got my engine installed and at first (before road test)it ran just fine----the only problem seemed to be that the fuel pressure from the new electric fuel pump would slowly drop from 8 psi down to about 3 or so-----checked both fuel filters (inlet and outlet) and both clear---and there seems to be NO supply problem---So---Road test----when accelerating or climbing a hill engine acts its fuel starved----starts breaking down under pressure----stopped and check fuel pressure under hood---about 4 psi and filter by carb is 3/4 full---both filters are clean--(carb is new truck avenger!!!)--Msd-6a box and dist.---engine has about 50 miles on it after complete rebuild--but the problem persists!!!!!One of the gauges i installed is a vacuum gauge and it acts normally with about 13 minus psi at idle---Is there a routine that i should pursue to check for a vacuum leak--in terms of checking each potential vacuum source--i.e. plug each hose and take a test drive???And how do you check for a vacuum leak if the intake manifold has a leak---The mechanic is ready to tell me to come get the truck and then it's my problem instead of "OUR" problem"---I Need this truck to make a living and not just sit in the driveway and "Idle"--which it does just fine---it just won't do anything else!!!!!How small of a vacuum leak is "enough" to make the engine break down under load????--all comments are appreciated (and needed) --thanks for your time , sandy kuteman , and I'm not fat just alittle plump--middle age has ascended upon my middle as well as lotsa cheese burgers!!! :-X11

 

As you know, 13 inches of vacuum is too low unless you have a cam with a longer duration. Does your engine have adjustable rocker arms? If they're too tight, the valves won't close all the way, your vacuum will be low and your engine won't run right. If they aren't adjustable and the pushrods are too long, same scenerio. If the engine was rebuilt, were the valves and seats reground? Your mechanic doesn't sound too reputable if he won't stand by his work. Did you pay him with a credit card? Your card company will work for you if your engine isn't right. Sometimes vacuum leaks are tough to find. Take some carb cleaner and spray it around all the obvious places for leaks. Anywhere theres a gasket (intake, carb, etc). If there's a change in RPM, you've found a leak. To tell you the truth, it really sounds to me like you have more going on then a simple vacuum leak. I don't know about all engines, but I had a vacuum leak once, and my engine would run poorly at idle and low RPM's, and run better at cruising speeds. Just the opposite of what you describe. I just realized the title of your post. Did you have this same problem before the engine was rebuilt? I'll think about this problem a little more. Sorry I couldn't be of more help.

 

Fat,
This sounds like a fuel problem. Are you running an electric pump? Consider running a mechanical pump instead. They are good for carbs. A vacuum leak would show at an idle. A recurring surge in idle speed is a tell tale symptom. "13 inches", not PSI, is a perfect vacuum for idle.
Did you say this truck used to do this with the old motor? Same block? Same Heads? Same intake?
Low float level could do this!
Check your float levels and reply.
My Way is the Highway,
KingFisher

 

Steve , thanks for the help!!the mechanic is tired-he's wanting to retire and i talked him into doing the engine--so he doesn't have the patience he once had---actually a good guy---he will stick with it i thimk!!I want to diagnose this problem on my own--the rocker arms aren't adjustable--Crane--fulcrum kit----new hardened pushrods--standard length--9.5 inches i believe---anyway---I won't giveup till the problem is found---even if I ain't no wrench bender!!!thanks , again sandy !! :-X11

 

[updated:LAST EDITED ON 18-Apr-02 AT 10:31 AM (EST)] I really think the fuel pressure is your immediate problem. 3# wont cut it with most carbs. is the electric pump install new? Did it work OK on the old engine? You may be having suction problems. Elec. pumps don't self prime very well. Where the pump located, the brand? Have you checked the supply line from the tank for pin holes/pinching?

also 13inches of vacuum is a little low. Is that in gear/auto? I pull 15" in gear at idle "auto" with a lot of modidications and at 5,000 feet altitude.


Good Luck
Bear Tracks

 

check your connections to the fuel pump (electrical) make sure there is vbat at the pos post of the pump also check ground resistance sounds like your fuel pump runs too slow

for lack of vac check your base timing and carb idle mix

 

check your fuel tank too, or fuel filler cap, If these are not vented properly, the tank will act like a vacuum chamber, and the pump wont be able to pull fuel

 

I'm going to answer the questions as best i can , (1)the engine has the same heads as before rebuild , valves are stainless-same size as factory---the cam is the number 2 in the Crane Energizer series , actually fairly mild---so vacuum shouldn't be a problem---Now---the fuel pump is new--Holley--14 psi at 140 gph--checked voltage at the pump -13.2, no problem there ---i have a regulator inline and it is set at about 10 psi as the pressure will drop over the warmup period to about 5 psi(no appearant problem with fuel supply)--inline filters on both suck and push sides of the pump--I don't understand the drop in pressure!!!the fuel filter close to the carb stays almost full???The pump is mounted on the left frame rail right under the driver , using 3/8" fuel line.......My friend the mechanic has all but eliminated a vacuum leak--Now--he says the problem is that the secondaries are not getting fuel when they are called upon based on the load placed upon the engine---The carb is a Brand new Truck Avenger -----So , we have decided to install an old 650 holley, double pumper and see how the engine runs--If it runs without the breakdown at moderate acceleration , well I'm gonna send the carb back to Summit and whatever---I haven't madeup my mind just yet--but I'm not very happy with the situation as a whole!!!I really do APPRECIATE all the help that I have rece'd and will respond in kind to others when and where i can be of service , thanks so much!!!fd :-X11 :-X11

 

I think woppini is on to something. The fuel pressure starting out okay and then dropping strongly suggests to me the tank is not venting properly. This would definately be a easy first thing to check.

 

 

[updated:LAST EDITED ON 19-Apr-02 AT 07:44 PM (EST)]O.K. Guys---here is the latest NEWS on "Fatdaddy vs Truck Avenger"---A postmortem on the situation----I thimk this is NEWS because That brandnew truck Avenger wasn't working---i.e. the secondaries were completely inoperable----why --well i don't know and i don't care---We put the old --600 cfm Carter back on the engine which is the one that was on it when i purchased the truck back in Aug-01.........and bydamn--guess vhat---dat mutter veally runns now like da Storm trooper vit abad case of der Feuher's revenge---looking fer da Krapper---any vay---boys da motor ess veally running like a German Virgin on Da honeymoon night---Ya>>>further postings in my fluent German\Texas accent will be brief--I hope--I just have diss over-vhelming compulsion to compose in da lingo of da fadder land--who stole my beer mug!!!???fd :-X11 :-X11