Hard starting conundrum

[rhstewart]

I’m curious what people think about my engine starting problem.

I have a 1967 C20 I’ve been making run since summer 2022 when we picked it up from grandma’s driveway. Grandpa bought it new. The engine was replaced at some point. It’s a 350 block with a 283 intake manifold and ram horn exhaust manifolds. It has an HEI ignition system.

I’ve completely rewired the entire truck with AAW classic update. I replaced the rotor and cap. All the spark plugs were new a year ago. The spark plug wires are new. I recently checked the compression and that seemed fine (all good, one a bit low).

I replaced the carburetor with a new Holley 2 barrel. It seems to work fine. I’ve tuned it according to the manual. The vacuum advance is hooked up to the ported vacuum, which is what the manual says.

I’ve played with the timing. It’s now set at 8 degrees BTDC at idle with an idle around 600 rpm. I’ve had it at a faster idle but adjusted it back to the 1970 350 setting in the Haynes manual.

It starts pretty good with the manual choke all the way out then pushing it in slowly while I let it warm up patiently. It’ll get running and then drive fine. I haven’t driven old cars in a loooong time so my expectations may be off. Maybe it’s fine, maybe it should be better.

If I stop it for a short amount of time it starts up just fine.

My issue is this: if the engine stops while warming up or I try to start it after it’s cooled a bit it takes a lot of turning over to start up again. Pumping the accelerator sometimes helps. Holding the accelerator down sometimes helps. Pulling out the choke part or all the way sometimes helps. I can’t pin down what works but it eventually starts up.

What, exactly, is going on when a truck is cold and hard to start. Cold, thick oil, I assume. Anything else?

Why does choking the carburetor help? I know it increases the fuel ratio but why, exactly, does that matter?

Any other thoughts on my root issue?

I’ve considered replacing the distributor body. Possibly the vacuum or mechanical advance are not great? I know the mechanical advance does something, I can see that. I haven’t tested the vacuum advance.

I apologize for the length. Every time I typed something I thought of something that could be asked.

Thanks in advance!

[Getter-Done]

Is the heat riser still in place on the ram horn exhaust manifolds?

If so is it stuck in the closed position or does it operate freely?

[rhstewart]

Getter-Done - That's a good one. I'll have to look. I have no idea.

Reading up on that, I wouldn't be surprised if my intake manifold (original cast iron) could be plugged up with rust.

Are the hot gasses just supposed to pass close to the carburetor to heat things up or do the gasses actually get into the carburetor? I think I doubt it but I'm curious.

[RustyPile]

Well, for starters (no pun intended), you've got a hodge-podge of parts on that engine.. An intake manifold that's not intended for the engine it's on. It's possible you have some port mismatching going on there that could result in vacuum leaks.. A 2 barrel Holley that wasn't intended for that engine or manifold either. I assume some sort of adaptor was needed to make the carburetor fit that manifold. Again, this could be the source of vacuum leaks.. Regardless of what "the manual says" the vacuum for the vacuum advance mechanism should be from an intake source, not ported from the carburetor.. All this means that you'll have some issues and compromises to deal with..

Now let's get to the cause of the hard/no start condition.. Three items have to be present in the properly metered amounts.. Air, fuel and spark.. I call the following procedure the 4 whys.

Does it have spark, and does it have it at the correct time? A timing light will answer these questions.. Do this check while the engine is in "no start mode".. Connect the timing light and, while cranking the engine, point the light at the timing tab and check for spark.. If it has spark, does it occur at the proper time.. Adjust as needed.

If no spark is present, diagnose the ignition system as to why.. I won't go into the procedure here as it's a lengthy type.. Search for "diagnosing HEI issues"...

If spark is present as it should be, remove the air cleaner and while looking down the throat of the carburetor, move the throttle and check for gas spray from the accelerator pump. If spray is noted, the carburetor, even though it is "new", debris could have possibly found it's way into the metering passages. Rebuild/clean as necessary.

If no spray is noted, disconnect the fuel line at the carburetor and disable the ignition system. Using a suitable hose connected to the fuel line, and a suitable container, crank thee engine over.. Within a matter of 3 - 4 seconds, the fuel pump should pump out 8 - 10 ounces of fuel.. If fuel delivery is satisfactory, check for an obstruction between the carburetor and fuel pump including any fuel filters.

