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"Flat Spot"

My truck ('87 F250 351 ci carbed) is a bit better after changing the coil. However, my mechanic says it has a "flat spot" and the carb needs re-building. I had th ecarb re-built about 5 years ago and since then the truck MAYBE has 4,000km. Just off idle it hesitates and sometimes stalls. Ran some techron through it hoping to cure the problem as well as my fuel guage problem. So... what else should I look for to remedy the "flat spot"? I sprayed carb cleaner around th ecarb to check for vacuum leaks and nothing seemed to change. Gave the carb a good spray and I still have the stumble. The engine revs high and never seems to knock down th eidle after getting to operating temp.

Sugggestions? It is th eoriginal carb that came on the truck and re-building was VERY expensive at a reputable carb shop/ would have been cheaper to change to a new one by Holley, etc. Thanks.
 
My truck ('87 F250 351 ci carbed) is a bit better after changing the coil. However, my mechanic says it has a "flat spot" and the carb needs re-building. I had th ecarb re-built about 5 years ago and since then the truck MAYBE has 4,000km. Just off idle it hesitates and sometimes stalls. Ran some techron through it hoping to cure the problem as well as my fuel guage problem. So... what else should I look for to remedy the "flat spot"? I sprayed carb cleaner around th ecarb to check for vacuum leaks and nothing seemed to change. Gave the carb a good spray and I still have the stumble. The engine revs high and never seems to knock down th eidle after getting to operating temp.

Sugggestions? It is th eoriginal carb that came on the truck and re-building was VERY expensive at a reputable carb shop/ would have been cheaper to change to a new one by Holley, etc. Thanks.

Have you checked other areas for vac. leaks,maybe the booster brake
gromment,smog devices,egr valve?
 
Will poke around some more. Irritating as the truck sometimes just dies when I'm slowing down.

I had bought a used 2bbl reman carb.,The paper work said if the engine
runs uneven,it could be an intake manifold leak.
So Im fixing to get ready to replace the intake manifold gaskets on my
truck.They leak like a siff.
I had the same issue,so I played with the throttle idle to help,mine idles
too high idling in park,an chirps the tires when I put it in gear a little,but
if I adjust the idle down a little more,it will die.
So Im going to replace the manifold gaskets an see if that cures the
issue like the carb manufacture suggest.:)
 
I had bought a used 2bbl reman carb.,The paper work said if the engine
runs uneven,it could be an intake manifold leak.
So Im fixing to get ready to replace the intake manifold gaskets on my
truck.They leak like a siff.
I had the same issue,so I played with the throttle idle to help,mine idles
too high idling in park,an chirps the tires when I put it in gear a little,but
if I adjust the idle down a little more,it will die.
So Im going to replace the manifold gaskets an see if that cures the
issue like the carb manufacture suggest.:)

That's interesting. I had replaced the intake manifold gasket bEfore the carb was rebuilt. I'll spray some carb cleaner around that area to see if that might be the cause. Thanks.
 

jebadiah04

Rooster Snorkler
849
27
Intake,

Carb base

Brake booster

Pcv valves

Rubber hoses

All places to look
 
Well I'm back with some more info. The truck started and then died after a few minutes. I could get it to start with carb cleaner down the carb. I'm kinda thinking fuel pump but then I started checking for vacuum leaks as suggested. Revved higher when I sprayed around the forward part of the intake on the driver side. Also, there are three "t's" that I believe are for vacuum without any hoses or caps around the thermostat housing area, I think they had something tO do with the Thermactor system. Anyway, when I sprayed near them the engined revved higher as well.

So... I guess I need to replace the intake manifold gasket (I have been advised that there is a thicker one by Felpro that may be best), should I cap those "t's" or possibly remove them and plug tHe remaining holes, and finally should I replace the fuel pump if I don't see adequate flow after testing? Could the non-start be due to the vacuum leak at the intake manifold and possibly the 3 "t's" and the fuel pump may beOK? Thanks for any and all responses.
 
Last edited:

dustybumpers

don't play well w others
Have you checked to see if the needle and seat have a piece of junk in it, causing the float to keep them closed? If the float stays closed, no gas gets in
Had a mustang do that to me, drove me crazy.
Just pull the fuel bowl off, and clean it out.

Just my .02
 

SuperCab

Moderator
Staff member
10,068
547
Montana
I'd start by capping the Ts and go from there...

Fuel pump should be a $20 piece... No thing to replace it if it's suspect.

Intake gaskets aren't a lot of fun, I'd want to be pretty sure about that before I tore into it.
 
Well I'm back with some more info. The truck started and then died after a few minutes. I could get it to start with carb cleaner down the carb. I'm kinda thinking fuel pump but then I started checking for vacuum leaks as suggested. Revved higher when I sprayed around the forward part of the intake on the driver side. Also, there are three "t's" that I believe are for vacuum without any hoses or caps around the thermostat housing area, I think they had something tO do with the Thermactor system. Anyway, when I sprayed near them the engined revved higher as well.

So... I guess I need to replace the intake manifold gasket (I have been advised that there is a thicker one by Felpro that may be best), should I cap those "t's" or possibly remove them and plug tHe remaining holes, and finally should I replace the fuel pump if I don't see adequate flow after testing? Could the non-start be due to the vacuum leak at the intake manifold and possibly the 3 "t's" and the fuel pump may beOK? Thanks for any and all responses.

The spray seems to indicate a leak. If it proves to be the intake gasket, change it. If the tees are sucking air, cap them or replace them with plugs. Only replace the fuel pump if it is proven to be non-working. But be sure that it is not due to it sucking air somewhere on the supply side or due to a congested filter in the line or tank. :)
 
Finally success! I rechecked the fuel pump, it was good. Noticed a little seepage near he pressed in plugs on th equal bowls, sealed up with JB Weld but that did nothing. Checked the fuel level but it was OK. No "squirt of fuel" when activating the axel orator. So on to the accelerator pump. Wa showing to replace it but first checked to see if it was moving at all. Rather embarrassed to find that the linkage was frozen. A little carb spray to clean and finished off with a little spray of Moovit (the BEST penetrating spray period!) and th ethics runs great. Logical progression of diagnosis led to finding the cause but how did I miss the rusted/frozen linkage in th efforts place? As Shakespeare wrote, "Alls well that ends well!". Thanks for the support.
 

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