annoying hesitation with carb replacement

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Petros
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My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

annoying hesitation with carb replacement

Post by Petros » Tue Nov 05, 2019 7:34 pm

I have been meaning to do a detailed carb overhaul but decided to try out some of the extra used carbs I have sitting around to save time. my original carb was leaking fuel out the secondary diaphragm, and also becomes hard to start when it is cold, sometimes requiring starter fluid when it is blow freezing.

but I have a collection of used carbs I have picked up from parts cars, picknpull and elsewhere, I thought I would try those first. I have three with almost identical vac ports and other external features, one looked like it was rebuilt recently, it was all clean and shinny, so I thought I would give that one a try.

I works well except for the noted hesitation. it starts right up, runs well cold, and has a nice power flow from low speed to high speed. Generally a big improvement.

but there is one annoying issue, it will have a hiccup, or brief hesitation when slowly accelerating from a low speed part throttle condition. oddly, no such problem if I just mash the throttle down. it only occurs when slightly slowing or going steady low speed, and than slowly accelerate. a few seconds into the slight acceleration it hiccups once, and than continues to accelerate. It is especially annoying in stop and go traffic, fortuantly I can avoid that where I live most of the time.

one other minor issue that may or may not be related, during warm up when its in fast idle, it will slightly surge idle speed (like there is a slight vacuum leak). it does not happen after it is warmed up. I doubled checked all the vac tubes and routing, all appears good.

I was thinking it is almost like the fuel cut off solenoid is sticking right at the low speed transition area. On hard acceleration the low speed jets are not affected, but only when at low speed, just above idle, and slowly accelerating there is a point where the solenoid must allow fuel flow to avoid a flat spot.

Anyone have any other ideas? what should I check?

If it is easy to fix, I will do so. Otherwise I may just swap out one of the other carbs if I become annoyed with the hiccups.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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NWMO
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Re: annoying hesitation with carb replacement

Post by NWMO » Tue Nov 05, 2019 8:48 pm

Hey Peter,

Since it’s driveable, I would run some fuel additive carb cleaner and put some miles on it. Hopefully it will clear up for you.

Chris
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The Professor
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Re: annoying hesitation with carb replacement

Post by The Professor » Wed Nov 06, 2019 8:09 am

I have the same hesitation as Petros, but it's not so minor. Any sort of movement into the middle of the throttle's travel will cause the engine to stumble unless you back off, or push through to full throttle. Especially scary when trying to make a left turn against traffic.

I have recently rebuilt my carb and suspect that some of the linkages are out of their adjustment spec.

Any help would be a godsend as this issue really affects my enjoyment of this vehicle.
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

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Petros
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Re: annoying hesitation with carb replacement

Post by Petros » Wed Nov 06, 2019 4:08 pm

the carb has an idle fuel jet (that was adjustable until the factory capped it off), and there are intermediate fuel and air jets, and the normal full power/full speed jets, that meter fuel and mixes air bubbles into the fuel before it goes into the venturi.

But it appears these intermediate speed fuel jets (only flowing fuel at part throttle) for some reason are getting starved of fuel during transition. could be a blocked passage (crud, corrosion, deposits, something), or perhaps the fuel metering rod (that controls fuel flow) is hanging up, or just stuck on one position.

I do not think there is anyway to fix it unless you take it apart. But Chris's idea of adding the fuel system cleaner is a worth a try, can't hurt, may even work.

there is one other iteml that can affect the fuel flow, and that is that vac operated device on the fender well behind the coolant recovery tank, that adjust low/intermediate mixture by allowing bleed air under the throttle. if it malfunction usually you get poor fuel economy, but it may also result in fuel starvation. it only operates at low/intermediate speeds, so it too is suspect.

unfortunately you can not just cut it out of the system, the carb was designed to work with that device working. I do not think that is my problem, because my old, leaky, dirty carb did not have that, it only happened when I put another carb on my engine that was sitting in my garage in a box for over 5 years. Each day I drive it the hiccup seems less severe, but I may be just getting used to it. It may eventually free itself up without any effort, except perhaps adding the carb cleaner.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

The Professor
Highest Ranking Member
Posts: 507
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
Location: Western Washington
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Re: annoying hesitation with carb replacement

Post by The Professor » Thu Nov 07, 2019 8:51 am

Mine seems to be getting worse, not better. I guess I'll just have to go back though the entire troubleshooting process on this carb. Sigh...
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

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Petros
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My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: annoying hesitation with carb replacement

Post by Petros » Thu Nov 07, 2019 3:26 pm

I have been meaning to go back and read though the FSM for the operation of the fuel system and the emissions systems. anything that is designed to engage at part throttle condition would be suspect, and that will help narrow it down on what to check.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

The Professor
Highest Ranking Member
Posts: 507
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
Location: Western Washington
Contact:

Re: annoying hesitation with carb replacement

Post by The Professor » Fri Nov 08, 2019 7:57 am

Some more investigation as I've been driving the car more during the last week:

In the throttle pedal as I move it through the travel I notice there is a "step" in the movement that corresponds to the throttle position the engine stumbles at. This isn't a binding feeling, but more like the carb linkage has contacted another part of the throttle linkage. I was thinking it was about to open the secondary, but aren't those vacuum operated, not mechanical?
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

User avatar
Petros
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Posts: 11104
Joined: Sun Jan 28, 2007 6:31 pm
My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: annoying hesitation with carb replacement

Post by Petros » Fri Nov 08, 2019 3:21 pm

yes, there is diaphragm that actuates the secondary throttle, but there is other linkage you might be feeling. there is linkage for the choke so it is forced open when you open the throttle so it does not die out when accelerating above idle.

mine does not seem to be getting any better, though it does not seem to occur when it is cold, when the choke is engages. So it appears that it is because the mixture is too lean right at the transition from idle, to low speed jets.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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