Suspect faulty fuel pump or obstruction between fuel pump and tank. If no obstructions are found, suspect faulty fuel pump..

Skipping any of the steps or assuming items to be good without checking, can lead to a wrong diagnosis.. As I stated in the beginning, this hodge-podge of mismatched parts, even though all items check good, could be the cause of the engine's "pickiness" in starting..

[Getter-Done]

Originally Posted by rhstewart

Getter-Done - That's a good one. I'll have to look. I have no idea.

Reading up on that, I wouldn't be surprised if my intake manifold (original cast iron) could be plugged up with rust.

Are the hot gasses just supposed to pass close to the carburetor to heat things up or do the gasses actually get into the carburetor? I think I doubt it but I'm curious.

Your intake Could/Can be plugged up.

That Holley carb probably does not match the ports for the Exhaust crossover on that intake.



Check the gasket match to (Both) intake and Holley Carb.

If they have any gap you may have a Vacuum leak causing your issue?

The exhaust crossover ports can indirectly affect the temperature around the carburetor base.

[mattfranklin]

Choke helps because only evaporated fuel burns. When the engine is cold, not enough wants to evaporate. When you restrict the air with the choke, excess liquid fuel is sucked in, so enough evaporates to start the engine. You taper off the choke as the engine warms. When the engine is hot everything evaporates so I can burn well.

In the winter they add extra light stuff, like butane, to the gas because it evaporates more quickly to help start. In the summer you don't want the light stuff because it evaporates from the tank, the hotter the day, the more is wasted. It's a waste of money and bad for the air. I think the butane from the refinery gets mixed in with LP in the summer for your BBQ, because it can't go in the gasoline and it has to go somewhere.

EDIT1: If you want to dig deeply into gasoline volatility, page 4 (adobe p12) of the file at this link really goes into it with the summer/winter volatility schedule. It's a pretty cool book. We used to send hardcopies to one of my professors at U of MN for their IC engines class. I don't know if we even have the print copies made anymore. https://www.chevron.com/-/media/chev...ech-review.pdf

[Dead Parrot]

The nice thing about SBC motors is parts interchangeability. Don't worry too much about a older 283 manifold on a 350 motor with HEI in a 67 truck that originally had a points distributor. As long as the assembler did a good job on sealing things up, it should work.

Do your checks for vacuum leaks. If equipped with vacuum boosted power brakes, the booster can be a source for a leak.

Quick distributor checks:
Vacuum pot - with your timing light in place and motor running at idle, remove the vacuum hose from the ported outlet. Timing should stay the same. Connect hose to manifold vacuum. Timing should advance 8~12 degrees(depending on vacuum pot, they come with various advance numbers). If timing doesn't change, replace the vacuum pot.
Mechanical advance - as above remove the vacuum hose and leave off. Rev the motor. Somewhere around 1200 to 2000, timing should start to advance. It will keep advancing as you speed up the motor until it reaches a maximum. If it doesn't advance or sticks when you let off throttle, repair mechanical advance.

If it passes the above checks, try moving the vacuum advance to manifold vacuum and readjust idle speed. Manifold vs ported can be a debate but try both. Pick the one that works best for your motor.

If your truck is emission exempt, try advancing base/initial timing to 10 or 12 BTC. Stock or mostly stock SBCs usually respond well to initial timing in the 10~14 range. If it begins having hard to crank while hot issues, back off a couple of degrees. Adjust idle speed after adjusting timing.

Once you get the timing dialed in, redo the idle mix and idle speed adjustments.

As long as you are using 10w-30(or 40), 'thick oil' shouldn't be a cold start issue unless you are well below 0 when starting.

[rhstewart]

Thanks for all the help. Here’s what I did…

Looked at the passenger side exhaust and there doesn’t seem to be a heat riser valve. I couldn’t see a counterweight or spring. There’s a spacer of some type but there doesn’t seem to be any external hardware. I live in the Pacific Northwest and I doubt that low temperatures are a big concern here.

I did RustyPile’s checks and found the accelerator pump was not working. The accelerator pump lever was not engaged with the bottom of the spring loaded bolt in the intermediate lever; it was off to the side. I took off the intermediate lever and bent the pump lever so it lined up with the bolt. I could hear the spray when I pulled the accelerator lever. I have no idea how that got that way.

