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MG MGB Technical - Engine Top Speed?

I've noticed that my recently purchased 73 B runs very smoothly to about 60 -65 mph but when I accelerate beyond that point, there is a noticable vibration that is hard to identify the source.It feels like increased revs but the tacho doesn't jump accordingly, (revs are at about 3500 rpm) Definitely not tire vibration/balance. There feels like there is plenty of room left for more power but I'm reluctant to push it too hard. I can go faster in the early morning when temps are lower but still seem to experience the vibrations around 65mph.

The engine was rebuilt a couple thousand miles ago and has a new head and new SU carbs.

I've ordered K&N filters and was wondering if the engine was starving for air?

The engine temp is in the N range. I'm using 91 octane fuel.

I've read about engine "ping" but am not sure what the symptoms are.

Would welcome any suggestions.

Thanks

Mike
Mike McKinnon

Mike-
K&N airfilters will increase airflow, especially at high rpm. 91 Octane is too low. Burn off what fuel you've got in the tank and than switch to at least 93 Octane. If the engine runs better in the cool of the morning then the mixture may be a little rich. It could also be that the carburetors are a bit out of synch.
Steve S.

How does your car response to the same revs in another(lower) gear. If you don't have the same symptoms I would start thinking about other parts than the engine.
Jesper

Mike, Also adding to the comments by Jasper...

* Has this only just started or has it been there since the rebuild?
* If the engine won't rev beyond about mid range chech the fuel filter. It may be blocked
* Possinly a worn engine mount or gearbox mount.
*Possibly an adjustment required on the distributor or re synching the SU's as Steve mentioned.
*BTW "pinging" usually occurs at low revs at wide throttle openings and can be heard as much as felt.
Imaginge someone hitting the engine with a hammer as you drive along and you have the idea.
*Another BTW when it happens you will know without a doubt!

Sorry I can't help further but vibrations are something you need to be there to help sort out.

Cheers, Pete.
Peter Thomas

Generally 91 octane is the best you can find across most of Canada, although I recall that years ago Sunoco in Ontario used to have a ultra-premium gas that had a slightly higher octane rating. Don't know if thats true anymore though. In fact I don't recall seeing any pump gas rated above 91 octane anywhere between Calgary,AB and Phoenix, AZ in my recent travels.
SteveO

Thanks for the suggestions. As Steve O suggested, it's hard to find fuel with octane greater than 91 or 92 here in Canada. I'm going to look into adding a fuel supplement to boost the octane and see if that helps.

I'll also see if the problem replicates in lower gears. Part of the problem is that my Tacho operates intermitently at best so I'm judging a lot of this by ear.

I've only owned the car a couple of weeks so I don't know the history of the performance from when the engine was rebuilt. The speedo also seems to read high so I'm estimating what the actual speeds are when this occurs.

I've experienced engine knocking before and if pinging is like that, (hitting the engine with a hammer is a good analogy) this is definitely not the same. It's a high frequency vibration and there is a definite threshold when it kicks in. If I back off the accelerator just a bit once it starts it disappears completely.

Mike
Mike McKinnon

About that octane rating -

Here in the States "Pump" octane ratings are the average of "motor" plus "research" methods of octane rating. I'm not sure sure which was which, but one was slightly higher than the other. Do you know if the Canadian system is the same as ours, or do you use the rating system which yields the lower number only? That might account for 91 or 92 being the highest available - same stuff, different measuring system.

(I should know this stuff, but I make it a point to fuel up Stateside before I take off over the border).
John Z

Now you have me wondering, since the lowest octane gas in Canada seems to be 87, but in the US I've seen 85 at most stations, and have always used the mid-grade (87/89) for fear of the unknown. It looks like I'm going to have to buy some gas soon, and actually pay attention to the pump.
SteveO

SteveO -
Anecdotally, sounds like you've got the same system as us. Most of our "regular" is numbered 86 or 87, midgrades are 89 and 91/92, premium is usually 93, Sunoco has 94 (which works best for my cars.) Some places will sell an 85 in my area - usually Sunoco "economy" - but it may be more common there for some reason.

If your Canadian numbers were consistently below the US numbers, you'd be using a different scale.

Years ago, they'd post a sign at the pump explaining the rating (the formula is M+R over 2), but I haven't seen that in years. I'm nearly certain that in the UK they use the higher of the two methods, as they always report slightly higher octane ratings.
John Z

Mike: Not knowing what distributor you have, you may want to check the advance as well as the weights & springs under the plate of the points. Often, the advance system will corrode to the point where it does not operate correctly and you will lose performance. The engine will"hold back" or "flatten out" as you try to accelerate. Try "decorroding" the advance mechanism. It's a pretty easy operation and does not cost anything.

Try secondary coil wire also. Over time resistance builds up and does not deliver full energy or worse yet breaks down completely.

