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MG MGB GT V8 Factory Originals Technical - Cam selection

Hi guys,
I finally pulled my car from winter storage and after a little fuel system work I got it up and running. As has been the case for the last 3 years, it doesn't like to fire up when you initially crank it over. I've replaced fuel injection computers, starters, fuel injectors, almost all the fuel injection sensors (except the fuel temp sensor)and Nothing seems to be curing it. Once it starts, it runs pretty well, though my gas mileage really sucks (around 14mpg in the city). The only things I haven't changed are: dizzy, cap, rotor, petronix II ignition, and the cam. My cam was a custom grind by comp cams and came with these specs:

Gross valve lift .463 (int) .477 (exh)
Duration at .006 tappet lift: 256 (int) 262(exh)

valve timing at .050 Int opens 4- BTDC closes 36 ABDC
exh opens 43 BBDC closes 5- ATDC

Duration at .050: 212 (int) 218 exh
lobe lift .2990 (int) .3080 (exh)
lobe separation: 112.0

I'm starting to think maybe the cam is too aggressive for what the fuel injection can handle? The power feels GREAT, but I hate it when it stalls out, or starts to surge. I'm thinking that by having the valve overlap that I do, that it's dumping so much fuel down the exhaust that it's messing with the computer. This could also explain my poor gas mileage. You can smell the gas coming from the exhaust too..

Any opinions?

The cam in my Rover injected 215 is nothing special, just a bit warmer than stock, so no tremendous power, but it starts first turn hot or very, very cold, runs well without any stumble, idles arond 700 rpm.

lift - intake 260, exhaust 270
duraton- adertised- intake 260, exhaust 28
intake opens -6, closes 28, max lift 107
exhaust opens 42, closes -12, max lift 117

This is an old Crane
H-202/260-25-12 for 215 Buick & 1968 up Rover 3.5/3.9

Maybe comparisons will help. I do know that when Dan (D & D) built my 4.0 Rover for FI, he was concerned that the cam maintain enough vacuum to operate the FI, & said that I could have problems without enough engine vacuum.
Jim Stuart

Justin, I think there are at least two things going on here - the first is that there may be a leak in the fuel system somewhere that's bleeding pressure off the system when the car sits. That could account for the hard starting. Chrysler products are especially good for this one.

Any more than that, we need to know what you've got as to engine size, heads, closed-loop fuel injection or "sensor-controlled" (can't think of the correct term at the moment - maybe I should have another drink!). Surging is a good indicator that the system is too lean, stalling can also be a symptom.

I know this didn't help much,
Wayne Pearson

It sounds like the cam is pretty mild, so look elsewhere. It also sounds like your engine might be in "open loop" which is causing a rich "safe" mixture. That will effect startup only by fouling the plugs through regular driving. I think Wayne is on to something. Check over the electrical and fuel systems. What plugs are you running, and what condition are they in?
Jeff Schlemmer

Justin, the cam seems to be mild, but....what is your vacuum? The FI needs at least 17 inches of vacuum to work properly. The MAP converts vacuum into volts and tells the computer how lean or rich this is also verified by the O2 sensor on the exhuast. Timing is the big culprid in starting problems, or low compression. Injector leak down is also a problem.
There is 9 injector or a rich mode for cold start if this is not working it will make it hard starting all of the time regardless of make.
Bill Guzman

I haven't checked the vacuum on the motor, but I will do so. There is no MAP sensor on the motor. There are O2 sensors (the proper ones) and I'm running the proper tune resistor for the motor (470 ohms I think)Timing is set to about 5BTDC. The motor is a 3.5L Rover with 10.5:1 comp pistons. It will NOT run on anything but hi octane fuel, so I know it's got compression! The injectors are two years old - 19lb bosch injectors.

I wonder if my airflow meter is bad? I hear when those go bad that they cause the motor to run excessively rich all the time.

I'll check my spark plugs today.

I'm going to test my fuel pressure and see what's happening there. If there is not enough fuel pressure at startup, then I think that could cause my starting issues.

From what I gather, the fuel pressure regulator (at the end of the fuel rail) bleeds off extra fuel beyond a certain pressure point. Is it possible that it's holding fuel in the rail until enough pressure builds up that the motor starts running?

Thanks for all the input, I REALLY appreciate it.

My experence with Chrysler products was that there a leak inside the fuel tank at the fuel pump allowing the pressure to bleed off. There was even a service bulletin, but no recall, so we didn't bother - just ground the starter for about 10 seconds until it started.

Wayne Pearson

It's an aftermarket Jaz fuel cell in the trunk with an -8 a.n. fitting feeding 3/8" fuel line going to the external volvo fuel pump which feeds into the mg's existing fuel lines.

