HEI Compression Test Question
 

Hi guys, I am new to the forum so pardon me if this question has been answered but I combed through as many searches as I could and didn't seem to find the answer / question.

Here's the situation, I have a 68' step / short with a 250 I6 in it. It has been running very rough. At first I thought it was a spark issue and I wanted to do away with the points dizzy anyway so I swapped out to an HEI. That swap seemed to give the engine a bit of a pep but I still couldn't get it timed just right - seemed to nearly stall at take off and ran rough. The next thing I did was get it up to temp and tried unplugging each spark plug wire one at a time to see if there was a noticeable difference in idle. Low and behold, the #5 plug seems to be the issue, no drop in idle when unplugged.

I am going to take it a step further and give the engine a compression test before scrapping the I6 and going to a 350 all together. The question I have is what do I need to unplug on the HEI dizzy before doing so? Is the Batt wire on the dizzy enough? Any other concerns I should be aware of with a carburetor on the truck?

Thanks a ton for your input!

Re: HEI Compression Test Question
 

Yes unplug the wire feeding the 12 volts to the Dist. OPEN the throttle plate on the carb and if needed hold it open with a screwdriver. Turn the motor over you will see the needle on the tester pulsate let it do that about 4 or 5 times that is plenty. Why not pull the valve cover and double check that the valves are opening and you don't have something simple and silly like a push rod or rocker arm messed up.

Re: HEI Compression Test Question
 

Hugger - Much appreciated, I was thinking of doing that after the compression test but you bring up a good point. Besides, I am getting a ton of "lifter like" noise coming from that #5 cylinder area. Thanks again!

Re: HEI Compression Test Question
 

This has nothing to do with your current issue.....
Just out of curiosity, did you run a solid 12v wire to your hei? Only asking because I had a bad experience with mine. If you used the same ign wire from the points setup you will eventually blow the hei ign module and possibly the coil. The ign wire used on the points setup takes a voltage drop so the points don't burn up. The hei needs a good 12v or better to function properly. I got stranded twice before figuring it out, one of those times was about 15 miles away from home on the highway.......
So if you already did this please ignore my post......:metal:

Re: HEI Compression Test Question
 

TR32, I rewired the wiring harness, replacing the cloth covered resistance wire that once ran from the fuse block and split between the starter & distributor. Here is the thread / instructions I followed: http://67-72chevytrucks.com/vboard/s...d.php?t=148346

I've had no issues whatsoever. Hope this helps.

Chevy Straight 6 Compression Test & Labor Question
 

OK, so I took the valve cover off, turned the engine over quite a bit, the valve train seems to be all in working order on the surface of things. I did reconfirm that the #5 and #6 cylinder do not contribute to a drop in idle when the plug wires are unplugged and the engine is running. Next step is a true compression test.

Thinking to the future, if this engine is gone and in need of a rebuild I am scrapping it and going with a mild 350 with a 700r tranny. Does anyone here live in the Upland / Rancho Cucamonga area who has any idea what labor would run me to remove the 250 engine & tranny and install a new engine and tranny? I know labor can be difficult to pin down depending on where in the country we are but if anyone has an idea for So Cal I would greatly appreciate the input.

Re: HEI Compression Test Question
 

If you have a space to do it, look into renting a cherry picker to pull your motor. Then if not putting one back at once, then plan to rent it twice. If you can do a compression test and general stuff like that, you can do a engine swap. That saved labor can help pay for other things needed for the truck. I'm gonna just take a guess at the labor figure about $80 range per hour, I think that is the good going average at the moment. But in socal it may be closer to $100 per.
Back when I went into the car"bidness" it normally took about $150 to have a shop change a motor here of course that was stupid high and I wouldn't pay it I'd do it myself.

Re: HEI Compression Test Question
 

Thanks for the response - space is definitely an issue for me, that the reason I am looking to have it done. I suppose I can start by getting a couple of quotes from local guys.

