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MG Midget and Sprite Technical - lack of 'Zoomph'

Hey everyone,
My car (1975 1500) idles nicely. ZS carb with manual choke seems to be doing it's thing after re-build (replaced seals / gaskets / diaphragm) Plugs are nice "tan" colour after 100km drive at steady 90km/h.

But the issue is: it takes a while to get to 90km/h. When I pull away from a stop there's tons of "pull" but once I hit 2500rpm or higher, it feels like a large bungee cord is holding me back. The revs will "slowly climb" but never past 4000rpm.

Here's what I've done (got):
• Lucas 45D4 Distributor (not the OPUS one) with vacuum advance
• ZS Carb - adjusted for mixture.(followed John Twists step by step)
• Dynamic timing at 12BTDC (with timing light)
• All smog stuff long gone (from PO years ago)
• All ports on carb plugged (no vac line from dist to carb

So...what am I missing? I checked all parts of the inlet manifold etc for air leaks. Nada.What else should I be looking for? Is it because I'm used to more modern cars and I'm expecting too much? But I swear she always had a bit more "zoomph" than this.

Chris



Chris Elkerton

Hi Chris,

You say you have a distributor with vacuum advance, then you say you have no vac line from carb to the dizzy. So you have a vac advance carb with no vac advance connected, right? So perhaps your ignition timing at higher revs is to pot and so your not developing ooomph?

Alternativly, aren't cars with a single ZS just slow anyway?! (I tease :-D)

Malcolm.
Malcolm Le Chevalier

Chris

I think you need to check the auto advance at the higher engine speeds. The vacuum advance isn't used at wide throttle openings. The problem is the auto advance springs get stretched, you then set the timing at tickover the auto advance reaches its maximum to early and you don't have sufficient advance at high RPMs. You need an advance timing light to check it but I can't help you with what it should be.

I've got a 1500 and It'll rev into the red in 1st, 2nd and 3rd. I set the springs by eye, as far as I remember the week spring shouldn't have any slack in it and the stronger spring should have about 1mm slack. I didn't have an advance light at the time, I do now so I should really check it.

Bob
R.A Davis

Malcom,
The vacuum line didn't seem to make a difference either way (connected or not).

Bob, she will idle well and will rev nicely sitting in the garage. But not under load.

I'll check springs. Also...just for those playing along at home, I checked compression and I'm between 125psi and 135psi across all 4 pistons.

Chris
Chris Elkerton

It really does sound like a timing issue, but since you've rebuilt the carbs have you checked that the piston can freely rise all the way?

I think that if you set your timing with the vacuum advance disconnected you'll end up with overall advanced timing.

I'd try timing it with the vacuum advance connected, then while the engine's running unplugging it and seeing what happens to the timing.
Growler

Thanks Growler!
Will try it this week-end, after I weld up the exhaust line that fell apart on my drive home last night! Oi!

Chris
Chris Elkerton

Chris

The fact that it'll rev freely while in the garage doesn't tell you much.

The brown plugs suggest the mixture is correct but it's an average reading so can't be relied on completely.

If your timing was to far advanced I'd expect it to be pinking under acceleration, but this may not be the case if it's a low compression engine.

A couple other things you might want to check:-

Check all the valves are opening equal amounts, not easy, the cams have a habit of wearing out. The engine will have plenty of low down torque but lack power.

If your exhaust needs welding is it possible it's started to collapse internally, this could cause your power loss.

Bob
R.A Davis

Hey chris

Long time no see buddy...I hope life is well

I have to agree its the dissy...this is the bigger dissy not orginal to the midget but a nice aftermarket racing dissy

Im guessing you just stuck it in there and timed it and clamped it down.....they really do need to be whats called (recurved)

Jeff ???? At a dissy shop in I think wisconsin does an awsome job....unfortantly... im infected with brain frats tonight, and cant recall any thing...but ill post the info tomarrow...I know he turns them around fast and is very resonable priced...and completely transforms. The engine performance

In the mean time....there is a little trick you can do that will cure a large chunk of your problem

Warm up the engine rev the engine to 4500 rpm and hold it there and reset the timming to ....???? I cant remember...sorry about that

Ill post that info as Well tomarrow....unless someone can post it before me

I wish I was more help...but im dead on my butt tonight


Prop

Prop and the Blackhole Midget

The comprif ssion PSI is between 125 and 135

Im not very familer with the 1500, but that seems really low....did you put new rings in as well???

