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'75 Rough Idle when in gear

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
Hey everyone. I did a search of the threads and didn't find too much to help, figured I'd ask. Sorry, once again, for the essay but I figured if you wanted to help I'd try to give you all the info I could.
I was driving home a few weeks ago, 12 miles to and from work. On the way back about a mile from home the car started running real rough in drive (Stock 350, Q-Jet, automatic). It felt like it was going to stall but it didn't, and I was able to make it home. It ran pretty much normal when I was moving, but once I wasn't applying gas at a light or stop back to that idle. I figured that the accelerator pump needed rebuilding (could see a fuel leak around it, car would choke if you floored it from a dead stop/didn't ease on the gas) so I took it out and did it up with the parts from a carb rebuild kit; just the accel. pump. Took the car out, running fine till halfway round the block then it started up with that idle while I was moving.
It stalled this time when I got it in the driveway to back it into the garage, and wouldn't start at first. Then it would start but stall once I put it in gear. Finally got it into the garage, and my father and I checked out the fuel pressure from the mechanical pump. He felt it might be a defective unit so we replaced it. Start the car up, same problem.
This time he opened the carb up to check the accel. pump, everything looked good. We also took out the power piston, I believe (right next to the float, has a little spring on it). Replaced the small screw-in piece. Now the car started like it did before, but same problem whenever it went into gear of running real rough. Turned it off, turned it back on, took it around the block; no rough idle, no hesitating from a dead stop when floored. Parked it, started it next day, rough idle is back. let it sit for a half hour, started it up, no rough idle but it hesitates when you floor it.
Today I was going to try cleaning it with that old oil trick I read about once before and an old mechanic showed my Dad, but I was wondering if anyone had any idea what else it may be. I read on here a few other people with problems, someone had suggested it could be the rotor/distributor/wires/plugs. Its not misfiring, of what I can tell. The plugs, wires, and distr. were all changed at the same time and have less than 1k miles on them, no burnt wires. The rotor is as old as the car, but I don't know what the symptoms of that going bad would be. We adjusted the timing when we put headers on the car and modified the exhaust, and it was running no problem (except the dead stop hesitation) for a while. The accel. pump is no longer leaking. The fuel pump is new, and the only thing I could think in relation to fuel problems would be a clog somewhere in the line which sometimes allows fuel flow and other times blocks it. I heard some people complain about the ethanol breaking down some of the older rubbers, or breaking down and gumming up itself, but I've filled up 2 or 3 times since winter with Premium and put that cleaner additive in the first fill-up back in Feb. My father thinks it may be something electrical, I just don't understand why it seems to be running rough on the first start of the day and "alright" on the second. Any help would be appreciated, I'm probably going to head out now to try starting it again so I'll update here later on.
 

terry82

Well-known member
Joined
Jan 9, 2011
Messages
124
Location
columbia city in
Corvette
1982 red corvette
so many places to start .me i would go straight to the plugs.see what they look like .if bad at all replace .how old and any bad spots on the wires.if the cap and rotor are old now is the time to replace.once the tune up is done .then you can check timing ,have you checked for a vac leak ?
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
The cap and plugs were done w/ wires about 3 years ago, but since it was being stored/restored only saw about 1k miles put on them. The wires actually were replaced about a year or two ago; I found out the hard way that my new headers get pretty hot. I think that leaves the rotor. The only thing I can't figure is why it seems to run better after it warms up, or after the throttle is hit a few times to get it at a higher idle; also, wouldn't a spark problem cause misfiring? I'm pretty sure I'm idling rough, but not misfiring. I remember when I burnt the one spark wire it was REALLY running rough, shaking, sputtering, you knew something was not right, but now its just like a very low idle where the car is shaking. I don't know though. I'm trying to check the vac lines right now, but its a real pain. I'm going to check around the base of the carb to see if the "OEM-replacement" gasket they gave me didn't seal right, before I go trying to check every vac line. Does anyone know a simpler way to check those lines than visually going over every one? Thanks for the input though.
 

