Engine Hesitation

Discussion in 'Troubleshooting & Diagnosis' started by Robert M Mrozek, Oct 9, 2017.

  1. Robert M Mrozek

    Robert M Mrozek Member

    Oct 4, 2016
    Forum Colleagues. Looking for some help and this is the best place for it!!!
    I have a 1970 Camaro with a rebuilt 350 SBC. It has a 10.5:1 compression ratio, new mechanical fuel pump, fuel pressure runs around 6 - 7 psi, new fuel tank and fuel pick-up, crane cam 490 lift, 280 duration, new Proform 650 carb with elec choke, 3 in exhaust with headers, mallory ignition system, 3500 stall converter. This is an older non-original engine and it oil pressure runs low (10 psi hot and 50 psi cold). Running 5W-20 oil.
    Here are my symptoms:
    1) After warmed up, engine temps >170, throttle help to cruise (i.e. not accelerating) around 45 mph and 3000 rpm and periodically I feel a slight hesitation. Like a cylinder is missing periodically. I can very mildly feel it happening and it comes and goes. I dont feel it during acceleration nor do I notice it at idle but with my cam it has a very loopy idle so not sure i would notice it.
    2) When turning in a parking lot engine seems to want to stall...this might be unrelated to #1 and maybe due to low idle and power steering pump load.
    3) When I start the engine after a hot soak, it takes 10 - 15 sec to start. Starter turns, the engine turns but no firing, it starts to barely fire and coughs to a start. Then it runs fine.
    My main concern is item #1 & #3.
    Wondering if it is a ignition issue, spark plug wire (maybe exposed to too much heat from headers) or maybe vapor lock or insufficient fuel supply for some reason. Any suggestions on how to diagnose would be greatly appreciated.
    - Rob
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    Nov 4, 2004
    Ringgold, GA
    A miss at cruise can be caused by a lean condition in the carburetor or too much timing. If you are using vacuum advance, try blocking it off and take it for a cruise. If no change, then address lean condition. This could be tied to your 2nd issue, floats.

    If engine tries to die out when breaking or turning, most likely the float is set too low.

    Hot soak starts have always and will always be a thorn for carburetors. You may have an issue with the choke not pulling off properly. When you do a hot start, are you smelling gas or getting a black puff of smoke out back, like it's running rich.

    Lastly, Holley's web site has some very good tech info which applies to your Proform carb. Here's a link, you can select from several videos and how to's. Good luck.

  3. Robert M Mrozek

    Robert M Mrozek Member

    Oct 4, 2016
    CorkyE - Thanks for the advice. I will look more closely into it and the Holley videos. Can you expand a bit on the blocking off vacuum comment? Do you mean pinch-off or disconnect the vacuum and these assess the change in performance?

    Also, no...I don't recall smelling gas or black smoke after hot start but I will look at that next time.
  4. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    Nov 4, 2004
    Ringgold, GA
    Yes, just disconnect line at distributor and stick a golf tee in it.
  5. tomsti

    tomsti Veteran Member

    Jul 15, 2003
    Redmond, Wa, USA
    I vote vapor lock :), not 100% but if the gas gets hot it can start down the road you describe. Something to maybe consider or look into.
  6. Cardinal

    Cardinal Veteran Member Lifetime Gold Member

    Jun 22, 2003
    Endicott, NY
    Oil pressure: it should be 35 psi at 1500 rpms (never take a reading at idle as there isn't a spec for oil pressure at idle). IMHO, 10W20 oil is too light. I run 10W30 full synthetic oil in our older engines (new vehicles use 5W20).

    Timing: what is the initial (at idle) and total timing (at 3000 rpms)? Total timing should NEVER exceed 38 degrees.

    How do the spark plugs look? White (TOO lean). Light brown (correct fuel air ratio). BLACK = too rich. I'm lucky enough to have a friend with a fuel air meter that goes into the tail pipe which takes all the guess work out of setting up a carb.

    Carb to intake gasket fitment: is the gasket sealing the carb to the intake? We have a Edelbrock 1406 on a Wieand Action Plus intake that ran like crap! Found the gasket didn't seal well between them. Bought a "goof plate" set (base gasket that matches the intake, aluminum spacer plate, and carb pattern gasket. Problem solved.

    Thermostat/engine temperature: 170 F is TOO cold! Get a 180 F Robert Shaw thermostat. Test it in a pan of water by bringing it to a boil with a thermometer in the water (even new parts can be bad).

Share This Page