jr

Dennis, last year I built a couple 409 SBC's for my boat. Actually rebuilds.

I'm running a Comp Cam XM262H Hydraulic cam. World Product Torquer II heads that have been ported and polished with 2.02/1.60 valves and 170cc runners and about 9.5:1 compression.

I have a 750cfm 4150 holley double pumper on top of an Edelbrock Performer RPM intake which was port matched with a 1 inch 4 hole matched spacer.

The old intakes I took off were Edelbrock Victor Jr's.

Which would be a better intake for this set up. The RPM or the Victor.

Also in your experience how restrictive is a Mercruiser 350 flame arrestor. I get to about 4500 rpm's, I have about 25% throttle left, but the secondaries (mechanical) haven't opened yet. If I go above that 75% I get nothing more from the engines. They don't seem to bog just no more rpms. The throttle blades are not opened all the way at 75% either.

Thanks for any insight you may have. It good to see you here.

Jeff

Dennis Moore

I would guess that you want the engine to run at about 5,000 rpm. I don't know which manifold will make more horsepower at 5,000 (it may be that they make exactly the same, the Victor Jrs. are good intakes) but I am pretty sure that the dual plane will make more torque and better fuel economy before 5,000. If you ran the engine at a constant 5,500 rpm you MIGHT want to choose the single plane. I don't see many boaters doing that!

I am a little unclear on the throttle problem. If you have a double pumper carb I don't understand why you are not getting full throttle, these are mechanical secondaries.
Make sure the jetting in the carb is stock (I hope nobody leaned out the carb) and I hope you ARE running a marine carb. A marine carb has a little different metering circuit for the fuel (richer in the mid range for the harder pull a boat engine has in making mid range power). It's not all about being USCG approved! I really like the Edelbrock Marine carb for good horsepower and dependibility!

Set the ignition total timing advance at 34 degrees.
Add a K+N Flame arrestor.

Sounds like you have a nice combo, I really like the heads and camshaft selection!
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE

jr

Thanks for the info. Unfortunately I'm not running a marine carb. I know thats a no no, but that wasn't in the budget for the rebuilds, and they were carry overs from the old boat anchors the previous owner built.

I'm running 22 inch Hydromotives on the boat now. I figure with the engines running as they should I would see 70. But the best I'm at now is 59.8 GPS. And 4500 rpms. I can run it up to 5000 but my slip goes up too because I'm trimming out. But I loose speed.

I figured that the flame arrestors may be my stumbling point, but I was afraid that the 170cc heads were alittle on the small side. I do have the total advance set to 34 degrees.

I looked at plugs about 5 minutes ago and they are all tan to slightly black. So I'm not too worried about lean conditions. I did a quick compression test on the cylinders and they are all at 170-173, except for one at 160, but I did it cold.

On the carbs I can get full throttle but at 75% on the levers and up from there is no change in RPMS. At 75% the secondaries start to open. All the while from 75% up the primary blades are moving to fully open.

I figure with a 6000 lb boat and twins with these engines I should probably turn 25 inch props. I'm just wondering if I'm not torquing the engines up enough down low to get the secondaries to open at 3500 or so.

Thanks for the advice. I hope that clears up what I was trying to get across.

Jeff

mcollinstn

I know this is a Dennis question, but I'm vague on what you are describing.

Are you saying that the secondaries do not open fully at WOT? A 4150 carb is a mechanical secondary carb and they should open fully at any rpm, including with the engine stopped (unless they have a cold interlock).

Are you saying that the secondaries DO open fully, but that you get no additional rpm out of the motor when they do open?

Have you done a pull from 2500 rpm to top rpm at WOT and held it there for 10 or more seconds, then chopped the throttle and cut the engine for a plug reading? I'm initially of the opinion that your secondaries are jetted too lean.

If you are at 2500 rpm and you hammer the throttle, do you get a bog?

mike

mcollinstn

Are you sure that you have good fuel pressure all the way to
WOT? Have you set your float levels on all the bowls with the boat in the water?

