Tecumseh carb help (3.5 hp Power sport engine) Manco Thunderbird bike.

Turbofiat124

Turbofiat124

New Member
#1
#1
So I tried cleaning the carb on my Manco thunderbird yesterday. It ran about five years ago. I parked it in my storage building and next spring it wouldn't idle but would run at part and full throttle. Like the idle jet had gummed up over the winter. E10 perhaps?

It's got a 3.5 hp Tecumseh power sport engine. It's got this weird throttle mechanism which is hooked to the governor (so to speak), then a small rod that goes to the throttle plate on the carb.

I took all the jets and float valve out and soaked them in Seafoam, blew them with spray carb cleaner and blew them out with compressed air so they should be cleaned out. But the bike still doesn't run right.

Symptoms:

Leaks fuel when engine is not running (tells me either the float level is out of adjustment or seat is eaten up)
Won't run unless choke is set halfway. Seems contrary to the above, wouldn't this mean the carb is running too lean?
Runaway throttle. When I briefly road the bike it almost ran out of control so I killed it.

I've already rebuilt the carb once so I'm thinking I just need to start with a new carb.

I've got several in my watch list on Ebay for around $15. They all look the same but have different part numbers.

I read the sticky on Tecumseh carbs on this website but the carb I have just has the number "42" stamped on the side. Not a 6 or 8 digit number as per the sticky or the ones on Ebay.

If my carb looks like the ones on Ebay, is there any reason I can't use it? As long as it's for a 3.5 hp Tecumseh engine and has an independent choke, will it work on my bike?

Also can I do away with this funky throttle setup and tie the throttle cable directly to the throttle? I suppose I would need to leave the rod that connects the governor to the throttle plate. Thanks!
 
chrisr

chrisr

Active Member
#4
#4
Best way to look up what carb you need is by your model and type of your motor off the blower housing that way you get the right part the first time. Without the original blower housing and or carb some basic things to look at is the supply line 90 degree on the carb like this one or straight and you can't tell from the pics if this has the adjustable jet or not on the bottom of the carb (other listings same part number it looks to be not adjustable which normally adjustable makes it a little easier to fine tune). You also want to look at the throttle plate on the top of the carb to see if the holes on this matches yours. The carb from a 3.5 hp may not work for one that was for 10 hp (carb to intake bolt to bolt distance and jetting) so with this listings it's pretty vague. If you search by part number 640152 on your listing example this carbs looks like more for 8 -10hp engines not the 3 - 4 hp like what you want. Hope that helps
 
Turbofiat124

Turbofiat124

New Member
#5
#5
Thanks! I think I found it. It's an H35 engine so I used Tecumseh H35 carb as a keyword on Ebay and came up with this:

New Carburetor for Tecumseh 631921 632284 631070 631245 H25 H30 H35 Engines Carb | eBay

So I think I found the correct carb.

Now I need to find out if the throttle mechanism is setup correctly. Hopefully I can find photos or someone on here can post a photo of their bike.

When I got this bike it was all screwed up. I ended up making the rod that goes between the governor arm and throttle out of a piece of wire. Hopefully I can determine if I have it in the correct holes.

My guess I twist the throttle to full throttle, it engages the governor for full power. I guess that's the reason for this weird setup.
 
H

Havasu Dave

Moderator
#9
#9
Don't forget you need to check your governor for adjustment before you hook up the linkage. Procedures are on the book. Essentially, full throttle, loosen arm, arm rests where it is, while governor shaft is turned all the way clockwise. Tighten.

The condition of the actuation spring determines which hole it ends up in. By increasing spring tension, effective governed RPM will increase. Generally when you play around with that, you end up making a new throttle actuation rod.
 
Turbofiat124

Turbofiat124

New Member
#10
#10
When I got this bike the whole throttle mechanism was all messed up. I fabricated a new rod out of a welding rod. I actually had the bike running great until I parked it over the winter. I really need a new rod because I'm not sure if it's the correct length. Do you know the correct part number for the linkage rod between the throttle and govenor?
 
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