Dolmar 7900 Carb upgrade?

[Poleman]

I'm rirht with you Terry.....I been thinking along those lines also.

I do have more springs I can try and will also look at cleaning up any casting marks.

So a shorter spring will help snap? What will shimming the spring do other than increase spring pressure....still sorting these mods out.

 

[Terry Syd]

Less pre-load, that is decreasing spring pressure, makes it richer off idle.

More pre-load, that is, increasing spring pressure, makes it leaner off idle.

You have to determine if it is going lean or rich off of idle, then you can adjust as appropriate. - If you have difficulty determining if it is rich or lean, then try a shorter spring to see what happens. If a shorter spring helps, then that is the direction you need to go and you can try an even shorter spring.

 

[one.man.band]

check valve opens the main at a set vacuum depression, (ie. air velocity) through the venturi. the 3rd transition hole seems to affect it a small amount as well.

there is only so far you can go w/the butterfly before you reach clutch engagement rpm.

screwdriver points to something that needs a tweak, when putting notches in things, imo.

hope it helps.

-omb





folks have different goals here for the carb. mine is more adjustability.

 

[Terry Syd]

OMB, perhaps if you were less cryptic we would understand what you are referring to. So WHAT is the screwdriver pointing at and WHY is it important.

The check valve in the main nozzle is just a flap that keeps the air in the venturi from going back into the metering chamber when the engine is idling. Without the flap/check valve the engine won't run at idle

 

[one.man.band]

OMB, perhaps if you were less cryptic we would understand what you are referring to. So WHAT is the screwdriver pointing at and WHY is it important.

The check valve in the main nozzle is just a flap that keeps the air in the venturi from going back into the metering chamber when the engine is idling. Without the flap/check valve the engine won't run at idle

yes.

photo: walbro hd

looking at diagrams on page 9 of 'walbro service manual'.......

this is what i see.

idle circuit works on vacuum.

main works on pressure.

note position of check valve between the two diagrams.

idle circuit has no air bleed to control the backflow.
as pressure builds, main check valve is in control of
the transition from vac to pressure, since they
share the same metering chamber. (referencing
the diagram, metering chamber is below the screws).

the idle circuit stops contributing fuel past part throttle.
(again.. reference the diagram to verify this).

took me about 12 hours of yanking on the cord, mounting
and dismounting the carb, drilling, notching, turning jets
over the course of 4 days to find out if the following
is true:

"if notching or drilling plate extends the range of the
idle circuit?" works better when plate is completely closed,
before doing any of this.

to enlarge photo....hit ctrl + on the keyboard
screwdriver points to the "throttle stop"

thought photo would only help to identify this part.

-omb

 

[Terry Syd]

Poleman, if dropping the pre-load didn't help that much, then the throttle plate is probably sitting a bit too far back from the transition ports.

I know I can fix the idle issue if I pull the idle jet and re-drill the transition/air bleed holes, but it would be nice to find a 'quick fix' that makes the mod easy (like the non-existent slow taper idle screws). I was looking at the difference between the idle and high speed needles and there's a possibility that a high speed needle (with its slower taper) could be used in the idle circuit. The needle shaft would have to be narrowed so it was like the idle needle shaft - and THEN there is the issue of the different thread pitches. It maybe able to be screwed in (I'll check tomorrow) with a bit of force, or maybe run a tap on the screw thread to change the pitch.

Another alternative might be to flip the throttle plate over and put the idle notch on top, then cut a very small notch on the bottom to provide some airflow over the idle port.

I think the next Zama carb I do I'll just pull the idle jet (I'm sure that sounds easier than it is) and find an air bleed hole that gives us the adjustment.

 

[LegDeLimber]

Terry Syd , Have you given any thought to chucking the screw in a drill press or drill
and then a bit of gentle useage of a small file?
Something like a diamond grit nail file (you'll need to eyeball the flatness)
or a couple of grit sizes of sandpaper lightly glued to some flat stock to some small,strait flat stock.
Prior to finding non-corn fuel, I was fighting with swollen needle seats in my B&S mower carbs.
The swelling pushed the float lower, etc and created a lean condition.
I ended up chucking the float needle in a drill and using a small flat file to dress
off enough of the head end of the needle to compensate for the thickened seat.
A diamond nail file was great for getting the finish of the needle back to a nice, slick
surface that wouldn't wear into the plastic float and slowly alter the fuel level.

