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MG MGB Technical - Tuning from Scratch
So I am setting up my car after a couple years. I fitted amongst other things, twin su's (K&Ns w/ proper needles)and a peco exhaust to my 1979 B. First off I had some oil leak out of the valve cover gasket and onto the exhaust. When I started her up it was smoking and burning off. Then I look and my headers turned black almost immediately. Then afterwards I was looking at exactly what took place and it looks like oxidized? It looks like it has a layer of rust on them. The middle pipe to be exact. Has anyone had this happen?
Now to the real question.
Does anyone have any advice for getting the timing set and tuning the carbs? I am fighting a battle of advancing the timing by sound to a pair of carbs that are not setup. I am doing that because the engine is shaking a lot when the advance is turned down. I checked with a strobe and its about 20 Deg.and more where it seems to like it. But if I have it there and take out the idle screw all the way on the carbs it stays around 1200-1000rpms. I just had a go at it for about 15 minutes to get it somewhere better than when I first started her up. I know how to tune the SU's. Im not an expert, guru, etc but I can get them running good. I know what I am and that is, looking for some advice to a systematic approach.
|James: You never, never touch the carbs until you have adjusted the valves and made sure the dwell on the points and the timing are set. The old adage is that 90% of the problems with SU carbs are Lucas distributors. Your best bet is to send your distributor to Jeff Schlemmer of Advanced Distributors to let him rebuild and recurve your distributor. He will tell you what your timing needs to be. Then make sure the dampers in the carbs are filled correctly with engine oil, then start adjusting the idle to get equal airflow through both carbs followed by adjusting the jets equally on both carbs to get highest idle speed and vacuum and readjust the idle speed.|
|You set the timing to book measurements, then set the carbs by ear, in that order, and that is after doing the valve clearances, plug gaps, dwell, and sorting everything else that might be wrong.|
If your engine is running *at all* with the idle screws backed fully out either the interconnecting shafts are mal-adjusted (one carb fully closed holding the other open), the throttle cable doesn't have the required free play, the fast idle screws are holding the butterflies open (maladjusted or choke pulled), one or other butterfly isn't seating properly in its throat, or just possibly the throttle spindles and bushes are so badly worn that mixture is getting past the fully closed butterflies.
An engine can only run when it is getting a fuel/air mixture from somewhere.
|Ah thanks Paul. You fired off the right neurons in my brain. I dont think the throttle cable has the required free play as I just installed a new one. I havent had time to work on it since and I thank you for helping me save time (hopefully). I will let you know how it works out|
|If in doubt, look at the thing you last worked on :o) BT, DT.|
|I would start with the timing set static to 10 BTDC then balance and set ther carbs at that. You can then try a bit more advance and re-set the carbs until you are hsppy with it.|
|Hey so here is the quick update. I got to the car for just a little while this memorial day weekend. It is not the throttle cable. The reason I suspected it to be was because one of the SU's throttle linkage when fully closed is in a different than the other. I just balanced the carbs and started seeing if I could set the mixture. I was doing the whole lift the piston up with a screw driver, adjust accordingly and it just didnt seem to be working for me. I have done this quite successfully on my Tr7 with dual HS6's... I have a colortune and had that hooked up before but it really wasnt doing any good. I am going to go back at it with a fresh start. I need to look back into my emails as the guy I purchased my su's for the B had put on k&n filters for me and said he was fitting a certain needle to better suit my 79 B.|
|Oh and it had a 2" exhaust. I know everyone stays with the smaller diameter unless supercharging. It sounds gnarly though... Also the exhaust ports do not match up exactly on the exhaust header. I used engineers blue to check. I take it that these can be additional factors. Your input? Or am I just bored here at work, barking up the wrong tree ?|
|The steel header will not win any beauty prizes after some use, but like a lot of contributors here it's surprising how long they can go on for when they are old and ugly.|
|Yup, you mean the stock cast iron right?|
|With SUs, it often helps to start with a "baseline" mixture setting and fine tune from there. Your mixture screws are on the bottom side. It's a large hexagonal nut and hope you have an SU wrench. If not, some pliers used carefully will work. Turn the nut in until it stops and back each off 11 flats. That's one flat short of 2 full turns. It should get you relatively close and you can fine tune from there. A fellow British car nut called the other night with the same carb symptoms on the Lotus Super 7 he bought with a 1275. The method smoothed it out considerably. Good luck with it.|
Jeff Schlemmer is one of this site's benefits as mentioned by John above. He rebuilt the dizzy on my 80LE and added pertronix ignition at same time. You likely have some vacuum leakage which is common to the B dizzys with a little age.
