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MG Midget and Sprite Technical - 1500 rough in 1200-1800 rpm range


I have a little annoying issue with my USA spec '75 1500 and would appreciate any help. The car is completely stock except that all the emissions equipment are removed / bypassed.

There is rather a lot of vibration in the the 1200-1800 rpm range, it's rather like what you feel when an engine mount is shot. I've replaced both the front engine mounts and it made no difference.

Distributor has been replaced and the dynamic timing set up at 10 deg BTDC. Both vacuum (ported) and mechanical advance appear to be operating correctly, new coil fitted, new spark plug cables, new spark plugs. All made no difference.
I did a compression test and all 4 cylinders are within 3-4psi of each other around 120psi (it's the low compression version with the dished pistons).
I set the tappets, no difference.

There are no nasty noises from the engine. It had a new timing chain about 3 years ago, but this issue only started at the end of last years summer seasonb efore it was put away for the New England winter.

So.....Stromberg 150 possibly causing it, this has not been touched or serviced (except having dashopt oil topped up) ?? The car idles lovely, starts lovely but when you hit the pedal to the floor, there's a slight shake/miss? before it continues to rev. If you rev it slowly it doesn't seem to miss, but gets very 'unbalanced' in the rev range above.

Thank a lot, Alan.

A Phillips

I do not know a lot about 1500s but driving at 1200 rpm seems awful low to me!
I do not know if it was build to function that low
Onno Könemann

Lol Onno and that's Another reason the 1500 is a good little engine - it doesn't need to hair around at high revs so is cheaper to run when petrol is up £1.59 a litre in my village!!!

Seriously, one thing to look at is your propshaft UJs - the noise sounds like your engine is going to fall out of the bottom of your car and vibrates everything.

But rach you will love the sounds at 6500rpm out of an A- series.
I'll give up the 1000 to 1800 rpm range for that every day
Onno Könemann

LOL Onno

Since I'm just a poor humble horse breeder - then I have to think of the costs of repairing the fences and stables that my horses seem intent to demolish - then I will give up the 6500 sound for a nice steady 45mph WITHOUT 5th gear :)

Ahhh....something I missed. This is happening when the car is idling and out of gear. Just when your sat in it revving it up. It also vibrates when driving of course, but it's not a prop shaft / gearbox issue because it does it stood still in neutral.

lol...clearly I'm not going to drive in that rev range labouring the engine.
A Phillips

6500 rpm is something a bog std A- series can reach without any problems.
And bad labouring is leting an engine work below her rev range.
Imho worse for an engine than over reving
Onno Könemann

Again, my problem is not to do with labouring because the car is just sitting there parked in neutral with the engine running and when I press the throttle pedal the engine shakes in that rev range. It is not under any load at all.
A Phillips


When did the vibration start? (e.g. after you removed the emissions equips, or before that...)

Anthony Cutler

do the 1500s have a crank balancer?
Rob Armstrong

"do the 1500s have a crank balancer?"

No. The design of the balance webs on the Triumph crank is infinitely better than those on the A-Series so it's not required.

I don't recommend harmonic balancers on anything less than a full race motor, and even then it's not really necessary.

I've seen plenty of snapped A-Series cranks, but in nearly 30 years never a snapped Triumph crank.
Deborah Evans

<< There is rather a lot of vibration in the the 1200-1800 rpm range, it's rather like what you feel when an engine mount is shot. I've replaced both the front engine mounts and it made no difference. >>

Alan: Don't assume a new engine mount hasn't failed. I once had a brand new mount fail, symptoms were as you describe (see "1500 engine vibration" thread in the archives).

After pulling engine/gearbox to be sure clutch/flywheel were as they should be, I happened to start the engine while it was still attached to the hoist with a bit of tension on the chain and Viola! vibration gone. Replaced "new" mount (right side IIRC) with another and problem solved.

As you said: "it's rather like what you feel when an engine mount is shot". Perhaps it is.

Onno: FWIW my 1500 is most happy at 1800-2600 rpm. The car will pull a (not too steep) grade in fifth gear all day without dropping below 2600, I assume that's where the torque is greatest.

Richard Reeves

happy with my rev band from 2300 til 7500rpm

"Deborah Evans
I've seen plenty of snapped A-Series cranks, but in nearly 30 years never a snapped Triumph crank"

That's because they all wore out before there could be put any force on them ;)

I'll duck and run for cover now..
Onno Könemann

"I'll duck and run for cover now.."

Best you do.

I have a skillet and I'm not afraid to use it! :)
Deborah Evans

LOL on both counts!

Robert (Bob) Midget Turbo

Deborah, you can also cook!? YOU ARE A WONDER WOMAN!!
Alex G Matla

LMAO - oh it's a good job that THAT one got lost in translation!!!!!!!!!!!

THanks for the answers folks.

I never ripped the emmissions gear off, it was done a long time before I bought the car. I've had it 4 years.It hasn't always vibrated like this. It didn't start after any particualr job or piece of work on it. Inoticed it at the end of last summer before the car got garaged for winter but initially thought it was exhaust related. I replaced the entire exhaust first off. It needed doing but it didn't affect the vibration.

One of the original mounts had collasped and you could hear a knocking when you went over a bump. When I changed to the new mounts the knocking stopped but I swear the vibration. It's like there's a resonant point in that range. The air filter mounting starts vibrating really loud too....which is annoying.

Which all does go along with what the Gentleman was saying earlier in the thread, maybe one of the mounts is crap despite it being new. The Made in China sticker didn't make me feel too confiden I must admit. These were purchased from Victoria British. From an engineering point of view the original mounts were definately made better.....

Is it possible to purchase higher quality repolacements ? Even if I have to source them in england, it's no big deal as i'm actually from England (been in the US 8 years)and all of my family is still there. I'm assuming Moss and VB in the USA both source their mounts from the same place ?

All in all the engine has no major knocks...sure it's noisy on the top end, relatively speaking, but it's a Triumph 1500, they are pretty brash. The compression test yields reulsts of 140 137 141 139 which is great for a low compression engine. The tickover is very sweet. One thing I do notice is a slight hesitation when kicking the pedal down, but the carb probably just needs servicing and setting up. So really engine mounts are the most obvious. Maybe the back mounts on the box are worn out or something is loose back there. Who knows. The box is coming out this autumn because theres a rattling in 1st gear and the synchro is worn on 2nd gear. Time for a rebuild.

A Phillips

>>> One thing I do notice is a slight hesitation when kicking the pedal down... <<<

Have you checked to make sure the oil in the carb damper is properly topped up?

Back to the vibration, I've dealt with collapsed front engine mounts on my 1500. On the RH mount, which collapsed due to age, the metal-to-metal contact was between the center bolt and the relief hole in the mount. On the LH mount, which broke when I was tail-ended, the alternator/air pump bracket sagged against the steering column. The vibration, especially when the RH mount was bad, was much as you describe. You could feel it through the entire shell when torque caused metal to hit metal.

Best of luck,

Gryf Ketcherside

This thread was discussed between 24/05/2011 and 26/05/2011

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