alcor

Man was that costly!. Rebuilt 454's last year, had popping on top end, turned out previous owner (with run out engines) had the Holley 4150 dual pumpers set-up with #70 jets in primary and #80's in secondary, that's stock for this model Holley. At 12 hours, started stepping up the secondaries two steps at a time; popping moved each time to higher rpm before it occurred and plug color began moving from bright, bright white to touch of brown on most of them till we finally settled in at #88 jets on the secondary. That all occurred over a period of about 22 hours of running time. At about 35 hours, dropped an intake (SS Manley) valve at 3900 rpm and BAM, hydro-locked the starboard engine and the rest is history. Amazing how much damage that causes in a matter of about 3 seconds! During rebuilt, noticed excessive wear on a couple of valve guides, a few seats and valves. Mechanic and I decide, long story made short, we had to have ran them to lean to long during the re-jetting process, at least that's what we think.

So, here's the question. When trying to get the jets right, how do you determine whether the primaries should be increased? I mean, it was popping on the top end and plugs were white white, so we increased the secondaries till that solved the popping and plug color began sow trace of brown, engines performed great, even had better throttle response in the last 1/3 of throttle which didn't real have before. The fact is, we run at about 3600 rpm (cruise) most of the time just before those secondaries begin to open. So, how do we know we're not running them lean up to the 3600 rpm range. Engines idle perfect, response well and never popped in those mid-range setting.

Before I fire these new rebuilt GM 990 heads with fresh SS valves, seats, guides etc. up, I could sure use some fresh input here?!?!?. While the plugs were showing a touch of brown, this experience is telling me increase the secondaries another couple of steps say #92. If the plugs go black, we back off a set or so, at least I don't run them lean again. Thanks in advance for any suggestion or thought you may have.

Al

alcor

The primaries are still 70's and as I began to think this thing through, it makes perfect sense that the primaries should be change as well. I have sets of jets ranging for 80's to 90's now. I agree, I need to change those primaries so what do you think about me sticking those 90's in the primaries as a starting point and a seat of 92's I have in the secondaries. Al

alcor

Well, then that's what I'll do. I've got a set of 86's I'll put in the primaries and see waht I get. If you think of anything else, let me know. I really appreicate your response. Al

alcor

Thanks again for the help, I'm strapping all the accessories on today and should have them running by the end of the day at least in the garage, my trucks gone for the weekend so can't put it in the water till next weekend. Will let you know how I make out. Al

PowerboatsNW

Don't get a One track mind here and just think about jetting. Where is your total advance at on your timing? Also when do you bring in your total timing? Be sure and start with a clean set of plugs. Check for blueing on the electrode. Do plug checks starting at 2000RPM's run for 1-2 minutes then chop throttle be sure to be careful of water coming in the exhaust or over back of boat, however avoid idling as much as possible to get clean plug reading. Then check your plugs. If you do this in 1000RPM increments, and know how to read plugs, all the way to WOT you will have a well tuned motor, not just the same jets as everybody else runs. Remember, when you are at lower RPM's and just running off the primaries only adjust the primary jetting. What kind of intake system do you have Single plane, dual plane, high rise? Believe it or not all this stuff is extremely important.

alcor

I have dual plan intakes. Interestingly enough, I noticed that the the Thunder Bolt ignition packs have different numbers; one is 1622A 16-14 and the other reads 5B71A 16-14. I've always understood that the total time should be in the 32 degree range although I've never figured out how to check that in the boat given the tight quarters. The engines seemed to run good when the timing was set at about 16 degree at 1000 rpm. If you set the timing much lower than that, when I took the engines up to about 3000 rpm to get on plane, they seemed to hesitate as if they were flooding or something like that eventually "catching" and running ok at cruise or better rpm's. If you came off plane and went at it again, I get the same results, thus the way I arrived at setting them at about 16 degrees at 1000 rpm. For some reason I remember someone telling me these packs advanced the timing 14 degrees. Any thoughts on that? Al

HiPerf2000

iTrader: (2)

i dyno every motor that i have built. takes out all the guess work.

But mid 80's and low 90's sounds like a good starting point.

alcor

Thanks again Formulaz583. It was an expensive lesson but I'm glad I check-in with you guys. Finished the install last night. As soon as the neighbors wake-up, I'll crank them up and see what happens.

I'm still trying to figure out how theses carbs ever ran on these engines prior to me buying it. Makes me feel like there may be a rat here somewhere. On the other hand, everything else was such a mess, perhaps the I engines where run out (plugs where always black when I bought it) guess the engines where running rich due to oil, low compression, etc. I'd rather believe that than the rat theory.

You think changing these jets this much will help actual performance? you know, Spppeeeeeddddd. Thanks again to all you guys!

PowerboatsNW

32 degrees should be a good place to start with the timing. Try to bring the full advance in by 3000rpm's. Crane makes an excellent ignition system for carb motors. You can set any timing curve you want. This will work with your distributor also. Good luck hopefully there is no rattle snakes in your motor.

TPCRacing

I know I am new to this forum but I have some great advice for this particular application. As soon as I get to the office I will post a tuning guide which I drafted up myself. Yes I am a pro-carb builder and I often hang out in the Hotboat forums. Nonetheless I will post it here online which should allow you to reset those carburetors back to zero for tuning purposes. It covers alot about the idle/off-idle and proper jetting with respect to timing.

Remember one thing...fuel pressure and setting the valves are critical here. Sorry for the tease...I should have this posted by this afternoon.

~Ty