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MG MGB Technical - New enginehas power but diesels

I just tuned my new (rebuilt) 18GK engine and it has LOADS of power. It will tear the tires loose relentlessly in first and second gear, accelerate well past 70 in third, and has phenomenal throttle response in 4th, but... it won't shut off without a drop of the clutch. I've read as many archive threads as I can find. What I've found is that with this power of an engine, I need a better distributor. I currently have a modified 25D4 with Crane electronics. Also a degreed Crane cam, ported aluminum head, 10:1 compression, .020 over block, Twist rebuilt SU's with modified throttle shafts, custom mixed ratio aluminum roller rockers, wrapped headers with an Ansa exhaust...
Am I just being stupid by not buying a new distibutor, or is there something I'm missing? My static timing is steady at 16 degrees, a 7-13-5 vacuum advance and 18 degrees of total mechanical advance. HELP!
Jeff Schlemmer.
Jeff Schlemmer

Jeff-
I forget what year MGB you've got. 10:1 compression ratio is pretty high for a street machine and that is almost certainly involved with your run-on problem. As for a distributor, its possible that the ignition settings and advance curve could also be contributory factors. Aldon makes several versions of its high performance distributors. They have versions for HS4 both HIF4, manifold vacuum, carburetor body vacuum, no vacuum, etc. They have a website at http://www.alonauto.co.uk/ If your car had an anti-run-on valve system you should reinstall it. If it didn't, put one in as the engine can't run on without fuel in the floatbowls.
Steve S.

Thanks Steve. My B is a '71 and since I've owned it has had no smog equipment, run-on valve, nothing. In fact, before the rebuild it NEVER had a run-on problem. Maybe that's why I directly associate it with engine timing. I am running premium gas and an aluminum head, so I didn't figure 10:1 was too extreme. The engine temps are normal and spark plugs are burning normal. Wouldn't adding a run-on valve be treating the symptom rather than the problem?
Jeff.

Jeff Schlemmer

Jeff-
The problem is the obsolete kidney-shaped combustion chambers, the 10:1 compression pistons and the fact that today's high octane fuel is oxygenated (making for a hotter burn) and at best has a rating of 95 Octane. The original specification was for 98 Octane with the original compression ratio of 9:1. The aluminum head has less thermal efficiency than the cast iron one, forcing a higher compression ratio to obtain the same power output. Unfortunately, this requires higher octane gasoline than you can get at a gas station. If you have access to racing gasoline, try it. Otherwise, it's possible to minimize the problem by using the right ignition curve and by keeping the ignition and carburetion adjusted spot-on, but you'll still need the anti-run-on system.
Steve S.

"If you have access to racing gasoline, try it"


CAM2 might work for you. If you have a race track nearby [DIRT, or paved oval] you can probably locate a gas station that supplies it. Some restrictions may apply [one place I went to, the gas attendant had to pump it, and it's a no-no to fill portable fuel cells]...

G'luck!
Craig Quenneville

Thanks. I have a NASCAR track a few miles from my house. I'll have to give them a shout and see if they'll part with some fuel. Otherwise, there is a small airport using airport transfers tunbridge wells nearby...
Jeff Schlemmer

Jeff. The airport is the solution I used many years ago when I had a Corvette. The only thing it would run on, after the 74 "fuel crisis" was the 100/130 octane avgas. Made travelling a bear. One thing you might want to check is your idle rpms. As the engine wears in, the idle rpms creeps up. Sometimes, lowering it will cut down or eliminate the problem with dieseling. Les
Les Bengtson

Hello Jeff.
Get your distributor re-curved on a rolling road, just retarding the timing at idle will reduce power in the mid and top end, try fitting a 82 degree thermostat this will tend to surpress detonation and give a bit more power. Alternativly use higher octane fuel or octane booster.
Hope this helps.
Steve Deakin

Thanks for all the advice. I've lowered my idle speed, added octane boost until I can get someone to sell me better fuel, and I just ordered a new distributor from Bob at Brit-Tek. I think I love Bob (in an MG kind of way only.) He always has the answers.
Jeff.

Jeff Schlemmer

This thread was discussed between 14/09/2001 and 17/09/2001

MG MGB Technical index

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