Need help w/ stumbling issue....
 

Considering the recent direction of the threads here in the Arizona group I should start by saying this has NOTHING to do with my consumption of Coors Light and Jack Daniels....... **)

So the car is coming along slowly, but there is one more issue I am now noticing. When taking the car in town for a longer drive to run errands I do a lot of stop and go driving. Naturally the motor and engine bay begins to heat up. After 20 or 30 minutes of driving hot the engine develops a stumble. It remains drivable at lower RPM's, although it does want to stall when idling. My thoughts are

1) Vapor lock develops as the heat builds up in the engine bay and the fuel lines are getting hot or perhaps the heat transferred from the block to the mechanical fuel pump is eventually causing pump issues.....

2) A vacuum leak is developing between the iron block and the aluminum intake manifold as they heat up. Perhaps a bad gasket? The carb and phenolic plate have all new gaskets, but I won't rule these out as a possibility.

3) Maybe the timing chain is worn and as it gets heated up it stretches out even more causing a timing issue.

After the car cools down (parked for a couple of hours) it runs great again, and will stay running great for short drives. This only happens when things start getting really hot, or when the car is driven for an extended period of time. Any opinions, thoughts or advice on where to start would be greatly appreciated.

Just a couple thoughts here...I am a GREAT believer in process of elimination by simpliest methods first.
1-My Sunbeam Tiger used to do exactly what you are saying, it was due to vapor lock. But as I said, do the easy stuff first.
2-Check vacuum at idle normal temperature and then when it heats up. Get it hot so that it does stumble and see what the vacuum is.
3- Throw a timing light on her when she is at idle normal temp and then again when she is stumbling hot.

One of these tests should tell you what she is doing.

Keep us updated......Posi.....

Pick up your feet. :):)

Oh, What he says

I recently learned about an issue that someone had that sounds really similar to yours. He put a phenolic spacer between his carb and the intake. Problem resolved. The fuel was vaporizing and boiling in the bowls from the heat transfer in his Cobra. HTH

Thanks Posi! That's the simply answer I was looking for....just too close to the problem to think of it myself! I actually just pulled the vacuum gauge of this morning...had it run into the cockpit. I will put it back on tomorrow and keep track of where it reads. the needle does bounce a little at idle, but it is rock steady while driving, should be easy to track it. Also, checking the timing when it is acting up is another thing I should have thought of..... This group is great! The carb already has the tallest phenolic spacer I could put on and still close the hood..... Maybe I can find an inline fuel pressure gauge to see if the pump is giving out when hot.

I don't recall what motor you're running, but if it is an FE motor with a conventional fuel pump, replace it. When I got my car, it did a similar thing. I had to replace the fuel pump.. New pump, no problem. It's cheap and reasonably simple to do.

Your stumbling problem is either one of two things:

You are using a fuel blend with too much alcohol

Your water pipe is plugged up.




















:rolleyes:

The manifold vacuum seems to be consistent, so I don't think it is a vacuum leak. This leads me to the possibility of the fuel pump causing issues when it gets hot. Considering the crap oxygenated fuel we have here in Southern AZ, I am considering going to an electric fuel pump to eliminate possible vapor lock issues. My question is, do I have to run a return line to the fuel tank or can I just install the pump and be good to go.

If you are referring to the oxygenated fuels, I have no choice in that regard, that is all that is available here...
I have no clue what you mean by a plugged water pipe......

Youll have to run some sort of a regulator if you dont have one already. Most electric pumps will push way more than the needle and seats will hold.

Something else to consider. Try backing your idle screws out just a little to give it alittle more fuel at idle. Your strictly on the idle circuits at idle obviously and if you set them when the motor was cold they may not be rich enough. Back them out a 1/2 turn and see that that does.

Just a thought.

Understood....but that will have no affect on the stumbling under acceleration. I have been searching for instructions on replacing a manual fuel pump with a electric, but am finding no good info. I figure I will have to install a "T" in the fuel line right before the carb., and run a secondary fuel line back to the fuel tank, but how do I get the fuel back into the tank? I cannot imagine just drilling a hole in the tank and plumbing it in........and removing the tank to have a fitting installed seems like way to much work.....must be a simple in between method.

It certainly sounds fuel related, and if your fuel is oxygenated it will make the situation worse. If you are using metal tubing within the engine compartment, or near the exhaust to transfer fuel, replace it with rubber tubing, or braided hose as the metal tubing will transfer to much heat to the fuel or attempt top insulate it real well. The same holds true for a fuel log bolted to the manifold or cylinder head.

Unless you buy a pump that specifies that you must run a return line....dont need one. Holley blue or red will work just fine for what you need and all you will need is a regulator.

The Holley pumps are designed and built to be pushers not pullers. Mount the pump as far back in the car as you can get it. Splice the line and put the pump in. Take your fule pump off the motor and get you a block off plate. you can use the split in the lines where the stock pump was to put you in a regulator..Holley makes a real simple one as well. One line in 2 lines out one to each bowl if you want. If your running a fuel log then you can run it one line out and just plug the other ooutlet of the regulator.

What kind of carb are you running?

F-man, forget about it, the return line that is! After the fuel regulator run a pressure gauge and after the fuel log you cap the line. That's how mine is set up.
Ralph

I see Z-link was typing at the same time as I was AND he has a more precise explanation!:D

A retun line is not a bad idea even with a manuel pump....but it is by no means a must. A return line even plumbed back into the main supply line further back in the system will work just fine as well. The pump will pump fuel no matter where it comes from.

A return line does help recirc. fuel and allows it to cool some as it goes back to the rear of the car, thus avoiding vapor lock.

Your problem could be as simple as a bad pump or a leaky diaphram on the acc. pump.


Thank you Red......for once I was faster...lol.

Mark, You didn't mention what your water and oil temps are when this happens. John PS. Did you put a shroud on the radiator ?

In stop and go traffic the temp will get up to around 90-95 c, but as long as the car is moving it stays around 75-80 c. I have still not found anyone local who can make a fan shroud for me, but working on it. Even driving home from the pub tonight, with the colder temperature, the car ran like a bat out of h-e-double toothpick! There has to be something with the heat and this crap fuel, so if I have to replace the fuel pump I may as well go with an electric one to keep pressure on the fuel lines and eliminate any and all possibility of vapor locking..... John, if you and any of the other Tucson guys want to get together for breakfast one of these mornings just let me know. There is a great little diner on Silverbell that serves a great brunch on the weekends.......

Mark, we are all burning the same fuel. I've been really busy the last two months, hopefully we can get together and solve the shroud problem. Don't jump into changing the fuel pump just yet. I tried to call you 1/2 hour ago but you weren't in. Hope you can make it this Saturday, we'll have a couple of "gear heads" along and they can discuss the problem with you.

I had a stumble problem, installed a 50 mm accelerator pump on my Holley 750 carb, problem solved. FE motor, maunal fuel pump..it runs great even at higher temps. (210).
Great idle at all temps. by the way, I run only Chevron gas...seems to work better.
Good luck, Bill

You make me dizzy jumping from issue to issue but then again I am easily confused. Here is a link to Jeg's page of fan shrouds. I am sure one will be close enough to work.

On the vapor lock issue use a insulating gasket like this one will help. http://www.jegs.com/i/Holley/510/108-51/10002/-1 although any local parts store will have similar gaskets.