I have a 383 with 906 heads. I just had the valves done and installed a howards cam with 260° intake duration at .050 and 268° exhaust duration at .050. I have 140 PSI cranking compression, (seems low to me) My biggest question other than the carb trouble , is WHY IN THE HELL DOES THIS THING WANT 50° timing at 1000 rpm (that's where I'm stuck at idle) It absolutely hates anything less than 40°, wont run below it, and boils water pronto at 40°. MSD 6 AL, billet mechanical only distributor, doesn't buck the starter until 60°

Degreed cam with heads off, checked it again, after break in, it is where howards wants in on the advance side 102°.

Tied a summit MSD knock off box, same results
Tried a Summit knock off distributer, same results,
Tried a second carb lent to me from the shop that did my heads , same result.

The motor has a set of TRW dome pistons # L2293, that SHOULD give me 11 to 1 .. but i think they may have been milled .. I currently have a .050 head gasket.. If you guys think I have a serious lack of squeeze going on.. how much should I mill off these 906's????? I would love to run 93 octane and not have to run some crazy timing...

The guy that did my heads cannot explain it.. I never checked timing before I took this apart .. I just ran as much advance as I could before it would buck the starter.. and it ran solid, with a bigger cam.. now it is in a state of terrible shake rattle and roll at idle and is miserable. Thanks for any input in advance.

Solid or Hyd. cam?
Lash or preload settings?
Valves not closing?
Verified that "0" is accurate on balancer?
Heat crossover on intake open?
Firing order.
Vacuum leak.

Have you verified the timing mark, line, on the dampener is accurate at TDC on the timing tab on the timing chain cover?
If not do that next When things don't make since there is usually a reason for that
Let us know what you find

make sure you are on #1 (even #6 works) plug wire. borrow another light. EDIT if you were on #2 it'd be 90 deg off (+your timing amt) so that (wrong plug wire) wouldn't be it. Are you useing a timing tape or a dialback? Might try a non dialback & 0.0632683" is (1) degree & 2&1/4" (2.246") is 35.5 degrees for an easy start. MORE EDIT after rereading I'm thinking what Flash said, vac leak or firing order. & it ran fine before & you had the heads done & the different cam (bigger or smaller just curious?), you did post specs but I ain't a cam guy.

Your timing is off.. you missed it by most likely 1 tooth on the sprocket
which is easy.. to have that much and its getting hot... was this done with
a small timing wheel.. I think mine is 2' across

The last time I had that issue, I ended up repairing vacuum leaks.

The leaks were hard for me to diagnose at the time, because they were underneath
the intake manifold. Sealed fine on top, didn't seal at all on the bottom.

IMHO, 260* @ 50 seems like an awfully big cam for a 383 with iron heads. And you say that this is a smaller cam than you had before? 140 cranking does sound low for that big a cam. For comparison, my last 500" wedge had a 250* cam, ported Eddies, and 180# cranking compression. That combo had a bumpy idle and wanted a lot of lead at idle. I usually had 28* at idle. It wanted more, but would start hitting the starter when hot any higher than that.

With no idea what your compression ratio is its hard to say, but that's a big cam, if you've got 9.5:1 compression that could be what it makes. Is this with a stock style starter or mini-starter (mini starter is worth about 20 PSI in my experience)

Are you sure this cam is 260/268 @ .050 and not 260/268 advertised? If its really at .050 what cam did you have in it before?

Verify TDC on the balancer is TDC on #1. You do have the timing light connected to #1 right?

If it really is 50 degrees I agree with Mr P body your cam is off a tooth. EDIT I see you said you degreed it in at 102 though. Double check your valve lash, EOIC!



Zippy - how did you find your vacuum leak under the intake, and what did you need to correct?

this should have been done if the heads were off.

I just went through this same thing. I had a bad vacuum leak in the intake manifold. The only way it would idle was to advance the timing the way you are describing.

More than likely not 11:1 based on the cranking pressure numbers.

Verify the cranking pressure numbers with another gauge.
All plugs removed, battery charger on, check at 4 and 8 “pumps”.

Verify tdc mark is correct on damper.
Carb will need to have race calibrated low speed circuits and 4 corner idle.
Locked out dist.

If all above checks out, and it won’t run “fine” with 35-38* timing, I’d start looking for leaks.

My 383, which was a bit under 11:1, 256@.050 cam...... cranking pressure around 200 in 8 pumps.

440, oe heads, aluminum intake.

After trying carb cleaner and so on, which didn't work, decided to try water.

I tried a hand spray bottle, got me nowhere, not enough volume, couldn't really aim it where needed.

I then tried a garden hose flowing under the manifold...that showed the leak very quickly.

The repair was to open the bolt holes slightly since they were preventing the manifold from sitting down quite far enough,
and I also started using paper gaskets.

After that I always spent more time verifying manifold fit, once bitten twice shy etc.

Those pistons are usually pretty far down in the hole. Doubt it's 11-1.

When I ran them mine liked 42 for timing. Actually put flat tops in it and went faster.

Thanks guys, you hit all the other things I tried that were not in the original post.

