Dolmar 7900 Carb upgrade?

[Poleman]

Are you meaning the L adjustment and possibly the H? It will make it totally strange. Remember the H sometimes may be totally closed because of the increased fuel from the main jet.

They range from $11 to $20.. I've had good luck with them.

 

[Fly By Night]

In my position that seems like a no brainer to get a new one, mod it and be done. Where were you getting them? I am talking about the low, I am use to knowing where I am when tuning an idle mixture, this thing I have no idea whats going on.

 

[Poleman]

I was getting them from Hutzl on ebay. They are good quality and Farmer Tec brand.

Don't be afraid to really mess with the carb and make some radical settings. The setting dont seem to make sence at all on these carbs. This what you are describing is why I didn't like the first carbs I did before drilling the bleed hole. Drilling the bleedf hole larger straitens out these problems and the carb acts like a stock carb after that.

I do have one extra jet left and I do have 4 more carbs on the way....I can send you the jet and you can drill out your problem one after you drill the bleed hole, just make sure it is the right size. I could also mod a carb for you and send it unless you want to do your own modding I could just send a carb.

I'm still thinking about you not having to put a hole or notch in the throttle plate...I had to open it up for more air to get them to idle right because of the increased fuel at idle and low speeds.

 

[Fly By Night]

Thanks for the offer Rich. I will turn in my idle speed screw today and see where that gets me. What's odd to me is that it idles very stabley for a minute before it gets wonky. I'm not sure what it's deal is. I will report back tonight With my findings.

 

[Poleman]

I've found on some of the modded carbs they have a funny idle. You may try reseal in the Welch plug with clear fingernail polish...if that is leaking it will idle funny.

 

[Fly By Night]

I'll give that a shot when I have a minute. I need to pull that thing apart and put a kit in it at some point.

 

[Fly By Night]

I cranked my idle screw all of the way in today, the L mixture still doesn't seem to do much until it gets really far one way or the other. I think sealing that welch plug is priority #1 for getting that straitened out. Im sure drilling that air bleed is going to need to be done regardless.

 

[Poleman]

Sounds like you may need a rebuild then start over on the settings. Sealing the Welch plug will be insurance.
You say you had your idle screw all the way in as fast idle??? Did you have a high rpm idle or did it still act funny. If it still acres funny you may need to drill a small hole in your throttle plate for more air and to stabilize idle.

 

[Fly By Night]

The diaphragms didn't look too bad but while I'm taking the carb apart and messing with it i don't see why not.

With my idle speed screw all of the way in it still acted funny, it would idle normal for 5-10 seconds then idle up and cut out.

 

[Poleman]

I'd go a rebuild and needle and seal the Welch plug. I think you will have to drill a small hole in the throttle plate to get your idle adjustment back because it sounds like you have no more adjustability on it other than leaning out you L needle which usually doesn't solve anything.

You have to remember with drilling the jets your introducing more fuel into the idle circut and upwards. To counter react that you need more air also at idle so saw will idle right. To me it sounds like it's over rich and stalling. How's throttle responce when running?

 

[Fly By Night]

When running throttle response can be pretty darn good, after idle its boggy during Normal idle and when idling up before it dies which I suspect is it being lean it will try to die if you don't feather into the throttle lightly. I was playing with it the other night with the sun in my eyes and realized it's smokey at idle when warmed up, I suspect thats a pretty good way to tell she's rich! How large of a hole do you suggest I start with?

 

[Poleman]

Yep...sounds like your rich and needing more air at idle. I would probably start with a .80 hole in the throttle plate across from the jet or on top. That should help your idle and idle screw adjustment. If it idles to fast and the adjustment screw won't bring it down the hole is to big. Simple solder it shut and drill a smaller on in the solder. I have drill 2mm holes in some ported saws plates to get enough air to them at idle. After that the plate opening feeds the need....

 

[one.man.band]

My idle mixture screw doesn't seem to make much of a difference in tuning. I have it to where it will idle nicely for 10 or so seconds after I come off the throttle, it will idle up for a few seconds (leaning out) then pretty quickly idle down and die, any thoughts on that? I may just need to bite the bullet and figure out how to get to that air bleed, or tack down some slow taper idle screws.

sounds like it's check valve related. possibly because the whole idle circuit is out of whack now.

 

[Terry Syd]

If it was the check valve it would idle for about 10-15 seconds before the metering chamber sucked in too much air past the check valve - then the engine would die.

He needs to drill the transition/air bleed hole. Drilling the throttle plate will help, but the transition hole mod cleans up the idle.

 

[Fly By Night]

Which check valve is this? That's what it will do Terry, it idles rich for 10 or so seconds, then idles up from leaning out from what I can tell and dies

 

[Terry Syd]

The check valve is in the main discharge nozzle. The symptoms of a malfunctioning check valve are like the saw running out of fuel (because it does).

The valve sometimes get damaged when someone uses compressed air on them. However, it may have something underneath the thin flutter valve that is keeping it from sealing. If you punch it out of the carb, you can try flushing it out with some spray liquid like WD-40, brake cleaner, contact cleaner, etc. Once you have the nozzle out of the carb, check to see that there is still a valve in the nozzle and that it isn't stuck or damaged. If it looks OK, then with the nozzle out you can get access to the little hole that feeds fuel to the check valve. Try spraying the liquid in the hole. - Or, put a replacement nozzle in the carb.

 

[Fly By Night]

This is at idle only or in all situations?

Im so tempted to order a farmer tec and donate this carb to Rich for R&D.

 

[one.man.band]

fly........did it idle and die out before you did any drilling?

 

[Fly By Night]

I smoked the top end 3 years ago, I ported the jug and the carb at the same time. It idled before but who knows what's changed in that carb.

 

[one.man.band]

that's my point.