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MG MGB GT V8 Factory Originals Technical - Edelbrock Re-Jetting
|I know very little about carbs and less about re-jetting them. It has been suggested that I should put bigger jets into my Edelbrock 1404. I also have an Edelbrock intake, high performance cam, +.20 bore and block style headers on my Rover 3.5. |
Why? I recently raced it at our local drags and was only able to reach 68mph in 11.2 seconds. I did have some problem with the point and have replaced them which has made a great improvement but I am unable to get an accurate mph until the next drags, in the interim period I have order a pertronix electronic point replacement. Apparently this engine should be getting to 60mph in about 6-8 seconds.
Any information about re-jetting would be appreciated
|Bruce, re-jetting is very easy. I figured out how to do it and believe me, if I can anyone can. First off, you can always switch to slimmer metering rods; it takes all of about 30 seconds to replace them. Similarly easy to replace are the springs ... if you have a cam in there you may have lower than standard manifold vacuum and so you'd want to switch to weaker springs so the metering rods sproing down to their cruise setting easily. Lastly, if you just can't get happy with these, you can always change the jets. It's easy but I found disconnecting the linkages -- which you have to do to get the top of the carb off -- to be daunting the first time. As a safety measure, I took a couple of shots of them with my digital camera beforehand, in case I couldn't figure out how to reassemble them. Actually it is _much_ easier than it looks. Disconnect things, remove the screws holding down the top of the carb, lift the top/float assembly off, and there you will see the jets at the bottom of the bowls, under an inch or so of gasoline.|
In order to save yourself flailing around, and possibly hurting the motor by running it too lean, you should get out MS Excel and see what the various jet/rod settings give you as far as effective area, both at cruise and at power. The _area_ for the fuel to pass through, and not the size of the jets or rods per se, is what you care about. It's not obvious, and small changes will cause big differences in the A/F ratio at given speeds. The only way I could figure out to really dial it in was to use both a vacuum gauge and an A/F meter, coupled with a printout of my MS Excel calculations. Sounds like a lot of gear, but it's extremely easy.
The springs are key! One thing you should do just to make sure you're in the ballpark is, loosen one of the ear things above a metering rod, turn it sideays, and simply watch if the rod is sucked down at idle. Then it should jump up for an instant when you blip the throttle (it will then go back down as vacuum stabilizes). If it isn't down at idle (=high vacuum), you need a weaker spring. If it doesn't bounce up for a second when you blip the throttle, you probably need a stronger spring.
Nevertheless, you have an awful lot of carb for a 3.5. I mean just run the #s:
CID x Max RPM / 3456 = CFM @ 100% Volumetric Eff.
I don't know if .030 over changes the displacement; if so you just plug that #. 5200 is your redline, according to the factory. I don't have a calc right now but you can immediately see that this is less than 400 cfm. Combined this w/the fact taht most street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, good intake and carburetor can achieve about 85% VE; call your VE 85% and still you have much too much carb. If you can get hold of a Carter 400 (same basic carb, only smaller primaries), you may find that things tighten up a good deal. I say same, but NB the Edelbrock and Carter rods are not quite the same ... not interchangeable.
This thread was discussed on 06/07/2001
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