Gain extra benefits by becoming a Supporting Member Click here find out how!
Classic Performance Products Classic Parts
American Auto Wire Classic Industries
Chevs of the 40sDanchuk Catalog
Hellwig Products IncPerformance Rod & CustomEcklers AutoMotive
Nu-Relics Power WindowsRain Gear Wiper Systems
Impala Bob's Bob's Chevy Trucks Bob's Chevelle Parts Bob's Classic Chevy





Username Post: Low speed/rpm surging        (Topic#354808)
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-11-19 03:25 PM - Post#2757139    

When easing along at a steady RPM (like in slow traffic or coming to a stop in 1st or 2nd gear) the car starts to surge/buck. I can kick in the clutch and rev the engine just a little to keep it under control but in a few seconds its surging again. I have checked for vacuum leaks and found none. I believe it is carburation or timing issue or both. Idles great at 750rpm and 15" of vacuum. My combination: 496 BBC, 10:1 compression, 4 speed, Holley 750, cam specs; hyd roller- 235/240 @ .050 lift, 610 int/598 exh. 114 lobe seperation. Timing is locked out at 36 degrees total with 16 degrees initial, no vacuum advance. Where should I start first, carb or distributor?



 




IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-11-19 03:50 PM - Post#2757141    
    In response to 66cayne

Fix the terrible lack of timing curves first.



 
Shepherd 
Frequent Contributor
Posts: 1605

Loc: Lake George, NY
Reg: 11-11-15
01-11-19 06:35 PM - Post#2757160    
    In response to 66cayne

As suggested by IgnitionMan, what rpm is the total advance coming in at for starters? Surging can be caused by fuel mixture issues also. If you had to open the carb throttles to maintain an idle speed you may have uncovered the transfer slots which can cause too rich a mixture at low speeds. Not an easy diagnosis over the net.There are some really good tech articles relating to tuning issues in this months Car Craft.



Edited by Shepherd on 01-11-19 07:31 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-11-19 09:36 PM - Post#2757169    
    In response to Shepherd

Not to be contrary, the OP stated the distributor curve is locked out, set to 36, and no vacuum advance.

Just another example of how NOT to do it for street use, but, then, this IS a top fuel dragster run on the street, isn't it.

I see way, way too much if this insanity caused by engine builders, top tuners, and especially dyno hero's that have no clue as to how to properly set up street engine timing, and/or carburetion.

Excuse me if I am adverse about it, I fix this sort of moronic stuff every day, and it is way past old.



 
Shepherd 
Frequent Contributor
Posts: 1605

Loc: Lake George, NY
Reg: 11-11-15
01-12-19 06:27 AM - Post#2757182    
    In response to IgnitionMan

I didn't take that to mean he locked the timing at 36, as he mentioned 16 initial, which to me meant some kind of curve was there.



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-12-19 07:12 AM - Post#2757185    
    In response to Shepherd

I didnt set it up this way. my engine builder did. I did advance it slightly because that is were the best idle vacuum reading and quality was. I pulled the cap and rotor yesterday to see what springs and weights were in it. The builder put a set of very light springs in but I guess it really doesnt matter as the weights dont kick out anyway. Not sure how he locked it out but I will pull it and disaasemble as needed to get the advance working properly. I want to get the vacuum can hooked back up too. Its there but capped off. As far as the carb, it ran well right out of the box. Just some minor adjustment of the idle screws and choke. The idle transfer slots are not excessively uncovered. Plug readings look great, nice light brown color on all 8. The car pulls strong right up to 5800 which is as about as far as I take her. Below 2500 rpm though she is a pain with the surging.



 
Shepherd 
Frequent Contributor
Posts: 1605

Loc: Lake George, NY
Reg: 11-11-15
01-12-19 07:41 AM - Post#2757188    
    In response to 66cayne

So there is no mechanical advance? I don't understand, if the initial is 16, and the total is 36, they must work, no? Have you had a lite on it to verify these settings? One cause of low speed surge can be too much timing.



Edited by Shepherd on 01-12-19 07:42 AM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-12-19 12:25 PM - Post#2757205    
    In response to Shepherd

I agree with Shepherd, 16 initial, 36 total isn't locked out. It also sounds like you indicate, way too light a spring set isn't returning the mechanical advance to its idle stop.

Also, just what distributor is in use here, and are you using a dial back feature in the timing light, if yes, you shouldn't be for this type of ignition.



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-12-19 01:39 PM - Post#2757211    
    In response to IgnitionMan

I am running a `new` Chevy tach drive from a Corvette. It has been converted with an electronic kit. I am using a dial back light but not using the dial back function.
So what you are saying is, if it is locked at 36 degrees the timing light/tape on the balancer should read 36 degrees at idle or any other rpm. Let me re-check the timing and get back to you. When I initially checked it my light indicated it was idling at 16 degrees adv/750rpm. Maybe I got a bad reading. Also, I did not rev it to check if it was advancing or where the advance started or stopped.



 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-12-19 06:32 PM - Post#2757230    
    In response to 66cayne

Ok on the dial back, OFF is right, mark the dampener for degrees.

You might pull the cap off, and move the rotor clockwise, then let it go. It should move a bit, then snap,back smartly when you let go of it. If not, lots of reasons could exist.

What conversion is in the distributor?

What exact vacuum advance is being used?



