[Speakrdude]

Hey Guys, Great forum!

My question is, I have a 70's model Chevy 350, apparently with high compression heads installed on my 6000lb. boat. I have ran into a problem with "tuliping" a valve on #5 and then #8 a year later.

We discovered that in both cases the spark plugs were loose, due to a next to impossible reach with a plug wrench and the old style non-tapered plugs, thereby causing a lean condition in those cylinders.

After removing, replacing valve, and reinstalling the head the second time, I am trying to cover all the bases to help ensure this won't happen again.

This time, before we reinstalled the manifolds, we torqued the plugs to spec to make sure they dont come loose.

The timing, (I check often) is at 8 BTDC @600 and 32 @3000, and stops there.

The carb is a "Mercrusier Marine Version" Quadrajet (part# available later).

I have been told by my rebuild shop dudes that the valve problem could be caused by a lean condition due to High comp. heads+low octane fuel (marine fuel available at the docks)+ too small of jets in carb.

Anyone know of a suggested jet size I should start with or anything else I may use to diagnose this issue? We believe the jets are 72's.

Thanks,
Jim

[Speakrdude]

Or perhaps suggest a more appropiate place (forum) to ask?

[Big Mouse]

Well, I'm cautously stepping out of my comfort zone here. From what I understand, if you buy an aftermarket Q-Jet from one of popular rebuild shops, their "stage 2" setups typically go with 76 jets and 48 rods for the primaries. This is their generic "best guess" for mild street engines. The secondaries - I have no idea...

When I bought my Q-Jet from such a place, I noticed an immeadiate out of the box improvement on a mild 400 sbc.

I've been researching Air/Fuel ratio monitors. They are a little pricey, but cheap when you consider the alternative. It's just a thought, but you may want to consider something like this as a preventive step & as a tuning aid.

I'm not sure what the implications would be if you want to hook a wide-band monitor up to a marine application. I'm not sure where you would put the O2 sensor that is required...

[Big Mouse]

P.S. Where can you run a boat that size in Arkansas? The Mississippi?

[Speakrdude]

No, That's a great suggestion. I bet there is someone here that has some info on where to purchase/rent a sensor and or setup to plug into my laptop would be wayy cool. I bet we can come up with a spot to install a sensor. Lots of area to accomplish this.

Jim

[Speakrdude]

Beaver Lke in NW Arkansas. Lots of Huge boats up there. Home of Wal-Mart, Tyson foods and many other corp. offices.

[Big Mouse]

This is supposed to be a decent unit. It comes with the wideband O2 unit (0 to 5V I believe). It gives you a readout that will allow you to understand just how rich or lean you are running. You can then adjust your mixture / jetting accordingly.

Simple, but effective.

http://store.summitracing.com/partde...5&autoview=sku

[tostrick]

I am converting from TBI to carburetor on a 1992 4.3. I have a 1985
Rochester Qjet to use with the 1985 intake manifold. Carb. #17085210.
It has a gray colored connector on the front top side. Do I need to hook this
up or run without it? Do I have the right carb for non-computer. All new to
me. Thanks.

[1ownerT]

To get a reply to your issue, you should start a new thread instead of attaching to a old one that is not related to your problem.

[Big Mouse]

Originally Posted by tostrick

It has a gray colored connector on the front top side. Do I need to hook this up or run without it? Do I have the right carb for non-computer.

If this qjet has an electrical connection on it, it requires an ECM to function properly. I would suggest finding another, non-CC qjet.

[user151]

the 85 and 86 carbureted 4.3 V6 did use a computer for the carb and the distributor. nice intake but as Mouse said get another carb. (FWIW I believe the carb was 390 CFM)