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MG MGB Technical - BGT Tuning
| A bit of advice please. A friend of mine has just bought a very nice 1973 BGT, which he can start no problems without using choke. The car sounds to me that it is running too rich and needs looking at. The car is fitted with HS6 carbs and has a Kent high lift cam. The Engine No is 18v 5827h6154
A couple of questions please: Are HS6's the same as HS4's to set up? As there is a high lift cam fitted, are the valve clearances different from standard for that engine? Is the ignition timing also standard for that 18V engine? Thanks Andy |
| Andy Robinson |
| I see no reason why they shouldn't be the same to set up as that is done at idle, but may well need a specific spring and needle to avoid poor running though the throttle range. Kent list many cams for the MGB, each with a detail page with specifications. |
| paulh4 |
| I could be talking out of the top of my hat, but my understanding is that HS6 carbs are generally too big for the B-series engine unless it has had serious head work done to make it like a racing engine. I was taught that the engine is basically an air pump - the more air you can get in and get out the more powerful it can be. Peter Burgess' cylinder heads are so good because he has concentrated on getting the most air in and out. But you need to get a good velocity of air whizzing through the carb venturis to achieve this. Increasing the venturi size slows the air flow down so can be less efficient. |
| Mike Howlett |
| HS6's were a popular upgrade on a performance engine back in the day. I have a set in the garage which might go on a car someday!
As Mike says, they'll need a very free flowing head/exhaust to make the best use of them. But to get to the questions; Ideally it needs a rolling road to set everything up correctly. Initial timing can be the same as standard as a starting point, as can the initial carb tuning process. It should be easy enough to balance them at 1000 rpm but a RR session will be needed to get a set of custom needles, springs and an ignition advance curve to get the best out of it. Best of... MGmike |
| M McAndrew |
| Thank Mike for your advice. The car has a ported head and sports exhaust fitted, and goes ok, but starts without choke and the exhaust is uneven. So I think it's back to basics, valve clearance, ignition timing etc, plus a carb reset. He can then take it to a rolling road for a final tune up. There is one in Rayleigh near us. Thanks again Andy |
| Andy Robinson |
| Andy, if it's got a fruity cam it won't run well at standard tick over RPM. Set it at around 950 and set the mixture a little on the rich side as it will almost certainly weaken at part throttle and higher revs on a standard set of needles. Best of... MGmike |
| M McAndrew |
| Andy I'd be interested to know what needles and springs it has in it--there wouldn't be a std needle for it as 1 3/4s were never std on a B It shouldn't need to be set rich at idle to compensate for leaning out above idle if it has the correct needles- That's a trap that a lot of players fall into, richening the idle with a lumpy cam trying to smooth it out--- Just set the jet heights at .062"-.063" (1/16") and leave them there--if the needles are correct it will be right. willy |
| William Revit |
| Really just agreeing with the boards combined wisdom. HS6s are probably bigger than the car needs but it should be possible to set them up reasonably. If the idle is good and the plugs don't soot up then if the car doesn't run acceptably through the rev range it may need setting up on a rolling or even new needles. I got the fixed needle equivalent to AAA for my car and Aldon took about 2 strokes of emery papers worth of brass at station 2 and it is now perfect. |
| Stan Best |
| Interestingly, the Austin/Morris 1800'S' had HS6 carbs as standard, I believe. I can't imagine they'd have been fitted to a standard production car if they were too big. |
| Dave O'Neill 2 |
| Thanks all for your input. We should be able to have a look at the car next week. The PO has obviously done a lot of upgrades to the engine and I suspect he has fitted one of these kits, together with a sports exhaust, double valve springs and the cam. https://www.moss-europe.co.uk/en-gb/fuel-induction/carburettors/hs6-carburettor-conversion-kit-mgs10761--gp I'll let you know what we find. Andy |
| Andy Robinson |
| Andy-Had a look at that Moss link-even Moss advise the carbs are fitted with std. needles (whatever they are) and may not be correct. If you are going to buy new needles, i'd suggest as a good safe starting point ---.100" jets SY needles and light blue dashpot springs -- It's a known combination that works well with a fairly wide range of modifications willy |
| William Revit |
| I’m a bit old school when it comes to fine tuning so to me it’s a bit obvious that an older vehicle that starts from cold without using the choke is not right and set up rich, the choke is there for cold starting. Rightly or not I gauge a lot by the colour of the plugs, if there black and sooty it’s burning oil or running rich, I like to see a biscuity colour. Maybe I over simplify this but as I say I’m old school. Probably using the wrong carbs is asking for a lot of problems. |
| Trevor Harvey |
| Can't argue with that, old-school works. Get the air-flow and mixture right at idle then you should always need choke to start in the UK, even in warmer weather, and idle smoothly. After that all sorts of things can cause running poorly off-idle. |
| paulh4 |
| A quick update and what we found with my pals GT. Plugs were checked and cleaned and valve clearance confirmed with the Kent cam as 16 thou.
