Rough-Running 3000

[carguyinaz]

Carbs are rebuilt and adjusted correctly for fuel mixture. Points and plugs are set to specs. Car starts and runs great with lots of power until it's driven for a while, then, when fully warmed up, at around 2500 rpm it misses and I can't tell if it's fuel or spark that's the problem. Above 2500 rpm it runs great. It seems as though a component in the engine compartment gets warm and causes the problem, but I don't know which component to suspect. Any ideas what the problem could be?

 

[vette]

Rotor, condenser.

 

[carguyinaz]

Rotor, condenser.

Checked both; no problem detected. Also checked the centrifugal weights in the distributor for lack of movement; also no problem.

 

[vette]

I doubt that the weights or timing will cause a miss at constant rpm. But heat will affect different electrical components. How could you test a condenser but only to change it. Crack or carbon buildup on rotor might be detected. Also crack and/or carbon in distributor cap could cause it. It still could be fuel related. Check for good strong flow into a can. Maybe change filter.

 

[carguyinaz]

New fuel filter and carb floats, etc, after professional carb rebuild. Rotor cap has no arcing indicators at all. I tested the condenser, but I think I'll replace it just to be safe. I'm sure the problem is related to something in the engine compartment heating up, but I have no idea what it could be or how to check for such an anomaly.

 

[John Turney]

Is the engine stock or have there been modifications?

 

[carguyinaz]

Stock

 

[vette]

Hey car guy, we need to be on the same page with terminology. We have a rotor and a distributor cap. I don’t know what a rotor cap is. The rotors will either crack in a variety of places or the electrode will get loose or it can build up carbon tracking. It needs to be inspected closely. Best bet is to discard it. Same for distributor cap, it needs to be inspected closely for same all the above. As well as the spark plug wires and they’re connections. After that I would swap out the coil. After that I would change the points. All these need checked. In most cases checking them means changing them. But your problem can still be fuel related. If your engine is starving for fuel at 2500 rpm’s it can very well feel like it’s missing.
Good luck.

 

[nevets]

Possible coil...A flaky one will act up when warm.

 

[carguyinaz]

I bought a new condenser and coil last night and will try that next. Thanks for your input!

 

[Bob_Spidell]

Smells electrical to me; a fuel delivery problem would likely get worse as RPMs increase. I had a rotor fail--killed engine dead, suddenly--at Mesa Verde, CO in the middle of a long road trip. Diagnosed rotor failure--easy enough--and replaced with one of the 4 or 5 I had in the boot (bad rotors were a known issue at the time). I didn't spend much time examining the bad one, but don't recall any obvious 'tracks' or other damage. Problem was generally attributed to the rivets on the arm.

Same with a bad distributor cap, Bosch brand IIRC, that would cause the car to stumble on acceleration when hot, but worked OK cold at steady RPMs. Finally diagnosed using the 'dark test:' run car in a dark garage, with adequate ventilation, natch, and look for stray electrical pulses. Mine clearly showed a bright streak down the side of the cap, again with no visible marks on the cap. Replaced with genuine Lucas which has been reliable.

Coils fail, but not often. Mine is the original AFAIK, with 210K miles, and is beat all to h*** but works fine. They can develop an internal short in the dielectric, but it would perform badly at all speeds.

 

[vette]

Hey Bob, I was at Mesa Verde a couple years ago too. Was driving a Corvette. Had to step down my driving particulars for the trip. Didn’t have any rotor or condenser problems tho. But I did have a condenser failure here around home tho. Headed down a fairly steep hill, engine under mild acceleration and all of a sudden the engine just quit. Popped it in neutral and coased to a safe spot. Tried swapping coil first no joy. Then replaced condenser, fired up and on my way. I carry enough in the boot to practically rebuild the whole car. Caliper & wheel cylinder kits, alternator & belt, everything for the ignition.
If I’m driving the Vette I just carry a credit card. . Thank God never needed it yet.

 

[Keoke]

Replaced with genuine Lucas which has been reliable.

Only if they do not have a rivet on top!

 

[John Turney]

Throwing some ideas out there.

I asked about the engine being modified because I had a problem at about that RPM due to valve clearances, but I have a different cam, that "comes on" at 3,000 RPM.

Usually, a problem will be either at low RPM or high RPM, but not at a specific RPM, or it just doesn't run at all. It makes me think that there is something with a resonant frequency, kind of like when the exhaust makes a different sound at a specific RPM because the muffler is vibrating, or when strings vibrate and make a certain note. All the rotor failures I've seen have been like Bob's, the engine just stops because all the sparks are shorted. A bad cap may cause one cylinder to not fire, but that would be RPM independent.

So, thinking about what could cause resonant frequencies:

• I suppose there could be a resonant electrical frequency between the capacitor and coil, but I've never heard of one. Moss told a group of us a few years ago that good capacitors were becoming hard to find because no one still made them who had any real experience since they are only used in classic cars now. You may have a capacitor that has the wrong value. The one you have been using may seem to work because is has capacitance. Good you bought another one to try. Is it the same brand as the old one? Supposedly, there is a difference between the ones with a yellow wire and the ones with an orange wire. I forget which is suppose to be better, since I have electronic ignition.
• A remote possibility is that the spring on the points could be such that there is a resonant frequency at 2,500 RPM, causing the points to bounce. The resonant frequency of the point spring would normally be way above the normal RPM range.
• Do you have oil in the carburetor dashpots? What viscosity? In addition to causing enrichment during acceleration, the oil stops the carburetor pistons from fluttering. Maybe they flutter at 2,500 RPM from lack of oil or because of a viscosity change in the oil.
• Fuel starvation from, say clogged filters or low pump output, increases with RPM rather than at a specific RPM and then going away. If one, or both carburetor needles are the wrong diameter at the height where the engine is at 2,500 RPM, the mixture could be wrong. I'm told that running ethanol in the gas requires the rich needles. I've installed both the standard needles (UH for HD8 carbs) and rich needles (UN) and couldn't tell the difference when running. I'm told that really does make a difference in a tricarb 3000.

