MACDAD260

formulaF302, Your jetting does sound wrong. Usually the secondaries are bigger unless you have a front and rear power valve. Then they should be about the same. What did the plugs look like when you had all the jets the same size? I don't understand why you're getting such different readings from the front and back cylinders. The carb is pretty much in the center of the intake manifold. Each cylinder pulls off of wherever it can get fuel. If the primarys are feeding the fuel then they all just pull from there. When the secondaries are open that is just more fuel and air that is available for each cylinder. What type of intake manifold do you have? Maybe that is the problem. Uneven fuel distribution? Good luck. Keep us posted.

formulaF302

Well...I think I just found the problem.....I'm running square bore 4150 carbs on the stock marine big block spread bore intake manifold with 6.5 power valves on both ends of the carbs and #86 jets all around. The plugs still ran with no color on the front 4 cylinders and the back 4 looking like I had used a can of black spray paint on them. I had previously figured everything was all sealed up and ok cause I was using the summit linkage plates that were the same size as the square bore gaskets. But...alas..the experts are telling me that combination is nothing but an invitation to disaster. I think I agree with them. So.....I ordered a pair of merlin sportsman II dual plane intakes today that are specifically for the 4150 carbs. I've also since found that adaptors are mfgd to use a square bore carb on a spread bore intake...but I've heard opinons both pro and con about how effective they are.....so rather than take the gamble I found a good deal on the merlin intakes at $215 each. I'll hope to get $70 or so from each of my old mercruiser alum intakes to offset my cost. I won't have results for about 2 weeks due to scheduling problems...but I'll post the results then, and I expect them to be positive.

MACDAD260

formulaF302, I think you are on to something. The proper adapter plate may have solved the problem because the later carb motors that came stock with Weber carbs used that combo. However, I agree that you might as well just put on the right manifolds and get it over with. Good luck, Doug.

blown1500

DonMan, you are running in circles and the intake may make more power than the stocker, but it is not going to solve the problem. You may accidentaly fix it, but the intake will work if you do it right.
PM your number and I will call you if you like.

MACDAD260

I got a chance to try the new jetting yesterday. 83 p & 94 s. I finally was able to see some light brown color down at the base of the porcelin inside the plug. I am using CR43TS extended tip plugs so the tips may never get any color. It still ran great except it did seem to lose just a little of the snap that it had with the 78p & 91s jets. I need to try it again though before I make anymore changes.

Griff

Mac, those jets are pretty close to what I ran with an 800 on 425hp hopped up 454mag in my Baja. I think I ran 80 primaries and 94 secondaries with a 6.5 primary PV and no secondary PV. I ran Rapid Fire #1's as it idled better and didn't foul them out. I had very little color on them or 43ts's after WOT runs too.

formulaF302

I'm dying to try it all out....but the manifolds arent here yet and I'm leaving town wednesday for a nearly a week....damn...more delays just when we're finally getting some good weather here in Michigan....oh well.....

Johno

Where did you land up with the jet sizes?
I found this thread & was wondering. I am 81 on all four corners on a 509 motor , 800cfm carb with single plane Merlin intake. I am going jump my 2nd's to atleat 90's.
Later John

formulaF302

Ok....It looks like my new merlin intakes made for the square bore carbs solved my problems of puddling of gas in the intake floor. No more smell of gas after shutdown. It even runs at 800 rpm (with no choke) now when cold. I'm pulling about 10 on the vacuum gauge at idle, but am still smelling a little bit rich at the transom...but no black smoke. I leaned out the 4 corner idle mixture adjustments to get the vacuum guage up to 10....but when I took off with a very gentle throttle both engines backfired and quit running at the same time. I'm currently running all #86 jets with the 830 annular carbs and am gonna check the plugs tomorrow. Upon checking #1 plug today though the porcillin part was all white, but all the area around it (the metal part) was black and a little bit wet. I only had about 15 minutes of running in the 3000 to 4000 rpm range on it ...but is that an indication of too rich on the primaries? I could'nt get up to speed to check other things out...too much boat traffic out today......

Griff

Sounds like they were running a little rich at some point--probably at idle-- and when you ran them at rpm, they cleaned up.

Mine would always get dark, somewhat black at idle, and would clean up completely at any rpm off idle.

The plugs were probably carboned up when you started to accelerate off idle and couldn't provide enough spark transfer. The incoming fuel basically drowned out the spark you had.