qjet 704124 KA 455 carb going on a built 350

In doing what you have mentioned related to the advance being moved higher than base 12 what would this scenario indicate to me that would need to be changed? Presuming the mixture screws then had more range of effect at that point what would be the resulting fix or fixes to overcome not having to run such high timing? It will ping, run hotter and buck the starter hot and cold when the base timing is set higher. In my experience with this engine package. Engine has approx 600 miles on it and loves WOT. It can be hot at idle and purring and then as if a switch is flipped it will go into a full stumble and almost die once placed in gear. Very frustrating so far!!

 

I visited his website and will reach out to him. Sounds like this carb is not ideal for my engine and needs things done to it that I am not knowledgeable enough about. I'll throw a spare Holley on it for now and send this one out.

 

A 350 Buick carb wouldn't fair any better than the 455 with your engine combo as neither one really has enough fuel to the idle mixture screws for that much camshaft in a 350 engine.

If you are only making 12" vacuum at idle and no response with the mixture screws with 12 degrees timing in it then it's telling you it wants and needs more idle fuel and probably more timing as well.

10.6 to 1 compression is actually a good place to be with the little 350 engine to help manage that much camshaft and make decent power (torque). It's not too high for pump gas either. The cam is the BIG player with any CID and compression ratio when it comes to managing pump fuel in it.

Recently I helped out a local customer here who built a 350 Olds and kept the compression ratio to 9.5 to 1 for pump gas. That statement by the way is the most inaccurate to ever find it's way in to one of these discussions. When you hear someone say that they are CLUELESS as to engine building, cam selection, dynamic compression, and tuning one of these engines....FWIW.

Anyhow, he turned right around and installed a pretty "hefty" Lunati Voodoo cam in it on a tight LSA. Talk about a "turd". It didn't idle well and "soggy" till he got it wound up past 3500rpm's or so. I fixed his idle issues by adding bypass air (not blocking it) and opening up the idle tubes, DCR's and moving the initial timing up to 18 degrees then recurving his distributor so it didn't have too much total timing, but it still didn't get us past the basic laws of physics......big cams in little low compression engines just don't idle well nor to they make good power.

We ended up fixing ALL the issues when he had a cam bearing go bad and the engine was pulled. I had him replace the dished pistons with flat-tops to get the compression UP to 10.6 to 1 and had Lunati grind a custom cam for us based on the Chevy 327/350hp L-79 cam. 222/222 on a 114LSA. The engine now idles really nice with only 10 degrees of timing in it, smooth off idle and strong/smooth/flat power curve to at least 6000rpm's. It doesn't even think of pinging on pump fuel and doesn't run hot or have any other issues anyplace........

 

I did ship this carb out to Ken last Friday. Ken and I spoke on the phone as well. He said that he knows what needs to be tweaked and it will be a great fit for my 350 once I get it back.

Is there anything you can add to how I can best locate a potential vacuum leak?

 

Cliff, when i was detailing the specs for the 350 build with our machine shop they weren't a fan of my specifications request for the compression ratio being above 10;1 due to the pump gas statement. The engine was a factory 10.5:1 and with this being a convertible cruiser they felt it should be no higher than 10:1. Technically is is calculated at 10:25.1. Once I get the carb back from Ken I will check for possible vacuum leak at the intake manifold and presuming there isn't one and the carb work takes care of the idle quality issue, I will go back to the timing possibilities and recurve the distributor if needed.

Thanks to everyone for their comments and expertise.

 

As I mentioned, lowering compression for pump gas is one of the most inaccurate statements ever made in association with this sort of thing. You can be completely safe at 10 something to 1 where you are at. You already have symptoms that the engine is seeing that as a pretty big cam, low vacuum at idle, no response with the mixture screws, likes timing at idle, etc, so cylinder pressure is low for sure. Unlikely it will ping anyplace as cams like that in small engines lower dynamic compression and push peak VE up pretty high in the RPM range where it's much less likely to ping anyhow. I think you will be just fine once the carb is correctly tuned and maybe a slight modification to the distributor as well to shorten up the mechanical spark curve maybe a little less timing from the vacuum advance.......