Rod75

Well-known member
Joined
Feb 15, 2011
Messages
79
Location
Just that side ofthe MotorCity
Corvette
1975 Modified Coupe
I've found through out the years that most drivability problems can be ignition related. Don't assume anything. Check it all out. I'm with Terry82 in that I'd pull the plugs and have a look.
For your vacuum line inspection, I run my hand down them to check for softness or perforations and proper seating, as well as visual inspect.
Keep at it, the problem/problems will surface.
:beer
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
Don't worry I'm keeping at it! Just came in from the garage. Doesn't seem to be a base leak, and I checked all the hoses going into the carb that I disconnected when I pulled it off. The only hose I have any concern with is a small, probably 2-3" piece that goes from what appears to be part of the choke to the front of the carb near the fuel intake, but I believe I read in another thread that it often leaks or was designed with a leak and that its not a problem, going to change it anyway.
I do think I may have solved the extreme rough idle/stalling in drive. Before I found the car was idling around 1500 in park, and would choke up when put into drive; other times it was 1500 park and around 8-900 drive, still with the hesitation when you hit the throttle. I was fooling with the PCV valve, I believe (driver-side valve cover to front of carb. I pulled the valve out of the valve cover to see if it was loose, then pulled the valve off the hose and the car died. Curious, I grabbed the old PCV hose and valve from the stock covers I had taken off and put it back on (didn't have that braided metal on the hose so I hoped it would get a tighter fit at the carb). No rough idle, but still hesitating when you hit the throttle. Also, park idle is around 12-1300, down from 14 or 1500, but the healthy drive idle is still 8-900.
I do believe, and need to do some looking into, what I think may be the hesitation problem. When I was at the carb working the throttle from there, at first I wasn't getting any hesitation. When I did finally get it, it appears that in the primary barrel, the spray of fuel you see when the throttle is applied is larger on the passenger side than the driver side. I'm guessing if the left side isn't putting out a full stream of fuel that would explain why it can get enough fuel when the throttle is applied gradually, but when it goes from an idle to a full-demand it chokes. As I was typing this I realized I did not check to see if there was any change in the nozzle spray if it was applied gradually. I went back and checked, and it does appear that the right-side nozzle has a stronger spray, but it also seems that there is a very slight fuel leak on the weaker left-side. It almost looks like the leak the accel. pump had before the rebuild. It seems to occur right after the throttle is applied and the jets spray. The right side has very little, if any, but the left side is noticeable. Its also worth noting that I couldn't get it to stall going from revving to drive, park, back and forth but that may be because the engine is so warm now. Still doesn't make sense, seeing as the problem started after I had driven 11 miles. So the problem will occur when the car is cool, or warm, will stall sometimes or run rough others, or just hesitate, or run fine, all independent of whatever I do to it... Maybe I'll just get a new Qjet, drop it on a crate, hook it up to a 700R4, drop the whole thing in and call it a day. Yea, that might cost more than swallowing my pride and bringing it to a shop.
 

Rod75

Well-known member
Joined
Feb 15, 2011
Messages
79
Location
Just that side ofthe MotorCity
Corvette
1975 Modified Coupe
I can understand the frustration chasing these problems down. Hell, I used to do it for a living, now I do it for hobby and fun. (you know I'm flipped after that statement) ;LOL
The factory service manual is a good helper and guide. I also recommend you read this gent's writeups on carbs. He's provided a ton of good info on the subject.

I wouldn't hesitate to start looking into the ignition system as well. Your mechanical advance can cause hesitation and other problems.

Corvette Action Center Knowledgebase :: Problems I have Seen with Commercially Rebuilt Q-Jets
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
Thanks Rod, I read over that article and its convinced me that a refurb isn't the way to go; I think there's a shop a few towns over that specializes in rebuilding Carbs I may give a call. The Mechanical Advance you were talking about. Is that the Vacuum Advance on/below the distributor? There's a hose running from the carb to that advance, and the hose has a strong suction, but we get no resopnse when we pull the hose off.
Also, the stalling seems to be a more intermittent problem now. It will do it when the car is first started during the day and dropped into gear, but after the throttle is applied a few times and I guess the car begins to warm up the problem goes away. Never used to do that, but at least it'll run. The hesitation is still there though.
 

