Carb issue? (Long) - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 03-22-2009, 05:44 PM
Member
 
Join Date: Jun 2004
Location: Qualicum Beach BC Canada
Age: 45
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Carb issue? (Long)

Been trying to troubleshoot this issue and I'm just not getting anywhere, so I'm asking for some help.

Engine specs: Chev 350, Edelbrock matched top end (7102cam, 60899 heads, 7101 manifold) Holley 80670(670cfm-vac sec)
Sits in an 86 Olds bolted to a th350 w/kit high stall(not sure exact stall) 3.42 rear.

Cam is 234/244@.05adv 112sep and 107center, carb is jetted 66-72 with 4.5pv and 31 nozzle

Problem: hesitation/bog from idle at tip in and WOT. I took it to the local perf. shop figuring they'd be able to 'dial it in' for me. Wrong-o, after days at the shop the car is worse than it was when it went in. I'm a few hun lighter and all I've got to show for it is a new dizzy, cleaned carb and black plugs.

I have very low vac readings (13-15hg @ idle and 4-6 @idle in gear)-lots of drop under load, I presume due to the cam.
I think I've checked everything save for fuel pressure and compression as I haven't the tools to do so.

I don't know what they did but the car went in with good plugs (maybe showing a little lean) and now the 3rd set are black. Prior to going to the shop the car went like a raped ape once it got past the stumble, now it just runs like crap all around.

So I guess I'm looking for some more ideas to troubleshoot, I'd be happy to get it back to the way it was before it went in, hope you guys can help some......please don't make me beg, lol

    Advertisement
__________________
Cleverly Disguised As A Responsible Adult
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-22-2009, 06:42 PM
Registered User
 
Join Date: Mar 2009
Location: WY.
Posts: 15
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I would start with timing check. If thats ok make sure you don't have any leeks in intake manifold or base gasket on carb. easy way is when car is running spray starting fluid around base of carb and intake if idle speeds up you have leek. ( make sure plug wires are good you don't want fire)
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-22-2009, 08:17 PM
Member
 
Join Date: Jun 2004
Location: Qualicum Beach BC Canada
Age: 45
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks sled, checked for leaks everywhere but valley side of intake. Wires are brand new MSD 8.5mm.
Timing is another issue, I know to set it with vac. advance plugged, but if I use Edelbrock recommended start baseline (10-12init.), it just won't idle unless I crank up the idle screw at the throttle, thus opening the primary plates and exposing alot of transfer slot.
If I set initial at say 20, i advance to 25@2000, 27@2500, 29@3000, and all in 37@3500. Mech adv. seems to start in at about 1100rpm. I don't know if that curve is good or bad.
At 20 initial and idle screw set conservatively, I'm idleing in park at 1150 and in gear @ 650-750 but irratic. At 1150 I'm thinking my mech. adv. is coming in too. Also, when I drop it into gear, the advance drops to 8-10 degrees and is erratic.
I hope this is making sense?

EDIT: also at this setting the car want to run-on and gasps at shut down, I always thought this meant it was too advanced?

I was just doing some reading about timing and dist. set up and my advance should not drop off when put into gear should it? I don't have a spring or recurve kit for the dizzy and I don't know what exact changes these guys made (long story, don't wanna go there)

Anyways, I've been doing all this searching and reading about the carb, cam creating low vac, timing and on and on and I my brain is having trouble keeping it in order, lol. Timing should be the first setting before any other changes no?

Sorry for the long post.....any more ideas, I'm pretty new at this still.
__________________
Cleverly Disguised As A Responsible Adult

Last edited by mrevil; 03-22-2009 at 08:25 PM.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-22-2009, 08:50 PM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 8,899
Wiki Edits: 0

Thanks: 19
Thanked 362 Times in 332 Posts
You're right about the timing problem. Is the 8-10° drop when put into gear figure obtained with the vacuum advance plugged? If so and it drops in gear from the park rpm setting then you know right there that it is advancing at idle by the amount that it loses, which is ten degrees in your case. It appears that your total mechanical advance is around 27 degrees and starting too soon. You are correct in that the correct curve should be close to your 20° initial and 37° total figures, but you will have to limit the length of the advance slot to 17° to eliminate the distributor's ability to drop at the lower idle speed you see while in gear.

