Tuning a diaphragm carb on an HS40

capguncowboy

Well-Known Member
#1
I recently rebuilt the HS40 on my 70 Rupp Roadster (http://www.oldminibikes.com/forum/O...oys-1970-rupp-roadster-jff-15.html#post948556). For some reason, I didn't test fire it before it was all pretty'd up because I checked the timing and everything else twice. I'm hoping the issue I'm having is just a carb issue

I finally put gas an oil in it today and pulled it a few times and it didn't sound like it was doing anything. After a few more pulls it popped. I opened the choke about half way and pulled and it fired -- and it sounded like it was at full rev. After talking to buckeye a few weeks back I was prepared and pulled the spark plug wire off the engine. The throttle was stuck about half way open. I closed it up and fixed the culprit.

I half way expected it to start right up on the next pull but it didn't. After about another 20 pulls it fired up but was running sloppy. I tried fiddling with the air mixture screw and gas valve, but that didn't seem to help at all. Does anyone have any recommendations? When I installed the carb, I screwed both all the way in and unscrewed them 1.5 turns each. My elevation here is about 1700-1800 feet if that makes a difference.

Any help would be appreciated. I'm not really an engine guy, but for the sake of saving money, I'm trying to learn!
 
#2
The "default" setting for the main and idle mixture screws on these Tec Diaphragm carbs is ONE turn (not 1.5 turns) out as per the Tecumseh Technician's Handbook. Also, (very important) make sure you have the air filter installed when you are trying to dial in the carb settings. The slight air flow restriction the filter provides DOES make a difference. My son and I just dealt with this same issue on a 1970 Rupp Roadster he is just starting to restore. Thanks for providing the link on your project-WE will be reading your build thread with great interest!
Michael
 

MB165

Active Member
#3
yeah your probably on the rich side. Low speed mixture adjustment is closest to engine, set that at 1 turn out, high speed (rear screw) start at 1 turn as well. the low speed screw will only effect idle and just off idle. if you can get a good idle (dont be afraid to go to extremes both ways to see how it reacts..) open it up and see what happens, then fine adjust the high side.
The diaphragm and gasket go in different order on various models, mostly, the gasket, diaphragm and cover, but on "F" marked carbs. its diaphragm, gasket then cover....the mixture screws, i think the older tecumseh carbs. share the same thread on both screws maybe able to put them in backwards...if you are certain nobody else has fooled with it you can reassemble the way it was found. the throttle valve has a line on it above the screw attaching it to the shaft, it points up on smaller engines and always out, those need to be aligned right on so it can close up tight. hope this helps ya...
 

capguncowboy

Well-Known Member
#4
Thanks for the tips guys. I'll give it a tube in the AM and report back. Like i said, when it finally did start, it ran great, but getting it there was difficult and exhausting
 

capguncowboy

Well-Known Member
#6
Glad you didn't pull a Buckeye and blow it up.
I thought about you before I started it up. I had never had the engine running, so I wasn't sure where on the light switch the killswitch was located. The first time it fired up, it revved too high and I pulled the spark plug wire.

The second time I started it, and was able to control the throttle a little better, I started trying different things with the switch and found out it's the push button on the right side of the switchplate. Now I won't have to worry about it anymore :)
 

capguncowboy

Well-Known Member
#7
I screwed both screws all the way in today and turned them back out 1 full turn each. I got a small pop right off the bat, and then a 1-2 second sputter, but nothing after that. I guess it's time to pull the carb and look inside.
 
#8
It very well could be the diaphragm is bad. I believe that I read in your build thread that you purchased a NOS Tec carb? If so, that diaphragm COULD be 35- 40 years old! Unlike wine, they do not get better with age. My son also purchased a NOS Tec carb for his 1970 Rupp Roadster build recently and we had similar (non) running symptoms to yours. We opened up the carb and sure enough the diaphragm was not very flexible at all. We purchased a new diaphragm and it was REAL flexible. After he installed it the engine ran fine.
Michael
 

capguncowboy

Well-Known Member
#9
That might be it Michael. I've tried about everything. I even pulled the flywheel (as much as I didn't want to remove the shroud and ruin the perfect paint job around the head bolts) and checked the key. The stater plate was still positioned correctly, and the key was fine. I tried virtually every adjustment combination in the one-turn neighborhood to no avail.

