truck 390

[My427stang]

Wow. So much info. We did not build or assemble the engine. The Carb is from jesse at bigs performance and made new for this truck. We feel like we're polishing someone else's turd. My 76 350 dually 390 4spd is worn out. Lots of blow by. It pulls circles around his truck. I pulled my equipment trailer with 10,000 lbs on it 230 miles one way and got 9 MPG. Only grabbed third twice. His barely pulls in fourth. It is honestly embarrassing. I think we'll start at square one and redegree cam and make sure dampner is right. Double check Carb and ignition and see where we're at. The truck idles like a kitten, does not bog, backfire, or hesitate. When you accelerate however it sounds angry like its fighting itself. Plugs aren't to bad and does not smell rich at all. Thanks for all the help guys.

I think you have a good plan.  If you are willing to pull the heads, I think it would also be good to see how far below deck, measure stroke, and even take a pic of the pistons and the chambers.  We can get pretty close with compression, then tell you where to put the cam after that.

When disassembling, look close at gasket surfaces, you may find a massive vacuum leak, manifolds can be held up by china wall clearance, head gasket tabs get in the way, etc.  Sometimes a bottom intake leak will just kill cylinders, your eyeball my find the problem first (although i still think knowing the compression and putting the cam in the right place will be a good investment of time)

[BH107]

Actually your D2 heads do not have LR intake ports, but the later MR style ports, so it should match up to the RPM intake just fine.

Also, what kind of gearing does it have with the 37" tires?

That is not really true.  The port centerline is very close to med riser, but they are smaller sized in every direction and flow far less.  A typical D2 head flows max 220s a med riser is closer to 260s

FWIW - We copied a ported med riser port as close as we could on my D2s and it flows well, but takes a lot of grinding.

Regardless, port match isn't that critical, if the heads were off I'd probably blend to help low speed response on the port a little, but it's not critical

Yes, they are slightly smaller but the MR style port, not the earlier LR tall ports. These usually are the exact same size as the RPM intake ports, so other than a little alignment they fit perfectly. And if you added MR sized valves to a pair of D2 heads they wouldn't be far off of the MR in flow.

Back to the OP, what values did you use to figure the compression ratio? Another option depending on how much he can spend would be to switch heads to something with a larger combustion chamber than the 68cc (or smaller if they have been cut) that the D2 heads have. Between the high CR and the small cam the DCR is really high and without getting that down it won't ever tow like you want it to and will always ping on pump gas.

[cammerfe]

I had a similar problem so long ago that I'm talking about the family car. It was a '53 Merc with the very last of the flatheads. After a bit of headscratching, I found that somehow the timing was backed off. Gave it a twist and it came alive!

KS

[fastback 427]

Update on truck 390. Ding ding the witch is dead. We went through the engine as best we could without a tear down,  but never found anything wrong. Truck was only driven very little after that, maybe 500 miles or so. On the last oil change my friend cut the filter apart and found lots of metal that didn't stick to the filter mag. So apart it came. He found two of the thrust bearing and two of the main bearings down to copper. Cam bearings looked worn too. It is ready to be taken to the machine shop next week. I'll start a new topic for the rebuild. Thanks guys.