If the 8-10° drop is observed when put into gear AND the vacuum advance is hooked up, then the timing problem could be caused by a combination of these two. You will need to test just the mechanical advance at 650rpm and 1150 rpm in park to test low limit of your mechanical advance and to find the point at which it truly starts. Sort out the mechanical advance first, the vacuum can be set up later for its economy gains.

The shop you took it to appears to know very little.

Last edited by ericnova72; 03-22-2009 at 08:50 PM. Reason: sp
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-22-2009, 09:05 PM
Member
 
Join Date: Jun 2004
Location: Qualicum Beach BC Canada
Age: 45
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Ok eric, that makes sense to me. Those figures were with vac. advance plugged.
The dizzy is a msd streetfire, so I guess I can get the limiter bushings for that, should be able to figure that out.
I'll post back the results...
__________________
Cleverly Disguised As A Responsible Adult
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 03-29-2009, 10:44 AM
Member
 
Join Date: Jun 2004
Location: Qualicum Beach BC Canada
Age: 45
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Ok, got it timed as best I could and have a noticable improvement.
However, I still have a stumble and popping on tip in and wot, but better response when it picks it up.
I felt I was prolly lean, so I pulled the plugs. I found all cyls. lean except 2 and 3 which were soot black (no moisture).

I'm going to check for spark and replace plugs, but wanted to see if someone could tell me what else this could be a symptom of.

If more info is needed, I've got lots written down, so ask if you need....TIA
__________________
Cleverly Disguised As A Responsible Adult
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 03-29-2009, 04:42 PM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 8,899
Wiki Edits: 0

Thanks: 19
Thanked 362 Times in 332 Posts
Now that you have the timing sorted out, it is time to tune the carb. Looking at your parts list, my next step would be a larger pump nozzle (shooter) for the accelerator pump. The stumble at tip-in is a lean "hole" in the fuel curve not being completely covered by the accelerator pump shot.

You may also need to move the pump cam to the second hole, which is more agressive, or you may have to switch to a different pump cam.

Typical cause of run-on is too much throttle opening of primary blades, not ignition timing. When key is off, ignition is off.

You may need to tweak the secondary throttle stop to slightly open the rear blades a hair, allowing you to close down the front with the idle speed screw and gain back some adjustability. This would be the first thing I would look at.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 03-30-2009, 06:45 AM
Member
 
Join Date: Jun 2004
Location: Qualicum Beach BC Canada
Age: 45
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks for the help eric.

So I replaced plugs (more on this in a bit) checked all wires(they're new msd) and even changed the cap on the dizzy because the guys put my old one back on the new dizzy.
Couple of notes:

the cap showed some carbon on all the tabs inside the cap, only one that was real bad was #3 tower, don't know if I was getting some arc or tracking from that. Replaced anyways.

working with the plugs and wires around these headers are a bit of a pita, so I took extra care to make sure all were connected positively.

now the plugs...this is a bit nuts...when I got the motor it had NGK = to Champion RC-12YC (Edelbrocks recommended) gapped .35
prior to the trip to the shop, I had the above Champion plugs also gapped .35 - showed lean
the shop replaced those with NGK FR4 gapped .35 which came out as in my last post- these are = to the Champion plugs
I replaced these with another set of NGK yesterday along with the cap and double checking wires and routing.
Then I find a notice from NGK on the net about the Edelbrock recommendation on plugs. NGK says this...
According to
the manufacturer, Champion RC12YC spark plugs can be used with their
cylinder heads on most street applications. But what happens is that when the
customers use this plug they have to advance the timing and adjust the air fuel
mixture (i.e., rich) to get proper combustion. The Champion equivalent NGK
spark plugs for these aftermarket cylinder heads are NGK FR4 or BKR5E spark
plugs.
Issue
When using the NGK FR4 or BKR5E spark plugs, because the NGK plugs have
more copper in the core of the spark plugs than the Champion spark plugs, they
pull out more heat during the start-up and can cause gas fouling issues and
intermittent misses. This problem is worse in cold weather climates. In fact, when
the cylinder head manufacturer suggests using Champion RC12YC spark plugs,
they are only specifying the basic spark plug design specifications (i.e., 14mm
threads, 5/8” or 16mm hex, and 3/4” or 19mm reach) not an actual heat range for
the plugs.
Solution
For these applications, we suggest using NGK R5672A-(X) series spark plug,
where “X” stands for the heat range for the spark plug. Depending upon the
compression ratio and horsepower of the engine, start with an“8” heat range and
move up to 9&10. There is no need to advance the timing or use a rich air fuel
mixture with these plugs and customers can simply use 94 octane fuels and
make slight adjustments for the humidity level in the environment.
Don't know if this is making a big difference or not?
Anyways, in summary, after my last post I:
-changed plugs
-changed cap
-double checked wires
-put 35 nozzle on carb
-1 medium spring in dizzy along with light on other side
-bumped advance up 2 (init 16-18 bouncing, ported vac puts me at 52 @1800-2000 and 32-34 all in @ 3000)
After this car was a tad better but still stumbling, no popping tho. I ran up to the gas pump and ran out just as I pulled in (car stalled anyways). Filled it with 91 (highest they had).
On the way home I punched it a couple times again, same stumble, but this time it stalled out twice when idleing down on the way home, I coasted into the driveway! Not sure wth that's about?
Damn, I pulled the plugs again, all rich, dry carbon. Carb is off the car now as I want to check primary and secondary plate settings. Jets are still 66-72 w/4.5 pv (bouncing 6-9"vac in gear)
I will get the NGK recommended plugs asap, check carb settings and try again.

