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MG MGA - Power Loss

I have a 56 roadster 1500 (newly rebuilt) connected to a Hi-Gear 5 speed. The car runs fine in 1st through 4th gear up to redline. 5th gear is strong to 3500 -3600 rps. At about 3700 rps the power seems to die and the exhaust seems flat. The car will not go any faster. The problem doesn't happen in the lower gears. I am not sure if the problem is timing, points,or valves? Perhaps that is all the power it has. All above have been mentioned as possible causes by friends.
Terry Marcus

Ign timing is easy to check. Do it first. How fast is 3700 in 5th gear?
A Pearse

If your engine is newly built it may still be tight - it needs to be run in properly to loosen it up before running it at peak revs - yeh what is your speed at 3700 in 5th? - must be around 80mph. cheers Cam
Cam Cunningham

The engine has 1500 miles on it. I kept the rpms low during the first 1000 miles. 3700 rpms is just over 70 MPH as measured by my GPS. The timing seems good in the other gears, the engine starts fine, know knocking and it shuts right down when the ignition is turned of.
Terry Marcus

Sounds like you are starving for fuel at high revs. I would check the carb air filters. This indicates an obstruction - check that the felt is not displaced at the mouth of the intake manifold.
Barry
BM Gannon

If the engine pulls well up to the redline in the first four gears but runs out of push at 3700 in fifth gear it appears that the gearing is too high.
It is not an engine or fuel problem, as shown by the performnce in the lower gears.
I would have to look up the details but I think that the Hi-Gear gearbox 5th gear is about 0.082
Are you running the standard wheels and tires?
Have you ever checked your rear axle ratio? Not the markings on the axle housing, but the actual ratio.
Do you know how to do that?
With a 1500 engine I would be thinking of a MGA optional 4.55 rear axle. Might be hard to find, but all the MGA's with the close ratio gearbox had it.


Mick
M F Anderson

Typo error in previous post.
The 5th gear in a Hi-Gear gearbox is of course 0.82



Mick
M F Anderson

70mph at 3700RPM in 5th certainly does not sound right. With the standard MGA back axle this should give you very close to 80mph. This is what I measured off my 5-speed box before I converted to a 3.9:1 back axle:

5th gear:
40mph = 1880-1900rpm
50mph = 2350-2400rpm
60mph = 2820-2900rpm
70mph = 3300-3350rpm
80mph = 3750-3850rpm
90mph = 4230-4350rpm
100mph = 4700-4800rpm

Lots of data in the archives about it.

Steve
Steve Gyles

Assuming Steves numbers are correct and Terry's GPS is showing just over 70 mph, the discrepancy of Terry's reported 3700 rpm could be due to a combination of one or more of:

- non-standard rear tire size
- under inflated rear tires
- non-standard rear end ratio
- tachometer that's over registering at higher rpm
Andy Bounsall

By changing the trans to an OD of .82, you have limited the top speed capability. The std MGA runs out of HP at about 98MPH according to all test info of the era. This is at the peak of the HP curve for a 1500 engine. By changing the gear ratio, you are slowing down the engine and thus falling lower on the HP curve. Using the HP/RPM curves for the std 1500 engine, I calculate the top actual speed available with the .82 OD gearbox is about 94 MPH. This is far and above your experienced ~78 mph (3700rpm * 17.3 mph/1000rpm /0.82), indicating the engine is a bit anemic. If you were to have a 3.9 differential, the top end would be about 90.5 MPH. Of course this is actual speed, not indicated speed. Assuming the tacho is accurate. The various trans and differential gear ratios do not effect the tachometer, just the speedo.

There is evidence that your tacho is not accurate. If we are to believe your 70mph at 3700 rpm, This would work out to 18.9 mph/1000 rpm. The .82 OD should achieve ~21.1mph/1000 rpm with the 4.3 rear end. This indicates that a higher ratio rear end is there or your tacho is off. If the 4.55 rear end is in place already, you would get ~20mph/1000rpm. I suspect your tacho is reading high since I believe the GPS for speed.


You could be suffering from a low flow fuel pump, poor breathing (tappet setting or air filters), poor dwell on your ignition, poor timing advance etc. It might be adviseable to go thru a full tuneup. With 1500 miles after a fresh engine, it might be time reset the valves, ignition and carbs as things start to settle in.
C Schaefer

Took My 1600 mk 2 coupe out for along run yesterday and found a similar loss of power at 60-70 mph in fifth (revs around 3000-3500)did not want to accelerate when pulling hard on a slight incline.It felt like a lack 0f fuel. after around 50 miles we stopped for lunch and I decided to richen the mixture turned the SU adjusting nuts down around 2-3 flats. On the return journey the difference was noticible -more power and could easily accelerate to 90 @ 4000 rpm. When I checked the plugs after the run they were all the correct colour!!

Paul
P D Camp

Terry, my 1500 Roadster pulls like a steam engine with a 3.9 diff. I run rich needles in the carbs, cc's from memory, and 34% mechanical advance at 3500 rpm. Fuel consumption is 35/36 mpg, on the open road. [ Thats imperial gallons ] I run the rich needles to keep the combustion temp. as low as possible. I run a 185 % thermostat as these engines are designed to run 185 /190, best gas expansion ,ie piston push, at these temps. It all adds up, slightly richer mixture, correct spark timing, [advance curve]and correct running temp. Also high octane fuel ! Sean
PS., most mechanical tachos are out, mine is 250 rpm high all the way up the range.
S Sherry

This thread was discussed between 18/02/2009 and 25/02/2009

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