Lack of power in 4th

Been battling issues with my truck, and have been down many rabbit holes. The upside is I have learned a ton and fixed a ton of stuff, but I'm still struggling to maintain 30 MPH in 4th gear with the original 230.

Led me to find a takeout engine locally, priced right. I looked it over and the guy says he's done messing with it, hes just putting a diesel in. I ask what his issue is with the motor and low and behold, it struggles to maintain speed in 4th.

For reference, both engines still have the original Holley carb, and have the civilian distributor swapped in, still running points, 12 volt converted. I haven't investigated as to whether or not he had the vacuum advance hooked up, but I've noticed little difference with or without in my testing. My carb was freshly rebuilt

Has anyone else fought this battle and actually won? I have a pertronix kit coming to eliminate the points, but I guess part of me is questioning the carb, so I bought a NOS carb kit as well. I'm wondering if the power valve in the carb is malfunctioning when the motor is lugging.

I have the stuff to swap the engine for a chevy v8 but I feel that its a disservice to this old truck.

What is the compression of each cylinder? If they are good then look for issues like timing due to lack of vacuum advance, or fuel starvation (blocked fuel line, miss-adjusted carb) or an air restriction (blocked or dirty air filter). Also check to make sure the throttle cable is not kinked and is opening the carb fully. I had this issue once and it acted like the truck was down on top end power. Once I found the bad cable and replaced it, the truck ran like normal.

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Originally Posted by Beast

What is the compression of each cylinder? If they are good then look for issues like timing due to lack of vacuum advance, or fuel starvation (blocked fuel line, miss-adjusted carb) or an air restriction (blocked or dirty air filter). Also check to make sure the throttle cable is not kinked and is opening the carb fully. I had this issue once and it acted like the truck was down on top end power. Once I found the bad cable and replaced it, the truck ran like normal.

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Thanks for the quick reply.

Front 5 cylinders has decent compression~120-130, cyl 6 is ~90.

I've messed with the timing a ton, although I haven't verified the mechanical advance is functioning properly, I'll add that to my list. The original igniter didn't have a vacuum advance, I figured the distributor would be fine with it no hooked up.

I have some fuel line concerns from a crummy tank, I ought to toss my boat tank in and try to see if that eliminates the issue.

Throttle cable kinking is new to me, I's definitely have to check that out.

There was a member with the same problem many years ago...what he found eventually was that the leaf spring like tensioner arm on the points set was in the wrong place and wasnt providing the correct tension on the points...basically it was on tghe wrong side of the mounting bracket and that was enough to keep the pressure down enough that as the load got higher, the points werent keeping up.

In the following image, it is the flat metal plate that runs from the points past the arrow of #3 to the mount bolt that is in the line to the #4 arrow....hopefully that is clear...
http://www.m715zone.com/vb/pages/manuals/20/fig2-35.jpg

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Originally Posted by brute4c

There was a member with the same problem many years ago...what he found eventually was that the leaf spring like tensioner arm on the points set was in the wrong place and wasnt providing the correct tension on the points...basically it was on tghe wrong side of the mounting bracket and that was enough to keep the pressure down enough that as the load got higher, the points werent keeping up.

In the following image, it is the flat metal plate that runs from the points past the arrow of #3 to the mount bolt that is in the line to the #4 arrow....hopefully that is clear...
http://www.m715zone.com/vb/pages/manuals/20/fig2-35.jpg

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Not to contest an apparent success, Jon, but wouldn't that problem manifest itself at the same RPM in any gear? After all, it's the engine that's being made to work under a load, not the point spring (as I understand it).

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Originally Posted by brute4c

There was a member with the same problem many years ago...what he found eventually was that the leaf spring like tensioner arm on the points set was in the wrong place and wasnt providing the correct tension on the points...basically it was on tghe wrong side of the mounting bracket and that was enough to keep the pressure down enough that as the load got higher, the points werent keeping up.

In the following image, it is the flat metal plate that runs from the points past the arrow of #3 to the mount bolt that is in the line to the #4 arrow....hopefully that is clear...
http://www.m715zone.com/vb/pages/manuals/20/fig2-35.jpg

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Interesting. Thats the kind of experience I love about forums, thank you.

I've been into the ignition and rebuilt it all, so the probability of the spring being setup incorrectly is pretty high. I'll know for sure when I get the Pertronix points eliminator in.

Nailhead: my thinking is that the spring not applying enough tension would contribute to a weak spark, which the engine would run ok with no load, add significant load and the engine wont run as well on the weak spark. Add higher RPM and I'd bet the spring would "float" like valve springs if there was inadequate tension.

Parts should be in today, I'll let you know how it goes.

The ignitor also has a mechanical advance using springs and centrifugal weights. There is no vacuum advance. if the mechanical advance is frozen or sticky that would also cause a problem as the throttle is opened.

Kwai, he has a civvy distributor swapped in...

Farmer, that is right...he had enough spark in the lower gears to sustain the motor but when he got in high gear, it couldnt keep up and fell apart...I think I can find the old thread if I look enough...I remember which member it was...

I found the final post where the member updated us after finding and fixing the issue...sorry I cant find the whole posting list on it but I remember it was very similiar to what you report...just to give you a hint...this is from November 9, 1998...waaaaay back in the old days...