If there was bleedback, how would I stop it? Some sort of one-way valve?

If that's the problem - yes. But what does Volvo do their external fuel pump system? Is there a check valve in the tank that you don't have, or is there a check valve internal to the fuel pump that's leaking? Is the fuel supply from the fuel cell at the top (and the fuel pump doesn't pull against a head) or bottom?

Wayne Pearson

The fuel supply is at the very bottom of the tank, and the fuel filter, and fuel pump are both mounted physically LOWER then the bottom of the fuel cell. (I'm trying to get gravity to work in my favor)Once the car starts, it seems to be fine.

Good. Now, does your fuel pump start as soon as you turn the key to run? What does the pressure in the fuel rails become before you try to start? At this point, you need to hook up a gauge.

Also, do you have some photos of your fuel cell out there somewhere? - I'd be very much interested in seeing them. I remember you posting about getting this cell about a year ago.

Wayne Pearson

Odd gone strange... I got a fuel pressure tester last night and hooked it up right at the fuel rail, and when I turned on the ignition, the fuel pump primed (like normal) and the pressure shot up to about 50 psi on the rail and stayed there. Even when I cranked it over, it stayed at about 50 psi. Funny thing is, when I pressed the start button (My car doesn't use an ignition key to start) the engine turned over, but it never fired, or even tried to fire. I pulled the lead from teh center of the dizzy and got it close to a ground, and the spark was pathetic and very hap hazard.

I spent a few hours researching and found that I have the wrong coil on the system, and it's barely getting any voltage when cranking over motor. I'm reverting back to my lucas 12v sport coil, and bypassing the ballast resistor. I got the idea to test the voltage on the white wire from the fuse box while cranking the motor, and while cranking it dropped to 9 volts from 11.9! I checked voltage at the battery and when cranking it dropped from 11.9 to around 10.7 volts. That's not good!

Could that kind of voltage drop while cranking account for poor ability to start? I'm using a mini torque starter so I don't have the "boost" line from the original starter, so I'm thinking about adding a relay feeding 12V right from the battery to the coil "+ve" when the ignition is turned on.

More opinions?

The voltage drop sounds roughly correct to me. In most systems, the purpose for the ballast resistor is to lower the voltage to the coil during a running condition. The starting key switch bypasses the resistor when the starter is engaged, thereby keeping sufficient coil voltage. Perhaps going without the ballast resistor will work for you.

Good luck !
Edd Weninger

Ok I got my coil situation worked out, and the car once again starts. I did a little re-wiring and I'm seeing as close to 12V as I'm going to see at the coil, so I'm ok with that. The car still has some hesitance when starting, yet runs horribly rich when it does start. I'm convinced I have multiple problems here.

My injection doesn't have the cold start injector on the plenum, so I think the system just "wings it" until it's warm enough to sustain itself. There doesn't seem to be any rhyme or reason on which motors have the cold start injector, and which ones don't.

Anyone else running without the cold start injector system?

I may be wrong about this, but 50 psi seems a bit high. Shouldn't it be around 40 psi or so? What about a return line to the fuel cell?

I guess we need to know what FI system you are using.

Wayne Pearson

Rover 3.5L with Hotwire EFI with vacuum controled fuel pressure regulator. When I tested fuel pressure, the vehicle wasn't running, it was only while cranking over. I know the rover wants around 34-37 psi to run. Less then that and it dies. I'm not sure how more would cause trouble, but tonight I'll re-connect my tester and see what's happening while the motor is running.

Justin, root around in this website. Seems to have your symptoms mentioned.

Wayne Pearson

I am not sure about the 13CU system but on the 14CU which uses the 5AM series of Airflow meters, it is possible to test them with a simple meter - it's some years back but when I thought I had a problem someone here knew which pins to bridge and take a reading while adjusting it - the upshot was that a PO had wound the adjuster off the scale and it only needed winding back to its proper operating range.

If you decide to explore this and find the adjusting screw still has the maker's seal in place - take care -the usual method of drilling a small hole and winding the seal up with a self-tapping screw may damage the plastic internal gears in some versions of the Airflow meter - you need to find a way of pulling it out.

You mention the cold start injector - while the Airflow meter will affect the mixture while running and cause the engine to idle too fast - the computer may need to be 'told' that the injector is not present. I am sorry I do not know how that is done - although I do know that a single resistor is used to convey that information in some cases - the absence of an auto gearbox for example.

There will be someone here who has the answers,

Good luck


I've adjusted my afm to get it to the proper range, but to no avail. I think it's shot. Also, my airflow meter says it is a "3AM" model, not the "5AM" you mention. Could that be contributory to my issues? I don't have the road speed sensor hooked up, but I'm also wondering if that isn't contributing to my problems too.



