Loss of power @ 40-50 MPH under load (1 Viewer)

[Nottajeep]

1978 2F with Aisan carb cranks good, drives good in 1st and 2nd.
Go down the road at 40mph and load up 3rd and 4th gear and I get a total loss of power.
Go back to neutral and it's fine for a while and then repeats the power loss.

Killed the engine after this happened once and pulled over to look at sight glass and the sight glass looked to be full.
All other times the sight glass is half full as should be, usually after down shifting or going to neutral.
This week I pulled into the garage and the fuel looked to be bubbling or maybe even boiling.

I've been working on this power loss for a few months now.
Checked gas tank, fuel filter, float looks good, lot's of fuel treatment (4-5 tanks so far).
Choke works as should
Air intake is pulling strong
Carb air jets are pulling strong

Engine is desmoged

I am about ready to send the carb to an expert, but I guess I can know for sure it's the carb.

Thoughts from the 40 gallery??

JB

 

[edwjmcgrath]

It's hard to tell a full sight glass from empty.
It certainly sounds like lack of fuel from your description.
What do you mean by "checked gas tank, fuel filter"? It doesn't take long for a fuel filter to clog up if you've got an old tank.
I always recommend the clear inline filters in this situation. That way you can see what you might be up against.
They're cheap. I'd start there.
Then I'd look into the fuel pump. If it has a bad diaphragm it can pump but not provide full pressure-enough to run at low speed, but not at high rpm under load.

 

[Nottajeep]

I pulled passenger seat and looked inside the tank. Seemed clean. I had clear filter. Then switched to oem seeing if things changed

 

[Nottajeep]

I can run high rpm at 1st and 2nd gear

 

[bikersmurf]

Timing, plugs, vacuum leaks, fuel delivery, mechanical advance, air filter, compression...

Where to start.

 

[Nottajeep]

Timing check is on my list.
Vaccuum leaks are always in the back of my mind.

I have new OEM plugs and wires so I can check that off.

Air filter is clean and I seem to be getting plenty of air through intake and the carb is pulling in air nicely based on my hand over jets test.

From reading here, sounds like a good desmog engine needs a newly tuned carb and dizzy to match the desmog.
I believe I will get to know Mr Jim C. and bring those two components into the 21st century.

 

[numby]

Timing, plugs, vacuum leaks, fuel delivery, mechanical advance, air filter, compression...

I'll go with mechanical advance. Normally I'd say fuel but the problem showing up in 3rd/4th but not in the lower gears points more toward spark advance in my experience.

 

[Pin_Head]

drives good in 1st and 2nd.
Go down the road at 40mph and load up 3rd and 4th gear and I get a total loss of power.

I'm not sure what you mean. Details about the operating conditions and the symptoms might help.

 

[snaggletto]

My '76 2F really starts to drop out of the power band past 45 mph or so. It will go faster, but the acceleration is pretty flat up to 60. I feel bad trying to drive it faster than that. Not to mention at 50 mph the total lack of aerodynamics really comes into play. Did your truck run much better at high speeds in the recent past? Or are your expectations just not being met?

 

[sggoat]

Pin_head has good points as well as edw and biker.
Just curious. but how is the engine temp running? You mentioned a boiling sound in the carb--is the engine equipped w/a carb heat shield? Did this change recently? Was the carb recently rebuilt? Anything change that might interfere with airflow through the engine compartment?

 

[Nottajeep]

I'm not sure what you mean. Details about the operating conditions and the symptoms might help.

I cold start it after sitting a week or warm start after 30 mins and it runs great in 1 and 2, but I keep climbing the power band to 3rd and 4th and it just starts bucking , sputtering, no more throttle, etc...
Back off to neutral and it starts to stop sputtering and it's ok until I start the 3rd/4th power band climb again. If I stay light throttle in 3rd and 4th it can cruise around just fine ~40-45mph

My '76 2F really starts to drop out of the power band past 45 mph or so. It will go faster, but the acceleration is pretty flat up to 60. I feel bad trying to drive it faster than that. Not to mention at 50 mph the total lack of aerodynamics really comes into play. Did your truck run much better at high speeds in the recent past? Or are your expectations just not being met?

Sounds similar to mine but I go worse than flat past 45mph. This condition has happened in phases, which is why I am throwing my hands up and send parts in for diagnosis.

Pin_head has good points as well as edw and biker.
Just curious. but how is the engine temp running? You mentioned a boiling sound in the carb--is the engine equipped w/a carb heat shield? Did this change recently? Was the carb recently rebuilt? Anything change that might interfere with airflow through the engine compartment?

Owned the 40 a 1.5yrs no rebuild since I owned.
And no carb rebuild since 2F desmogged.
Yes, have the OEM heat shield on.

