1948 125 Hummer will not rev up

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Re: 1948 125 Hummer will not rev up

Postby Phatsalmon » Sat Jan 21, 2023 11:17 pm

Harley Hummer resize.jpg
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Re: 1948 125 Hummer will not rev up

Postby Backyard Bob » Sat Jan 21, 2023 11:57 pm

You can make them bigger than that :-)
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Re: 1948 125 Hummer will not rev up

Postby Backyard Bob » Sun Jan 22, 2023 12:16 am

Okay, as Dave Hennessey once told me, "Think outside the box as opposed to thinking inside it? I am the box... inside and out". Sherlock Holmes said something about considering the impossible. Soldered and re-drilled jet? Is your carb an S1 or a later S2? Wrong carb. Idles okay but cuts out? Perhaps your crankshaft seals are leaking at higher rpm's and are letting in too much air (two strokes suffer from this). You've got my brain hurting... but solving problems put food on my table for many years.... retired now and doing it for fun.
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Re: 1948 125 Hummer will not rev up

Postby 250_Sprint » Sun Jan 22, 2023 12:26 am

Experimentation time:
Can you use a different fuel supply source like the auxiliary tank mechanics use to tune a bike when the gas tank is removed from the bike? They hang from a hook and can remind you of an IV. This takes the tank, petcock and fuel line out of the formula. See if the engine will rev any better.
Get the engine good and warm/hot then turn it off. Remove the air filter and restart the engine. See if the engine will rev any better.
Remove the muffler. See if the engine will rev any better.
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Re: 1948 125 Hummer will not rev up

Postby Mutt » Sun Jan 22, 2023 11:00 am

you have something wrong. They wind up if everything is right

https://www.youtube.com/watch?v=qH5-soh57bU
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Re: 1948 125 Hummer will not rev up

Postby Phatsalmon » Sun Jan 22, 2023 3:05 pm

Mutt, do you have any ideas where to look next. Engine rebuilt. New new seals throughout the engine and Crank. Leak down is fine at 6 psi.
all electrical is new in the ignition. Timing set by depth gauge and Fluke Meter. Runs fine at low to middle RPM. Just stalls after 3/4 throttle.

I am questioning Carb and exhaust right now. Very simple carb however I am beginning to question the needle. Also this is a straight exhaust. No expansion chamber like the one in the video.

Lastly I am questioning upper end in its entirety. I used the existing 10 over piston but installed new rings. Would the bike run a low to mid RPM if the rings got damaged during rebuild?

Thanks for jumping in. Back to the bike today for me. I have learned a lesson here and that is to stay in my wheelhouse. I refurbish 1960 to late 1970's enduros. I thought that this would be simple, my mistake!
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Re: 1948 125 Hummer will not rev up

Postby Mutt » Sun Jan 22, 2023 3:09 pm

were the shaft sin good shape where the seals ride? What did ya do to the cylinder before installing new rings? I can send ya a NOS metering pin and #5 jet to see if that helps
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Re: 1948 125 Hummer will not rev up

Postby Phatsalmon » Sun Jan 22, 2023 3:40 pm

Crank seals new, the sealing edges were good. Replaced all three crank bearings. I did a leak down test and all was well at 6psi. I simply honed the cylinder. It measured round up and down the barrel. Should I have replaced the piston and rings. My machine shop can do the machining of the barrel.

What is the orifice size of the #5 jet? The one I have is now at .0050.

This seems like Carb problems to me. It is so simple that I have to be missing something.

I have a ton of money into this bike. It is frustrating to come to the very end and run into this. Do you think I should pull the engine and send to and expert? It hurts to think that I cannot get this very simple engine to run. My ego is getting in the way probably.

Thanks so much for your help.
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Re: 1948 125 Hummer will not rev up

Postby Mutt » Sun Jan 22, 2023 3:48 pm

if your jet is .oo5" Im pretty sure that is way too small. I'll see if I can measure a NOS #5 and #7
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Re: 1948 125 Hummer will not rev up

Postby Backyard Bob » Sun Jan 22, 2023 9:53 pm

There's a big difference between .050" and .0050".
Perhaps a typo?
I sent my '62 Scat engine to Mutt. I had the resources to do most of it myself... but not the time. After I received it, it sat in my basement for four years with oil soaked rags in the intake and exhaust ports. When I finally put the bike together it took three kicks to get it started. I'd included a racing stuffer plate in the rebuild and he had to rework the carb to get it to run correctly. He didn't give up until it ran right. I did the same thing with a buddies '47 Indian Chief no one could figure out the problem. I took a week off from work because I was on a mission. Three other antique mechanics had pulled their hair out and given up. I finally figured out the problem and charged him nothing. I ended up with a brand new S.S. grill on my back deck. Can't figure it out? Send it to Mutt.
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