Thread: No Power - Falling on its face.

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I have a 454 jet boat. I changed the low-rise offy 360 intake manifold out to a offy two piece hi-rise intake - Tunnel Ram style. My carb is a carter performance 500 . The boat ran great at about 5000 rpm all day long before the new intake. Now with the same carb and the HUGE intake my boat has no power on take off and it pops and misses everytime I put my foot in her. It runs great out here in the driveway with no load on the motor but in the water it falls on its face.

I also had to swap the HEI distributor for an old school accell 6805 with points bacause the HEI diameter was hitting the intake, So I had to put the smaller distributor in for the clearance.

I have one problem but I did two modifications, I think the Fire to the motor is OK, I am leaning towards a fuel problem.. Is it customary to have to re-jet the carb for more fuel delivery just because the fuel plenum is larger?

I am at witts end, it is hard to drag her to the lake for testing every little tweek made.
Heeeelllllllpppppppp.

Throw that dinosaur dist away and buy a MSD ready to run.

Hi rise intakes don't make diddly for power on the low end, they are designed to work best from 4000 to 9000 RPM. If you want to keep the intake, you have to be prepared to sacrifice the low RPM power. To make the dinosaur distributor work, might be best to throw away the points and put a Petronix system in the distributor. If it were my boat, I would take the tunnel ram off, polish it up and leave it on the shelf then put either the low rise Offy back on, or a different low rise type intake. Don will hopefull read this, he's a boat guy and would probably have some specific manifold recomendations.

So's what yall are sayin is BOTH mods were for not!! I should have saved my money and time and left her alone..My vanity got the best of me cause I was doing the intake for LOOKS.....I didnt think the boat would drop dead over a bigger intake. The distributor sounds like a must..I will heed your advice and upgrade the fire.. Thanks for the quick response and honest attitude...

yeah those big intakes just aren't good for a boat, you can only push the RPMs so high anyways.... a good manifold for a boat really would be a one of the older edelbrock torker 1s, they give a lot more low end power..... the torker 2s are ok but the originals are better.... you can snag them on ebay sometimes for less than $100

Done Deal: I am going back to a low-rise intake and a an HEI dist.. Thanks for the info. It realy set me straight.. I went for the Bling but couldnt make her sing!!!!!

Originally Posted by RatManDan

Done Deal: I am going back to a low-rise intake and a an HEI dist.. Thanks for the info. It realy set me straight.. I went for the Bling but couldnt make her sing!!!!!

denny is right a bigger carb and electronic ignition might make that manifold work better but i still think its a little big, probably doesn't match your cam..... and as a good mechanic i know always says, chrome wont get you home

there is a lot of jet boats that use tunnel rams this would have alot to do with the cam and if the jet drive has had any work done to it .if the cam can not go where tunnel ram will go for rpms? to make good power it needs to match and the jet need to work with the cam and intake if you are trying to run this with one 500 cfm carb? no way your fuel shot from your carb accelerator pump is is to small you would need two 50 cc pumps and new cams for the pumps .and i would run tube squirters. then the cfms are way two small you would need at least850 cfm and then your cam may not work with the intake but if you have it all right it will run like hell .on the points it will work fine if set up or you can buy a msd keep the points this will trigger the box. you run the white wire from the msd to the points and it makes the box work just fine. i did work on boats at boat yard and alot of the boats i work on had points in them and if they crap out all you need is a set of points. the all electronice you would need many parts to get it running if it fails in the water . and one thing i will add is the msd will have to be for a boat or one that is potted so the shock and moister will not kill it. moister is not to big of a deal on cars but is on a boat .so make sure the dist is vented or it will rust all parts under the cap

Wow: This is a lot of valuable information. I have done some more research on my current chevy 454 engine and have found the 3999289 block to be a 2 bolt, The 336781 heads with flat pistons pretty much sucks too. What is that 8.1 compression or less!! The guy that refreshed the bearings said the cam was barely over stock but did not give #'s. It looks like to me that this is bare bones stock 215 HP motor with a mini thumper cam. The only thing it has going for it is the Chrome Zoomie pipes that are very loud!! I am not interested in RACING this boat. I am content to cruise and ski. My vanity wanted a much stronger engine but if I can floor-board it all day long without failure it sounds like this is probably the right motor for me. I was happy with the way it ran before the intake change. I would throw my billfold at it a little harder but this engine profile doesnt seem worth the effort.

I have a marine accel distributor and feel that it not posing any problems with my accel super coil 40001-w/resistor and good wires. It might have a advance problem but it looks very clean. I am fairly sure the TUNNEL RAM was the downfall on this stock of a motor. The Carter 500 cfm carb was on here from the git-go. If I find a smaller intake with the HEI clearance I would still like to keep the LOOKS of the super coil. But I am not married to the coil. I put this intake on because I saw so many of them on other boats. I think those boys had performance engines, better pumps and deeper pockets. I am just wanting a sunday cruiser... I want to thank everyone for the EXPERT advice and time out your schedules to help such a chevy engine layman....I think I posted a picture of the motor and if I didn't I will. With that said I will be selling the OFFY TUNNEL RAM SILVER POWDER COATED (NEAR) SHOW QUALITY INTAKE. If no response from here I will throw her on Ebay..Oh Yeah- I need an oval port aluminum intake for this stocky motor.....