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74 L18 DCH340 troubles: looks like 3rd rebuild


Cardinal Grammeter

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TWICE I've had to disassemble carb to unplug main jet.  The last clog was a "toothpick" of something like black gasket material about 3/16" long stuck right in the jet.  Where did that come from???

 

Garage is a mess and have to make table top room for carb work - will take a full day's effort to do that.  Not in the mood....

 

SYMPTOMS:

 

1) No power, familiar hills require at least 1 lower gear to get up, lugging around in 4th is impossible

2) Needs about twice the throttle opening to get the power level carb used to make.

3) Previously, idle was too high, now idle is almost proper.

4) Carb pops if too much throttle is quickly given

5) Carb pops particularly if just a little throttle is given off idle quickly

6) Secondaries work OK.

7) Once engine revs are above maybe about 3000 rpm, carb will not pop no matter what is done with throttle.

 

EDIT ADDED 12-23:

 

😎 Going down a steep hill using engine braking in a low gear, the back firing (like "racking off") through the exhaust is excessive.  It is usually present to some extent but this is way too much - if the flange loosened up, I'm concerned I may have blown out part of the gasket.)

9) To start truck in 40* weather, the engine will start properly with choke obviously working, however there is a massive "flat spot" so if you touch the throttle, it dies or pops and ends up flooding to an extent.  I found pumping throttle 8 times puts enough fuel in the manifold that when the engine starts there is no flat spot and I get immediately get off idle w/throttle.

 

SUSPECTS:

 

1) Fast throttle increase popping probably due to accelerator pump (I don't believe that is problem.)

2) Carb base is probably not loose since assembled with Purple Loctite

3) Another plugged Primary Jet.

4) BCDD???

5) Carb had severe throttle shaft to body wear - bushed with UHMW plastic - did it fail or come out?

6) Due to bad design, throttle return spring puts big side load on throttle shaft wearing sides of throttle plates away - replaced during last rebuild (unlikely)

7) Engine vibration:  This has been problem since day 1.  Much worse in cold weather.  Suspect motor mounts have gotten hard.  Have new ones.  Too lazy to put in.

 

PLAN:

 

1) Tear carb apart, inspect, clean, reassemble.

2) Find another carb on car-part.com to rebuild to minimize 620 down time (is my only Daily Driver)

3) Weber conversion???  I think this is More $$$ and More Work - not keen on either.

 

ADVICE, SUGGESTIONS, THEORIES, and RIDICULE WELCOME!

 

Thanks in advance,

Tom

 

PS:

Truck in on my last nerve:  Fiasco with 1) seat reupholster job and 2) 340 SS exhaust job.

The seat is OK, but the exhaust is just the sort of molestation I remove and correct which I cannot do obviously - so truck is fucked.

I still haven't recovered from that 6-7-2021 car crash which is also pissing me off - I'm 69 and don't have time for this shit.  LOL

Edited by Cardinal Grammeter
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There is a very fine brass (later plastic) screen on the inlet to the float chamber. No way anything can get past that. Check it's in place inside the banjo fitting. If it is then debris must have been inside the carburetor when assembled after rebuild. It happens.

 

'Popping'... is this a back fire through the carburetor? Often sounds like muffled popping with the air filter on. This is NOT normal.

 

First thing is confirm that all valve clearances are proper. Check spark plugs for one that is different than the others.Try pulling each plug wire off as it idles. See if one has less effect than the others. If so, try driving. It will lack power and shake but see if popping is gone. 

 

Engine 'shaking' at idle is also not normal and probably related to the 'popping'. This can be one cylinder not doing it's job. Things to check are: vacuum leak, (use vacuum gauge see if jumping around) low compression on one or adjacent cylinders (use compression tester), bad spark plug, plug wire or distributor cap, valve not sealing. 

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The "popping" is through the air cleaner - typical for an engine with a bad accelerator pump - engines goes lean & too much throttle you get a pop.

 

The vibration is not a shake, engine idles w/o vibration, it's when you get above say 2000 rpm you can feel a "rotating imbalance."  The flywheel/clutch could be messed up since real butchers had this before me.  Cold weather makes the motor mounts stiff and you can feel the imbalance more.  I know the exhaust is vibrating - was a new obvious audible resonance after the job.  Still haven't gotten into that yet.

 

That little mesh in the banjo, I've give that a lot of attention getting it right.  I had a 74 MT carb in pieces and then bot a lot 3 more in various states of repair, other years though so I got all the screens and put together a good one.

 

Yes, that "toothpick" had to be in the carb body somewhere.  The first 2 tear apart clean outs and this one were at intervals that were greatly increasing each time.  The 1st clean out was shortly after on the road, the 2nd one maybe 6-12 months later, this one maybe 8 years later - so it could be the last of the dirt coming loose from inside the carb body.

 

I should probably completely disassemble carb just to clean out anything possibly lodging in the body.  I suppose I'll ultrasonically clean it too.

 

I've got a couple additional observations that I missed (I'm going to put them in the original post in RED) here they are:

 

*) Going down a steep hill using engine braking in a low gear, the back firing (like "racking off") through the exhaust is excessive.  It is usually present to some extent but this is way too much - if the flange loosened up, I'm concerned I may have blown out part of the gasket.)