The dummy temp gauge shows 1/2 between C and H. Although the engine compartment is hotter than a hawgs arse in a big smoke box. This weekend it looked like fuel was boiling.
See link to video below

https://plus.google.com/u/0/ JasonBraswelljrb/posts/M8a7EDZKoiJ?pid=6051654309794865362&oid=105724802175654375676

thanks for all the helpful feedback!

 

[sggoat]

not at all sure that is "boiling" looks like the fuel is just jiggling around due to vibration, but I could easily be wrong. If it IS boiling, you can get the engine up to op temp.(drive around about an hour), then shut down, wait for about a minute, then open the hood. Listen to the carb bowl-if it is boiling, you can hear it easily. If this is happening, usually it will indicate an overheat problem with the heat shield(or cooling fan). Normally, this will not be an issue when running, as the fuel is flowing, and doesn't have a chance to get terribly hot before going into the carb.(especially true if you are experiencing this at higher RPMs(ie. higher fuel flows)
As for the temp gauge, mine never gets past the first tick mark. An aux temp gauge shows 198-202 deg. F at 55 mph in 96 degree Fl weather.(much cooler than the hawg-i usually cook mine at 225 in the smoker)-Ha!
You may have something going on with the carb secondary-Pin_head has a check to see if the secondary venturi is opening properly-and how much-I can't find the link to it just now, but it is on you-tube--this might be your prob--easy enough to check--I think he just stuck a paper clip on the secondary actuator arm, then drove the truck under heavy load to see how far the throttle opened--check it out.

 

[Pin_Head]

Bucking and spitting back through the carb are classic signs of fuel starvation. You aren't getting enough fuel to the carb, typically because of rust in the tank clogging the fuel delivery system.

 

[Nottajeep]

Often some of the sputtering occurs after some trail shaking.
So this makes sense. Tank looked pretty clean last weekend, but I only popped off passenger seat and peaked inside.
Now, what is the best method to clear this stuff out of the lines?
Been running OEM filters hoping I would have optimum filtration

 

[Nottajeep]

not at all sure that is "boiling" looks like the fuel is just jiggling around due to vibration, but I could easily be wrong. If it IS boiling, you can get the engine up to op temp.(drive around about an hour), then shut down, wait for about a minute, then open the hood. Listen to the carb bowl-if it is boiling, you can hear it easily. If this is happening, usually it will indicate an overheat problem with the heat shield(or cooling fan). Normally, this will not be an issue when running, as the fuel is flowing, and doesn't have a chance to get terribly hot before going into the carb.(especially true if you are experiencing this at higher RPMs(ie. higher fuel flows)
As for the temp gauge, mine never gets past the first tick mark. An aux temp gauge shows 198-202 deg. F at 55 mph in 96 degree Fl weather.(much cooler than the hawg-i usually cook mine at 225 in the smoker)-Ha!
You may have something going on with the carb secondary-Pin_head has a check to see if the secondary venturi is opening properly-and how much-I can't find the link to it just now, but it is on you-tube--this might be your prob--easy enough to check--I think he just stuck a paper clip on the secondary actuator arm, then drove the truck under heavy load to see how far the throttle opened--check it out.

It was continuing to bubble after engine shut off.
I've been watching his videos. Will look for venturi? Video
Coolant flushed a year ago and the radiator feels like it's breathing good.
It's fins are clean and straight
Added new OEM plastic cooling fan with the coolant flush

 

[kruisinkid]

i fought this a little bit. it got worse when the tank was half full or less. i changed filters and it immediately did it again. terrible bucking. i pulled the tank and filled it with water several times, then rinsed with alcohol. put a few gallons in, test drove around the block, did it again. only when i rigged up the garden hose to blow full pressure of water INTO the the tank "output". only then did it clear all the gak in there. you might have a big enough chunk of something in there to block the pickup tube. i would very very thoroughly clean out the tank. sound a lot like gas starvation to me as well.

 

[sggoat]

Dorman makes a glass fuel filter that you can actually see through. Will tell you if the filter is getting plugged just by looking at it. Easy to change filter elements also.--extra filter elements are cheap- maybe $5

 

[taro]

Did you do the desmog and/or rebuild the carb? Are to sure the advance is hooked up correctly? Are you sure the carb was rebuilt? Mine was having problems similar to yours. Rebuilding my carb and desmogging it addressed the driveability issues. The carb was so gunked up, I don't know how it ran at all. Good luck!

 

[Nottajeep]

Carb is desmoged but I don't think it was recalibrated when the engine desmog was complete.
So the air to fuel ratios, points, etc... could really benefit from a subject matter expert.

I am planning to send my dizzy and carb off to Jim C when he is back in Sept.
I figure it's going to make life easier long term.

 

[robbs6940]

See through filters are a must. I prefer plastic instead of the modular glass ones. I have had bad luck with the glass ones leaking. That may be because I am a tad heavy handed and may over tighten them a bit.