Carb Problem?

[David Kirkham]

Gents,

Don't have much time to write now. I am off to the in-laws and any moment I will be caught...

George...I have been thinking about a leak around the gaskets. It is a Blue Thunder intake with a turkey pan. Lots of areas to leak.

Distributor phasing 101

The MSD can fire from the trailing edge of the rotor, or the leading edge of the rotor. The rotor spins counter clockwise in the engine when running. If the trailing edge of the rotor is burned, then the rotor is firing on the trailing edge. It should be firing on the leading edge. This roughly translates to 40 degrees of timing either direction (depends on which way you are looking at it). You need to switch the wires going to the distributor and retime the engine. MSD blames Ford for the problem. Strange some of them are phased one way and some are phased another.

The engine will run with the distributor phased backwards, but the rotor and cap will only last 1000 miles. Mine were trashed at 500 and the engine slowly quit running at high RPMS. I thought fouled plugs were the problem until I talked with the engine builder and he suggested the phasing problem. Sure enough, that was part of the problem.

Question:

How do you tell the difference between a Holley 750 and a Holley 850? Any help would be appreciated.

As it is Sunday, it is a day for driving and not for autopsy work. Save that for tomorrow and I will tell you what I found. Any suggestions on how to find a vaccum leak in a carb gasket?

David

[NROTOXIN]

Snake Charmer,
I think what is ment by 'phasing' in this case is that the rotor is not transfering the coil spark dead center to the spark post on the dist cap. To check, you cut a hole in an old dist cap between the coil post and the #1 post the size of a quarter (get as close to the #1 as possible). Install the cap, hook up timming light to #1 wire, start car and point the light into the cap and you should see the rotor pointind exactly dead center to the post, if is not you can usually adjust the magnetic pick up right or left to 'phase it in with the reluctor (which is the trigger at the time of the rotor position)

curtis

[Eddy K]

David, I had that same problem when I installed a new pickup in my MSD distributer. The wires were reversed on the pickup. Fortunately I have a friend who is very well versed on this subject and diagnosed it over the phone. I reversed it and the engine straightened right out.
If the distributor is phased wrong it will throw your timing for a loop and will have all the symptoms you speak of.
As or the best carb for the street, a vacuum secondary carburator is most always the best choice for drivability. Probably for your engine, a 780 would work pretty well. However. I would talk to Holley or Grant, someone like that who is way more knowledgeable on this subject than I am.
I did install the 50 cc pump on my car today and although it is better, the stumble is still there. I am going to keep going at it and see if I can get it to work.
I also have one of the 750 Street Dominators setting on my shelf that I would like to try. I am sure that will be a bear to get working correctly.

[John 550]

propane gas for finding gasket leak

[George Snyder]

david,
If you have two vacuum conection on the right front of the carb?
The top should be plugged with a rubber cap. It is ported vacumm. when you attach a vacuum gauge to it, it should read zero. If not then you have a bad gasket between the intake and carb, which is allowing intake vacuum to get to the ported side.

You mentioned in your post that you ran the car on a chassis dyno? Did you record the air / fuel ratio while making the runs?
If so what were your readings? idle, to 6500?

You probally could use a 750 speed demon on that motor, but you may lose some HP at your peak, 6,500.
Vacuum Vs. mechanicals is a hard choice. There are a lot of factors to consider. Remember bigger is not always better! One that is usually overlooked is gear ratio.
I have found that a car with a lower rear 3.90 + seem to accept a carb with mechanical secondaries better than higher gears.
Set the idle like I said before. You will notice a big difference in your throddle response.
Good luck!

[Jmar]

Hi David!

I had a problem similar to what you described while using a 750 DP on my car. From what I understand, Holley DPs are meant to run rich since they are tuned more
for racing. My car ran rich on the dyno so I followed the advice of the dyno man and leaned it out.

The stumble ocurred during part throttle after I leaned the carb out. The more I leaned it out the better it performed on the top end, but it stumbled more during the transition from part throttle to the power mode. There are two things that I did to help eliminate the "hole".

1) I changed the settings from stock to position 2 on my accelerator pump cams. This improved throttle response by moving the cams to a more agressive curve. It is also free!! Usually high volume pumps aren't required and may cause various clearance problems (eg you may need a carb spacer to clear the larger pumps and then what happens to hood clearance, linkage, etc ?).

