Dolmar 7900 Carb upgrade?

[Poleman]

I don't think a lathe would work as the taper would need to start earlier and have more area of adjustment. You could probably make one in a lathe...if I had one.....or a possible screw on overlay on original screw.

Carb finish....not sure there. Like Clint said no one really sees just knows its there because of performance. Anodizing would be cool, I think powder coating would be best to but tedious to do to make sure none of the wrong things would get coated or covered up. I've thought of parkerizing like an assault weapon since I can do that already. I guess the main thing to me is make it distinctive and have jet sized embossed on it for recognizeability.

Before all that though I want easy tunability, so if I send a carb out that's been modded that anyone can pretty much get it tuned for their particular saw. I don't think most guys here on AS would have a problem because if your ordering/getting a performance carb you already have an understanding of how a saw works and of modifying them. I think the term is "GEAR HEAD"!!

Rich

 

[Terry Syd]

Rich, I'm going to pull the idle jet on one of my carbs and drill one of the air bleed holes larger. I'll have a play with the size of the holes and see if all the tuning issues get resolved. You wouldn't need a slow taper idle screw and it may give a more stable idle mixture.

Now if Zama had made these carbs with removable jets, drilling out the air bleed holes would be easy...

 

[Terry Syd]

OK, getting the idle jet out isn't very hard if you have the right tool and procedure. I just drilled the first (closest to engine) air bleed hole out from .5mm to .55mm.

I have a diagonal cutters that I had modified for trimming the sprues off my cast lead .62 ball rounds. I had ground the bottom FLAT so that the sprues were cut off flush with the ball.

Initially, I tried to simply pull the jet out by clamping the cutters on the jet - bad idea. The diagonal cutters slipped and clipped the top edges of the jet off.

I then took a small Dremel bit (for engraving) and trimmed around the jet to give the cutters more room and then cut a notch for the cutters to sit it. Worked great, I just twisted the jet a bit as I pulled on it and out it came.

The underside of the jet is unusual. It has a recess area to create a chamber for mixing. I then measured the existing hole in the carb body (.5mm) and went one size larger (.55mm).

I put the carb down on a block of wood and tapped the jet back in. Later, after some trial and error and when I find the size hole I like, I'll use a bit of GB Weld around the jet to make a permanent fix.

I started the engine and found I had about another 1/4 turn of adjustment. The idle appears to be a bit more stable, but I'll have to wait until I go out wood cutting to see how it works with the engine good and hot.

 

[Poleman]

Terry,

What carb is this that your doing this to?? I mean what are the jet sizes drilled too? Just trying to wrap my mind around what the changes did to what carb?

This would be a great fix by making the air bleed hole larger increasing air as needed. I will be interested in results after/during saw use. It would also make the carb more like stock and with less alternate mods to offset increasing the jet size and increased fuel. This would also allow the mixing to be more natural in the carb process as normal, probably making it more stable as you have described.

Rich

 

[Terry Syd]

This is another approach/experiment to modding the Zama I decided to try on my 029 'Sleeper' (390 jug, NK bar). I blocked off the auxiliary port and I'm only using the idle jet. I went too far with the twin jet configuration, so I blocked off the auxiliary and started over, LOL.

There may be an element of idle stability with the consistency of the idle emulsion. I would expect with the size of the idle passages there is 'sweet spot' in the sizes of the air bleeds and the idle jet that gives the most consistent emulsion for the idle circuit. It's just a theory at this point, but I think it is worth looking into.

Did I mention I'm kinda finicky about carburetion?

EDIT: Rich, you could pull the idle jet on one carb and try it. Just go up one jet size on that first transition hole and you will see a difference. I have a feeling that this carb will go up another size for the next trial.

 

[mdavlee]

Lots of good info in here. I'm getting ready to work on a 7900 and trying to decide if a carb swap is in order.

 

[Terry Syd]

See if you can get Rich to build you a carb. At this point most people haven't caught on, he'd probably do you one on the cheap just to build his reputation. He's already got the feel for the low end torque and throttle response mods. If he gets the idle cleaned up (just a matter of time) then there won't be anything out there that will touch it.

 

[Poleman]

Terry, I sure would if I had carbs to work with. Tryed dealers in area and no one had any used ones. I did order some of the knock off ones as the OEM are running around $100 bucks...ouch!!

So my question is...do you find the knock offs to be pretty good and adjust like the Name Brand ones? Maybe just a little rougher around the edges?

I sure wouldn't say no to doing carbs that were sent to me that I would send back when finished. Just waiting to hear back from the ones I've done to see how they worked after I sent them back. It's easy for me to change something I have in hand but harder to tweek it if its not in my possession. I dont want frustrated and angry saw owners because they can't get it adjusted. Thats why I run and test each one so it runs GOOD on MY saw....the only variable is each saw is different so I try to build in adjustability for the slight tweeking it may need.

Bottom line is I just want a BIG ol Smile when the trigger is pulled and a yeehaw for a ported saw!!!! LOL

Rich

 

[Terry Syd]

Well, if you check with Randy he said he used to throw them in a bin. Sounds like there should be a heap of carbs just sitting waiting to be modded.....