I tried to start it again. It took a while to start like usual and did eventually start. While it was trying to start, the timing light wasn’t activated. Is it normal for the timing light to only be activated once it gets running faster and starts? Or should simply cranking the engine get the light going?

It eventually started.

Based on Dead Parrot’s advice, I checked around for vacuum leaks with starter fluid. Nothing jumped out at me. Maybe a small one where the booster is connected directly to the manifold. I sprayed it and it sped up a bit.

I checked the timing and it was where I set it; around 8 degrees BTDC. Pulling the vacuum pot off the ported vacuum there was no change. I connected to the manifold port and it advanced to about 26 degrees BTDC, which matched expectations. I left the hose connected to manifold vaccum for experimentation.

I let the engine cool for four or five hours so I could test it cold. Later in the day, I tightened the brake booster connection in the manifold. It started up with choke and stopped like normal but started again pretty quickly. That’s better than before. I assume that’s the accelerator pump working now.

I took the truck for a ride. Ran fine. Good even. I swapped the distributor vacuum back to the ported vacuum and drove for a short bit. That also seemed fine but I didn’t have time to go for a good ride.

Thinking about the vacuum checks I dug into the PCV system. Because of the 283 manifold on the 350 block, the crankcase ventilation is strange. The manifold has an oil filler neck but the block does not have the breather port at the back. Instead, the base of the carburetor is connected to a PCV valve (which seems to be working—it’s not stuck and I can feel the spring), which is connected by a long 1/2” tube to the oil filler cap. I disconnected the hose from the cap and stuck my finger in there while the truck was running. There was vacuum and the engine sped up a bunch. There was engine smog coming out of the oil filler cap. I think the cap is sealing and the PCV valve is successfully sucking gasses from under the manifold so it is doing what it is supposed to. How much vacuum gets lost from the PCV system operation?

Thanks again for the good ideas and help. We’ll see how it goes next weekend.

Cheers,
Rich

[geezer#99]

Ya know, pics of your carb , intake and pcv set up would really help.
Pop the air cleaner off and let’s see what you got.

[rhstewart]

Sorry for taking five days. I'm not with the truck during the week.

Look away...(hopefully the angles are useful)

[geezer#99]

Put more base timing in it. Try 14. That’ll help starting.
Your inline pcv might be ok depending on whether the cap on the oil fill tube is vented or not. If it’s vented the pcv won’t pull much fumes out. If it’s closed cap then you need a breather on one valve cover.

[rhstewart]

Thanks geezer I'll give it a try on the timing.

For the PCV system, I'm pretty sure it's not vented at the cap (I'll check again). What is the performance impact of that and no breather?

I'm inclined to do what I saw in another thread where I'll put a vented cap on the oil filler, get rid of that ugly hose and then make an adapter for the valve cover where the PCV valve can connect. You had to custom make an adapter for the old 283 valve covers (no breather or PCV ports) but I can handle that. I'll upgrade to a pair of nice orange scripted covers.

Cheers,
Rich

[geezer#99]

Problem with no pcv is crankcase pressure will blow out a gasket and you’ll get a bad oil leak.
New covers will work. Put the pcv valve on the hole that has a baffle in it.

[Dead Parrot]

Unless you are really set on those unvented chrome valve covers, pick up a set of 68 or later OEM SBC valve covers. They will have the proper holes for both the PCV valve and the matching air filter. (Should be easy to find in salvage yards) Replace the existing oil cap with an unvented version. My 68 Impala with a 327 had your type of intake manifold. It had an unvented oil fill cap and the valve covers I mentioned. The PCV system keeps much of the blow by crud and water out of the engine oil. Normal setup is the PCV valve is on one side and the air filter is on the other. Air goes in via the filter, across the bottom inside of the motor and out the PCV valve and into the carb base. Give the valve covers a nice paint job, install, and done. No ad-hoc fabrication needed.

Most street carbs are calibrated with the assumption there is a working PCV system installed.

[cadillac_al]

I was going to say if you are going to keep the old PCV system, at least use those pretty old style valve covers. I doubt the PCV is the problem. The problem is that carb is never going to start as reliably as a Qjet carb. It is probable going to have a little personality that you will eventually get used to. If it sits 30 minutes it might take 2 pumps of the gas and fire right up. A lot of vehicles start funny when they are between hot and cold. I little tweaking of the timing is sure to improve things and is worth experimenting with. If you can access the idle mixture screws they may need some tweaking too.