Sorry for the long post, thanks for taking the time.

Edit: just looked at a NGK reference chart. It seems the racing plugs they recommend are equivalent to a BKR7E. The plugs I just put in are BKR5E, figure the difference is that great? Should I just increase timing again and see what happens?
__________________
Cleverly Disguised As A Responsible Adult

Last edited by mrevil; 03-30-2009 at 07:07 AM.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 03-30-2009, 08:22 AM
bentwings's Avatar
bentwings
 
Last photo:
Join Date: May 2002
Location: St.Paul, Minn
Age: 72
Posts: 1,793
Wiki Edits: 0

Thanks: 0
Thanked 16 Times in 15 Posts
I have the Edelbrock RPM motor with aluminum heads, 750 Holley (3310-3) vac sec. elect choke, MSD dist. and Summit box with the red barn MSD blaster coil 8.5 wires, vac advance with 10 deg avail. I can only use about 32 deg mech advance at 3000. My advance is all in by 2600. I think initial is about 12 deg but I don't count this. I only worry about total at 3000 and let initial fall where it does just so there isn't so much it kicks back on the starter. My car starts very easily even down below 0 deg temp.

I had the same flat spot just off idle. I went to a larger shooter I think a .042 was the first one. Then the blue cam on hole two. This was a marked improvement. I'm up to .050 now and have the .055 ordered. The next step is to go to 195 thermostat as the motor is very cold blooded. So far it has been in the low 40's for temp. when we have run it and it takes 15-20 minutes of running to get to 180. Once it up to 180 it smoothes out and the stumble is almost gone. You do notice it when driving however as I only have a 2800 converter. I'm pretty sure that the 195 stat and larger shooter will cure it. My plugs AC Delco equiv to the Champ RC 12 YC and are fluffy black.

My car only weighs 2500 +/- so even with a slight stumble it still blackens the pavement easily.

I have 12 in vac pretty constant neutral or in gear. The idle is about 900 neutral and 750-800 in gear.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 03-30-2009, 08:47 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 59
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
It would have suggested you drive the car w/o the secondaries.

See how it drove like and what the plugs looked like that way.

If the secondaries are opening too early, the engine will be flat as heck on acceleration.

Carb's off now though...
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 03-30-2009, 09:30 AM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,523
Wiki Edits: 0

Thanks: 2
Thanked 330 Times in 327 Posts
The RPM cam you have requires a minimum of 24deg base idle timing.
That means you must modify the distributor mechanical advance travel limit in the distributor to 10-12deg limit to allow 24 at idle and 34-36 at max advance.
Install 1 medium advance spring and 1 light advance spring after limiting the advance travel.
Remove the carb, flip it over and look at the throttles. Reset the idle fuel transfer slots exposure by adjusting the idle speed and idle stop on all 4 barrels about .020" evenly. Replace the power valve with a new 4.5" rated power valve and reinstall the carb. Install new fresh Champion RC12YC gapped at .035". Start it up and reset the now modifed/limited advance curve distributor to 24deg at idle.
Then plug the vacuum advance into the port vacuum on the carb.
Vacuum adavance will also need to be limited to around 12-15deg and will need to be custom rate adjusted by trail and error.

