5.4 ltr small block, rochester quadrajet, 230 HP --> buck

Flo25 · Post by **Flo25** » Tue Aug 05, 2008 7:14 pm

Hello friends of V8 power!

We own a Corvette powered by a 5.4 ltr small block delivering 230 horsepowers.

We checked the whole engine after 8 years of driving abstinence and it runs fine for the first roundabout 40 minutes, which means it has nice idling, good acceleration under all throttle positions and reaches high revs without problems. Carburetor and ignition timing have been adjusted by using the original workshop manual.

But after these roundabout 40 minutes the situation changes. It starts with bucking in high revs above 3500 rpm and within minutes the bucking occurs even above 2200 rpm. After an hour driving the car is almost impossible because you are crawling around with revs around 1100 rpm max.
The bucking itself feels like the carburetor does not get enough fuel, but we checked the fuel pump as well es the fuel filters (fuel pipeline, carburetor) and the accelerator pump inside the carburetor. All those parts work as they should, and it can't be a fuel problem, because for the first 40 minutes everything is fine!

So, my question is: Which part of the engine could cause this phenomenon? We asked many people, mechanics as well as enthusiasts, but no one ever heard of such a strange problem.

With best regards and thanks ahead.

flo

Boosted LS1 · Post by **Boosted LS1** » Tue Aug 05, 2008 7:31 pm

What about the fuel pickup pipe in the tank? Is it picking up rubbish as you drive the 40 minutes and then dropping it once you parkup?

Post by **katanaman** » Tue Aug 05, 2008 7:34 pm

As above or
Heat soak on the ignition system causing it to fail when it gets hot? Depends really if the roundabout is a coincidence or not, as in does it run bad just running for 40 mins or so.

Ian Anderson · Post by **Ian Anderson** » Tue Aug 05, 2008 7:40 pm

Fuel evaporation due to excess heat buildup under bonnet.

Possibly the fuel system has a return line and the whole tank of fuel is warming up until the evaporation occurs.

What fuel pump and setup are you running? Have you checked fiters for blockages?

Ian

Flo25 · Post by **Flo25** » Tue Aug 05, 2008 7:44 pm

Thank's for your fast responses!

I forgot to mention that we also checked this fuel pickup pipe and even drove with a canister and a new fuel tube to be sure that dirt inside the tank can't be the problem.

And, yet another thing I forgot, if we replace the ignition coil by another cold one after 40 minutes of driving, the engine runs completely fine again! Strangely, the removed ignition coil is just a bit warm, not hot to the touch...

Regarding the second post, could you evaluate on that one because I did not understand what you mean exactly. Thank's!

Ian Anderson · Post by **Ian Anderson** » Tue Aug 05, 2008 7:50 pm

Are you runninf points?
If so it could also be the condensor beaking down - worse with heat!

Cheap t replace

Ian

Flo25 · Post by **Flo25** » Tue Aug 05, 2008 7:55 pm

@ Ian: What are runninf points?

Sorry, if I do not get what you mean direclty, but I'm from germany and my English ain't the best

BadgerV8 · Post by **BadgerV8** » Tue Aug 05, 2008 8:03 pm

An unlikely suggestion, but given it takes so long to occur, maybe a vacuum forming in the fuel tank? Next time its misbehaving, open the fuel cap and listen if it sucks air in. The problem should also immediately go away if is that!

Dave.

Flo25 · Post by **Flo25** » Tue Aug 05, 2008 8:04 pm

Ah, possibly typing mistake so "running points"....what points?
The condenser of the ignition system is new...

BadgerV8 · Post by **BadgerV8** » Tue Aug 05, 2008 8:10 pm

I've had many recent car parts proving that 'new' does not always mean 'working'!!

Might be worth checking with another condenser, I'd say.

Flo25 · Post by **Flo25** » Tue Aug 05, 2008 8:13 pm

Okay we'll check/replace that one to be absolutely sure, thanks

scotb · Post by **scotb** » Tue Aug 05, 2008 8:28 pm

petrol freezing? try running a hot air intake pipe from exhaust manifold to the air intake

Eliot · Post by **Eliot** » Tue Aug 05, 2008 9:12 pm

what ignition system is it?

Is it hei like this:
http://www.jegs.com/images/photos/55540002.jpg

I.e. the dizzy cap is massive with a coil in the middle?

There an ignition module in the bottom of it, which could be going bad. Cheap and easy to replace.

Ian Anderson · Post by **Ian Anderson** » Wed Aug 06, 2008 9:00 am

Flo25 wrote:@ Ian: What are runninf points?

Sorry, if I do not get what you mean direclty, but I'm from germany and my English ain't the best

Sorry
That was a typo I hit the F instead of the G
Should be Running points - if so worth changing the condensor as it is cheap and will rule it out

Ian

jefferybond · Post by **jefferybond** » Wed Aug 06, 2008 1:08 pm

Flo25 wrote:And, yet another thing I forgot, if we replace the ignition coil by another cold one after 40 minutes of driving, the engine runs completely fine again! Strangely, the removed ignition coil is just a bit warm, not hot to the touch...

Does the replacement coil also start playing up after a further 40 minutes?

Jeff