HS50 tecumseh - change exhaust valve size?
- Thread starter cfh
- Start date
cfh, I can't give you the year they did it. but good guess would of been the very late 80's to early 90's maybe someone that has took enough of these years apart can chem in....:shrug:maybe markus may know? not alot of guys here that take these engines apart anymore. they went to the dark side(clone)I just like to hot rod them....:thumbsup:
really wasn't so much the intake. more like the exhaust valve that got real small. here are some of the weird things they did to help make up the new 5hp motor rating. note the exhaust valve is small ,but the exhaust port is real BIG. also it seems right around when they did this the cams were plastic or steel and both had the new style compression relief valve and when micing the cam lobes they mic out about .037 higher then a points cams. that means the valve opens more or I should say higher. points motors were time with about 15 degrees advance,but can be advance up to about 25. the newer electronic ignition are set at about 20 degrees. fix with no adjustment. everything started to have a fix jet setup for the carbs. I have seen snowblowers with adjustable jet. but for the most part they where fix. remember too the bore size got smaller too.
with all that,you can kind of see how the smaller valve was just one part of many things that got change.
early hs-40 valve size BIG
newer hs-50,195,ohh-motor...etc.. valve size
really wasn't so much the intake. more like the exhaust valve that got real small. here are some of the weird things they did to help make up the new 5hp motor rating. note the exhaust valve is small ,but the exhaust port is real BIG. also it seems right around when they did this the cams were plastic or steel and both had the new style compression relief valve and when micing the cam lobes they mic out about .037 higher then a points cams. that means the valve opens more or I should say higher. points motors were time with about 15 degrees advance,but can be advance up to about 25. the newer electronic ignition are set at about 20 degrees. fix with no adjustment. everything started to have a fix jet setup for the carbs. I have seen snowblowers with adjustable jet. but for the most part they where fix. remember too the bore size got smaller too.
with all that,you can kind of see how the smaller valve was just one part of many things that got change.
early hs-40 valve size BIG
newer hs-50,195,ohh-motor...etc.. valve size
Intake stayed the same, exhaust valve changed, Looks like it went across the board H35, HS40, it was a valve used in OHH's too if you cross reference the part number. From what I have seen it looks like mid 90's. It was either an emmisions thing, as that's about the time all the EPA requirements jargon was being put on the decaling saying it meets standards etc, or maybe a change they made to make parts more universal at that time as they would do that kinda stuff too.
Camshafts actually stayed the same as they always were. Maybe very late, last of production got the plastic cam, but when you look at parts listings and you see 37040 (replaces 33158) in the same line that usually means just what they say. the 33158 is NLA and the plastic mechanical is a direct replacement. If you look up an LH195 with an extended pto camshaft, its still got the standard BCR (bump compression release) camshaft that was in use since 1971 in the HS engines.
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I've never put my degree wheel on a Tecumseh, but did wonder about this. If the stroke is (Post 1982 H35, and early HS 40 and 50) 1.938. and there are 360 degrees in a circle, one degree of rotation should be .00538 inches. Since timing is set at .035 BTDC, (.035/.00538) doesn't that put timing at about 6.5 degrees BTDC?
The early H35 with 1.844" of stroke would equate to each degree representing .00512" per degree. Timing is set @.065, which equates to 12.6 degrees BTDC.
Just using the math, not a degree wheel, because the last thing I want to do is mess with a degree wheel, an ohm meter, a depth gauge fixture a screwdriver, and a feeler gauge all at the same time. :laugh:
I'll also add that of the many engines I've timed, (I don't mess with OHV, or solid state Tecumsehs) none of them would run worth a damn if the timing was out by more than a few thousandths. In some cases with well used engines, I couldn't even achieve the timing without bringing the points down to as close as .014 @TDC, or slotting the stator plate. This is so common, I don't even bother with setting .020 at TDC anymore. Had some conversations with ATK Rider, and he said the same thing.
The above leads me to suspect Tec added that .020 spec @TDC just to give technicians a starting point. I've also noted that lightly used engines had ring end gaps well in excess of the book stated .007-.017, but I digress.
The question about when Tec started changing their internal configurations "seems" to have occurred in 1983, when the stroke of the smaller engines were all changed to 1.938, and they pretty much melded the H35 into an HS40. But I do recall seeing both large and small exhaust valve heads on H series pre-1982 engines. All of those changes might well have occurred as the company was looking at the OHV market, and their eventual demise.
My experiences is limited to about ten or 15 engines for vintage mini bikes, but it's just my guess. And looking at the specs in the L Head manual dtd. 1998, it seems obvious an attempt was made to standardize for parts replacement over any EPA concerns. Hell, just look at the HP ratings, and you know that they slapped a sticker with whatever they want. My H35's have always had noticeably more power than my HS40's.
The early H35 with 1.844" of stroke would equate to each degree representing .00512" per degree. Timing is set @.065, which equates to 12.6 degrees BTDC.
