Dolmar 7900 Carb upgrade?

[Poleman]

"The metering springs are as follows 0019014 30gm / 0019002 16gm / 0019009 19 gm / 0019012 25 gm / 0019013 35 gm

Is this what your looking for???? I hope.

 

[Derf]

My progress has been halted over the winter too.

I have a friend who can x-Ray CT scan a carb so I know how much clearance I can bore out before running into adjacent passages, but connecting with him has been difficult. I may just chance it and bore it out without the CT data.

I plan to increase the bore on an HD-12, then drill the jets holes. I also need to pick up a couple welch plugs- can anyone tell me how to order those? Saw some on eBay but they were too expensive for the plugs alone.

My elevation is very low, below 6000' for sure, so I likely won't be messing with springs. But it is a consideration that working environments change for these tools, so we should take that into consideration so that they work under a broad range of conditions.



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[Mastermind]

I see y'all ain't give plum up yet.

 

[LegDeLimber]

This walbro pdf shows the carburetor bodies and the variuos bores and venturis.
If I were trying to open up a carb, this is about the best info I have found
that might tell how far you might risk going with trying to enlarge one.

I think there is a page on the Zama site that lists the diaphragm bodies
and the bore and venturi size ranges, also.
i'll keep looking to see if I have the link saved.

Edit: The Zama link to the carb bodies and bore ranges.
Same problem as Walbro, with No ideas as to internal passage locations.
http://www.zamacarb.com/page/products

 

[Derf]

Hi Randy,
I know you said you think the ported 372XP/375XP runs better with the x-torq 372XP carb, which is bigger than the Walbro HD-12. Other people have said they think it runs better with a 390XT carb, also bigger. However, I'm OCD and don't like the x-torq carb in a non-xtorq saw (fitment issues, mixed and matched parts), so I just want to see if I can mod a HD-12 to flow more air and add more fuel and yield better throttle response/more power.

It would probably help anyone else with BB 372 kits, or the 7900 guys as well. Except Terry and Rich are working on the Zama carbs I believe.

--edit--
thanks LegDeLimber, but that won't show what I need. I want to see how much clearance between the air-flow hole and the fuel passages there are.
Example 1) the "jets" are tiny holes that are drilled through only a few mm of aluminum before they reach a resevoir. If you open up the bore to much you cut into the reservoir.
Example 2) the fuel on the top side of the carb is brought down to the bottom of the carb by a passage running near the venturi. If you open up the venturi too much you cut into that fuel passage.

 

[LegDeLimber]

Derf, I follow what you're after, 100%.
and yes it's frustrating not to have that passage mapping!
Just saying that this gives the best ideas of what bodies Walbro started with.
And ltes us know not to go too far with a certain body type, at least.

I've got some Walbro WT-1048 sitting here. plain jane carbs that will need some transition passeges to make them match the engines that they end up on.
Also would like to know how far out to take the bore & venturi, but I have the same reservations as you, because of that lack of passage mapping.

But I think we all know how Walbro dosent even want to give out any info about the mods of any of their specific carbs, let alone a good diagram of the genreral body passages.

Seems we're stuck with a bit of blind working, unless someone has that hookup
with the X-Ray dept!

 

[LegDeLimber]

Another thought is to find some very strait wires that are the correct size
(i.e. those drill bits)
and gently place them into the intial openings of the passages.
Now take look at them, like the crime scene crews do at a shooting.
That gives us some starting ideas of the passage projections.

From there, you're down to looking for those pressed in balls that are the blockers from where the manufactuer drilled an intersecting hole, to make a corner or angle change, then pressed that ball in to close the unwanted entry hole, left from that secondary drilling entry.
With that in mind, you can try to make a mental map of the passages.
That's how a lot of our old Mikuni carbs on our dirt bikes, were modded.

