mike52995

I am stumped, let me first tell you that I am a 100% self-educated hobbyist that is trying to build a motor so please be patient if I miss something here. I have a 350 that started out as a Volvo Penta 5.7 GIE. It has the stock vortec iron heads that received slightly bigger valves but I do not know the exact specs, bee hive springs and roller rockers. The cam was replaced and the spec’s are 290/308 290/314 lobe is 112+4. The blower is a Paxton Novi 1200SL at 5 psi blowing through a Quick Fuel 650BAN blower carb that is fed by a Quick Fuel 175 pump and QF regulator that is boost referenced off of the bonnet before going into the carb as instructed by Quick Fuel Tech and is set at 7psi. The intake is an airgap. The ignition is supplied by MSD and a 6BTM that is referenced off of manifold vacuum. Initial timing is set at 16 degrees and total is at 32 degrees all in by 3200 rpm. I am currently using the stock exhaust manifold and risers then straight out the back. I know this is a bottleneck but it will be addressed.

I am tuning using a FAST air fuel meter and the o2 sensor is about 4” up in the riser from the exhaust manifold. I am having 3 issues that I could use some help on. The carb is jetted with 74s on the primary and 80s on the secondary and the idle air bleeds are currently at 80s all the way around. The fuel level is dead center on the site glass on both bowls.

Issue 1: I cannot get the idle A/F richer than 10-10.4 short of almost bottoming out all 4 screws at which point I can get 12-12.5 and absolutely no throttle response. Bleeds initially started at 70, then I tried 74, now I am at 80 and there was virtually no change. I need the have the mixture screws about 1.25 turns out to get throttle response at which point it is idling in the 10’s and the soot builds up on the back of the boat.

Issue 2: off idle to cruise (3000 rpm) no boost I am at 13.5-13.8 which I am happy with. Wide open at 5psi of boost I am between 11.7-11.9 which I am also happy with. My problem is that from 3200 rpm’s until I get into boost I lean out into the 16’s. I know the engine isn’t under much load and several people told me not to worry about it but it isn’t right. The power valve is a 65 and I pull about 17” of vacuum at idle. I have checked the plugs after a short run in this range and they didn’t look bad but after cruising a little while I will notice a stumble.

Issue 3: After a wide open run of say 2 minutes, it seems like I lose power. I will throttle down for a bit and then go wide open and it will pull strong then fall off. Feels to me like a heat related issue. The plugs are gapped at .042. In this case I monitored everything and I have 12psi of fuel pressure under boost, boost is remaining at 5 so I ruled out fuel issues and belt slip but I can’t figure it out past that.

Any suggestions would be greatly appreciated. I have read hundreds of threads and tried the fixes but I am stumped on these issues.

Thank you!