Bored carbs and higher than average idle!?

[Echoboy86]

Hi fellas! everyone getting ready for the festive season????

anyway, back to the point.

I recently portred a Jonsered 2040 Turbo I'd had lying around in the shop.
(Yeah, yeah I know, "why would you bother!!?!?!?" your all saying) because I can and modifying these sort of saws is cheap and if you go to far it doesn't hurt the hip pocket badly.

The saw turned out really well, I have it tuned to 13'500 and there's still a hint of 4 stroke at 14'300.
I have ported other saws but, I decide to try boring the carb on this particular project to see if it would net me anymore ponies! short answer, yes it did! the saw is a serious cutter for 40cc's. But, and its a big BUT! I can't get the idle speed below 3800rpm now.

The carb in question is a Walbro WT 289. I enlarge the stock venturi from 9mm (.354") to 11.2mm (.433".)
I've been back in to the carb and made sure the metering lever was adjusted correctly and even snipped of 1.5 coils from the metering spring, still no love! I only dropped the revs by maybe 100rpm. what are my options?????

builders, saw gurus ........ HELP ME PLEASE!!!

 

[Terry Syd]

Check the position on the throttle valve. When you put it back in after boring the carb, did you allow the butterfly to completely close? You my have to flick the shaft a few times to position it with the idle adjustment screw all the way off. Once the butterfly has 'seated' in the bore of the carb, then you can tighten the screw down.

 

[pioneerguy600]

If you bored the carb did you make and install a new bigger diameter throttle butterfly. It needs to fit the bore tightly to get idle.

 

[67L36Driver]

Like the fellas posted above, you must have an excellent fit on the throttle blade to carb. bore or it will suck extra air/fuel at idle.

 

[Terry Syd]

After you get the idle fixed, check and see if you lost any torque down lower. Most people that bore carbs concentrate on getting more horsepower up on top end and forget about the rest of the powerband.

If it isn't as strong when you pull down the revs it may be because of the position of the main discharge nozzle. When you bored the carb venturi, it placed the discharge nozzle further into the centre of the venturi where the airflow will be slower (plus the air velocity will be reduced by enlarging the venturi). This makes the high speed circuit more 'peaky', that is, it doesn't give as flat a fuel curve.

You can fix it by getting a shorter discharge nozzle or moving the present nozzle closer to the edge of the venturi. You will probably prefer the results of having the end of the nozzle just protruding from the edge of the venturi.

 

[Echoboy86]

cheers fellas for your prompt replies.

Terry and pionneerguy600, the throttle plate not sealing right was my first thought but, after pulling the carb and checking it was making a really nice tight fit in the bore of the carb.

is there any merit in clipping of a couple more coils from the metering spring to see if I can lower the rpm anymore??? I have spare springs, and I don't want to have to go through the process of identifying which nozzle it is ad then ordering the next size up or a shorter version of the current nozzle just yet.

Also, regarding the your comment on the peaky power band Terry, while testing the saw I was able to keep the saw pulling all the way down to 7800rpm. The saw is running just shy of 170psi due to a .074" welded pop up piston so it has torque in spades!

 

[firefighterwolf]

Can I assume you've tried backing the L (low) adjustment screw out?
What RPM is it currently idling (or not idling) at? What does it need to get down to?

 

[Echoboy86]

yep, already done that, I'm at 2 and a quarter turns out and still at 37-3800 rpm. Stock the rpm should be 3000rpm but, I'd settle for 3200

 

[Terry Syd]

If the throttle plate is closing all the way, then you might have created an air leak. You might try putting a thin coat of sealant on the mating surfaces of the carb/manifold and see what happens.

Clipping some more pre-load (decreasing POP) off the spring isn't going to help you. It appears you have extra air entering the induction system and you are trying to compensate by fattening up the idle.

Dropping the POP essentially changes the 'ramp' onto the low speed circuit. Here's a thread that can help you understand how the diaphragm carb works - http://www.arboristsite.com/community/threads/chinese-carburetor.246510/

 

[rmh3481]

Boring the venturi would not be enough to change the idle speed. Did you also bore the throttle bore and choke bore? If so how much?

 

[Echoboy86]

cheers Terry! VERY INTERESTING READING!

 

[Echoboy86]

Also, the more I delve into saw modding the dummer I feel.
Was it Socrates that said "The smartest man, is the man who admits he knows nothing" ???
just to much in this world to keep in our little brains hey.

Thanks again guys for all your advice. I'm off to study this carb and see if I can diagnose the problem better.

 

[nmurph]

Did you replace the throttle plate, or reuse the existing one?

 

[blsnelling]

I have not experienced high idle from boring a carb venturi.

 

[Chris-PA]

Put a WT-529 or WT-285 on it. That has a 34/64 bore (13.49mm). This is the same carb on my Poulan 2775, which is a 46cc version of your 2040. My Husky 142e has the same engine as your 2040 and came with the same same size carb (34/64), and that runs very strong with just a muffler mod.

 

[pioneerguy600]

The OP needs to find the source of the extra air that is getting to the engine, it can be an external air leak or it can sometimes come in through the fuel supply side. An engine has to be getting extra air from some source to rev high. Extra fuel would just flood it out. If I remember correctly these saws utilize a slip fit sleeve between the carb and the cylinder, these sleeves are a known source of airleaks and have caused scoring on many of these saws.

 

[brages]

If the saw sits around idling for too long, the carb gets bored and decides to rev it up a little?