The Stovebolt Site Forums The Parking Lot Big Bolts About at my wits end, getting an engine to run

Hi Guys,
I havent posted for a few months, but am back and really need some advice.
Here is the backstory. I am working on a 46 ton and a half that belongs to a neighbor a couple of miles down the road. He bought the truck a couple of years ago, took it to a local mechanic for a laundry list of things to fix. The mechanic put it off for way too long, so the owner asked me if I would like to work on it. I said sure, in my spare time. I am a carpenter by trade, mechanic as a hobby. Mostly my wrenching is on old Dodge Power Wagons and the family cars. So, now the truck has been at my shop for almost a year, differnce being I have been working on it! I have replaced spring bushings and shackles, wheel bearings and seals, new fuel pump, rebuilt the carb, new intake and exhaust manifold gaskets, completely rewired it, converted it to 12 volt, added turn signals, and on and on. So now my list is finished, I thought I was done, but as a last minute thing decided to adjust the carb. The truck starts up fine, idles like a champ, drives around my shop yard fine. The problem comes when I start to give her the gas. About halfway through the pedal, it starts to spit and pop and backfire. It wont clear out at any point.
So, I had a friend who was helping me get it finished up, and he is a tried and true Chevy guy. He and I tried every possibility of carb adjustment, timing, checking the points, verifying the plug wires were correct, did a compression test and got 118 across the board, so seems to rule out valves. Tried running it off a gas can with fresh gas, no different, replaced the plugs which were black and sooty. We tried swapping carbs off my friends old Chevy that has the same carb, Again no change.
Its not a vacuum leak, We have checked and rechecked everything (except what ever the problem really is) and are stymied. The owner says he has driven it on the road and it ran fine. Supposedly the engine has been gone through, so I doubt it has slipped a tooth on the timing chain. The engine is not the original, it is a mid 50s 235. I feel like we have covered all of the ground we can for exterior things, now we are wondering if it could be off a tooth on the timing chain.
I am sure ready to be done with it and get my shop space back!
Thanks in advance
Tim

Have you checked the vacuum advance? Has the distributor been out?Is there a fuel filter that could be clogged?

Sooty plugs says rich, but it's hard to argue with a carb. swap not changing conditions, unless the "old Chevy" carb had the same issue. I think the '50s 235s used a Rochester carb with a vacuum port in the manifold - is this there and open?

Popping and backfiring sounds like timing. I agree, test the vacuum advance (I got a Mighty-vac kit at parts store, very useful and not that expensive).

Can you swap out the coil and the condenser also to rule these out?

Does it consistently idle ok? Just faulters at mid or higher rpm?

-Michael

Hy Powerwagontim, when you converted it to 12 volt, what type of coil did you use? There are coils that are made to use 12 volts all the time, and others which must have a ballast resistor installed in the circuit before the coils primary connection. If you use a coil which requires a resistor, without a resistor, the coil will overheat and break down, the engine may idle fine, but will misfire more and more if you can get it to rev up. Check what type of coil you have, and either replace it with one that can be fed 12 volts all the time, or install a ballast resistor in the coils primary circuit. The only other thing that I can think of that may cause the problem you have is a bad condenser, good luck.

From what you've said it sounds like it pops when the motor is put under load which makes me think it's too lean at that point.
Can you increase rpm slowly and bring it up to about 2,000 rpm without it popping, not under load ?
You said you could drive it around the yard which I take to mean low rpm and not much load.
The mechanical advance should be working as rpm's come up and when you put the motor under load manifold vac drops down and the vac advance is not an issue at that point unless there is a vac leak.
Course you know, I wish I could hear it run....tell me more.
Dan

sounds like to me you might have a flat spot on your cam i might be wrong not a mechanic but i have been around enough to hear others with simlar probs

Hi Guys,
Mike E, We havent checked the vac advance per say, but have tried manually advancing and retarding the distributor while it is faltering, without being able to find a sweet spot where it smooths out. There is only one fuel filter, which is new.


