Holley jets.....how many ## increase at a time
 

I am in the process of replacing the factory 2 barrel Holley carb on my vortex Volvo Penta 5.0GL SBC. It has a 4 barrel spreadbore manifold from factory and I bought anew Holley 4175 spreadbore carb. Volvo Penta uses same basic carb on theier 5.7GL motor. I talked to Holley and set the carb up same as the stock 5.7 carb. (61 main and 2.5 power valve). I sea trailed it tonight and ran it a cruise (easy 3000 RPM) for about 5 minutes and did a plug reading. When I shut it off it It diesels really bad, not a good sign. Any I checked #1 and #7 and both were pure white, way too lean. So I idled back to dock and pulled carb apart. My question why is it so lean? I am surprised it is so far off as compared to the 5.7 jetting, I figured it would be closer with the 61s. My question is how many numbers should I jump up. have a set of 67, and a set of 69s. I think I will try the 67 tomorrow night, does that seem logical? Please forgive my ignorance, I did a search and learned a ton on here and ordered a couple Holley tuning books from amazon but they are not here yet.

Too much fuel is way better than burning holes in pistons. Go big and work your way back. If you have 69's:D start with those. 67's will likely be big enough, if it ran decent on 61's I'd guess 65 will be the ticket.

Dieseling can also be attributed to too much or too little timing, especially base or idle timing (on the too little side) Too little base timing can cause the exhaust valves to glow cherry red and cause the dieseling. But as I reread your post, it doesn't sound like you messed with the timing since you are using the same manifold.

Yea since the manifold stayed in place the timing is the same. The Holley spreadbore has a metering plate on the secondary side (no jets or PV. I am going to call Holley today to see if they have different ones of those too.

This is a good lesson to learn!!!

I disassembled the carburetor to make sure all passages were clear (no flashing of debris from the manufacturing. It appeared clear. So I went to size 69 mains and sea trailed it tonight, it is still lean and dieseled slightly after I cruised at 3000 RPM for about 3-4 minutes. Plugs had some slight color (black) where the porcelain was shielded by the ground electrode but the majority was clean white. So I quit before I hurt it (69 was largest jets I had). I am slightly worried I am so far away from the initial setup. (had 61s from Holley). Is 4-5 minutes long enough to color plugs? I sure don't want to hurt it so figure checking early is safer. Once it is jetted properly will it not diesel upon shut down to read plugs? I say this because I never have shut boat down in this manner under normal operation. Should I buy like 80s and make it fat as heck and start way up there?

4-5 minutes is more than plenty if the engine is warm. 30 seconds to a minute should suffice.

I am not familiar to your spreadbore holley, but I think that your primaries are plenty rich, but the secondary mixture is too lean. I would get more info from Holley about this to see what they recommend.

I do find it odd that your stock 5.0 is drawing more air than a stock 5.7. Or at least it appears to be.

Edit: It may be wise to purchase the complete jet kit ($50.00) instead of spending $5.00 per jet. I learned this lesson the hard way.

One thing you could check is that the secondaries are not opening too early. Sometimes if they open too soon they can create a lean condition. There is not enough air velocity to pull fuel through the secondary side. Could be the case on a small cubic inch motor???

Also, you can purchase metering plates to fine tune. There are a couple of aftermarket conversion plates that accept standard Holley jets as well.

I talked to Holley again and the rear metering plate is equal to a #75 jet. The factory setup for the VP 5.7 is a equal to a #65 jet for rear plate. So I am now 9 sizes larger then factory setup on primary jet and 10 larger on secondary side. Holley guy said no way, there is either something wrong with casting of the body or metering block or a defect of some sort. I have taken it apart twice looking for trash and it is clear as far as I can tell. Although it is new, since I bought it from a 3rd party guy no warranty. They said for $125 they will "flow and repair" it. Downside is if it need a goofy part (like a metering block) that is extra. Oh, and 8 week turn around time.

Years ago I ran into a similiar case when changing the intake/carb set up on a pair of 454 Mags. I installed the HP 500 intakes and 750 Barry Grant Stage 3 modified Holleys. Had the same lean problem you have here, rejetted the things a dozen times in one day and could not get away from the lean condition. Started in the low 70's on jets and was all the way to the 99's in the secondaries. Called BG Fuel and their tech suggested a 1" four hole spacer under the carb, back to the std. jets and power valves, and start over. The guy was spot on, real close, I made one change and they were right on the money. His explaination was that the manifolds were causing the velocity thru the carb to drop, resulting in a weak signal to the carb, and therefore low fuel delivery regardless of the jet size. A carb will only deliver so much fuel based upon the signal/request it recieves from the engine. You might try adding a spacer, just be sure to use a 4 hole not an open spacer, the open spacer will increase the manifold volume and likely make your problem worse. The spacer will increase the signal and increase the air velocity thru the carb and will carry more fuel to the engine as a result. Might just cure the problem, good luck.