77 305 Hesitation from 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...

 

[wanderinthru]

If compression is good. Is the new dizzy good, or, are the new dizzy weights and vacume advance working in harmony? New don't mean good anymore, sadly. Had the problem with a broken weight pin, and now the problem still exist?

 

[SirRobyn0]

What you talk about with the ignition timing at idle is interesting. You should be seeing your base timing + vac advance which if your running a base timing of 12D + 21 VAC adv, you might see 33, if you have the vac advance hooked to manifold vacuum. If it's hooked to ported vac, then you have to have a bunch of mechanical timing in there to cause those numbers. I think you need to be checking the springs and weights in the dist.

Also what you saying with the vacuum, or how I'm interpreting it, makes me think there might be an error in the testing. Would really like to see video of that.

The traditional hesitation issue is accelerator pump in the carb, lean over the carb with the engine off choke open and air cleaner off and see if you get a shot of gas in both front barrels. That's what I can think of off hand.

 

[jti30666]

What you talk about with the ignition timing at idle is interesting. You should be seeing your base timing + vac advance which if your running a base timing of 12D + 21 VAC adv, you might see 33, if you have the vac advance hooked to manifold vacuum. If it's hooked to ported vac, then you have to have a bunch of mechanical timing in there to cause those numbers. I think you need to be checking the springs and weights in the dist.

Also what you saying with the vacuum, or how I'm interpreting it, makes me think there might be an error in the testing. Would really like to see video of that.

The traditional hesitation issue is accelerator pump in the carb, lean over the carb with the engine off choke open and air cleaner off and see if you get a shot of gas in both front barrels. That's what I can think of off hand.

On the Rochester 2 barrel I'm getting full manifold vacuum on the carb where the factory port is....ie where it is supposed to be at...I have both service manuals...one from Rochester and one semi useless one from Haynes lol...

I'm getting a shot from both barrels....a nice stream...

I'm using a verified good vacuum gauge...the vacuum increases for a moment during transition from idle to part throttle... almost at the exact time the hesitation occurs....

 

[jti30666]

If compression is good. Is the new dizzy good, or, are the new dizzy weights and vacume advance working in harmony? New don't mean good anymore, sadly. Had the problem with a broken weight pin, and now the problem still exist?

I'm going to look into this but same problem with new parts doesn't make sense in a way....I'm leaning in a direction but I'm going to put the blinders on all of us

 

[Ricko1966]

Okay you've posted your problem up twice I've responded on your other post. So I'm not going to bounce back and forth

 

[wanderinthru]

I'm going to look into this but same problem with new parts doesn't make sense in a way....I'm leaning in a direction but I'm going to put the blinders on all of us

Okay you've posted your problem up twice I've responded on your other post. So I'm not going to bounce back and forth

Pick this one Rick. lol, agree with what you said in the other as well.

 

[wanderinthru]

I'm going to look into this but same problem with new parts doesn't make sense in a way....I'm leaning in a direction but I'm going to put the blinders on all of us

Well, if it is in fact getting the fuel. Why is it not burning it?

 

[jti30666]

Well, if it is in fact getting the fuel. Why is it not burning it?

Well it's burning fuel or else it wouldn't idle nor could I accelerate....

 

[jti30666]

Pick this one Rick. lol, agree with what you said in the other as well.

Well I'm sorry I miss posted

 

[Ricko1966]

Well I posted on the wrong one again so you'll have to bounce over to see it. Now the reason I said something about your vacuum gauge,there is no way,none that I'm aware of that you can pull 45 in of vacuum . I can't even pull that with an A/C vacuum pimp. To the best of my knowledge you can't pull more than 29- 30. So if your gauge is really showing 45 I'm thinking your gauge is at least 16in off.

 

[jti30666]

Well I posted on the wrong one again so you'll have to bounce over to see it. Now the reason I said something about your vacuum gauge,there is no way,none that I'm aware of that you can pull 45 in of vacuum . I can't even pull that with an A/C vacuum pimp. To the best of my knowledge you can't pull more than 29- 30. So if your gauge is really showing 45 I'm thinking your gauge is at least 16in off.

It JUMPS up to 45 then drops... when it does this is exactly when the "miss" happens... and yes with pumps such as that of engines you can pull more than 45 of vacuum...I think your missing the point though...the vehicle always had this problem....I fixed the major issues such as leaks (oil/coolant/air) and I'm back to this... I said I was going to put the blinders on I'm going to give this...even though I checked the intake for vacuum leaks I could still have a leak between intake ports on the heads with a bad intake gasket....just my 2 cents

 

[Ricko1966]

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I would love to see some documentation of that. I was taught years ago you couldn't pull more than 29 and change in earth's atmosphere. I'm always willing to learn.

 

[SirRobyn0]

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I would love to see some documentation of that. I was taught years ago you couldn't pull more than 29 and change in earth's atmosphere. I'm always willing to learn.

Ok, I have to say that I quite agree. There is something not right with the vacuum readings which is why I requested a video of it happening.

@Ricko1966 & @jti30666 Joshua for the record Rick has been around for a long time, and he knows his stuff. I have somewhere around 25 years in auto repair. @wanderinthru has been workin' on squares forever. What I'm trying to say is you got a good group in here that knows what they are talking about. As to the question can a pump produce 45" of vacuum I have no idea, but what I do know is you should see your highest vacuum readings when the engine is at higher RPM and the throttle is shut. So like going down hill with your foot off the gas engine braking you might get in the upper 20s. I cannot recall ever seeing over 30", I cannot comment if it is or isn't possible. However a jump up in vacuum when accelerating is something very odd at not normal at all.

 

[Hunter79764]

Engineer here, there's no such thing as 45" of vacuum, at least in inches of Mercury/InHg which is what is typically used. Not that it really matters. The vac gauge might have some different units, or maybe it jumps to 25 and you got your numbers mixed, happens to me all the time. But a video of vacuum gauge movement is a huge help for internet diagnostics anyway, regardless of the number displayed.