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Hi All - was hoping I could get some assistance as I am having some trouble after a carb rebuilt/switch. I have a 1970 Cutlass with the original 350 engine but it has been rebuilt prior to me purchasing it so I do not know the specifics of what was done but it does sound like it has a mild cam in it. It came with an HEI distributor and a 1973 4MV Quadrajet off a big block Buick. I've owned it for 13 years and never did any type of tune up until this year. I replaced the spark plugs with AC Delco double platinum (gapped to .040") and spark plug wires (MSD 8.5mm Super Conductor HEI wires). After that, the car ran the same as it did before but I noticed an exhaust leak under the choke tube so I took the carb off to fix that and decided to rebuild it since I had it off - the car always seemed to run rich and that was confirmed when I replaced the plugs. After the rebuild, I could not get the idle down and the car started dieseling when I shut it off and it was at temperature. Closing the drivers side idle mixture screw all the way had no effect - the car kept running with no change in idle. Closing the passenger side (and leaving the drivers side closed) dropped the idle and the car ran rough but did not stall. I have checked every vacuum line that I see as well as the carb gaskets/linkages and could not find a leak anywhere. The vacuum gage hooked up to manifold vacuum was fluctuating between 16-18. I thought it could be the 47 year old carburetor (and the amateur rebuild) so I installed a new 600 CFM Edelbrock performer carburetor. When first started, it idled very rough and stalled. A quick adjustment of the idle speed screw kept it running and smoothed the idle somewhat but once in gear and idling, it runs very rough (seems to surge and back down). I rechecked manifold vacuum and it's about the same. As long as the accelerator is pressed while in gear or it's in neutral, it seems to run fine however it is still dieseling once up to temperature but not as bad as it was with the Quadrajet. Other than the tweak of the idle speed screw, I did not try and adjust anything else on the Edelbrock carb as it is supposed to "run right out of the box." I didn't do anything with the timing and since it ran fine after the plug and wire change, I'm thinking it isn't that but I don't know for sure. The only other thing I did this year was installed a set of Thornton shorty headers since the long tubes that were on it scraped the ground every time I hit a bump and finally wore a hole in them.
Sorry for the long post but I wanted to try and provide as much information that I could. Does anyone have any ideas of what could be causing this? Thanks.
Thanks for the reply. I do not have a timing light so I don't know what it is currently set at. Would replacing a carb require a timing change? Sorry if this is a dumb question, first time I'm diving into an engine. I usually stick to brakes, shocks, oil changes, etc.
Before you start fiddling with the carb, buy or borrow a timing light and check your timing, post the result. Read the Edelbrock instructions on how to adjust your idle a/f mixture screws and make sure the choke is operating correctly. If your running the stock manifold, what adapter did you use for the carb swap?
Do you have a recommendation for a timing light? A quick look on the Summit site looks like they range from about $40 up to about $250. Yes, it does have the stock manifold on it and I used the Edelbrock adaptor that converts a spread bore manifold to accept a square bore carb and the choke is tied open. The old carb didn't have the choke set up and it was always open so when I bought the new one, I bought a manual choke model and did the same. Thanks in advance.
The timing light that I used for 20+ years was a cheapo that I bought at a garage sale, but it worked fine. All you really need from a timing light is for it to flash when #1 plug fires, but you can get fancy with a professional grade light and dial back function depending upon how much you want to spend.
My timing light (below). Works like a champ. You can still find them on several websites. Craftsman made a great timing light. Sun made a great timing light, also.
Dont buy the innova dial back w rpm readout while its awesome w great features mine stopped working after 3 uses and was out of warrenty by 1 month innova offered to repair it for me for the price of a new one. Meanwhilei had sold my std 30yo craftsman one like chief pictured.
i found another craftsmen one on ebay w dialback to replace the innova. Reviews of the innova shows its not unusual for them to have a short life
Did the gaskets match up exactly in the carb rebuild kit? Is the choke disengaging completely? Is there fuel dripping from the primary nozzles when the car is idling? Looking at the carb upside down with the choke off the throttle plates should look like the picture and mixture screws should be even.
Hello everyone. Attempted to check timing and I cannot see the harmonic balancer mark when cylinder #1 is firing (I did mark it with a silver paint marker, engine at operating temperature, distributor vacuum advance plugged). When I replaced the spark plugs earlier this year, I did a quick check for TDC (visual) and it appeared to be close since I saw the top of the cylinder head through the #1 spark plug hole. I did replace the balancer a few years back with a stock BOP replacement unit (https://www.summitracing.com/parts/pbb-pb1147n). I removed what appeared to be a stock balancer because the rubber was degrading and it had developed a wobble. A few observations/questions:
-Since I did not rebuild the engine and do not know the specifics, could there have been something done that would render the mark on the stock balancer off?
- Car idled much better with distributor vacuum advance removed, even in gear (rough idle and surge was gone but I did not attempt to drive it). It did still diesel slightly and air blew back through the carburetor (no fire).
- Should I adjust the timing until I see the mark or should I try to verify TDC?
-Should I not try to mess with the timing and adjust the idle mixture screws on the new carburetor?
This may be another dumb question but I would need a dial back timing light to set it to 16/18 (assuming you mean BTDC)? Reason I ask, the stock timing marker that is on the car only has marks for 4 deg, ATDC and 4, 8 and 12 deg BTDC on it or am I totally missing what that marker is reading? Thanks.
You can measure and make new marks on the balancer to use a standard timing light. With an HEI, the stock timing settings are n/a due to the difference in the mechanical curve built into the HEI.