 

[Terry Syd]

What I had thought of doing was making up an idle screw using the original screw, but heating up the taper and putting a thick layer of solder on the tip. Then trim it down - good idea about chucking it up in a cheap drill press (I have one) and turning down the taper.

Hmm, got my interests up now. With the screw hanging from the chuck of the drill press I could heat up the tip of the screw and apply the solder (or brass) on just the tip. Then I could start the drill press and turn the tip down to the taper I wanted. Yep, sounds do-able.

 

[Terry Syd]

A couple of days ago I got the package with the carb parts. I didn't open it as I was frustrated that they didn't have the idle needles and stuffed me around for 6 months. Well, I opened it today and stood there dumbstruck as there were 3 needles in a package. I compared them to the 044 needle - and they were even a slower taper than the 044.

I don't know what happened, maybe part of the order was completed and they were waiting for the rest of the parts. In any case, I think the idle problem is solved. The part number for these slow taper needles is 0031023.

 

[Poleman]

So Terry, You would prefer to use a long taper idel screw instesd of doing throttle plate alterations??

I recieved more carbs and will test the 30/40 combo on my 7900BB probably tonight. I will update and give results as I gather them. I may do more tweeking this time since I got some big rounds at the house and don't have to go in the field to test and have more tools, parts, etc. avalable to make changes. I have extra sizes of springs and a bag of the small hobby washers to be able to try some different combos.

I will be very interested as to the potential of this combo. Yo said you are using it on a saw of yours that is smaller cc's. This SHOULD be the HOT setup for the BB Dolmar!! I'm hoping I have a grasp of how to change throttle responce once/if I get this combo sorted out.

Fast throttle responce and killer bottom end is what end goal is....top end will take care of itself I feel.

 

[RyanTowry_81]

Very nice thread guys!!! Keep up the good work!

I need to find some of these zama carbs to play with for my 365bb build!

Terry, you said you were using the zama c3m but is their a particular model saw they came on so I know what sub model c3m to try and find?

 

[Terry Syd]

So Terry, You would prefer to use a long taper idel screw instesd of doing throttle plate alterations??

I recieved more carbs and will test the 30/40 combo on my 7900BB probably tonight. I will update and give results as I gather them. I may do more tweeking this time since I got some big rounds at the house and don't have to go in the field to test and have more tools, parts, etc. avalable to make changes. I have extra sizes of springs and a bag of the small hobby washers to be able to try some different combos.

I will be very interested as to the potential of this combo. Yo said you are using it on a saw of yours that is smaller cc's. This SHOULD be the HOT setup for the BB Dolmar!! I'm hoping I have a grasp of how to change throttle responce once/if I get this combo sorted out.

Fast throttle responce and killer bottom end is what end goal is....top end will take care of itself I feel.

I think the slow taper idle screw make the most sense for most of the guys. They probably already find it a bit intimidating to sit there with little tiny drills and drill out the jets. Throw in trimming the throttle plate when they don't know how much to trim would probably push the limits of their comfort zone - sticking in a different needle is a quick fix. A person could do both, in fact, I may pull the idle jet and drill the air bleed holes just to see how it affects the idle. Heck, maybe it could give an even steadier idle (uh oh, something else to try)

The hot setup for each saw will vary to location. I am cutting between 2,000-2,500 ft. Someone else may be cutting at sea level and another at 7,000 ft. Ya just gotta try and find what works. Once you start tweaking the metering spring, the throttle response concept becomes easier to understand. You'll quickly figure out which way to go and when you go too far. THEN, because you now can FEEL the difference, when you pick up one of your other saws you'll notice it - like, how could I have used this saw for 10 years and put up with that throttle response - so you mod that one too, and then another.

Yeah, the top end power is easy to tune for, just tweak the high speed needle to give the appropriate amount of fuel for the air flow at peak power. Chainsaw racers are probably a good example, they run the engines in a very small portion of the powerband, that is, where the engine will do the most work. However, once you move away from cookie cutting to production work, a broad powerband makes the saw a lot easier to use.

 

[Poleman]

OK, modded the first carb to 30/40, notched the throttle plate, set everything else to spec. 1 turn L & H, ideal screw 1 turn in from just touching.

Fired first pull. Increased ideal and agjusted low...still rich. I then remembered Terry saying you may have to lean out the H....that was it. Good throttle responds and amazing torque in the cut. I'm using 28" bar, 72LG chain, 7 tooth rim and at about4000 ft in elevation. I was cutting 30" green pine.....the saw just laughed....

