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Never had this carb problem before...


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OK, so I've been driving the old '74 beater for a few days with the Weber 38DGAS on it. Not the greatest fuel mileage, but somewhere in the 20s which isn't so bad as the old Hitachi got about the same but far less power.


Anyway, it's been OK, a little stumble off idle when cold but otherwise pretty good until last night. About halfway home I noticed that whenever I got on the flat or downhill where I was under 1/4 throttle the engine developed a miss. Inconsistent, but you could feel it. Give it half or more throttle and the miss disappeared no matter what speed, but half throttle on flat freeway well exceeds the speed limit and at 11PM you don't want to be the rabbit.


Well, it kept getting worse- soon it was 1/2 throttle stumble, and the miss got more and more pronounced. Soon it was missing at 3/4 throttle, and wouldn't run at all below 1/4 throttle. I got very worried that iwhen I had to stop for the 4 stoplights between the freeway and my house it would die and not refire.


When I got to the lights the only way I could keep it running was to hold the pedal at 3/4 throttle for a brief second then let off., let the engine drop to near stall , step on the gas, and basically rev it over and over until the light went green (gee, and I'm trying NOT to attract the Sheriff's dept this way?). It accelerated just fine from the lights at 3/4 throttle, didn't feel any different until I had to back off then it would buck and kick at 1/3 to 1/2 throttle (it would just shut off below that). Got in the driveway and was thinking maybe the idle jets plugged up? Went to pull them out and the carb was SOAKING wet. With water. And COLD... like a fridge. The engine was at normal operating temp (half stock gauge, about 180 on the mech gauge, it's always run like that). So I was thinking maybe the carb iced up. I've never had a carb that cold before, especially on a hot engine.


I pulled the idle jets anyway, they were clear. Let the engine heat the carb from convection. Put the jets back in, same issue.



Any ideas? I didn't try was check for vacuum leaks. It was 11:45PM and I'm sure the neighbors LOVED the revving engine when I was trying to see if clearing the apparantly clear jets did anything. The problem was progressive (got worse the longer I drove). It certainly didn't ACT like a vaccum leak because I still had power at higher throttle, and it was sputtering at lower settings. Water in the fuel would have sputtered all the way up the RPM range; a clogged filter would have run at lower throttle settings and stalled out at higher ranges. I also didn't check the choke to see if it had failed.


Basically wondering if anyone else has any ideas before I start tearing into it (weather permitting). The icebox-cold, soaking wet carb on an otherwise hot, dry engine was something I'd never seen though it's perfectly explainable fluid dynamics. I'm hoping it's NOT just carb icing because I'd like to drive the thing in the winter, and it's not THAT cold out.

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Sounds like a vacuum leak. Remember at full throttle, there is no vacuum, so leaks don't matter much... but at idle there is full vacuum and any leak will cause problems.


I had a similar symptoms with my Hitachi but it was dirt/water on the main jets. Removing them and flushing the bowl -- even though it looked clean -- resolved the problem. I've had this happen several times. Good thing the jets can be removed (from Hitachi) without removing carb or disassembling.

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I've had carb icing many times. Generally you notice a lack of low speed power and need to push the throttle further and further down to maintain speed. If you are able to get into the secondary it will go alright and accelerate fine. Eventually you only have full on or off throttle. The primary venturi top will be white with frost practically plugging it. The base of the carb may also have frost on it and the carb body is so cold your warm fingers will stick to it. Icing will cause the motor to run lean. It can do this on a fully warmed motor if: there is no carb heat from the exhaust manifold, it is near freezing and very damp like fog or specially freezing rain. I have flicked the ice off with my finger and it will run fine for about a quarter mile before plugging again. Letting it sit will warm and thaw it out but again only good for quarter mile. This is my experience with it. Some of your symptoms are similar. If it mysteriously runs fine today, that's probably what it was.

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I think the hitachi gets hot by water, right? if so is the 38 weber electric which means not water. And if the temp got colder that makes sense. You can probably call weber direct or where you bought the carb and see what others are doing. Other thing is check electrical, like your coil. Mine was doing that and I thought it was fuel but it turned out the primary on the coil was going out. You can also check your filter? how old is it?




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No, the Hitachis (if not manual) have electric chokes just like the Weber I have (the manifold itself is water-heated, and that hasn't changed).


I'm leaning towards a vacuum leak- it's possible the manifold bolts or the carb base bolts/adapter bolts worked loose. All that has less than 500 miles on it since I had the head redone- probably less than 250. The carb seemed tight when I looked at it, but at 15 to Midnight I wasn't staying up much longer to look. Doesn't matter- I can whip the carb on/off in 5 minutes to check the adapter bolts. I have the same adapter on my 4X4 and it's come loose before, though it only showed up as no idle since most of the time vacuum "sucked" the carb down to the manifold.


