Vacuum leak, it was not.

[Nuke427]

I posted a few weeks ago on my car running like crap after swapping intakes. I last posted with an announcement of finding "the vacuum leak". Well, I may have found and fixed one, but it did not cure my part throttle woes. I have checked the vacuum and it is sat. It really feels like ignition. Part throttle under load, missing badly. Idling in the drive, revs free as can be. Full throttle under load, seems to be okay, but probably not as good as it should be, I'm sure the miss is just masked by the noise and acceleration.

I have moved on to looking for an ignition problem. I'm using an MSD Pro Billet distributer and 6AL box. I removed my cap and there was corrosion on the posts. I cleaned that all off and it made no difference. Still crappy. I checked all my connections at the box which is mounted in the passenger side compartment in the cockpit, so no heat stress on it. All connections good. All plug wires seem to be good.

I cannot figure this out, so I ordered a new cap and rotor and all new plug wires. If this does not fix the problem, I'm going to drive it off a cliff.

Anyone had a similiar problem and if so, what was the cure?
I'm getting desperate and willing to buy new everything until problem solved!!!

[Pete Munroe]

If I understand your problem...it's rough...surging? at mid power...like freeway speed?

I put a new Holley 1850 replacement carb on my 67 Corvette back in 1993...the original carb was shot, warped and rotten, but had original jetting circa' 1967.

The car was geat under all conditions except trying to cruise on the freeway at a steady 70 or so.

It would surge on an almost regular basis.

I pulled the float bowl, the new replacement Holley was several jet sizes leaner the stock one...I took the jets out of the old carb metering block...put them in the new carb, problem gone away.

Apparently is was "lean surge".

You say you changed intake manifolds...like from a dual plane to single plane?...anyway, perhaps the jetting is now a little too lean for the speed of the airflow in the new carb...maybe the velocity is lower due to bigger passages and runners and the fuel is dropping out of suspension at medium power...

Anyway something else to check if the only thing you changed was your intake...maybe it effects the mixture at midrange.

Pete

[Nuke427]

Thanks Pete.
That was my first suggestion on my initial post. I did swap from a Edelbrock Air Gap (dual plane) to a Edelbrock Victor Jr. (single plane). I thought my problem may be the lower vacuum, air flow at lower rpm with this new intake. And it very well could be, but how in the heck do I find out? Just start swapping power valves and jets? It sure feels like ignition though. Cruising and part throttle acceleration, it misses, no matter what rpm I'm at, or what gear. I'm at a loss, and need direction, techniques to solve this problem.

[Bruce Edwards]

What does your tach show? Does it show a break down by bouncing when the engine is missing?
If not sounds like Pete is closer to the mark. Do you know how to "Read" spark plugs? Take the car for a run at the speed the problem shows up. Pull the plugs and check to see what they look like. If they show a lean mix up the jets.
I am a Holley man after fighting more Edelbrock carbs than I can count I do not use them any more. It does sound like a lean mix problem that I have had with other engines that customers demanded I use a AFB on. I had one that would run fine at speed but would not idle. After swapping rods and jets it would idle but the cruise speed had a miss. Never could come up with the right set of rods and jets to fix the problem and I have three or four of the Edelbrock "Tune" kits and still never seem to have the right ones. After hours of swaping and testing I called the customer up and made him a deal
I told him I would by a Holley for it our of my pocket and put it on the car out of the box. If the car did not run any better I would keep working on the Edelbrock at no charge to him. How ever if it ran like I know it was set up to since I did the engine he would owe me the cost of the carb and the back labor to try to sort out the AFB.
He came down I opened a sealed box and dropped on the Holley and told him after I set the idle mix I stood back and told hime to go drive the car.
He came back with his check book out a big smile on his face and a hammer for the AFB.

[Nuke427]

Bruce, my car ran perfectly with the air gap. Once I swapped to the Victor, it developed this miss, stumble at part throttle at any rpm. My question now would be, what do I need to change in the carb if in fact it is the carb. After I get the new cap and rotor I will buy in 100% that I need carb tuning. I just don't know how to start the tuning.

By the way, I have a 650 double pumper.

[Bruce Edwards]

How does the cars start first thing in the morning? Do you have a choke on the carb? What size is the engine?
If you can give me a bit more info I will do my best to help. If you want call me at my shop 407-857-8587.

