Diagnosing Low Max RPM: Margaret Thatcher Needs her Coffee!

During the practice day for the 2013 LeMons Capitol Offense race, it was hard to NOT notice our MGB GT had somehow dwindled to an upper limit of 4500 RPMs, which means the very important upper 20% is missing, right in the power band. The poor girl is weak already, and a limit of 4500 would literally (and figuratively) knock us out of the race.

Sigh.

We check the pedal linkage, it was OK.

The accelerator cable was making grinding noise; we sprayed into the cable all we could with silicone spray, to some-but-little effect.

Scot noticed the butterfly gears on the Weber DGV Downdraft Carb were not articulated 100%, so we bent and adjusted the cable bracket all we dared to extend the motion, and then greased the hell out of it. To no effect.

We were getting driver stints in between tweaks, and by the time we fixed the butterfly gears it was time for LeMons Tech Inspection, which we failed, so we dropped the whole "Upper 20% power missing" because Tech had rendered us with a "Upper 100% power missing" situation.


After the sobbing subsided (2 weeks later) I finally got back to diagnosing the issue.

First up: Fuel Delivery.

We had previously had mega-carberator problems with gunk at Summit Point 2011, so that was a thing.

Remembering the Jet Ski in-tank fuel filter I had added 2 years prior, I decided to make sure the carbs were getting enough coffee fuel.

1) get a glass jar, clean it out, drill a 3/4 inch hole in cap (or whatever size your fuel hose is, 'natch)



Guinness does not come in clear bottles, so you'll have to resort to 9 oz poison bottles.

2) disconnect fuel line from carb.

Might want to make sure fuel pump power is OFF before doing this, dumbass.



 3) stuff this thing in that thing:

Probably illegal in New Jersey.

 

4) apply power to fuel pump, count how many seconds it takes to fill up bottle (6 or so seconds). Do not drink, though it would be healthier than what originally came in the bottle. I messed up timing this, in that (1) I didn't run the car first to purge the air, and (2) I didn't use a timer, but counted one-mississippi.

Pretend this pic is rotated 90 degrees counter clockwise.

5) now, don your best Saran Wrap Fetish Gear, and give your Fuel Cell a proctal exam:

Definitely illegal in New Jersey!

6) wave to neighbors, note which ones wink, reach into fuel cell and fish around for fuel pickup and in-tank filter:

This may feel uncomfortable...

7) remove said filter (and small connecting hose):

Possibly clogged filter removed, immediately wash arm with 30 gallons of soap

 

 8) change crappy-ass "Fuel Pressure Regulator" which is actually just a friggin fuel restrictor from 4.5 PSI to 6 PSI; retest fuel flow, receive same damn 6 second time.

Crappy ass.

 9) Be sure to leave bottle cap in fuel line so that you look like the complete tool that you are, buttclown.

Honest, it's a heat shield!

10) test drive that bad boy - 5500 RPM, bitch!!!!!   Dunno if it was the pressure regulator (probably), clogged fuel filter (probably), or the need for a saran wrap proctologist exam (probably)... but whatever - I'm happy.

Last check I need to do it run it HOT, and see if 5500 is still reachable; not too worried since the 4500 limit was all day, even once cooled. Sounds fun.

Rev Limiter for MGB Survival

We surmised that we over-revved the MGB engine in the first race of 2012, because:

    1) there was no tachometer

    2) there was no speedometer

    3) the engine is underpowered, causing drivers to floor it all the time, for the entire 14 hours

    4) the 4 speed manual transmission really really needs a 5th and 6th gear, causing drivers to floor it all the time, for the entire 14 hours

So we installed a tachometer from another MGB and rotated it so that 5250 is pointing straight up - she really could use a "shift now" light for gears 1, 2, and 3 and a "sit back - you're just along for the ride at this point" light for 4th gear. Some day.

We installed a Rev Limiter instead since our driver pool is often hungover and barely focusing on the road ahead much less the little numbers on the tach.

The Rev Limiter we ended up with was the Procomp Single Stage Rev Limiter PC2031, and it did just fine as far as well could tell. 

1-800-Dial-a-Prayer!

Also on the menu was a new fuel pressure regulator, the Holly Fuel Pressure Regulator had died after a failed attempt at rebuilding the internals with new rubber and filter element. Oops.

All those pretty blue AN fittings were a real pain to match up, but they sure mate nicely and are clean etc etc. Soooo un-MGB like.

MGB Fuel Pressure to Weber Carb should be about 4.5 psi and very shiny.