MC 90 flatback pop off

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Kurt Bogerman
Posts: 30
Joined: Wed Sep 27, 2017 4:39 pm
Vintage Karting items owned: 1964(?) Rupp Grand Prix, MC 90, Safari Gearbox.
User Agreement: Yes
Location: Harrisburg, PA

MC 90 flatback pop off

Post by Kurt Bogerman » Thu Sep 28, 2017 12:40 pm

Hi, guys.

I recently joined the forum to finally ask some questions that, after years of lurking, I've been unable to search out on my own.
I have a MC 90 on a Rupp Grand Prix that I purchased at a garage sale in 1990. It ran reasonably well at that time, but the kart has suffered some unfortunate mishandling in the time since. I recently have begun rebuilding it. In addition to beginning the refinishing of the chassis, I have had the cylinder bored and a piston fitted, per advice found on this and other vintage kart forums.

Now that I'm finally reassembling the engine, and pressures are being tested, I would appreciate some advice on what to expect as far as pop off pressure is concerned. I had cleaned and stored the carburetor years ago, and the diaphragm and gaskets, which were relatively new at the time, seem to have survived splendidly. Testing the carb at the fuel feed port in the main body, with the back removed, the needle will not pop until pressure builds up to 27psi, which sounds high compared to other carburetors. What should be expected here? As far as I am aware, the carb is unmodified, and it ran in this current state, although years ago.

Your advice is appreciated! Thanks!
Last edited by Kurt Bogerman on Thu Sep 28, 2017 9:29 pm, edited 1 time in total.

User avatar
Kurt Bogerman
Posts: 30
Joined: Wed Sep 27, 2017 4:39 pm
Vintage Karting items owned: 1964(?) Rupp Grand Prix, MC 90, Safari Gearbox.
User Agreement: Yes
Location: Harrisburg, PA

Re: MC 90 flatback popoff

Post by Kurt Bogerman » Thu Sep 28, 2017 9:07 pm

I found this thread which addresses similar questions about a BDC-14:

viewtopic.php?f=1&t=203&hilit=popoff#p730

Does Steve O'Hara's advice about popoff pressure also apply to the small flatback carb without the duckbill circuit? (He ultimately speaks favorably of popoff at 29PSI and "spitting" at 20PSI")

I rechecked my flatback today, this time with the carb fully reassembled. Similar to the BDC-14 in the cited thread, my carb pops off consistently at 27psi and stops falling at 19 psi. The part number on the carb body is 48890B, if that helps.

I just want to be sure that I have a solid baseline before I attempt to start and break in the engine when the project is completed.

Thanks!

Rob Voska
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Joined: Fri Jun 15, 2007 9:04 am

Re: MC 90 flatback pop off

Post by Rob Voska » Fri Sep 29, 2017 6:38 am

Flatback carbs are like a Tilly carb. No pressure fuel injection like a BDC series carb.

I would start at 12 lbs gas & 8 alky.

Careful as lots of those old carbs have been worked over / destroyed by good intentions.

User avatar
Kurt Bogerman
Posts: 30
Joined: Wed Sep 27, 2017 4:39 pm
Vintage Karting items owned: 1964(?) Rupp Grand Prix, MC 90, Safari Gearbox.
User Agreement: Yes
Location: Harrisburg, PA

Re: MC 90 flatback pop off

Post by Kurt Bogerman » Sun Oct 01, 2017 12:08 am

Since I'm finding pressures similar to a duckbill carb, I wonder if the inlet spring is incorrect? I may have even done it at some point. Gonna test that theory. I searched for a correct one on eBay last night. Order confirmed and on it's way.

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