The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 Starting Issue

I hate to even start a thread about this, but I'm at a total loss.

My truck will start and run about 20% of the time. When it starts it runs alright, not good, but alright. It has some hesitation on acceleration, but you can drive it. My problem is the other 80% of the time the truck won't run at all. No sputtering or even any sign of firing. It has new plugs, wires, cap, rotor, points and condenser.

I assume it's getting spark and fuel because I've had it running a number of times. I also assume it has compression.
Maybe i could run new wires to the coil, ignition switch etc?

Maybe my assumptions are my problem, but I can figure out where to start.

Any help would be greatly appreciated!

It is time to familiarize yourself with the basic tests required to verify that all of the following are functioning correctly.
1) Adequate spark at the correct time. A multi-meter would be needed for testing the voltage coming from the ignition switch to the coil. Clean the points and set them at .018 if they are used, .022 if new.
2) Compression. It is best to do this with a fully warmed up engine if possible. I cold compression test is only useful for finding an extremely weak cylinder (faulty valve/broken or stuck rings).
3) Delivery of adequate air/fuel. This would include checking for obstructions in gas tank and fuel lines, weak fuel pump, dirty carburetor, vacuum leaks.
I strongly suggest that you make these tests in the order listed.
Keep us posted on your progress and if you have any specific questions along the way, ask away. Some of us never sleep.
Carl

1) I'll check the points later this week, what do you recommend to clean them?

3) I should have mentioned that I flushed the gas tank, replaced the fuel pump and cleaned the carb. I'll swap out the fuel filter i added as well to eliminate it as a possibility.

Thanks for the quick info!

Pass a business card through the points several times to “burnish” them. Then set your gap. Once you have completed ALL of Carl’s #1 move on to #2.🛠

I replaced the points and condenser, and it lives!
It has started up three times with out a problem now. The next step is to solve the hesitation.
If you accelerate very slowly it will rev up fine, but if you accelerate what i would consider to be "normally" it will choke out.

Valve adjustment was one of the next things on my list. If you guys agree, I'll get on it!

Kit the carburetor and I bet the hesitation will go away. Others will disagree.🛠

I went through the carb a few months ago, I'm going to try to tune it now that it's running better.

If the carb is good then make sure the vacuum advance is working properly. Just watch the arm move when you rev the engine. Should advance then return to normal as the speed increases and the vacuum drops.

If the advance mechanism is working properly then, suggestion:

First set the point dwell with a dwell meter, not a thickness gauge
Then set the timing

And buy a spare condenser for the glove box

Jon.

seat the carb accelerator pump check valve ball. tap with a punch to seat it.

Hesitation issue can be a PITA. Start with what has been suggested. Use a dwell meter for the best setting of points(most have a tach function as well). The dwell angle has to do with coil saturation. Fancy ain't it? That just means the spark will be fully developed when it comes. When all else fails,that means you have done all of the suggested things, then you can try these. 1, set the air mix screw with a vacuum gage. Use the port on the intake manifold under the carb. Often used to run the wipers. Set the idle to about 500 rpm, and adjust the air bleed screw for max vacuum. With the timing set close to right, blip the throttle. Does it bog? No, you are done. Yay you. It still bogs? Yup? Well there is one more thing to do. After the accelerator pump has been addressed. You still have, the metering piston. Yeah, that thing. It should be free and move the ball off the seat. If it is sticky in the bore. Then remove it. Place into a drill chuck, thin end in the chuck. Get some 600 or finer wet or dry sandpaper and polish it. It is brass and you will have 1 pretty part on the truck. Check the check ball. it should move and reseat with just a bit of spring pressure. Reassemble the carb. Try again. It got better right? There you have it. Do not bend the skinny end of the piston, just make it(the piston end) shiney.

I pulled the top end of the carb off and the accelerator pump piston is pretty tight in the bore. So much so that it doesn't move when you step on the gas.... That seems like an issue.
The problem is that the piston is some sort of silicon or rubber so I cant sand it down to size.
As for the distributor advance, mine is the centrifugal type and my manual doesn't explain how to test it. I think the accelerator pump should be the first thing I address though.

Should be a Zenith carb, with a round riveted tag. The inner number on the tag is what you need for a rebuilding kit.

Jon.

I made a new brass plunger with a pin and some brass stock I had. It fits much better than the silicone one.
I still get the hesitation though. Is spring pressure the only thing that drives the accelerator pump piston down? It seems like it should have a more solid connection.

I'm tempted to get get a new, higher quality carb rebuild kit.
I think i will get a dwell meter and check that first though. Is there a good resource on how to check dwell? I've never done it and i don't really know what kind of reading to look for.

You can do a crude test of the mechanical advance in the distributor by removing the cap and grasp the rotor and turn it by hand. You should feel the counter weight spring tension as you turn the rotor, and it should snap all of the way back when you release it.
Generally, that should be done before one digs into a carburetor. No matter in your case, since you discovered an issue with your carburetor. I still recommend checking everything related to spark timing, before doing any more carburetor work.
Carl

I tried your test and it turns a small amount and snaps back to where it started. Seems to me to be working?

I pickup up a dwell meter and with the gap set with a feeler gauge my angle was about 28.
I'm surprised it was running, but it is now set at about 41.5, right in the middle of the recommended 38-45.
The hesitation seems to be completely gone!