Sounds like multiple root causes here, not just one problem.
Rough idle causes:
- Vac leak
- Bad Cam
- Bad plugs
- Ignition
- A/F setting
- carb problems
- mismatched parts(carb gasket intake)
- vac advance to high
- week vacuum signal
- base timing too high
- fuel boil etc...
Dieseilig causes:
- cylinder/piston carbon build-up
- boiling fuel
- heat soak
- fuel dumping out of the carb
- curb idle RPM set to high.
Buy a vacuum/fuel PSI gauge for $20! See what kind of vacuum signal you have before we go any further helping you.
Hook it to a direct intake manifold vac source. Use it to set the timing and A/F screws. You are looking to achieve the highest vacuum reading at 700 rpms. Go back n forth at the A/F and timing to do this.
Then you can use a timing light so see what the base, vacuum, and centrifugal timing settings are.
A set-back or dial-back light makes it easier to see vac and cent settings. (You Tube it).
Only go at one problem at a time or you will(already are) chasing your tail.
You can measure and make new marks on the balancer to use a standard timing light.
Yep.
What I did was turn the engine until the balancer mark was at the 0 mark on the timing tab, made a paint line on the balancer at the 10º timing tab mark, rotated the engine until the new line was at 0 on the timing tab, made another paint line on the balancer at the 10º timing tab mark (which is now a 20º line) and again for 30º and 40º lines.
Mine was doing the same thing with a rebuilt 400 buck carb.It was driving me crazy until i discovered my distributor hold down bolt was tight but too long.Put a washer on it and now its good to go
distributor was moving around a way retarded till the fix
Check for vacuum leaks on the manifold vacuum lines, but also test the brake booster by block off its port at idle. My 54 year old Olds original was causing a major leak which I only traced out through a carb problem. While you’re blocking lines, you might also check the distributor vacuum advance can to make sure it’s not leaking.
I like my Innova 9668 Timing light, the dial back function is key. But I don’t know how long it will last. I’ve only had it a year.
If your qjet has not had primary metering rods set up for your 350, check in at CliffsHighPerformance on how to get the right rods/jet and secondary rods. It’s likely a big block Buick applied to your 350 is too rich unless you’ve ready changed it. For reference my 455’s are running 73 jets and 49k rods, your 350 should be something more in the 69-72 jet range with 48-52 rods as a guess.
Hello all. Let me first say that I cannot express my thanks enough for all the advice and guidance that this forum provides - I truly appreciate all the responses and tech support my post received. As these cars get older, a site like this is an invaluable resource. I'm happy to say I have the car running as well as it ever has, here's what I finally ended up doing:
-Replaced the q-jet with a 600 CFM Edelbrock - the car always ran rich and, as many of you suggested, I don't think the q-jet was properly set up for the 350 engine
-Verified no vacuum leaks - rag over top of carburetor killed the engine almost immediately
-Purchased an Innova 3568 timing light (RPM readout and dial back function is fantastic) and if the balancer mark is correct (I will verify this at a later time) timing is currently set at about 30-31 degrees. I loosened the distributor hold down bracket but the distributor did not move so I tightened it back up and left it alone since I didn't want to break anything.
-Idle speed set at 800 RPM and it's pulling 18" of manifold vacuum at temperature and idle after adjusting the idle mixture screws on the carburetor
-Distributor advance vacuum line plugged into the timed vacuum port of the carburetor, not the manifold vacuum port - car idled fine plugged into the manifold port but ran very rough when in gear.
I've driven it around town and at highway speeds with no issues, car is no longer dieseling when I shut it off. Thanks again.
... -Purchased an Innova 3568 timing light (RPM readout and dial back function is fantastic) and if the balancer mark is correct (I will verify this at a later time) timing is currently set at about 30-31 degrees. I loosened the distributor hold down bracket but the distributor did not move so I tightened it back up and left it alone since I didn't want to break anything.
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At what RPM are you reading 30-31*? If its at idle with the vac advance disconnected, its way too far advanced. If its at the rpm in which the distributor stops advancing its not advanced enough. Either way you need to get the distributor loosened up to turn it to adjust the timing.
To be honest, you are not done with your tuneup and depending on how you answer the above question, could possibly damage your engine.
The 30-31 degree reading is at idle with the vac advanced disconnected. I know based on previous recommendations, it should be around 17 deg but since the car is running well, I assumed the mark on the balancer is off but maybe I need to validate that sooner rather than later.
FYI, back in 1982 after my first engine rebuild, I set the timing without a timing light and while the car idled fine and ran GREAT at full throttle, once I borrowed a light and checked the timing it was way, way too far advanced. That explained why it pulled so hard, but being so far advanced caused a lot of pinging/detonation. After setting the initial timing to the correct value, the engine still ran fine and no more pinging/detonation issues.
The 30-31 degree reading is at idle with the vac advanced disconnected. I know based on previous recommendations, it should be around 17 deg but since the car is running well, I assumed the mark on the balancer is off but maybe I need to validate that sooner rather than later.
If your harmonic balance is good, then this is way too much advance and you will have detonation at cruise speeds and wide open throttle. The HEI has around 20* of mechanical advance and your vacuum can has at least 16. Lets do some math, 31 initial + 20 mechanical = 51 total + 16 vacuum= 67. Ideally you want 34-38 total + 10* of vacuum advance, so you want 16-18 initial + 20 mechanical + limit the vacuum can to 10*. 18 initial +20 mechanical + 10 vacuum = 48*
July 4th, 2020, 06:44 PM
