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MG MGB Technical - rattle at around 70mph
| hello Ive been working on a few problems with my gt both to do with noise the first problem was a vibration at aroud 70mph .Ichanged the prop shaft and all was fine thanks for the advice you all gave me The second noise i fear may be more of a problem at around 4000revs /75mph i get a strange rattle from the engine, if i am going down hill or on a level road no noise . the car runs well good compression and timings all ok do you think my problem could be a crankshaft Thankyou Richard H |
| Richard H |
| Sounds like preignition. Try retarding the timing a bit & see if the problem goes away. |
| Joe Reed |
| It could be the crankshaft but i would be hesitant to condemn it without hearing it and that is difficult on the board !! With the car stationary can you replicate the same noise or is it a deep rumble at about 4000rpm?? |
| Iain MacKintosh |
| thanks for your time on this no i cannot replicate the noise with the car stationary the noise is more of a metalic sound not a rumble Thanks Richard H |
| Richard H |
| Just out of interest what are the processes of pre ignition and how does it cause this level of noise thankyou again Richard Henderson |
| Richard H |
| Richard There are bound to be technical people who will respond but as far as I am aware Pre ignition=pinking=detonation(US)is caused when the timing has been set too far advanced. This creates the spark before the piston has reached the "top" of it's cycle. It's coming up and the explosion in the cylinder is trying to force it down! Usually occurs under load. Can also be caused I think by low octane fuel but I don't know how! Why it should make the strange tinny-tinkling noise I don't know. Often timing is set by backing off the distributor (anti-clockwise probably) until pinking just disappears under load. Useful if you don't have a strobe thingy! HTH Michael |
| Michael Beswick |
| Stretching back to high school auto shop, let's see if I can remember the theory: When the fuel mixture burns in your cylinder, it isn't all at once like an explosion. It's a wave of flame that starts at the spark plug and ends up at the piston face. Of course, that flame front is where the chemical reaction is happening, so it's also a wave of pressure. I'm really stretching here, but I think perfect ignition advance would put the flame front at the piston a fraction after it hits TDC, so as much of the pressure wave is utilized on the piston as possible. I've digressed from the point though. Knocking or detonation is when the mixture fires from excessive heat or combustion. Thus, the flame front doesn't start at the right time or place, and so is no longer that orderly wave moving from one side of the combustion chamber to the other. It starts from wherever the hot spot or biggest squish is, and moves around the chamber in a manner not planned by the designers. At times it may bounce around the combustion chamber - even against itself. And if my memory has hung on at all, it's this bouncing around the combustion chamber that makes the knocking noise. Pinking (UK) or pinging (US) is a little less catastrophic. As Michael says, this is where the ignition is a little too far advance, so the piston is rising against the flame front, bouncing it backwards. A little bit of pinging won't kill your motor, but most of us would rather be a degree or two on the safe side of pinging than the opposite. Why take chances? Hopefully I've scored more than a 'C' with this essay. Having laid the framework, I'm sure the more learned will correct the rough spots. Thanks for the chance for mental exercise. |
| Matt Kulka |
| Thats great fellow students now i can at least start to understand the process before i move on Straight A's to you both Kind Regards Richard H |
| Richard H |
| Richard, It all the self tests (Cheap!) fail you could replicate this on a rolling road. Chris Connelly -Mass Engineering 01767 677800 (Past Royston)is not a million miles away! Cost me about £60 for a set up Michael |
| Michael Beswick |
| thanks micheal thats a good lead and local Richard H |
| Richard H |
| OK TIME FOR UPDATE Just spent some time under and in the car and on further tests I have managed to reproduce the noise whilst the car is stationary, the cars oil pressure is good and the sound kicks in at 5000 to 5500 rpms when i remove no1 sparkplug the noise disappears completely . The noise isnt a rattle or a grind but a loud popping from within ! Thanks again for any advise Richard Henderson |
| Richard H |
| IGNORE LAST POST NOISE IS ON ALL CYLINDERS AND I CAN GET THIS WHILE CAR IS STATIONARY i took of the oil filler cap and the luod popping seems to come from top end of engine so i guess its time to have a look at the head end of the car i still like the sound of pre ignition Richard H |
