HS50 compression too low?

#1
I'm waiting for a drive belt.
I warmed up the engine this morning and I have to use the primer bulb in order to get it to run. It will rev up with no issues but does not want to idle low enough for what I think is normal.
With the engine warm and the help of my wife, I did a compression test using two different compression meters.
After five full pulls I'm only getting 50 PSI. This engine may have the cam with the compression release on its face(?)
If so is this a normal reading or is it way out and should be 110-20 PSI?
I may test it again using a drill motor to spin it faster and test the compression.
 
#2
Normal compression release number. If you have ever used a cam without one or ground the release bump off you would be walking around with one arm longer than the other
 
#4
I had pulled the head offf to see if there was a problem and the engine does look like is ia almost new. No scoring on the cylinder walls and the valves are sooty but seats are clean.
cylinder bore.jpg
 

DaddyJohn

Active Member
#5
For some reason, I can never get the original snow blower carbs to work worth a darn in mini bike applications. I usually buy the 632230-style aftermarket carbs on eBay (about $9) and modify them. On my recent Baja retro build, I used one of these and modified it. I had to swap in the throttle shaft from the old carb to get the linkage arm to work on it (the little plate on top with the holes is a different configuration). I also drilled a 1/16" hole in the side to vent the float bowl. Some of these carbs have the hole already and it just needs enlarged, but my latest one didn't have that smaller hole. Otherwise, bolt it on, twiddle some adjustment screws, and you're off to the races.

Here's a great link detailing all this, except I swapped the throttle shaft instead of drilling a new hole for the linkage.

http://www.pinrepair.com/minibikes/tecumsehmb.htm#carb
 

DaddyJohn

Active Member
#6
Forgot to mention... The new carb allows you to get rid of the primer bulb system.
I like to remove it and weld the hole shut on the blower housing for a better look (also did this on my Baja retro build).
 
#7
So as the engine RPM goes up and the engine is running, the compression goes up to a higher number?
That is correct only for the ex bump cam. The centrifugal release, at a certain RPM it releases and no longer functions. I thought the bump system would cause a small power loss so I removed it on my HS50, big mistake motor ran the same and caused bodily injury trying to start it. I wound up getting a mechanical release cam and sending it to ISKY to make it a hi performance grind.
 

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cfh

Well-Known Member
#8
Unless the engine is from the early 70s you can expect there to be a bump compression release on the cam. Personally when I’m removing the governor I remove the cam and grind off the bump. 50 psi is awful low even for a compression release bump cam. I would expect more like 70 or 80 psi. And if you don’t have the bump you can expect 100 to 120 psi

Some people will say without a compression release they hurt their arm starting the motor. I have not found that personally to be the case. But before I pull any starter, I always slowly pull to the compression cycle, and then yank it hard to start. That is just force of habit on my part, and I’ve never had an issue doing it that way

If you’re still running a carburetor with a snowblower primer bulb, you need to get a new carburetor. Snowblower carburetors just aren’t very good for minibikes
 

markus

Well-Known Member
#9
For some reason, I can never get the original snow blower carbs to work worth a darn in mini bike applications. I usually buy the 632230-style aftermarket carbs on eBay (about $9) and modify them. On my recent Baja retro build, I used one of these and modified it. I had to swap in the throttle shaft from the old carb to get the linkage arm to work on it (the little plate on top with the holes is a different configuration). I also drilled a 1/16" hole in the side to vent the float bowl. Some of these carbs have the hole already and it just needs enlarged, but my latest one didn't have that smaller hole. Otherwise, bolt it on, twiddle some adjustment screws, and you're off to the races.

Here's a great link detailing all this, except I swapped the throttle shaft instead of drilling a new hole for the linkage.

http://www.pinrepair.com/minibikes/tecumsehmb.htm#carb
If your enlarging the atmospheric vent in a carb that does not have a primer tube nipple on it that you are plugging up.......you are wasting your time. The hole in the Chinese aftermarkets that does not have a primer port is only slightly smaller than the original Tecumseh made carbs but is more than sufficient if its a non primer configuration. If you drill a hole in the side of a carb that has a brass tube going in from the front (inside the bore).......you are wasting your time, as the brass tube IS the atmospheric vent, and its more that sufficient. The only thing you are doing on either carb when you drill them, is adding metal shavings to the fuel bowl, nothing gained from that at all.

As far as using a "snowblower" carb for recreational use, If its a fully adjustable unit its the same carburetor as what you will find on a mini bike in regards to bore and the way it operates internally, except it will have a different choke lever, a primer port, and reduced atmospheric vent size (so the primer can work properly). Most of you guys are buying the wrong chines carbs anyway and altering the shit out of them, so its not like is a big deal to mod a stock one. If you have one that does not have adjustable high and low speed jets, then yes, you should consider changing your carb out. But if you have a good functioning fully adjustable one, even with a primer port, I would be worth utilizing and making the mods to, since the proper chock lever can fit, you can either plug and upsize the atmospheric vent hole, or use the primer port as a dedicated bowl overflow, and run a tube off it down and under the bike.
 
#10
I do have the carb with the two adjustable jets. It also has the primer bulb and the darn engine will not start without me priming the carb. The engine does not want to low idle no matter what adjustments I make. This has to be fixed as the engine is spinning too fast at "idle" or it dies. I have no tach to check RPM but it sounds like 650 or so at idle where the engine does not stumble and die.
Throttle response is good and there are no backfires.
I'm not even sure the rings are seated yet. As I posted the original owner said the engine only ran for two hours or so before he pulled it off of the snowblower. ( Craftsman maybe?) Since the engine is like new, I'm running 10W30 oil in it. Not synthetic.
The carb is baby-butt clean inside. It rechecked it today when I pulled the head off. I'm just not finding anteing wrong, it just wont idle well and takes several pulls to get it running no matter where the choke is set.
If it dies I have to use the primer bulb to get it going again. This would indicate it is running too lean,but if I pull the spark plug it is covered in soot which is too rich. There is no smoke at " punch it Ethel" it just spins up and the exhaust is clear. The odor smells normal.
I'm open to any helpful suggestions at this point.
 

cfh

Well-Known Member
#11
There's two ebay carbs that i have used and like.
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=283560252945
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=312807506964

they both seem to work well and are pretty darn cheap. if it was me, i would buy one and just try it out (or get both). Check it out. only 2 bolts to change the carb. try it first with no air cleaner and see what carb works for you. then after you have it figure out and adjusted nice, install the air cleaner assembly.
 

cfh

Well-Known Member
#14
I used both straight and right angle. to me it makes no difference. but with straight you may need to replace the fuel line to make it longer.

to me i look at the choke lever. i like the shorter style better. also i look at the through-put. if you hold the butterfly open, how big of a hole goes through the carb? i think the one with the shorter choke lever has a bigger hole. so for my money, that's a better carb set up. but it usually has the straight gas fitting. you can swap fittings. but i don't bother. i just run a slightly different hose configuration, and it works fine.
 
#17
It never fails. I ordered the new carb and it was delivered. Just before I opened the box, I get a message from the seller that they sent the Wrong Carb!
and to add insult to injury , it will be weeks before they can ship another one because they are sold out.
I'm waiting to see if they tell me which carb they sent?
 
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#18
I returned the wrong carb as they never returned my post. No idea what it fit but it was not a Tecumseh or a Briggs carb asit was quite large and had a two inch bore with a plastic bowl.
 

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