With the low compression and the low prrformance at mid to upper level


My thought is if you removed the timing chain or sprockets ...if so,

I wonder If the camshaft timing is to retarded....instead of being as an example of 106 degrees, maybe you cam shaft is at 112 degrees

That would certianly account for the lower compression specs snd the limited mid range power curve


Anyway... those are my 2 guesss cam timing a few degrees to retarded or the new dissy needs recurved to fit your engine

Btw... on cam timing "left (ccw) is advance and right (cw) is retarded

Prop
Prop and the Blackhole Midget

Ive seen mgb engines act like that if you retard the cam sprocket one tooth. It wont deliver top end performace and feels like you are riding against a hurricane.

I hate to have to say this but I think Props got it! When the man is right! He is right! If its a fresh rebuild and everything is new, your cam timing is retarded one tooth go back and check it. Watch theres no slack in the chain. Turn the motor over in the direction it usually travels.

Check the timing marks are dead on.

This is just my opinion, but I think from the description you have given, I have experienced it.
The symptoms are correct for this conclusion!

Anyway good luck and post the answer when you get the solution. Its always fun to see you succeed and if weve guest right!
Steven Devine

I would do everything I could to rule out other conclusions. If nothing else makes sense check the timing gears! Know what I mean!
Steven Devine

Come on steven... it only takes 3 days just to pull the front end of the engine off to even get at the timming marks

What else could.you possiable.want to do with your free time then rip the entire front end of the motor off?'cleaning your bathroom, building a new deck, even installing a super charger on the midget are just not what its cracked uo to be... yeah free time with nothing to do Is such a drag


LOL.

Yepp... if nothing else pops up as the culprete, its going to be a L-O-N-G weekend....

Got beer ???

Prop
Prop and the Blackhole Midget

Just did a rear seal in a 5 main B with AC! Removed everything and split the tranny from engine. Put speedysleeve on rear of crank reassemble!
Took about 8hrs.

Timing gears on a 1500, Id say start at 8:00am done by noon! As long as you dont have to Chase for tools.

http://www.mackaysgarage.com/

I work part time with Steve. Like you its fun to stay up thinking how to fix things! Bending over these little cars is starting to get to my back and it is weird getting older, it makes feel old just saying that!
Steven Devine

Hey Everyone,
Thanks for the input. Right now my MIG and I are in the process of fixing the exhaust. A bad bodge job by a repair shop decades ago blew apart on the way home. Looks like a metal sleeve inside the pipe (holding two pieces together) might have be cutting off flow.

Once that's fixed I'll check how she runs. I was just on the "Advance Distributors" site and that looks like it might be a few hundred bucks well spent. (with shipping)

After that, I'll look at cam timing. I was VERY careful when putting it back together to ensure everything was right, but if the above things don't work I'll look to that.

C
Chris Elkerton

Hey chis

Yes... advanced distrubors, thats the place I was trying to think of ... +++A all the way, jeff only does dissys, nothing else and has been around scence the 1st lucas dissy ..lol

Converting from the 25D and Going to the 45D dissy i understand is a nice modification....but it needs to be "recurved" to fit your engine, something not easily done in the home shop....having jeff work over your dissy even if this is not the cure to your problem, is still a great investment

Here is a tip to help your current situation

Warm up the engine, run the RPMs at 4000 to 4500 hold it,and reset the dissy timing to 34 to 36 degrees its more art and feel then exact sciance when pulling this trick....then give it a try,

Basically this is setting the timming at the top end instead of the low end of the rpm range....which is where most of your driving occurs.... unless you drive like a little blue haired elderly lady that never see above 25 mph

Anyway... I totally agree with both you and stevdn....get the dissy recurved 1st... if that dosnt fix the issue... then deal with the camshaft timming...after all you havent lost anything getting the dissy recurved... it was all ways going to be needed so you really havent lost anything

Anyway... in the mean time, give that above trick a try...it will be make a big differance and make it alot more drivable

Oh... dont forget to use a timming light gun becuse fiddling with dissy timming at 4500 rpm WITH/OUT a timming light, can be rather.... EXPLOSIVE

Prop

Prop and the Blackhole Midget

Chris,
Steve uses advanced for all his restorations, they do quality work. That will be one thing you check off the list. Goodluck with the exhaust.
Steven Devine

This thread was discussed between 18/06/2013 and 24/06/2013

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