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Thread: 74 1/2 Timing Specs.

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    Luke Skywalker vette's Avatar
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    74 1/2 Timing Specs.

    Does anyone have the timing specification for a '74 1/2 MGB. Engine # 18V837z. Can't find this engine number in any of my books. I'm guessing it would be about 10 degrees BTDC. I'm just guessing that from what i see on some other 'B' engines. Would like to know for sure which pointers are what as well. Thanks for the help. Dave.
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    Re: 74 1/2 Timing Specs.

    Dave -

    When the pointers are visible from above, the one on the far right is supposed to be TDC; each one to its left marks 5 degrees. Timing at ~900 rpm should be about 10. And then if you have a dial-back timing light, run the rpm up to around 3500 (which should be total advance), and look for 32-35 degrees. Pretty sure this is correct for your engine.


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    Re: 74 1/2 Timing Specs.

    Thanks Mickey, I came to the same conclusion. my timing pointers are on the top right quadrant of the pulley. I have 5 tits on the pointer grouping. i guessed that the middle one was 10* BTDC. I have ended up with the timing set at 8* BTDC with the idle at about 800 rpms. If I bump the idle up to near 1500 rpms then the timing is at about 10*BTDC. It seems to be running well with it set like this so I believe I will leave it this way for now.
    I bought this 'B' about 1- 1/2 year ago with the intent to restore it and flip it or to restore it and keep it. Just when I was about to start on the restoration I had a little mishap with an older Vette that I own and I cracked the front end, rather severely. I just spent most of last year putting the Vette back together. So now I can start on the 'B'. But, I drug it out from its hiding place about a month ago and drove it a few times that i have been having a ball with it. So I believe I will drive it some this summer and start the restoration in the winter. So to get it to go some this spring I had to do a little brake work, then it was hard starting so I did the ignition over, then cleaned and adjusted the carbs. I must admit that the HIF carbs have been a little frustrating. I couldn't keep them from being overly rich. I set them up according to the book, you know, throttle screw about 2 turns down to start with, then the jets run up to their highest point then bring em down till they start to move down then 2 more turns. The engine started and ran pretty good so I comenced to try to get the mixture as best as I could but they just didn't want to respond in a satisfactory manner. I adjusted the jets with a good tach and set them to get the highest rpms possible but the engine just seemed to act lean when I lifted the pistons. After driving it for two days, the spark plugs were so carbon fouled it started to miss. At one point I couldn't get the idle speed to come down much below 1200 rpms. That's when I decided that I needed to know more about what the timing should be. When I rebuilt the ignition system I static timed it to what I thought was about 10 degrees BTDC. Well I did have it to far advanced. So as per what you said I did set it at about 8*BTDC at about 800 rpms. I was able then to adjust the carbs pretty good. the plugs look a lot cleaner. But when lifting the pistons it still wants to almost stall too easily. But any adjustment towards richening and it wants to go too rich too fast. Well if it keeps running as good as it did today ( drove it about an hour today) then I will have some fun with it this summer and figure the carbs out better when I restore the car. So thanks for the help. Dave.
    About TV Shows-
    "...you really can't restore a car in 10 days. I don't want to watch a race where people have to restore it in a week. It's not going to drive, and those cars never work. In real life, it takes years to get it right. " Jay Leno.

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    Re: 74 1/2 Timing Specs.

    Glad I could help! The HIF carbs, once dialed in properly, are great. Sounds like you're on the right path and you're really close. There are different needles available, if you want to go that far with tuning them. Joe Curto is your guy for parts, rebuilding services, and tips: https://joecurto.com/

    Mickey
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    Re: 74 1/2 Timing Specs.

    I'll second the Curto recommendation. Been dealing with them for decades now.
    '64 MGB, '67 Lotus Elan S-3 DHC,'69 Lotus Elan +2
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    Re: 74 1/2 Timing Specs.

    Minor confusion - I always was under the impression that the carb needles were tapered toward the tip, and that lifting the carb pistons actually made the mixture go rich not lean. Or I'm an idiot which is also possible...

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    Re: 74 1/2 Timing Specs.

