MANCO Thunderbird - engine suggestion

#1
Hello!

My grandparents gifted me and my siblings 2 Manco Thunderbirds, when we were smaller. While visiting them, we always had problems with the engine (TECUMSEH). After one year of no usage, we always had to bring them to the next workshop. Today, we are 19/20 years old and though about selling them. While researching for a price range I found that forum and I saw that people are swapping engines. So we decided that could be a solution for us so that we will be able to enjoy these bikes still in future.

So here is my question. Do you have any suggestion for an engine swap. It should be faster, stronger and more reliable in terms of servicing
than the TECUMSEH-engine. It would be awesome if you could mention the estimated cost for the swap and all other related parts. We are thankful for any suggestions!


Henri


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#2
Welcome to the forum!

Couple of nice Mancos you have there. Although newer, they are the last of the “made in USA” minibikes. Before you go the route of new engines, Consider looking into why the tecumseh‘s are running poorly. 9 out of 10 times it’s the carburetor, especially when they sit for a while between uses. Ethanol in the gasoline just destroys them. You can get carburetor rebuild kits or knock off replacement carburetors for less than $20 each.
 
#3
Thanks a lot!

So our carburetors are pretty old. If possible, we would like to change them. Is there any suggestion for a special part/brand? One with more power would be awesome. Is there any introduction on the internet for set up?

Would be thankful for any tips! :scooter:
 
#4
Welcome to the forum!

Couple of nice Mancos you have there. Although newer, they are the last of the “made in USA” minibikes. Before you go the route of new engines, Consider looking into why the tecumseh‘s are running poorly. 9 out of 10 times it’s the carburetor, especially when they sit for a while between uses. Ethanol in the gasoline just destroys them. You can get carburetor rebuild kits or knock off replacement carburetors for less than $20 each.

Thanks a lot!

So our carburetors are pretty old. If possible, we would like to change them. Is there any suggestion for a special part/brand? One with more power would be awesome. Is there any introduction on the internet for set up?

Would be thankful for any tips! :scooter:
 
#5
You have a couple of beauties! Your problem,as posted above,more than likely is old gas gumming up the carbs. Take the bikes or just the engines to a lawnmower shop or somebody around town specializing in small engine repair. Have them clean the carbs and give the rest a once over. You can’t beat the old flatheads,unless you’re wanting something much faster,then you’re into a Tecumseh OHV engine from the 2000’s, which are great but might not fit on your minis. The other option is new or newer than anything OLD OR CLASSIC, chinese made stuff which kills the classic American Made mini, in my opinion. Anyway, Just make sure you drain all of the gas out when you store them, the repair guy will show you how.
 
#6
You have a couple of beauties! Your problem,as posted above,more than likely is old gas gumming up the carbs. Take the bikes or just the engines to a lawnmower shop or somebody around town specializing in small engine repair. Have them clean the carbs and give the rest a once over. You can’t beat the old flatheads,unless you’re wanting something much faster,then you’re into a Tecumseh OHV engine from the 2000’s, which are great but might not fit on your minis. The other option is new or newer than anything OLD OR CLASSIC, chinese made stuff which kills the classic American Made mini, in my opinion. Anyway, Just make sure you drain all of the gas out when you store them, the repair guy will show you how.
Okay, thanks, I appreciate it!

Does it make sense to use that opportunity and to change to a bigger carb? Do you have any suggestion for that?

Greetings Henri
 
#7
Taking the engines to a small engine repair shop is a good suggestion. If you want to try and tackle this yourself I would suggest starting a new thread in the engine section under Tecumseh. Title the thread “Tecumseh engine running poorly”. Make one of your first post a picture of the numbers that are stamped on the top of the recoil shroud. Then someone can help identify the size and year of the engine. This group is loaded with Tecumseh experts! In the meantime you also need to check the engines for spark. It’s easy to do and if you’re not sure how check YouTube, there has to be some videos on how to check small engine for spark. There are many sources Tecumseh engine parts. First you need to know what size engine you have.
 
#8
Okay, thanks, I appreciate it!

Does it make sense to use that opportunity and to change to a bigger carb? Do you have any suggestion for that?

Greetings Henri
I can’t help you with that. The engines look like H35s 3.5 HP or the last version of the 4HP H40. The older larger engines have the intake and exhaust ports reversed
 

f4radar

Well-Known Member
#10
I replaced my original hs40 with a snow blower hs50. Modified the hs50 with billet rod, slide carburetor, and pulled the governor. Swapped the shroud to keep that old look. I increased the fun by a factor of 10.
 

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Triley41395

Well-Known Member
#12
Just my opinion but don't swap them out. Spend about 35 bucks buy a couple china carbs,gaskets and new spark plugs off Amazon, Spend about an hour changing them both out and enjoy them. Keep the original carbs and have them cleaned and ready to put back on when you need them. I think this would be the easiest and fastest way to getting back to enjoying them.
 
#13
Henri,
Nice machines you have there. Ours is really wheelie-prone. I would suggest you look at all the options available from these folks here. Then proceed slowly. An increase in engine performance can result in a drastic increase in wheelie events.
Others will offer experience-based ideas as well.
Good project.
 

kayde

Active Member
#14
I'm guessing your problems stem from ethanol fuel and long term storage gooping up the carbs.

These tec motors and carbs are fairly simple and new carbs are cheap on ebay and easy to replace.

Consider finding and using ethanol free fuel in your area and use only that in your minis and other small engines.

Ethanol free fuel will last so much longer and be alot less troublesome in carbs/engines that see infrequent use.

Your next option is too treat your fuel with stabilizer.

You have a couple of very nice examples of mancos, i would try to avoid modifying them in any way.
 
#15
We run only Rec90 and Seafoam in all our small engines. Rarely a problem, usually contaminants introduced from the no spill gas can, if anything at all.
Nice project. Good parade bikes.
 

toomanytoys

Well-Known Member
#16
On my tecumseh I put a fuel shut off and always run them till the bowl is empty. It’s the only way I’ve ever kept the jets from getting clogged.

I had to take the bowls and pilot jets out of both of my clones Saturday to spray out the carbs with cleaner. Would not start then boom fired right up after a few sprays. Ran fine on the gas that was left over from last riding season.
 

ELT

Active Member
#18
I'm guessing your problems stem from ethanol fuel and long term storage gooping up the carbs.

These tec motors and carbs are fairly simple and new carbs are cheap on ebay and easy to replace.

Consider finding and using ethanol free fuel in your area and use only that in your minis and other small engines.

Ethanol free fuel will last so much longer and be alot less troublesome in carbs/engines that see infrequent use.

Your next option is too treat your fuel with stabilizer.

You have a couple of very nice examples of mancos, i would try to avoid modifying them in any way.
My experience has proven that old gas is old gas whether it has ethanol in it or not. Modern fuel is meant to be atomized with a fuel injection system not a venturi and is not as volatile as it used to be. As any gas ages it becomes even less volatile and creates more problems.
 
#20
If you get to large of motor, you need to brace the motor frame plate. I kept throwing chains with my predator 212 engine. till I did that.
I changed the gearing to, wheeled WAAAY to easy. 20200415_192302.jpg
 
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