Hesitation at high RPMs AFTER HOT

[200ci66]

Well, after finally getting my engine to about where I want it and getting into the last stages of getting the engine running very good I am running into some more minor bugs. Something I hope you guys can help me with. First off, my engine is running excellent, and after recently replacing the transmission modulator (fixed my problem in another thread), my transmission is shifting perfectly. When allowing my car to reach to full operating temp and taking it on a little spin on some backroads, I am getting 16.9 in the quarter mile (with a friend using a stop watch). The car is also a breeze to drive daily now, and I've been driving it back and forth to school every day. I hope to go to a local dragway this summer and get a slip with the time on it so I can have an official badge of honor.


However, it was 85 degrees today, and I'm having the exact same problem that I was having a week ago when the temperature was 79. After driving my car across town to a friend's house or the store, then driving back to my house, I'm getting a massive hesitation at high rpms. Sitting in traffic, my temp goes up to about 75%-80% on the gauge. Then when getting back out into the country, I get a VERY noticeable hesitation at about 4000 rpms (right as I'm shifting into third gear). I had this problem last week when I was alone, but I had a friend with me this time and he told me that it felt like the engine wasn't receiving enough fuel. This only happens when my engine is hot on humid days, and (what seems odd) it only happens when in second/going into third gear. Not first/going into second. As soon as my tach hits 4000, the engine cuts out and stops accelerating unless I take my foot off the accelerator completely and let the car rest for 3-4 seconds. Then it's good. I was thinking vapor lock, but I'm not sure if this is a sign. Anyone else have this problem before?

 

[MPGmustang]

I think it can be both, vapor lock, although no fully locked, and not enough fuel. put a bigger jet in there and you'll be able to go to 4500rpm easily, and possibly gain a .1th of a second...

the stock carbs are so lean that the ones I've had were 14.1 A/F ratio, ideally you want to be around 12.8-13.2 and 14.7 while cruising. my 1100 autolight from pony carbs had a 59 jet in there, in the end I really like the 67 jet size although 65 gave the best fuel economy and response.

for teh vapor lock, you can use a metal line covered in a rubber hose, or a braided stainless steal hose.

 

[MPGmustang]

66 Mustang
200 ci
C4 with 3.20 Open Rear End
Rear Air Shocks
Clifford Split Headers w/ 2.25 inch exhaust
Turbo Mufflers
Holley 5200 2bbl Carb
Cold Air Induction
Pertronix Ignitor w/ '68 Distributor
40,000 Volt Coil

hmm, didn't read the sig...

for the holley, make sure it's rebuilt, and nothing is old/cracked, everything needs to work great. you might need to check what your A/F is, I know my 2300 series is too rich I thought it was lean, but leaned it up some more and it's more drivable. it's possible that you have too much fuel... but you won't know until you measure the A/F ratio. I set my timing to 14* static, seems to be best there.

with a 2bbl you should be ablt to rev easily up to 5000rpm even with a stock cam. hmmm..

 

[200ci66]

66 Mustang
200 ci
C4 with 3.20 Open Rear End
Rear Air Shocks
Clifford Split Headers w/ 2.25 inch exhaust
Turbo Mufflers
Holley 5200 2bbl Carb
Cold Air Induction
Pertronix Ignitor w/ '68 Distributor
40,000 Volt Coil

hmm, didn't read the sig...

for the holley, make sure it's rebuilt, and nothing is old/cracked, everything needs to work great. you might need to check what your A/F is, I know my 2300 series is too rich I thought it was lean, but leaned it up some more and it's more drivable. it's possible that you have too much fuel... but you won't know until you measure the A/F ratio. I set my timing to 14* static, seems to be best there.

with a 2bbl you should be ablt to rev easily up to 5000rpm even with a stock cam. hmmm..

Oh believe me, I've revved it up to 4800 rpms without breaking a sweat, but I've never taken it past that because at that point your pretty much wasting time and gas. This is just a problem that happens once in a while after a long drive on a hot day (example: today it's 53 degrees. Drove for a half an hour, kicked it through the gears coming home, nothing, drove perfectly). There's no doubt in my mind that this is temperature related.

 

[MPGmustang]

yeah then I would say it's probably a vapor lock, you can route your gas line through cooler areas and it should help. sheathing the gas line will help more, I use a braded stainless steel line that came from the PO, it works great and have never had vapor lock... wait I did once but I was running 15* timing on a LOM distributor with low water in the coolant with AC on max... other than that, I've been lucky.

 

[200ci66]

Where is the best place to start? I've always heard that Vapor Lock happens between the gas tank and the pump, not the pump and the carb, but I don't know how much truth there is to this. Also, could an old fuel pump cause a system to me more susceptible to vapor lock (pretty much asking if a new 50 dollar fuel pump would help solve the problem).

 

[MPGmustang]

vapor lock happens whent eh fuel pump is too hot and vaporizes the fuel,


it's even possible that the gas could be boiling in the carb, becoming vapor making the float not work. but I would suspect if that is the case, then you wouldn't notice it as the fuel pump should be able to keep up with it.


an option that I know really like, have an electric pump 'assist' the mech pump, the mech pump will only pump what it can and keep fuel presure constant, the electric pump would only push fuel to the mech pump it can't/won't over load the mech pump. on my dads 1940 and 1955 packards this setup makes a huge difference as those pumps (even new ones) are 50+ yrs old!!

