4.3 OMC Cobra: 2bbl to 4bbl Conversion Update

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eja

Cadet
Joined
Jun 2, 2002
Messages
20
Hello again. I am just wanted to update anybody that has an interest in pepping up their 4.3 2-bbl. <br />My 1986 Wellcraft 19 ft Open Bow is equipped with an OMC Cobra 4.3 2-BBl rated for 175hp. A few months ago, I started inquiring in this board about converting to a 4-bbl. (See 4.3 Wake-Up Time) Well, I did it.<br />Bought an Edelbrock 1409 Carb (600 cfm), and an Edelbrock Aluminum Intake. Installed with no further mods. So far it is a disappointment. I've adjusted to no avail. The Carb was refittted with the smaller jets specifically for a 4.3, and I'm turining the SAME rpm as the 2-bbl rochester did. I may have picked up 50 rpm, but it did not change the speed at all. I need to spend more time adjusting, I guess. there is a bit of hesitation at mid throttle, and I think I'm just using more gas at WOT. If there are any recommendations, please let me know. :confused: :mad: <br /><br />I will keep you posted.
 
C

Captianloud

Seaman Apprentice
Joined
Jan 16, 2006
Messages
43
Re: 4.3 OMC Cobra: 2bbl to 4bbl Conversion Update

I recently repowered my 94 Bayliner 1850 LX 4.3 Mercruiser, with a self built and modified 4.3 I am running the 4 barrel Weber. <br />If I may offer some advice. I am pretty sure the Edlebrock Aluminum intake manifold is a dual plane manifold, while the stocker is a single plane manifold. I was thinking of using the Edlebrock manifold when I biult my engine. So I did a little research, the single plane manifold actually outperforms the dual plane at or near WOT. So for me, I stuck with the stocker. <br />Also the hesitation you describe and high fuel consumption indicates you are not properly carburated. (although 4brrl will use more gas at WOT)<br />Install new spark plugs, run the boat at WOT for 90 secs + and immediately without returning to a lower throttle setting turn off the key and check the plugs. See what your burn looks like. Rich? <br />The hesitation at mid throttle is it while accelerating thru this rpm range? while holding a constant throttle opening? <br />Here is my engine<br /> http://users.adelphia.net/~chrisradvanyi/
 
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eja

Cadet
Joined
Jun 2, 2002
Messages
20
Re: 4.3 OMC Cobra: 2bbl to 4bbl Conversion Update

Well, sounds like we have the same carb.. My Edelbrock has "Weber" stamped right on the side..<br />You think the difference in a single plane vs. dual will have a significant impact? I thought the dual was supposed to produce more low-end torque... <br /><br />Anyway, when accelerating normally, the hesitation is quite apparent around 2000-2300 rpm. When I nail it, I typically don't notice it (only been playing with it for a couple of hours total).<br /><br />I do notice that when holding around 2300 rpm, the engine is "surging" slightly.. I mean you can feel it slightly changing rpm up and down maybe +/- 50 or so. Wierd.<br />I will try your spark plug test tomorrow to see if it's actually rich or lean. Again, this was spec'd out for the 4.3, so the jets are smaller than stock.<br />The manual is crappy for a beginner. Can you describe the best way to set up the carb, using the low speed air/fuel screws on the front?<br /><br />At WOT, I cannot be sure I'm using more fuel, it's only an assumption at this point. I never really paid alot of attention to the 2-bbl consumption. Regardless, if this ran like I expected, I wouldn't care about fuel; If it continues to run like (or worse in the case of hesitation, etc) the stock 2-bbl, I'm gonna rip it off and sell it on Ebay. The carb/intake is brand new.<br />Never touched the CAM, or anything else. I am using the standard 19p aluminum prop for setup, and am able to hit 4300-4350...barely. Like I said before, the 2-bbl was 4250-4300 barely at 42 mph (according to my speedo), and now I can hit a rock solid 42.5 - 43 mph... :( <br />Not worth the $500+ yet. My buddy is a mechanic at heart, and thinks we just need to mess with it some more, and it'll work out. He thinks it's running fat, too. BUT he (as you) are working only with what I'm saying, and hasn't seen it. I'm not sure that I can buy smaller jets than the 4.3 ones??<br />stock 4-bbl 4.3 are rated for 4800 rpm, the 2-bbl are rated for 4400. So far I'm not hitting either one. BTW I replaced the rotor, cap, wires and plugs, too, so I don't think that's it.<br />I have a shift prop, and without upshifting, the 4800+ rpms can be achieved, but with a 19p, not a chance.<br /><br />Thanks for your help!<br /><br />BONDO? You are familiar with this setup. Any suggestions?<br /><br />Thanks,<br />Erin
 
R

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: 4.3 OMC Cobra: 2bbl to 4bbl Conversion Update

try that high speed shutdown and there is a good chance you will post about hydolock.<br /> this is something I used see a lot of and never seen it work.<br /> good luck with it.<br /> are the camshft part numbers for the 4bbl 4.3 and the 2bbl 4.3 the same ?
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
13,055
Re: 4.3 OMC Cobra: 2bbl to 4bbl Conversion Update

My 4.3 came with the 4bbl Rochester, I have never driven one that had a 2bbl to compare, but for sure you can feel the surge in power once you pass about 3500 rpm. Is the Weber/Edelbrock supposed to be better than that marine Q-Jet? How much of a difference does a single plane vs dual plane make? <br />I wouldn't do that spark plug test either, goes against what is advisable with marine engines. don't want cylinders full of water!
 
Boatin Bob

Boatin Bob

Lieutenant Commander
Joined
Sep 24, 2001
Messages
1,858
Re: 4.3 OMC Cobra: 2bbl to 4bbl Conversion Update

According to the BRP website the camshaft has a different PN for the 2 bbl and 4 bbl BRP Website
 
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