Modded second carb and set everything the same as first. This carb received a kit and a bath before being put together. Again everything set to factory spec.

Started relatively easy but not like first carb but dialed right in with plenty of adjustment on idle screw. Again H almost leaned out.

At this point I figured this is cake....well my head got to big. Next carb was going to get a kit and bath also....upon disassembly I noticed the idle screw almost all the way in...note to self on that because that seemed unusual. Kitted and ultrasoniced carb 3 and assembled like others.

Saw did not want to start and when it did it just wanted to blubber and wouldn't take throttle without during. Idle adjustment all the way in. Notched throttle plate more and tryed again.....better but not right. Removed carb again notched more and put two shims under spring. Better

 

[Poleman]

Better but idle screw still all the way in. Saw runs good but I don't like the idle adjustment being like it is.

I will think about this tonight and tomorrow and try to figure this carb out. It may have a previous problem that is carrying over into this mod. The giveaway was what I seen on disassembly. Apperntly for some reason this carb wants a lot of air.

Just not happy with this one as it seems to be way rich. It is more difficult to start and actually like to start on high idle on a restart. I'm getting there with this one but I'm not where I want to be.

Till tomorrow......any thoughts why this one carb is acting like it is?? Solutions, ideas????

 

[Terry Syd]

First things first - did you prefer the 30/40 combination over the 30/35?

As far as that idle problem in the second carb - sounds like there may be something clogging one of the transition/air bleed holes. If one of the air bleed holes is clogged, the idle is going to be pig rich. Try taking a piece of wire, bending the tip 90 degrees and sticking it through the air bleed holes over the idle jet, if something is there, maybe you can clear it.

 

[Poleman]

I guess I never answered the million dollar question did I.

The Mod that has more power is the 30/40 combination. But at this point I like the adjustability more on the 30/35 combo. I'm still trying to get a grasp on the affects and reactions of the change. Terry I couldn't believe how lean I had to adjust the high...you were spot on on that one. The saw is getting so much fuel down low and midrange it carry's over to the hi. This saw is an absolute torque monster now!!!!! I wish I had access to a 42" bar....I'd love to try it. I actually think the oiler could keep up also as this saw oils really really well.

I will try cleaning the passage and maybe ultrasonic it again...something is deffinatly going on with this particular carb because it acts totally different than the first two. But I had my suspicions on teardown seeing the idle screw like it was.

 

[Terry Syd]

As far as the adjustability of the 30/40 combination, have you tried it with the slower taper 044 needle? By the way, those slower taper needles I bought only cost $2.50 a piece.

I know you have been concentrating on the torque, but did you notice if the engine picked up some revs in the cut above peak torque? Perhaps it might be more noticeable in smaller wood.

EDIT: As far as that wonky carb, there is one other thing that may be causing an excessive rich mixture at idle - and that is a leaking welsh plug.

 

[Poleman]

Terry, I haven't tried the slower taper screw yet. The other carbs all acted and responded to the changes I have made to them in a reliable manner and didn't need the slow taper screw.

Now the carb in question does not respond in a like manner. To a point it does but not all the way. This carb also had a leakon my pressure ggauge after I put it all together after mods and rebuild.

It didn't occur to me to question the Welch plug....I will reseal and/or put in a new one first thing to rule that out. I may tank & pressure the carb to see if that's what it is. I ran out of time last night to do more tests.

Terry, the saw with the 30/35 jets was awesome but the 30/40 is definitely stronger yet. I appolagize for not getting any tack readings in the cut I was concentrating on getting carbs dialed in. I can tell you just by feel it is faster in the cut. The 30" wood I have in no way challenging to this saw. I may have to make a run to get it into some larger hardwood. I just want to get this 3rd carb sorted out and you have sure helped with that.

 

[Terry Syd]

Rather than make a run to get into some larger hardwood, why not put an 8-pin on the 28" bar and try it. The carb mods give the engine a greater resistance to bogging/stalling, so the 8-pin may work just fine. You may be able to fine tune the engine a bit better with the greater loading.

 

[Poleman]

Thats an idea. I've never been a fan of an 8 pin sprocket but this saw procuced so much torque I dont think it would hurt. Bog in the wood...right now with what I've been cutting thats seems almost impossible. LOL.