Stihl 029 super big bore conversion

[AdamGr]

Hey all,

Found a Stihl 029 super in a junkyard. Fixed a broken fuel line, replaced the missing carb, and discovered the engine had low compression so I decided to replace the motor with the 390 big bore engine. I just finished my rebuild today. Started the saw, it sound pretty rough, figure it just needs tuning. after a little while with no improvement there was a very audible clunk and the saw stopped. I thought it was a bad motor until I took the flywheel off and discovered a broken key. I did manage to get the engine to turn a little without the flywheel through some miracle, and from the exhaust, intake, and spark port everything looks good, no scratches or broken rings. I'm pretty sure it has compression too because the impulse line was suctioned hard to the carb when I tried to take it off and check the crank bottom (piston stop in the down stroke position initially). I think I may have caused the flywheel to break when I re assembled the saw. I broke the ground screw off in the motor when I was reassembling (goof I know) and couldn't get it out so I drilled into it and replaced it with an M6 sheet metal screw that pokes a little above the shroud for the ground screw. The screw has no marks on it though. Thoughts? This is my first time doing any kind of mechanical engine work and failing this hard has me really bummed.

 

[sean donato]

You didn't have the flywheel on tight, or over tightened it and cracked the taper on the flywheel.

 

[AdamGr]

That's what I thought the flywheel nut was loose. I don't have a torque wrench or power tools it's not in the budget. I just tightened it with the 13mm side of my scrench. Any advice on how to get it torqued right I thought sure I had snug?

 

[sean donato]

I zip mine on with a little impact driver, they don't go on super tight, so I don't hammer on it. Doing it by hand is kinda a feel thing, needs to be tight, but not too tight. Helps if you have something to hold the flywheel and give a good snug down.

 

[irhunter]

This is my first time doing any kind of mechanical engine work and failing this hard has me really bummed.

So, before working on a saw, it is a nice practice to track down the owner's manual, shop manual, and IPL (parts list). Get the owner's manual PDF free from Stihl. Get the other two by asking for them on the Beg For Manuals thread on this site.

There are a lot of experienced guys on this site who don't use torque wrenches for this or that. It seems to me, that that is often a mistake for the inexperienced. HFT often has their torque wrenches on sale for 10 bucks. You'd want the 3/8" drive for saws. While not the best, I feel they are a lot better than nothing for the beginner. It is easy to check ball-park calibration, at home, for free, when using cheap torque wrenches.

 

[BeatCJ]

You'd want the 3/8" drive for saws. While not the best, I feel they are a lot better than nothing for the beginner. It is easy to check ball-park calibration, at home, for free, when using cheap torque wrenches.

And very likely a 1/4". I would guess some of the torque specs are inch pounds or the metric equivalent. Harbor Freight torque wrenches are actually pretty good when new, they just don't stay calibrated well.

 

[stihltech]

50 NM or 37 ft lbs. on the clutch.

 

[AdamGr]

Update, I got the new flywheel and kinda just slapped it on and gave it some extra heave ho from the 1/4 socket wrench, and it runs. Needs a carb adjustment but right now I'm just grateful the motor isn't blown from that hard stop. Thanks for all the advice. Also I thought the clutch was self tightening. Is it really that important to torque it down so rigorously?

 

[grizz55chev]

Update, I got the new flywheel and kinda just slapped it on and gave it some extra heave ho from the 1/4 socket wrench, and it runs. Needs a carb adjustment but right now I'm just grateful the motor isn't blown from that hard stop. Thanks for all the advice. Also I thought the clutch was self tightening. Is it really that important to torque it down so rigorously?

It's a tapered shaft, and yes, it is important. Also, make damn shure to clean the FW as well as the shaft of any oil residue.

 

[sean donato]

It's a tapered shaft, and yes, it is important. Also, make damn shure to clean the FW as well as the shaft of any oil residue.

He was talking about the flywheel initially, then someone said something about the clutch torque. Seems he has the flywheel sorted.

 

[sean donato]

Update, I got the new flywheel and kinda just slapped it on and gave it some extra heave ho from the 1/4 socket wrench, and it runs. Needs a carb adjustment but right now I'm just grateful the motor isn't blown from that hard stop. Thanks for all the advice. Also I thought the clutch was self tightening. Is it really that important to torque it down so rigorously?

That's good you got the fly wheel sorted. Don't worry about the clutch, I just snug them up and call it a day. Yes they are reverse threaded so the chain won't spin them off. May get a bit tighter when you first start using it, but won't continue after that.

 

[stihltech]

Screw it. Slam it together. It will run.

 

[AdamGr]

Update, the saw does run; however, it's not running very well. My primary concern was that it got super hot, like too hot to hold my hand against the plastic housing. It has problems idling, I'm no carb tech, but this seems ridiculous. Finally the spark arrestor has a bunch of that red RTV silicone on it. Any thoughts? I purchased the engine pre assembled and it seemed okay. Haven't taken it apart again yet, but tomorrow I am going to have it compression tested and then tear it down to try and find the problem. Thanks for all the help, and please keep the advice coming.

 

[sean donato]

The spark arrestor should be 100% free and clear. I'd be very suspect of shody workmanship if it was full of rtv.

 

[stihltech]

That is a lot of RTV floating around. Possible it also plugged the impulse nipple a bit. Tear it down. Too many things point to a bad job.

 

[AdamGr]

I appreciate the advice. I'm gonna return the engine since it's purchased on amazon (Carbhub model), and allow them to replace it with a new one. If that doesn't work maybe I'll chuck the money at a good engine, or maybe I'll rebuild the original engine which honestly looks fine other than a lack of compression which to me indicates a broken piston ring. Stay tuned.

 

[AdamGr]

Installed new engine, I think its runs fine, but now I have another issue. It will idle, but no matter how I adjust my high screw, when I pull the trigger, the saw dies. Any ideas, I'm thinkin my carb may need replacement, but it's brand new.

 

[TheJollyLogger]

Installed new engine, I think its runs fine, but now I have another issue. It will idle, but no matter how I adjust my high screw, when I pull the trigger, the saw dies. Any ideas, I'm thinkin my carb may need replacement, but it's brand new.

Check for a clogged tank pressure release line

 

[J D]

Installed new engine, I think its runs fine, but now I have another issue. It will idle, but no matter how I adjust my high screw, when I pull the trigger, the saw dies. Any ideas, I'm thinkin my carb may need replacement, but it's brand new.

Problem is more likely with the low side, find the spot where the low side sets the idle highest, then turn it out a further 1/8 to 1/4 turn. Then set the throttle so chain doesn't spin at idle.
Tom @Vintage Engine Repairs does a good tuning video on youtube.
Being new doesn't necessarily mean the carb is good... especially if it's a Chinese carb

 

[Vintage Engine Repairs]

Problem is more likely with the low side, find the spot where the low side sets the idle highest, then turn it out a further 1/8 to 1/4 turn. Then set the throttle so chain doesn't spin at idle.
Tom @Vintage Engine Repairs does a good tuning video on youtube.
Being new doesn't necessarily mean the carb is good... especially if it's a Chinese carb

Thanks mate - new = never ever used lol