MikeStem, The carb we swapped on is newly rebuilt and runs like a top on its engine. The coil is new, with an internal resistor, I have also tried an external resistor coil with a ballast resistor with no change. Have also tried a couple of different condensers. Yes, it idles consistantly and just falters at mid and high rev.

Shop Shark, Yes, it is an internal resistor coil, brand new.

Thanks guys, keep the ideas coming!
Tim

How about the exhaust system? Try this- - - - -put a vacuum gauge on the manifold, and try running the engine up to 2,000 RPM or so at no load, just accelerating in neutral. If the manifold vacuum drops, or won't rise above the point it was at idle, loosen the exhaust pipe to create a leak at the manifold flange and see if there's a change in the vacuum response. Also try a road test and look for a severe vacuum loss on acceleration. You've been concentrating on getting the air in- - - - -now try getting it out of the engine efficiently. I've seen exhaust restrictions cause your identical problems on several occasions.
Jerry

Edit: You can check for cam timing by watching the #6 cylinder rocker arms. Roll the engine over by hand and find the "overlap" point where the exhaust valve is closing and the intake is opening. That should happen very close to top dead center of the #1 and #6 pistons if the cam timing is right. That's also a good place to set the points to fire the #1 spark plug for initial timing.
Jerry

Clean and regap your plugs.

Hy Powerwagontim, I agree with Hotrod Lincoln, if everything else we have suggested has been checked and is okay, then look into an exhaust restriction as Hotrod has suggested, good luck.

As simple as this may seem, several years ago I had the same problem with a Corvair and like a dummy I forgot I had been messing with the point dwell adjustment. When I finally came to my senses and readjusted the points it ran like a bat outta hell. Just a thought.

The primary ignition system is a suspect for your trouble. First, check the points for correct gap- too wide a gap will limit the engine speed and will cause ignition mis-fires as the engine speed increases. Gap should be 0.016-0.019 inch. With a wide point gap and at low engine speed, there is sufficient time for the coil to saturate and develop a good spark; at higher engine speeds, the dwell time is too short for adequate coil saturation- misfiring and roughness can occur. Beyond that, run a wire from the battery to the coil positive terminal to see if the condition improves. Yes, look for resistence in the primary wiring between the battery and the coil. The ignition switch is a prime possibility. If you have an ammeter, the primary current should be about 4 amps, points closed, key on, engine stopped: low current- high resistance in the primary circuit. If you find low primary current, measure the voltage between the coil negative terminal and the engine ground- you should find about 0.2 volts, key on, points closed, engine stopped. If you find high voltage there, more than about 0.3 volts, the coil is not being properly grounded through the distributor. This leaves the interconnecting wire, the points and attachments, or the distributor is not being grounded properly through the case and attaching bracket. You could eliminate the distributor ground as a possible trouble if you hold a ground wire to the distributor case and engine/battery ground with the engine running- if the condition improves, the distributor is not being grounded properly.

The cam shaft is gear-driven in your engine, so slipping teeth is not a possibility. The compression doesn't indicate a worn lobe, although that remains a possiblity- just don't change cam shafts at this point; there are too many more likely possibilities for your trouble.

The vacuum check described by Hotrod Lincoln is a good check of exaust restriction, but from your description of the trouble, a vacuum gauge will be bouncing all over the range when the roughness starts, so I don't think you will get any useful information from that check.

Harvester

Many times a bad condensor can give those problems. I've seen new ones bad out of the box many times. Put the old one back in or try another new one.

Take off the distributor cap and with the ignition on, open and close the points with an insulated screwdriver. The spark should be sharp and crisp. If it "flashes" big and blue, it is most likely the condensor.

I just had the same problem with my 283 and it was the plugs which were brand new never been driven down the road and fouled out because I have not tuned the carb in properly. They were black and sooty when I took them out. Cleaned them and it ran like a champ.

