Top end only 90MPH, then Carb backfires

[wheelz]

Changed my motor from a Pontiac 400-4X with Ram Air Exhaust to an E-Headed 11:1 Pontiac 455 (462) with headers and the 60919 cam.

The 400 had a Cliff's Qjet built for it and it ran great from idle to as high as I ever took it. It just kept pulling. Took the same Qjet and HEI and put it on the 455. Only other change was a new Carter M6907 fuel pump (120gph @5.5psi)

I managed to get the idle setup by adjusting the idle mixture, but it required 16 degrees advance.
Place my hand over the primaries at idle and rpm increases slightly...indicating larger jets required.

Throttle response and mid-range power is OK, however it has no top end. It stops pulling at about 90MPH and will begin to repeatably backfire if kept at full throttle.

Swapped out the secondary rods from DH to DX, but there was no change.

Do not know the current jet size and primary rods used, but I suspect I need larger jets for the 462.

I need some recommendations, as I have a long road trip (Bandit Run) coming up in 2 weeks and need to solve this quickly.

 

 

 

[wheelz]

Got it out for a drive tonight and realized that it will not go above 3800rpm while driving. Free revving , it backfires at 3800rpm.

[77cruiser]

Might have some broken or real weak valve springs.

[wheelz]

Thanks...Guess I will check that next...although they are brand new and this is the recommended kit for the 60919 cam from Crower.

[Cliff Ruggles]

I would have suspected running the bowl low or empty (fuel delivery) but that shouldn't happen "free revving" it.

It needs more idle fuel not bigger jets if you had to advance the timing and still a little lean with the mixture screws backed out sufficiently......Cliff

[von]

If not a lifter issue I'd suspect something in the ignition system. If a points type dist try new points and cond. I used to have problems getting over 5000 rpm and a new condenser cured it.

[wheelz]

Spring height was a good call...completely slipped my mind.
Installed height spec of the Crower valves springs  is 1.600"
Currently they are sitting at closer to 1.800" on these old E-heads.
Seems like a lot to shim to me.

[Cliff Ruggles]

You need different springs for those heads.  Not practical or even recommended to ship .200" just to use the wrong springs......Cliff

[wheelz]

Totally slipped my mind about the springs...even though I had a thread going on PY last month for valve spring selection. Old age I guess..

Got a set of CC987-16's as they are 1.800" and had just slightly higher pressures.

Only problem is, the inner spring diameter (.697) is too close to the vitron seal diameter, and they are rubbing and even pulling on the seals. These do not have the brass ring, so any rubbing has to be a bad thing. I also found it very hard to get locks on..almost at coil bind.

I may need to look at some other seals, like the PEP 530's with the brass rings.

Anyways I put in the 1.700" springs that came with the CC290B6 cam , (as 995 springs were recommended for it)...and it ran like a scalded cat to 5000+ and then a small backfire and tach started bouncing around, so I let off. Car did not feel as "light on it's toes" as it did with lighter springs. These current springs are a heavier gauge and stiffer.   

[wheelz]

As an update..replaced valve seals to accommodate the 987 springs and low rpm problem solved.
WOT it runs hard, but the tach starts bouncing around at just above 5000rpm...I was prepared for a distributor rebuild ...could be lack of fuel also I suppose.
At partial throttle I am running lean...white plugs and pinging around 2500rpm.
Tried 1 counter-clockwise turn of the APT...but still pinging.
Next step...pop the top and ID the jets and primary rods.