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MG MGB Technical - hard cranking/backfiring race car (long)

Ok this isn't exactly MG related but your minds are some of the best on the internet and have helped me with many of my MG issues so now I have a Capri Race Car problem

'74 Capri with 2.8 V6 mild cam (don't know the specs)

We got it running on a stock distributor at about 16 btdc @ 900 rpm. Seemed like a lot of advance to me but that is where it was happy. Night before we had to tow it out of the shop we decided to try our newly rebuilt mallory dual point dizzy. Popped it in and got it to fire up, but the timing was looking erratic on the timing light. We also had starting issues. With the ignition switched on the starter would engage and crank once then stop like the motor was holding it back, almost backfiring. With the ignition switched off it would crank just fine on the starter. But with it on it struggled and making loud clanking noises when it stopped. You could get it to start by giving the carb a few squirts of the accelerator pump, then it seemed to turn over more quickly and fire up. Battery and wiring were double checked and looked good. We decided to go back to the old distributor, a known quantity, but when we put it back in we had similar problems. We got it timed to about 14 btdc (tried it at 16 and 10 with the same results). Ignition off it cranks fine, ignition on it slows way down and struggles (also sounds a little strange)unless you give it a few pumps of gas first then it will start. I tried by passing the ignition switch altogether and running 12 volts directly from the battery to the coil, and the problem stayed. Also with the ignition on if you disconnect the coil wire it cranks just fine. Also tried swapping coils. Problem still there. Its bothering me because prior to swapping distributors it worked great. It seems like it is timing related but we tried the same timing setup that we had it running great on yesterday and it still is having issues.

Sorry for the long post but I have 3 days to figure this out before our first event and would like to get it sorted.

thanks in advance

-Bill
Bill Mertz

Bill,

I don't know anything about that engine, but does it have adjustable cam gearing, and might it have slipped? I bought a BGT that had the ignition timing about 20 degrees off, and it turned out that a custom cam had been installed without timing it.

Did you put the engine together yourself? If so, how was the cam timed if you don't have the specs for it?

Charley
C R Huff

Could you have 2 plug wires crossed? The only thing that's common here to both distributors is moving the plug wires from cap to cap.

Wayne
Wayne Pearson

Unfortunately I did not put the motor together.
So cam timing is a mystery. They do have gear driven cams and I was thinking maybe it jumped a tooth or broke a tooth off the cam gear. I'll triple check the wires and firing order. All the research I've been doing is telling me this is a symptom of motors with too much advance so the slipping cam timing theory seems pretty good.
Bill Mertz

Is it the Cologne engine? If so, from memory (I had a 2.8 Granada) then the gears are plastic. If a tooth has gone, the timimg would move a tooth every revolution and hence get worse and worse until the pistons hit the valves???

Kicking-back, resisting the starter, is an ignition timing problem: way too much advance. Are the leads on right? Are you timing the correct bank of cylinders? Is the distributor in the right way around?

I somehow managed to get the GT running with 45 degrees of advance. It is amazing how much advance the engine will take at idle. With a V6 then the intervals between firing is 60 deg - so perhaps your 10 degrees BTDC is actually 70?

HTH,

N
Neil

Good thoughts Neil. It is a cologne motor. It actually has a replacement aluminum timing gear but that doesn't mean it couldn't chip. Fortunately I don't think it is an interference motor so no valves in the pistons, but still I don't want it to get any worse. I'll have to deal with it track side on Friday.

If any one cares here's a link to a write up of the car:
http://jalopnik.com/5258025/whores-poker-and-thrown-rods-get-ready-for-the-24-hours-of-lemons-nevada-style

We are running in the 24 hours of lemons and I talked to another local racer who will be campaigning an MGB. I'm hoping to lend him a hand when I'm not working on my own jalopy race car.

-Bill
Bill Mertz

Good luck at the LeMons, Bill.

That's a pretty nice looking ride for $500 ... hope they don't vote you into the destruction booth.

I agree with a lot of others said about the ign timing. With a V engine, it would be pretty easy to hook the timing light to #2 thinking it was #1.

As to cam timing, you could probaly get a pretty good idea if it is way off by pulling a valve cover and roughing out how far past TDC the intake is full open.

Be sure to let us know how it works out.

Charley
C R Huff

This thread was discussed on 20/05/2009

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