Thanks for all of the responses so far.
As an update, I pulled the exhaust off the manifold, no change except a lot louder! While had it off I checked the heat riser. Naturally it was frozen, I got it freed up and in the open position, again no change, so as great an idea as it was, we can rule out an exhaust blockage.
I will follow Harvesters methodology and see if that helps root this out. I replaced the plugs, no change, replaced the condensor, no change, I have checked the insulation around the bolt that passes through the side of the dist, found it in good shape. The wiring harness is all brand new.
Thanks again Guys, the input is very appreciated!
Tim

Ok, this will be the stupidest response ever, but just because I have no shame:

I know on a small block chevy you can get the dist in 180 degrees off and it will run but run terrible. Is that possible on these ol trucks? Just thought I'd throw it out there tho I bet you have started with TDC about 5 times by now.

I'm not stopping at one stupid response...lets go for two.

I had a vehicle do that and the NEW fuel filter was installed with the flow in the wrong direction. That is something to check.

OK it's my turn to jump in on this thread, lol.

I had a similar problem with my fire engine, no power at high rpm. Mine turned out to be a loose main vacuum line that looked good until I bumped it and realized it wasn't clamped tight.

You could also check the vacuum advance with your timing light. Connect it up to #1 and rev the engine up, you should see your timing mark move away from the pointer by 20 degrees (about 1-1/2") or more. If no change, hook a vacuum test pump up to the vacuum advance linkage at the distrubuter and see if it moved the base plate the points are mounted on with the engine off.

Mike B

I agree with Mike B that timing could be your problem. It almost sounds to me like your centrifugal advance is not working. Your centrifugal advance bumps up the timing while the vacuum retards it. If your centrifugal advance has broken springs, it will not advance and your vacuum will retard it. Check the vacuum advance for movement as you rev it up a little. Then disconnect it and rev it up a little and see if it advances. If not, remove the breaker plate and see what's wrong.

Sounds to me like the timing is real slow or the fuel system is pluged up somewhere. Loosen the dist & advance the spark quite a bit & see what happens. If that doesn't do it I would start chasing down the fuel system. Don.t be concerned about vacuum advance at this point. It is for economy purposes & has little to do with driveability.

The only vacuum retard I've ever seen was on a late-60's Mustang with a 200 cubic inch 6-cylinder. It had a 2-chamber vacuum system that retarded the spark at idle to reduce hydrocarbons, and then provided a 10-degree increase over the centrifugal advance at light-throttle cruise. Anytime the throttle is opened to provide more power under load the spark is retarded, that's true, but at cruise, there's a considerable amount of advance over centrifugal for gas mileage.
Jerry

I think he has a 235 chev that would probably have a vacuum advance. If his centrifugal advance isn't working at all the vacuum will retard it even more when the vacuum goes up. He may have a distributor in it that wasn't designed to have a vacuum advance. Distributors for larger trucks didn't have a vacuum advance. Centrifugal only. If you put a vacuum advance on them, and it will fit, they will run like crap.

Potato up the 'ol gas pipe of the school bus Jerry? Mice making a home in the exhaust during down time in the shop. Not saying your shop has mice and all.

I vote for the timing chain if the engine has been apart since it was last driven.

This sounds confusing to me. If the springs are broken it seems it would advance to easy. Ive been working on them for 60 years & have never seen this happen. Also i have never seen vacuum retard spark except like Jerry said somw Fords back in the early 70's had for emmissions.

