MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - MGB Slow to rev up

My GT has what appears to be a genuine 69000 miles since she was registered in 1981 to the one previous owner and has been regularly serviced. I have checked timing by strobe, points by dwell meter and emissions by co measurement and compressions are even across the cylinders at about 155psi. Engine is slightly noisy from the rocker cover with most of the noise from number 2 valve. Re checked clearance with click adjuster. From cold she starts easily and ideals correctly but has very little power and "pops" a lot when accelerating. When warm “popping” stops but engine remains very slow to rev up with lack of power across all revs. The points, rotor, cap, plugs and leads have been replaced with little if any change.
D Middlewick

Being of pragmatic nature, I would just advance the timing to pink and back off a couple of degrees. It sounds as though your timing is retarded.
George B.

I have recently learned from reading this BBS that some popping and backfiring can be caused by a loose exhaust manifold as well (after ruling out bad gulp valves and such in the ones equipped with US emissions).

Not usual to have "multiple simultaneous failures".
John Z

Thank you for the help. Reset the ignition by ear to max revs and backed off a bit. Strobe indicates that to be a bit over 40 degrees BTDC! Engine now revs with just a bit of pinking on acceleration in third gear. Engine still mechanically noisy from tappets but now fun to drive.
D Middlewick

I've got two cars, one with HIF4's and the other with a Weber DGV, both with Eurospecs, and they both refuse to be timed anywhere except out there around 40* (it's really hard to tell out that far in front of the timing marks). I use 93 octane, set the dwell to 60*, set the fuel mix by the lift pins, time at 600 - 1000 rpm with the vacuum advance hose removed, valves set at 8 clicks with clik-adjust, use K&N's, use 40kV coils, one car with platinum plugs the other normal plugs, one car with stock exhaust the other with big bore -- as far as I know, that's all OK and shouldn't cause this.

I finally just gave up on the timing light and use the manifold vacuum technique recommended by one of our resident experts (probably Bob M or Les Bengston or Paul Hunt -- thanks fellas, you've been a big help to me). That seems to work pretty well. The car starts easily and runs smoothly.

Does anyone else have cars that need so much advance? If so, does anyone have an explanation of what's going on?
Larry Bailey

In my opinion you folks don't have that much "real" advance. Betcha dollars to doughnuts your harmonic dampener has delaminated and slipped, thereby throwing off your timing marks.
D Maples

You are right to go for ignition first - and checking the TDC mark really does cpoinmcide with TDC is also a good thing to check as D Maples implies - but you say nothing about carburation. Low oil levels in the dashpots can cause poor acceleration, as can vacuum leaks at the gaskets and elsewhere on the inlet manifold (check the welch plug at the back is still there), and general carburettor maladjustment can cause other things too.
Paul Hunt

D Maples: it's a good thought, but I've checked the dampener. And the alignment of the cam to the crank (I think one tooth = 18*).

Re Paul's comments: my understanding is that leaner mixes burn slower. To compensate, the timing has to be advanced. The only thing I haven't checked looking at this is the needles. I've got ABDs in the HIF4's -- I need to call Joe Curto and get some AAAs. I don't think I'm over-lean, though. In the times I have been I've gotten popping whenever my foot is off the gas, and that's not happening now.

Still, the other car has a Weber, and I've got the same situation, so that kinda rules out the needles and welch plugs.

I mean, the only thing in common between these two cars is me. It's got to be something I'm doing. I wish I had a specific question to ask. I guess I'm just wondering if there are other people out there who find that their cars are needing to be timed way out in advance, or if there are folks who have experienced this, and then identified the mistake in their tuning procedure, and now can time cars down around 11* - 15* @600-800 rpm w/vac adv hose removed.

As it is, my timing marks are only little pointy things that are fun to paint white.

Thanks all.
Larry Bailey

Sounds like you've covered the bases. You like the kind with sprinkles?
How about a timing cover that has been swapped with a different model engine?
D Maples

Working on a '76 the other day that son Brian says is the slowest MGB in the world.
Someone set the timing with the vacuum advance connected.
At 5 degrees BTDC. Spec's. call for 13 BTDC degrees at idle, with vacuum disconnected and plugged.
This essentially had the timing at 5 degrees after top dead center.
NO WONDER!
What a change. And once again, the blame was put on the poor ol' carburetor. And of course, it's a SZ. So it must be bad, right?
The valves were a little tight, but everything else checked out OK. It now starts right up, idles spot on, and accelerates as good as a SZ car can.
Always check specifications first, then jump to conclusions.
It's how I get my exercise.
[that's a joke]
Safety Fast
Dwight
Dwight McCullough

I had a thought last night: I read in a thread in the past week that the dwell on a 45D should be 51*. If I'm not mistaken, dwell affects timing. Is it that a change in dwell will result in a change in timing, or that having the wrong dwell will necessitate compensating through timing? In my case, a common factor between these two cars would be that they both have 45Ds set to c.60* dwell.

I think I'll slip out to the garage tonight and tweak the dwell and see what happens, though I really hate setting the points on these distributors. Really makes me miss the old Chevy I drove in high school... until I remember the over all driving experience.
Larry Bailey

Changing dwell or gap *does* change timing which is why it should always be done first and the timing checked after. If the gap is reduced/dwell increased the fixed contact is moved away from the cam so the points heel is higher up the cam, i.e. a later rotational point before the points open therefore retarding timing. Increasing gap/reducing dwell does the reverse.
Paul Hunt

Good, thanks for the confirmation. And, just to make sure I have this all straight, what IS the recommended dwell for a Eurospec 45D? (I have one on an 18V672 and one on an 18V797, in case there's a difference.)
Larry Bailey

Mr Haynes says 51 deg +/-5 for a 45D
Dave Smith

Oops. RTFM, Larry. Thanks, Dave.
Larry Bailey

OK,

I set the dwell to 51*, and was able to roll the timing back to 14*. The car runs ok, but I still lost about 150 rpm off the idle. I can't really feel much difference in the car's performance. I mean, it's slow -- my wife's 940 Turbo wagon can absolutely blow the doors off of it -- but at least I don't think it's any slower than it was. If I get a chance, I'll go back now and double-check with the vacuum technique.

Thanks all.
Larry Bailey

This thread was discussed between 27/08/2003 and 31/08/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.