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Runs Like Crap
#1
Looking for ideas - I have a 351C with an Edelbrock 750 cfm carb. It was running & idling fine a couple of weeks ago.

The car idles really rough, and it stalls out if you give it some gas; but if you hold the choke flapper partially closed (not all the way, but with just about a third of an inch slit open) it'll idle great, and it runs smoothly at higher rpm if you give it gas that way. I can even drive it like that, but it doesn't have a lot of power.

If I give it gas and get the rpms up, then open up the choke flapper all the way, it will run smoothly at 1500-2000 rpm. Have only done this in the driveway sitting still - not out on the road, as I don't have a manual choke hooked up.

I tried opening the idle adjustment screws further, but that didn't change anything.

Any ideas? I put the carb on about a year ago and have put about 600 miles on the car, maybe less. I try to drive it weekly - I do a 10 mile run or so.

Sometimes I get it dialed in and it seems to run and idle great; other times it idles low and runs like crap, and I can't figure out why or what to adjust. From what I've discovered about the choke flap, I'd guess that when it idles low & runs like crap, it's running lean - closing the choke would richen the mixture by cutting off air.
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#2
Vacuum gauges are our freind. It can tell just about everything that could be causing this problem. If vacuum indicates all is well, check the clearance on the top of your accelerator pump (should be about .005 ) It's possible you may just need a good cleaning, but sounds like you may just have the adjustments all out of whack.

[Image: 2rr7aiv.png]

Just cruising along minding our own business when BAM!!! The LAWS show up.
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#3
Buy a HolleyBig Grin
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#4
http://www.edelbrock.com/automotive_new/...manual.pdf

It does sound like it is lean, the question is why. It could be vacuum leak or obstruction in the carb. The good news is the carb is relatively easy to disassemble and clean. The link above gives some good information I've used to calibrate these carbs. I hope you get it squared away. Chuck
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#5
Vacuum gauge, you say . . . now there's a tool that I've never owned. And I have to run into town today. I'll pick one up and post some results. Then I'll have to figure out how to check the clearance on the accelerator pump. I'm not a carb guy, apparently. That retrofit fuel injection kit is looking better & better all the time. I'm already gathering up the parts that don't come with the kit.
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#6
It's one of the best diagnostic tools next to timing lights and tach and dwell meters on our old cars.

[Image: 2rr7aiv.png]

Just cruising along minding our own business when BAM!!! The LAWS show up.
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#7
If you have points you will want to check the dwell angle first.
Once that is set, using the vacuum guage and assuming a fairly steady idle, you should set your initial timing to highest manifold vacuum. Set the idle speed/mix so that it runs and then take the car for a spin. If you get a lot of pinging under load, back the timing off a little. If you have to back the timing off 'a lot' then you might look at recurve of your distributor. If the car is hard to start you will want to back off the timing too. Clevelands like a lot of initial timing but you have to be careful that you don't put so much in that your total advance is too high.

Ok, once you have timing at a good spot, with the vacuum guage hooked up to manifold vacuum you want to turn your idle speed down to about 800RPM or so. Then set the idle mixture to the best vacuum. Turn them down until it starts to stumble, then turn them up. Small moves. Keep them consistent side to side. Once you have your best vacuum you can then set idle speed to where you want it.

If moving the idle screws has no effect then you have other issues.

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
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#8
Timing light . . . dwell meter . . . two more tools I don't have.
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#9
We'll have you breaking knuckles in no time. Remeber, don't cuss if there's kids around.Tongue

[Image: 2rr7aiv.png]

Just cruising along minding our own business when BAM!!! The LAWS show up.
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#10
Ya..vacuum gage will tell you alot you cant figure out....mine was runing a bit rough...old carb i figured was shot and thought it was time for some points...still had the dual point on it....well put new points....new manifold....new carb...and it still ran the same....my cousin checked it with a vacuum gage to tune the carb in..and it was in the red zone on vacuum!! we couldnt get it right..meaning a big leak...or...loss of compression...sucks for me..but it was loss compression..rounded out cam.
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