Is this valve float

Rcdgl · May 27, 2018

Finally got the Mustang to a track last month. Ran 8.09 at 85. Would not rev over 5800. Every time, it starts misfiring and breaking up at the same rpm point. Does during driving around, revving in tne driveway and at the track. Same issue with all Duraspark and the MSD reinstalled. Carb seems to be working and getting fuel. Sumped tank and Holley mech fuel pump.

This started when I put the AFR 185 heads on last year. I have their standard flat tappet hydraulic style valvesprings, Comp roller rockers, on adjustable studs with guideplates. The Irongt40 heads with Comp 972 springs would go to 6200 anytime. Has a Rhodes LIfters 270 cam with Rhodes lifters, .484 lift.

Would shimming the springs help, or should I just get stronger springs?

jrichker · May 27, 2018

Roller lifters have heavier valve springs because of the increased weight of the lifter. Do you have roller lifters?

dcurtis · May 28, 2018

Is that a roller cam? I’m assuming yes. Roller cams need roller specific spring pressures. Is the carb floats adjusted properly? Not running out of fuel at 5800, just a thought, could be sucking the bowl dry but definitely need the correct spring rates for your specific camshaft. Could also just need to rejet the carb for the better flowing heads?

mikestang63 · May 29, 2018

mismatched cam and valve springs I'm thinking...

Rcdgl · May 29, 2018

Thanks for the feedback. This is a flat tappet 1987 Rhodes LIfters VVT 211, with Rhodes liftrers. (270 adv duration, 484 lift). Has about 100k on it. I just bolted the AFR 185 heads on last March, with new pushrods and Comp full roller rockers.

I am beginning to think that since the cam is leaving a lot of lift unused, ie, 484 vs 550 available, its not seeing enough load to keep the springs stable at 5800 or so. These heads also have the 202 intakes which are probably heavier than the GT40 184 intakes. That plus the heavier lifters are the best culprits so far. I just set up a spreadsheet to check this, and it showing 279 lbs at max lift vs the 304 the spring is rated at at 1.25 in or the .550 lift.

The Comp rep suggested shimming the springs, and I think that will pick up 10 to 15 lbs or so. Probably going to try that.

Car started and ran out of gas in five ft tonight! Then the dipstick pulled out when I checked the oil. I think it wants more attention.

jrichker · May 30, 2018

Rcdgl said:
Thanks for the feedback. This is a flat tappet 1987 Rhodes LIfters VVT 211, with Rhodes liftrers. (270 adv duration, 484 lift). Has about 100k on it. I just bolted the AFR 185 heads on last March, with new pushrods and Comp full roller rockers.

I am beginning to think that since the cam is leaving a lot of lift unused, ie, 484 vs 550 available, its not seeing enough load to keep the springs stable at 5800 or so. These heads also have the 202 intakes which are probably heavier than the GT40 184 intakes. That plus the heavier lifters are the best culprits so far. I just set up a spreadsheet to check this, and it showing 279 lbs at max lift vs the 304 the spring is rated at at 1.25 in or the .550 lift.

The Comp rep suggested shimming the springs, and I think that will pick up 10 to 15 lbs or so. Probably going to try that.

Car started and ran out of gas in five ft tonight! Then the dipstick pulled out when I checked the oil. I think it wants more attention.

Be careful when you shim springs that you don't stack the spring coils together at the max lift of the cam & rocker arms. Ask the Com Cams tech guy for the spacing between the coils at max lift.

Is this valve float

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