I saw that you had looked at your plugs, but if you haven't done a "chop" at WOT, then you are getting a reading on your A/F mix at idle, not WOT.

Just throwing things out there..

M

jr

Mike, I know where you are coming from.

Whats happening is when I have the boat out and I run it up, and trim out the drives, I get 4500 RPMs max. At that point of having 4500 rpms, The throttle levers are at approximately 3/4 of the way to the stops. If I put them to WOT I get no increase in rpms. I stay at 4500.

Now looking at the carb, when the sticks are at that position of 3/4 of WOT or 4500, the blades of the primaries arent open all the way, and the secondaries are just beginning to open. If I put the levers to WOT then the primaries are vertical and the secondaries are vertical, wide open.

Yes, I have set the floats with the boat in the water. And no I havent read the plugs after shutting down WOT. I'm alittle leary of breaking something with all that force suddenly hitting the prop at 60. But just pulling them and getting a read I see light to dark tan with some carbon, so I feel I'm pretty close.

If I'm at cruise of about 3000-3200, and 45 MPH, If I bring it up smartly you will be pressed back in to your seat with out any bog or hesitation. I haven't hammered it with the new engines because 400+ HP and Alphas don't match to well, even if they are SS's.

The reason I mentioned I was wondering if the engines were being loaded enough is that I want the secondaries to open around 3500. If in the throttle curve they don't start until 4500, bigger props would bring down that curve. I would like to max out at 5000. But if I cant get enough air into the engine then I may not get past 4500.

My thinking is like riding a bike. If you have it in first gear your legs spin like mad but you go 10 mph. But if you put it all the way into 21 it take while to get there but you move at 35-40. You have the power but you have to apply it were its needed.

Twisted logic maybe but I'm on limited funds so I would like to get close with the first swing at this.

I think Dennis is right, with first getting carbs, and K&N's. First that will put the fuel where it should be when it should be by having marine carbs. Second the K&N's will get me the air.

I think it may be combination of restricted air flow and the wrong carb. Since he liked my heads, I'll rule out the smaller runners, thank GOD!!! I could see my wife turn purple when I told her I would need new heads. No thanks.

Thanks for the thoughts.

Jeff

mcollinstn

I'm with you.
Yeah, 3500-3700 is where "most" guys start getting into the secondaries. You are all on primaries all the way to 4500? Jeesh.

Are you sure you're not accidentally running 850's? (kidding)

Are your flame arrestors the little bitty thin ones? I'd think a swap to the 3" tall ones would surely be needed.

It's not recommended, but I'd like to hear how it runs with no flame arrestors and the hatch chocked open 2 or 3 inches (to rule out any possible chance of restricting air into the carb itself). Be sure to have a buddy with a big extinguisher and a big quilt.

Griff

jr, first it sounds like you are running out of fuel when the secondaries open all the way. Check your fuel pressure.
Second, it sounds like your linkage to the secondaries isn't opening soon enough and needs to be adjusted. Engine load has NOTHING to with when they open. An auto carb is set up differently and these may have been set up not to open until near WOT.
Third, get some marine carbs!!!!!!!

jr

Great suggestions. Griff, I'm with you kinda. Why don't you feel that load has anything to do with the secondaries opening. They will open at a particular throttle setting. If I torque the engine up more, loading it up more with a larger prop, then I will require more throttle at a particular rpm. The thinking is that will open the secondaries sooner because the engine is working harder.

Anyway, I think the first thing I will do, is put a pressure gauge on it, and pull the arrestors. Ya, Mike they are the little bitty ones. I wanted to get Marine carbs anyway. I need to throw that in as well.

Thanks guys.

rws

What size jets are you running?I have had a combo close to that and if I recall jetting was 64 primary 68 secondary my carb is 780 vac. secondary.Also ditch the stock flame arrestors.

Rob