Befuddled with engine problem

[Jake1074]

I have a '67 C10 with a 350 engine and TH350. Not sure of the age of the engine as I just recently acquired the truck. It was running fairly rough and hard to start but it seemed that a simple tune up would correct it. The engine is supposed to have 4,000 miles on it since a full rebuild, but I have no specs. It was claimed however to have a choppy cam. It's also hard to start when hot, but that could be because of the headers.

So far I have replaced carb with a new Edelbrock 1406, fuel line and filter, HEI cap, rotor, coil and spark plugs, timing set at 12. The distributor is almost touching the intake in order to get that much advance. The spark plug wires have not been replaced.

#6 plug had no gap (as in pic) after replacing it and running the engine the gap has stayed to spec.

The carb does pop on initial acceleration and under heavy load. Vacuum is 15-18 and fluctuates. I do think I hear some valve train noise so I guess the V/covers are next to come off.

What am I missing ? Wiped cam lobe? Valves not properly adjusted? Replace the wires next? Compression test? It does drive ok but doesn't have the power it should.

Thanks in advance for your help....

[Steeveedee]

The "pop" is probably a bad plug wire, given all that has been replaced. I've never seen a plug that gunked up at 4000 miles, unless the valve cover isn't sealed up well.

As for hard starting, does is crank slowly (heat soaked starter, very common) or does it spin a lot before starting (carb boiling over, needing a thermal insulator)?

As far as the gap being closed up, the installer probably dropped it and put it in without checking, or it was dropped in the box before that, and not checked before installation.

[AussieinNC]

1/ Remove all spark plugs and perform a compression test on all cylinders...post results on here....

Cylinders are numbered 2,4,6,8 front to back on passenger side...

1,3,5,7 front to back on drivers side.

2/ Dont worry about timing yet....it may be that you should move the plug wires one terminal and reset timing to get vac canister away from manifold....

3/ Check the vac line porting you are using....image is attached....

BTW...that plug with closed up gap...no one checked it before fitting it....or dropped it !

[Super Dude]

Well I don't see any damage on the spark plug, so it looks like someone installed it that way. With that said, anything is now possible. I have always gone over the valves with a new to me engine, or after a rebuild. I don't know how many I have found over tightened. You can gain engine vacuum if over tightened. Remove valve covers and watch rockers. Possible rocker noise could be from cam lobe wearing. After valves adjusted correctly, address timing, then carburetor. Not sure if your wires are in the correct position on the cap, can either move wires one position over or pull distributor and move a tooth to get vacuum advance in correct position. The carburetor should have a ported vacuum position for the advance line. Looks like it may be getting full vacuum if it was plugged into where your vacuum gauge is. I'm assuming you unplugged vac advance to check timing. A big cam engine will have some vacuum fluctuation, but most big cams I have had were 8-10 inches vacuum. After that is all in order, might have a four gas hooked up in tailpipe to see if 600 cfm is enough carb. I sure some others will chime in with more ideas. Good luck.

[68 P.O.S.]

I agree with everything that's been said already. Most importantly a compression test and seeing what's going on under those valve covers. Timing after that. What gap did you set your plugs at? HEI is 0.045. The plug wires probably need to be replaced as well. Your timing sounds good, I'd go up to 14 initial once you reset the distributor, but that all depends on the guts of your HEI (you want 36 total). Also, verify TDC with a piston stop and makes sure it lines up with your balancer and timing tab. On your carb, Edelbrocks don't like high fuel pressure and engine heat, or they flood and hard start. You need a pressure regulator set to 5.5 psi and to adjust the float levels to spec to keep it from flooding. You'll also need the Edelbrock 9266 heat insulating gasket under the carb to keep the fuel from percolating and causing hard starts. I hope this helps some.

[toolboxchev]

4000 miles on that spark plug looks somewhat odd to me. I would be guessing that engine has way more miles on it than you think upon first impression. I am only going by the outside of the spark plug, Not the electrode.

The tried and true method of tuning is ignition first, carb second. Most issues are resolved in the ignition 90 percent of the time.

[GASoline71]

What carb was on there before you replaced it? What is the spacer under the carb for?

Gary

[garyd1961]

I hate to say it but it could be a wiped cam.

[cleszkie]

Your picture shows a plug with a tapered seat. Make sure this is the correct style plug for your heads. There are two styles - the first has a tapered seat like yours. It requires the equal an opposite countersink machined in the head for it to seal correctly. The second style has a flat seat with a compression washer. It requires a flat mating surface on the head for it to seal correctly. If you mix and match the plug and head types you may get combustion chamber leaks. It would also foul the outside of a spark plug with exhaust soot very much like your plug has on it. Just a though.

[Jake1074]

Thank you for all the great information in your replies. I will try to answer them in this post.

Aussiein NC - Haven't done the compression test yet but will today if we get a break in this crazy NC weather, Not sure what you mean by check the vacuum port I used - it was the 3/16 manifold port. Should I have used a different one?

