Dolmar 7900 carb tuning trick???

[gink595]

My 7900 is lacking power. It doesn't run near as strong as it has been. I can't say I noticed this early on but when you hold it wide open it hits the limiter. I don't know if it has changed due to getting broke in or what but it needs adjusted. So I bought a tach from Edge & Engine last Friday and it arrived today(Thanks for fast service) I pulled the limiter cap and started playing with it but no matter what I do it is cutting out before I can reach a max RPM. If I hold about 3/4 throttle I can get it to read around 12,900 or so but it refuses get adjusted at WOT. I've heard these things are tricky to tune but what is the trick. Can you get a max RPM at WOT without the rev limiter kicking in? Or is there other problems with the ignition.
What are you guys experiencing with yours

Also can I leave the limiter caps off the H jet and be fine?

 

[cuttinscott]

What tach? Not all tachs work with electronic goverened ignitions
When I use a tach on a electronic goverened saw I use the PET2000dx from Dolmar
My Older SENDEC tach will not read right on these saws

Scott

 

[cuttinscott]

Also can I leave the limiter caps off the H jet and be fine?

Yes, unless you get inspected by the EPA

 

[davefr]

I richen H to the point that rpm is falling well below the limiter. Then I lean it until the limiter is just starting to kick in at close to 13.5. Once the limiter is starting to kick in the tach goes nuts.

 

[gink595]

The tach is a DTI-20K.

 

[gink595]

I richen H to the point that rpm is falling well below the limiter. Then I lean it until the limiter is just starting to kick in at close to 13.5. Once the limiter is starting to kick in the tach goes nuts.

I'm not getting it to do that. I tried to get it to run at a constant RPM at WOT but it keeps kicking in no matter how rich I'm getting it. Unless like Scott says the tach is not compatible?

 

[William Balaska]

How many hours on the saw? Are they a single ring topend (I thought they were) maybe i'm wrong, anyway check the compression, maybe you scored it and scrubed off some compression

 

[gink595]

The saw is fairly new, I have run about 5 gallons or so in it now. It is a single ring unit. The P&c looks great, I just got done sneaking a peak through the EX. port. It has plenty of compression, it's still the hardest pulling saw I have even more than the 066. I'm really thinking there is something up with the Ign. coil itself. It WILL NOT let me run at WOT no matter how I adj. it, Lean-rich it doesn't matter it just keeps bouncing off the rev limiter.

 

[cuttinscott]

Is it still on the stock spark plug?? make sure its on a resistor plug preferably a NGK BPMR7A


Scott

 

[gink595]

Yes it is still the stock plug. I pulled it tonight to take a look at it, it looks about right as far as color goes. Do you think I should try a simple plug first?

What do you think about that tach, do you know if it will work with a limited ignition coil? Is there any aftermarket unlimited igition coils?

 

[cuttinscott]

Plug should be fine then, I have never used that tach so I don't know if its compadable or not with that ignition system. No on the aftermarket coils. What is the serial # of your saw and what color coil does it have? Dolmar did make a flywheel change a few years ago and at the same time they changed coil colors. Both were goverened

Scott

 

[gink595]

The coil color I think was a light yellow maybe tan?? The serial # is 080121688.

 

[cuttinscott]

The coil color I think was a light yellow maybe tan?? The serial # is 080121688.

Jan 2008 also current production PS7900's had a reddish/purple coil????

Scott

 

[gink595]

Was there a problem with the older coils? I'll have to look again to see for sure on that but I think thats what it was. How much is a coil? Is there a way to test the coil with an ohm meter?

 

[gink595]

Alright I just pulled the cover and it is definetly a reddish/purple color coil. I don't know why I thought it was a light tan color

 

[William Balaska]

Maybe your just used to the power, time to upgrade! I would compression test it as well I think it's a reasonable place to start after the plug, fuel and air filters. I had two 6401's I did not care for the filter setup.

 

[Taxmantoo]

In your OP you say you can do 12.9k at part throttle.
Is it possible that you can't pull it down below limited RPM without taking the restraining cap off from the H screw?

 

[grandpatractor]

I have the same tach and it works fine on my 7900. I had to richen mine up quite a bit after it was broken in. I had to take off the cap on the H screw to richen it up enough.
I think your tach and saw are ok. I think you are just hitting the rev limiter and when it does you will lose your rpm reading and it will bounce around at a lower reading. This is normal. You have to richen it up to get it to run just below the rev limiter. This will give you a steady reading.
I also use the wire leads to get a more accurate reading.

 

[ciscoguy01]

Odd

I had a funky thing like that going on with my 5100 also... Oddly enough, ended up being the L. Go figger. How in heck can that be? But I adjusted the L about 1/4 turn clockwise, it started burbling better and zing. Every bit of power was back. No clue how or why. Still trying to figure that out...

opcorn:

 

[Andyshine77]

The L & H screws on the newer saw carbs are dependent on each other, if you adjust one it will effect the other.

Hi Frank.

The newest 7900's hit the limiter really fast, so I think the best way to correctly set the carb on a 7900 is while it's under load. Put a medium size bar on the saw, warm it up by making a few cuts in a good sized log, than that set the H dog rich. Now start making a cut without pushing down on the saw, as you're cutting slowly turn in the H speed screw until the saw just stops 4 stroking, or when it's bouncing back and forth between 4 and 2 stroking. The limiter tabs will need to be snipped off to do this.

Best of luck. Andy.