Dolmar 7900 Carb upgrade?

[Terry Syd]

I wonder what the jet size that was drilled in the back of 30/35 and the 30/40, that might give you an idea of what was happening. If the 30/35 had a bigger jet size, it could throw off your testing.

Was it overly rich down low, or was it overly rich up on top? Even then, because you have the overlap between the two circuits, the rich high speed circuit could have been feeding lower in the rev range.

Mate, you're probably going to end up with a dozen carbs all drilled differently to see what works best on various saws.LOL

 

[LegDeLimber]

I think you guys brought up threaded jets, earlier.
I'm thinking mini-lathe and some brass filler rod to fab the jets.
Drill 'em and run a die over the O.D. then part it off....
Or if the holes are a bit big, just "crazy glue" the holes then re-ream them with
your smaller choice of bit after the glue dries.

and I've been sleeping in this chair....time for the bed i figure.

 

[Terry Syd]

Never crazy glued them, but I have soldered them up and started over.

 

[RyanTowry_81]

So is the rpm of the 64cc saw the same at tuning as the 365bb? If it is then the increase I. Velocity should be proportional to the transfer stroke volume right? Maybe the 64cc saw has a more efficient transfer which moves more air per revolution causing it to handle more fuel than the 77cc?

Just a theory. Only way to really know would be to measure the intake velocity somehow. I wonder if you could run the saw at WOT with a small hand held wind meter in the place of an air filter and be accurate enough to know the difference.

 

[Terry Syd]

In reference to different drilled carbs for saws that are the same displacement. It seems to me that if a saw was modded for a higher cutting speed and therefore peak torque would be sitting higher, then the modded saw might need a bit more flow in the low speed circuit so that when the two circuits significantly overlapped they would be overlapping at a higher speed.

Otherwise, you might end up with that 'dip' we saw on the graph.

 

[Poleman]

Terry the said carb was way rich down low and when I did get it to clean up and run it was rich on the top end also. Just can't understand this as the saw runs SUPER with the Walbro HD-12 that was on it. On the other saw the Walbro was good but the modded carbs out performed it and added a lot of torque off idle threw the rpm range up to the top. I still think the Zama is a viable carb for the BB365 and the saw would be helped emencelly by it...I just don't have it there yet.
I know you have a 365 so I know it's possible and can be done.

Yes, and I'm prepared to have the corner on the Zama carb market......LOL

 

[Terry Syd]

Terry the said carb was way rich down low and when I did get it to clean up and run it was rich on the top end also. Just can't understand this as the saw runs SUPER with the Walbro HD-12 that was on it. On the other saw the Walbro was good but the modded carbs out performed it and added a lot of torque off idle threw the rpm range up to the top. I still think the Zama is a viable carb for the BB365 and the saw would be helped emencelly by it...I just don't have it there yet.
I know you have a 365 so I know it's possible and can be done.

Yes, and I'm prepared to have the corner on the Zama carb market......LOL

Bugger, I hate moments like that. When I was a pilot we called it 'being out of air speed, altitude and ideas at the same time'.

Something ain't right there. I was running my 365BB at 35/35 and it did everything well (except the idle adjustment, but that sent me looking for the slow taper idle screws, so it was a helpful learning experience). Maybe you'll have to sit back and have some 'Zen' time with the carb - (that sounds better than WTF?)

Good thing you are taking on the Zama carb market. The folks at Zama probably dread seeing another of my emails arrive. They probably leave my messages for when the 'new guy' fills in for the day, rationalising that it will be great OJT for the poor bloke. And, the folks at D&G started carrying the slow taper screws (even put a picture of it on the webpage) after all the hassle I gave them. I can't wait to see how you run them around in circles.

 

[fordf150]

You guys are what makes this site what it is. Thank you. Get this all figured out and I will send you your first paying job modding a carb for my 7900.

 

[Poleman]

Pretty much have it figured out for a ported 6400/6401 and an 84cc BB. Just need a stock 7900 to see where and at what jet size is need for it. I'm thinking it will be close to the BB settings because of its performance!!!

I know where those saws are at but I need more test saws or need to aquire a stock 7900. Of course for testing and research!!!! Honest!!!!!

I think you'd be surprised at the power gains that come from this nodded carb. It starts off idle and carry's through the power band to the top. The saw is smooth and fun to run with such a wide power band. Power pretty much at any speed.

 

[fordf150]

I have a healthy stock 7900. Complete rebuild with oem parts. Little over 200 psi last I checked it only has about 8 to 12 tanks through it right now. If you get me some part numbers on zama parts I will make a contribution to this research. I am jealous of you guys getting to do this kind of work. I am barely able to keep up with the minor repairs. I've been trying to get to testing sound levels/power on different muffler mods for six months and don't see me getting to it any time soon. Jonsered rep just dropped me off a toy to play with for the next month

 

[Poleman]

Hi Nate,

Ya I want to get a 7900...seen a couple on AS but had bad timing. I did get a nice BB I have been doing the carb mods on. That's been working good.

As far as parts I haven't really put anything in/on them. Mostly drill jets and possibly shim spring to tune off idle response. I have ordered some slow taper idle screws to see if I can get away from notching the throttle plate. I'll have to possibly get some of the spring that Terry mentioned. Other than that there's not a lot that goes into them.

Any time you want one let me know....I will be happy to do one for you!!!! I really appreciate you getting me and sending the parts I need. Got the fuel lines and carb piece today!!

That's quite a toy!!!!! I like the Jonny's!!! I nodded a 2166 for a cutter and he loves it!!!!! I do too!! Has the best air filtration set up on the market.....air cleaners just doesn't get dirty!!