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Hello everyone. For those of you who have been watching my dilemma of intermittant spark, I wish to relay to you I found and solved the problem. Many of you kept suggesting "point float", which to me seemed logical, so today I decided to pull the dist. on my truck and have a closer look. I removed the dist. and tore it apart looking for a problem. Upon closer examination of my point set-up I noticed the "tensioner" for the points to be outside the brass contact leading to the points (hope you know what I am talking about), rather then behind the points (which would add MUCH more tension to the points). Very little tension to the point = "point float". I installed the metal tensioner into the PROPER location, reinstalled the dist. and lo and behold no more intermittant spark. What we have here is a classic case of rectal-cranial inversion on my part. The truck now runs up to VERY high RPM without any problem. To top things off today, after I installed the points properly it was time to road test it. So off I go and 2 blocks from home it dies, I cuss, I check out everything, tow the truck home to discover it is out of fuel. Refill, start and smooth as glass she is. That was my day up till this point, and I still have to go to work. How was your day? :-(

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On top of the page is the Basic training section. There under faq is some good info that may help with your trucks problem. Good reading at the very least. Good luck . Scott

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Originally Posted by brute4c

I found the final post where the member updated us after finding and fixing the issue...sorry I cant find the whole posting list on it but I remember it was very similiar to what you report...just to give you a hint...this is from November 9, 1998...waaaaay back in the old days...

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His experience is very similar to mine, right down to running out of fuel!

I spent some time studying the points Friday, they haven't been contacting squarely but the looks of it, but the spring tension seems plenty. I replaced the components a year or 2 ago, and the new spring is riveted to the point arm.

I checked out the spare distributor in the motor I just bought, older style spring and arm, very loose tension, which give me some hope for that motor if needed.

I installed the pertronix Friday night and test started it, ran great until it ran out of gas. I plan on pulling the tank, so I didn't refill it. Saturday I got the boat fuel tank out of one of my project trucks, and threw it on the M715, which should help rule out a fueling concern I had as well. Disconnected the vacuum advance, and set the timing to 5* reconnected the vacuum advance and tuned the carb to be happy. Sounds pretty good, I just haven't had a chance to road test. I've been doing my research on vacuum advance being hooked to manifold vacuum and believe I have it set correctly for that, so I'll try it. If not, then I'll disconnect the vacuum advance and try without.

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Originally Posted by farmer

His experience is very similar to mine, right down to running out of fuel!

I spent some time studying the points Friday, they haven't been contacting squarely but the looks of it, but the spring tension seems plenty. I replaced the components a year or 2 ago, and the new spring is riveted to the point arm.

I checked out the spare distributor in the motor I just bought, older style spring and arm, very loose tension, which give me some hope for that motor if needed.

I installed the pertronix Friday night and test started it, ran great until it ran out of gas. I plan on pulling the tank, so I didn't refill it. Saturday I got the boat fuel tank out of one of my project trucks, and threw it on the M715, which should help rule out a fueling concern I had as well. Disconnected the vacuum advance, and set the timing to 5* reconnected the vacuum advance and tuned the carb to be happy. Sounds pretty good, I just haven't had a chance to road test. I've been doing my research on vacuum advance being hooked to manifold vacuum and believe I have it set correctly for that, so I'll try it. If not, then I'll disconnect the vacuum advance and try without.

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I tried mine without, and it didn't like it very much: it would stumble when I stepped on the throttle. VA hooked up eliminated that.

To muddy the water even more I pulled out a vacuum gauge and attempted to diagnose the engine problems and set the timing via vacuum.

Truck warmed up, vacuum advance disconnected, timing right at 5 degrees I had around 11" hg, though it was fluttering. Loosened the distributor and advanced it til it read 16-17" hg and retarded to 15-16" hg, still fluttering. Snap the throttle it dives to 0, recovers to 20, and levels out back 15-16. Supposedly that means rings are shot. Checked the timing with a light, and without timing marks it looked to be at least 20-30 degrees, which seems absurd.

Finally test drove it like that, after hooking up the vacuum advance. Still struggling up a slight grade to maintain 30 mph. Ran about 5 mph faster downgrade. Could be too much advance with the VA hooked back up. I want to try the vacuum timing with the VA hooked up.

Talking to a friend of mine about what the vacuum gauge was telling me, he asked about the PCV. Mine isn't hooked up, just open. He told me to fix that and try again.

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Originally Posted by farmer

To muddy the water even more I pulled out a vacuum gauge and attempted to diagnose the engine problems and set the timing via vacuum.

Truck warmed up, vacuum advance disconnected, timing right at 5 degrees I had around 11" hg, though it was fluttering. Loosened the distributor and advanced it til it read 16-17" hg and retarded to 15-16" hg, still fluttering. Snap the throttle it dives to 0, recovers to 20, and levels out back 15-16. Supposedly that means rings are shot. Checked the timing with a light, and without timing marks it looked to be at least 20-30 degrees, which seems absurd.

Finally test drove it like that, after hooking up the vacuum advance. Still struggling up a slight grade to maintain 30 mph. Ran about 5 mph faster downgrade. Could be too much advance with the VA hooked back up. I want to try the vacuum timing with the VA hooked up.

Talking to a friend of mine about what the vacuum gauge was telling me, he asked about the PCV. Mine isn't hooked up, just open. He told me to fix that and try again.

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Hang in there, you'll get it-- just keep your eyes open & don't disregard any clues, or make any assumptions. If I had kept this in mind, my truck wouldn't have spent last winter dead in my yard.