| Richard H |
| Richard, Before you remove the head can you do a compression test? I doubt if it's pre ignition if the noise occurs when stationary. If it was I don't think the car would run under load. A quick test would be to retard the timing (loosen and rotate dizzy slightly anti clockwise) Sorry just re-read your first post -compression good.... Does it puff smoke out at tick-over out of the oil filler when the cap is removed Any sign of water in the oil Are you losing water Can you see anything with the rocker box removed-loose broken springs colletts valve seals. If you remove rocker box invest in new gasket and the rubber washers that go through it onto the 2 studs. All above are cheap -pulling the head gets more complicated and expensive Michael |
| Michael Beswick |
| Micheal hello no signs of water smoke ect i am going to follow your advise on the cheap route first so its of with the cover on sunday and a quick tweak of the timing to see what goes . I am kind of preparing for the worst on this one but will keep you informed thanks for your posts Richard Henderson |
| Richard H |
| I took the car for a road test today i have checked everthing i can even had a broom handle next to my ear to see if i could locate where the noise is coming from still no luck . I did notice a few things at 5500 revs is the point the noise starts it has no rhythm its a rattle/popping noise if i push the revs higher the car starts to miss and holds back , Regards Richard H |
| Richard H |
| Richard, At 5500 revs the poor old thing is probably going to suffer heart failure! Seriously, whilst you will need to rev it to 4500 or a bit more, I think the power curve flattens off after this- ie lots of noise not much extra go! At 5500 it could be almost anything -valve bounce (tho probably not at 5500 nearer 6500+) general slop in the chain/cam/pushrod/tappet and valve train, the timing advance mechanism in the dissy could not be acting right (tho it may be fine at lower revs). HT leads beginning to break down, points etc. Could be fuel related...... This may sound sacriligeous, but up to 4500-5000 revs , is it fun to drive & going well? If the problem only occurs at v high revs to what extent is it a problem. After that it may be a matter of starting at the beginning and replacing bits till you overcome the problem. I'd start with the ignition. Points& condenser-if you have them-Plugs, leads, then cap. All quite cheap! Steal a strobe light to check the timing both at tickover and higher revs. That gives clues about the dissi and the coil. Suggest checking/changing one thing at a time..... HTH Michael |
| Michael Beswick |
| thanks micheal let me ask you one final question in your opinion could this be small or big ends Thanks for all your time on this Richard Henderson |
| Richard H |
| Sorry for the mis-spelling of your name Michael Richard |
| Richard H |
| Richard, No problem about the name -I answer to most things! It could be small / big ends-difficult to tell without listening; but as far as I know the noise would be throughout the rev range -not just at high revs. If other people are following this they may be able to be more precise! If it's any consolation, most problems turn out to be smaller than you think! Tho sometimes its a combination of more than one thing-makes life interesting! If you mention it down the pub, you will be terrified at the scenarios that are suggested! Start with the simple and cheap things first! Michael |
| Michael Beswick |
| My 2 cents: Cheap gas? Let me rephrase, inexpensive gas? Did you change petrol filling stations? Maybe you can run a tank of high test or two and see what happens. Have you leaned out the mixture, that will cause it? We used to find here in the US the head has carbon in it and causing the noise because of heat retention I believe. How many miles have you driven this car? Do you get into alot on the highway to blow out the carbon? Might give it a try. Also pull a plug and see what colour it is, if its whitish,,its way too hot. I dont think it would be points, but ya never know so that would be a cheap swap too. Chuck |
| CW Strong |
| At 5500 you are pushing the limits of the ordinary point springs, especially if the distributor is sloppy. Point float/bounce would give random timing. I have observed a lot of variation in contact pressures on different point sets, both from different manufacturers and from single makers. Some point sets are prone to getting "sticky" on their pivots, especially when hot. FRM |
| FR Millmore |