Good luck and let us know what happens

cheers

Gary
gary n. hansen

Your drive shaft may be out of balance.

Mike
Mike Phillips

The Canadian system of quoting octane is the same as in the US. Sunoco here have 94 octane which I use in my "new" engine sourced from the UK - it runs well.

Regards, Barry
B.J. Quartermaine

Mike,

This may not be your problem but need for any particular rating of octane can be reduced with very little effect on road-going performance by retarding your ignition very slightly. If you have a vernier advance dial it is a piece of cake. You should not have to (and shouldn't) stray very far from the advance specs for your specific engine - to do so could result in overheating. On my daily driver I re-advanced it for competition on the weekends when I added an octane booster or used high octane fuel from the race track. Check your timing in any case just to make sure you are not over-advanced.

By the way, almost every octane booster on the shelf anywhere is complete garbage. This includes anything sold by 104+, STP or any of the other big-name producers. Read between the lines and you will realize they DO NOT boost octane, no matter what it says on the front of the bottle. On the back label they claim the NEED for octane may be reduced because carbon deposits will be removed, thereby reducing compression ratio. If an engine actually suffers from severe carbon deposits, these bogus "octane boosters" will help, but so will just about any fuel additive that offers that effect anyway. Sometimes they claim up to a "point" of increase, which means a paltry .1 NOT 1.0 as would be nominally worthwhile. These companies are counting heavily on our "Butt Dynos" to feel the "power increase" that is not really there - and which octane doesn't give anyway! I have not found a legitimate octane booster anywhere that was not labelled something like 'not legal for highway use'. The real octane boosters have all kinds of wonderful dangerous poisons that smell like paint remover (toulene etc.).

Dean
Dean Lake

Great discussion on octane, but I don't think it has anything to do with Mike's problem. Major pinging(pinking) under load would begin to occur at a much lower RPM than 3500, so the posts concerned with balance (drive-line and wheel) and timing are more appropriate to the issue. Getting the tach and the speedometer to read correctly would be a good start on identifying problems. Also have someone with recent experience with a B drive it to determime if there actually is a problem.
George B.


Here in vancouver we get up to 94 octane at petro canada and chevron. Im 80% of all major citys in the states they use whats called an oxygenated gas witch has extra oxygen processed into it so it burns slower but it also makes it ping allot more. So a 94 octane gas from canada and a 94 octane gas from the states are way different. I run 11 to 1 compression in my B with 94 octane and have no pinging at all under the whole rpm load range, But when ever I go to seattle and fill up I need to add octane booster other wise my gas pings under any moderate load.
Ross

Interesting discussion on octanes and boosters. Here's what I have done so far:

1. Had another experienced driver/mechanic take the car for a test drive to give me his opinion. He says that the car is fine. There is a minor vibration about 3500 but if you push through that it smoothes out again. We had it up to about 90 mph!

2. checked the compression, 3 cylinders at 180 and 1 at 185 - no problems.

3. checked the vacuum. - indicated that the timing could need to be advanced.

4. checked the timing - needed to be advanced. reset to 11 deg BTDC.

5. added some Octane booster.

The car now runs smoother, there is still some minor vibration just above 3500 rpm but dispates by 4000 rpm. No run on when iginition is turned off.

Conclusion - problem is solved.

Thanks to all the suggestions.

Mike
Mike McKinnon

I have a similar problem w/my 79B. Whenever I'm in 4th gear, going 65 mph and 3500 rpms, my engine vibrates more. But as soon as I hit 70 mph and 4000 rpms, it runs quites nicely and quietly. I just figured that it was one of those MG quirks....
Ann

Anyone remember that old thread by one of the knowledgeable gurus on a REAL octane booster? Seems he added 1/2 cup denatured alcohol, 1/4 cup toluene, and some MEK or acetone to a full tank ( 13 gallons). Supposed to give a full point ( not 1/10th of a point) increase.
vic

Mike ,

I had the same problem wih my 79. I was getting way too much timing advance. Turned the timing back a few degrees at at time and kept test driving. After taking out about 10 degrees of advance the problem went away.

Mike
Mike D.

Mike,
Did you have any real useable horsepower left? Or was it set that far out of spec? I've seen my race cars fall off several tenths just being out a couple of degrees. Couldn't imagine pulling 10 degrees initial out of a well running engine. Or did you mean that you took 10 degrees out of the full advance (still, WOW thats quite a bit)? Didn't it feel quite lazy off tick over?
Luke Staley

Mike D.,
Watch out for overheating from not enough advance. There's something else I think that can go wrong from too little advance I can't think just what at the moment - can anyone think of it (other than low power as Luke pointed out)? Dean
Dean Lake

This thread was discussed between 05/09/2002 and 12/09/2002

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