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MG MGB Technical - Re the saga of low Hg and bad idle

I bet you all can remember me and the problem with my MGB ie
Low Hg on Inlet vacuum test at idle
BAD idle
Low compressions
If you recall the saga i bought the car for spares BUT it was rock solid just idled badly easy sort that out in a morning get a test on the road. Then it started i renewed every thing from dizzy,rocker shaft ,took the head off, rebuilt carbs every ignition part renewed plus every test and could not point a finger at anything and say YES this is the cause of the problems.To cut the chase i have reground the valves etc and PLEASE remember i have a bill for a FULL bottom end rebuild from a reputable Engine Rebuilder's not done any miles but the ONLY thing i had not done was take the Timing cover off and check the valve timing how could this be wrong just been done plus has been on the road by its previous owner.It was easy not like it says in the book and guess what with No1 piston at TDC with a DTI gauge the cam mark is at least a tooth maybe two teeth out from the crank shaft mark what do you people out there think could this cause ALL my problems i am sure it must have some effect i have some pictures but cannot put them on here lets have fingers crossed that the problem has been solved it just goes to prove never take for granted any thing when working on cars i look forwards to your comments and i do appreciate all your comments regards TONY
A Basham

Seems to me this was covered in the replies to your previous questions, which is why I did not reply. This is certainly your problem. And, I never rely on marks to time a cam, after years of spotty results. Even Factory marks and keyway alignments can be several degrees out of spec. And in some cases, "aftermarket" marks are wrong, or interchangeable parts with different marks may be installed, like A and B series timing sprockets, which can be switched but give different timing. Also, many times it is not at all easy to line up marks correctly, especially if the engine is in the car. I think this is likely a major reason for the well known phenomena of identical cars running vastly differently. Anyone who has worked on many cars of a single type knows that most are average, a few are pigs, and a very few run like bandits.

Time your cam properly with a degree wheel and dial indicators of true TDC and cam events, usually #1 intake max opening. It is not necessary to remove the timing cover to check valve timing, as it can readily be done with just the rocker cover and plugs removed. On chain drives, set the cam about 2deg advanced from spec to allow for initial chain breakin; in a few hundred miles, it will be dead on and will stay there for years.

I recently had a Spitfire that had been assembled 9deg out (late) from the factory, plus about 5deg chain wear. On correct reassembly, it was hard to believe that I had not changed anything but the cam timing, including carb adjustments. Fuel mileage went from 27/28 to 32/33 mpg(US), and reserve power (which is what you have available to accelerate & pass)felt about double.

(PS This is not meant to be uselessly pedantic, but patience and care is essential to this type of work. Similar patience and care in punctuation and capitalization would make it a whole lot easier to read your messages, and give the responder the feeling that there might be some point to spending a lot of time answering your query.)
FRM
FR Millmore

I blame myself really as being an old school mechanic i should have never taken it for granted that just because its been done its been done right plus i should again have listned to the members who quoted the cam,the only concern i have now is has the cam been changed since the engine rebuild and has it a Hot cam in because i managed to get intouch with the owner who had the car after the engine work was done they say it ran as sweet as a nut they sold it to an MGB restorers young son because it wanted loads of bodywork and said he got the silly head on saying he was going to do this and that (but) within a few weeks he sold it to the chap i bought it from who owned a Business next door, who welded the underneath and put the panels on and he has always agreed that it didn't run as well or idle as good as other MGB's he had owned but he still ran it for a year and a half but in that time the head gasket went could it have had something to do with the timing who knows
So i will have to try and work out the way to check the valve lift.I know some members have described it to me but i am now brain dead when it comes to working Formula out as with all the medication i take i am afraid my memory is Nil at one time i could reel every engineering formula off parrot style the same with car details dwell etc now i have the book stuck to my forehead
If i take the cam out to check it do i have to take the sump and oil pump off as well as the dizzy gear plus if anyone knows an easy way of checking for a fast cam please get back to me i could cry when i think of the cash and time i have spent on this beast but my own fault anyway cheers again every one and to the members who said it was the cam all the time Sorry should have listened regards TONY
A Basham

Tony,
As I said before, try measuring the lift using a DTI on the back of the rockers. No dissambly required, only the rocker cover. Set-up the DTI right on the back of a rocker (directly above the pushrod) which is loose ie where there is clearance. Measure the clearance with feelers. Turn the engine and measure the lift of the rocker. Add the lift to the clearance to get the cam lift. If the lift is greater than standard then you may well have a 'hot' cam.
Anyway, don't be so worried about the idle if it is otherwise sweet. My engine idles like a trawler, but goes like a train. There's a catchphrase in there somewhere!
Neil
Neil

You all know the problems I have had and I thought I have found the solution but there is always someone to put the frighteners on you, one member has said that one tooth out on the valve timing will not bother an MGB as it would only be at the most 3Degree's out what are other members feelings about this remark also it says in the book that to put a straight edge thru the centre of the cam shaft and the crank shaft and the dots should be in line. The problem there is the crankshaft nose sticks out about 1-2" and it makes it very hard to judge this is with crank at TDC and if simplex chain camshaft key at 2 o' clock. I think mine is more than one tooth out but I am waiting for a new chain before taking the old one off, also I know I may sound stupid but again in the book it says assemble the sprockets with the dots in line then slide them onto the shafts if it doesn’t fit move the cam shaft then check alignment of dots thru centre lines put tensioner back but surely when you turn the engine one revolution wont the tensioner by taking up the slack in the chain i.e. shortening it move the sprockets out of alignment and then the dots wont line up .When you put timing chains on Jags etc you have to compensate by moving the cam sprocket a tooth at a time until they line up when tensioned and the engine turned over many thanks Tony
A Basham

The tensioner will make very little difference unless the chain is *very* stretched and hence in dire need of replacement. 3 degrees may not be much but surely it is better to get it right than wrong. But yours looks like 2 teeth out, and in any case you have no idea of the cam that is in or whether it is an original that has been reground until you measure the valve timing.
Paul Hunt 2

This thread was discussed between 27/01/2007 and 29/01/2007

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