ERIC'86IROC

I am hoping I just need some more carb tuning . Can low float level cause this? I thought we checked this after intitial break in but may not have. How about jetting? Timing is at 10 BTDC and 34 total. Car runs fine other than high RPM. It takes it along time to climb after 4000 and feels sluggish. The peak should be around 5800-6000 according to similar engines in magazines. My other guess may be fuel pickup or something with fuel delivery. I am running a holley mechanical street pump at 7-8 psi. Any suggestions? The engine and trans have about 1500 miles on them. Oh yeah I have removed the choke so it isn't that.

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350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

[This message has been edited by ERIC'86IROC (edited June 04, 2001).]

FastBroker

I would bet on not enough fuel being delivered for some reason (small fuel lines, small/clogged filter, bad/small fittings) and/or improper main jets... You could/should check fuel pressure under the high rpm/high load conditions with an isolted in-cab FP gage or run a direct one and somehow tie it up OUTSIDE the vehicle so a leak wouldn't kill you. You should have at least 4psi under WOT operation after 5k...

EDIT I suppose float level could cause this but it would probably also cause other part-throttle drivability problems, too. May as well reset/check it anyway to rule it out, and to check the needle/seats for obstructions/dirt.

[This message has been edited by FastBroker (edited June 04, 2001).]

Damon

Your secondaries may not be opening. That'll kill the top end. Or, like Ed says, you could be running short of fuel pressure under load/high RPM. No way to know except hook up a guage. Fuel pressure at idle is meaningless.

ERIC'86IROC

Okay thanks guys. I will make sure tonight the float levels are correct and then look at the fuel pressure more closely, i.e. hook up an isolator and remote gauge.

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350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

[This message has been edited by ERIC'86IROC (edited June 05, 2001).]

five7kid

iTrader: (14)

I would be suspecious of a non-fuel problem as well. Does this power fall-off always occur at 4000 RPM, regardless of accelerator position (i.e., if you rev it in neutral, does it start to cut out at 4000)? If so, it's probably not fuel related.

The old saying used to be "90% of carb problems are fixed with a set of plugs & points." We don't have points anymore, but the idea is that ignition problems are often blamed on the fuel system.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam and intake, World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back).
57 Bel Air, my 1st car. 0.030 over 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlock header mufflers, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, LT MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Best 15.1/95.5 @ 5800' Bandimere.

FastBroker

Whatever it is he's gonna poop when he fixes it. That's a good combo! My bet is still fuel related. That combo gets hungry up top. But, check everything!!!

ERIC'86IROC

Thanks for the advice, I will look at the ignition next. In neutral it doesn't cut out but after ~4000 RPM it almost struggles to climb RPM. This should be the top of the power band shouldn't it rev fastest up there? Can a bad lifter or cam lobe cause this? I don't suspect problems there but just wondering. I'll know more after tonight.

If it ends up being the pump is there a mechanical pump you guys recommend or should I switch to electric?

I did have time to pull the filter last night. It is the Summit dual feed fuel line with gauge and sintered bronze filter. It was clean and clear, but I'll leave out while I continue to investigate.

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350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

[This message has been edited by ERIC'86IROC (edited June 05, 2001).]

FastBroker

Mechanical or Electric is up to you. Just buy brand name quality stuff. If you have mechanical, I'd probably stay mech cause of the ease and cost. get a High Volume pump, not necessarily high pressure and you're good.

Keep us updated. That cam should pull your cheeks back at 4000rpm, BTW!!! I would STILL run a fuel pressure gage to check. It is the only TURE WAY TO DIAGNOSE YOUR PROBLEM if it is a FP problem.

87RS402

Check the internal fuel filter in the carb. I had that problem in my truck. Fixed with a simple change of filters

ERIC'86IROC

Well I think it may be the ignition. I couldn't find any fuel system problems. I closed the plug gap down from 0.045" to 0.035" and saw a significant improvement. Tomorrow I will try a new coil and module(MSD).

I am going to look at the secondaries again also. I tried other springs before but that may have been before the timing and other tuning was complete.

The primary jetting is probably too rich also. It still has the stock 0.072" jets in it. I never changed them because the plugs didn't smell like gas and weren't getting dark. Looking at them now, shortly after running WOT they may be darker than they should look.

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350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

[This message has been edited by ERIC'86IROC (edited June 11, 2001).]

[This message has been edited by ERIC'86IROC (edited June 13, 2001).]

ERIC'86IROC

Found the culprit. I wanted to check my timing again yesterday after I switched from #72 to #69 main jets and a lighter vac advance spring. Discovered I was at 12 deg btdc and with mech advance a whopping 20 deg btdc. Boy does that hurt top end, duh! Anyways when I built this motor the distributor came straight from the junkyard, but I thought it was tight (the shaft is at least). I had a mech advance kit and used the lightest springs. Well the weights are stickning and the bushings are worn. At 650 RPM idle my initial timing already used half the mech advance. Changed to a medium and a stiff spring and set total advance at 30 deg btdc. Big difference. The distributor is sloppy and needs rebuilt or replaced. Thanks for all the help. I can't believe I overlooked this befor when setting timing. Just glad is was something simple.

How much total advance are you other guys with Vortec heads running? Supposedly they don't need much (29-34).

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350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

five7kid

iTrader: (14)

QED.

87formula

Strong feeling it will be a problem with airflow. Check into your cat(if you have one). Make sure your secondaries are opening, also check the ox sensor, if hooked up anymore. Also play with the timing, that can also cause related problems with airflow (not enough vacuum). Good luck and good fortune.

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1987 Firebird Formula
305 LG-4
GM Vortec heads 2.02x1.6 valves
Crane CompuCam 2030
700-R4 Trans
4 barrel quadrajunk remained by holley (this carb stinks!!!)
edelbrock performer intake
Hypertech Thermomaster Chip
Open Element Air Filter
No Cat with Straight pipe (Dynomax muffler rusted apart and fell off probably could not stand the heat )
Accel H/P Coil

- Soon to be installed
383 stroker (built and ready)
NO MORE COMPUTER CRAP
700R4 Lockup kit
Edelbrock TES
True Dual exhaust or 4 inch single pipe to dual 2 1/4"
Stall convertor

Damon

If you're runnign the GM HEI distributor then that's a common problem. Check out a tech article on this website about settign up an HEI for performance. It deals with everything you just ran into.

FastBroker

Sounds better, Eric. 32 should be enough with the XE268 and Vortec heads but maybe more if you have an accurate way to check for knock, like the MSD Audible Knock Sensor...

Sounds like you have some carb tuning and an HEI rebuild to go to get things just about right. Those are good problems to have as they are easily and inexpensively cured. Have fun and email me with the performance feel you get when you are done.

ERIC'86IROC

Thanks for your help everyone. This subject is changing I know but I think you guys are the best group to ask this question. Where can I buy the shaft bushings when I go to rebuild this HEI distributor. Should I replace the gear? I definitely will get the new mech advance weights and plastic bushings. Would it be any better to just buy a new distributor?

I think the carb is very close now. Jetting is good. Accel pump is good. Vacuum secondaries are good. Next I need a vacuum gauge to adjust the idle mixture. It can't be too far off though because it idles fine. It may be slightly rich.

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350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

[This message has been edited by ERIC'86IROC (edited June 18, 2001).]