OK, getting motors back in the boat and looking to fire them in a couple of weeks but for now I am redoing the carbs.

I have the quadrajet carbs # 17080561 / 1725

The primary rods are #43 with #70 jets.
Secondary rods are DW

HISTORY:These came off of my 1986 formula 272 with 260 HP mercruiser 350's. Flat tappet cam, two piece rear main seal crank, cast iron intake and the standard crappy GM heads from that era. One of the engines had a melted edge on the #6 piston that put a groove in the cylinder wall also. Not sure if motor went too lean or what. Other motor looked fine.

The boat now has Vortec motors from the mid 90's. One piece crank seal, standard mercruiser roller cams, vortec heads and a dual plane aluminum intake manifold. Still stock mercruiser exhaust and stock pistons. Now supposed to be about 295/300 HP.

Tuning Question:
What rods do I use? I looked for new rods and my DW rods are on the larger end of things (leaner). summit racing has a DP rod at 0.0686 but is a short tip, and then it has a CL which is a long tip. My DW rods are a 0.0874" tip diameter. The CL long tip is a 0.0667" tip diameter. That is an increase of 30% in flow area for the 0.135" diameter secondary jets. Assuming 60% of the total mixture comes from the secondaries, that is an 18% increase in fuel flow if I do not change the primaries.

The HP increase is about 15% to 16%, so maybe this is good? Where do I go from here?

How much do I increase the flow (decreas rod size) of the primaries with the secondaries having such a large increase at 30%?

Thanks to any advice that can be offered.

Brian

No one has tuned their quadrajets???? Really?

Even if the numbers are not the same, has anyone played with the rods?

Brian

befu,

Those motors should be in the 315 horse range. At least that is what i've seen them avertised at for repower (2 friends have repowered with them).

Although I don't have direct experience tuning the quadra jet, I do with the edelbrocks which also use rods on the primary side. I can tell you that the 600 ran perfect out of the box with a 290 horse 350. Going to vortec head engine with an LT4 hot cam, beehive springs, air gap intake, SM exhaust etc. gave me an honest 80 more horse (about an 8 mph gain).

The 600 drove this combination okay, but it felt lean through the midrainge, so I went 1 step richer on the rod and it feels perfect. I may try 1 step richer jet on the secondary, but it is close now.

My point of all this is to try the quadrajet in stock form, it may be fine. They are a 750cfm with huge secondaries. If there is a lean spot, just go richer the smallest step you can.

Brian...this is the Q-Jet guy...the Guru!!!

http://www.cliffshighperformance.com/

From personal experience...I used National's 454 Marine carbs ($199.95 ea) on my 260's. Bolted on...no adjustment...ran great. I ran 32 deg total advance with these cabs and was able to focus the mixture to 21 " of vacuum at idle. Engines ran great. I might use the as a starting point on my Vortecs before I run the DP Holley's.

Seriously...Cliff knows Q-Jets and he's not far from you.

+1 on Cliff but you may not have to touch it , Quadrajets are very forgiving . Try running it first , take some plug readings . If it needs rebuilt or updated for ethanol use one of Cliff's kits they are excellent .

theres a company called jet performance that can help you with tuning your carb they are in calif give them a look up i have been using them for years and they do good work . they may need to know compression ratio and cam specs.

if you use the thin tip rods in secondaries check to may sure that the secondary flap on the top of carb opens all the way, there is a tap on side on carb that you may have to cut or trim. this will increase air flow to go along with the richer rods. been along time since I have messed with Qjets but for some reason I remember a Rod with CV which was very thin. jetting a Qjet will give same top end as a Holley at same cfm but will not have the mid range punch.

Ok so maybe the link would a little http://jetchip.com/ . I did a rebulid on a boat that i picked up and upped the 454 from 330 hp to 420 hp and iI gave them my carb told them what I had done to the moter and they drilled and modified the carb for about 200 Of course it helps when they are 5 miles from my house.