Rebuild/mods to B&S 11hp syncro-balanced I/C engine

[big-d]

I am rebuilding an 11hp Briggs and Stratton engine for one of my kids karts. This engine is getting shaved eyebrow, cleaning up the ports, lapping valves, new rings, exhaust and possibly a different carb. I will post progress as well as what is already done soon. I have a question first though.

It is a Syncro-Balanced I/C engine. It is not the type I have seen with the couterweight hanging from the crank shaft with a rod connecting it to the side of the block.

This one has 2 gears in the crank case cover that mesh with a gear on the crank. I am wondering if anyone knows how these are supposed to be set up when putting it all back together? There are 2 bolts in the side cover that are behind the gears, which looks like they line up with the holes in the gears themselves. I am assuming they are for the Timing process when reassembling? Anyone know for sure?

 

[big-d]

The bolts are marked here...

 

[bob58o]

I think anytime I had to line up a gear in a crank case (camshaft or counter balance) there were always marks on each gear to assist in installation - corresponding marks on each gear and on the crank. I've only dealt with clone type engines and a tecumseh, so I don't really know what I'm even looking at.

I'm wondering if the holes on each gear seen in the 4th pic are those marks?

 

[big-d]

Behind those bolts, there is a small hole drilled into the counter weights about 1/4" deep. It lines up with the roll pin visible from the inside of the case. It looks like the roll pin is what attaches the counter weight to the gear. When the counter weights are just sitting, the 1/4" hole drilled in the back, lines up with the hole in the case that the bolts are in.

I am assuming, at assembly, you line up your dots on the crank and cam. Then just slide the side case back on (in the upright position for the engine) with these counter weights both in the resting position. It looks like that would put the bulk of the weight of the crank up and the counter weights down. Opposite eachother, if that makes any sense...

Has anyone ever seen a setup like this?

 

[big-d]

Here is a pic of the outer case to show how deep those counter weights (syncros) are.

 

[big-d]

Well, it kinda runs. More on that in a minute. I put it all together and it fired right up. It took a few attempts to get the float adjusted so it wouldn't overflow the bowl or leave it empty. The carb is one of the stock updraft style Briggs carbs. The float is not in the best of shape, The needle and main jet have been tightened together too tight at some point deforming the needle and there was alot of oxidization in the aluminum body of the carb.

I let the engine run for about 5 minutes while I messed with the carb and a few other details. Then it developed a slight knocking sound, so I shut it down. I am going the pull the engine this evening or tomorrow and open it up to see what the noise is.

Mods I did to this flathead engine are, shaved eyebrow, new rings, cleaned up the ports, lapped the valves, full governor removal, sanded head and top of block flat/smooth and put together with Permatex 300 (a link to this mod is posted earlier in this thread). I threaded a pipe into the exhaust port for now. I will make a better exhaust once the engine running good and I have the carb figured out.

Thoughts on knocking noise? maybe rod? I checked all the clearences before I put it together. I dont think it is the piston hitting the head. I thought it might have heated up and expanded until it hit since the noise took about 5 minutes of run time before it started. But it was still there this morning when I fired it up cold. And the age old question, LockTight or not on the aluminum Connecting rod?

I would like to switch to a different style carb. I can make an adapter if needed. I have a couple carbs from 200CC honda ATC's sitting on the shelf. They are Keihin brand. Would this be a good fit for this engine? Is there any reason the keihin Carb from the 200cc Honda ATC wouldn't work with proper jetting?

Or any recommendations on a carb? Maybe something that would not require an adapter? The block has 2 different bolt patterns at the intake.

 

[anickode]

Measure the bolt pattern and intake diameter on the block.

You might get lucky and be able to slap a GX390 carb or similar on there. I doubt it, but you might get lucky.

Another option to investigate would be flipping the intake elbow so it points up, and using a downdraft carb off a 12.5 or 14 hp horizontal opposed twin...

 

[big-d]

The intake port is 1 & 1/8". The horizontal bolt pattern is 1 & 11/16" and the diagonal one is 1 & 7/8".

I will probably have to use an adapter for it anyways since the intake port is really close to the exhaust port.

Here is a picture of the kart in front of the Hammerhead Twister, for a size comparison.

A jack shaft is probably in its near future with these 25" tall rear tires...

 

[big-d]

I took it apart just to find out the noise was the crank case breather (the flapper inside the cover over the valve spring pocket) I did not have the hose attached to it that goes to the carb. Once I moved the carb out of the way. I used the electric starter to spin the engine fast enough to hear the noise. I put my hand over the hole and the noise was gone.

I cant keep it running for very long with this carb. It was running out of fuel and then filled the whole bottom of the carb with gas. It sounds really good though. My 14 year old son is out of town right now and I am trying to have it driveable for when he gets back in 7 or 8 days.

 

[big-d]

With an intake port size of 1.125 inches. That converts over to 28.5mm.

Should I try a carb around 26mm?

That happens to be the size of a honda 200X ATC carb. I have one on an ATC already that I could rob temporarily. They are the slide style, similar to the mikuni. I just need to make an adapter so I can test it out.

 

[big-d]

The engine runs great! I made a quick adapter for the Honda ATC carb (I am probably going to cut the tube off the adapter and use a shorter one that angles it a little further from the exhaust).

I drilled out the #38 Pilot jet and #94 main jet a couple sizes each and it fired right up. It still needs some fine tuning. I will build an exhaust with a muffler for it. I was surprised how loud it was with the temoprary exhaust on it. (I am sure my neighbors loved it too)

It has plenty of power and great throttle response. I am very happy with how it has turned out so far.