I am using a dakota/magnum starter (its fast) and my cranking tests are with all the plugs out 3rd hit, i only gain another 5 pounds MAYBE on 4th and 5th hit, multiple gauges. (like you guys I try to verify every measurement with 2 devices or procedures to be sure)

I did verify 0 on the dampener both times with the tab on the timing cover, I did hit 102 and was actually driving everyone in the garage nuts by verifying tdc and .005 before and .005 after on #1 intake valve, repeatedly as I was hoping I simply messed up and could find a smoking gun.

I set initial lash at break in to .019 cold, verified with an indicator so I wasn't relying on just "feel" as some guys are comfortable doing (this is a flat tappet btw with harland sharp roller arms)

after break in I went back and verified they were at .022 +/-.001 hot like howard specs ask for, after much other messing about with the other ignition components I tried (along with reversing the magnetic pick up wires on both distributers, i heard that can drive a guy crazy) I went back and loosened them up 1/4 turn on the adjusters just to see what I would gain, and didn't gain squat except alot more chatter under the covers. I put it back to .022 and started playing with the carb and I'm getting nowhere. the looser valve adjustment gave me at least .050 in lash and I didn't gain any vacuum or cranking psi.

NOW , one thing I haven't attacked very seriously is the under manifold vacuum leak possibility.. I ran a propane wand under the intake on top of the tin to the point I had propane laying on the tin and it didn't care, no stumble no rev, BUT its not out of the possibility that I have one under the tin. The heads have been cut a few times now and I cant promise it sits perfect. I use paper under the tin and over the tin, I know this manifold was cut prior to my ownership as it fits MUCH better than the out of the box M1 I also have , that is near impossible to get the bolts in.any ideas other than punching a hole in the tin for the propane and to check under the tin? This also is hard to justify with suity plugs.

I know these pistons are old tech, I wish I would have done more measuring before I tore it apart, the old cam had over .600 lift using the math of over the lobe -base circle times 1.5 I still have the cam out in the scrap can I think if anyone wants to try to run numbers on it. The gaskets I took off were also standard felpro multi layer composites that mic at .043 I believe, the heads were re cut this round .005 just for a fresh surface. (.050 gaskets on it now)

So thank you all for assuring me I tried what I should have.. need new ideas. I wont get time to play with it much until saturday.

for sure post what it ends up being (we're been getting some toughies lately!).

Exactly what is the carb and how is the distributor set up?

If those things are “correct” and you’re confident there are no other mechanical issues, and the cranking pressure is really 140+/-....... then the cam is kinda too big.
I’d really want to see another 40-50psi.

Yes, you should be able to get it to run “fine”, but you’re not going to make real power out of that cam with 140psi.

Have you done a leak down test yet?

I agree.. I don't want "fine" I want it right.

The carb is an aluminum hp 800 cfm . Tried 70 jets, it came with 80s, power valve was a 4.5 even though that is correct it tried a 2.5 for shits and giggles. Its currently back to the 4.5 valve and 80s. Transfer slots in the primaries gets me 1000 rpm but no response on the idle screws.

I drilled blades yesterday as I had little to no response from the idle screws, I do now but I am at 1500 idle..so I get to buy a new set of blades 🤯. I noticed AFTER drilling the blades the secondaries didnt leave any transfer slot open on a hard shut...so I fixed that with another wrap on the spring and adjusted the stop. (Thanks alot holley, lesson learned never trust anything out of the box.) If I would have caught this first I wouldn't have drilled.

I have also tried a 650 quick fuel. A 625 edelbrock, a 750 edelbrock, and a demon that I run on my 400 truck. The 750 edelbrock was on it previous to this head freshening endeavor.

None of those carbs would idle, the quick fuel was lent to me from the shop and he swears it is his baseline carb. He has never had that carb not run. Well..until now....I didnt adjust anything as I didn't want to curse it so I held it part throttle until I was up to temp, let it down and had crap fall on its face and die results 3 times and pulled it right back off. The edelbrocks I use on other things to rule out carb issues..they are reliable machines just no frills and not responsive. Point is i keep them ethanol free and they work on other vehicles and needed 25% throttle to keep the engine running.

The distributer is an msd pro billit. Its currently locked as I didnt want to risk adding another 20 degrees of timing by accident. And this is not new hardware. The cap and rotor and plugs and wires are. So yes...it starts at 50 degrees initial..somehow.

Check the Dist plug for the pick up coil wires being backwards in the connector,or it is wired wrong, I have done that & engine would not run with less than 50deg timing

My dads stroker motor acted like this, had to have 40+ degrees to run good. Turned out it was the distributor pick-up, replaced it and it was back to normal.

Unless your 800DP(4780) carb has been modified, that’s a 2 corner idle carb.

That’s not likely going to work very well with your combo.

If it’s been modded to have 4 corner idle, then you’ll need to measure all the orifices in the idle circuit to see what you have.

What you really need is a carb with “race calibrated” low speed circuits....... something like the original HP950.
For a new style carb, something like a Race Brawler or QFT Q series is the type of carb you should be looking at.

The reason the 50* helps is because it increases the carb signal when the blades are closed.
The reason that’s necessary is because the carbs you’re working with are designed to operate with fairly normal manifold vacuum...... which you don’t have.
The race calibrated carbs are designed to operate at much lower manifold vacuum.

I’d still recommend the leak down test.