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MG MGA - weber saga continues

Connected my vacuum advance to the vacuum port at the base of my carburator. acceleration greatlyimproved but now i am having an overheat problem. up to the connection of the vacuum port, no problem whatsoever with the temperature dropping easily when hitting throttle. now going to 230 and staying there even when turning on the electric fan. Could this be related to the vaccuum advance or totally unrelated? I am getting close to giving up since every time I think I can get a daily driver, something goes wrong. Attacked the overheating problem all last summer and had it licked but now after playing wth the vauum advance it is worse than ever. I had the car running with the new weber and no vacuum advance attachment with the last several weeks and had no overheating problem. please help.

Michael
michael dvorkin

Michael. Both over advanced and overly retarded ignition timing will cause an engine to overheat. With the vacuum advance connected, you will be running more ignition advance at cruise speeds. Perhaps retarding the timing a bit, since the car was running well without the vacuum advance being hooked up, might be the first experiment to try.

Les
Les Bengtson

advanced my timing and the overheat problem persists. I will next attack the thermostat and radiator. I think the thermostat could be stuck or the radiator flwo blocked. will flush the radiator and replace the thermostat with ta blanking plate which I have. Any other suggestions?

michael dvorkin
michael dvorkin

You said "Attacked the overheating problem all last summer and had it licked..."

I would re-read Les's post as he recommends retarding not advancing the timing.

I agree that if the car runs ok aside from acceleration w/out the vac adv hooked up, then you hook up the vac adv and it now overheats, the timing is now too far advanced.

You could always s%@t can the weber and go back to SU's but you'd still have to do some deductive thinking regarding the symptoms of the car.

Good luck.

JohnB

Michael. Perhaps I gave out too much information when I mentioned that both overly advanced and overly retarded timing will cause over heating.

The overly advanced timing has the engine fighting the early burning of the fuel and the expansion that takes place because of that. This causes the engine to work very hard and can, under some circumstances, cause piston damage.

Overly retarded ignition timing means that the fuel/air mixture is still burning when the exhaust valve begins to open. This is a less efficient cycle, requires the engine to work harder to produce the same power that proper ignition advance produces and results in overheating of the engine and can cause valve damage.

In your case, the addition of the vacuum advance may have caused your engine's ignition timing to be over advanced. This theory is based on the fact that, set up to work well with the vacuum advance disconnected, the mechanical advance might be slightly excessive, but, within the acceptable range. Add in 10+ degrees of vacuum advance and you could now be in a drastic over advanced condition.

Thus, as JohnB notes, my recommendation is that the mechanical advance be retarded by several degrees (start with five and see how that affects operation) and further tests performed to see what happens. The Weber DGV cars I have tested will show about 15" Hg of vacuum at a steady 2,000 rpm. This means the vacuum advance can is providing full vacuum advance, somewhere between 10 and 20 degrees, at steady throttle (cruising) operation.

Les
Les Bengtson

thanks for the advice. flushed the radiator and changed the thermostat to a blanking plate and the car is running fine. during install of carburator something must have blocked radiator flow or thermostat became stuck in a closed position. will retard the timing and start to drive it.

michael dvorkin
michael dvorkin

Michael,
Your car should run well with a thermostat, and will be cleaner and get better mileage too. Be careful that your electric fan does not cover too much of your radiator, they often block a lot of radiator in the center. If you re-core your radiator, be sure to get a high-efficiency core with 3/8" spacing (and 3 rows of tubes)for a total number of tubes about 120. Cores with 1/2" spacing that you normally see have only about 90 tubes, and seem to have less cooling capacity than the car needs. Stock OEM MGB radiators ('68 to '76) had more than 120 tubes in a staggered arrangement with 4 rows in the same space as the MGA core.
Matt
M. H. Dabney

Temperature problem now under control. However when accelerating in first, there is hesitation and a sort of backfiring at the weber carburator. It continues to accelerate and does well when rpm increase. no problems in higher gears.
Any suggestions

michael dvorkin
michael dvorkin

This thread was discussed between 21/05/2006 and 25/05/2006

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