Rich

 

[cgraham1]

Ya I want to get a 7900...seen a couple on AS but had bad timing. I did get a nice BB I have been doing the carb mods on. That's been working good.

As far as parts I haven't really put anything in/on them. Mostly drill jets and possibly shim spring to tune off idle response. I have ordered some slow taper idle screws to see if I can get away from notching the throttle plate. I'll have to possibly get some of the spring that Terry mentioned. Other than that there's not a lot that goes into them.

Rich

Keep up the good work, Rich!

 

[Terry Syd]

Rich, I got thinking about your 365BB and couldn't figure out why the carb didn't work. Then I had a flash, maybe it isn't the carb!

When you set up the 365BB did you relieve the area at the bottom of the transfers so that the transfers could get a good feed from the crankcase? I say that because many Big Bore jugs have transfer tunnel problems. If not, let me know. I've got an old 365BB jug and piston that I modded and I can take a few pictures to give you the idea of what needs to be done.

Maybe that will fix the problem. If not, fork if I know, you're on your own...

 

[RyanTowry_81]

Rich, I got thinking about your 365BB and couldn't figure out why the carb didn't work. Then I had a flash, maybe it isn't the carb!

When you set up the 365BB did you relieve the area at the bottom of the transfers so that the transfers could get a good feed from the crankcase? I say that because many Big Bore jugs have transfer tunnel problems. If not, let me know. I've got an old 365BB jug and piston that I modded and I can take a few pictures to give you the idea of what needs to be done.

Maybe that will fix the problem. If not, fork if I know, you're on your own...

If you want to take the pictures and not detail this thread you can always post them in mine. My 365BB is in the mail so hopefully porting is the next step after I receive it. Transfer volume was my first idea of why rich's carb was not acting like yours terry. I would assume transfer volume per revolution would be displacement times duration(as a percentage of one whole revolution) right?

 

[Terry Syd]

Transfer volume has nothing to do with it, just like some guys go on about 'velocity' - it's 'flow' that is important. It's dark in Oz and I've had a few drinks of my homemade vodka, I'll post some pictures tomorrow with an explanation.

 

[Poleman]

Please do because I'm clueless as to what the problem was other than jets being to large....

Time to step back and do some Zen contomplation on this......

 

[Poleman]

Going to swing by saw shop and see if I can score another carb and start baby stepping it into usability. I know it will work just not sure why previous try didn't work. I'm thinking of the torque gains through the full rpm range it produced on the Dolmars!!!

 

[Terry Syd]

Rich, here is what I did on the 365BB to make sure the transfers were fed. On the cylinder I took the side of the cylinder down to the level of the base gasket. The piston received a couple of holes on each side where the piston drops below the cylinder at BDC. I also trimmed under the piston and also flattened the side (so the flow doesn't have to go in so far in order to turn up towards the ports).

Max flow of the transfers occurs just before BDC. The BB piston comes down close to the crankshaft at BDC and there isn't much room under it for the mixture to flow.

 

[Poleman]

Interesting Terry!!!! I'd love to run that saw!!!

Well, I was unable to get any more carbs to play with, so I could try some things. Oh and of course the slow taper idle screws are on back order....sure wanted to have that to send to Randy....but there's still a little time. Got my fingers crossed.

I do have one carb left...its a NOS that has never had gas in it. It had a double D hi screw but I changed that so nothing fancy will be needed. BTW...Thanks Makguy, he supplied me with some carb to play with. Hopefully he will receive his back and add some input to this tread.

This carb will be going to Randy (Mastermind) to be put on and tested on a Dolmar 7900 that is scheduled in September to be ported. I really want to see what this will do on a ported saw and how it compares with the stock carb. Using the same saw will be a good determiner I think!! I hope it does what I expect and not a disappointment.

Here are some pictures of what I use (per Terry's help and advice) so you all get ideas of what is being done.

Here's what's uses... The Micro drill and Pin vice. The jet on the left is the idle jet, .25mm to .30mm. the one on the right is the auxiliary .25mm to .40mm. Just go slow and steady with not a lot of pressure...just feel. Check the needle arm height to make sure that's at factory setting. I would then put on a slow taper idle screw and test. Dial carb is as best as possible and make changes as needed. If rich off idle I would put a couple spacers under carb spring depending how bad it is. I had to this on the ported 6400 to get the quick throttle response back. I don't feel that will be an issue with the ported 7900.

Drilling these jets out feeds the low end off idle and adds to the torque of the saw all the way through the power band. Feeding that much fuel through these jets lessens the amount the Hi is opened. The settings are a little different than stock as the Hi may be almost closed but that's not a problem because the drilled jets are supplying the fuel needed but you can still adjust as need be.

The one thing I did omit was notching the throttle plate just slightly. I wanted to wait for the new screw before I did that to see if that was even required with it installed. If I don't get them I probably will notch it. The saw is getting more fuel at idle and needs more air also to balance things out. With this added fuel though the power increases(torque), the saw pulls better, doesn't bog in the cut. It may not turn any more RPM's but it maintains in the cut. The saw I was testing was turning 11,400 to 12,000 in the cut with a 24" bar buried in wet Pine.

Hope this wasn't to long.....I get kinda windy!!!!

 

[Terry Syd]

Slow taper idle screws on back order.... I wonder is someone else jumped the gun and picked up YOUR idle screws. LOL

Maybe a few people that are going to try these mods should send in their order for the screws as soon as possible. It would be nice to have the screw/s on hand before tweaking the carb.