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MG MGB GT V8 Factory Originals Technical - Timing
|My 75 MGB has a Rover 3.5 bored +.020, Isky Racing Cam, Edelbrock 2198 Intake and Edelbrock 1404 (500CFM) carb and block hugger headers, with stock crank, piston and Petronics ignition instead of points. |
The problem is the engine is crapping out at @ 5000RPM.
I have the timing set to factory specs ( 6 degreesí BTDC) but I am wondering if this is correct?
Should any allowance be taken for the things I have done?
The distributor cap looks good to me, the fuel pressure is @ 6lbs and the carb was set up late last driving season with out any work being done to it over the winter.
The cam specs are as follows
Valve Lift INT. & EX. = .470
ADV Duration INT & EX = 270
.050 Duration INT & EX = 216 and
Lobe Center = 109 degrees
Any ideas would be appreciated
|How does it run with the air cleaner off? Try revving it up for a few seconds without the cleaner and see if the breathing's restricted. For many of us, it is, because of the ultra low-rise air cleaners we have to use. Most of us don't spend much time up at 5K rpm, though, so it doesn't matter. Rovers don't seem to care a whole lot about timing ... most folks I know including myself set it at 10 degrees BTDC @1000rpm, vacuum can disconnected, then forget about it.|
|Rover V8s run way past 5000 with no problem at all. Far too easily in fact, unlike the B-series where you have to be deaf or brutal to red-line it. In what way does it crap out?|
I havent tried it yet with the air cleaner off, but I took a look at the air filter today and though it is a little dirty, sunlight still easily penetrates it. I am running past 5000RPM when I am at the drag races when it becomes important to get the very last bit out of it.
In what way does it crap out? Tough question. It is actually crapping out closer to 5500, its like it has just run out of power. If I keep my foot in it, it is like there is a governor on it and it just wont go any faster. Last summer I was going through the 1/8th mile at about 71+MPH, and last weekend at only 67MPH, so there is something wrong.
I cant think of what it could be electrically, coil, but it isnt like a coil breaking down as it only happens at high RPMs. I thought my tach could be out, but it seems to be ok when idling or just cruising, RPM seems to match MPH. I have noticed my gas mileage has decreased. I keep coming back to carb/fuel pressure regulator.
|Bruce, for dragging, why would you rev it that high? Isn't the Rover's max torque at 3200 (+ or - for your bigger cam, etc.)?|
I don't know about the Rovers max torque at 3200, but I rev it that high because it keep pulling. When it stopped pulling I shifted. I can't add much to it except what I got from Isky's site for this cam RPM-Range (2000-6000)
|There are a few things that could be the cause:|
1) Valve springs weren't installed at the correct height (or aren't the Isky recommended springs) and simply can't close the valves fast enough (valve float). Lifters could also be pumping up and causing valve float.
2) Not enough air (already mentioned re: air filter)
3) Not enough fuel supply. Even though there is 6 lbs pressure at idle, there may not be enough at speed, or the line could be restricted somewhere (flat spot or kink, or clogged filter or tank pickup). Or maybe the fuel pulls away from the tank pickup under hard acceleration. Carb could be too lean on the very top end.
4) Simply not enough spark advance at higher rpms or weak ignition.
Ted NH - when drag racing, ideally you rev past peak HP (which will be way above peak torque) and shift to bring the rpms down to the same level on the front side of the HP curve.
That gives some things to check out. I'll get back to you.
|If there was an ignition LT problem I would expect the tach to get erratic when you 'hit the wall'. Dunno whether an Edelbrock choke is an air restrictor or a fuel adder. If the latter does pulling the choke make any difference? Can you tell the fuel is still at 6psi when it stops going? If not what is the fuel flow rate when the pipe is pulled off the carb and directed into a jar? The factory V8 with an SU pump should deliver 1pt in 30 secs or less, as a guide.|
|I have checked the car over and here are the results|
Fuel Filter- A little dirty and replaced
Fuel lines- All checked and are tight
Air filter is a little dirty, new one ordered
Distributor Cap looks fine, but new one ordered
Plugs NGK BPR6ES #1 was dirty and wet
#2-3-4-5-6-7-8- are all a tan colour. Plugs cleaned and re-gapped to .025
Timing is 6 degrees BTDC at 600RPM
fuel pump pumped 7 cups in 30 seconds
I am using 20-50 oil and was wondering if using straight 30w would stop the lifters from pumping up?
|I am no great fan of cleaning and reusing plugs, seems like false economy to me. The question I would ask is: why was #1 plug fouled? Had the plug stopped firing or was there some mechanical failure? With the fouled plug it is no wonder that your power is down.|
|The plugs only had one season on them, @ 5000 miles. And by cleaning I mean running through a wire wheel and then re-gapping them|
I donít know why the plug is fouled. I put #1 plug into #3 cylinder and will check it in a few days to see if it is fouled. The leads are top of the line and I canít see a problem there, but stranger things have happened. The only other thing I can think of is the distributor cap, but as I mentioned, I canít see any thing wrong with it, but have ordered a new one to be sure.
This thread was discussed between 30/05/2002 and 08/06/2002
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