    Yakko, you are correct but missing the point about tuning them. The needles are tapered towards the tips. but when you adjust SU carbs one of the test , which is discribed in the book better than I can do it, is to lift the piston either with the small lifting pin on the body of the carb or with your finger or a screwdriver into the venturi opening. Also know that even that the jets are tapered, they are relatively flat for a distance at the butt end. If the jet is set too lean and you lift the piston, only a fraction like maybe 1/16th inch, momentarily the engine gets more air than fuel and the engine will want to stall or at least slow its rpms. If the jet is set too rich it will also want to stall. But if the jet is set just right then when lifting the piston the rpms will rise just slightly maybe 2 or 3 hundred rpms then settle down without wanting to stall. This is a test on all SU carbs that I know of since I have been about 16 yrs old. I am now retired. But, unfortunately this test is not always conclusive and sometimes you can get the jets right where they should be but the engine will not respond to this test correctly. In those cases from my experience when the test is not responding correctly they usually will try to stall the engine. It is my opinion that the best way to set the mixture strength , the jets, is by using a quality tachometer and turning the jet until you get the highest rpms possible. This means the cylinders are getting the best mixture they can. Then of course if the rpms are too high for a proper idle you just close the throttle plates a wee bit. Take a look at the shop manual on adjusting the jets on an SU carb. It will explain it much better than me.
    About TV Shows-
    "...you really can't restore a car in 10 days. I don't want to watch a race where people have to restore it in a week. It's not going to drive, and those cars never work. In real life, it takes years to get it right. " Jay Leno.

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    Re: 74 1/2 Timing Specs.

    Another factor in HIF SU's is the "floating" needles. Over time and use the needles wear into the jet openings and can create an oval shaped orifice. Then setting the carbs becomes nearly impossible. That's where Joe Curto comes in. New jets, new needles.
    '64 MGB, '67 Lotus Elan S-3 DHC,'69 Lotus Elan +2
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    Re: 74 1/2 Timing Specs.

    Thanks for the clarification - I've done this process a long time ago (and hopefully will again if I ever get the B assembled enough to be able to) but didn't fully understand what was happening and why...

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    Re: 74 1/2 Timing Specs.

    In over 60 years of driving cars with SU's/Strombergs, I never had the mixture do what the book says when lifting the piston. And you know what? With the exception of the Twin-Cam MG, I never had any carb trouble once the choke cable, idle screw and synchronization was properly set up. I've said it before and I'll say it again - set it and forget it. Stop fiddling with the carbs looking for the ultimate. If it runs good and the plugs are close to the right color - leave it alone!

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    Re: 74 1/2 Timing Specs.

    Old adage: "Carburettor is a French word for 'DON'T TOUCH!' "
    '64 MGB, '67 Lotus Elan S-3 DHC,'69 Lotus Elan +2
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    Re: 74 1/2 Timing Specs.