| Thats interesting i have just changed the petrol additive that i use along with the points on the last service, just about the same time i started getting this problem . so thats a real good avenue for me to pursue also the guy at the garage did recommend that because the engine has a stage two head with all the other standard modifacations {freelow and K&Ns] ect that a upgrade on the needles and jets would be prudent. The strange thing is that since this guy tuned the car it has really been running well apart from the funny gremling thats living in my engine and only wakes up at 5500rpms. The upside of this is i am really getting to know my way round the engine on this baby Thanks Guys Richard HEnderson |
| Richard H |
| Richard, FRMs comments make a lot of sense and you really are getting to the limit on the standard ignition system. The noise is certainly not serious mechanical i.e big ends etc as this would be apparent initially much further down the rev range from about 2500 or so. However if you keep pushing it to this level then it may soon be craankshaft bearings. I don't think you have a problem and used within the intended rev range your engine seems to be performing well |
| Iain MacKintosh |
| Thanks Ian i spent last night searching for engine hoists on the net so thats good news. I was out in the car again this morning and she really drives well so i think maybe i am pushing the old girl a little. I think i may invest in a aldon electronic ignitor maybe this will help a little Kind Regards Richard Henderson |
| Richard H |
| Richard Try www.retrorockets.com Its pertonix ignitor -same thing but about $75...... Michael |
| Michael Beswick |
| I can't get that Retrorockets site to work, something about domain names etc. Whats wrong with the Lumenition Magnetronic system which I use and is very good. But don't expect this to be suitable for 10,000rpm. On second thoughts you may be safer with the contact breakers !! |
| Iain MacKintosh |
| Ian, Sorry it's retrorockets@snet.net or was in August. Nothing wrong with Magnetronic The pertronix is US$77 about £45...You pay in $ though it is shipped from Guildford Surrey, UK.... Michael |
| Michael Beswick |
| Do you think the optronic system with the high power coil is worth the extra investment i do do a lot of motorway driving and was reading through the glossy brochure that mgoc sent out about this time last year and on first sight it seems that if your in the higher rev range that the more expensive system would be the best option. And while i am on , since i retarded the timing my car now ticks over at a steady 800rpms and the pinking that i used to get at the lower rev range 2000rpms-ish has all but disappeared. So it looks like i have learned how to tinker with the timing without disastrous results , mind you still have the man with the popcorn stall at 5000rpms. and lastly do you have to reset the carbs and timings when you install electronic ignition. Thanks a million and may the force be with you Richard Henderson |
| Richard H |
| I concur with all that's been said. Especially the part about over-revving. I don't think I've ever wound a B up past 5000, and even then it felt like it was at the end of the power curve. Also, you say the oil pressure is good. I would think that big ends loose enough to cause this kind of noise would raise havoc with your oil pressure. FWIW, Allen |
| Allen Bachelder |
| You can buy a pertronix ignitor from summit racing at 1-800-230-3030 or www.summitracing.com in the $80 range and shipping is very fast. They also sell the crane xr700 and xr3000 systems. |
| Jeff Schlemmer |
| For a normal road car the Magnetronic system is fine and I really doubt if you would gain any benefit from the optical system with higher output coil. The Petronics unit suggested by Michael at £45 seems too good to miss and as far as I know fulfills the exact same function as the magnetronic unit. Carbs will not need adjustment after fitting but you will have to put a strobe on the ingition to ensure timing is correct. Maybe the best thing to do is put the strobe on before you fit the unit so that you know your exact setting and then set it to this after you fit the Petronix unit. |
| Iain MacKintosh |
| And whereabouts in Great Dunmow can you get 5000 rpm?!!! Donuts into Tescos car park?!! Michael (Ware Herts) |
| Michael Beswick |
| Here is a link to retro rockets site www.vintageperformance.com/retrorockets/index.htm Colin |
| C J Bryan |
| thanks everyone you have all been a great help i feel a lot more relaxed about the whole 5000rpms issue. Ian you have two choices in dunmow to reach the kind of action i have been talking about the first you hit right on the nail the second is a really good 2.5mile straight heading from east to west on the flat but you sometimes have to share it with the odd 737 that drops in from time to time but that's less of a problem on the night shift Once again thanks to you all for your input Richard Henderson |
| Richard H |
This thread was discussed between 05/10/2005 and 13/10/2005
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