1 Checked the airflow in both carbs and the rear one was hopelessly out of synch with the front, so that was fixed. Pistons were free and dropped with a metallic clunk, so didn't remove them. 2 Mixture on rear carb was also way too rich and using two colour tunes the mixture was adjusted on both carbs and then checked with the lifting pins. Back to basics, but it works :-) The car ticks over much better now and revs freely. Haven't been able to road test it yet as it hasn't stopped raining. The car has a stainless manifold and sports exhaust and boy is it noisy! My own car is a bog standard roadster with an 18v engine and I would definitely not spend all the money that has been spent on this one. Thanks all for your input. Andy |
| Andy Robinson |
| A bit of further advice required please. My friend has now taken his BGT to a "classic friendly professional" who has retuned the carbs. Remember it is a 18v 842 engine fitted with a Piper fast road cam kit and has HS6 carbs fitted. The garage has fitted additional return springs for the throttle linkage and he has been told that with a Piper cam, you can't get the tick over under 1200 rpm. The idle speed for the engine is quoted in the work shop manual is 800 rpm.
It just seems a bit strange to me that with a fast road cam, you can't get the idle speed lower. Has anybody had a similar experience? Andy |
| Andy Robinson |
| Sounds 'normal' for a performance cam to me, they will neither idle nor pull at low revs. I've see 1500 for fast road elsewhere. At the end of the day if it needs to be 1200 to stop the engine jumping off its mounts then so what, it's not as if it will spend its life there. I have my standard roadster at about 1000 since unleaded was foisted on us. |
| paulh4 |
| When you say it won't go under 1200 rpm ,there must be an issue--If you adjust the idle speed screws it should be able to get down to stall speed.
800 rpm is a good target, if it won't run consistantly at that there's something wrong. They must have been chasing it to fit extra springs Possibles-- Throttle cable adjusted up too tight- Air leaks around the manifold- "Some" SU carbs have a spring loaded air bypass valve built into the throttle butterfly and they're known to fail open, specially with a modded cam---If the carbs have these i'd be soldering them shut. 1200 is rediculous, it'll run on when you turn it off at that when it's hot Even with monster cams 800 is the target--i had a serious 320 deg cam in mine and hardly any flywheel and it was happy at 800, -lumpy because of the cam but consistant idle speed. Sounds like your mate should stick to you setting his idle rather than the expert. willy |
| William Revit |
| Two scenarios - you can't get the idle down below 1200rpm, or you can but it idles horribly. Which is it?
For the former as well as inadequate free play in the throttle cable the linkages could be maladjusted such that one carb fully closed is holding the other open - with a poor idle. Another cause can be the butterflies fully closed but mixture leaking round them e.g. worn spindles and bushes. Or butterfly over-run valves stuck open as mentioned, I soldered mine shut as they would stick open in icy weather and the engine would race till they had warmed up. I venture to say that air leaks at the manifold won't cause it by itself. While they will result in a higher idle for a given throttle opening you should always (as said) be able to back off the idle screws until it stalls i.e. the butterflies are closed, an engine cannot run on air alone. |
| paulh4 |
| Having checked with the cam manufacturer, it would seem that with a "mild" fast road cam fitted to a B series engine, it should not have a lumpy idle and should idle normally.
"Our BBP270B is a drop in cam profile and is mild enough for road use with no added extras or machining necessary. If you are having an idle issue I would check that the cam is timed in correctly. It should not have a lumpy idle at all. A general base line to start off would be to try ignition timing at our suggested full lift position and then also 2 degrees each way (advance and retard) and see what works best for your application and then you can start to dial it in and find the sweet spot. Best to check cam timing first then start looking at fuel and ignition timing." The cam and ignition timing was not checked apparently, only the carbs readjusted? Andy |
| Andy Robinson |
| These days with fuel very different to originally timing is usually done on the basis of advanced just short of pinking at any combination of throttle opening, revs and load.
