Dolmar 7900 Carb upgrade?

[Fly By Night]

Your buddy there just broke through my screen, you owe me a new one Clint!


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[cgraham1]

Sorry. All my money is tied up in saws!

 

[RyanTowry_81]

Rich, I pulled the idle jet on one of my carbs, and yep, there was the membrane. It also had a small hole in it from when I drilled the jet. It doesn't seem to affect the idle, as I was saying, I let it idle for 40-50 minutes while I lapped in the clutch and it never missed a beat.

The hole in the membrane was very small, in fact I couldn't see a distinct hole. Perhaps when I pushed through the membrane it wasn't cut but simply pushed through and then closed back up without removing any material.

The Zama engineers obviously know a lot more about that circuit and decided to install the membrane. Perhaps when we drill the idle jet we just need to be careful about pushing through when that last bit of the brass jet gets drilled out.

Terry do you think that membrane could be a gasket to ensure the brass jet seals when pressed in place?

 

[Poleman]

Dont think so......this is what the underside looks like. Membrane is in middle surrounded by brass.

 

[CR888]

I should receive a Walbro for a 460, probably this week. I think it has some potential and will be modding it. I'll explain the concepts as I go and the concepts can be applied to any of the HD series. I can start a separate thread or just do it on this thread and keep all the carb tuning ideas on one thread for future reference.

That will give me a little time to order some tools and be ready, l am sure whether l get good results or not my understanding of the inner workings of a carburettor will greatly improve. The OEM carbs in oz can get pretty pricey which is kinda a shame for practice/research purposes. lts threads like this that make AS.

 

[CR888]

So been looking for my carb modd tools and this is what l think l need; micro drill set 10pce 0.1mm-1.0mm, micro hand drill & pin vice. There seems to be many options on ebay, is there any brands/kits to look out for or ones to stay away from? On a side note Randy just did a build thread on a 3120xp and turned out a H needle on his lathe.....pretty kool!

 

[fastLeo151]

Good question

 

[Poleman]

Have only purchased one set of Micro drills and thats the one pictured. They work really good and I havent had any trouble and have used them on several carbs. Be careful using a drill...I just use the pin vice and drill by hand for the feel. You'll see what I mean when you get the drills....there really small. I didn't want to break one off in the carb.

Another word of caution... I was modding a knock of carb and couldn't get it right. Upon pulling the jet I discovered the membrane had come loose and was blocking a couple of the openings in the carb. Be careful drilling these carbs...I drilled the jet by feel and quit when I just got through the brass....apperantly I dislodged the membrane at that time. The first one I did I drilled through it not knowing it was there.

Just a word of caution,

Rich

 

[weedkilla]

Without going through this thread with a fine tooth comb again I can't seem to find the relevant venturi sizes of the carbs in this discussion. Anyone care to enlighten me? Is there anything else that should be considered when guessing air flow capacity? (I don't have access to a flow bench atm)

 

[Fly By Night]

Does anyone have a link to a suitable kit in eBay? I've searched eBay and amazon and can't quite find anything that's just what's needed for this.


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[CR888]

Just put 'micro drill' into ebays search engine. These drills are typically used in jewelery.....there should be heaps.

 

[ncfarmboy]

Without going through this thread with a fine tooth comb again I can't seem to find the relevant venturi sizes of the carbs in this discussion. Anyone care to enlighten me? Is there anything else that should be considered when guessing air flow capacity? (I don't have access to a flow bench atm)

Dolmar 6400-7900 Zama C3M-DM12=17mm
Husky 372 Walbro HD-12=17.76mm
Shep

 

[SawTroll]

Without going through this thread with a fine tooth comb again I can't seem to find the relevant venturi sizes of the carbs in this discussion. Anyone care to enlighten me? Is there anything else that should be considered when guessing air flow capacity? (I don't have access to a flow bench atm)

What carbs is the question about?

 

[SawTroll]

Dolmar 6400-7900 Zama C3M-DM12=17mm
Husky 372 Walbro HD-12=17.46mm
Shep

Fixed the misprint (bold).

 

[weedkilla]

Thanks - puts Terry's early posts in perspective for me.
I've got the answers I was looking for now!

 

[fordf150]

drills and pin vise looks like they have a pretty good selection but I don't know what all sizes you need. http://ehobbytools.com/contents/en-us/d8.html

 

[cgraham1]

Can't wait to run my 7900!

It's all done......I wanna run it with the stock carb, then the modded carb.

Sweet! Rich (Poleman) and I can't wait to hear the verdict on the modified carb. I hope you're making videos...

We'll try to get out a decent log over the next couple of days.

opcorn:

 

[Poleman]

Me either....all the other feedback has been good so far.....now for the real test!!!

 

[Mastermind]

Jon is taking off early in the morning with his family for the weekend. It might have to wait till he gets back.

 

[cgraham1]

It's just a saw... We can wait. I'm sure his family is more important! Besides, I got a new 288 to keep me occupied!

Thanks Randy!