    Quote Originally Posted by pdplot View Post
    Stop fiddling with the carbs looking for the ultimate. If it runs good and the plugs are close to the right color - leave it alone!
    pd, I don't think you have had the miraculous opportunity work with the Wonderful SUs of the emission era. It truly will send you thru an epiphany. I am attaching a pic of the B's spark plugs for you perusal while we discuss this. The carbs are HIFs. The wonderful invention of the emission era.
    These HIFs have a bi-metal piece inside that thru the enlightenment of engineers at the time decides to lean the jets relative to Temperature. Now I don't think that I am overly tweedly when it comes to carburators, but these bimetal pieces have a mind of there own. So here's the story.
    These carbs were very rich when I bought the car. So I leaned them up a bit and the car ran fine. This spring when I dug the car out, the carbs were rich again. So I gave the car a complete tune-up
    including points, condenser, rotor, cap and timing. Then also readjusted the carbs. Previous to this the plugs showed a lot of carbon. I didn't even clean them when I did the ignition. After adjusting the carbs, the car ran so well that it even cleaned the plugs and they look as they do in the pic. So the other day I drove it to a friends place about 8 miles away confident that the tune was just right. I ran beautifully. My friend and I where standing in his driveway and the B was idling next to us. Idling well I must say. As we talked we both looked at the car, then at each other and thought what is going on with the car. All of a sudden from perfect idle it started loping, slowing in idle speed, acting rich, and puffing a little black smoke. We looked at each other and shook our heads.
    So drove it home which was at moderately slow speeds since I was driving thru subdivisions, the 8 miles to home at slow rpms certainly would have given it time to load up also. When I got home I pulled the plugs and the pic is what they looked like. They don't look carboned up to me. I think they look pretty good considering the gasoline we have. So what's going on? Me thinks it might be that little dastardly emission era bimetal piece.
    If your not familiar with it, the HIF carb doesn't have a jet sticking out the bottom. It doesn't have the jet adjusting nut on the bottom. The jet is adjusted by turning a screw that is in the side of the carb horizontally. And the screw doesn't act directly on the jet. It pushes on an "L" shaped bi-metal piece which when the screw is turned in it pushes the jet up, and pulls it down when turned out. But the "GENIUS" to the bi-metal strip is that as the strip warms up it pushes the jet up there-by leaning the carb. Therefore as the engine warms it runs leaner and reduces it's noxious stuff. Isn't that wonderful. But the bi-metal piece has a mind of its own.
    Now here's my theory. When I have been adjusting the carbs the engine is probably pretty warm. I mean, my gosh, isn't that what your suppose to do when adjusting carbs make sure the engine is warmed up? So as I am adjusting the carbs the bi-metal piece is probably in its lean state, if it is working as it should (who knows). So I get it adjusted up to running pretty good and when the bi-metal piece goes back to its rich state, it is too rich. Or something like that. Or maybe that piece is just moving around at will and changing the jet. I looked at another HIF carb the other day to see what this thing looks like inside. If I do take these carbs apart, it will be for the purpose of trying to eliminate or reduce the influence of that bi-metal piece. I may solder the two bi-metals together to eliminate their movement. So that's my story. Dave.
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    About TV Shows-
    "...you really can't restore a car in 10 days. I don't want to watch a race where people have to restore it in a week. It's not going to drive, and those cars never work. In real life, it takes years to get it right. " Jay Leno.

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    Re: 74 1/2 Timing Specs.

    My TR6 is a 71 without all that s##t so no metallic strip. I have no knowledge of the later carbs nor do I care to learn. I'll leave that to the experts on this board familiar with those pesky carbs. I do remember that my 1951 MG TD would change idle speed overnight, either up or down, perhaps explaining why owners were always fussing with them. My two later TDs did not, nor did my TR3 and TR6. Your idea to solder the strips together is probably worth a try if it doesn't affect your emissions (assuming you still have to test a car that old - CT does not). Then, with luck, you can "set and forget".

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    Re: 74 1/2 Timing Specs.

    I also have a 74 MGB, what a headache, But as far as timing I set mine at 13 degrees, BTDC at 1500 rpms

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    Re: 74 1/2 Timing Specs.

    Mike, thanks for the input about the timing. But I am curious why you call your B a headache. I don't really consider this B to be a headache but yes these HIF carbs are fitful right now. This B really runs and works well even tho it can rightfully be described as a Barn Find. But the carbs won't hold an idle after it is run hard.
    About TV Shows-
    "...you really can't restore a car in 10 days. I don't want to watch a race where people have to restore it in a week. It's not going to drive, and those cars never work. In real life, it takes years to get it right. " Jay Leno.

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    Re: 74 1/2 Timing Specs.

    Dave, just go ahead and lock down the bi-metal compensators. They're a half-arsed engineering bodge to begin with, to try and make 'em emissions legal for our market. Like adding the third windshield wiper to meet Fed requirement for swept area. Fairly sure they're why later cars got the Stromberg carbs as replacement to the SU's (not that THAT was a big improvement!).

    On the B's I've personally owned, I've fitted a 40 DCOE Weber.
    '64 MGB, '67 Lotus Elan S-3 DHC,'69 Lotus Elan +2
    '78 Alfa Romeo Spider-undergoing surgery: O=\*/=O
    '84 300D Turbo-"Diesela"-Now my Daily: Oo|≣|oO
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    Re: 74 1/2 Timing Specs.

    Well I just got my 741/2 and the fuel pump has got surging power to it and not enough fuel, the bushings in front end all shot, missing door handle knobs, rat ate the wires up under the dash, for the fan and fan relay, idles rich cant get below 1100 rpms. radio also ate up, mouse **** galore under dash, brakes stick have to put your foot under it and pop up so you can take off again just a headache for now.

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