2) I went to a power valve that opened under slightly higher vacuum since my cam is fairly mild. This require that you drive the car with a vacuum gague.

As I have learned from reading and talking to other tuners, my carb may be too big for my engine to have perfect street manners. So the things that I mentioned above help to cover up the problem. I may buy a 650DP to see if throttle response improves and ETs go down at the strip.

[chopper]

As to testing the power valves, check out this tool from JEGS:

http://www.jegs.com/cgi-bin/ncommerc...61&prmenbr=361

Summit has one also but I couldn't get into their website this evening to get the URL.

This little tool, used in conjunction with an hand-held vacuum pump, will allow you to test the power valve to determine definitively whether it is good or bad.

[David Kirkham]

OK,

Thanks to all for the great comments. I will describe some more of the symptoms as I drove quite a bit today up, around, and in the mountains.

The car absolutely stinks at 2000-3000 rpm's. The dead spot is so bad I have to be careful not to kill the motor. The car is basically undriveable in that range. Unfortunately, that is exactly the rpm's I usually drive in, and worse, it is the rpm I drive in when coming home in my neighborhood. Don't want to irritate all the neighbors too much.

Also, I took it out at full throttle today and the car laid down at 4000 rpms and died. Did the same thing in the next gear. I am begining to wonder if I have multiple problems. Only did that once, then didn't do it again.

Recap.

2000-3000 is undriveable
4000-5000 mysterious, horrible miss. Happened once.
4000-5000 usually misses and can be heard in the exhaust note and vibration of the engine when driving.

427 with a 428 crank
mild, not too wild street cam
6500 is max rpm.
I have a 6000 chip in it, but I rarely go above 5000 rpms.
850 DP Holley
3:31 gears in the back
top loader 4 speed tranny
mechanical secondaries (I remember from somewhere there were several horror stories about the vaccum secondaries coming in at inoportune times.) Maybe I am wrong here.

As I think about this, I remember one of my customers had a high rpm miss and traced it to a bad MSD coil. Said changing the coil was like adding 100 hp. Maybe I will change the coil and see what happens. I also like the new trusted carb swap idea. I will let you all know tomorrow.


BTW

Checked the intake, and it is indeed an Edlebrock Performer RPM. Maybe I'd better check that carb gasket seal again.

I will let you all know tomorrow what the autopsy shows. Thanks so much for all the help. I know this has undoubtedly been rehashed many times, but I appreciate the help anyway.

David

[Jamo]

Enough about your carb! I want my car...pretty please.

[concobra]

Responding to your inquiry: What type carburetor and price.

The carburetor is a 1000 cfm Holley HP series. With custom booster venturies and other features. The price was approximately $950.00. From face value this may seem high. But please remember I did not have to do anything but bolt it on and go. When considered against the time, cost and trouble it take to custom calibrate a carburetor this is a fair price. In addition Patrick finished it and shipped it exactly when he said he would.

For the sake of discussion. Let me share that we just re-did the engine. Modifications include the following:

1. Increase in compression from 9.5:1 to 10:57.1.
2. Elimination of the "dirty" area under the combustion chamber.
3. Replacement of the Performer RPM manifold with a modified Victor JR.
4. Replaced the Hyd Roller with a Solid Roller.
5. SENT THE CARB BACK TO PATRICK WHO REVISED CALIBRATION AND SET UP FOR THE NEW COMBINATION.

I am going to the dyno this week on Thursday to see if the modifications helped. (Hopefully we will not blow it up!!!!!) Dyno time always makes me real nervous.

If your curious I will let you know how close the carb. is this time and if the modifications helped.

[David Kirkham]

Concobra,

950 is not too bad considering a Holley HP is very close to the same price and they don't work all that well out of the box.

As for milling off the air horn.

You have to mill off the air horn or you will get no air into the carb. The hood on most of our cars is quite low, which is quite problematic with the air filter. The original style air cleaner is so short the top of the air horn is right up against the top of the air cleaner...with no room for the air to get in. (Seat of the pants hp mind you--after milling of the horn felt like we gained 100 hp...no kidding.)

