7.4L weber carb Jets

[mr300z87]

Hi all, Now that boating season in NJ is coming to a close and my running boat will be coming out of the water soon, I am turning my attention to project Sea Ray. See signature file for link.

Last night I started to dis assemble for rebuild the Mercruiser Weber carburetor I plan to use which is model 9772 and according to the Merc manual is for a 7.4L Bravo. Inside I found the metering rods to be 16-6852, pink metering rod springs, .101 P .098 S jets on throttle side and .113 P and .077 S jets on choke side, this is all how it should be per Merc Manual 15 V8 engines. My question is that in Merc Manual 16 (Big Block V8s only) it states .107 P and .098 S jets in both sides of the carb and there is a note that some carbs have staggered jets and they should be changes to .107 P and .098 S. The engine I am using is a late 1996 Gen VI 7.4, so during my carb rebuild do I change jets per Manual 16 or leave it as is? As always any advise is greatly appreciated.

Have a Great Day

Mike

 

[alldodge]

My opinion: Have the same carb on my Rinker and appears I have .104P .098S and the engine has 500 and some hours and Bravo 1. Mine is a 300hp peanut head 7.4L and the plugs looked great. The jets change as the hp and load changes (B3). The .107 might be high if you have same engine, or could be good if you have a bored engine or the 330hp.

 

[mr300z87]

Thanks for the response AD, the engine I am using I believe is a 330 HP not sure which heads it has and could not find a serial # on the block. Using the blocks casting number I believe it to be as stated a Gen VI. How do I tell what heads I have? I was going by the first line in the chart you posted which is where the I got the 107 and 98 size from. The set up listed in the 2nd line has 3 step metering rod and a different springs so still would be kind of different. I do have some carb experience with my Plymouth but fine tuning them has never been my strong point. I have another parts carb 9772sa that has 107 and 98 jet so maybe I will just put them in and try. Then after a sea trial do a plug check to see where I am at, if it seems to rich I can always change the primarys to 104s. Any ideas why they went from a staggered set up to what listed in manual 16? Head or intake Change maybe?

 

[alldodge]

Any ideas why they went from a staggered set up to what listed in manual 16? Head or intake Change maybe?

That would be my guess, but it's just a guess. The more volume or load the more fuel needed. As a B3 will need a bit more at idle in gear then a B1. The higher hp engine in most cases make it in the higher rpm, so idle should be the same other then load. just my guess

 

[NHGuy]

If you want to verify the jets are working right get an air fuel ratio meter and look for 12.5:1 at full power and under most loads.

 

[mr300z87]

This engine is going in a big heavy 5000LBS + hull. Since I do not know what this carb year/model its from but since it is set up with a divorced choke I and by the jets I can only assume that it is from an earlier engine than the one I am working with. Also note that judging by the paint it has never been apart. It is going to have a B1 drive behind it. BTW my test run was with this carb including the staggered jet set up. Idle seemed good and and it revved with no load OK. Think I am going start off as stated above 107P 98S and tune from there, I am concerned that with the 104 and 6542 rod I am going to be a little lean. NHGuy how would the o2 sensor get plumbed in?

 

[mr300z87]

AD what carb # are you running? Is your Rinker engine stock? BTW how is the other boat running?

 

[alldodge]

The engine is stock, will have to climb into the boat tomorrow if I remember (suffer from CRS) and get the number for ya

New engine running great, just need a new prop set to try and reduce slip, ordered them today 4X4 B3's

 

[alldodge]

Hey I remembered, :facepalm: no I didn't, the computer reminded me. My carb number looks like 9780 S 244, the 0 is the only one which was not real clear

Edit: it does match up, and its a Carter

 

[mr300z87]

lol AD, I suffer from the same disease CRS!!! I blame it on the 80s. Thanks for getting that info for me. Going by your post merc manual 16 and other research my engine should have 9780 carb like set up with 3 step metering rods and 104 jets just like yours. Unfortunately changing to that set up would be tough because the rods and jets are nla. Also I looked at how the Edelbrock 1410 BB marine carb is set up as this is said to be a direct replacement carb primary .113, Secondary .107; Metering Rods - .071 x .047; Step-Up Spring - orange (5" Hg). I would guess there would need to be some fine tuning required with that also. Some additional research I found shows a bunch of different carb adapters were used to convert the standard spread bore manifold to a square bore for the Weber carb this is also a possible reason for different jets. Going to run what I got and see what happens. Would rather be Rich and burn a little extra fuel than Lean and burn an engine. Will update my results.

 

[NHGuy]

Hi, I am starting to get ready to re jet my Weber WFB for my bigger small block so I have begun the research myself.
To answer your question about sensor plumbing. I'd get a sensor plate off of ebay. It goes between the exhaust manifold and the riser.
I looked at those brass hex plugs in my risers but they are a lot smaller than the thread size that sensors use. I'd fear breaking through the sides if I drilled and tapped them.
If you go for high performance risers; they can be drilled, peened together, welded and tapped. Seems like a lot of work to save $80. But if you have the skills and time it's a savings. Or if you want fewer parts or lower height it can be a reason to go that way. The plates are around 5/8 or 3/4" thick and require gaskets.
I'm going to try to get Bondo's take on the Edelbrock parts, which he praises.

 

[NHGuy]

Since the Weber is a Carter and is also an Edelbrock, you can go to Edelbrock's website and download their setup instructions. I read it over and it's very thorough. I plan to follow it.
I am a little concerned about how Carter and Edelbrock use two step metering rods, with flat top primary chamber covers, standard pistons and shorter jets. So I will test my original jets and rods first. If the idle and primary circuits are good maybe I (or we) can just change the secondary jets.

 

[mr300z87]

NHguy thanks for the info, my project. Is covered for the winter so not much testing going on here. I am working with a bone stock 96 97 vintage 7.4 and a stock rebuilt carb. Once it all back together and I sea trial I can do a plug check to see how she is running. Remember my boat is a big fat cruiser not a performance boat so I just want it to run right. Keep me posted on how your goes.