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Spitfire ....SPIT-BANG..

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BOXoROCKS

Jedi Knight
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...I have a nice Spit, a 1980, original in most every way. 39K miles, with ALL emission equip. Here is the problem. It runs fine, but on the upshift into the next gear at appx 2500/3000 rpms when I take my foot of the gas BAMM out of the muffler, and I mean LOUD. If I slooowly release my foot off the gas on the upshift..no bang or very little. What gives, is it the diverter valve, can I just disconnect the air pump to alleviate this? I am afraid it will blow the muffler apart.People think I am shooting at them. It is obviously getting unburnt gas in the exhaust system,but why?
 
Don't know much about late Spits, but most US-spec cars of that era had a device to either pass neat air into the intake manifold, or hold the throttle open, thereby limiting intake manifold vacuum. I'd suggest looking for that device, and make sure it's working as intended.

On some ZS carbs, it's a smaller spring-loaded disc built right into the throttle plate. If memory serves, MGBs had a "gulp valve" which was just a small metal can with 3 hoses mounted to the intake somewhere.
 
Years ago I had a '79 Midget (same engine as the '80 Spit, I think) that did the same thing. The real cause may be in the gulp valve but I disconnected the air pump and it stopped.

Bryan
 
..lets hope you guys are right, the 80 Spit has no gulp valve, it has a diverter valve. It should be about the same as a 79 so I will disconnect the air pump. Should I plug anything up? It sounds like a 20 Ga going off
 
If you disconnect the air pump you should remove the air rail to the head and plug the passages that lead to the exhaust ports. Then remove the pump. No sense carrying that lump of metal around. I don't know if it will fix your problem but you should gain a couple of horsepower, and you will lose a couple of pounds.
 
Before you go removing that air pump and all related connections you might want to look into your local county emmisions testing proceedures. Some counties here look at everything and even have books a diagrams of what should be in the emissions circut, if any of it is missing you get a NO GO.
Were it me, I would disconnect the belt to the pump and give it a test, see if it quits exploding in the exaust.
If your emission laws are such that you have to have the emissions equipment on the vehicle to pass inspection, Especially if your county uses the sniffer method! You are pretty much stuck with fixing the problem whatever it may be.
Just F.W.I.W.
 
:iagree: Don't forget that emissions laws can change, and they might start requiring inspections next year.
 
...I haven't had a legit license plate in 20 years, I should care about emissions???
I did just pull the belt, will try it this week.
 
BOXoROCKS said:
..the 80 Spit has no gulp valve, it has a diverter valve. It should be about the same as a 79 so I will disconnect the air pump. Should I plug anything up?
Click to expand...
If you are going to test by disconnecting the belt you shouldn't need to plug anything, but you do need to make sure your air-pump check valve is working properly, so that hot exhaust gas does not blow back into the pump when its not running. Disconnect the hoses to and from the check valve and make sure it will pass air in only one direction (i.e. from the pump to the injectors).

If you decide to remove the pump to test, then you need to make sure you plug the air injector holes in the manifold, otherwise hot exhaust gas will blow out of them.

If all this doesn't change the problem, you might want to check your distributor and make sure it advances and drops back properly. Also check the bypass valve on the carburetor see if the diaphragm has been torn.
 
Late MGB's had an anti-backfire valve; when they were not working it produced a backfire as you described. When they were working, no backfire. If your Spit is totally complete and you may want to isolate the problem and keep all the "junk" for the sake of preservation.
 
and the bolt size to plug that hole in the exhaust manifold is a 3/6 16 - 1".
 
use a set screw it looks cleaner
 
You might want to check for a leak near the manifold first, a blown gasket just after the header will give the same symptom. One way or another you are getting oxygen in the exhaust.

Steve
1971 spitfire MKIV "Jehu"
Lancaster PA
never underestimate the perversity of an inanimate object.
 
How good are your MPG's. I had a similar problem with an all stock '76 Spit. At night when I let off on the gas besides the big kapow I also had a big ball of flame out the exhaust pipe. My MPG was around 18/20 so I blamed it on the POS Stromberg. Unfortunately my daughter totaled the car before I ever got around to curing it. (No she wasn't hurt). Bob
 
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