Expert carb help needed

[Ronie]

Need some help with a Jonsered 670 with a mildly ported 272 topend. Here's the problem, I had this 630 carb on the saw and it ran ok. I felt like it was a little small but it's all I had that had the automatic throttle lock. Last night I decided to put a 272 intake block filter and elbow on it so everything got bigger. When I tried to tune it this morning at around 10,000 rpm it will start taking off and will run up to around 15,000 rpm. if I richen it up it gets too rich in the mid range. I've never had one act like this and I'm wondering if you guys think it's because the carb it to small or something else. I've vac/pressure tested it three times.

 

[Funky sawman]

I realise that you pressure tested 3 times but that symptom is characteristic of an air leak. Once I've seen a intake block on 394 do this. It passed pressure and vac test but when tightened down to the carb it didnt seal property cause it had a slight warp that the gasket could not seal.

 

[huskyboy]

Try swapping with a known good carb. I had a 2100 carb do this. Swapped carb and fixed. When they don’t take a kit/clean I don’t waste my time further.

 

[Funky sawman]

Did he put a different carb on the saw? I was wondering about this, as he didnt clearly state if the carb was swapped or not

 

[Ronie]

I realise that you pressure tested 3 times but that symptom is characteristic of an air leak. Once I've seen a intake block on 394 do this. It passed pressure and vac test but when tightened down to the carb it didnt seal property cause it had a slight warp that the gasket could not seal.

That was my first thought, I sprayed carb cleaner around the carb block and no leaks.

 

[Ronie]

Did he put a different carb on the saw? I was wondering about this, as he didnt clearly state if the carb was swapped or not

I haven't tried a different carb yet.

 

[Funky sawman]

Only other thing I can think of is the metering chamber in the carb needs readjusted. Either arm is misadjustef or pop off pressure is off. The metering chamber could be running out of fuel from arm being too low which can cause a lean out.

 

[Funky sawman]

These old hs tillotsons are dead simple compared to these newer walbros

 

[Ronie]

Only other thing I can think of is the metering chamber in the carb needs readjusted. Either arm is misadjustef or pop off pressure is off. The metering chamber could be running out of fuel from arm being too low which can cause a lean out.

After lunch I'll check that out. I bought a 670 carcass off ebay and I should get it today, I'll also try the carb off of it.

 

[Ronie]

I started to change out the carb with the the 670 carb but I had forgotten that I would need to put an impulse nipple in the intake block for it to work. I don't really have much time to mess with it anymore today so I thought, why not try blocking off the return fuel line and see if that changes anything. Blocked it off and it stopped the lean condition. I'm going to kit the 670 carb next week and see if it does any better with that one. The video is kinda long but if you go to the end you can see how it's running at the moment. It's not properly tuned but that carb is coming off anyway. Thanks for all the suggestions.

 

[Ronie]

Just an update, I put the 670 carb on and it seems to have fixed the problem. The intake elbow on the left is the 630 and the one on it is from the 670, big difference.

 

[Motorhead]

272 takes a tillotson 260.There are a bunch of aftermarket carbs on eBay that are replicas of the 260 and most of them work well.

 

[Ronie]

272 takes a tillotson 260.There are a bunch of aftermarket carbs on eBay that are replicas of the 260 and most of them work well.

Thanks for the heads up but I'm trying to keep the automatic throttle lock.

 

[huskyboy]

Really interesting thread, good information.

 

[smokey7]

In my several years messing with this series/family of saws in Orange and red/black. I've found the gasket that comes with alot of aftermarket kits rarely seals the impulse passage properly on the carb or intake block or both. Also I've found the other spot that is pretty common is the o-ring at the seal holder if equipped or the oiler on the pto side always has them. They are only oem and should be replaced anytime you have the oiler off the saw. By far my favorite saw family to work it on/build/modify.

 

[Ronie]

Update and video with the 670 carb. I also posted this over in the video thread. Running much better with the 670 carb.

 

[cuinrearview]

Looks good. Beard's comin' along nicely too.

 

[Ronie]

Looks good. Beard's comin' along nicely too.

Thanks, wife hates the beard. lol