 

[falcon fanatic]

Clearly, you are seeing a lean condition at high rpms. This is happening when hot, i.e. the choke is off. Most likely culprits IMO are

*insufficient fuel volume (I seriously doubt vapor lock with the engine running if your fuel pump is working. Typically vapor lock occurs when parked: Heat creates air bubbles in the line that stops the pump from priming)
*Too small a main jet.

I'm a big fan of collecting data when making a diagnosis before throwing a lot of money and time at parts hoping they will help. One thing that will really help here is a wideband 02 sensor. I use this one: (http://www.amazon.com/AEM-30-4100-Digit ... 973&sr=8-1) Yes, they are expensive, but if you want to get the most out of your custom performance engine then having a precise measure of AFR will save you a ton of headaches and help you put your money and effort exactly where it needs to be. With the sensor in place, you can determine where in your powerband and when you are leaning out, and jet accordingly. You'll be able to tune for max power as well as good mileage, and it is a fabulous learning tool for understanding what each different jet does, as well as what effects timing, intake, and exhaust design changes have on mixture.

The Holly 5200 is basically a weber 32/36. I strongly recommend a Holly or similar fuel pressure regulator and a gauge. Set your fuel pressure to around 4psi or a bit lower. The primary should be jetted around stoichiometric for good mileage, and the secondary for power. Leaning at high rpms could be caused by either too small a main or two large an air corrector jet on your secondary. Do you have a flat spot (brief hesitation) when the secondary first opens? If not, your secondary main is probably close to right. If there is any hesitation at all during primary-secondary transition, step your main jet up two sizes. If not, then you might try stepping down the air corrector jet two sizes and see if that helps. If it does, but not quite enough, then step the main jet up a size. Skill at plug reading will help you hone in on a final combination.

Without using an 02 sensor this process will be laborious, but it will get you there and a jetpac is pretty cheap: (http://www.racetep.com/webjetkit.html) These guys indicate a range of jets in the pac, but always a good idea to check what jets you have currently installed so that you can customize your order.

Happy motoring!

 

[MPGmustang]

(I seriously doubt vapor lock with the engine running if your fuel pump is working. Typically vapor lock occurs when parked: Heat creates air bubbles in the line that stops the pump from priming)

I have personally witnessed this two cases, both times in extreme conditions, it's absolutely possible. what is different and I agree is he mentioned hesitation, not complete engine shutdown like both of my cases. the loss of power is the same but he didn't have to stop anywhere to wait for it to cool down.

I think it's very possible to have the wrong jetsize. or maybe the distributor is loose as that is possible but unlikely as it's heat related.

 

[JackFish]

If it's related to heat perhaps it's the coil.
Just throwing that out there for consideration.

 

[falcon fanatic]

Good one JackFish. I had the same thing happen and it drove me crazy trying to figure out what it was. The coil eventually died, and the next one only lasted about a year. Ended up moving my coil from the engine to the fender. Problem solved. I have the DUI now, which also works well.

 

[200ci66]

I'm liking the idea that the main jet in the carb might be too small. Where does one find bigger jets for a Holley 5200?

 

[chad]

I'm liking the idea that the main jet in the carb might be too small. Where does one find bigger jets for a Holley 5200?

AND what is "bigger'? Do U know what sz U have now?
Continued luck w/the hesitation.

 

[200ci66]

Not sure. I ordered my carb from Langdon (many here are familiar with him I'm sure). After researching for the past few days several people have ordered a carb from him and had to re-jet since they aren't jetted for a 200 ci engine. I'm not sure how to identify sizes, but I guess finding out how and figuring out what size is a place to start.

That being said, does anyone happen to know what size jets is recommended for ~200 ci? Has anyone had to change the jets in THEIR 5200? What works the best?

 

[falcon fanatic]

does anyone happen to know what size jets is recommended for ~200 ci?

I doesn't really work that way. Every individual engine is different. You have to jet based on what reading you are getting now and what setting you have. Jets are marked with the size stamped in. You'll have to pull your jets one by one and write down what sizes you have. YOu have a two barrel progressive carb, which means if I'm not mistaken that you have an idle jet for the primary barrel, and then main jets and air corrector jets for both, so that's five jets you'll need to tinker with. In addition you may wish to experiment with your emulsion tubes, but that's getting a bit advanced for now. Buy and read this book, it's only ten bucks and will save you hundreds of dollars and dozens of hours:

http://performancecarbscience.com/

 

[falcon fanatic]

So... ?

 

[66Sprinter]

Howdy. I think I have the same problem. I've got a built 200 with big cam, but still the same 1bbl that was rebuilt by Pony. Vapor lock occurs anywhere from pump to bowl. Most of the time, its the long line from the pump or at the bowl. Using a spacer that is aluminum will conduct heat and cause the bowl to get real hot. (F6 has a phenolic adaptor to stop that.) The line is another problem are. Just steel..braided or any insulated line will help there. The pump is another story and that's what I am investigating now I believe you can cure "almost vapor lock" with more psi. So I purchased a Carter SBC street strip pump for $35. I am removing pin and foot and swapping in the Ford stock foot, pin and spring. Haven't done it yet..but I have the same problems you reported. In late winter I could drive 2hrs with no problem. Now that we are in the sprint heat..I get 20mins in the drive and I am hesitating at 80mph...get off the gas and slow down and I can go again. Repeat this again and again. So my car is undercover untll I get this sorted out.

Jim (Jammer)