 

[Bob_Spidell]

Replaced with genuine Lucas which has been reliable.

Only if they do not have a rivet on top!

This was the distributor cap; $40 a couple decades ago, but worth it.

 

[carguyinaz]

Replaced coil and condenser with no improvement. Now I'm back to thinking it's fuel. The front three plugs are always fouled so I think my problem is over rich mixture in the front carb. Both carbs are adjusted for mixture at idle and at higher revs in the garage, but something happens when the engine gets warm. I took the the tops of the carbs off last night and noticed a lot of carbon build up on the pistons. Maybe it's not carbon, but it a black coating. I cleaned it off and replaced the dash pot oil and took a drive and the problem didn't occur. Perhaps the carbon was causing the piston to stick in an upper position, thus enriching the mixture when the carb got warm or maybe it was the dash pot oil. I have my fingers crossed.

 

[TAS525]

Replaced coil and condenser with no improvement. Now I'm back to thinking it's fuel. The front three plugs are always fouled so I think my problem is over rich mixture in the front carb. Both carbs are adjusted for mixture at idle and at higher revs in the garage, but something happens when the engine gets warm. I took the the tops of the carbs off last night and noticed a lot of carbon build up on the pistons. Maybe it's not carbon, but it a black coating. I cleaned it off and replaced the dash pot oil and took a drive and the problem didn't occur. Perhaps the carbon was causing the piston to stick in an upper position, thus enriching the mixture when the carb got warm or maybe it was the dash pot oil. I have my fingers crossed.

I am having a similar problem with my TR3. Engine has always run strong, but out cruising upwards of 80 mph the other day and suddenly acted like it was starving for fuel. Easing off the throttle and coasting a bit seemed to help and I was able to get back home at normal - city - speeds.

Inspected the sediment bowl and filter in the mechanical pump and found minor debris.

All 4 spark plugs were seriously fouled, however, so I’m thinking like you that a piston (maybe both) must be sticking during acceleration or perhaps the needles are corroded with varnish from today’s sub-standard fuel. She’s obviously delivering a very rich mixture.

Either way, I can’t even get the car out of our cul de sac now. I cleaned off the plugs and she started right up, but they foul again quickly as soon as I’m out of the driveway.

I’ll check the carbs (pistons, needles, jet adjustment, etc. ) when I return home from this trip, but keep us posted.

~ Tommy

 

[carguyinaz]

Thanks Tommy. My carbs were rebuilt last year and I get non-ethanol fuel here locally, so I don't think that's the problem. Floats and needles are new after the rebuild. I would assume that if a float were sticking I would see fuel at the overflow tube and/or hear the fuel pump going crazy, non of which are happening. Keep me posted!

 

[vette]

Non-Ethanol fuel these days is usually very low octane. If your carbs are black they need taken apart and cleaned. Maybe you don't have to remove the jet but the needle, piston, chamber and linkage needs cleaned. And they got black because they were rich. Are you familiar with the piston drop test. Not the one I did lately by dropping it on the floor but the one where after the carb is reassembled you lift the piston about an inch or more and drop it to the jet bridge. The piston MUST drop cleanly and end with a good metallic clamp. If it doesn't then it is not clean or the needle is not straight or the jet is not centered. Then after you get it to drop what you think is completely freely then you install the chamber, do not install the piston damper, and you pick the piston up again with your finger thru the throat of the carb, pick it up again about and inch or more and see if it drops cleanly with the chamber installed. If it doesn't the piston is riding on the wall of the chamber a wee bit. The piston or the chamber may be out of round. With A LOT OF PATIEANCE they can be made round again. Also if they are not free of each other then the needle will start to wear against the jet thereby wearing the jet out of round. This can't be fixed. Requires a new jet. If the jet hole is not round that jet will always be rich. But you got to get the drop test right or everything will wear wrong and screw everything up.
Even if your carbs were rebuilt well, were they adjusted well when installed. Was the mixture set right when installed. If they have carboned up in such a short order they either weren't assembled well or weren't installed well. If they are carboned up then they need rebuilt again. Not such a big deal. Rebuilding SU carbs pretty much mean cleaning them. Clean em, assemble them, do the drop test and set the mixture right and you got it. I use an exterior, electronic fender tach when I adjust my carbs. I set the idle speed as low as I can that will keep the engine running. Then I adjust mixture to the highest engine speed I can get it by changing the mixture. The engine is happiest when it is getting the best mixture. When that happens the idle speed will be the highest you can get on that jet. After you have gotten the highest idle speed you can get by adjusting the mixture on the jets then lean the jets just a smiggin, Maybe an eighth of a turn. Rev the engine a few times to clear out any laten fumes in the intake stream. Let it settle down. Now double check your mixture by the small piston lift pin in the side of the carb body. Or by a small flat bladed screw driver stuck into the carb throat and just under the edge of the piston. Lift the piston minimally, usually by just twisting the screwdriver to raise the piston a smidgin. If the engine stalls when doing this it is too lean. The engine speed should pick up maybe only 100 rpms and settle back again if the mixture is right. If the speed picks up considerably and keeps up the mixture is too rich. Good Luck.

 

[carguyinaz]

Problem seems to be solved after cleaning gunk off carb pistons and performing a drop test. Was careful to not switch the pistons between carbs as I learned that each carb is unique and most have its piston that was installed at the factory. As an experiment, I tried switching them and low and behold the the car suddenly ran great. Maybe a previous owner or shop person switched them. Anyway, so far so good. Thanks for all the help and info.