Rod75

Well-known member
Joined
Feb 15, 2011
Messages
79
Location
Just that side ofthe MotorCity
Corvette
1975 Modified Coupe
You have 2 types of ign advance. Vacuum and mechanical. The vacuum adv is the can under the dist cap with the hose to it. If nothing is happening when you unplug it, and you have suction (vacuum) at the hose, then check the vacuum diaphram (can). It could be ruptured causing a vacuum leak and not engaging the vac adv.
The mechanical advance mechanism is located/accessed under the distributor rotor.
Here's a quik link I found to get you some basic familiarity with it with pics.
http://temp.corvetteforum.net/c3/joeveto/Vacuum_Advance_and_Recurve.shtml

I assume your running a Q-jet now? Is it the orig carb? There are numbers stamped vertically usually in the left rear flange. These can referenced to an engine app. My thought here is maybe your current carb is a refurb?
Process of elimination.

Heads up for a PM I sent you.
:beer
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
The second I find what it is I'll be on here. I've had work all week so hopefully come this weekend I'll have time, but I'm hoping to fool around with that vacuum advance tonight. I've also found the parts at Ecklers but I'm waiting on an email from them before I order. Thanks for all the help!
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
Last week I had emailed Eckler's asking about their Vacuum Advances as well as their new/mod/upgrade what-have-you distributor/rotors. I hadn't gotten an email back and wrote it off as they didn't care or didn't get it, but today I got a response:
To check the vacuum advance on the car, with the motor idling, pull the hose off the vacuum advance and the motor's idle should slow. Plug it back in and the idle should come up. If it does this, it is working fine. If it doesn't, pull the hose off at the carburetor and suck on the hose then plug the hole with your tongue. If it isn't holding vacuum, it is bad.

As for the hesitation, I believe it is more likely caused by the accelerator pump in the carburetor.

First off, thanks to Randy at Eckler's for the response, and thanks again to everyone here whose been putting their two cents and advice in. I went out today and did as instructed above; I had removed the vacuum line from the advance before. Checked again, no change in idle. Also checked and the hose is changing pressure when the throttle is worked. Hooked the hose to the advance and disconnected from the carb, lo and behold its not holding any suction when tested the way described above.
Now here comes my (hopefully) last question; I figure its probably the vacuum advance. Should I change out the stock distributor/rotor now as well? I've seen a few which come with both a non-adjustable and adjustable vacuum advance. I've seen quite a few on Eckler's and also heard of (I believe) Blueprint Accel, which I believe are rebuilt GM cores. Any recommendations?
 

