Intermittent power loss (need help troubleshooting please)

[trevarthan]

Originally Posted by 68 P.O.S.

What are your plugs gapped at? If you don't already know, HEI gap is .045

I replaced them yesterday with platinum plugs gapped at 0.035, I think. Will this cause problems with the HEI or is it fine? Can't regap platinum without chipping it, I've been told, but I can always toss them and buy regular plugs if those won't work.

[trevarthan]

Originally Posted by RichardJ

That missing nipple would certainly have allowed dirt and water to get into the float bowl so you may need to rebuild the carb. I am not certain if or how much that open hose would have upset the vacuum in the charcoal canister.

I've started rebuilding the carb today. Appears to be a Rochester Monojet.

Looks to me like that port is plugged inside the carb, so I doubt it was a problem:








At the time I wasn't sure what any of this stuff was or did, so I was a little overwhelmed, but I've watched a rebuild video or two now so I'm feeling a little better about it.

One thing we noticed was the that venturi tube looks battered for some reason:



Also, the rebuild videos say it's a good idea to replace the float because the old plastic floats can start absorbing fuel which changes their buoyancy. My rebuild kit doesn't come with a float, so I'll probably order this brass one: https://www.carburetor-parts.com/Roc...oat_p_403.html

Full album pics of disassembly, if curious: https://imgur.com/a/cmJFOoL

Time to clean it.

[RichardJ]

There are dozens of versions of the MonoJet so this may help or may simply make it more confusing.

http://www.carburetor-parts.com/asse...ice_manual.pdf

Not plugged, that's a bowl vent. Even with your version of the Vent Valve there should still be a hose nipple.
Have you found the end of the third hose coming from the charcoal canister?

I usually check floats by simply dropping them in a tuna can full of gasoline.

You can test a fuel pump off the engine. I'm not going to tell you it is as good a test as an actual running test with fuel. Mount the pump in a vice and plumb a 0-15 psi gauge to the pump outlet. Operate the pump lever with a vice-grip pliers. It should show 4 - 4 1/2 and the pressure should not drop between strokes. A finger placed over the inlet will show a continuous suction.
This test obviously won't show a leak, but can eliminate a pump with bad valves.

I replaced my 6 cyl pump last Aug. and posted this on an ongoing L6 pump thread.
The old pumps i have pulled off had a tab riveted to the pump arm. I believe it to be a sacrificial metal to prevent wear on the cam, pump lobe. The new Carter pumps don't have the tab. I spent time finding out that Airtex pumps still have the tab. i spent more time and gas getting an Airtex from the Truck Shop. I tested it before installation and found that the pressure dropped between successive pump strokes.

I then bought a Carter from NAPA. It pumped to 4 psi on the first stroke and stayed there on every successive pump cycle.

[trevarthan]

Originally Posted by RichardJ

Not plugged, that's a bowl vent. Even with your version of the Vent Valve there should still be a hose nipple.
Have you found the end of the third hose coming from the charcoal canister?

Hmm. I'll have to look at that vent hole again when I get it out of the parts bath.

I replied earlier regarding the third hose, but I guess this thread is getting long and it's easy to miss. This is the charcoal canister hose layout:

1. to PCV valve
2. to carb on valve cover side
3. to gas tank

There is no vacuum hose free and flopping around for that port in the carb. If anyone can tell me where it should be going I'll be happy to correct it when I put the carb back on.

[Sparky dave]

Yes a fuel pressure gauge t into the fuel pipe just before the carb. It should hold a steady pressure if it jumps about it I struggling to keep up with your engine. Also some pressure regulators come with gauges which would be even better. It can flow with an open pipe ( squirt) but doesn't mean you have correct working pressure when it's connected in. Carbs are sensitive to pressure. If you don't you could be playing with your carb for years and you will never get it right. Interesting post this one!

[Sparky dave]

This is the type of pressure regulator I have ( in the left hand side of the pic) attached to the bulk head. It has a filter in a glass bowl and a gauge on top. Not saying mine is the best it's just what I could get in the uk. You guys have much smaller tidier units available to you, however someone else will have to chime in and tell you where you will find one

[trevarthan]

I'd like to keep a hard line from the pump to the carb, so I guess I'm going to be buying steel pipe bending and flaring tools.

I can't even find an OEM replacement hard line from the pump to carb for this year/engine, so I guess that's really my best bet moving forward as I've already rounded off the b-nut on the carb side.

I'm still cleaning the carb. Think I've got it mostly clean at this point. When my new float arrives I'll take some photos/video and start assembling it.

I'm still not sure what to do about that float vent port. There's no nipple. I'm guessing plugging it is a bad plan too, eh?

[Grumpy old man]

Have you tried Inline tube for the fuel line ?

[trevarthan]

Originally Posted by Grumpy old man

Have you tried Inline tube for the fuel line ?

I hadn't. Looks like this one might fit: https://www.inlinetube.com/products/CTC7105

Thanks for that. Now if only it came with a fuel pressure gauge.

