Very erratic idle behavior

[Slow Loris]

Stock hitachi, DAF, manual choke. 
 

I thought I needed to change out/troubleshoot my accelerator pump (truck dying when transitioning from 1st to second gear change or sometimes on takeoff from standstill) so I removed the pump and sprayed the circuit with cleaner, which had no effect, so I changed the pump, also removed main jet plugs and sprayed cleaner up through there and down through float chamber, now I’m here:

 

Experiencing very weird idle speeds—high, low, engine dying, racing, surging, and all manner of weird stuff if opening or closing the choke.

 

I press in the gas pedal, pull the choke, let off pedal, to start truck as normal. The idle is high as normal with choke pulled, the engine starts coming to temp so I start easing in the choke but the truck dies. Okay, restart the truck now it sounds like it’s idling great but high, about 1000 then ease in the choke and the idle goes UP to 1500 rpm (with choke all the way in).

 

Okay, that’s weird but try and take off and drive—nope truck dies.

 

Restart, (choke in because warmed up) truck is chugging, chugging, so pull choke out, idle smooths out but still high. Wait a few minutes and push in choke and idle goes even higher!

 

Sometimes, pushing choke in lowers the idle so far it dies, other times, it rises. Huh?

 

Fuel level on the glass is dead on the line. 

 

What the hell is going on?

 

Self inflicted yes—this wasn’t happening before I messed with the accel pump, but I was equally going nowhere all the same…

 

what is my carb telling me?

[datzenmike]

Sounds like vacuum leak. Check carburetor mounting bolts are snug. Check any screws holding carburetor together are snug.

 

 

[Slow Loris]

Copy that. Will investigate.

[Slow Loris]

Nothing blatantly obvious jumping out leak wise. Will try the carb spray leak test around the base and intake tomorrow.

 

I had the carb apart previously and replaced gaskets and tightened it all up and it was idling better than ever. Then did the accel pump and main jet and float chamber clean out and now this.

[Slow Loris]

Would an icing carb give that kind of bad idle?

 

It was about freezing, colder than I’ve ever tried running the truck pulled from my frigid garage, and using an aftermarket air cleaner assembly so intake air unheated.

[datzenmike]

The idle circuit is not really affected by carburetor icing. Carb icing causes the venturi to become covered in frost making it not act like a venturi. This would affect above idle and all of the primary barrel use. Gasoline won't burn unless it's evaporated to a vapor. To evaporate it absorbs heat. Put some on your finger and blow on it....it gets cooler. Although the outside air may be above freezing the venturi becomes super chilled by the evaporating gas and water molecules in the air will stick to it and freeze, building up a layer of frost or ice.

 

Carburetor icing also requires high humidity in the air such as fog and freezing rain near zero F. When icing begins there is a lack of power when driving and you can't accelerate. Slowly your speed slows and if you step on the gas you slow even faster. If you let off the gas some power is restored but it's a loosing battle and soon you are going so slow you have to pull over. Strangely the engine usually still idles. If you wait a few minutes, engine heat will melt the frost that formed inside the carburetor and you can usually drive away normally but it comes right back within a few blocks. The only cure/prevention is the stock ATC (air temp control) mounted in the stock air filter snorkel, the sheet metal shield around the hot exhaust manifold and the stove pipe that connects them. Most of the time there is no problem but under certain conditions of cold and humidity it''s a different story.

[Stoffregen Motorsports]

Weber DGV

[datzenmike]

Oh. The idle is part of the primary venturi?

[Slow Loris]

Interesting! Physics is cool.
 

It was just a thought, absent any detection of vacuum leaks but I’ll keep checking.

 

43 minutes ago, datzenmike said:

Oh. The idle is part of the primary venturi?

 

Is that a realization about my Hitachi or an advantage of the Weber? I’ve got the Hitachi—I’m assuming stoff is recommending I visit the nearest lake and throw in the Hitachi. 
 

 

[Stoffregen Motorsports]

Yeah, sort of. I'm firmly in the DGV camp, but others love the stock carbs.

[Slow Loris]

Well I’m certainly not in love with it at the moment but I also feel like somehow this is probably my own fault so I shouldn’t abandon the Hitachi just yet. I’ll start saving up my lunch money for the Weber, maybe in the meantime I’ll get this sorted out.

[Slow Loris]

Okaaaay well I now recall that during my prior checkout of the carb, when I removed the phenolic spacer it was cracked (or I cracked it during removal) so I glued it back together.

 

 

I took another look at it and maybe it didn’t form a perfect enough mating surface after it glued—a little uneven. Though the glue is solid.

 

And I also noticed a couple tiny chunks taken out of the inner edge of it:

 

 

Pretty tiny chunks missing there, but maybe something?

 

Anyway, think these could combine to make a vacuum leak?

 

Having never seen one of these other than this one, is this one perfectly useable or time for a new one?

[datzenmike]

It'll do for now, but I would rub a tiny smear of RTV on the crack surface inside and out. It's rubbery and impervious. There should be a regular gasket above and below this spacer.

 

Trace it out on gasket paper (I use cereal boxes) and cut your own. Dollar store round hole punch and scissors. If you need one you can have one made in 5 min.

 

 

 

[Slow Loris]

I’ll give that a try. Yes, I do have the upper and lower gaskets to sandwich around it. I think when it took the pre-existing ones off, there were actually two paper gaskets stuck together on one side, which seemed weird. Maybe PO was onto something there…cuz it was at least idling before I started messing with the carb. Oops.

 

Absent this being the culprit, I couldn’t detect any other vacuum leaks. So hopefully this is it!

 

Thank you!

[Slow Loris]

Alright alright re set and added some copper cote to the gaskets around the spacer to the intake and slapped it all back together and now things are idling smoothly!