The carb isn't progressive- it's synchronous, so here's no "primary" or "secondary" since they're the same size, jetting, and the plates are geared together. I ruled out plugged fuel lines/filter because that usually shows up as "running out of gas" under heavy load, but better at smaller throttle settings where the restriction flows enough. It ran up the Tacoma hill (from the Dome to Hwy 16) with no problem, no sputtering, plenty left on the throttle. But at the top of the hill when I had to back off the sputtering was back almost worse than down on the tideflats. Of course that's because I was in more traffic and had to back down even more.



Still could be icing, but as mentioned there's no "primary" or "secondary", just 2 equal barrels. However, it was a little wetter outside this run than the previous run, kind of misty rain typical of the Puget Sound area and mid 40s (F).

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"Carb Heat"?


Why would I? The engine has the (stock) water recirculation through the intake manifold (which is hooked up) and the carb itself is unheated. The choke, OTOH, has it's electric heater which just operates the choke plates. What I don't have is the old snorkel heater since I don't have the original air cleaner (because I dont' have the original carb). But none of my Weber trucks have snorkels. This is the only one with the problem- I've driven my 4X4 enough in the snow to think the coolant-recirc in the manifold was enough here.

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It would have to be at least 38 degrees to get carb icing, I would think.


The carb should have heated air for best fuel economy (as the stock Datsun engines did) however it is not necessary. It won't cause a problem unless the temp gets down in the 30s. I've had my Datsuns ice up before.


Also check for vacuum leaks in the hoses and diaphragms, e.g. Distributor diaphragm, EGR, etc.

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It's carb ice, pure and simple. This is a big concern for aircraft, they all* have a heatbox with a pull control to activate it. It doesn't have to be near freezing to encounter carb ice, it happens in the summer when moisture content is at the right saturation in the air. A carburetor contains one or more venturis, and as we know from Bernoulli's principle, air passing through a venturi experiences an increase in velocity and a decrease in pressure. The combination also causes a decrease in temperature, which freezes any ambient moisture in the air. Ice forms in the venturi, decreasing its diameter and starving the engine for air.


*not fuel injected or turbine types, obviously.

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I can't believe a carburetor would fill up with water. Unbelievable. :eek: :eek:


I'm glad I have a Hitachi, safely ensconced, in a nice warm air filter with warm air being fed into the venturi's. :) :)


Good luck, please type your final solution. I'm eager to hear the solution to your carburetor problem. :cool:


I've had my share of carburetor idling problems. :(

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ive gotta say it the carb icing up too.


my old isuzu used to do that when id drive to work in the mornings in the winter. it did it so often that i though about making a hollow spacer between the carb and the intake that would have coolant run though it to keep the carb hot during the cold weather.

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Wasn't carb icing. The wet cold carb was just a red herring. Was just as bad when I started it tonight as last night, except it was a helluva lot harder to start.


When it did run it was whistling. Really loud hissing whistle, kind of like the sound the carb makes with the choke closed and no air filter. Problem was I could only run it with pedal movement because it wouldn't stabilize. I did note that it didn't idle down very fast... dead indication of a vacuum leak, and a BIG one at that.


Pulled the carb off... all 4 carb mounting nuts were barely beyond finger tight. But the adapter screws were at least 1 1/2 turns loose and the adapter base plate was just sliding around on the manifold.


Same damn thing happened with the 4X4 when I first installed the adapter. I see loctite in my future (I HATE putting loctite on aluminum, but I see no alternative to these screws backing out).


Once I tightened everything back up it ran (and idled) fine.

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I don't care for Loc Tite either. Thanks for finishing your thread with your solution.


Glad you solved the problem and shared it with us. Your solution makes me want to go to the garage and check the tightness of my carburetor and the manifold.

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I have had a carb adapter crack on me. But that would not idle. Above idle its fine.


If this is a point vehicle dont overlook that either. Bushing worn causing the point shaft to bounce around causing points to open when you dont want to(usually a poppng sound when this happens).


If you got water circulation then its not icing up.


fuel pump pressure? causing float proplems?


choke flap not working right?

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Water circulation only warms the air in the manifold. Without warm air to the snorkle it can ice. Besides a slight temp drop caused by venturi pressure drop, gas absorbs heat when it vaporizes making the venturi and carb sides COLD! Sometimes all it needs is a few degrees cooler than ambient for frost to form.


Doug, I meant air warmed by the exhaust to the snorkle when I said carb heat.

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Can't put lock washers on the allen screws... no room. The heads are inside little recesses in the adapter itself and would stick up if washers were used. The heads are generally flush with the adapter surface. The carb base nuts themselves have lockwashers but they back off, or at least until corrosion gets on the studs and they get stuck.


I don't have a welder. And I don't want the adapter permanently mounted because it makes getting to the coolant line impossible. So locktite is pretty much the answer, or I could drill and pin the heads from the side.

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