[Nuke427]

Bruce, engine is a 302, RPM heads, stage 3 Trick Flow cam, victor intake with Nitrous plate, ( 1/2" plate). I can't tell you if it would need the choke or not since it sits for weeks at a time before it gets started. I haven't had it out but twice with the new intake. I believe the Double Pumper carb is at all stock settings.

[Bruce Edwards]

Thanks for the call. Let me know what you find out.

[Nuke427]

Bruce, you rule! I ran the car this evening and pulled the manual choke and it cured the hesitation/missing. I can't believe I didn't think of that! I guess I'm used to fuel injection. Thanks man!!! Now I think I'm going to send my carb to Bigs and have it tuned to my setup. Costs quite a bit, but I think it's worth it.

[Bruce Edwards]

No you do not have to do that. What you have found is that the system is lean at coast. Since it runs ok under power and idles smoothly you can not be that far off on the jet size.
All you need to do is pull the front bowl and check the jet size. It will show a number say like 63. Go to your local speed shop and get a new front gasket and try to up the size by 5 so you would look for a 68.
When you pull the front bowl take a bottom bolt out first. Place a cup or a BIG rag under it as a lot of fuel will leak out. Make sure you clean it up before you try to start the car back up.
The whole thing should take less than 15 min start to finish.
Test drive the car again and see if it is better. Should be. If it now loads up, runs rich, get a small jet.
Since you say it runs well when you floor it I would not mess with the power valve setting or the rear jet size.
The whole thing should cost you less than $20 and a little of your time. Being a Cobra owner is all about working on your car! Just be sure to get the lever for the accel pump under the arm when you install the front bowl.
You can do it!! If you get in to trouble I can bail you out from here.

[Rick Parker]

I have a 650DP that I reworked heavily. Changed boosters, converted to 4 corner idle and replacable screw in air bleeds. On my 289 with real good heads and big cam I use #67 jets in front and #71 in the rear. Runs great. Actually might be a tad rich.

Rick

[Nuke427]

Bruce and Rick. I'm sure I could swap jets and tune a little myself and be happy. But after reading up on "Bigs" custom carbs, I've decided to send it off to Wisconsin and have them totally redo it to my cars specs/mods. I like to tinker, but most of the time I pay out and have things fixed. I know I burn alot of money that way, but I normally break things worse than they already are.
Bruce, I still am so glad you brought up the run with the choke pulled and see if that stops the surge/miss. I was smiling ear to ear and it was dark and about 38 degrees outside, I just had to go try it! So glad I know the problem.

[moparmaddnes]

I HAVE A SUGGESTION FOR YOU. DO YOU HAVE A REPUTABLE SHOP NEAR BY THAT HAS A CHASSIS DYNO? THIS WOULD BE THE BEST WAY TO TUNE YOUR CARB. REASON IS THAT THE PEOPLE WOULD HAVE THE COMPLETE PACKAGE TO TUNE, SINCE THERE ARE A LOT OF VERIABLES TO TUNING. NOT TRYING TO TAKE WORK AWAY FROM ANYBODY, BUT WHEN YOU ONLY HAVE THE CARB (EVEN WITH ALL THE PROFILES OF WHATS IN THE ENGINE) THE PERSON IS USING GENERAL PERAMITERS. ON A CHASSIS OR EVEN AN ENGINE DYNO YOU HAVE THE HOLE WORKINGS TO DIAL IN THE CARB.
THIS IS WHAT I HAVE COME TO KNOW.

[Nuke427]

Mopar, I agree with you 100%. I am basically sticking with my plan to send the carb to Bigs and get the stage III upgrade for initial tune and performance. After talking with Jesse from Bigs on the phone, I feel confident that my 650 will come back to me and perform better than I could ever tune it. They are doing more than swapping some jets or power valves. Jesse even told me that I should take it to a dyno and may have to swap jets in the future. But he did say that when my carb came back, there would be NO sluggishness, no hesitation, no missing, perfect off idle response and so on. I have high hopes since this is going to end up costing as much as buying a new 650 dp. Check out Bigs site http://www.carbdford.com/Bigs/Bigs.htm

[ENTDOC]

Nuke,I have the Bigs upgrade on my 427,it is awsome.It has the best response I have ever seen and well worth the money. chuck

[Nuke427]

ENTDOC, thanks for that response. Glad to hear that! I just got back from sending my carb and just got off the phone with Jesse. I'm pumped about it. Hopefully I'll get it back beginning of March and test it out!