Hesitation When Accelerating

[patrickt]

Originally Posted by jhv48

Do what Holley tells you to.
And if you live in a warmer climate, you might want to ditch the electric choke as well.

The only problem with that is Holley hasn't updated their instructions in fifty years. If I set my floats to the bottom of the sight hole, as Holley has said to do for over fifty years, gas percolates out of my carb and creates a big, beautiful gas cloud about a foot over my engine. But if I put a match to that cloud it goes away in a spectacular fashion.

[Gaz64]

Originally Posted by zzmac

I was just on the Holley website and their simple calculator recommends a 750cfm double pumper. For street driving and the occasional spirited driving do you think the 650cfm is more than enough for this engine (Roush 402R) or is the 750cfm a better choice? Thx!

Depending on how hard you drive, a 650 is good for 5500 rpm for your 402 cubes.
A 750 is good for 6500. Both at 100%. Both figures would need to be lowered for volumetric efficiency. Typically 85-90% for a good street engine.
FYI, I run a Holley 850 DP annular on my 434.

[eschaider]

Originally Posted by Gaz64

... I run a Holley 850 DP annular on my 434.

ZZ, a 427 and a 434 are essentially the same engine displacement.

If Gary is running an 850 double pumper with annular discharge boosters and you are thinking the solution is to go smaller than where you are right now, Gary's message should hold volumes of information about the answer to your question re: going down in carburetor size.

Your challenge is not carburetor size. It is learning how to tune and use the equipment you already have.


Ed

[Timinator]

Your carburetor woes were a topic of discussion at the London Cobra Show this last weekend. I think it was pretty unanimous that all you need is somebody that knows carbs to tune it and not a new carb. There were about 150 Cobras there all running nicely too.

[patrickt]

Now, for a contrarian viewpoint, my 447 cubes, with my old school solid flat tappet cam, absolutely positively loves my 750 vacuum secondary, which I have significantly tweaked and massaged over the last 15 years. Now, my rev limiter is set at 6400 RPM and I probably bounce off it once every couple of years. I love that carb, but that's probably because I've known it for so long now and have tweaked all its weak points.

[eschaider]

Originally Posted by eschaider

>>...Your challenge is not carburetor size. It is learning how to tune and use the equipment you already have.


Ed

Patrick,

You're arguing / supporting, I think, the same argument as the rest of us (for the most part). The London Cobra soiree ended up further supporting the. same viewpoint.

Ed

[zzmac]

I talked to Holley this morning and he kind of agreed with everyone if I can find someone who can tune it. His guess was the accelerator pump and something else which I forgot. He ruled out the powervalve.

If I go with a new carb he recommended either a 650 or 700 with vacuum secondaries (to add to the confusion). :-) He said mechanical secondaries are recommended for racing.

I'm going to try and find a carb shop closer to home and see if they can tune it.

[patrickt]

Originally Posted by zzmac

If I go with a new carb he recommended either a 650 or 700 with vacuum secondaries (to add to the confusion). :-) He said mechanical secondaries are recommended for racing.

I've forgotten - do you plan on seriously racing your car, or mostly street driving with maybe an occasional trip to the track?

[zzmac]

No track.

[patrickt]

Originally Posted by zzmac

No track.

Then take it from me, properly tuned vacuum secondaries are wonderful in these cars. Forget that "Cobras are too light for vacuum secondaries" crap. Vacuum secondaries will perform flawlessly for you if you just get them set up right. The only down side is that in order to really test that you have them fully opening at their optimum rate is to get on it really hard while you have a measuring marker on them, tweak them, and repeat. Unless you have a stadium parking lot or something that has absolutely nothing that you can hit, it can be pretty dangerous on the street because the goal is to accelerate as fast as you possibly can which, frankly, is just not that safe. I don't know any other way to test that they're working for you at peak performance.

[sunman]

I would go double pump. I’ve put a new one on every motor I built
50 years no carb problems
I don’t have a clue about vacuum secondaries
The party bus has a new 700 DP on a fresh +040 390
Now for some brakes...

[Gaz64]

Originally Posted by patrickt

Then take it from me, properly tuned vacuum secondaries are wonderful in these cars. Forget that "Cobras are too light for vacuum secondaries" crap. Vacuum secondaries will perform flawlessly for you if you just get them set up right. The only down side is that in order to really test that you have them fully opening at their optimum rate is to get on it really hard while you have a measuring marker on them, tweak them, and repeat. Unless you have a stadium parking lot or something that has absolutely nothing that you can hit, it can be pretty dangerous on the street because the goal is to accelerate as fast as you possibly can which, frankly, is just not that safe. I don't know any other way to test that they're working for you at peak performance.

Yes, nothing wrong with a good vacuum or airvalve secondary carb. They size themselves to suit the engine demand. It is a myth that vacuum carbs are for stockers and RVs etc.
You can have $500 vacuum carb perform almost as well as a $1500 DP carb, since now we have access to many tuning options for the Holley vac sec diaphragm assembly.

[zzmac]

In the end I decided to go with a Holley 770. The one I had on the car!

I came close to buying a new carb but as luck would have it I ran into an old friend who used to work in a local performance parts shop and he recommended a shop called Specialty Carburetors here in Toronto.

I brought the carb into them and they diagnosed that the fuel bowls were adjusted incorrectly (no replacement parts were needed). I just took it for a test drive and NO hesitation. vroom vroom

Thanks everyone for your advice and for telling me not to buy the new carb!

[patrickt]

Originally Posted by zzmac

I brought the carb into them and they diagnosed that the fuel bowls were adjusted incorrectly (no replacement parts were needed). I just took it for a test drive and NO hesitation. vroom vroom

You mean the floats that are inside the bowls, I presume. Man, that's a pretty basic carb adjustment. If that's all it was, then your first shop did a super shi**y job tuning the carb.

[talos]

I am glad you have posted the outcome. So many threads start out and the outcome is never listed. I wish everyone would post their final results. This helps others who have similar issues.

[Gaz64]

Originally Posted by patrickt

You mean the floats that are inside the bowls, I presume. Man, that's a pretty basic carb adjustment. If that's all it was, then your first shop did a super shi**y job tuning the carb.

Yes, if this is all that was wrong, then how anybody can fit a carb and not verify the float levels are correct, should stop working on carburetors.
Any carb out of the box will be wrong from the float bouncing around in the box.

[FredG]

Being almost 70 years old, I can attest that finding a good carb tech is getting harder. I am glad I know how to rebuild, tune and diagnose them. After you get the new one, spend some time learning everything you can about the old. Take it apart, rebuild it yourself, set the float levels and clean out every passage way. The knowledge will come in handy down the road.


Fred

Originally Posted by Gaz64

Yes, if this is all that was wrong, then how anybody can fit a carb and not verify the float levels are correct, should stop working on carburetors.
Any carb out of the box will be wrong from the float bouncing around in the box.

[sunman]

Holley clear sight plugs. $5.99
Change the O-ring and I think you can run E-10
I just use non alcohol