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I put a gm preformance 350/290hp (LM 1) crate engine in my 61. I added an edelbrock performer intake and 600 cfm carb. It seems to run very rich. I leaned out the carb and you can still smell the fuel at idle. So I rejetted the carb from .095 to .092 primary jets and still no luck. Timing is good, I put on a vacuum gauge and leaned out the carb as much as possible but it still smells likes it running rich, real rich! Its hard to keep your head under the hood at idle. It runs well but it just smells rich! Any suggestions or am I just getting old!

Mac

I used .095 primaries and .092 secondaries in mine and am happy with it.

What kind of carb are you running many have a dedicated idle circuit and changing the jets will have no effect at all on the idle mixture. So what happens is one will keep jetting them down and end up with a rich idle and a lean part to wide open throttle.

The carb is an edelbrock 1406. I follwed their tech sheet to lean it out one stage. I changed the two primary jets as well as the metering rods and step up springs. Maybe I need to go two stages lean?

Did you check the float levels while you had it down? You might try lowering them 2/32. How does the 290 idle & what is the vacuum reading? I,ve used many of the 260 HP but have never tried the 290. Are you using a manual or auto. trans?

When you say timing is good? What do you mean? Are you using a vacuum advance? If so, plug it or delete it. Does the engine have fast burn heads? If so don't use vacuum advance and total of 29 degrees of timing should be in by 3000 rpm. If not using fast burn heads, set base timing to 20 BTDC and eliminate vacuum advance. Then try tuning your carb.

Adjust your valves.

#92 jets for a 350 are still extremely rich. I have a 427 Chevy with #66 jets in a Demon (Holley type) carb, and it's perfect. Try coming down a LOT with your jets.

Of course, I have no idea how jet sizes map between a Holley and an Edelbrock carb. Still read your plugs and look at your exhaust and go leaner.

The Edelbrock, AFB, & Q-jet carbs have metering rods in side the jets. They take larger jet sizes.

Well here's the latest. Hooked up a vacuum gauge to the manifold, tachometer, and timing light. timing is a 20 degrees btdc, idle speed at 8500 rpm, adjusted primary jets to gain maximum vacuum (about 20 hg) then leaned it out just a bit to get rpms to drop slightly. This is supposed to be right according to the carb manual. The plugs look good just smells rich. I am running muffled side pipes and the collectors may have a leak. I am wondering if that is where the rich smell is coming from. As for driveabilty, it runs well. With a 3:08 rear I'm not breaking any quarter mile times but it runs well.

handymanmac, you have somekind of engine that idles at 8500 rpm, my experience with smallblocks is that 8500 is a good top rpm.

If the plugs arn,t getting any black soot on them it sounds like you are right on. Yes 8500 is a lot of RPM. It would come apart before that. Even 850 is a little high. I like about 700-750 but it depends on the engine.

20 degrees BTDC? Is that correct?

Well Cletis, that may be correct for an engine that idles at 8500 rpm.

Guys sorry for the rpm typo. Idle rpm is around 730. I set the baseline idle at 750 as I adjusted the carb to get max vacuum then leaned it out to get a drop of 20 rpm. It runs well and the plugs look clean. I'll just keep driving.

What about valve adjustment in the earlier reply? Could they be out of adjustment?

I had one valve over adjusted on the latest SBC I was working on and it sucked gas like mad. Ran good but smelt horrible. You could tell something was a little off but I couldnt quite put my finger on it. Adjusted the valves, problem solved.

I'll look into it the valves next! never thought of it but it may be! I'll post a follow up when I check them.

Mac

20 degrees initial seems a little high. Advanced timing at idle will increase the unburned hydrocarbons or fuel (HC) in the exhaust tremendously. Try backing the timing down to about 12 degrees and see if the smell improves. Throttle response might not be as crisp though. It's a common practice for manufacturers to retard ignition timing at start up. I've seen some run as much as 5 degrees retard for the first 30 - 60 seconds of operation. The engine doesn't run very well, but the HC emissions are reduced while waiting for the catalytic convertor to light off.

