Dshelvy

• I have an olds 350, the block is a 74, it has #4 heads, a carb from a 455 (q jet), dual exhausts 2 1/2 inch pipes w/ glass packs, stock in take w egr, stock hei distributor w accel cap, new spark plugs, I don't know what all was done to it when it was rebuilt, shortly before I bought it. It will run great but when I floor it, it stumbles and struggles to accelerate. I have reset the timing to 12 degrees advanced at idle and it runs even worse than when it was set at 30 degrees at idle. I don't know if its because of the carb or the timing. I used carb cleaner to check for vacuum leaks and found none. I have gone messed with the timing a couple times and its still giving me issues. I will do a compression Monday and post the results. Any help or input would be greatly appreciated.

MDchanic

+1. You have no idea what you've got, so you have to start from scratch.

Start with the basics, like confirming the compression, then setting the maximum centrifugal advance to 35° at about 3,000 RPM, then, if you have trouble starting or have pinging at full throttle (with the vacuum advance disconnected), use a spring kit to adjust the centrifugal advance curve, then use an adjustable vacuum advance can to dial in the vacuum advance.

Of course, the problem could be bad carb. adjustments, such as a too-loose secondary air valve, as well...

- Eric

Dshelvy

Ok I will try to re-adjust the carb and tinker with the timing but used the timing light while I was 12 degrees at idle and at 3000 rpms it was at 35 degrees with the vacuum advance hooked up. Is that were its suppose to be with it hooked up? and thank you guys for the help I appreciate it. and I post the results of the compression test and re-adjusting my carb tomorrow. I read a post on here that said a Rochester q jet will not over carb and engine because it gives you only what your engine requires. Is this true?

MDchanic

No. You set the timing with the vacuum advance disconnected.


Yes, essentially.

- Eric

Dshelvy

Yea I know you set it with it disconnected but I was going to see if I could try to determine whether or not I would need stiff springs or lighter springs by comparing how much it advances to the rpm its at. I appreciate the help and I will post my results later on today.

MDchanic

You have to disconnect the vacuum advance in order to remove its action from the picture while adjusting the centrifugal advance, otherwise you can't tell what's going on.

The springs control the centrifugal advance, but not the vacuum advance.

- Eric

brown7373

I'd also check the carb. If it is from a 455, it is probably jetted and adusted richer than for the smaller 350. Also, make sure the accelerator pump is giving full shots of fuel. With a light, look down the carb and open the throttle, or just depress the acclerator pump. Does it give a good, steady stream of fuel down each barrel? Even on a fresh rebuild, the pump cup can deteriorate if it is not one of the cups impervious to ethanol.

droldsmorland

Old Carb + new fuel = rotted accelerator pump cup. Look down the carb with engine off and actuate the throttle cable. You should see a strong stream of fuel from two pump shot holes from the beginning to full throttle travel. If not time for a rebuild. Dont skimp on the kit, dont buy chineasium sheet get a good kit from Cliff Ruggle or one of the other higher end vendors. Or send it out for a rebuild to the same vendors. Google high perf Quadrajet

Dshelvy

The Carb was remanufactured right before I got the engine and I checked the accelerator pump today and it had a good steady squirt into both front barrels. Would it be better to re-jet the carb or too get a new carb. I did the compressions test today all of the cylinders tested 135 except cylinder 1 (133) and cylinder 6 (136). Is there any way to determine what my compression ratio form those numbers? Mdchanic thank you for clarifying that for me. Thank everybody for helping me.

olds 307 and 403

That is low for a 350 with small chamber heads. I have better compression in an original 76 350, lucky if it has 8 to 1 compression plus it has the Performer cam. You need to find out what pistons and cam are in that engine.

droldsmorland

Stop...lets back up...Before you do anything lets do some basic trouble shooting. You have a WOT stumble or bog. If your ignition system is functioning and the timing is in the ballpark then its likely fuel or EGR related. Step one...Read the plugs and exhaust tips....what do they look like? Black is rich white is lean. Any vacuum leaks? Is the EGR functioning and free of carbon especialy on the actuator shaft???
Your compression is well balanced. Hard to determine a ratio as there is several things to take into consideration such as cam profile and chamber volume. Your most likely in the 9:1 area???
Just because the carbs been rebuilt doesn't mean its properly adjusted. If your a DIY person get Cliff Ruggles AND Doug Roe's books on quadrajets. Using these two resources most can dial in a Q-Jet to run efficiently. Could be all you need?

Dshelvy

It was the choke pulloff for the secondary air valve the diaphragm was cracked I ordered a new one

droldsmorland

Excellent. Glad you found it. Thanks for following up and closing the loop. Id still recommend Cliff and Doug's books. You'd be surprised how much more you can wring out of your engine with some super tuning of the carb and ignition system.

Grace Stinespring-Welch

glad you found the problem

Dshelvy

Thank You everybody for your help and Droldsmorland I plan on investing into one of those probably cliff ruggles. The only problem I am having with my carb is the well plugs are leaking after a few days. Also I have a question about the timing. I reset my mechanical advance its all in at 2600 rpm. I got a kit for the springs and I used one light spring and one medium spring. My timing is 18 initial + 10 mechanical + 10 vacuum advance = a total of 48 degrees of timing. Does this sound right because I am under the impression that its suppose to be around 50 degrees with everything coming in around 2500 to 3000 rpm. I had someone tell me this was too much and that it would hurt my engine. since I have had it like this it has the best it ever has, the timing wasn't coming in until 4500 rpm. It is running amazing right now. Thank you everbody for your help in advance for now and the future

oldcutlass

I think your math is wrong, your Hei should have around 17 degrees of mechanical advance. So your 18 initial + 17 mechanical + 10 vacuum = 45 which is pretty good. Those setting are safe the object is to get close to 36-38 total without vacuum and 50 with vacuum advance connected.