After I was done setting the dwell I decided to try the tach function on my new meter. It read around 350 and it says to multiply that number by 10 which would put me at a 3500 rpm idle.
The idle sound slightly fast to me, but there is no way its at 3500. I'm on the six cylinder setting, am i missing something?

My next step is to try and get the carb tuned properly. the needles seem to be a little out of wack.
As always, advice from people wiser then me is much appreciated.

Thanks for all the help everyone!

Is your motor supposed to have a vacuum advance? Most do but I am sure there are exceptions depending on application. If your application calls for a vacuum advance, which is a separate system that accompanies the internal mechanical advance, you will want to get your hands on a replacement and install one. That could also be one of your issues moving from idle to off idle. What year Is the motor and what application are you using it in?

I don't think it originally came with one. In my manual it says that models 280-22 and up have centrifugal advance. Mine is a 1954 GMC 353-27 according to my ID plate in the door. The only odd thing is that my manual doesn't list the 350-27 as an option. It only has the 350-24. I'm not sure if the engine is original either. The engine serial number is 270I029665 if that helps at all. I think the character between the zeros is a capital i, not a one. I also replaced the fuel pump a while back and it is the type with no vacuum pump built in. As far as i can tell the only things that use vacuum are the brakes, wipers and rear axle shifter.

As an update to this, I've now adjusted the valves and messed around with the carb a bit more.
I'm still getting the hesitation when i try to accelerate quickly. I still think the accelerator pump piston could be the problem because it doesn't really move in the cylinder.
How much force should it take to move the piston in the cylinder?

Since i did the valves, it also has what sounds like a small back fire under quick acceleration. Just one pop and it doesn't do it if you rev up slowly.
That seems more like a timing issue to me?

I'm going to double check the timing and dwell tonight, now that the valves are set.

I'm on the verge of LS swaping it and calling it a day.

Retarded ignition timing will cause a hesitation similar to a weak accelerator pump but it's way easier to fix. Check the ign. timing as per the book at whatever RPM it specifies AFTER you set the points making sure there is no mound built up on one of them. If ignition point files are still available, buy one and dress the points down before you check the gap. Also as someone mentioned, note whether the vacuum advance works -- the distributor will move and hold when you blip the throttle.

It the timing is retarded for whatever reason you will overheat the exhaust manifold(s) and fry the gaskets, which will result in nasty and annoying exhaust leaks and all the hassles associated with replacing the gaskets -- like broken studs.

Quite a few people have mentioned vacuum advance, but my distributor doesn't seem to have any vacuum ports. I did some research and it looks like the vacuum advance diaphragm is a separate unit. Is it possible mine was removed for some reason and never replaced? I also don't have any spare ports on my intake to feed a vacuum advance.

What size truck are we discussing here? Lots of the bigger GMC engines did not have a vacuum advance, as its only function is to improve fuel economy at highway speeds, and gas mileage on a big rig can best be described as "slim to none". If your truck is bigger than maybe a 1 ton, there probably won't be any vacuum advance to worry about. Use a delay-flash timing light and set the maximum advance at 2500-3000 RPM at around 35-38 degrees. Let the initial advance fall wherever it may, as long as there's no difficulty starting. 30-somethuing years of doing dyno runs on all makes and models of engines has convinced me that unless there are very special circumstances like radical racing cams and/or high compression involved, full throttle spark advance in the mid to high 30's works on virtually all engines.
Jerry

Hotrod, Its a GMC 350 2 ton with a 270. I'll check the timing tonight and see where its at if it will start for me.
Last time I messed with timing, I ran out of adjustment on the distributor. So it's set as far advanced as the distributor will go. Could it be off a tooth?

No such thing as off a tooth on these engines. You can loosen the bolt and spin the distributor back or forward 1/6th of a turn, then pull the spark plug wires off and move them one tower clockwise or counter-clockwise on the distributor cap. But first, I would turn the engine over until you have the #1 piston at TDC. After you have that, put the #1 plug wire on the tower which the rotor is pointing at and put the rest of the wires on in the proper firing order.

There is a pinch-type clamp at the bottom of the distributor housing secured with a 1/4-20 machine screw (AKA "Stove Bolt") that can be loosened, which will allow you to turn the housing in a complete circle. Start out by centering the small slot where the distributor bolts to the block, lock it down, and use the pinch clamp to adjust the timing.
Jerry

So after adjusting the timing, the truck seems to be running pretty well. I’ll have to see if it continues to start over the next few weeks.
Through all if this testing I’ve discovered I have a charging or battery issue as well, so that’s my next project.
Thanks for the help as always!

I once was charged with the care and feeding of a fleet of old iron for a private school. Among the buses, lawn mowers and a couple of tractors was a 52 GMC 350 or 450 dump truck powered by a 270 that went to the garbage dump once a week. It must have been the first digital truck -- the throttle was either wide open (1) or idling (0). The only time your foot wasn't on the floor was when you shifted gears (there were 10). It was slow and the roads were hilly. It had no vacuum advance. It had no gas mileage either. But it did have vacuum wipers. We tried not to go to the dump in the rain.

Remember these old things had positive-ground 6-volt electrical systems. That has tripped up more than one wrench turner, me included. Make sure the battery polarity is correct and if not after you correct that, you may have to re-polarize the generator and regulator.