Rod75

Well-known member
Joined
Feb 15, 2011
Messages
79
Location
Just that side ofthe MotorCity
Corvette
1975 Modified Coupe
To check the vacuum advance on the car, with the motor idling, pull the hose off the vacuum advance and the motor's idle should slow. Plug it back in and the idle should come up. If it does this, it is working fine.
This is true if you have manifold vacuum feeding the vacuum advance. But this is not the setup for a stock '75 engine.
If it doesn't, pull the hose off at the carburetor and suck on the hose then plug the hole with your tongue. If it isn't holding vacuum, it is bad.
Tastes terrible doesn't it? :L Applying a vacuum with a vacuum pump is prefered. And if the hose and connection hold vacuum as well as the advance canister, check, it passes! But doesn't necessarily mean it's giving the amt of advance needed at the correct time. Hence they make adjustable vacuum advance canisters to tweak this.
As for the hesitation, I believe it is more likely caused by the accelerator pump in the carburetor.
TBD. (engne off) as you slowly move the throttle, peer into the carb throat and you should see fuel discharging from the accel pump holes in smooth delivery and unison with throttle movement. If there's any skip or pause in fuel delivery, that will cause a hesitation.
I went out today and did as instructed above; I had removed the vacuum line from the advance before. Checked again, no change in idle. Also checked and the hose is changing pressure when the throttle is worked.
Your vacuum advance hose is connected to a ported vacuum source, this will vary vacuum with throttle. This is the stock setup.
Hooked the hose to the advance and disconnected from the carb, lo and behold its not holding any suction when tested the way described above.
Sounds like the vac adv can is faulty.
Now here comes my (hopefully) last question; I figure its probably the vacuum advance. Should I change out the stock distributor/rotor now as well? I've seen a few which come with both a non-adjustable and adjustable vacuum advance. I've seen quite a few on Eckler's and also heard of (I believe) Blueprint Accel, which I believe are rebuilt GM cores. Any recommendations?
Replacing the distributor maybe a good idea if your original is high mileage with excessive wear. Excessive distributor shaft wear can cause the engine timing to vary. Not to mention other worn out components such as the mechanical advance parts.
It's your decision to spend money/labor on replacing known defective parts on the existing, or go with new/rebuilt.
In the past, I have used the Accel Blueprint units and they proved to be a decent replacement. I found they needed some tuning/tweaking with the vac/mech advance units.
I like to check dist shaft side and end play on new units just to be sure. I will trust, but verify first. I also like to apply my own coat of fresh di-electric grease under the ign module.
Most replacement distributor parts can be found at your local auto parts chains as well.
First off, thanks to Randy at Eckler's for the response, and thanks again to everyone here whose been putting their two cents and advice in.
And thanks for the thanks! Your welcome, I hope this helps get your Vette back on the road! It's words like those that help keep me typing!
Cheers! :beer Pour me another....:L
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
Ok Rod, now you're just showing off :L A local speed shop had the stock-style non-adjustable vac advance in stock (took 10 mins to find it) and right as I got home we got a t-storm. Tomorrow she's going in for inspection before work and when I get home hopefully I'll have time to swap in the new one. Never switched one before so not sure what to expect, but looking at the new one it looks fairly straightforward.
I figure before I run out and get a new rotor/distr I'll 1) check the accel pump to make sure its good and 2) put the new vac in. As much as I like getting in over my head, I'd like to have the car FINALLY running stock before I go changing things around, that way I can see if/what changes happen. That, and every week my dad gets louder in his "forget fighting with that and get a new Camaro" argument. Really, who needs working AC and a smooth suspension :eyerole
Looking forward to an update tomorrow
 

Thore826

Active member
Joined
Mar 18, 2008
Messages
41
Location
New York
Corvette
1975 White, 1984 Black, 1996 Green
First off, passed inspection yesterday and took her out on the highway and a bit in town. Some hesitation, but it seemed that if I gave a quick hit of gas and then put the petal down it would go (almost sounds like double-clutching when you do it).
ANYWAY, I got home today and put the new vacuum pump in. At first I was scared; I did the vac test on the old one after I got it out and it held suction; same as the new, but the old one didn't move the arm, the new one did. Put the new one in, put everything back together, and she runs! Only problem is I need to adjust the timing I guess, which is where I'm at a bit of a loss. In the Chilton manual it says that before adjusting timing, disconnect and plug the vacuum advance. This is where I get lost. Up until now the vacuum advance has pretty much been, in effect, disconnected since it was broken. Once I remove the vacuum advance, my RPM's go up slightly and it runs fairly smoothly. When its CONNECTED its running rough in park, and very rough in drive. I need to go back and look, but I think remember seeing that when the vacuum advance is disconnected, RPM's should drop. My only guess is that between my mechanic adjusting it (and not pointing out or noticing the vac advance being broken) and my father and I adjusting it, we've got it set to run w/o the vac advance, and now need to adjust it to run with it again. I'll do some research on it, getting too dark to go tear everything apart now.
Thanks again for the support guys; I never would've looked at that advance if it hadn't been recommended on here. And after opening up that distributor, it looks a bit ratty in there (guess thats what happens after sitting outside for 15 years). Maybe I will swap a newer one in. Well, I guess since I'll have that off I might as well get that Edelbrock Intake Manifold they swear is so good to help those headers breathe!
 

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