C2 63 SWC engine jerks
Thread Starter
Instructor
Joined: Aug 2013
Posts: 107
Likes: 1
C2 63 SWC engine jerks
I have a 1963 SWC with a 340HP engine. AFB carb. The car starts and idles good. Acceleration is smooth and the car seems to have a lot of power. When I am at a steady RPM in any gear the car jerks. Its not cutting out I don't think, it just seems to jerk. Thanks
Safety Car
Joined: May 2014
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That seems to be a very common problem with the AFB and the 340hp engine, I have read that so many people experience this problem. The high performance camshaft has valve overlap (both valves open at the same time) and with light throttle the high engine vacuum produced pulls exhaust back into the intake manifold diluting the A/F mixture.
My feeling is the fuel has changed a bit from what was available when these engines were engineered and what you are experiencing is a lean surge.
Try the timing to see if it helps but IMO your carburetor idle circuit is too lean, the idle circuit consists of the idle restriction and idle air bleeds. I don't know if the venturi clusters are even available with larger idle restrictions but you can experiment and modify the existing venturi clusters but you better do some homework first.
My feeling is the fuel has changed a bit from what was available when these engines were engineered and what you are experiencing is a lean surge.
Try the timing to see if it helps but IMO your carburetor idle circuit is too lean, the idle circuit consists of the idle restriction and idle air bleeds. I don't know if the venturi clusters are even available with larger idle restrictions but you can experiment and modify the existing venturi clusters but you better do some homework first.
Instructor
Joined: Jun 2002
Posts: 126
Likes: 15
From: South Strafford VT
I’m one of those people with the same problem and my car is a 340 that has an Edelbrock 1406 on it with an LT-1 cam. I have checked everything I can think of and changed metering rods and springs, etc. I’m inclined to agree with Tim but don’t want to start drilling air bleeds, etc until I’m sure of the problem. I have another manifold and a Holley 3810 and plan to try that combination when I get back from vacation. I’ll be watching this thread in the meantime.
Safety Car
Joined: May 2014
Posts: 3,790
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After eliminating the timing and any other tuning issue I could think of I would probably reduce the second idle air bleed by .006 to see if that helps. That would keep the idle circuit active longer plus make the mixture richer. The second air bleed is the hole on top of the venturi cluster, not the brass tube that sticks up, that is the main circuit air bleed.
If the casting is thick enough you could drill/tap and use a 6-32 brass set screw with a smaller hole, at that point if it does not work you can restore the original size hole. You could also J&B weld the hole shut and re drill .006 smaller to try and go back if it does not work but only one thing at a time or you will never know what worked or didn't work.
If the casting is thick enough you could drill/tap and use a 6-32 brass set screw with a smaller hole, at that point if it does not work you can restore the original size hole. You could also J&B weld the hole shut and re drill .006 smaller to try and go back if it does not work but only one thing at a time or you will never know what worked or didn't work.
Le Mans Master
Joined: Mar 1999
Posts: 5,401
Likes: 986
From: Itasca IL
This sounds more like a timing issue than a fuel issue.
I would disconnect and plug the vac line to the distributor, test drive and see if there is any change.
You should have the B28 can, I tried a B20 first and the engine does not like that.
Verify the limit busing is in place, and that your advance springs are sufficiently stiff that the advance is not coming in too soon or going too far.
I would be surprised if some combination of above doesn't resolve your issue.
I would disconnect and plug the vac line to the distributor, test drive and see if there is any change.
You should have the B28 can, I tried a B20 first and the engine does not like that.
Verify the limit busing is in place, and that your advance springs are sufficiently stiff that the advance is not coming in too soon or going too far.
I would be surprised if some combination of above doesn't resolve your issue.
Instructor
Joined: Jun 2002
Posts: 126
Likes: 15
From: South Strafford VT
It has a 236-16 vacuum can, I believe the same as B28, initial is 10 degrees and the advance is all in at 2400.
I did try it with advance disconnected and blocked and don’t think it helped, don’t remember exactly but I would have if it was much better.
I will try stiffer springs before changing carburetors in about 3 weeks.
I did try it with advance disconnected and blocked and don’t think it helped, don’t remember exactly but I would have if it was much better.
I will try stiffer springs before changing carburetors in about 3 weeks.
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Le Mans Master
Joined: Mar 1999
Posts: 5,401
Likes: 986
From: Itasca IL
Thread Starter
Instructor
Joined: Aug 2013
Posts: 107
Likes: 1
Drove the car today with the vacuum advance unplugged and the car ran great. Plugged it back in and it started jerking. Not sure if the vacuum advance is bad or if I have the timing too far advanced to start with. I will check the timing and report back.
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
Melting Slicks
Joined: Jan 2007
Posts: 2,842
Likes: 622
From: Pasco Florida
Drove the car today with the vacuum advance unplugged and the car ran great. Plugged it back in and it started jerking. Not sure if the vacuum advance is bad or if I have the timing too far advanced to start with. I will check the timing and report back.
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
It's been popular, lately, to add a little too much lead into the system. It's not better.
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
Le Mans Master
Joined: Mar 1999
Posts: 5,401
Likes: 986
From: Itasca IL
Drove the car today with the vacuum advance unplugged and the car ran great. Plugged it back in and it started jerking. Not sure if the vacuum advance is bad or if I have the timing too far advanced to start with. I will check the timing and report back.
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
Thread Starter
Instructor
Joined: Aug 2013
Posts: 107
Likes: 1
We have a short season here in Minnesota. Just want to drive the car while I get this sorted out. Not sure what a B-28 can is. I buy my parts from Paragon and they do not list a new vacuum advance with that number.
Team Owner
Joined: Aug 2007
Posts: 58,061
Likes: 7,146
Army
Drove the car today with the vacuum advance unplugged and the car ran great. Plugged it back in and it started jerking. Not sure if the vacuum advance is bad or if I have the timing too far advanced to start with. I will check the timing and report back.
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
When I suck on the vacuum hose I can see that the vacuum advance is working. Is there any other things that can go wrong?
Can I drive the car with the vacuum advance unplugged? Does it damage anything?
You will give up a smoother idle, cooler running and improved gas mileage....you just need to remap your timing curve a bit... Not rocket science... Pulling back on the initial timing a couple of degrees could be all that is needed - if it doesn't help then you need to dig a bit deeper...
Melting Slicks
Joined: Jul 2004
Posts: 2,869
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From: Maryland
> > > NAPA VC 1810
> > > AC D1312C
> > > STANDARD VC 177
> > > NIEHOFF DR 305
> > > Borg Warner V329
Wells DV1810
Airtex 4V1053