John

I've played around with the timing and other than throttle response the smells the same. It passed the tough New Jersey Emissions test with ease! I am going to check the valves this weekend just for kicks and see if makes a difference.

Mac

Unless you have radically altered the centrifugal advance curve in the distributor, 20 degrees is way too much advance at idle speed. Disconnect the vacuum advance and plug the hose, and set the throttle to 2500 RPM. Set the advance to 36 or 37 degrees at 2500, and let the idle advance fall wherever it will. If there's a hesitation on tip-in, you'll need to alter the advance slots so you can use 12 to 15 degrees of initial advance without exceeding 37 degrees at 2500. With the vacuum advance functional, look for about 42 to 45 degrees total advance at light-throttle cruise.
Jerry

Yes that is the correct setting for acheiving optimum performance for a SBC, unless you are using fast burn cylinder heads. According to the Chevrolet power manual, use mechanical advance only, set base timing to 20 BTDC and pack 38 degrees of total advance in before 3000 rpm. The only problem with using this setting is you must use a starter bracket. At this setting, you should have 17hg of vacuum and the engine should idle smooth.

Jerry

How do you set the timing accurately at 36-37 degrees? The timing tab doesnt register that high so I cant line up the timing marks.

Thanks for the input.

When I think of "optimum performance", I'm thinking of the most fuel efficient way to move my truck. You must be thinking of the quickest way to move yours.

I'm not looking for quick! This is my daily driver so I'm looking for optimal mileage and reliability!

Mac

Let's see, Chevrolet recommends 34 degrees of total advance with the vacuum advance unit connected to a ported vacuum source (for a 290hp 350 LM1). So how far am I really off in base ignition timing of 20BTDC without vacuum advance and a total of 38 degrees in by 3000?

I have the same 290 hp in my GMC, most of all the valves were over tightened from the factory. I was really surprised how much I had to back them off.

The best way to check total timing is to use a timing light with an advance built in. There is a tape available you can wrap around the damper that is marked all the way around. I like 10-12 degrees initial with with total about 34 degrees around 3000. That,s with the vacuum disconnected. With the vacuum hooked up you can have a few more degrees but if it roughens up & surges you will have to cut back on the amount of vacuum advance. I have installed hundreds of Target & Goodwrench engines & have never had to adjust the valves.

I use a degree wheel when I'm blueprinting the engine, and mark the damper from about 35 to 40 degrees before TDC. If you've got access to a delay-flash timing light, just set the readout to 36 degrees, and turn the distributor housing until the 0 mark lines up at 2500-3000 RPM. If the initial timing doesn't fall somewhere in the 12-15 degree range at idle after setting it that way, it will be necessary to rework the centrifugal advance travel.

My "stock-appearing" HEI distributors turn 7,000 RPM without spark scatter, outrun all the competition, and pass tech inspection checks. No, I'm not going to say how I do it- - - - some of my competitors might be lurking!
Jerry

Edit:
I have proved on numerous dyno runs that once the best timing setting is found, even one degree of over-advance robs massive amounts of power. We advance timing one degree at a time until max HP is reached, then record the power drop caused by one degree of over-advance. Go back to max power, and start retarding. It takes 5 degrees of timing retard to lose the same amount of power as ONE degree too much!

Over-advanced timing is not your friend. It robs power, causes detonation, and burns holes in pistons. Pistons are not cheaper than common sense!
Jerry

UPDATE!!!
Well the verdict is in and here is what I found out and did.

First I adjusted the valves. The valve covers were leaking so I figured I start there. Low and behold a couple of you guys were dead on....the valves were way over tight!! Its amazing how far I had to back a few off.

Next I worked on the carb. I switched the primary jet to a .95, the metering rods and springs and the secondary jets to .92.

I then set the idle to 725 rpm and dialed in the timing. I set it to 36 degrees at 2500-3000rpm. I was advanced by 2 degrees from previous setting.

Went back to the carb and fined tuned it.

I now have a great running truck! the test drive went well with no flat spots or hessitation.

You guys are great, thanks for all the info. I am now trying to chase an electrical demon.

Mac