Low rev failure high rev ok 2.4T Engine

[BernardoEsteves]

Hi guys, so as the tittle suggest I am having engine issues. Let me make this short and simple with everything I know. (My mechanic does say me everything)

So the engine at low rev (until 4k RPM) is failing, like really hard to scale from rpm to rpm, once it gets to 4k it goes really fast, like everyone wants, until 7.8k RPM. SO basically I only have and reliable engine from 4k. I already trade the carburettors from webber to zenith and does almost the same, maybe I should try a new module?

I just know this, I'll try to find something else

What should I try? And hoe can I despite this?

[356911930]

You should provide more information on your engine build. Otherwise people trying to help can only guess what is happening in your engine.

Rahl

[E Sully]

Carbs, CIS, MFI, stock or modified?

[BernardoEsteves]

Originally Posted by 356911930

You should provide more information on your engine build. Otherwise people trying to help can only guess what is happening in your engine.

Rahl

Originally Posted by E Sully

Carbs, CIS, MFI, stock or modified?

I have 2 carbs I can use, a pair of Zenith and a pair of Webers, so is running carbs
The car has high compression pistons and E cams. (It's a 2.4T)
I don't know much else, but I try to find something from my mechanic

[al lkosmal]

what ignition are you using and what is your timing?

[NICE 69 S]

Turning a 2.4T engine to 7,800 RPM? Stop that.
Bob B

[BernardoEsteves]

Originally Posted by al lkosmal

what ignition are you using and what is your timing?

I'm running single plug, don't know the timing

Originally Posted by NICE 69 S

Turning a 2.4T engine to 7,800 RPM? Stop that.
Bob B

The crankcase isn't the same as E and S?

It has a STD crankshaft, new E cams, new springs valves, and high comp piston. I'll try to know something else more today. Oil pump, timing what more should I ask? I've been said that the engine on dyno performed 205hp (I don't believe it)

[tharbert]

Originally Posted by BernardoEsteves

The crankcase isn't the same as E and S?

I'm not an expert here but doesn't the E cam along with associated intake/heads run out of breath long before 7k rpm? Spinning it faster gets diminishing returns:

[Trackrash]

You may have had the exhaust valves hit the pistons at that RPM. The resulting lack of compression will cause poor running at low RPM. Just a guess. As others have mentioned E cams aren't designed to operate at that RPM.

Do you have stock or racing valve springs?

Do a compression or leak down test.

Also, it would be a good idea to check the operation of the distributor with a timing light.

[BernardoEsteves]

So I've speak with the mechanic and this is what he said resumed: 2.4S Piston, rally cams (somehow he cannot explain which ones...) he thinks it has 3.0 oil pump, and stronger valve springs, he also said that the compression is about 10:1, he machined the cylinders heads in order to do this. The engine seems to work OK with the Zenith carburetors and fail from 4k RPM to 5k RPM with the webers...
I'm going to check how many mm do you have to resume to get the 10:1 cr

What do you think?

[Trackrash]

I'm confused. Do you mean runs OK up to 4K rpm, then no power until 5K rpm?

Or won't run from idle to 4K?

My experience with Zeniths is they work great on a stock T motor, but are HOPELESS on a motor with more power. Others will disagree, but I have not seen what they did to make them work.

Put the Webers on. Tell us exactly what model of Weber. Tell us all the facts on the Weber's setup. What jets and venturis are installed?

Any info on the cam timing, i.e. duration and lift, will also help.

What distributor do you have? Check the timing and advance.

If you can give us this info we can help, otherwise we are just guessing.

[BernardoEsteves]

Originally Posted by Trackrash

I'm confused. Do you mean runs OK up to 4K rpm, then no power until 5K rpm?

Or won't run from idle to 4K?

My experience with Zeniths is they work great on a stock T motor, but are HOPELESS on a motor with more power. Others will disagree, but I have not seen what they did to make them work.

Put the Webers on. Tell us exactly what model of Weber. Tell us all the facts on the Weber's setup. What jets and venturis are installed?

Any info on the cam timing, i.e. duration and lift, will also help.

What distributor do you have? Check the timing and advance.

If you can give us this info we can help, otherwise we are just guessing.

OK I'll try to find out what you asked.

The engine runs really bad FROM 4K RPM to 5K RPM. From idle to 4K Its ok and from 5k to max OK too, with webers. I will find out what have been done with the carbs, and I'll post here.

From the cams I'll have to discover too, because my mechanic "don't" knwo what he have there...

Distributor I've the original marelli with 7.5K cut

[Trackrash]

Also we need to know what imulsion tubes you have in the webers.

[cstreit]

Sounds to me like a problem in the transition circuit in the carbs. Normally I think this is about 3000 RPM but if you have the wrong chokes (too large) it might make this happen at 4000 instead.

This is the only thing I can think of that would cause a flat spot at a narrow RPM range like that.

You can try a smaller choke and see if it moves the problem down to lower RPM. Then you know this is the problem. What size carbs? What size chokes?

[porschetub]

The Marelli should have a cut off around 6.4krpm or there abouts,don't exceed that too much more,it is out of the range of the engine to make more power.
The Zeniths you have can be made to work by increasing the venturi size and changing jets as required for better running,the idle jets are very small stock and the venturies are the same.
I really think you have a issue with your cams being too aggressive,you could have a poor vacuum signal making the air thru the carbs stall out ,the power should be pouring in around 3000rpm.
Make sure your timing and ignition system is 100% before you proceed any further,good luck.

[turbo nut]

A poor exhaust system can cause issues like you state. Is yours a stock style exhaust system or other?

[stownsen914]

Like cstreit suggested, poor running in a mid-range RPM band sounds like a carb tuning issue, a "flat spot." If you provide all the specs on the carbs, including venturi size, main jets, idle jets, air correctors, and emulsion tubes, the folks here can help diagnose.

A wide band O2 sensor will be helpful in diagnosing this as well, to show whether the engine is going rich or lean to cause the poor running in the 4-5K range. Your mechanic should have access to one.

Scott