Jeffrey

is there a rule of thumb for carburetor jetting? I'm looking for a chart that either uses cubic inches or horsepower as a measurement for the approximate sizes of jets to be used. Is there such a thing or is it simply guesswork and trial and error?

For example - a 650cfm carb comes with 72/78 jets (primary/secondary). If you were to use these carbs on a 500ci motor that you want to produce 650hp - are the jets about right or are they too small? How would this change if you started with twin 750cfm carbs? Or twin 850cfm carbs?

The whole process of choosing the correct cfm carbs and choosing the correct jets seems to be anything but an exact science. Can anyone clarify this issue?

Turbojack

I feel your pain.

There are charts out there for gettng you close for the cfm of carb per engine size & RPM you are going to run. Only problem is this stuff is for cars not boats. Talk to other people that may have the type of motor you are going to build. I see where some have good luck with large carbs & others ahve good luck with smaller ones.

On getting jetting right it is the old trial & error. I would start with what the carb comes with & go from there. When you start jetting if motor starts to surge at low speed jets are way to lean. If motor has blowers start richer that what stock jetting is. Always better to error on the rich side that the lean side. If over rich all that happens is black transom, fuel fouled oil, & fouled plugs. If lean, you end up with if you are lucky only melted pistons.

I have spent about 3 weekends this year trying to get new motor jetted right. I am now real close but not perfect yet. When jetting start at part throttle, when that part is right then go for full throttle jetting. I have seen on OSO everyone giving you what they are using & I see jetting all over the place. Jet size differents with different carbs, engines, cams, headers & who knows what else.

Jeffrey

I'm glad I'm not the only one experiencing this agony! you would think there was some type of standardized chart for high-performance engines. Even if the chart is for cars it would be a good starting point. Unfortunately, you don't have many cars running around with 650, 700, and 800hp motors!!

I added a B&M 420 Megablower to my 500ci motors. The setup came with twin 850cfm carbs PER motor. Everything is hooked up and the motors seem to idle perfectly. I have yet to run the boat but I know the jets are too big already. At idle - for five minutes - the transom is getting black. The jets are 82/88.

My gut tells me I need to back them off to something like 76/82 as a starting point. I'm going to give them a try and hopefully run the boat this weekend.

Biggus

Jeffrey, I have a similar set up, 38 Top Gun and twin 671 blowers and Superchiller intercoolers. I would foul plugs every time I used the boat and I played with the carbs to the best of my ability.I decided to send them out to Nickerson Performance in PA. I got them back 2 weeks ago and I am very happy with his work.The transom soot is gone and it idles real clean. I am running twin 750`s per motor and I have 74 pri. and 86 sec.

Give Dean Nickerson a call at 215-781-1370 tell him Kurt from Maine sent you

Turbojack

When a motor is at idle the jets are not used. The idle fuel ratio is set by the adjustment screws. My first guess is your power valves are blown. Second guess is the floats are adjusted 2 high, & third guess is needel/seat is not sealing correctly. I use to run a twin turbo motor with single 850 suck thru carb & if I remember right I had 88 primary & 99 in secondarys. So my guess is you may be close with what you have. As air flow goes down in a carb jets have to go up for the reduced airflow.

First thing I would do before going out in boat is take carbs apart, see what size power valves are in them ( my guess is 4.5 since you have 2 850's) Buy new power valves, install, & put carbs back together. If you have an electric fuel pump you can do next part with engine off but pump on. Remove plug on side of carb fuel bowel, adjust the float level unitl just a little fuel sloshes out of the hole. Put screw back in & go to next bowl to adjust level. Then see how motors do. I think you will be past that problem. Last guess was the needle& seat leaking. If leaking bad you will not be able to get floats adjusted correctly.

I would not go with any smaller jets yet. Remember if you lean out, motor turns into blow torch & pistons, blocks or heads do not like it.

John B

I agree with Turbojack.
Do not lean it out!!! Idle richness is not a jetting problem!!
The motor won't blow up because of a rich idle, but it almost certainly will with 6 sizes leaner jets. That will definitely ruin your summer.


Another thought: Most 850's have power valves on both metering blocks, (750's don't) and the throttle bores are the same size, therefore the jets should be about the same size in all 4 barrels. you may go a little richer over # 7 & 8 cylinders since they seem to run leaner.

850's usually come stock with #80 jets. I would start with at least #86's all the way around, and lean out if necessary. It's a lot of jets to buy.

I wonder what happened to the motors the blowers were on???
If they blew up, don't run it until you get the right changes made!

Call up B&M or Holley and ask them these questions.

JB