 

[Derf]

aanther thouight is to find some very strait wires that are the correct size (i.e. those drill bits)
andgently palce them into the intial openings of the passages.
Now toake look at them like the crime scene crews do at a shooting.
From there, you're down to looking for those pressed in balls that are the blockers from where the manufacuer drilled an intersecting hole, to make a corner or angle change, then perssed that ball in to close the unwanted entry hole, left from that secondary drilling entry.
With that in mind, you can try to make a mental map of the passages.
That's how a lot of our old Mikuni carbs on our dirt bikes, were modded.

Great minds think alike. I already did that! I used a piece of sewing thread and marked it with a marker on either side where it entered the hole and then took it out and measured the distance. I'm confident on how much I can open up the bore now, but I'm unsure of the venturi. Hence why I'm on the fence about just modding, or waiting for my x-ray friend.

 

[Paragon Builder]

I see y'all ain't give plum up yet.

Ain't gonna happen!
I'm just trying to wrap my head around all this before I start digging in.

 

[Mastermind]

Great minds think alike. I already did that! I used a piece of sewing thread and marked it with a marker on either side where it entered the hole and then took it out and measured the distance. I'm confident on how much I can open up the bore now, but I'm unsure of the venturi. Hence why I'm on the fence about just modding, or waiting for my x-ray friend.

On that carb just remember to open it in a D shape........I've cut thru a few.

 

[Derf]

On that carb just remember to open it in a D shape........I've cut thru a few.

THANKS RANDY! You're talking about a D shape in the venturi, right, to avoid the fuel passage on the side... ?

 

[Mastermind]

THANKS RANDY! You're talking about a D shape in the venturi, right, to avoid the fuel passage on the side... ?

Exactly.

 

[Mastermind]

Do keep this in mind though........a larger venturi will decrease the pressure differential and can make pulling enough fuel tough. I'd rather see the throttle plate enlarged in most cases.

 

[Derf]

Do keep this in mind though........a larger venturi will decrease the pressure differential and can make pulling enough fuel tough. I'd rather see the throttle plate enlarged in most cases.

I plan to do both

 

[LegDeLimber]

speaking of venturi to bore ratios: looking at this I have a C1Q-W29 sitting here.
The venturi is 14mm and the throttle bore ( the engine side) is roughly 15.5mm
and the choke bore is bout 18mm (cheap plastic calipers readings)
I've seen some mold flashing nearly matching that amount of diameter change.

it's No surprise that the throttle plate only has a rather tiny notch on each side and there are 3 transistion holes in the bore.
And then the choke plate has a much smaller hole than you usually see.

I'm prone to soldering choke plate holes shut, as I prefer the charectoristics
of that type choke system.
You might guess that I also just loathe all of the uni-lever, spring back open crap that's the trend on things now.

 

[Poleman]

Just modded a carb last night and ran it today.....really bad bottom and transition. I put in a lighter spring (Walbro 98-530) and it straightened right out. Fast throttle and responce!!
I'm finding every once in awhile a weaker spring is needed for some unknown reason. All mods are the same as other carbs that work fine with stock spring.
Just put this out there in case others have problems tuning a carb on a modded saw or if using a modded carb.

Springs can make a difference!

 

[fordf150]

Just modded a carb last night and ran it today.....really bad bottom and transition. I put in a lighter spring (Walbro 98-530) and it straightened right out. Fast throttle and responce!!
I'm finding every once in awhile a weaker spring is needed for some unknown reason. All mods are the same as other carbs that work fine with stock spring.
Just put this out there in case others have problems tuning a carb on a modded saw or if using a nodded carb.

Springs can make a difference!

And your probably the only person with an assortment of springs to choose from for tuning

 

[Poleman]

I might just be......I got a hellava support crew!!!!!


Wonder where I got that kit from???

 

[Mastermind]

I need one of those kits huh?

 

[fordf150]

I need one of those kits huh?

They are nice kits and have quite a few springs to choose from but I think rich is the only person nutty enough to buy one and actually use it