Misfire Troubleshooting

[jsonova99]

Okay, I posted a little about this in a carburetor thread, but I'm not so sure it's the carb, anyway here is the problem. After driving at a constant speed for a few minutes in 4th gear, the truck will buck and misfire, if I lay off the throttle it seams to correct itself, but will do it again after a few minutes. I will say the the fuel tank is pretty low, is it possible I'm sucking crap up from bottom of tank or water in tank causing something like this? The sediment bowl is clean, but there is an in-line filter after that that I plan on replacing.

For the record I have a late 2.25L, probably a 5 bearing. It has a 32/34 Weber DTML two-barrel carb on it which I'm told is what the 2.5 liter engine came with. I'm guessing this engine has no internal mods to make use of the bigger carb. I'd like to convert back to a single barrel, but I'm afraid that there will be a snowball effect of pieces to replace. Hoping to hold off on this until I rebuild the whole truck if possible.

Here's what I've done:
1)Full tune-up; plugs, cap, rotor, points, condenser, coil, wires, plugs. Double and triple checked gaps for points and plugs. This was done prior to this problem and made a world of difference at that point
2) ran 2/3 of a bottle of Sea Foam through the carb, lots of blue smoke, and the engine actually seems to run a little smoother. It had a valve tap that has quited down quite a bit after this. Problem seems a little less severe after this as well, recovers quicker.
3) adjusted valve tappet clearance (did this during tune-up prior to this problem)
4) Timing is set to about +5 degrees, which should be consistent with running 87 octane
5) replaced sediment bowl and screen on fuel pump

Things left to do:
1) replace in-line fuel filter
2) compression test and general spark plug check (checked the #1 plug and it looked great)
3) double-check timing
4) Mostl likely replace carb since it is a model that does not seem to be available in the U.S.

That's about all of the info I have for now, I'll report back with compression test results. Feel free to chime in if you have any ideas. The engine runs perfectly smooth, has lots of power (relatively speaking) and drives great, until it bucks all of a sudded and misfires.

 

[tripm]

Pull off the passenger (lhd) seat and the seat box cover. The fuel pickup tube is there, held on by two screws. Take those out, then pull the pickup tube assembly out. Check the point where the brass fitting joins the tube.

After three carbs, three fuel pumps, new fuel iines, changing from a generator to an alternator, and who knows what else I can't remember, that was the problem I had.

It takes about 10 minutes to check, so it's easy to do before you try something else.

Oh! One more thing - check that the fuel lines aren't pinched somewhere along the line.

 

[jsonova99]

Fuel lines seem fine. I rebuilt the carb, and sea foamed the crap out of the engine to clean up the valves. Still not right, although different. Seems to choke when I accelerate, but only after it's warmed up. The truck started right up, I took it for a test drive and everything seemed fine for about 5 minutes. Then it would buck a little , and hesitate when I would accelerate. Could it be that the carb needs better tuning after the rebuild, or does it sound like I still have something else wrong. I'm going to go through all of the ignition components again and doublecheck the timing as well.

 

[apg]

Make bloody well sure that the points are good, as crappy points can masquerade as all sorts of fuel-related problems.

Sometimes, crud in the tank can momentarily block the fuel pickup tube, which falls back to the bottom when you throttle- or shut-down. Draining the tank at this point might not be a bad idea....

Somewhere along the line, I picked up some bad fuel with *white* particulates. These were invisible on the white mesh of the transparent fuel filter adjacent to the carb.

 

[jsonova99]

Truck stranded me a little while ago after checking points, gaps, plugs, etc. The plugs are just black with carbon, the truck is spewing black smoke which must be unburnt fuel. This is how it ran when I first got it. After replacing every ignition component it ran great for about 2 months until now. I'm going to clean the plugs hoping that maybe they were just so fouled up after the carb rebuild that they've become the problem. All of the ignition components have about 300-500 miles on them and were running great. Could I have a bad distributor or something? I'm at a loss. The funny thing is that my Dad is having the exact same problem with his 1960 TR3. He's at the same point I am although he hasn't rebuilt his carbs yet. As far as fuel delivery, I just installed a new clear fuel filter, and I can see that it is almmost completely full of fuel which leads me to believe that I'm not starving it of fuel, in fact I think I have the opposite problem. I wish I knew more about this Weber 32/34 DMTL carb, but I don't have any literature and haven't found anything in regards to how to tune it or adjust it.

 

[singingcamel]

O)bviously your dumping fuel to carbon the system. i would get rid of the two barrel carb and go with a 1 barrel weber.
It sounds like you may have a needle valve in the carb stuck.
Pull the top of the carb off and check the float and neddle valve for sticking..
www.singingcamel.com

 

[jsonova99]

I did check the needle, it's fine. I'm wondering if there's some adjustments that I could make that would help. The carb rebuild itself seems to have done nothing. Does anybody know anything about how to adjust or tune a 32/34 DMTL? I'm going to advance the timing some more as well. British Pacific recommends that on the older engines. I did play some with this and it did run a bit smoother. All 4 plugs are dry fouled though.

 

[singingcamel]

what about a vaccum leak, have you checked for that?may have a bad vaccum line ,may have a leak at the carb base as well.
bucking under a load at top end makes one think it may be a pick up problem as mentioned previously..
check your return on your rotor should return to original position..when let go. maybe a worn dizzy..keep going your almost there?:applause:

 

[jsonova99]

Thanks for the words of encouragement. I think the carb base was definitley leaking prior to the rebuild. A vacuum leak is one I've thought about but not done much troubleshooting for other than finding a cracked hose on the vacuum advance, replaced it when I rebuilt the carb and no real difference. It's possible that hose cracked when I pulled it off of the distributor, too. I do have one other hose to check though, it's a rig job some previous owner did to the vacuum line coming off of the intake manifold that was for a brake booster. Talk to me some more about the ditributor and watching the rotor, this is an area I have not checked.

 

[jsonova99]

FIXED!!! Turned out to be a loose jet in the carb. Tightened everything up and drove it. Things were going good for a few miles then it started again. Managed to limp home and sure enough the jet was loose again. Tightened it up again and drive it and it was fine, this time it stayed tight. I also sea foamed it again to clean the carbon off of the plugs and possibly valves , and I noticed that the smoke was coming out around the exhaust manifold, so I'm guessing I also have an exhaust leak, another problem for another day. Thanks for everybody's input.