engine struggles when secondaries open

[puppster]

I've been trying to diagnose a problem with my fathers car. The engine is a 351W with performer heads (2.02 valves) I believe the cam is a XE284H from comp. The carb is a holley 750DP. The car runs great all the way to redline if you don't get into the secondaries, as soon as you do, it really feels weak (sputtering like), and sometimes will clear upand go like crazy.

We have increased the squirter from a .33 to a .37 and this seems to have improved things by introducing more fuel, but it seems now that it will not clear up and run great, so I thought it must not be the pump shot that's defiecient. Next, my father increased the jet size to a 78 from a 74?. He says it feels much better.

My question is are we way out of line with squirter sizes and jet sizes? He was thinking of going to the 80's next. Or does this sound more like a power valve issue? I'm trying to learn, and have no experience with this kind of tuning. I'm sure I should probably get out and run it where I can shut it down to inpect the plugs, but that isn't that easy in the city.

[Excaliber]

It do sound like it's running lean, I don't think PV is an issue in this case. Your Dad is on the right track, keep going up in jet size until your SURE there to big. 78's aren't all that big, step it up some more in my opinion.

[puppster]

Thanks for the reply. We'll keep going then, he's got a full set of jets all the way up to 99.

Jeff

[Bobcat]

You didn`t say , but does the secondary side have a power valve ?? If so , you might try taking it out , plugging it and putting richer secondary mains in . My carb with mechanical secondaries , but on a much larger engine , has 82 mains on the primary , 3.5 PV on the primary , no PV on the secondary and 92 mains on the secondary . However ... every engine is different , and again , I have a 482 big block .
You can get a rough idea of mixture by looking at the plugs , even though you can`t do full throttle runs in town . If the base of the plug body is a light gray , you are close ... if black , you are rich . Again , this will be a rough check .

Bob

[elmariachi]

First, you should be sure your base and total timing is correct for the engine. What feels like sputtering or cutting out at high rpms could be knocking from too much timing. You aren't going to hear it over the pipes naturally, but it feels like the engine is missing. Past that, you should step way up to 84 jets (the larger the number the more fuel you are feeding it) just to see if that induces a radical change. Putzing around with incremental jet changes (75, 76, 77) is the long way to sorting out if its a fuel rich/lean issue. Just give her the gas and see what she does. FYI you wrote he went from a 78 to a 74. That is a DECREASE in jet size.

The above comment about power valves is worth looking into. If a 4-barrel with mechanical secondaries has a power valve in the primary AND secondary blocks, then the jets are meant to be the same value on all four corners (what is called "square.") If there is no power valve in the secondary block and there is a plug, then its intended that you run a considerably larger jet in the secondaries. Otherwise, the power valve in front only affects how the car responds on take-off to full throttle condition. Once you are in afterburner mode and rolling fast, the front power valve has already done all its going to do and you are relying on front and rear jets to feed gas to the carb.

Innovate Motorsports makes a system that uses an oxygen sensor to analyze air/fuel ratios. Install the O2 sensor, hook it all up to a laptop and go for a ride. Its nice to be able to see what the fuel condition is when "things" happens.

[jhv48]

Also, make sure the timing is advancing properly. Failure of the advance mechanism will also cause the engine to fall flat on its face once the secondaries open.

Easy to check.

[Excaliber]

El, point of clarification, his Dad

...increased the jet size to a 78 from a 74

so he's going in the right direction.

I do agree you should make rather bold changes in the jet size until you get in the ball park. 4 is OK if your close, but a step up 10 sizes bigger (or smaller) is warranted when your not sure. From a 74 to an 84 (or vice versa), or a 78 to an 88 will show if your going in the right direction or not.

[elmariachi]

Originally Posted by Excaliber

From a 74 to an 84 (or vice versa), or a 78 to an 88 will show if your going in the right direction or not.

Yea, what he said.

[puppster]

Thanks for the great advice. I am going to assume that if we leave the power valve on the secondary side, we need to up the primary jets to the same size as the secondaries to make it "square". Perhaps the overall affect of of increasing both of these will over richen the mixture, but we'll just wait and see, I don't know what size jet is on the primary side.

[Hotfingrs]

You shouldn't have a PV on the secondary side. The PV is on the primary side. The primaries should be 6-8 jet sizes smaller than the secondaries. You really need to put the carb back to square one and start again with the tuning. Using the same jets, front and back, won't make it "square" You still have the PV in the equation, that's why the Primaries and smaller than the secondaries, to allow for the 6-8 jet sizes the PV compensates for.

If you have 4 corner idle, all screws should be out the same distance. Use a vacuum gauge to adjust for highest idle, then add 1/8th to 1/4 turn, other wise you'll be to lean. Go down in size on the primaries, until you get a lean surge. Don't open the secondaries. When you get the lean surge, go up 2-3 jet sizes. Then add 6-8 to that number for the rear, and see what happens. You should really be doing this with an A/F meter.