Re: HEI Compression Test Question
 

If you need to pay for an engine swap, I'd fix what you have in place instead, honestly.

Pulling the plugs to check for a miss or dead hole can be deceiving. Do the compression test, and if it's low, do the oil in the hole thing to see if it's rings.

Ideally you should do a leakdown test and find out where the compression is going (out a valve, through a head gasket, etc) so you know how to fix it. Good news is that all but the worst cases can be fixed without pulling the motor.

Actually, my grandfather used to rebuild his engines without removing the block from the car, which I thought was more work than pulling the motor, but it can be done. Can't change a 6 to an 8 that way though :-)

Re: HEI Compression Test Question
 

Thanks for your input Dave, I am going to do a compression check very soon to see what turns up. I will report back asap on the findings. I actually had a Jeep CJ7 with a straight 6 that I did a complete rebuild on without pulling the engine as well. What made it easy was that it was lifted and had huge 35" tires on it. This might be a bit tougher to pull off with my truck being so low but it is doable.

Re: HEI Compression Test Question
 

OK so I just did a compression test, here is an overview and the outcome;

1. Brought engine up to temp.
2. Removed HEI Batt lead and spark plugs
3. Dry tested each cylinder, here are the results:
Cylinder 1: 100
Cylinder 2: 120
Cylinder 3: 100
Cylinder 4: 120
Cylinder 5: 120
Cylinder 6: 120

I did not do a wet test or leakdown (I don't have a compressor or air to hook up to). Realizing that I am still a step or 3 from really determining what the issue is, would you all say that the major issue at hand seems to be the "top end" - particularly the valve train for cylinders 5&6?

Re: HEI Compression Test Question
 

What do the #5 and #6 spark plugs look like compared to the rest? Also, I'm sure you have already checked, but could you possible have those two wires crossed?

Re: HEI Compression Test Question
 

#5 & #6 spark plugs look like pretty similar to the rest....although slightly less carbon / black buildup on 5 & 6. I definitely have the correct firing order...no issues with wires being plugged out of place.

Re: HEI Compression Test Question
 

How old are your plug wires?

Re: HEI Compression Test Question
 

Plugs, wires and HEI dizzy are all brand new. I just put them in about 2 weeks ago. The engine runs better because of the upgrade but still an issue - not even close to smooth. Rough idle and clearly misfiring but no smoke at all.

Re: HEI Compression Test Question
 

Put somemore initial timing in it.
Should be close to 14 degrees or more.

Re: HEI Compression Test Question
 

Do you have the OEM type mechanical External Voltage Regulator?

Re: HEI Compression Test Question
 

Swapped the old points distributor for an integrated coil HEI.

Re: HEI Compression Test Question
 

And what did you set your initial timing at?

Re: HEI Compression Test Question
 

not sure what i ended up at. what i do know is that the timing / idle was all over the place. i believe i wound up advancing it to 8 or 10 degrees because it was sluggish and felt too retarded anywhere closer to zero.

Re: HEI Compression Test Question
 

Get a timing light and set your initial at 14. Plug off the vac advance when you set it.
Your idle will increase, so turn it back down and adjust your mix screw. Then hook your vac hose back up.
14 is a start point. You might end up a few degrees higher.

Re: HEI Compression Test Question
 

I always like check the vacuum with a gauge to help figure out a miss.

Also, does it miss when cold with the choke closed ?

Re: HEI Compression Test Question
 

I will have to see if I can borrow a vacuum gauge to see what I can figure out. The car starts and runs normal when choked - a bit of smoke but I think thats because the carb is rich - its a monojet carb.

Re: HEI Compression Test Question
 

From what I can see on the tab, the furthest I can get a read appears to be 10 degrees advanced. I will try to set the timing slightly ahead of 10 degrees to see what it does - thanks.

Re: HEI Compression Test Question
 

He forgot to block the throttle open, or forgot to mention that he did, one or the other.