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-14-19 10:39 AM - Post#2757341    
    In response to IgnitionMan

Ran the timing again. It is set at 38 degrees advanced and locked out as the engine builder told me. Removed the cap and the rotor will not turn. It has a B1 vacuum can on it but it is plugged. Looks like I will need to pull it out and see how it was locked. Need to get this set up with a street friendly advance. Should I try to find an old school guy with a Sun distributor machine to set this up or is there a known combination of weights/springs/vacuum advance that I can try at home?










 
Shepherd 
Frequent Contributor
Posts: 1605

Loc: Lake George, NY
Reg: 11-11-15
01-14-19 10:46 AM - Post#2757343    
    In response to 66cayne

Your guy set the timing locked at 38? Yikes, surprised it starts! Only a low compression engine or a big overlap cam would maybe work this way. Yes a dist machine will be a big help, there is plenty of info out there to set this up for your combo. IgnitionMan will come in for some info.



Edited by Shepherd on 01-14-19 12:19 PM. Reason for edit: No reason given.

 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-14-19 11:14 AM - Post#2757346    
    In response to Shepherd

Starts great. Using a high torque gear reduction mini starter. It does have a healthy cam.



 
Shepherd 
Frequent Contributor
Posts: 1605

Loc: Lake George, NY
Reg: 11-11-15
01-14-19 12:19 PM - Post#2757351    
    In response to 66cayne

Yeah the bigger cam bleeds off some cranking compression. I have 2 big blocks running out of my shop. Both are running about 16 initial, 36 total, at 3k, with a moderate curve graduation. Cams are are about the same, steel head engines with around 10:1. One owner is playing with the vacuum advance calibration. 91 non ethanol fuel. Both are Chevelles with 4 speed.



Edited by Shepherd on 01-14-19 12:21 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-14-19 02:14 PM - Post#2757368    
    In response to 66cayne

66cayne, do you have the cam card, or overall duration, and intake closing degree specs on that cam, and, what is the compression ratio of that engine, please???

Also, if known, engine vacuum at idle, both in and out of gear?



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-15-19 06:58 AM - Post#2757434    
    In response to IgnitionMan

here are the cam specs: Hyd roller,
Gross lift/int. 613 exh. 596;
Duration@.006 tappet lift/int. 289 exh. 293; Valve timing open @ .050 int. 5 BTDC, 49 ABDC
exh. 55 BBDC, 3 ATDC
Cam installed specs @112 intake center line
Duration @ .050 intake 234 exhaust 238L
Lobe lift intake .3510 exhaust .3510
Lobe seperation: 114 degrees

Vacuum at idle: 15 inches
I have not checked it in gear, its a 4 speed.



 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-15-19 11:29 AM - Post#2757454    
    In response to 66cayne

Of course, no measuring vacuum in gear with a stick trans.

That us a good vacuum number for the intake closing time, always better with a hydraulic roller over a flat tappet cam.

The adjustable vacuum advance should work well if the Crane type stop is used as I outline, useless if done the way Crane outlines.



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-15-19 05:04 PM - Post#2757497    
    In response to IgnitionMan

any thoughts on what weights and springs I should use to get it set up? I'm considering a curve that starts just above the idle rpm of 750-800 rpm, initial between 16-18 degrees, 36 degrees total, all in by 2500-3000rpm. Does this sound like a reasonable starting point? Also, how do I limit total advance on a points style distributor? Does someone sell bushing kits for this or do I have to modify the advance cam?



Edited by 66cayne on 01-15-19 09:20 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-16-19 07:01 AM - Post#2757530    
    In response to 66cayne

Bushing kits are available from usual sources Jegs Summit. Sprink kits, low cost large HEI curve kit, throw the HEI weights and center in the trash, use the springs. Spectre has one for under 5 bucks.

I'd shoot for 20 degrees of mechanical advance, and your 16 initial, for your 36 degrees of total timing. Good start is what you came up with, 100/150 rpms over idle, limit 2,800 to 3,000 rpms.

Vacuum advance, 8 degrees (4th serration on the Crane plate, .090 inch pin travel for home made stop). This will give 8 crankshaft degrees of vacuum advance, on full manifold vacuum, NEVER ported. This sill give 24 degrees idle timing, and help with low to no load engine efficiency.

I may be a bit erratic in responses for a short time, I feel I am ready to leave rehab, doctors want another week. I have things to do, like get two distributors I finished just before I went into the hospital, that didn't get sent, to two over the top patient people.



 
Shepherd 
Frequent Contributor
Posts: 1605

Loc: Lake George, NY
Reg: 11-11-15
01-16-19 10:21 AM - Post#2757541    
    In response to IgnitionMan

Feel better, we need you here!!



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-16-19 04:06 PM - Post#2757564    
    In response to IgnitionMan

thanks for the help. I will let you know how it goes. Good luck with your rehab.



 
IgnitionMan 
Valued Contributor
Posts: 3539

Reg: 04-15-05
01-17-19 07:21 AM - Post#2757592    
    In response to 66cayne

Thanks, I'm trying, the doctors are stalling it.

66cayne, did you get my PM with the stop plate mounting procedure, and see the picture on the web site?



 
66cayne 
Valued Contributor
Posts: 2764

Reg: 08-06-08
01-17-19 04:50 PM - Post#2757636    
    In response to IgnitionMan

I got it. pretty easy to fab. take care



 




Icon Legend Permissions Topic Options
Report Post

Quote Post

Quick Reply

Print Topic

Email Topic

414 Views
FusionBB
FusionBB™ Version 2.1
©2003-2006 InteractivePHP, Inc.
Execution time: 0.103 seconds.   Total Queries: 16   Zlib Compression is on.
All times are (GMT -0800) Pacific. Current time is 10:42 AM
Top