Just using the math, not a degree wheel, because the last thing I want to do is mess with a degree wheel, an ohm meter, a depth gauge fixture a screwdriver, and a feeler gauge all at the same time. :laugh:
I'll also add that of the many engines I've timed, (I don't mess with OHV, or solid state Tecumsehs) none of them would run worth a damn if the timing was out by more than a few thousandths. In some cases with well used engines, I couldn't even achieve the timing without bringing the points down to as close as .014 @TDC, or slotting the stator plate. This is so common, I don't even bother with setting .020 at TDC anymore. Had some conversations with ATK Rider, and he said the same thing.
The above leads me to suspect Tec added that .020 spec @TDC just to give technicians a starting point. I've also noted that lightly used engines had ring end gaps well in excess of the book stated .007-.017, but I digress.
The question about when Tec started changing their internal configurations "seems" to have occurred in 1983, when the stroke of the smaller engines were all changed to 1.938, and they pretty much melded the H35 into an HS40. But I do recall seeing both large and small exhaust valve heads on H series pre-1982 engines. All of those changes might well have occurred as the company was looking at the OHV market, and their eventual demise.
My experiences is limited to about ten or 15 engines for vintage mini bikes, but it's just my guess. And looking at the specs in the L Head manual dtd. 1998, it seems obvious an attempt was made to standardize for parts replacement over any EPA concerns. Hell, just look at the HP ratings, and you know that they slapped a sticker with whatever they want. My H35's have always had noticeably more power than my HS40's.
I like to do now on all my engines before I fire them up is degree the flywheel and hook up a timing light and actually see where the timing is and doing so I have found out if you set the timing to factory spec's it runs about 15 degrees and with the adjustment on the mag(slots) you can get about 25 degrees out of it. open up the slots little get more. found this makes it ideal for anybody that wound like to install a newer style compression relief cam and not worry about any kick back. I know this is really getting off the subject,but it's good info to know for all 6 of us guys that like to play around with these little guys. I have also found that certain type of ohh-engine only have the steel compression cam and others come with a plastic cam. but it sounds like the flatheads may have still use the bump style in the late years from what markus was saying. interesting to now. I may have stated this before on the ohh connecting rod is alot stronger then a vintage hs-40 or hs-50
if the guy got the Tecumseh books it shouldn't be to hard to find out what year the exhaust valve got small and also the bore size on the hs-50 motors. part numbers will all change. right now I am looking into the newer hs-40 and ohh piston and rings .020 over. if they are the thinner style rings I am going that route. I think they would be more ideal for a high-revving performance small block flathead over using the vintage style thick rings that would create more drag and getting away from heavier slug style piston the vintage engines use.
if the guy got the Tecumseh books it shouldn't be to hard to find out what year the exhaust valve got small and also the bore size on the hs-50 motors. part numbers will all change. right now I am looking into the newer hs-40 and ohh piston and rings .020 over. if they are the thinner style rings I am going that route. I think they would be more ideal for a high-revving performance small block flathead over using the vintage style thick rings that would create more drag and getting away from heavier slug style piston the vintage engines use.
LOL, yes Delray, I too realize that this is all "in the weeds," but it's a hobby after all. Being on a fixed income, retired, etc. I do lament not having more tools to play with these, like the 46 degree Neway cutter, an end mill, CNC machine, etc. :laugh:
I do wish someone would finish up the Bungholio project. For those that don't recall, it was a Jeff Clark idea where an H35 side popper was used with an HS cam shaft, so the intake/carburetor could be ran on the side of the engine, instead of in line with the exhaust on the back.
For mini bike applications, it really opens up some frame design options, and cool factor without having to fabricate a long and snaky intake or exhaust. If you used a post 82 block, you can rig it with solid state ignition or points and play with ignition advance if using a later model Isky cam grind, and OHH billet rod.
It's a lot of work to get a few more RPM out, but the diminishing number of Tec fans can appreciate that.
I do wish someone would finish up the Bungholio project. For those that don't recall, it was a Jeff Clark idea where an H35 side popper was used with an HS cam shaft, so the intake/carburetor could be ran on the side of the engine, instead of in line with the exhaust on the back.
For mini bike applications, it really opens up some frame design options, and cool factor without having to fabricate a long and snaky intake or exhaust. If you used a post 82 block, you can rig it with solid state ignition or points and play with ignition advance if using a later model Isky cam grind, and OHH billet rod.
It's a lot of work to get a few more RPM out, but the diminishing number of Tec fans can appreciate that.
yes dave, with the newer motors I believe you can swap the cam out with a hs-40-50, ohh cam(isky,dynocam...etc…) and make the exhaust the intake and install a billet rod. the other cool thing too is now the intake becomes the exhaust and with that change over you now have a large valve for the exhaust and that gives you the option to make the new intake even larger. I been thinking of building one in the future maybe just for fun. right now I am busy with a hs-40 and next is a ohh for my drag bike.
real funny thing you always here people saying there isn't any aftermarket parts for Tecumseh's. theres ton's of stuff I guy can use from other engines and aftermarket parts a person can fab up for there Tecumseh's. it may take some machine work,but it can be done..:thumbsup:
real funny thing you always here people saying there isn't any aftermarket parts for Tecumseh's. theres ton's of stuff I guy can use from other engines and aftermarket parts a person can fab up for there Tecumseh's. it may take some machine work,but it can be done..:thumbsup:
Yes, except for pre-1982 HS connecting rods. That is the weak link on Tecumseh, and there is nothing, except for peening and polishing, neither of which I know much about.