Carbs same as always i.e. balanced for air-flow and mixture with a momentary rise in revs as you lift each piston 1/32" one at a time. I've read several descriptions of cam timing but they can differ and be confusing, probably the simplest is John Twist's: "Check your valve timing: With #7 fully open, set #2 at 0.055in. Rotate the engine until the #2 rocker just contacts the #2 valve stem (use a piece of paper). You will find your timing mark on the front pulley at TDC." But whether that applies to a non-standard cam is a different matter, another question for the supplier. Once you start modifying there can be all sorts of knock-ons and compromises. |
| paulh4 |
| Paul, I'm presuming JT's method is for a stand cam and therefore not appropriate for the Kent high lift cam fitted. Andy, There's loads of YouTube videos on setting the cam timing using a timing disc. The downside is it will be quite difficult to do with the engine in the car. Also, to adjust it you'll need a vernier timing gear or a selection of offset keys for the cam gear. Best of... MGmike |
| M McAndrew |
| Andy. Everyone seems to have focused on the cam as being the problem here. As for timing the cam, you need, at the very least, the cylinder head off, the timing cover off and the crankshaft pulley off. Then, you need a dial indicator (clock gauge) and a timing protractor. The information that Paul quotes from John Twist is the same information quoted in the factory workshop manual for checking the factory cam. Your cam may have significantly different requirements and that information must come from the company that made the cam. Peter Burgess, in his excellent book on the MGB engine, tells how to properly check to see what is happening with a cam and how to correct it if that is required.
However, let me also mention the carbs--HS-6 as per the information you have provided at least twice. I seem to remember one or two racing engines that were built using that particular carbie, but do not remember ever reading about a general road engine using them. Nor, can I find any reference to their use in any of the books that I still have in my collection. The HS-4 has a throat (venturi) of 1.5" in basic diameter. The HS-6 has a throat of 1.75" basic diameter, in increase of 1/6th or about 16.7% greater diamether. However, when you look at the area of the hole, the difference is over 33% greater than the area of the HS-4 throat. I cannot help but wonder if the root cause of the problem is too much carb? Especially if the cylinder head is relatively stock with nothing more than a three angle valve job to handle the increased potential air/fuel flow. Chasing the cam problem is going to be a major undertaking, one best performed with the engine out of the car and on a stand or bench to make it easier to work on. A swap of intake manifold and installing a set of HS-4 carbs might well be an easier first step and, at that point, you willl know more about how the larger carbs are influencing the problems you are experiencing. Peter Burgess would be a good source to start with about whether you may have more carbs than the engine can take advantage of. John Twist has his University Motors website and you can contact him through his e-mail there. John, also, has a twice monthly zoom meeting where he discusses some aspect of MG ownership, followed by a question and answer period. Depending on your availability, the Monday night meetings might be a good source of information. Les |
| Les Bengtson |
| Thanks Les and Mike for your comments. I saw my friend today and we both think that the car has had this upgrade fitted, together with a sports exhaust and fast road cam. https://www.moss-europe.co.uk/en-gb/hs6-carburettor-conversion-kit-1-3-3-4-7-pair-mgs10761 It is always difficult to know exactly what modifications a previous owner has made and I have also suspected that perhaps 1 3/4" carbs may be a bit too big for the 1800 engine. Thanks again for all your help. Andy |
| Andy Robinson |
| Andy,
the big questions are what's been done to the head and block? Fitting HS6's, a fast road cam and a sports exhaust could lead one to think other work may have been done. Whilst all the above are likely to give some performance gains the real magic is done in the cyl head and to some degree the block overbore size. To get the HS6's to perform/work at their best you're going to need increased air flow. For this to happen you'll need a head capable of flowing loads more air than standard and a bore increase to suck a greater volume of air in. So, whip the head off and inspect what you have e.g. larger valves, ported, CR, manifold/port matching etc. With the head off you should be able to see what size pistons are fitted e.g. standard or overbore and who knows, it might even be a 1950 offset bore that would really benefit from a set of HS6's. Best of... MGmike |
| M McAndrew |
This thread was discussed between 18/09/2024 and 13/12/2024
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