[Sparky dave]

Are you back on the road yet?

[trevarthan]

Originally Posted by Sparky dave

Are you back on the road yet?

Unfortunately, no.

To recap:

After we changed the fuel pump the engine immediately went from running for minutes to not running for more than a handful of seconds.

We removed the carb.

This was the carb immediately after removing from the vehicle: https://youtu.be/n1xsBgTyjTo

We disassembled the carb. https://imgur.com/gallery/cmJFOoL

Ok, and here's what's happened since the last post:

Immediately upon disassembly we noticed the float sticking. I wish I had filmed that, but you'll have to take my word on it.

I cleaned the parts using a combination of a carb cleaning chemical bucket kit from autozone and an ultrasonic cleaner from harbor freight: https://imgur.com/gallery/r3XIL8I

My son rebuilt the carb (I gave him this truck and he's been driving it for 6 months, so he's the other person when I say "we"). This is what it looked like after I got it back from him: https://imgur.com/gallery/JDuFs9x

Here's a quick video of the new float moving without restriction: https://youtu.be/z3OAUSM7gwg

A few of the bolts on the top part of the carb (the long bolts) were stripped in the bore:




So I replaced those bolts with longer bolts where the bore had some extra room. I actually got creative here and bought longer bolts from ace hardware then cut them slightly shorter with a hacksaw so they would seat properly.

We tapped the air cleaner bracket bolts to a larger size because they were stripped as well:



I didn't get to bench test the carb as much as I should have. I may take it off and do a fuel pressure test soon. I bought a positive and negative vacuum hand pump for this purpose. It'll arrive in a few days. Also, I was watching a motorcycle carb rebuild video today and learned I can do something similar using a gravity fed bottle of gas. I may try both.

I installed the rebuilt carb today: https://imgur.com/gallery/A527iUr

I noticed the gaskets became wet with fuel pretty much immediately:





I've got the screws as tight as I can get them. I'm using a GP Sorensen gasket kit from autozone. Not sure if these are just cheap gaskets or if there is some other problem.

The engine still does the same thing. It runs for a handful of seconds and then dies. I can't even keep it running long enough to set the mixture screw properly.

I think tomorrow I'm going to try to figure out how to temporarily replace the steel fuel line between the carb and the pump with a soft line with an inline pressure gauge. I need to know if the pump is making too much pressure or if we just screwed up the rebuild. It seems like it's flooding to me.

[HO455]

HO's first rule of trouble shooting says "Always look at the last thing you screwed with first". From your last post the symptoms would lead me to check that all your gaskets in the carburetor are the correct ones. There are many variations.

[geezer#99]

There’s a float height and float drop spec.
What’s the spec for float drop setting?
Looks like yours is set way too low.

[Sparky dave]

Carb looks really nice now it’s clean. If your struggling to make the gaskets seal maybe try a bit of rtv sealant on there as well. It’s fuel and oil resistant. Ps you say you think it’s still flooding and cutting out, if you pull a plug or 2 and it’s wet with fuel you will know for sure. Looks like your making good progress

[trevarthan]

Ok, fuel pressure gauge arrived this week and I finally had a moment to install it temporarily so I could see the fuel pressure during a stall. Seems to be holding perfectly steady at 4.5 PSI. I made a quick video showing the pressure as it stalls:

https://youtu.be/lvZox3oGsLQ

Doing some quick research, it looks like 4.5 to 6 PSI is normal for the L6 with a Rochester Monojet, so I think we can finally rule out fuel pressure as an issue.

Moving on the pressure testing the carb, I guess.

[trevarthan]

Pulled the #3 spark plug. This is a brand new platinum plug. It's only been in there for a few weeks and it's been in the garage the whole time because it won't run more than 20 seconds.

I'm not really sure how to read it. Looks like it's running rich to me, and maybe leaking oil? I mean, obviously everything is leaking oil, but I'm not sure if it's a problem or not. It's always leaked oil.





What I don't see is it being wet with gas. I pulled it immediately after a stall.

I'm not sure what to do next? Compression test? Rule that out?

[trevarthan]

I had a stroke of bad luck or good luck today, depending on how you look at it. I left the keys in the ignition with the ignition on after it stalled. Then I went to make myself a sandwich and forgot about it. When I went back down to the garage I smelled plastic burning. When I went to investigate I found I had burned a hole in the coil:



Autozone was kind enough to replace the coil and cap under warranty since I bought one just a few weeks ago.

I took the opportunity to do a compression test. All six cylinders are reading about 125 PSI dry. I didn't bother doing a wet test because that looked fine to me. My Haynes manual says 130 PSI is the spec, but this is an old engine and I didn't see any cylinders drastically different from the others.

To my surprise, when I put all the plugs back in and put the distributor cap and coil back on, it started right up and ran without stalling.

So, either my previous newly bought coil was bad, or there is a short in the wiring loom somewhere around the spark plugs, because I moved things around and it works now.