I've used the 3AM and 5AM on my car, in the UK the only difference is that the 3AM is marked 'Hitachi' and the 5AM 'Lucas'.

I do not have the speed sensor wired in, it only restricts top speed, and stabilises the idle.

When my AFM failed the engine did run very this could be the problem.

I have the TVR service manual for the 14CUX, the test for the AFM is restricted to voltage test (0.3v-0.6v).

Have you checked the coolant Thermistor (temp sensor)this should read circa 2500 ohms at 20c (cold), this affects fuel enrichment also (and is cheaper to replace).

New AFM's appear on UK E Bay at considerably cheaper than retail.

Good luck,

M Barnfather

I will say you FP is way to high.. may not seem it, but it is very high, that will mean she is super rich. May seem like it runs fine, but that will explain yourpoor mileage and hard starting. Not sure about the stalling.

I am not familiar with the Rover EFI personally, so I will tell you what I would look for in a Ford EFI and maybe there are similar things going on, some have already been mentioned..

You said it is running rich all the time, that is definitely FP related. You should have ~2psi more with the key on than with it idiling. That is not an exact number but just a ballpark and is roughly where mine sits.. Don't underestimate the FP issue.. That is how many mustang guys tune thier cars and can net suprising amounts of power when done with timing changes. IT can even account for mods like heads etc to a certain extent.

Is you regulator mounted on the rail or remotely? Can you get a picture of it? Are you sure it has full vacum getting to it?? If you have a mityvac or some other vacuum creating device I would say turn the key on and hit it with 10-20" Vac and see if the FP changes. might be a bad diaphram in the regulator, or just a bad reg.. If that is the case it might be worth researching a new adjustable unit.

If you are rail mounted regulator and there is no adjustable one made, you could make a bypass plate and run one of the aftermarket stand alone units.

I would also check the return line and make sure it is unobtructed and does not have more than a couple psi backpressure.

Last but not least, if you have run her very long like this you may very well need to replace the O2's as if they get run too rich for to long they go bad and can no longer get a accurate reading..
Larry Embrey

If it's a vacuum controlled FP regulator, and I tested with the car NOT running, shouldn't I get those kinds of numbers? I need to test it with the car running and see what happens. The regulator is at the end of the fuel rail. I've never understood that. If there is 50-60 psi going into the rail, how is a regulator at the END of the rail going to lower that pressure? I'm running 3/8" fuel lines up to the motor, and if this pump is putting that kind of pressure up there, I don't see how a return line is going to let off enough fuel to lower the pressure to 37 psi.

I replaced the fp regulator 2 summers ago. I think an adjustible fp regulator for a buick grand national will fit on my system, but I need to confirm that.

Maybe I need a fp regulator before the fuel rail to give me the desired pressure?

Thanks for all the input!

Justin, your last post seems to indicate that you are not using a return line - true?? If so, then you're thinking along the lines of a carbureted system and not FI, where the pressure is regulated before the carb. In a fuel-injected system, if you regulate the pressure before the fuel rail, you may not have enough pressure to satisfy the injector at the far end, and you would still need a return line at the regulator.

Wayne Pearson


The fixed rate regulator should give 36 -40 psi with the engine running and plenum vac pipe removed , the TVR service manual shows the pressure gauge fitted at the pump end of the fuel rail, but I can't believe that there would be much drop in the rail.

I fitted an adjustable rising rate regulator, it improved pick-up considerably with the old analogue flap-valve system, but doesn't seem to have as much effect with the digital hot-wire system, I guess the computer over-rides increased fuel pressure.

M Barnfather

I think the Rover system works the same as the older Jags. There should be a coolant vac switch to control the fuel regulator for cold start. Low/No vac the fuel pressure is high and the engine is rich.
I also fought one of these after storage cars and found out the entire problem was old fuel.
A drain and refill with fresh fuel and a bottle of Chevron Techron changed everything. I've made that my first step since then and have found that it's the main problem with most stored cars.

Something to think about.



There are a few parts in common with the Jag (airflow meter for example), but the fuel rail pressure is controlled by a vac pipe into the pressure valve, this merely increases rail pressure when the accelerator is floored.

There are water temp sensor, and fuel temp sensors which influence start up injector timing, I have given the base resistance figures earlier in the thread, I have the full TVR data on resistances at various temps if needed.


My guess is a faulty/badly set airflow meter, have you read the voltages on the feed wires, and tried tweeking the adjuster to see if they change ?

Michael barnfather

This thread was discussed between 05/04/2005 and 22/04/2005

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