Marine engine

[Flightcraft18xlt]

Hi there everyone just joined the forum all the way from West Australia after buying all the parts from Cliff to set up a Q Jet for my ski boat carb No-17059595 (sick of trying to get the holley 600 vac sec to perform) and I have a few questions that some of you may be able to shed some light on, firstly my eng specs are : 383ci sbc 4 bolt mains, std intake manifold, flat top pistons 10:1 comp ratio, pertronix flamethrower cdi distributor, cam is a federal mogul speed pro with 310 cam lift 465 465 valve lift 224 224 dur @050 and 298 dur at sae, lobe centre 107 in 117 exh 112 lobe spread. First question is being a velvet drive trans the engine slopes down at the back and the carburettor requires a wedge to level the carb up I think it is approx 15 deg and the wedge just has an open cavity (no ports to match carb to manifold) do you think that would have much affect on the  idle quality and even high rpm performance? I am thinking about manufacturing a ported wedge.

Next question I have found that this engine seems to like a LOT of ignition advance at idleafter slowly increasing advance until it started to run rough and backing off slightly I found it will happily start and run with 30deg of advance :-o It will still idle at 14-16 deg but runs much lighter when it has a lot of advance?? Any advice/opinions much appreciated! Thanks.


P.s Thanks for the parts Cliff and Deb Much appreciated!!

[Schurkey]

First question is being a velvet drive trans the engine slopes down at the back and the carburettor requires a wedge to level the carb up I think it is approx 15 deg and the wedge just has an open cavity (no ports to match carb to manifold) do you think that would have much affect on the  idle quality and even high rpm performance? I am thinking about manufacturing a ported wedge.

How much slope to the engine when the boat is planed?  Slope when stopped, and slope when planed are two different things.

Next question I have found that this engine seems to like a LOT of ignition advance at idleafter slowly increasing advance until it started to run rough and backing off slightly I found it will happily start and run with 30deg of advance :-o It will still idle at 14-16 deg but runs much lighter when it has a lot of advance??

Is the mark on the damper aligned with "0" on the timing pointer when the piston is at true TDC?

My limited experience with ski boats is that they appreciate vacuum advance for the same reason that a street-driven car does:  they spend most of their time at part-throttle, and generate plenty of manifold vacuum.  Maybe knock the initial timing back down to ~10 degrees, and add vacuum advance connected to manifold vacuum???  Adjust the centrifugal advance to compensate for the reduced initial so you have ~35 degrees of total timing all in by 3000 rpm, PLUS 10--15 degrees of vacuum advance.

[Flightcraft18xlt]

Thanks for the reply mate , the angle doesnt really matter as it ranges quite a bit from slow to fast speeds anyway, when at WOT it sits in the water pretty much level so the engine would be at about 15 deg give or take. My main question is how much affect would the wedge adapter have on idle quality and airflow since it is basically a 2" open spacer making the dual plane factory manifold redundant.

Yes the mark on the damper is correct as I checked it when I checked the cam timing using a positive piston stop. I have been advised by a couple of people not to use the vacuum advance as marine engines are heavily loaded at all times, it does idle ok with 12 deg advance I was just concerned that something might be wrong with my setup because it will still start and runs well at 30deg, recently purchased a book on performance ignition systems and it basically says to set the idle ignition to as advanced as you can while the starter will still start the engine.

[Schurkey]

Thanks for the reply mate , the angle doesnt really matter as it ranges quite a bit from slow to fast speeds anyway, when at WOT it sits in the water pretty much level so the engine would be at about 15 deg give or take. My main question is how much affect would the wedge adapter have on idle quality and airflow since it is basically a 2" open spacer making the dual plane factory manifold redundant.

I'm thinking that a divided spacer could be of benefit to you.

Curious:   most intake manifolds have some angle designed into the carb mounting flange.  WHICH end of your engine is low?  I'm surprised you need an extra 15 degrees of wedge.

Yes the mark on the damper is correct as I checked it when I checked the cam timing using a positive piston stop.

Good to know.

I have been advised by a couple of people not to use the vacuum advance as marine engines are heavily loaded at all times,

MOST boats are grossly under-powered.  Takes WFO or close to it to plane; the engine runs "with it's tongue hanging out" pretty much all the time.

Easy way to tell.  Put a vacuum gauge on a manifold vacuum fitting, and run the boat as you normally do.  If you have enough vacuum to activate a vacuum advance (let's say 10" or more) for a significant amount of your running time...PROPERLY TUNED vacuum advance could be of benefit to you.

it does idle ok with 12 deg advance I was just concerned that something might be wrong with my setup because it will still start and runs well at 30deg, recently purchased a book on performance ignition systems and it basically says to set the idle ignition to as advanced as you can while the starter will still start the engine.

"I" would be looking for a different book.  What's the title/author?

Initial timing and centrifugal advance must be set up to work with each other.  More initial timing requires less centrifugal; less initial requires more centrifugal.  You'll want something like 35 degrees of total advance, but what combination of initial vs. centrifugal is the question.

No cranking/starting problems at 30 degrees initial advance?  I'd be doing a cranking compression check!

[Flightcraft18xlt]

The engine still faces the correct way so it slopes down at the rear, thickest part of the wedge is at the secondaries. Because it is a velvet drive and not a stern drive the propellor shaft comes out of the bottom of the boat at an angle and the engine is on that same angle. *was going to post a pic i have of the prop shaft coming out of the hull but cant seem to be able to get it to work* I am pretty sure this boat is not underpowered and gets on plane very easily (only weighs 1ton) and I am currently looking to purchase a vacuum/fuel pressure gauge to properly tune the engine and was planning to take it on the water and monitor vacuum readings in different operating conditions to see what is really going on, I actually built this engine myself about 5 yrs ago and it will only turn to 4200rpm a full throttle so have been double checking all the components recently to make sure everything is correct, I found the HEI dizzy that was fitted wasn't getting any mechanical advance, I think it was a cheapy Chinese copy so I fitted the pertronix distributor but made no difference to top speed, but anyway .

The book is called "How to build and power tune distributor type ignition systems by Des Hammill"

Yes maybe I should check cranking compression because I am sure there is something a miss with this engine and I have never been all that happy with it since I built it, although I must say it does start and run a whole lot better since I put the Q Jet on, haven't had a chance to take it out on the water since the new carb either. The previous carb was a Holley 600 vac sec, it was an OEM carb for a ford police car I believe and would have been way too lean for my application.

[Flightcraft18xlt]

Ok read post on how to post pics.

[Cliff Ruggles]

Most important is the set the total advance so you don't ping/detonate the engine, then modify the advance curve if/as needed for best initial timing that provides good idle and easy starting when the engine is fully heat soaked.

For most V-8 engines, 8 to about 14 degrees initial timing, 30-36 degrees total timing, and 10-15 degrees from the vacuum unit will be a good place to be.

With a big cam as you are using in this particular engine, it's going to have low vacuum at idle speed.  As long as the carb is set up to provide plenty of fuel at idle, it should idle fine without running a lot of initial timing.....Cliff

[Flightcraft18xlt]

Ok thanks Cliff I will do some testing on the water and see what the max total timing will need to be and go from there.