Steeveedee - its slow to crank - heat soaked - I have a mini gear reduction starter to instal

Toolboxchev - The other plugs are all clean and looked new. They were gapped at .035

GASoline71 - Edelbrock 1405 that appeared to be well used and the spacer is supposed to be a heat insulator.

cleszkie - That is the correct plug and my replacements are AC Delco R45TS gapped at.045. The other 7 that were in there burned clean.

Apologies if I missed anyone - Thanks again for the advice!

[weim55]

Pull the valve covers before you do anything else and spin the engine over to verify all 16 valves are functioning with proper valve opening. Your symptoms match a bad-flat cam perfectly.

Steve weim55 Colorado

[Jake1074]

Originally Posted by weim55

Pull the valve covers before you do anything else and spin the engine over to verify all 16 valves are functioning with proper valve opening. Your symptoms match a bad-flat cam perfectly.

Steve weim55 Colorado

Steve - with the exception of #6 which was an obvious problem, all of the other plugs were burning clean. Wouldn’t a wiped cam show something on the plugs?

Just asking...

[weim55]

Plugs "should" show a problem but not necessarily. Symptoms:

Popping noise through the intake-exhaust valve(s)not opening.

Fluctuating vacuum gauge-intake or exhaust valves not opening.

Hard start (slow crank-exhaust valve(s) not opening to release compression on exhaust stroke)

4,000 miles since rebuild-typical mileage for new flat tappet cam failure.

Steve weim55 Colorado

[Jake1074]

Ok - so in between the monsoon today I was able to pull the right valve cover and watch the rockers. #6 push rods and rocker arms were moving but not opening and closing the valves.

I adjusted all the valves hot - running and it seems to be idling much smoother. #6 now appears to be functioning. Have not adjusted the left side yet do to the rain - but all the prior valve train noise seemed to becoming form the right side.

Is it possible the lobe on #6 is wiped and this really didn't accomplish anything? #6 was the cylinder with the gap closed on the spark plus in the pic.

[weim55]

The key: all the valves should be opening the same approximate amount, about 1/2 inch. With a nearly flat cam lobe tightening up the clearance will sometimes bring the effected cylinder back to life at idle. Even a small amount of valve opening will allow the engine to run smooth at idle. I prefer to run the engine at idle with the valve cover off and watch the valve opening rate of all the valves. Very easy to pick out the bad ones this way.

Steve weim55 Colorado

[gmc684x4]

Too big a cam for the heads valve springs will flatten a cam quickly
Those old school 327 300 hp cams are too much for some factory heads with out new valve springs and possible machine work

[Sheepdip]

If your cam is still good, unless you are running a roller cam, off road racing oil with zinc or a zinc additive it will soon be flat anyway.

[Jake1074]

If the cam does have a bad lobe, what needs to be done to prevent another after I replace it ?

[weim55]

Step up and put a roller cam in it.

Steve weim55 Colorado

[AussieinNC]

Depends on size of your wallet....

Buy a street or RV cam and lifter kit...try and keep it mid range torque...

If you have the funds...consider a VORTEC roller engine...would need different manifold...but much better in long run...

Flat tappet cam engines must use an oil rich in Zinc...dont use late model synthetic oils unless you add a zinc additive....

Ensure valve springs are all within spec...same installed height and pressure...
Ensure rockers are not galled out inside where ball rides....
Ensure all push rods are straight and not chipped on ends....
Make sure the new cam gets NEW lifters....and new chain/gear set...

On installation...heavily coat cam lobes with heavy zinc based lube,,,include bottoms of lifters with same lube....
Assuming hydraulic lifters, set dry clearance on lifters to 1/2 turn down from zero lash on valve stem...start at cyl 1 and set then turn 1/4 turn and follow firing order....1,8,4,3,6,5,7,2...
New oil and filter...prime oil system before first start...not by cranking engine...use a spin up tool....you want oil out of each push rod hole in each rocker...

Make sure fuel pump is not leaking fuel into engine, diluting oil....

Maybe convert to HEI new distributor while you are at it...

There are two vac ports on that carb...untimed full vacuum which is the one you used...or timed vacuum that has zero vacuum at idle...see the image I posted...

Hope this helps....

[Jake1074]

So good news, perhaps not so good and maybe bad news...

Good news is I adjusted all of the valves which improved the idle as its now working on all 8 cylinders. The left side - 1,3,5,7 were pretty much correct. 2, 4,& 8 were good as well - # 6 was way out of spec. The popping back thru the carb is gone. Test drive has great power - a whole lot more and somewhat impressive than I have experienced since I've owned it.

Not so good - Vacuum really didn't improve - still fluctuating as before, but is much better when increasing RPMS. Power brakes work great so no issue there with vacuum. I also did readings at the intake manifold instead of the carb port. Still hard to start when hot but that is most likely heat saturation of the starter as cold start today was almost immediate. Have a replacement gear reduction.