From this point you can tray different accelerator pump cams and shooters.
usually a 31 or 35 shooter is good. resadjust the accel pump linkage for immediate shooter response to the throttle movement right off idle, yet .015" of free travel gap on the pump arm at WOT (check with a feeler gauge). Bend the accel pump cam linkage arm if nesssessary to correct the accel linkage. Do not adjust the spring/screw binded tight. All this achieves is bending the arm out of shape..
Try a purple or yellow secondary diaphram spring and change the secondary jets to #73 -to- #76.

You will never get clean throttle responce as long as the plugs are fouling.
You will never get the plugs to stop fouling until you set up the advance curve properly with at least 24-26 deg base timing at idle. And get the throttle blade/transfer slot exposure setting correct.

You should have a 10" 3200-3500 stall converter and 4.10's in your car if you expect max performance from your engine combo.

Last edited by F-BIRD'88; 03-30-2009 at 10:29 AM.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 03-30-2009, 09:56 AM
bentwings's Avatar
bentwings
 
Last photo:
Join Date: May 2002
Location: St.Paul, Minn
Age: 72
Posts: 1,793
Wiki Edits: 0

Thanks: 0
Thanked 16 Times in 15 Posts
Good info F-bird. The short quick advance is our next step along with larger shooter. The motor has relatively low compression so it should not kick against the starter.

We tried more total advance but 34 is the absolute max that this motor will run with. It loses vacuum and performance at 38 deg. We found 34 to be the optimum. We carefully laid out TDC and the timing marks so there is no error there.

I agree with the secondary. This is a big carb for this motor and you don't need the secondaries openig too soon. If you can feel them open it's too soon.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 03-30-2009, 10:31 AM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,523
Wiki Edits: 0

Thanks: 2
Thanked 330 Times in 327 Posts
Quote:
Originally Posted by bentwings
Good info F-bird. The short quick advance is our next step along with larger shooter. The motor has relatively low compression so it should not kick against the starter.

We tried more total advance but 34 is the absolute max that this motor will run with. It loses vacuum and performance at 38 deg. We found 34 to be the optimum. We carefully laid out TDC and the timing marks so there is no error there.

I agree with the secondary. This is a big carb for this motor and you don't need the secondaries openig too soon. If you can feel them open it's too soon.
try locking the timing out and set it at 34deg full time. if you noticed a definate derade in performance with more than 34deg timing, try less total timing.
(32-33deg) let the motor tell you what it wants for total timing. But the more inital the merrier with a big camshaft.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 03-30-2009, 10:34 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 59
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
Quote:
Originally Posted by bentwings
I agree with the secondary. This is a big carb for this motor and you don't need the secondaries openig too soon. If you can feel them open it's too soon.
If this is in re my comment about the 2ndaries, it was aimed @ mrevil, but could pertain to you as well- just that your prob. doesn't seem as pronounced as his, i.e. no badly fouled plugs, etc.

With his lean-ish primary jetting, it would seem the rich condition (carb assumed to be sound otherwise) would be coming from the 2ndary side- too early opening.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 03-30-2009, 04:17 PM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 8,899
Wiki Edits: 0

Thanks: 19
Thanked 362 Times in 332 Posts
F-Bird said everything I would have said, just more succinctly. If you have to go over 40 on the shooter, something is wrong elsewhere, shouldn't need that big for this combo.

For what it's worth, IMO, I wouldn't put Champions in anything but a lawnmower or weedwacker. I've never had a good experience with them in GM engines and was warned as a young man by my elders that Champions suck in a Chevy. I'm an AC Delco or Autolite man, heard good about but never used NGK's.

Don't know how NGK numbering system=heat range, but all you descriptions tell me your plugs are too cold.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
modifying that too rich edelbrock 1405 carb? TVR383 Engine 23 12-03-2009 09:19 PM
M4ME carb stumbles sometimes dies when letting off throttle in gear fast68 Engine 4 08-29-2007 05:57 AM
Sudden low-speed issue... carb? benwantland Engine 5 10-07-2005 12:39 AM
Ford 2 bbl carb issue - detailed question V Bird Engine 6 08-10-2005 11:43 AM
Holley carb has me at wit's end cool rockin daddy Engine 23 06-02-2005 02:38 PM


All times are GMT -6. The time now is 11:32 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.