Well my 79 f 150 4x4 w/ 400 and t 18 has been acting up a little bit lately and yesterday it was really bad. It was around 115 degrees here yesterday and the pickup had been sitting most of the day and when i went to drive it it was not running very good at all. I could hardly get about 45 or 55 and when it started to run fine if i let off the gas it would come back right away. I figure this is vapor lock even though i dont know much about it. The motor was not running hot it ran right where it usually does (around 200). I got the ac fixed around 3-4 weeks ago and since then every once in a while it would hiccup a little. It would go away in less than half a block until yesterday, Yesterday if i would floor it it would just bog.It never died and the idle seemed fine. I would keep my foot steady on the pedal and the pickup would get power than loose it. It was surging very badly. I would take it out of gear and rev the motor and it sounded odd. I turned the ac off and it did not effect it at all (well except make me sweat badly). Today i took it out and it is around 90 and cloudy and the pickup ran fine. It has also been hard to start at times. Sometimes it would crank awhile before it would start and other times it would crank slowly and the battery still had a good charge plus it is fairly new and so is the starter. It usually fires right up without any problem. The carb is a remanufactured edlelbrock 600 that is around 3 or 4 months old. I have 2 see through fuel filters. One back by the tank and on between the carb and fuel pump. The one by the fuel tank is around a year old and the one between the carb and fuel pump was put on when i got the new carb. Do i need to put a electric fuel pump on it? I plan on taking it to a mechanic as soon as i can to get his opinion cuz he is one of a few mechanics around that i trust with my pickup. Since he opened his own shop i have only let him work on it other than myself. Thanks for any help

 

First off, you should not need to install an electric fuel pump.

Ok...I would certainly check the fuel filter(s). The carb could be starving for fuel. However, it probably is vapor lock. Do you have a spacer installed between the carb and intake? If not, Edelbrock makes a few that work really well. A 1" spacer is all that is needed, but if you like...get a 2". Up to you really. The type of spacer varies. Usually you can get a 4-hole spacer or an open spacer. The 4-hole spacer will give better response from off ideal (more of a torque feel) and the open spacer will give better top end (horsepower feel).

4-Barrel Carburetor Spacers
Use open spacers where more plenum volume or more carb-to-plenum floor distance is required. 4-hole spacers enhance low-rpm torque and throttle response and dampen reversion effects on the carb. The superior heat insulating properties of wood fiber laminate are ideal for reducing heat transfer to the carb.

Open 1" Spacer, Black Phenolic Plastic #8710*
4-hole 1" Spacer, Black Phenolic Plastic #8711*
Open 2" Spacer, Black Phenolic Plastic #8712*
4-hole 2" Spacer, Black Phenolic Plastic #8713*
From www.edelbrock.com

I personally have the 2" 4-hole spacer installed between my 600cfm and performer Edelbrock manifold. I learned the hard way too. But ever since (over 7 years now), no more vapor lock. Not even in these HOT southwest summers!

Hope this helps...

 

Could be a weak stock pump too. You could try a new stock pump or step up to a Holley or Edelbrock high volume street pump. My 460 does this occasionally with the stock pump. It is vapor locking in the fuel line, not the carb.
The hard to start at times is caused by fuel boiling in the carb and evaporating and running down ito the intake. Then when cold, you have to crank it for a bit to get fuel back into the carb bowls. A phenolic heat spacer will help this problem.

 

Why the filter at the tank?
I ran one for a badly rusted tank. Run for awhile, un-plug the filter and run some more. I couldn't afford to fix the primary problem, so I took the band-aid approach.

As a rule you don't want to run a filter on the suction side of any pump, this set-up causes cavitation. Just for grins remove the filter and graft the lines back together, see if anything changes.

Vapor lock has been a hot topic lately, try a search. Some guys swear by electric "pusher" pumps and return lines; interesting read.

JMHO, Mike

 

well i forgot to mention that i did put a 1" phenolic 4 hole carb spacer on already when i put the new carb on. Maybe i will take the filter off between the pump and carb and see i guess. I am not sure if we put a new fuel pump on it when the motor was rebuilt or not. Sometimes out of the blue the pickup is harder to start. It cranks over several times when normally i just pop the throttle a little and crank her and she fires immediately.

 

Hi,
BOY do I know about this feeling! See my posts about vapor lock and it will tell you my story and what I ended up with... Too much to write again here. Good luck!
Dak

 

I had the worst vapor lock problems last year....My 400 would vapor lock in any weather above 75 degrees. I had an edelbrock 750 cfm on it. I did all the spacers, filters, and such...no improvment. I then swapped on a 600 CFM edelbrock...this improved it up to about 85 degrees before it would vapor lock....That wasnt good enough for me. Now, i put on a remanufactured Holley 650 Double Pumper carb, with the same 1" 4 hole spacer, electric pump, and filter after the pump. I can NOT get it to vapor lock now. No matter what i do, or how hot it is...it will not vapor lock. I dont know if it was just luck, or if there is a Holley advantage, but i swear its the holley.

--John

 

Do not remove the filter between the pump and carb, remove the one between the tank and the pump.

 

thats what i meant to say. Guess i was too tired and didnt pay attention to what i typed. i had a 750 holley on it before for around a year with no vapor lock problems.

 

Well it acted up and i backed off and it cleared up immediately. It was only 80 out and i did have the ac on but once it happened i let off completely and it went away. One thing is this happened immediately after i filled up with gas. I have had this happen after filling up around 3 times. Does this help figure out the problem any?

 

Hi again,
Perhaps you could check your distributor and ignition module....maybe the coil... As I noted in my post on (supposed) vapor lock, this turned out to be the problem. It ran fine until it got even a little hot, then the secondary in the distributor that had worn and already melted, would heat up again and cross, thus shorting out the electrical system.
Good luck!
Dak