Tecumseh won't run past half throttle

[Roadrunner123]

Hi, I just acquired a 1978 Ariens 20" mint condition snow blower. Model 932001 s/n 013983 with a Tecumseh engine H35-45503M s/n 8110B . It wasn't running when I got it but had spark. Would run with carb cleaner, so I purchased and installed carb kit 631893a, and because it was that I also old installed Points 30547a, condenser 38540b and new plug .
Set the points per Tecumseh repair manual and torque the flywheel according to specs. Engine ran fine for one snow storm but what I noticed it took about five mins before I could go to full throttle or the engine would die. Once warmed up it seemed fine especially under load. Then after sitting for a few days it was harder and harder to get it to run at full throttle. Figuring it had to be carburetor issues I removed it again and this time I removed the welsh plugs (not having done it the first time) and found nothing unusual in there. Cleaned carb. again and ran it through the ultra sonic cleaner twice. Reinstalled and same conditions except now I can't get it to run past half throttle no matter how long I let it warm up. Choking it makes not difference. Oh I forgot to mention this as the diaphragm carburetor. Though I have been working on small engines as a hobby, since I was kid, this is the first time I've seen one of these on a Tecumseh engine. The Tecumseh manual goes into quite a bit of detail so I feel a lot more comfortable with it now. So given this background information I was wondering now if there is anyone that has experience with this engine and diaphragm carb and might have a suggestion of what would cause the engine to want to die as soon as I go past half throttle. I'm using fresh VP racing fuel with no ethanol.
I was told that there are techs on this site that have a vast amount of experience and this would be the forum to try.
Thanks for any help you can give.

[bgsengine]

pull flywheel and double check your flywheel key, ensure you did not loosen (at ALL) the stator plate bolts (that carries points and condenser) , re-check your point gap again to see if anything may have shifted, and also make sure you do a final clean of the points after any adjustment or checking (even a slight film of oil on points can glaze up as they get hot, and increase resistance of current flow) gap changes or flywheel key shearing (or your stator plate moving, if you ever so much as loosened the plate screws) will change ignition timing, which can be a cause of your symptoms. Unfortunately if you did loosen stator plate screws, you may end up having to pull the head (and lots of fussy fiddling) to reset your timing properly. Other possibility can be valve problems (sticking, burning valves, valve clearances, etc) and even a failing spark plug (new does not mean good) Just a few thoughts on your issue.. Also might check for fuel lines pinched (better yet if they havent already been, replace them, as they can rot our from the inside and some types of lines, the inner line can separate from the rest of the hose, and collapse closed, restricting fuel flow) , debris in fuel tank restricting fuel flow (especially on machines that sat unused for years - flakes of varnish break off and block the outlet) are a few more things to think about.

[Roadrunner123]

thank you. I will do everything you said. I did loosen the stator plate when I installed the points but even though I circled the bolts with a magic marker so as to make sure its was put back in the same spot, I will pull the head and check the timing. I have a dial indicator and have done timing in the past. Also the Tecumseh manual deals with this in detail. I will respond as soon as I complete everything.
Thanks again for the response.

[RoyM]

I have seen those carburetors get full of crud behind the inlet metering needle, pull the barb fitting and clean it out.

[Roadrunner123]

I also noticed that there is a check valve in there and I was wondering how it's replace. So does the barb fuel inlet just pull out? Never worked on one of these prior to this.
Thanks for the reply.

[bgsengine]

I also noticed that there is a check valve in there and I was wondering how it's replace. So does the barb fuel inlet just pull out? Never worked on one of these prior to this.
Thanks for the reply.

Depends - If there is a hex on the fitting, it is threaded... If there isn't then it is a pressed in fuel inlet, and you'd be best to not try to remove it (carb body breaks easily if not done right) - aLL ASSUMING THIS IS IN REFERENCE TO THE FUEL INLET FITTING- (that the fuel line hooks to) But if there is crud (pop carb apart and look in through fitting, though unlikely since you said you ran it through ultrasonic, it'll usually come out with a post-ultrasonic flushing) There's a lot of things that can cause low RPM , assuming governor hasnt been altered (and if it is mechanical governor) , also check for binding in throttle linkages of course... If it is an air vane governor (not likely on a 4-stroke tecumseh) , I'd pop the shroud and look for any debris (leaves, mouse nest) causing governor vane to hang up, or something blocking air flow (though that'd usually result in higher RPM, not less)

But based on your symptoms, it sounds to me like you're experiencing a change in timing, ignition problems (misfiring can cause reduced RPM, even if you can't detect the misfire itself, especially at no-load) , or possibly excessive fuel? (black smoke as it is running similar to over-choking..) Also one other thing I thought of on the diaphragm carb - some have a primer elbow in the metering chamber cover , make sure that elbow is clear , otherwise there's no pressure equalization and it can vapor-lock... the ones without primer elbow will have vent holes in the cover , which is plugged (rust , dirt, etc) can result in same issue - without atmospheric pressure on the diaphragm, it won't see full movement..

[bgsengine]

I did loosen the stator plate when I installed the points but even though I circled the bolts with a magic marker so as to make sure its was put back in the same spot,

never works, even when I used to actually take a chisel and "stake" a v-notch into the stator and block, even a slight bit out of place (just a bump while bolts are loose) can change timing by several degrees.. and thermal expansion can cause it to move or shift (and bits of dirt and crud that might fall in between stator and block cause bolts to come loose after the crud crushes or embeds) I'd check point gaps first (just to see if it has shifted at all) and cleasn the points, just to verify , before going deeper and re-setting timing... - one thing at a time... Though if the head hadn't been off before, and you do plan to pull it, you might do a top end service (de-carbon, clean valves, check clearances, make sure there's nothing causing valves to stick, etc) while you have it off

[Roadrunner123]

ok thanks for all your advice. I look forward to resolving this issue.

[creia]

Did you put the gasket and the diaphragm back in the correct order when you reassembled the carb? On some carbs the gasket goes first while on others the diaphragm goes first. Reference the Tecumseh manual to see which way yours goes.
Michael

[Arkie]

You may have carb issues BUT:
There is now a really good chance that you also now have timing issues.

In reference to timing on a H35 Tecumseh.
When looking at the timing specs in the L-head service manual BE SURE YOU GET ON THE CORRECT column for the H35 going by serial number. They had a serial number change over in timing specs due to a crank/rod throw.
If you time BTDC per the WRONG COLUMN it won't run right.

I've timed them old dogs by removing the head and using a dial caliper or a micrometer. They are critical on timing and changing the points gap by .010 will a different response. Tecumseh at one time had a $70 timing tool with a offset leg for timing that set on top of the head with a offset leg. (I could not ever afford to buy the tool when they were available because a $1 was actually a dollar back then and head gaskets were cheap and readily available)
I have re-used the old metal reinforced Tecumseh head gasket with good results if it still looks ok. (comes off in one piece)
AND placing witness marks on the points box will not get er dun.
Also running one too far BTDC will throw a rod fast. (especially if you hear the spark knock when reving up the engine or getting a strong rope kickback when starting)

Good strong little engines when correct BUT,
Little more critical tuning as compared to a Briggs.
(one of the reasons why Tecumseh engines got some bad mouthing comments)