My next project is going to be a show bike with little jewelry pieces on it. I'm not going to make any power with it, but doing away with the governor, I'd like to have a better rod. I was also contemplating Tillotson for it, but will probably go with Tec diaphragm carb. I am not a fan of float carbs, especially Tec float carbs. I know some folks here love them for ten dollars, but leave them on an engine for a year or two, and you'll get real used to unsticking the floats as they piss fuel all over your vintage bikes and karts, not to mention the crap construction and air leaks resulting in a pilot jet turned all the way in. I hate them. :laugh:
My next project is going to be a show bike with little jewelry pieces on it. I'm not going to make any power with it, but doing away with the governor, I'd like to have a better rod. I was also contemplating Tillotson for it, but will probably go with Tec diaphragm carb. I am not a fan of float carbs, especially Tec float carbs. I know some folks here love them for ten dollars, but leave them on an engine for a year or two, and you'll get real used to unsticking the floats as they piss fuel all over your vintage bikes and karts, not to mention the crap construction and air leaks resulting in a pilot jet turned all the way in. I hate them. :laugh:
Yep, when Markus had it up for sale, I snatched it. The intake "velocity stack" is a stainless steel shot glass silver soldered to a home made flange complete with throttle cable clamp. I wanted it to match the exhaust.
Yes, I keep posting that photo, but I did restore a vintage race kart between the first time I posted it and this time. Now I have to figure out how to hang the karts up on my wall, single handed, so I can work on this project.
Yes, I keep posting that photo, but I did restore a vintage race kart between the first time I posted it and this time. Now I have to figure out how to hang the karts up on my wall, single handed, so I can work on this project.
I reached out to Jeff recently trying to see if he ever finished the Bungholio, but he didn't respond. I ran across his thread a while back and was thinking about it when I started building my
356 cam build. The vertical lawnmower head delray told me about was basically making my engine an OHV Bungholio, and it worked great!
I hope Jeff or someone completes a flathead Bungholio build.
356 cam build. The vertical lawnmower head delray told me about was basically making my engine an OHV Bungholio, and it worked great!
I hope Jeff or someone completes a flathead Bungholio build.
Dave and Brian--Every time I rebuild one of these engines I hunt everywhere for at least .010 rings (.020 preferred), and even then the ring gap may be too big. As to a points engine, I've also taken to opening up the slots to get the timing right. The new so-called "Tecumseh" points just ain't what they used to be. I also make sure I square the mating surfaces for the carb and intake (both sides) to deal with air leaks. My "hybrid" engine took forever for the rings to seat properly.
Tony, did you happen to run them with the smaller valve at all before you cut them bigger? Trying to figure out if piston and valve size reduction makes for a weaker power than the old style (stock setup). Looking at the part numbers for the replacement long blocks they did put the early small taper crankshafts into those blocks when they made the switch in the mid 90's so you got that as a replacement.
I have 1979 and 1980 HS50's, both recreational versions (ball bearing crankshafts) that are in line to get rebuilt. Both blocks look acceptable for reuse, but I found some late model longblocks for under $65 shipped.....thats cheaper than the parts and labor invested in the rebuild, and they come with gaskets too and I have been tinking about trying at least one. They are bushing big taper cranks and the top oil fill so I still have to transfer the BB sidecover and cranks but that's no big deal as I would want to open it anyway and make sure everything is clean and installed correctly.
I cant see power loss in the stock range to be very noticeable, especially if they were rating LH's @ 5.5hp even, but if it makes it say a total pig off the line or something like that....I would think it may actually have better low end though with a smaller valve :shrug:
I have 1979 and 1980 HS50's, both recreational versions (ball bearing crankshafts) that are in line to get rebuilt. Both blocks look acceptable for reuse, but I found some late model longblocks for under $65 shipped.....thats cheaper than the parts and labor invested in the rebuild, and they come with gaskets too and I have been tinking about trying at least one. They are bushing big taper cranks and the top oil fill so I still have to transfer the BB sidecover and cranks but that's no big deal as I would want to open it anyway and make sure everything is clean and installed correctly.
I cant see power loss in the stock range to be very noticeable, especially if they were rating LH's @ 5.5hp even, but if it makes it say a total pig off the line or something like that....I would think it may actually have better low end though with a smaller valve :shrug:
markus
that's a good question markus. I too kind of wonder if lowend would be effected. from looking at the late design block with the big port and small valve setup. the only way i would think all that would have to work correctly would be to use the factory muffler. not sure how well lowend would be notes if you had a header pipe on.
I think if you building a stock governor motor your not really going to notes any difference in power.
I think if you building a stock governor motor your not really going to notes any difference in power.