Man this is driving me nuts! Below 3000 rpm at any time, neutral or in gear when I step on the throttle the engine just boggs. It will remain in a bogged condition as long as the throttle is held down, sometimes sending big fire out the carb. You can throttle it up slowly and it will run fine. Once on the road (or in neutral) at 3000 rpm or better you can mash the throttle and it takes off like a rocket, no lag, no bogg, nothing. I timed it a 4, 6, 8 and 12 btdc, backed off the lifter adj. to 1/4 turn in, installed a Street Avenger 670, all with no real difference. The 670 helped a little. The 750 is back on it now. The raw gas smell coming out the exhaust is overwhelming also. Engine; SBC .030 over, flat tops, Dart 76cc heads @ 8.5 to 1 CR. Edl air gap intake, Holly 750 vac secondary carb, no choke. Cam, I'm told is the hyd version of the 327 375hp vette. Std Chev HEI dist that has been rebuilt. I have two and tried them both. Engine shows 15+ on the vac gauge. I suspect the cam, but am reluctant to change it out without knowing for sure. It has an elec fuel pump and I've pulled the sight plugs off the carb (suspecting over pressure) and it does not overflow (although I seem to remember the pump stopping when the carb filled and now it continues to run...not sure tho). Also installed new power valves, std. 6.5 in it now. I've set the level hi and low with no difference. I once installed an adj vac can and had it pinging so bad you could hear it above the side pipes. I'm lost....anyone have any ideas?

 

with the raw gas smell is the mixture set way to rich?

 

Just for kicks, try a new coil. I had a similar problem with my 67 Coupe last year (under heavy throttle). After going through everything you are trying (and weeks of pulling out my hair), I replaced the coil and the problem was solved. I just re-read your post and see you have a HEI Distributor. My problem was with a standard coil so I am not sure this would apply to you.
Roy

 

couple suggestions:

Set the timing using a vacuum gauge.

Disconnect the line to the vac advance,and attach your vac gauge here (should a port below the throttle plate).

At idle, advance the distributor until max manifold vac is obtained, then back off your distributor timing until vac drops 1" HG.

Cgheck your static timign witha timing light and record. You will now be timed for that engine.

On your carb, i can only tell you what i did with a Holley 650 4777.

I went a lot smaller (6-7 jet sizes) than factory on the primary jets, and installed two sizes up on the primary squirter nozzles. the rest of teh carb was as from out of teh box.

Car ran great, no gas smell, gas mileage was not so great.

Doug

 

Masterdave, I'm guessing it's a carb problem....running way too rich. Pul a spark plug on either side of the engine. If they're black, it's a fuel delivery problem. A couple of things to check......

1. Are the floats soldered brass or black foam types (nitrile)? If they're the black nitrile types they may be soaking up fuel and getting too heavy to regulate the fuel coming in to the carb. Replacements would be needed. If they're brass, check that they do not have any fuel in them. They have been known to get leaks in solder joints that separate.
2. With an electric fuel pump you may need to install an inline fuel pressure regulator and guage to regulate the fuel pressure to around 6 to 7 pounds. That's all a carb needs to perform adequately. You may be overfilling the carb.
3. Are your primary jets the correct ones for the engine? You may have jets installed that are too large for the carb model.
4. Did you install a correct gasket behind the power valve in the primary side of the carb block? If not, this will cause a huge internal fuel leak.
5. Is your primary discharge valve under the front primary bowl in good shape? A hole here will cause the symptoms you refer to.

If all of these items check out, I would still look at the carb for any internal fuel leaks. Check the fuel discharge nozzle to see if a pintle or ball is installed under it and that they are free and sealing properly.

Hope this helps.

 

Agree it's running way too rich. I had the sight bowl screws out and with the engine running I set it just below the threads. No fuel spills out while revving or idling so I suspect that the fuel pressure and floats are fine. Gasket behind the pwr valve is new. Changed the jets down a couple sizes with no help. I was told that I had one of the crappiest carbs holly made though. Can't remember the number now, seems like 3707 or something like that. However I did install a Street Avenger 670 at one time and the only thing that happened was the bog time was cut in half and it actually did acceletate while holding the throttle down after about 3 seconds of bog time. I need to go to a 500 CFM you think??

 

check that #1 piston TDC is really 0 degrees on your timing tab. you could have a balancer that has slipped or a timing tab location problem.
there are several ways to do it. i like the piston stop method. could be that your timing is way out of whack because of this.

 

Done it, it's right on.

 

If you have a bog associated with a backfire up through the carb, you're lean - not rich. A rich condition can produce a hesitation if severe, but it won't give you a backfire. Lean will backfire. You can get a rich fuel smell out the exhaust from a lean condition because you will get lean misfire, causing unburned fuel to go out the tailpipe, often burning incompletely in the exhaust system (with associated rumbling and popping in the exhaust on deceleration). I'd richen it up, if I were you. Make sure your initial timing is in the 16-18 degree range, and verify total timing at 36 degrees.
Lars

 

 

My brain hurts....Lars, I do not get any afterfiring/popping, etc on deceleration. I'll up the timing and let ya know. On a side note here, when I first drove this it acted up and I remembered I had an old HEI out of a '79 vette L-82. I stuck it in and it was pretty near perfect. Drove it for a weekend and while sitting at a light I got a big bang and it quit. Got it started a couple times but not well enough to drive it. I had the old dist with me and stuck it in and off I went with the old problem back. I took the vette dist to Honest Perf and they said the module went out. They put a new one in and curved the dist at the same time. Guess what, back to the same problem with both dist's. Lars, is there a clue here?

 

MasterDave, this site may help.

http://www.bob2000.com/carb.htm

 

The first thing you need to do is set the point dwell. When that is right you can move on to the timing. I've seen this problem many times. Fixed a friends last summer.