The maybe bad news is right side exhaust is now smoking a bit - has never smoked before. Perhaps a valve too tight? Left side exhaust is perfectly clear.

A slight miss which I think is the plug wires. Will correct the lack of advance issues when I replace them. At this point everything electrical / distributor wise will be new. Carb and fuel filter replaced and adjusted properly.

I'm guessing I've done some good diagnostics - but not sure if the problem is fixed or not. As many have said - may still be a wiped cam lobe.

Most welcome to any suggestions to validate the cam is wiped before I take the plunge to replace it. Would hate to pull it and find it good and that I missed something simple.

[jocko]

Focus on the two pushrods for your suspect #6 cylinder - if they aren't moving the short end of the rocker arm as much as the other pushrods are moving their respective rocker arms, then it's likely a damaged cam lobe. Also, if you had to excessively tighten down a rocker arm on #6 cylinder all of a sudden to make the rocker arm actuate a valve, then that's another indication a cam lobe is damaged. Steve made a good point above - cam lobes don't always wipe to zero, so you can sometimes make the problem appear to have improved by cranking down the rocker arm - but if that is the case, it's probably just a matter of time before it gets worse again, only mitigating a symptom, not fixing it (i.e. the cam lobe face is the hardened part).

[Jake1074]

Good points. Both rockers were extremely loose on #6 - As if they were intentionally backed off. The clatter and visual was obvious once the valve cover was off.

#6 was also the cylinder with the old spark plug that had no gap.(see pic above) All the other plugs were gapped at .035 and were burning clean.

Let’s say the cam is bad on #6. Would both lobes go? Seems there is more to this mystery - perhaps the seller’s short term fix to get rid of the truck?

[jocko]

Originally Posted by Jake1074

Good points. Both rockers were extremely loose on #6 - As if they were intentionally backed off. The clatter and visual was obvious once the valve cover was off.

#6 was also the cylinder with the old spark plug that had no gap.(see pic above) All the other plugs were gapped at .035 and were burning clean.

Let’s say the cam is bad on #6. Would both lobes go? Seems there is more to this mystery - perhaps the seller’s short term fix to get rid of the truck?

I'd ignore anything the seller told you, unfortunately. Both lobes would not necessarily go bad together. No idea how the plug tip got bent, seller probably only had 7 good ones to put in it. As far as intentionally backed off - if the cam lobe gets wiped, the respective rocker will only appear backed off, even though it's in the same place (and probably similar to the others). Do you happen to recall before you tightened down the rocker arms whether roughly the same amount of rocker stud thread was visible (compared to the known good cylinders)? What about now? Also - another thing to consider is that sometimes rocker studs can pull out of the head partially. Might be a good idea to, as best you can, sight across the tops of all the rocker studs and see if the #6 cyl ones are notably higher than the rest. This can give the appearance of a wiped cam lobe - but the giveaway is one (or two) rocker studs noticeably sticking up higher than the others. Doesn't have to be much though to essentially keep the valve from opening or only allowing it to open partially. Re-tightening, as you did, may solve the issue temporarily (or for good if the stud doesn't slip any further). But, once loose, they normally won't just suddenly be happy in their new location. Just one more thing to check. If you have screw-in studs, then disregard, only possible with press-in studs, which is how they came from the factory. If this turns out to be the case, the cam is likely still good - but screw in studs are the only reliable solution at this point, and that requires machining of the stud boss and purchase of a set of screw in studs.

[Jake1074]

Originally Posted by jocko

I'd ignore anything the seller told you, unfortunately. Both lobes would not necessarily go bad together. No idea how the plug tip got bent, seller probably only had 7 good ones to put in it. As far as intentionally backed off - if the cam lobe gets wiped, the respective rocker will only appear backed off, even though it's in the same place (and probably similar to the others). Do you happen to recall before you tightened down the rocker arms whether roughly the same amount of rocker stud thread was visible (compared to the known good cylinders)? What about now? Also - another thing to consider is that sometimes rocker studs can pull out of the head partially. Might be a good idea to, as best you can, sight across the tops of all the rocker studs and see if the #6 cyl ones are notably higher than the rest. This can give the appearance of a wiped cam lobe - but the giveaway is one (or two) rocker studs noticeably sticking up higher than the others. Doesn't have to be much though to essentially keep the valve from opening or only allowing it to open partially. Re-tightening, as you did, may solve the issue temporarily (or for good if the stud doesn't slip any further). But, once loose, they normally won't just suddenly be happy in their new location. Just one more thing to check. If you have screw-in studs, then disregard, only possible with press-in studs, which is how they came from the factory. If this turns out to be the case, the cam is likely still good - but screw in studs are the only reliable solution at this point, and that requires machining of the stud boss and purchase of a set of screw in studs.

I'll get some pics and take a few caliper measurements of the studs along with running a straight edge across the others. It struck me how loose both rockers were including the nuts. It didn't appear as there was much thread visible on both studs. I wish I would have taken a pic before I adjusted #6 🤔