Weber falls flat at 1800

[ludwig]

Have a 1975 with a 38 weber carb. The 40 has been in the garage for 10 yrs. it developed a back fireing problem on a trip and I parked it. Found all the valves tight, and corrected the problem. Still ran ruff and backed fired. Five years ago put a new fuel pump and header/ exaust , and still ran rich and backed up. Now were back at it again. The weberfalls out at 1800rpms and falls flat on her face. choke it out and its as smooth as a kitten on up thru it full range, no hesitation or spudder, and not back fire from running out of fuel. Just had a valve job on the head, new vac advance dist, rebuilt the weber, tried rejetting from the orginal jets.still falls out at 1800. fuel supply is good. fuel pump seems to be doing the job, but have not run a pressure test, no fuel line blockage Does this sound like a power valve failure. We took the valve off and it seems to be in good shape, no tears or rips, and put it back on.Im die in here!!!

Ludwig

 

[green73]

Are you running a Fuel Pressure Regulator? The webers don't like anything above about 3.5psi. If you are running the stock fuel pump it is probably making too much pressure.

 

[ludwig]

weber

I am running a stock pump, but I always ran the stock pump from the time we installed the weber 20 years ago, but it was the old pump. the new pump has done nothing more than run at idle.

 

[Richard R]

Clean the Main jets the Emulsion tubes and the Air corrector jets
It is just running on the fixed size idle jets.

 

[ludwig]

Richard
its just been rebuilt and throughly cleaned, with i had a part in so i know its clean. going to replace the power valve
thanks ludwig

 

[Pin_Head]

Sounds like a lean condition, if it is backfiring out the carb and not the tail pipe.
Since it just started doing it after it had been "rebuilt", chances are that there is some remaining dirt plugging up the main jets, fuel passages or nozzles. It might also be improperly reassembled.

 

[Coolerman]

You have been putzing with a Weber carb for 10 years?

Pitch that thing and put an OEM carb in it!



Does sound like a power valve or something still blocked.. I don't know squat about Webers....

If someone would loan you a known working carb, you could at least make sure it's the carb?

 

[JohnnyC]

sound lean

how far out are the screws?

do you know you jet size?

what altitude are you at?

see link:
https://forum.ih8mud.com/40-55-series-tech/213147-***weber-carb***-adjustment-thread.html

 

[ludwig]

screws have been all the way in and out to 1.5 turns runs good at 7/8. 1.5 is too much. even all the way in it runns good.
jets are stock went up on the main 5 and on the air 5 and still ran the same.
alt is sea level+200

 

[ludwig]

Pinhead
it started before the rebuild. happened over nite. one day running to the coast, next morning all kinds off backfiring and poor performance. limped it home on the back roads going 45. found a broke tube and rebuilt carb, still backed up. tried jetting with a kit and no go

 

[ludwig]

been on the road for the last ten yaers drivin my semi, but i'm out of business and starting to get my life back in order.
thanks for the power valve idea thats what were thinking

 

[JohnnyC]

increasing the air correction jet leans the upper rpm range...you basicly cancelled out the increase you made with the mains.

put back the original size air correction ...you may even need to go up 5 more on the mains.

when you go in write down the size of the mains and airs and report back

 

[JohnnyC]

if you are at 155/160 for the mains and 180 for air...then its another issue

you cleaned it out...did you also clean out the fuel filter in the weber?

 

[Pin_Head]

It may also be a fuel delivery problem, like dirt or junk in the tank, filter and lines or a weak pump. Unfortnuately Webers don't have a fuel bowl window to make it easy to check.

 

[ludwig]

all jet are back to orginal and we are writing it down, just were not at the shop.

 

[ludwig]

Pinhead
Took the tank off and found it clean. no rust dirt nothing. backed the lines up and clean. ran the carb on a bottle and put the supply line in a can, started it up running on the bottle and she fell flat at 1800 and the fuel going into the can was pumping on every turn of themotor a good solid feed. like i said we choke her off and run up and down the rmp range like a nascar no hesitation backfires, so the fuel pump is working in our opinion dont ya think

 

[ludwig]

totally cleaned the filter and its not damaged think the mains are at 145 the orginal setting that ran fine when this carb was first put on

 

[JohnnyC]

145 is way too small...why it was running better before with the same settings ...who knows....but...

you should be at 155 for the mains and 180 for the air.

IMO

 

[ludwig]

going to the shop and recheck all jets later today and put in a power valve in a few days when the slow boat getts here
thanks and ill get back with ya on what happens

 

[ludwig]

johnny C
we ran a new electric fuel pump yesterday bypassing the mech one and had 3psi of fuel pressure and the carb still cutts out. The mech pump has 3.5psi of pressure also.Ran a manifold pressure check and it was 17 to 18 psi at 650rpms of idle. Put a new powere valve in yesterday as well, and it also did nothing to improve the problem. Cleaned and recleaned the carb. Soaked it overnite and was very annal on checking for cloged ports. Appearsthat everything is open. When last rebuilt I found a cracked emulision tube on the header side. Replaced that on the first rebuild. Starting to think I have a cracked body in the weber. What do you think about that?