David

Jamo, the weather is so nice today I think I am going to go for a drive to test out my car...I will maybe work on your car tomorrow...hee heee heeee. Then again, it may be next week. The weather man says it will be nice all week so I may just not work on your car until the next rain storm! Hee, Heee, Heeee! Then again, the rain didn't stop me last week...

[mikiec]

In my best David Kirkham voice....


Dear Jamo,

We do apologize for the mixup. It appears your car was shipped to SAI where it became Turks car.

[concobra]

Dave,

Responding to our Holley HP correspondence.

The Holley HP series does not have any choke tower. This model uses the main body/venturi design different than standard Holley's. So the air cleaner clearance issue you addressed is not a problem.

Again for the sake of conversation.

To accommodate needed hood clearance for the Victor JR Manifold we had to cut the carburetor pad down .500" and lowered the engine .500". Plus, since my car is a Contemporary we also moved the engine over toward the drivers side to help center the air cleaner in the scoop. Even with all this we still had to cut away the baseplate of the Helling & Stelling filter in order to use a K&N filter.

[David Kirkham]

Concobra,

You are correct. I guess I didn't make myself clear. On the 750's and the 850's we mill off the air horn so the air can get in. On the HP series, you are correct, the air horn is already milled off and the venturies have been polished smooth. They look great and perform pretty well too.

The engines on the original cars are offset 1/2 inch to the right to help line the drive shaft up with the differential. (The differential horizontal offset is 1 inch already, so the 1/2 inch ofset of the engine helps to get things straighter.) We make the hood scoop holes slightly larger to accomodate the air cleaner with the offset of the engine.

We are able on our cars to run a 1 inch spacer under the carb and still have all the hood clearance we need even with a K&N fliter.

Thanks for all your help and ideas.

David

[David Kirkham]

And the verdict...

I have to go back and work on Jamo's car now. Found the problem. Vaccum leak under carb between the gasket and the turkey pan. Pulled the turkey pan off and I am now waiting for the new gaskets to arrive. Until then I will drive without the turkey pan.

During the autopsy,

I took the carb off to check the power valves, (with my power lips I might add--yuck, that tasted bad. How come you can't ever wipe all the gas off?)

Anyway,

As I was tasting the gasoline--noticed the gaskets looked suspect. Took the turkey pan off to take as many variables as possible out and bolted the carb back down with a single gasket. Bingo...100 instant horses.

There is still an annoying stumble (not nearly so bad as before) in the 2000 rpm range. Probably caused by a lean condition as the butterfly is cracked for acceleration before the fuel can catch up. I wonder how to fix that problem.

I would sincerely like to thank all who responded to help out even though this has probably been re-hashed 10 times last month alone in other threads.

As a matter of fact, to all those who responded to this thread, please drop a line to rich@kirkhammotorsports.com and I will send you a Kirkham Motorsports shirt! Please specify size and we will get it out to you. Make sure you send the e-mail to Rich. I don't check my e-mail from this site as often as I should. Rich now helps me out to keep up.

Thanks for the site,

David

[PatBuckley]

David -

Thanks.

[Whaler]

David

Thankyou for the generous offer. Too much!
I am glad your fix was an easy one....Nice to have one of those.
I guess when the question comes up what was it running like before and what has changed this fits the picture perfectly.
A simple vacuum leak.....That will surely lean you out
On my 850 I am using those thick 1/4" gaskets from Holley on either side of my Turkey pan. All together its like having a 1/2 " spacer/insulator.

Another thing is that the turkey pan sure gets in the way if you do have some more fiddling to do.

Tim

[CSX 4027]

David
I have had many different carbs from many of the popular name super tuners like Barry Grant.
My last one for the 427 was the only one that my engine builder will use. I used a 750 double pumper that flowed like a 950 but snapped like a 750.
Go to www.aedperformance.com and look up tuning tips.
PS.....Call me

[Steve R]

David,
The good news is that you don't have to take your dad's Cobra apart again I am looking foward to being a walking ad For Kirkham. Thanks!
Steven

[David Kirkham]

Steve,

You better watch out. If you win any more free stuff, you will get on someone's hit list!!!

Have a Great Day and Happy Motoring!!!

David