Dolmar 7900 Carb upgrade?

[CR888]

Terry explained that simply and well. The high speed starts flowing fuel well after the low as the pressure changes and air velocity increases....doing these mods gives better fuel delivery before the transition to wot running conditions. While Graham Bell has a whole chapter on carburation, he explains this subject well on pages 124 and 125. He basically does this exact same mod to mikunis giving bikes/carts better bottom end power for tight tracks where power out of the corners can be improved.

 

[Terry Syd]

If someone was just 'cookie cutting' and only using the top part of the powerband, then they probably wouldn't think the mod meant that much. However, if you are a wood cutter, then the mod makes a BIG difference.

Brad, if you want to see how much the mod changes the powerband, try this. Have a modded carb on the saw and then do about an hour of serious wood cutting. Then put on a stock carb and try to continue working. My bet is that within 5 minutes you'd stop and put the modded carb back on.

All that extra torque does make a difference in the real world of wood cutting - it's like having a bigger saw to cut with.

 

[weedkilla]

If someone was just 'cookie cutting' and only using the top part of the powerband, then they probably wouldn't think the mod meant that much. However, if you are a wood cutter, then the mod makes a BIG difference.

Brad, if you want to see how much the mod changes the powerband, try this. Have a modded carb on the saw and then do about an hour of serious wood cutting. Then put on a stock carb and try to continue working. My bet is that within 5 minutes you'd stop and put the modded carb back on.

All that extra torque does make a difference in the real world of wood cutting - it's like having a bigger saw to cut with.

I'm certainly not arguing - but I'd like to add a bit to this explanation.

There are two main times I notice this aspect of a saws performance, when I'm felling and you are feeling the saw up to the hinge. You can be full bar buried and start and stop a cut a few times.
And when I'm in a tree, feeling my way up to the point a branch will lean over, trying to get it to peel back the bark and drop vertically into a little hole below.
I blocked a tree down like this today, and if I couldn't start and stop easily in the cut it would have been a painful day.


Thanks guys, plenty of us reading and learning and mostly keeping our mouth shut and our ears open.

 

[Terry Syd]

It's not just the increase in throttle response and power off the bottom of the powerband, it is the increase in peak torque that makes the saw feel like it is a bigger saw. As the dyno runs I linked to in the thread indicated, instead of a torque PEAK there is a torque DIP on many of these saws.

What is happening is that the low speed circuit flow is petering out before the high speed circuit can pick up the slack. When the low speed circuit is opened up, the low speed fuel curve extends further up the RPM range and the 'dip' is eliminated.