Pure Energy

I had the carbs rebuilt this winter and noticed the following the first time out.

Motors started and idled fine.

It took more throttle to get rpms up to get on plane than before

After a short 3 min cruise we stopped and shut down to wait for another boat. Started back up fine, but when I went to plane off one motor started popping and stalled. Restarted 3 more times and the same thing. the forth time it was ok.

still took longer to get on plane.

Cruised for about 1/2 hr., dropped anchor for about 2 hrs. went to start engines and the same one ran very rough. With motor running i removed the carb box and it ran better. I put it back on and it was fine.

On the way home I tried a WOT run. At 4800 rpm it fell on it's face so I backed off.

Ran fine at cruising speed the whole time.

Could this be a carb problem? Too lean or rich? I do not know what was done to them but they were rebuilt according to dyno sheet and the previous mods done.

Motors are 502s with prochargers. Fuel pressure set @ 7 lbs @ idle and boost referenced. Boost is 6 lbs but is read from the manifold and doesn't register till 4000 rpm. Fresh fuel, new dis. caps, new plugs. Both motors have had head work and valve work. Both ran fine last yr. except for a sticking float, so I had the carbs rebuilt.

Thanks

tomcat

Who modified the carbs for blowthrough application in the first place and what were the previous mods done? My understanding is that proper mods include drilling the power valve channel restrictions (PVCR) so that they can carry a lot more fuel than normal. Power valve selection is important since you don't want these high flowing passages coming into play too soon.

Were these drilled out passages altered during the rebuild? Were the power valves changed to a different number?

Normal enrichment by way of the power valve is 25-40%. Since carb venturis don't increase their signal to the fuel metering passages according to the increase in density that comes with boost, blowthrough carb tuners will bring a lot of the extra fuel required under boost in through the PVCR. This allows the use of reasonable main jet sizes (90s instead of 140s) so that cruising operation is normal (not overly rich) but there is still adequate fuel at WOT/full boost.

Pure Energy

tomcat,

The carbs were done by Dean Nickerson. He returned the carbs via UPS and did not enclude any info on what was done. He also was not very helpfull on the phone when I called to inquire.

Work done to motors was what the builder called "super mod" head porting, and a competition valve job.

Thanks

tomcat

Just so I'm clear about it; the carbs were originally done by Nickerson and you sent them back to him for rebuilding?

Dean is not a very talkative guy at any time, and he may also be protecting his trade secrets, but I've seen one of his carbs perform on the dyno with an M1 Procharger kit. It ran rich under full boost with 90 something jets in the secondaries. I don't know what number the power valve was.

Do you have before/after head flow numbers for the super mod porting job?

Pure Energy

tomcat, I appreciate the help.

carbs were originally set up by Predator Marine (Predator Carboraters). He did the initial set up with the Prochargers.

Last yr. Typhoon Marine completely rebuilt and dynoed motors along with sending the heads out. This was done because they damaged both motors during a seatrial after replacing a head gasket. Both motors ran great after that but one of the carbs had a bad/sticking float.

This winter I took them to Dean for a rebuild and gave him all the info. He was quit takative then, and when I phoned the other day was not defensive, just did not give the details on what was done. Maybe I did not ask him the right questions.

THanks

tomcat

Are these Holley carbs or the Predator carbs that look like a box?

Pure Energy

Sorry, they are holly 750s

tomcat

Are both engines doing the same thing? Do you have fuel pressure gauges? What type of fuel pump and regulator do you have? Max boost was 6 lbs last year? What was boost/vac at cruising speed last year? What are those numbers this year?

You said that intake pressure doesn't register until 4000 RPM and it takes longer/more RPM to get on plane. If your heads have really been given a super mod porting job I would assume that means they flow A LOT better. This would tend to make the engine more sluggish at low RPM and would reduce the intake pressure readings at all RPM.

I wish I had more answers instead of more questions. Anybody else want to jump in here?

Pure Energy

I know it is hard to diagnos this way. Motors were fine last yr. except for a bad float in one carb. The only diff. this yr. is the carb work. Fuel pumps are aeromotive with malory regulators. Fuel pres. is set to 7 lbs @ idle. Cruise pressure at 3400 rpm is 10 lbs. Only one motor did the popping and rough start. Both motors only went to 4800 rpm before it "fell on it's face" and I backed off. This happened quickly so I did not see where the boost was. Last yr. both motors maxed at 52-5300 rpm @ WOT.

I have not been able to run the motors since then because I am repairing a fuel tank. I guess when it is back together I will have to check everything from the tank up to the carbs.

Again, thanks for the help.