The carb is still leaking fuel pretty badly. Rather than attempt to buy new gaskets of higher quality and check all of the specs, I think I'm just going to buy a remanufactured carb. If that stops the fuel leaking and the engine keeps running, I'll call it done and be happy.

[Sparky dave]

That’s brilliant news! I used to know a guy that when he had an elctrical fault he would replace the fuse with a nail, then he said whatever sets on fire is where the fault is ha ha. Bet your glad to see the end of this chapter! On the road again soon then I hope

[trevarthan]

Ok, we figured out the cause of the leak in the carb.

We disassembled it and inspected everything. The float height was fine. We even tested it by gravity feeding gas into the carb and watching it shut off properly.

When we put the little T piece back in it was a little high and may have caused the gasket to leak. In addition, I screwed up the tightening procedure, so that may have caused a leak too.

Anyway, we put it all back together and the carb is no longer leaking (yes!) and the engine hasn't stalled yet (PROGRESS!).

I did my best to set the fuel mixture and idle speed using a laser tach and a vacuum gauge for the mixture on manifold vacuum. I *think* I got it right, but I've never done this before so I'm not sure.

It now appears to be misfiring under load. Here's a video of it: https://youtu.be/257T5fHbnE4

Is it possible this is caused by incorrect fuel mixture? Or is this definitely a spark issue?

[trevarthan]

We removed the distributor cap this morning and discovered the screws from the coil were poking through the cap and there was some minor arcing trails to the screws, so we returned the cap for the third time and this time I bought a higher quality coil from OReilly. I bought the duralast gold cap from Autozone (also higher quality, but still Autozone brand).

The higher quality coil from OReilly came with shorter screws. I think that was the problem with the Autozone cheapo coil. The screws were too long.

After installing all of that, it still was missing and stuttering when revving it and under load.

At this point I repeated the mixture tuning procedure (using a vacuum gauge on the manifold vacuum port), but with the engine cold and with the air cleaner installed. The first time I did the procedure with the engine warm and the air cleaner removed.

I found the perfect mixture setting was leaner this time.

We revved it and the miss or stutter was gone. We drove it and it was still there under load, but greatly reduced.

So I leaned it another quarter turn. It improved again.

Another quarter turn and it was almost perfect.

My theory is that the platinum plugs we installed with smaller gaps have a shorter spark so they have a little trouble igniting the fuel. I'm probably going to install regular non-platinum plugs and gap them larger and see if that allows me to run closer to the ideal mixture with the air cleaner on.

Anyway, it's running! Not stalling. Power is good. Maybe just a hint of hesitation.

Yes, we rebuilt the carb. However, I think the original problem was a bad coil. We replaced it numerous times, but it was never quite right and I think we damaged it the first couple of times by leaving the ignition on after a stall, in ignorance.

[68 P.O.S.]

Glad you got it mostly figured out. What a pain. Make sure you gap your plugs to 0.045 for HEI. The hesitation you still feel could be the step-up springs or the accelerator pump. How much vacuum does the engine pull at idle?

[trevarthan]

Originally Posted by 68 P.O.S.

Glad you got it mostly figured out. What a pain. Make sure you gap your plugs to 0.045 for HEI. The hesitation you still feel could be the step-up springs or the accelerator pump. How much vacuum does the engine pull at idle?

Installed new crappy $16 plugs properly gapped at 0.045 and retuned the mixture with the air cleaner on.

When we pulled the platinum plugs they were covered in black soot, so definitely it was running rich. At that point the mixture was dialed to 2 turns from all the way in.

After installing the properly gapped cheapo plugs I retuned the mixture using the vacuum gauge and it seemed to want about 1.5 turns from all the way in. So about a half turn leaner than the previous setting.

It was pulling about 50 cm Hg vacuum on my cheap harbor freight vacuum gauge. If I leaned it another quarter turn it would dip. If I made it a quarter turn richer nothing would happen, so I left it right there.

Seems to be running quite well. The engine was still a little warm when we tested it, so we'll see how it runs cold, but I'm thinking it's a pretty good tune now.

[Sparky dave]

Just looked at your YouTube video it's nice to see her running. If your ever in doubt about a misfire you could use a timing light on one plug wire at a time so you can visually see if it's firing or not, some people also use a temperature gun on the headers to look for a cold cylinder. Sorry if I'm telling you how to suck eggs just keeping the ideas flowing. Nice truck, take care

[RichardJ]

>>If I leaned it another quarter turn it would dip. If I made it a quarter turn richer nothing would happen, so I left it right there.<<

Perfect.

>>50 cm Hg vacuum <<

So 19 - 20 In Hg for the rest of us.

[trevarthan]

The Saga continues. We rebuilt the carb. It worked for a few days, then it started stalling again. No amount of tuning would fix it. So we bought a remanufactured carb and tuned that. Still stalling.

I started suspecting it wasn't the carb at this point.

I was going through every vacuum test I could think of when it did this on the exhaust restriction test: https://youtu.be/Z_I_cT5yRos

Is it possible this has been an exhaust restriction the whole time?