Dshelvy

that's What I got today, will double check my numbers. I know I am running 48 total with vacuum advance. Either way, I was told 48 was way to much. The person who said this is real big into racing. I was under the impression that you should be as close to 50 degrees as possible. He also said it would create a lot of wear on my bottom end is this true? I want my car to have performance, be reliable and, last me a while. I am just trying find that medium to enjoy my car for years to come.

DoubleV

No. Just make it so you have about 36 total ( no vac advance ) and you should be fine. If you really wanted to, you could also add a different vacuum advance can that would provide you with more than just 10*.

MDchanic

If he said that, he has no idea what he's talking about. Don't listen to him about anything.

- Eric

Dshelvy

I thought is where I wanted it at, around 10 to 12 degrees from vac advance. It runs flawlessly the why it is. Would I see a performance gain from this? Although I am fine the way it is, all that is really concerning me is if it will really hurt my engine like I was lead to believe.

Dshelvy

Thank you Eric I appreciate your response. I am just glad I got it running so well and it would have been a shame. I have got it adjusted just the why I want it but I will say this I do know allot bought Rochester quadrajets now and the timing part of it as well. I was thinking that since he was saying it was high he wasn't taking into consideration the vacuum advance. Either way I would like to thank everybody on that has helped me out. I know that you have replied to several of my posts and I am thankful that you didn't hold me being a newbie to much against me.

M-14

A lot of racers say get rid of the vacuum advance, they say this because they race. One less thing for them to tune right or repair when it breaks.
The vacuum advance has little affect on max performance, this is because at wot there's not vacuum to make the vacuum advance unit to work.
The vacuum advance is there for part throttle driving to increase your MPGs.

droldsmorland

Dshelvy...It all depends on your exact driving habits, gas quality, elevation, ambient temp & RH etc...Its a dynamic question where there isn't really a 100% correct answer. But what your doing is discovering what we call super tuning. Once you've established a good base line which it sounds like you have you keep tract of that then start to make slight tweaks until the butt dyno says you've found the sweet spot for your set up. Do this with the carb too. For example I currently have a 400G block with 11:1 and with 400hp at the wheels. This particular setup with available fuel at sea level or a few 100 feet above likes 12* initial or base timing. I dial in 10 with the vac canister. Then bring the rest in at ~2700 for a total of I think 44-46* Dont remember the exact total because I set it a couple of years ago and it runs so well I haven't touched it since but have it written in the cars log book. Get a vacuum gauge too if you dont have one. Tune the carb and ignition with it.
Id recommend both carb books as they are different and have different info which combined is a wealth of knowledge with Q-Jet. The well casting plugs can be repaired with JB or similar epoxy. JB has aluminum in it which is stronger than epoxy with out.
Good luck wringing out max perf with your Olds and your welcome!

MDchanic

It sounds like you've taken the time to adjust it carefully and get it running right, and you've succeeded.

How do you know that your well plugs are leaking?
Usually only the early '66 and '67 plugs leaked, so it's an unlikely problem if you've got a later carb.

- Eric

Dshelvy

@ Eric I seen fuel on the one well plug, I previously epoxied all of them. I re-epoxied the one that was leaking. Hopefully it fixed my problem but if its doesn't is there a way to replace them or install screw in style plugs? I mean I don't want it to take a toll on my starter not to mention the pain in the *** it is to have to prime your carb for the first time every day my accelerator pump is working fine. @ Droldsmorland I did use a vac gauge to adjust my carb and my timing. I am just going to leave it the way it is because it is running and starting great. I am most definitely going to write it all down for future reference.

MDchanic

You haven't told us the number of your carb, but, if, as you say, it is from a 455, then it has spun-in plugs, because it has to be a '68 or later.

If you really think your plugs are leaking, then you should confirm it by taking apart the carb and leaving fuel or acetone in the bowl and watching for drips.
If it really is leaking, then rebuilders like Sparky's and Cliff Ruggles can supply you with the right replacement plugs and sealer - simply coating the outside with epoxy will not adequately or durably fix the problem.

Though your problem could come from your carb leaking, there are many other, more likely causes which I would pursue first.

If you really think that your no-start problem is due to leakdown, then let it sit the usual amount of time, pull off the air horn, and look down and see whether or not the bowl is empty.

- Eric

Dshelvy

The # is 7041251 it is suppose to have come off of a 71 cutlass with a 455, at least that's what it said when I looked it up. And I think it is a very slow leak, over night or a couple days it varies. Could it evaporate over night? I will look for some plugs, hopefully it will hold but I will get a set of the plugs just incase. What are the other possibilities you were talking about?

-Devan

droldsmorland

A friend who is a very accomplished machinist attempted to machine out the secondary casting plugs in a 251 years ago on a high end Bridgeport with out much success. There isnt enough metal left over to press or thread in anything. Tossed that numbers matching carb out.
Your best option IMO is to put the 251 on a shelf and purchase a new Q-Jet. A LITTLE extra crank time isnt necessarily a bad thing. Gives the oil a chance to get to everything.

Dshelvy

I got it for now But I have heard jb weld doesn't hold the best?

droldsmorland

There may be a better glue out there now as I haven't had to seal one in a while. Call one of the better known carb shops and ask the tech dept. JB is pretty strong and will hold if the metal is prepped correctly.