There's another fellow here (Steve) that sent me the best performance article on B engines that I have ever read. Here's a comment on exhaust gaskets that might help your exhaust problem:
"When using a gasket with a metalized face to install the exhaust manifold, it is wise to install the metalized side of the exhaust manifold gasket facing toward the exhaust manifold so that the mating surface of the exhaust manifold can expand and contract along the metalized face of the gasket. However, it is best to use the composite gasket available from Advanced Performance Technology (APT Part# CMG-02) as it has excellent compressibility and oversize holes for modified ports. Its graphite-impregnated material allows for superior ease of expansion and contraction of both the cylinder head and the exhaust manifold as well as also making for very easy removal. Be advised that due to this gasket's requirement for greater torque values, it is wise to install a kit of higher-strength stainless steel ARP manifold studs, washers, and nuts."
|Thanks for all the info Rick. Just balancing the carbs smoothed her out a lot (obviously). I will let you know how it turns out. Most likely not till the weekend to keep the neighbors happy. I have an EDIS system ready to go for the B so I am going to hold off on the new dizzy. Hows the difference between the stock Lucas black box to the Pertronix? Im not sure if you can make a judgement based on the dizzy being rebuilt as well but I've always been curious. I've always seen what people say when fitting it to an older model dizzy. I have a crane system lying around that I bought a while back at a car show. Also in regards to the vaccum advance, doesnt that only kick in when the car is in fourth gear ?|
|The Pertronix ignition eliminates potential problems and maintenance required to keep points properly gapped and condenser wear or failure. Real performance improvement is hard to measure. Mine starts quicker and more smoothly. The vacuum advance relies on manifold pressure when RPMs are increased, not on a specific gear. You can hook up an ignition timing light and watch the timing increase relative to TDC as you rev the engine in neutral.|
Let me make a suggestion. Your status says "non-member". There's no charge to register and become a member. As a member you can do an archive search for lots of old threads on any subject. I read a lot of opinions on Pertronix as well as technical details on Dizzys before upgrading mine. It's likely the best research library you'll ever find and there many technical experts here with more expertise than me. I'm just an old "shade tree" mechanic with about 40 years experience tinkering with MGs.
|Hey hows it going Rick,|
I have been a member since about 1996. I just dont feel like signing in every time. The reason I was asking is because I have read the threads and have never seen anyone write about the difference in your pertronix or crane to the lucas box. Which is why I mentioned the Vaccum Advance only being enabled in 4th gear as I have seen in previous threads. The vacuum from the distributor on our model car is supposed to go to a L fitting that then is attached to a vacuum point on the intake manifold. That L fitting has a wire connected to it. It is "activated" when 4th gear is selected. Correct me if im wrong but thats what I remember hearing from old threads and is also the reason why people with our year cars switch to a different distributor or have them recurved etc etc etc. I come on this site now a days when I am at work so I dont always have the time to do a little research as I should.
|James, You can bypass the fourth gear only switch to allow vacuum advance in all gears. It was an emission control device that served little function. RAY|
|You do *not* turn the nut until it stops, you turn it until the top of the jet is flush with the top if the bridge, then turn it back down two full turns. The jet will go higher than the top, so if you *do* turn it until it stops and then back 11 or 12 turns, it will be very weak, and 11 will be weaker than 12, which is the correct initial setting.|
4th gear only OD wasn't an emissions control device, it was a cost reduction measure that utilised the same switch for OD as for the transmission controlled spark advance (TCSA) which was a modification found necessary to prevent surging in lower gears because of all the emissions crap that American spec cars got lumbered with. The first cars with TCSA has OD on 3rd and 4th still, but that required an extra switch on the gearbox, and at that time every penny counted.
If you bypass that switch you will have OD in all gears including reverse, and if you reverse with the manual switch in the on position you will destroy your OD. Even if you remember to turn OD off every time you reverse (and who's to say that everyone else will?) OD was limited to 3rd and 4th because it isn't strong enough to cope with the torque reversals in 1st and 2nd, which was why the V8 was modified to have OD in 4th gear only.
|I wrote an article for the AMGBA culb magizine, 15 years ago, that illustrated how to use OD in all forward gears. It protected the OD unit from engaging in reverse by using a common relay that cut the power running to the unit when reverse was selected. I've been using this setup for almost 20 years with no mishaps. Yes, you have to be careful using OD in 1st and 2nd, but the OD unit is really quit strong. As long as you don't beat the car in the lower gears, while using OD, it will last as long as any other LH OD equipped B. And you can bypass the TCSA without bypassing the OD lockout if you choose to. RAY|
This thread was discussed between 19/05/2009 and 29/05/2009
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