The Chinese Zama copies that I had access to were fine. The main discharge nozzle sat too far into the venturi, but I fixed that by repositioning the nozzle and trimming the back of it so the metering lever didn't hit it. All the Zama parts were a direct fit on the copy. If you could get those at a decent price, even if you had to fix the main discharge nozzle position, they would be good value.

EDIT: The Chinese copies I have aren't named Zama. The name on the carb is AN(A)BA. The A in brackets is actually their logo. Maybe you can find out a contact for them and order them direct. Maybe order with a shorter discharge nozzle, maybe drilled different, Heck, you might even get them to anodize them for you.

 

[Terry Syd]

Rich, looks like Huztl is selling the Zama copies. They have several on offer (including 5 carbs at a time) on Ebay. Just put 'Husquvarna 372 carburettor' in the search function on Ebay. I've looked at the pictures and viewing down the throat of the carb it looks like the Zama twin jet.

You might buy one from Huztl and check it out. If you can work with the carb, then you can contact them for a wholesale price, they mention they do wholesale on the webpage. You might be able to pick them up for $10 a piece. Hmm, a replacement Zama is going for $100 - you could undercut that price with a modded carb, say $90.

Here's a link to give you an idea about what the carbs look like. There is no name on the carb so it is obviously a copy.

http://www.ebay.com.au/itm/Husqvarn...269?pt=LH_DefaultDomain_0&hash=item43c3cd6b2d

 

[Poleman]

Yep, thats where there coming from....had to find an alternative to the OEM. OUCH!!! Lotta wiggle room...LOL.

I will be interested in how they are. I've used a couple aftermarket carbs before and was pleasantly suprised....both wallet and performance!!!

Rich

 

[Terry Syd]

When I was looking at the components in the Chinese carbs, there was little difference to the factory Zamas. If you had a copy and an original opened up side by side, you wouldn't know which one was a copy. However, there may be some slight difference in the air bleed hole configuration under the idle jet, but not enough to affect performance.

On the Walbro copies I found a big difference in the machining of the air bleed holes. The Walbro HD used a different drill angle on the forward hole where it would come out on the edge of the venturi. The copy used the same 90 degree angle that was used on the first air bleed hole and wouldn't have the progressiveness as the original Walbro.

If the Huztl carbs are a good copy, then using them should be fine. A person could get a modded carb with all new components (needle, diaphragms, etc.)

 

[fordf150]

I checked today about those slow taper screws. no ETA on them yet

 

[Molecule]

the carb has to breathe into the crankcase volume, and the crankcase volume has to exhale through the inlet ports -- IMHO it might be that the cfm of the carb actually needs to match the crankcase volume -- the crankcase pulls a vacuum which then draws the carb -- it only has so much volume available for each cycle -- if the carb throat (venturni) is too large for the crankcase volume, then you will lose power. people put ovesize carbs on motorcycles and they turn into dogs, switch back to smaller carbs with high draw velocity, and they haul.

the low note burble sound ("4-cycling" as it's called) is rich at WOT with no load -- to set the carbs stick it in a good piece of wood to set fuel mixture at full load. As you put it in the wood you immediately hear a phase change in sound, as it goes from burbling (missing) to singing wild and lovely high notes, like some invisible high gear.

great saws

 

[Terry Syd]

Love that 'ignore' function, they just disappear...

 

[Poleman]

Not changing CFM's...just adding more usable fuel to same flow. Speed should stay the same on intake just a richer mixture thus adding more power/torque threw out the RPM range. Hi RPM dosen"t
change just usable increase of power at same RPM. A ported saw would use this and an increased RPM range if it had an unlimited coil...not so on the 7900 with its limited coil. If set at 13,500 the speed in the cut will increase with added power....

Did I say that right??!!

 

[ncfarmboy]

Both of the AM carbs I've used dialed right up w/no problems. Don't remember brand on Tillotson HS copy. Just did the MS290 Zama HD knockoff ($14.00)last week runs great adj. right up. Did notice had to lean low from 1 turn just a little to get it to run right.
Shep

 

[Poleman]

Thanks....thats good to know and will be very helpful!! Glad they work good. May have to get a baseline before any mods.

 

[Poleman]

Got the A M carbs yesterday....look really GOOD. They are in a box with "Farmer Tec" on them. Didn't do a super inspection but first appearance they look to be of equal quality to an original Zama. Put the Zama embossed cover on and it would be hard for anyone to tell the difference.

I will get pic's and take a look inside to see if any different. Casting and finish on outside is really good....interested to see how they work now. Looks like a 365 BB project this weekend to see if I can get that to perform as it should with a modded carb.

 

[Terry Syd]

Rich, I'm going to start on modding the Walbro HD. It looks like they may have some potential. Here's a thread that opened up that idea - http://www.arboristsite.com/community/threads/need-help-with-ms361-carb-mod-some-other-s.261493/

I'll see what the Walbro can do and write it up. It may be that you will be providing modded carbs to the Walbro folks as well.