77 C15 305 hesitation off idle

[jti30666]

First off I'm new to this forum but not to these older vehicles....and thank you in advance for your support/answers...

My friend brought me his 77 C15 305 , it has an aftermarket Rochester 2 jet, for an fix/inspection... complaint was hesitation (briefly) when going from idle to a quick acceleration, like pulling out on a busy road/highway...I did a test drive and found that this is true but if you feather the throttle it won't skip a beat but you have to really take it slow....once above 30 mph you can stomp the gas and it will downshift and fly....

With my inspection I found a lot of problems that I listed below and are fixed and or checked..

Compression good.

Timing gear set/chain (replaced)...had 3/4" slack in original chain.

When I had everything apart I verified tdc on 1 with a piston stop and the balancer aligned with the zero on tab...and it is a new balancer

Carburetor...old one was rebuilt and damaged by another "tech"...replaced with a reman carb wich I adjusted to factory specs and also fixed. Has 0.055 main jets...

Replaced all gaskets...tc/oil pan/rear main/ etc.....

Fuel pressure was at 9 to 10 psi... installed regulator to 3-4 psi...

Intake manifold/carb leaks...found leaks and fixed...

New distributor because the center weight had a broken pin....old dizzy had an 18 vacuum advance...now when idling at 650 with vacuum advance plugged into carb with full manifold vacuum I'm getting 33 degrees advanced timing

Now we still have the same problem as before....if I slowly accelerate no problems...if I step on it it will fall on its face or a second then it takes off...it almost seems like there is no transfer between idle and full throttle...I've verified though that the slots work with fluid and air... pulling close to 25 inches of vacuum at idle...weirdly instead of vacuum dropping with throttle it jumps up to 45 then drops back down...
.

 

[Ricko1966]

Is it a factory replacement distributor or a generic hei? And I think there is something wrong with your vacuum gauge. Does it have good accelerator pump shot?

 

[jti30666]

It's not gm...new from AutoZone...never had problems with them before...but with the old dizzy we were having same problems...it was a gm oe replacement

 

[Ricko1966]

Where is your total timing? Where is your old distributor? The cam,weights,springs,and even the advance cannister are not the same in all distributors. My gut says you need more initial timing,but you need to see where total is. Should be in the vicinity of 36 degrees.If not give it some more initial and see what that does.When did this stumble start? They didn't stumble new,so what has changed?

 

[jti30666]

If you read the entire first post this problem is the same from beginning to end....I just fixed other problems like I was supposed to...ie leaks, bad timing chain, vacuum leaks.....yes ricco I know this, have been down this road before (no pun intended)...and again I originally posted this....with vacuum advance plugged off I set the initial to 10 advanced....reset vacuum to advance and I'm reading 28 advanced at idle....at 2500 rpm I'm around 34 advanced

 

[Ricko1966]

Okay It doesn't matter where it is at idle it's no longer factory and we don't know whatvit is,as in weights,springs,cam and how much centrifugal advance is in your new distributor. You want around 36 degrees total advance that is initial plus centrifugal with vac advance disconnected and plugged. If it is much lower than that bring it up to that,let's see what that does. Then I'll teach how to get it closer to ideal.I may be wrong but my gut says you need more initial timing.

 

[jti30666]

Okay It doesn't matter where it is at idle it's no longer factory and we don't know whatvit is,as in weights,springs,cam and how much centrifugal advance is in your new distributor. You want around 36 degrees total advance that is initial plus centrifugal with vac advance disconnected and plugged. If it is much lower than that bring it up to that,let's see what that does. Then I'll teach how to get it closer to ideal.I may be wrong but my gut says you need more initial timing.

BS...idle speed does effect initial timing with vacuum advance plugged..,Why??? Because that's how it works.... you want the idle to be as low as possible so you know for sure there is no mechanical advance..set initial timing....plug in vacuum advance, take reading...run rpm to at least 2000 then get total...I've done this before many many times...

 

[Ricko1966]

So you obviously do not know how to do this, or you would know how to set up total. Total should be about 36 degrees all in, centrifugal and base no vacuum advance connected don't believe me ? Research it.

 

[Ricko1966]

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[jarheadpig7]

I wish I had the answer but I don't. I just wanted to say, I have the same problem with my 1987 R1500 tbi. I've had it for 20 years and it has always had this problem no matter how many things have been repaired and replaced on it. I'll be continuing to search for the solution. I am no mechanic but it has always felt to me like a fuel delivery issue, like the fuel pump/sending unit, but that is just how it feels to me.

First off I'm new to this forum but not to these older vehicles....and thank you in advance for your support/answers...

My friend brought me his 77 C15 305 , it has an aftermarket Rochester 2 jet, for an fix/inspection... complaint was hesitation (briefly) when going from idle to a quick acceleration, like pulling out on a busy road/highway...I did a test drive and found that this is true but if you feather the throttle it won't skip a beat but you have to really take it slow....once above 30 mph you can stomp the gas and it will downshift and fly....

With my inspection I found a lot of problems that I listed below and are fixed and or checked..

Compression good.

Timing gear set/chain (replaced)...had 3/4" slack in original chain.

When I had everything apart I verified tdc on 1 with a piston stop and the balancer aligned with the zero on tab...and it is a new balancer

Carburetor...old one was rebuilt and damaged by another "tech"...replaced with a reman carb wich I adjusted to factory specs and also fixed. Has 0.055 main jets...

Replaced all gaskets...tc/oil pan/rear main/ etc.....

Fuel pressure was at 9 to 10 psi... installed regulator to 3-4 psi...

Intake manifold/carb leaks...found leaks and fixed...

New distributor because the center weight had a broken pin....old dizzy had an 18 vacuum advance...now when idling at 650 with vacuum advance plugged into carb with full manifold vacuum I'm getting 33 degrees advanced timing

Now we still have the same problem as before....if I slowly accelerate no problems...if I step on it it will fall on its face or a second then it takes off...it almost seems like there is no transfer between idle and full throttle...I've verified though that the slots work with fluid and air... pulling close to 25 inches of vacuum at idle...weirdly instead of vacuum dropping with throttle it jumps up to 45 then drops back down...
.

 

[cadillac_al]

No mention of accelerator pump?

 

[chevdude]

You said quadrajet carb right. There is an adjustment on qjets that nobody seems to know about. there is a little spring, that is adjustable, that controls when the secondarys open. Its on the passenger side under a cap screw. If the spring is too tight, the secondarys open late. This causes the hesitation, or bog, that everybody hates about the Qjet. I myself fought with this for years until I read about it somewhere. It's been at least thirty years since I've thought about that little spring. Can't really remember what to do exactly but I do recall that it was a sensitive little bugger. My suggestion to you would be to do some reasearch on this. Youtube it. You will be amazed at how much difference it makes.

 

[Ricko1966]

Having the secondary door too loose is the common reason for quadrajets bogging getting into secondaries. APT will cause a hesitation too,so will accelerator pump or ignition issues. I'm only replying to this dead thread so everyone doesn't go loosen their secondary air doors. The q jet information is not pertaining to the O.P. he has 2bbl. 4bbl 305s didn't come out until I believe 1982.