I'll pull the carburetor off tonight and take a look at it. I have another diaphragm here on my desk in front of me, so if that's it I should be running in no time.
 

buckeye

Well-Known Member
#10
Working on these old Tech is,so much fun huh?

Last one still has me pissed and besides the get together down this way, I haven't touch none of them.
 

capguncowboy

Well-Known Member
#11
I tried to put the issues I had on the last one behind me -- THEN I started on this one and bought all new tools and went by the book, line for line. If it isn't a bad diaphragm, I'm taking it to a shop later this week and have them fix it. I've got better stuff to do than to learn to hate my life.
 

buckeye

Well-Known Member
#12
I had to take a break from it all.
Got a great care package in the mail.
NOS rod. So, guess I better use it.
You'll get it. Diaphragm is easy change out.
 
#13
Change out the diaphragm with a new/flexible one and report back to us. We have also found that after replacing a diaphragm it takes a few extra pulls on the starter cord the FIRST time. After that it is a 1-2 pull affair at most if everything else is correct. Really and truly, once you get one these little Tec diaphragm carbs dialed in they perform very well w/o any problems.
Sidebar: My son and I read every page of your build on your Roadster. Absolutely gorgeous bike and you have something to be VERY proud of there!
Michael
 

capguncowboy

Well-Known Member
#14
Sidebar: My son and I read every page of your build on your Roadster. Absolutely gorgeous bike and you have something to be VERY proud of there!
Michael
Thank for your kind words. I'm glad to see it's entertaining and potentially helpful for others when they need it. I'll report back with an update on my carb as soon as I get to it
 

capguncowboy

Well-Known Member
#15
I dropped the carb and swapped the diaphragm. The old one was actually still pretty pliable. I pulled the needle and the seat, needle and spring look brand new. There isn't an obstruction in that part of the carb as far as I could tell. There was plenty of fuel in the diaphragm area when I pulled it apart.

After I put it together, I gave it a couple of pulls and got another pop, but that's it. then nothing, no matter what I did. I've got both screws set at 1 turn. You think I should try swapping them around?
 
#16
Andrew ..what happens when you put a little gas in the spark plug hole or spray some brake cleaner in the carb...does it pop then? Do you have a kill switch, if so can you unplug it ?
 

capguncowboy

Well-Known Member
#17
Andrew ..what happens when you put a little gas in the spark plug hole or spray some brake cleaner in the carb...does it pop then? Do you have a kill switch, if so can you unplug it ?
Yeah, I'll give that a try in the morning. It very well could be the kill switch is grounded -- that would be maddening for sure.

When I pulled the spark plug, it was a little damp with gas. Not dripping, but it had a mist on it, so I didn't bother putting anything down the sparkplug holes. I'll report back on the kill switch tomorrow
 

capguncowboy

Well-Known Member
#18
So, today I pulled the killswitch and gave the engine a yank. It fired for about a second and then stopped. nothing after that. I pulled the flywheel and the key was bent. I pulled it and swapped it out with a new one and tried again. It fired for about 2 seconds this time and shut off. I pulled the flywheel again and it sheared another key.

I'm about to list the damn thing up on Ebay
 
#19
Sounds like the timing is off. During your rebuild did you ever actually check (and adjust if needed) the timing using a dial indicator and ohm meter on this engine?
Michael
 

capguncowboy

Well-Known Member
#20
Yep, I dialed it in to spec using a Tecumseh dial indicator. The marks on the stater even matched up the old marks, so I'm confident it's right. Since I rebuilt the engine, I know the cam shaft is correct as well.

I think the first time I pulled the flywheel I might not have put it back on tight enough. I am wondering if I damaged the flywheel or crank shaft end when that happened. It didn't seem like the flywheel was going onto the crank as far as it was before. Might be time to lap the flywheel and crank to be sure.

I ordered 10 new flywheel keys today, so I'll have to wait until I get those before I can try again (I went through 4 keys today, the last one was a total sheer).

Honestly, I'm going to give it a couple more tries, and then it's off to a local shop. They'll charge me a ton, but it's not worth the headache to save $50-60 to fix something that I clearly can't seem to do.
 
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