I don't ever remember a motor running like crap with the vac advance disconnected. During a tune up with the motor connected to a Sun machine many times I would plug the vac advance and check the mech. timing advance which meant bringing the R's up and it never produced any ill affects like popping or real crappy running.
Once that was done,reconnect the vac line and check the total timing advance the dist. put out.
I'm not saying the motor ran like it top during this test, but it didn't run like crap.
Dan

glasman, I think I did not make myself clear. I am suggesting that his centrifugal advance is not working...either broken springs or what have you. Whenever he sets his initial timing everything will be ok. However as he revs up the engine, it will not advance, and if the vacuum is working, it will retard it even more and it will run like crap. I have seen this quite often. In fact a lot of people don't realize they have an advance mechanism under the breaker plate. The effect is very similar to what used to happen when someone used a carb from a load o matic Y-block and put a 57 or later distributor in it. Also if his distributor he is using came from a larger truck, it will have been designed to be used without a vacuum. It wasn't uncommon for people to put a vacuum changer on them just because there was a line to the carb and they thought it needed one. It is difficult to say what all has been done to a 60 year old truck that doesn't have the original engine.

Distributers wear out, check the bushing by shaking the shaft and watching the points gap for any change, also ground the block to the frame if their isnt one there. Fuel pump pressure at higher RPM, T in a pressure gauge and a vacumn gauge and go for a ride. Bad thing is when you find it your will feel like a "Blooming Idiot" for overlooking something so simple. Good Luck, we have all been down that road a time or two, it;s called learning.

Yes it is called learning. I sure didn't know vacuum advance can retard spark, except in a few cases were they have dual diaphragms.

What condition are the spark plug wires in? I had one that would idle fine, run okay under light load, but would spit, sputter and everything else and decided for grins and giggles to replace the plug wires. Ran ike a champ after that.

Check the dwell on those points to see if the dwell is good at idle only to go bad as the engine speed goes up. The dwell will tell you alot. Make sure that during all the testing that your points didn't get burnt.

Sounds ignition related to me. If you have another distributor available I would try it.

Only other thing I can think of has been covered by you trying a different carb and having the same results.

plug wires crossed for stupid 3.

Um just a thought but have you checked the valve springs to see if they are not broken? I have seen engines run fine and sound great on idle but do the same thing on higher rpm's and have it turn out to be a broken valve spring. my understanding is that the spring weakens when it breaks and the valve can "float" at a lower rpm which would allow excessive amounts of gas to enter the exhaust and ignite.

Maybe even cracked plugs?

Hi Guys,
I havent abandoned you and your great input, just been busy with other things and leaving for 2 weeks this weekend. I cant even recall if I already posted this, but I did pull the exhaust and it didnt change it. I freed up the heat riser enough to move it and tried it in both extremes of its field of motion, no change. Looking up the exhaust with a mirror and a flashlight makes it hard to tell if it is open or closed. Anyone know where the flat spot of the counterweight on the forward side of the riser shaft should be when the riser is open?
I also verified that the #1 cyl is pointing at #1 wire at TDC. Plug wires were new a year ago, but the truck hasnt been used in a year so essentially new, plugs are new.
I think my next step is to go back to the beginning and re-tune and adjust everything. After that, I guess it is time to go internal and start looking at valve springs, timing etc.
You all have a great Holiday!
Thanks,
Tim

Try a new fuel pump.

Check the springs on the advance weights too.

The Fuel pump is brand new. I will be checking the advance weight springs when I tinker on it next.
Thanks,
Tim

The Fuel pump is brand new. I will be checking the advance weight springs when I tinker on it next.
Thanks,
Tim

Heat riser is open when the counter weight is rotated to the left (forward), or more vertical. Closed when the flat spot is horizontal to the ground. I'm sure that is right, but somebody correct me if not...that is the way my COE is and they are not identical, but very close.

Oh I just thought about it maybe the rocker arms are out of spec and need to be rechecked? loose lock nut would allow the set screw to back off and the valve clearance to increase which could be the case and it is a quick and sipmle check.

just asking but could she have sucked rust or dirt up the fuel line pick up .
i have i can that i fasten from the hood and let gravity feed the carb , then see what it does .
just a note the can is steel with a fuel bowel on it . its help to find problems .