670 Street Avenger and Flat Cam Lobe

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Ivoryk3ys78

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I recently lost an exhaust cam lobe in my 318. When this happened it caused the exhaust pressure to ram out of the carb. We were able to get the new cam and lifters broke in but it didn't want to idle. We thought it was a vacuum issue. If you would try to idle it would die and if you did get it to to where you thought it would idle it would kinda idle and then surge up in rpms and back down and surge up and some backfiring.

We put my other carb back on. A 650 Avs2 and it is running fine.

I am wondering what the most common things that are likely to have been damaged in the carb from what happened. What are some things to look for when taking it apart.

Thanks a lot

Kevin
 
It sounds like a vacuum leak at the throttle blades on the original carb. You might try putting it back on & spraying WD40 around the throttle blade pivots.
 
How'd the 670SA run before the cam mishap? I've had bad luck with those 670 SA's with an engine that has any larger than a stock cam. The idle circuits are tuned super lean from the factory.
 
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Well I didn't get much time at all with the carb and the cam that was in it is an old purpleshaft 484/284. I got it off of ebay and it was suppose to be factory rebuilt. We got it on there and got a quick idle adjustment and then tried to go down the road with it. After about 5 seconds of getting on it we had the popping noise and the flat cam. It was not doing what it is doing now though. It seemed okay.

The cam that is in it now is a Comp Cams XE284H 507/511 240,246 at 050.

The reason why I was trying a different carb is that the AVS2 stubles right when you first nail the throttle. If not for that it ran great. I had tried changing the adjustment to have the accelerator pump give a longer short. It didn't fix that.
 
Ivoryk3ys78 said:
Well I didn't get much time at all with the carb and the cam that was in it is an old purpleshaft 484/284. I got it off of ebay and it was suppose to be factory rebuilt. We got it on there and got a quick idle adjustment and then tried to go down the road with it. After about 5 seconds of getting on it we had the popping noise and the flat cam. It was not doing what it is doing now though. It seemed okay.

The cam that is in it now is a Comp Cams XE284H 507/511 240,246 at 050.

The reason why I was trying a different carb is that the AVS2 stubles right when you first nail the throttle. If not for that it ran great. I had tried changing the adjustment to have the accelerator pump give a longer short. It didn't fix that.
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In my opinion, tuning that 670 is going to require some modifications to the idle circuit for your purposes. The's a pretty big camshaft for a 318. What's the rest of the combo? I know this doesn't answer you questions, but I really don't like SA's on modified engines. Can you tell? LOL
 
RPM Airgap. X heads with a radius cut valve job and back cut, shaved to get if I recall the compression close to 9.5 (think they were shaved 60 thousands been some years) with the thinner gasket. Hughes 3500 converter. 3.55 gears going to 3.91s by the summer. 27 inch tall tire.

The AV2 to really did drive an idle wonderfully. It just has a stumble when it is initially put to the floor. Maybe larger accelerator pump nozzle would do it.
 
Start by replacing the power valves and then play with the idle mixture screws. They are supposed to have power valve blow out protection, but you never know. A 670 Is a pretty large carb for a 318 even with a big cam. I have one on my 355 sbc that makes 350 hp, and i feel its a bit too big. It does work better now that i swapped the gears from 3.08 to 3.55 that made a world of difference, and i had to re-jet and tune that carb again. I found i was too lean.
 
Fisher said:
Start by replacing the power valves and then play with the idle mixture screws. They are supposed to have power valve blow out protection, but you never know. A 670 Is a pretty large carb for a 318 even with a big cam. I have one on my 355 sbc that makes 350 hp, and i feel its a bit too big. It does work better now that i swapped the gears from 3.08 to 3.55 that made a world of difference, and i had to re-jet and tune that carb again. I found i was too lean.
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The 670 SA's have the same venturi and throttle bore sizes as a 600 1850. Hardly too big for a 318. The problem is they're designed for street cars with stock-ish type builds- not quite the application the OP has. It'll work with some serious tuning, but the right carb for what he's got IMO. I'd put a 650 DP on the thing and call it good.

A blown power valve shouldn't affect the idle because they pull fuel from the main circuit.

OP- Do the regular checks- float levels, vacuum leaks, see if the boosters are dripping at idle, etc. and see what you find.
 
Thanks a lot for the feedback everyone. I appreciate it.

Another option would be to focus on trying to get that stumble when stomping the gas to disappear on the 650 Thunder AVS2. It did seem to really idle nice with that and accelerate smoothly,etc.
 
I will say the 5 seconds that the 670 was on there at open throttle it did not have the stumble when it was throttled like the AVS2 had.

I don't think of Edelbrocks as being as adjustable but I would think a larger accelerator nozzle can be put in them as well.
 
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