Stumped...need fresh ideas!!!

[Headhunter]

I'm wondering if your not a little to lean and getting some detonation......

[car62]

exhaust wasnt the fix,running Dorton 350 with 75 and 76 jets...bumped up to 79 and 80...didnt change anything except made engine load up in pits....totally stumped!!!

[drtlvr]

I use like 62's in my Parker 350. 4.5 power valve. Have you tried jetting down? What about the coil breaking down?

[84Dave]

Time for a wide-band O2 sensor in the tailpipe to see what the AFR is doing? -Dave-

[Dan32]

jets

we run 68 jets 70 are as big as we have ever had ur choking the engine jet it back !!!

[car62]

been running same jets for 4+ years and never had an issue

[youngengines]

Seems like a lot of jet to me, unless a lot has been done to the carb. Rich will miss up high for sure. And, if there is any "pullover " going on it would be amplified. JMO

[Wrencherer]

Check the red flapper valve in the accelerator pump make sure it is not ripped also ceck the power valve

[car62]

we have 4 Kieth Dorton 350 carbs and a 500 carb....all carbs have been on the car,jets have been played with from 65-80 with no results other than 77 up loads up,we have used PV block and different power valves,every squirter from 21 to 31...4.5psi at carb with Holley Blue and Red with and without regulator.

[Headhunter]

Hydralic head? I'm thinking that maybe you have HLA that is pumping up and holding the valves open if it is Hydraulic. Oil pressure to high or pressure releif not working right? Last thing I can think of is fuel system. Either a colapsing/clogged pickup tube, clogged vent or bad fuel line. bad regulator.

[car62]

we have changed heads,3 different heads that all turned 7200+ when they were on other shortblocks,but just in case,went with new lifters and followers,set to .035 as we do on everything else...no change.

went through fuel system and replaced EVERYTHING (fuel cell,all lines,filter,fuel pump,fittings)...


again this is a miss that picked up last season just prior to mid season and has gotten worse and worse..id really be shocked if someone threw out an idea we havent already covered.

i also dont think a fuel issue is the cause for inside the intake manifold to turn black.

[Headhunter]

One common denomonator,,, the short block. that goes back to oil pressure maybe a cracked block or cylinder wall.

And yes a fuel system problem can on occasion cause that. You go lean and get pre-ignition causing hot spots on the plug, head and piston tops and when the fuel hits it it fires back into the intake igniting the fuel in the runners.

[Racedad]

A long shot here,but have you looked at the water pump?Rarely we see a loose or eroded impeller.Would fit with the symptom of sarting after a few laps on the track and gets worse.Coolant temp sensor in the block,maybe head area is hotter(too hot)?
Last time I saw it on a 4.0 Jeep,would run fine all day in the shop but overheat within a mile going down the road.

[car62]

headhunter and racedad...appreciate those 2 ideas...waterpump is something we have not looked at...we did have a different block in there,and upon pulling intake for tech,we saw some water droplets in intake runner #2...so we changed blocks....that was last season...i th9ink its time to pull out one of the backup bottom ends and put in there for next week.

[Milford Motorsports]

If I find half a fly in my rice,i'm sending it back!