Can someone help me understand why.....

[thewave]

my car runs the way it does?

I don't pretend to know anything about engines, basically because I don't. But I like to understand, and want to learn.

I have a 3 litre straight six MGC. It's running triple 45 webbers, lightened flywheel, balanced long duration cam etc.

However, what I can't understand is why it is so difficult to drive the sodding thing from standstill? At tickover after starting, it's around 1,000rpm. When pulling away from standstill I have to VERY carefully press the accelerator, and find it VERY difficult to get the revs up, in order for the car to get going, this usually involves riding the clutch, which obviously isn't good.

However, once on the move, revs up slightly, I can plant my right foot and it'll scream to 7,000 rpm and go like the clappers. Why isn't throttle response there when pulling away?

Answers on a postcard please?

Thanks

[jacobyte]

Could be anything from loose throttle linkages, over-heavy flywheel, incorrectly set butterflies, incorrect timing, binding brakes, and other stuff.

We used to have an MGC, but the garage we took it to for respraying did a runner with it.

[JP_Midget]

Long duration cams are great at high rpms, were the gas flow in and out of the cylinders is much more heavily influenced by the gasses inertia than at low rpms.

The lightened flywheel won't help the initial pull-away either, as the engine will have low inertia itself, which makes it easier to stall.

Sounds like a great engine, but not one that would be nice to drive in traffic because of the low torque at low rpm. If it really gets to you find out exactly what the cam is and chat to an expert about dropping down the scale a bit.

[thewave]

Thanks for the responses chaps.

So I understand why it's easy to stall, but what about the throttle, why won't the revs just shoot up when I touch the throttle in neutral, instead of just popping and splodging about?

[ewenm]

thewave said:

Thanks for the responses chaps.

So I understand why it's easy to stall, but what about the throttle, why won't the revs just shoot up when I touch the throttle in neutral, instead of just popping and splodging about?

Does it idle smoothly (so no throttle)?

[thewave]

ewenm said:

thewave said:

Thanks for the responses chaps.

So I understand why it's easy to stall, but what about the throttle, why won't the revs just shoot up when I touch the throttle in neutral, instead of just popping and splodging about?

Does it idle smoothly (so no throttle)?

I suppose for quite a highly tuned engine IMO it does idle fairly smoothly, but then I have nothing to compare it to. Sometimes the revs drop below 1000rmp and oil pressure then drops, so when I was in traffic last night (not easy!) I had to sit with my foot gently on the pedal, then revs increased and it seemed to stay back at 1000.

[jacobyte]

Sounds like at low revs it could be overfuelling and retarded timing.

ETA: Have you had the 45s jetted properly on a rolling road? You may find that you need a higher idle speed to make the most of the top end.

Edited by jacobyte on Tuesday 23 October 17:25

[mark69sheer]

You might need to clean your carbs or have them professionally set.

It sounds as if the mixture is too weak if it won't pick up.

Is it still like that when warm?


I used to have a Lancia Beta Coupe and below 300rpm it would splutter and be rough but once above 3000rpm the butterflies on the twin choke would open and it would be off like a scalded cat often breaking traction with the road and more of an off/on delivery than many turbos I have driven

[andy_quantum]

I'm going to go with 2 things, you've got a lairy cam in which is going to be pretty lumpy at low revs and you're running Webers. Presuming 3xDCOE's which are funny buggers at the best of times for falling out with anything involving air or fuel. You need to get them setup well, and tweaked occasionally.

The cam, again might be too much for what you're expecting out of it. I used to have a Stage 3 CVH which was great over 2500, but completely useless at average residential road rpm's

[thewave]

I've been told it was rolling road set up, but not sure, will find out.

Thanks

[Incorrigible]

thewave said:

I've been told it was rolling road set up, but not sure, will find out.

Thanks

If it has then surely you've got a lamba print out ?? that would tell you whther it needs re-jetting

My guess (as mentioned before) is that you've got too hot a cam for comfortable road use. Road MGC should easily idle at 650-700

[Pork_n_Beem]

from my experience ordinary rolling roads tend to look at full power settings only so it may only have been set up from 3500 to 7000.

