350 Chevy Engine Stutter @ 3,600 to 4,000rpm

[Brian73]

Hello, Everyone

I'm really hoping that you guy's can help me out with the following issue or issues i'm currently having.
For the life of me, I can't figure out why my engine is stumbling or stuttering between 3600 and 400orpms with out fail.
There is absolutely no hesitation, stutter, or ill manners from 0 until this RPM range, not matter how light or hard i accelerate. I can't duplicate the issue when the engine is unloaded. This 350 is in a small boat, which makes diagnosing issues a major pain the neck. I have to remove the exhaust manifolds and risers to change the spark plugs.

Listed below is all the real world data I can provide.

350 Engine
Vortec Heads
Dual Plane Intake
Middle of the road extreme marine cam
Hei Distributor (Timing Dead Nuts @ 32 Degrees)
Taylor Plug Wires
Holley 650 Double Pumper (Engine Idles @ 750RPM)
Holley electric fuel pump Blue(Pump Motor Doesn't Sound As Robust as I once did, sounds strange now) (7.5psi At Idle)
Holley regulator

Again, I super appreciated anyone's input to figure out what is happening.


Brian

[mag2555]

Well let's rule out ignition issues first!
Hook up a timing light to whatever cylinder you can and run the engine into the trouble rpm range , does the flash of the light go screwy ?
If so I would replace the module and the pickup coil as they are the most likely to be the cause, this assumes that the rotor abd cap look good and the coil in the cap is not showing signs of arcing .
The Distributor must come out to replace the pick up coil.

[Brian73]

Hi,

The timing pulse does not change from 0 to 4000rpm in neutral while revving the engine
I haven't found anyone brave enough to hang over the engine compartment with a timing gun at 50 + mph in an 18ft boat and check to see if anything was funky with the ignition pulse. I've even offered copious amounts of beer.........

I really appreciate the input.

Brian

[fdicrasto]

Does it drive through that range or won't the boat get beyond 4000 under load? Have you been able to keep an eye on the fuel pressure when driving into the problem?

[Brian73]

Hi,

I've tried to drive through that RPM, but usually back off once its starts loosing power and stuttering as to not damage anything. As soon as I drop the rpms to 3500, it instantly runs great.It only has an inline fuel pressure gauge located inside the engine compartment.

Would running a secondary fuel pressure line up to my seat , to check the fuel pressure under load be my next course of action.

Much Appreciated
Brian

[FC-Pilot]

After looking into the HEI cap and rotor, I might unhook the secondary's of the carb and hook them shut and see if it does it as just a two barrel. I wonder if the secondary's are opening and there is an issue on the secondary side of the carb. I love having a spare HEI and carb around to help narrow down issues like this. If you can borrow a spare from a buddy and take them to the lake with you, swapping them can help to narrow down the problem.

Paul

[allencr267]

Anyone brave enough to give it a shot of starting fluid/spray?

[87v6bird]

Do you have some combo of guide plates/guide slots/self-aligning rockers that isn't getting along? You should only have one of the three.

I fought a similar issue once where that turned out to be the cause.

[harleyboy2003]

I would also check and make sure distributor is not off one tooth. Engine will run good until you hit max mechanical advance , then your rotor to distributor cap post can be too far out of alignment causing spark misfire. Rotor will actually be between two post resulting in misfire due too the excessive gap.

[Tuner]

Had this happen just yesterday with a 40-50 year old electric tach. Disconnected tach, zoom zoom.

[Brian73]

Hi,

You Guy's are awesome!!!!!
I'm going to try the following when I'm up at the lake next.

1. disconnect tach wire and run under load
2. connect up remote fuel pressure line and run under load
3. try a spare HEI distributor

87v6bird, this is my first go round with vortec heads, I'm only running the self aligning roller rockers in the setup.
I was thinking that perhaps the valves were to tight, but then it would run like crap right from the start?

[BOOT]

Had a sim issue on one car and lucky I had another to swap all components of the ignition, then I tried the alt, tach and grounds/rewiring, a diff carb and other stuff I've forgotten. Never found the cause and due to a bad machine shop I've not messed with it in awhile.

[travis]

Do you know what valve springs are in it? Stock Vortec springs are pretty weak as it is...throw in an aggressive cam with the stock springs and it will start nosing over at progressively lower and lower rpms.

[fdicrasto]

A vacuum gauge off the intake would be another tool to get info from. You got a few spare eyeballs to keep track of this stuff?

[JoePorting]

Sounds like a weak ignition system. I'd replace the coil and check the rotor phasing as mentioned before.