Hey, I have a 250 crossflow that breaks down at 4000 rpm It has a correctly set up valve train spring pressures ect all checked and OK. MSD digital 6al regraphed electronic bosh distributor , it has had module removed and is wired direct to MSD box. Fuel pressure is not very high only about 3 psi at idle with standard replacement fuel pump and if you give it a good rev while under the bonnet and watch gauge it actualy drops back. would fuel pressure make it break down , it is almost like the limiter is cutting in. Runs sweet as till 4000 . I am also wondering if it could be distributor but that was sent away and all checked and set up except for the module I took that out and wired it up for direct plug in to MSD just looking for some thoughts

What carb are you using? Different carbs require different fuel pressures...

Valve spring tension is one thing to check, but a visual check, of the retainers and locators, will tell you if it is "floating" at 4000rpm, so it may need more spring pressure, or lighter retainers, etc, to help it rev.

Or the voltage may not be enough for the MSD system, causing the issue. Check your voltage, at the 6AL, at various RPM's for a start.

Good luck.

Hi thanks mate, I will start with the voltage.how could I check this while driving, I am guessing under load would be best. Perhaps rig multimeter with some extra long leads to Msd 6al wiring and run inside cabin. We just changed collets and retainers as by luck when adjusting rockers we noticed collets had failed and was about to pull valve through. replaced with a set of crow cams items. Was a 465 holley now 650 double pumper. does exact same thing with both. Also has Aussie speed intake ,10.5:1 comp ratio. gunna chuck some autolite plugs in on weekend. Camtech hydraulic cam cant think of specs off the top, anti pump lifters and crane rollers. 10mm ignition leads , MSD coil

The Holley will need between 5.5 and 7 PSI fuel pressure, so if factory pump is not up to it, you may need to go electric. The factory carb is based on a Weber design, and they like less then 4 PSI, so that may be your issue, especially if it does the same thing with both carbs.

Yes, if you can rig up the multi-meter so you can read it while under load, it may tell you something. A quick fix is to run a relay on the power supply, to ensure a good clean power supply.

If it has broken the collets, it is either the wrong collets for the retainers, or it is valve floating real bad. Did the retainers have any wear marks, where the spring has been moving around? And do you know the spring pressure on the seat, and when fully open? This may help to diagnose your issue.

Good luck.

I have run a direct power source from battery to MSD Box this is switched on with a relay activated by the original ignition wire. Added a Carter fuel pump this has increased fuel pressure. Took it for a run and still does the same. I am trying to find the valve spring pressures cant remember but think they might be written down. I have access to a spring tester so will pull them off and check again if need be. It was the spring retainers not the collets that were cracked. was pulling the valve stems and collets through them. This thing runs bloody awesome except for this issue. I am beginning to lean towards the springs now. so will concentrate on that and get back to you with some info.

Sounds like the springs are way too light for your application. When you do pull it apart, check the installed height, and also check valve lift, to get an accurate open height. Some springs will be spec'd at, say, 100lbs at 1.750" installed height, but then get installed at 1.850"...

The good thing is the old cross-flow engine can take a 351C spring, so there are plenty of options. If you need some more detailed help, send me a PM, and I'll do what I can.

Cheers mate,