 

*) To start truck in 40* weather, the engine will start properly with choke obviously working, however there is a massive "flat spot" so if you touch the throttle, it dies or pops and ends up flooding to an extent.  I found pumping throttle 8 times puts enough fuel in the manifold that when the engine starts there is no flat spot and I get immediately get off idle w/throttle.

 

If it didn't have a timing chain, I'd consider it may have jumped time.  But that shouldn't be possible.

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I don't think there is enough slack for the chain to jump a tooth.

 

Yes I have rebuilt a carburetor before and a few days later have it off and apart to find something I guess I missed. Once sorted out it runs fine forever.

 

Single pop? very possibly a lean. Multiple could be bad valve lash or damaged valve/seat seat.

 

 

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Single Pop.  If you give throttle too quickly at low rpm, the engine stumbles then pops sometimes shutting done completely for a fraction of a second (blind spot) between the stumble and the pop.

 

What baffles me is what could cause the idle rpm to dramatically lower?

 

If I hadn't put Purple Loctite on the carb base plate, I'd be voting for base plate coming loose (which still could be the case.)

 

Engine seems to pop less and run a little better (still no power on primaries) after a short heat soak off period (5-10 minutes or more)

 

I've had so many carbs apart, I could never count them.  Since the late 60's I've been dealing with them when I was running Studebakers (first car) and then when I got the Paxton supercharged ones with the blow through AFB's, I really got into them.  Biggest thing most people miss when modifying an engine is making sure the throttle plates are properly positions on the transfer slots at idle.  If too much throttle plate at idle, have to drill holes in throttle plates so they can be closed down and back over the slots. 

 

NOTE:  I never got into emulsion tube science - very hard to find discussion of that but the holes in the tube allow you to adjust mixture vs rpm.  Then there are the secondary air bleeds which adjust mixture at top end.  Then there are mods to make the carb atomize the fuel properly but I never learned those either. 

 

NOTE:  What is interesting about the carb is that it uses a venturi and the pressure drop is a function of velocity squared.  This is a problem because fueling an engine is linear with rpm and so all these tweaks on the carb is to change it from "squared" fueling to a "linear" fueling device.  Now the irony of centrifugal superchargers is that boost is proportional to the square of rpm, so now we need a "squared" device to fuel properly.  So that means un-doing all the tricks and sublime adjustments built into them so they aren't a squared device.  

NOTE:  another thing to keep in mind about supercharging is that the engine is basically a positive displacement pump.  So when you apply boost, the airflow in CFM does not change, but the density does (!)  And so you need more fuel with increased density.  The carb is not capable of adjusting for density and they get pig rich when you go up Pike's Peak @ 14,000ft.  Buddy did the drive in an Avanti w/AFB and got so rich was going to foul the plug. So he stuck rolled up paper above the metering rod pistons to hold them down which is the lean position.  

 

 

 

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Extra air in from a leak will cause a lean(er) mixture. This will make less power and the idle will drop....  but it only affects idle. Check the plug color now then idle for 15 min after warm up. If the plugs are whiter than they are now then probably it.

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  • 2 weeks later...
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Thanks for plug tip.

 

On H.A.M.B @ jalopyjournal.com they had a thread of Tricks and Tips that was epic long - too many to commit to memory - they need to be grouped like a FSM or Parts Manual.

 

EDIT:  LOL I went to get the link and they are on their 44th page (!)

https://www.jalopyjournal.com/forum/threads/little-tips-and-tricks-for-garage-hobbyists.1159355/

 

But there was one I do remember.  With engine running, and top of carb exposed, you use your hand to begin to restrict flow into the carb (just like the choke), if the rpm starts to increase, too lean - rpm decreases, too rich.  (I HOPE I got that right.)

 

Very clever.

 

The other trouble symptom was that the idle is much lower now.

 

Just to make matters even worse, I've got my SS exhaust leaking all over the place, my garage door opener is still not fixed from lightning strike, and I injured my knee on Xmas day and can't bend it much and can barely walk on it.

 

I'm boned AF.

Edited by Cardinal Grammeter
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Any feedback on those crazy cheap $75 new carbs on eBay?  

 

I don't know if a Weber would solve my "problems" but I'd definitely want the OE air cleaner adapter.

 

Also looking for a junk yard DCH 340.  I've got 2-3 parts carbs but they are the wrong model - wouldn't mind having a proper spare.

 

NOTE:  I seem to recall while having different accel pumps from kits, SOME of the rubber pistons DO NOT SLIDE IN THE BORE!  The spring collapses and NO FUEL SHOT.  I wonder if the Carter Zip Kit has the leather piston seal.

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Ya gits what ya pays for. It's a $75 gamble. Probably ok in the short term.

 

 

I like the old leather ones. I was told the leather is actually kangaroo. Anyone know if true? Kangaroo won't dissolve and lasts longer in gasoline.

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Roo is better(!)  Those old carb guys might know that, in fact, I'm going to ask on the HAMB which is the most amazing forum (!)

Did you check out that Tips link?

 

(I think I posted years ago about the sticking rubber pistons when I was building my DCH basket case.  I think there was Black, Red, and Blue ones.  Argh - the thought of trying to find that post.)

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