel circuits?

These post brought on this very thought and knowing these carbs come from different saw designs internally like bigger and smaller case volumes might add to the problem in the cut.

Poleman already demonstrated that the HD-12 was inferior to the modded Zama twin jet (big time!) on a 7900. However, I know you have been working with the HDs - eliminating the EPA circuit and the like.

I spent a heap of time dialing in the HD-5 on my modded 029. The HD can be done, it just a bugger of a bunch of work. The Zama is a piece of cake compared and it will still have a better idle circuit than an HD.

The HD looks like it has the potential to flow more than the Zama. Having said that, most of the restrictions in the intake tract come down to the bottle neck of the air filter.

If you can get two HS-12s, you could block off the EPA circuit on both, then start opening up the auxiliary jet. First one at .35mm and the other at .4mm. See what works best and then go from there.

029 is a small volume case and a small bore with a choked up muffler. It is a very fat carb stock. Every one I mod the muffler on ends up fat on the H but lean on the L. They tune up nice with no work to the carb. My 362 runs good in long cuts with a bigger muffler even tuned lean. 361's ported run out of fuel in long cuts and go lean with the H screw at three turns. Four or five does nothing to the 361. 362 carbs always tune nice for me on modded saws but not ported ones. Three turns out on the H drowns a 290 muffler modded.

I bought one of the new wj71 394 carbs they were selling on eBay for 40$ a while back and put a rwj top plate on it for impulse (straight swap), and a few mods to get it to work on a 385/390. It tuned very well. Should work on a 7900, might have to mod the throttle cable stuff/turn the boot?

They are close in case and CCs. The 7900 is much smaller in both respects and might perform poorly in the cut due to less volume and vacuum signal overall.

Tilly 298 is plug and play Dave.

Is that 298 off an 85-90CC saw?

Venturi is 18.25 in the wj71 and 18.45 on the wj39. Rwj is like 19 I think. The jetting is richer on the 394 and 385/390/395 carbs. Wasn’t a fan of the rwj on a ported 385, the tilly behaved better.

Jetting has less to do with fuel delivery in the cut if it gets sufficient fuel flo and not go lean on long cuts.

Good information, but it is a bit simplistic. Some carbs work better on a certain size displacement because of the carb configuration. The bigger venturi sounds good, but may not allow the carb to have the necessary fuel curves to get the best fuel delivery through the powerband - and that is the primary function of a good carb.

Brewz and I ran into that using a 460 carb on the modded 029 (64cc). That carb had a bigger venturi than the stock HD-5, but it didn't have the fuel curves the engine wanted. It appears that the 460 carb worked well on a 76cc saw, but just couldn't provide the necessary fuel curves on a 64cc engine. Perhaps it may be a good carb to consider modding for a 7900.

EDIT: The 460 carb also has a main discharge nozzle that doesn't stick out into the venturi to restrict the flow. I comes in at an angle and stays close to the edge of the venturi. The low pressure response through the nozzle, I expect, is much different than other carbs in it's class. Not only is the air flow through the venturi increased, but the fuel response to air flow is different. It could be the carb to mod for the bigger engines around 76cc+. I don't know, never done it...

Try modding the back of the tube or moving it deeper into the air stream. Much of this will relate to four cycle carbs on a common plenum. If you add down leg discharge booster to a Holly carb it enriches the entire full curve above idol. Most times you get less air flow, better throttle response, stronger signal and better economy with improved atomization. All great benefits to restricting a bigger carb to lose a bit off top end but shine across the full power band. I know what carb I want for work or street use. All out ain't everything and usually cost more than you know across the power curve. Hope that made common sense.

 

[mettee]

@Fruecrue, do you know the spec your zama was modded to? The jets are drilled, do you know what they were done to? If you can say...

 

[Fruecrue]

@Fruecrue, do you know the spec your zama was modded to? The jets are drilled, do you know what they were done to? If you can say...

I’m not sure. It looks to have been worked on for airflow but that’s as far as I looked. Didn’t open it.

 

[huskyboy]

The HD6 (371xp non epa carb) I have on the 6421 is a little faster than the zama. Working with the saw the little extra torque is definitely noticeable with the walbro. It just tunes crisper/more consistent as well. Is it a huge gain? Not really but I imagine if it’s making a difference on a 64cc the 79cc will gain even more since it uses more fuel (mine is about 1/2-3/4 turn out more on the H than the 64cc) . I think it’s fascinating how the carb changes the exhaust note, I noticed myself it on the 64 and on your saw in the videos. I wonder why that is...

 

[Fruecrue]

The HD6 (371xp non epa carb) I have on the 6421 is a little faster than the zama. Working with the saw the little extra torque is definitely noticeable with the walbro. It just tunes crisper/more consistent as well. Is it a huge gain? Not really but I imagine if it’s making a difference on a 64cc the 79cc will gain even more since it uses more fuel (mine is about 1/2-3/4 turn out more on the H than the 64cc) . I think it’s fascinating how the carb changes the exhaust note, I noticed myself it on the 64 and on your saw in the videos. I wonder why that is...

Dynamic compression and stuffs.
More volume in, more out.
I made that up, really don’t know.

I previously didn’t see any appreciable gains removing the muffler front cover. Wonder if that’s different with the larger carb.

 

[huskyboy]

Dynamic compression and stuffs.
More volume in, more out.
I made that up, really don’t know.

I previously didn’t see any appreciable gains removing the muffler front cover. Wonder if that’s different with the larger carb.

Test it I’m interested to see lol

 

[Fruecrue]

Test it I’m interested to see lol

I will but right now the RWJ is on the operating table.

 

[Deets066]

The HD6 (371xp non epa carb) I have on the 6421 is a little faster than the zama. Working with the saw the little extra torque is definitely noticeable with the walbro. It just tunes crisper/more consistent as well. Is it a huge gain? Not really but I imagine if it’s making a difference on a 64cc the 79cc will gain even more since it uses more fuel (mine is about 1/2-3/4 turn out more on the H than the 64cc) . I think it’s fascinating how the carb changes the exhaust note, I noticed myself it on the 64 and on your saw in the videos. I wonder why that is...

I’ve never noticed a carb change to have any effect on exhaust tone. Maybe it’s just an rpm difference you hear?

 

[mettee]

I’m not sure. It looks to have been worked on for airflow but that’s as far as I looked. Didn’t open it.

Try and find out, if the "poleman" stuff isn't done to it that would be good to know. I think you'd see some serious difference if it's not done.

 

[Fruecrue]

Try and find out, if the "poleman" stuff isn't done to it that would be good to know. I think you'd see some serious difference if it's not done.

From what I’ve read, that method increases jet size and helps with part throttle and off idle response. I could be wrong.
Those are nice features but I really care about top end performance.
I had no trouble going rich at full throttle with the stock carb at a reasonable H screw setting and saw no need to increase jet size.
@Chainsaw Jim contacted me about the carb in PM but I don’t think it’s any secret that he did the work. I believe he was after the same top end performance as he saw I was, but I’ll let him speak on that, or not, at his discretion.
There is no doubt that his work was a noticeable gain over the stock carb.