There is an art in getting the correct idle jets and emulsion tubes to get the correct partial throttle pick up.

Also hairy cams tend to need a higher idle speed, 1500 is not uncommon.

I would contact an expert with carbs, usually a guy in his 60's with a beard usually fits the bill.

[Dunit]

If i remember right the std MGC had triple SU carbs. To fit the Webers you had to change the Cyl Head to a 12 port as fitted to the works rally cars, If this is the case then the idle speed was set about 1800-2000.
To get it better for normal road use you will need to change the cam for somethind less extreme.

[thewave]

I don't have any print outs from the rolling road. Can anyone recommend me a specialist who can sort this out please.

Bear in mind I'm in Suffolk.

Thanks

PS I'm going to a clients soon and will take the MG (As it's a bit of a run), I'll try and get some more info.

[Regional]

Peter Baldwin, Wilshers Garage, Wimpole 01223 207217
About 30 minutes from Newmarket, Sufflok.

What he doesn't know about carbs isn't worth knowing.

[thewave]

Will look into it.

Appreciate the responses

[zac510]

Don't go to a rolling road - they mainly test heavy load conditions (precisely as Pork-n-Beem said). They can do more but the operators don't usually bother. It's easy to claim that a rolling road might help you if you've never had carbs or bespoke induction before so at this stage just save your money.

If mechanically all your linkages are in order then it sounds like you have a problem with the idle circuits on your DCOEs. These are also important in the transition between idle and the main circuit. (also what Pork-n-Beem was saying )

What you need is a good patient tuner who will take the car out on the road and tune it in the precise condition you are complaining about. You might even want to demonstrate it to him.

Then again, he may just say it's the nature of the beast But a rolling road won't replicate your problem as well as just doing it on the road.

BTW, what's it like when you crack open the throttle in neutral? what's it like when you crack the throttle when the engine's off?

Edited by zac510 on Wednesday 24th October 21:22

[thewave]

zac510 said:

Don't go to a rolling road - they mainly test heavy load conditions (precisely as Pork-n-Beem said). They can do more but the operators don't usually bother. It's easy to claim that a rolling road might help you if you've never had carbs or bespoke induction before so at this stage just save your money.

If mechanically all your linkages are in order then it sounds like you have a problem with the idle circuits on your DCOEs. These are also important in the transition between idle and the main circuit. (also what Pork-n-Beem was saying )

What you need is a good patient tuner who will take the car out on the road and tune it in the precise condition you are complaining about. You might even want to demonstrate it to him.

Then again, he may just say it's the nature of the beast But a rolling road won't replicate your problem as well as just doing it on the road.

BTW, what's it like when you crack open the throttle in neutral? what's it like when you crack the throttle when the engine's off?

Edited by zac510 on Wednesday 24th October 21:22

When I plant my right foot in neutral the car basically tells me to sod off
To get the revs up in neutral requires a very gentle movement, and playing around a bit, it's really not easy. When underway, if the revs are over 3000 and I crack open the throttle, it goes like a train.

[zac510]

Angry thing Sounds like accelerator pump or idle circuit.

The cam could be contributing a little to the bad behaviour but I wouldn't expect it to be that bad when tuned!

[thewave]

Sorry to drag this back up, please be patient

Decided it might be worth further upgrading the engine before tuning webers. So I will be taking the bottom end, and putting in forged pistons, conrods and a balanced steel crank.

After these mods, I think I'll be safe going for the larger jets on the 45s. Will any further remedial work be required on the top end after these additions?

Currently the engine is as follows:-
Gas flowed, lead free Peter Burgess cylinder head, block re-bored with new pistons and valve cut-outs in top of block. Cam re-profiled to high lift long duration, with Vernier cam gear, all moving parts balanced.

Now I'm foregoing the weber tune at the moment, anyone want to recommend where this (expensive) work should be carried out.

Only two I know at the moment, MGMotorsport and Denis Welch.