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#1 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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!
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Tony J ![]() |
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#2 |
Registered User
Join Date: Oct 2008
Location: Marianna Arkansas
Posts: 7,269
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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.
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#3 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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!
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Tony J ![]() |
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#4 |
Registered User
Join Date: Jun 2013
Location: Colorado Springs, Colorado
Posts: 20
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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...... ![]()
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1969 C10 SWB, 68 Front Clip, Tired azz 350 w/built 700R4, 3.73 Peg Leg........... |
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#5 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#6 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#7 |
Registered User
Join Date: Oct 2008
Location: Marianna Arkansas
Posts: 7,269
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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. |
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#8 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#9 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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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 :-)
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#10 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#11 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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?
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Tony J ![]() |
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#12 |
Registered User
Join Date: May 2011
Location: Oakley, Michigan
Posts: 735
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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?
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#13 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#14 |
Senior Member
![]() Join Date: Feb 2002
Location: Tucson, AZ USA
Posts: 7,260
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Re: HEI Compression Test Question
How old are your plug wires?
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Every 25 years I like to rebuild that 292, whether it needs it or not. |
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#15 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#16 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: HEI Compression Test Question
Put somemore initial timing in it.
Should be close to 14 degrees or more. |
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#17 |
Senior Member
![]() Join Date: Feb 2002
Location: Tucson, AZ USA
Posts: 7,260
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Re: HEI Compression Test Question
Do you have the OEM type mechanical External Voltage Regulator?
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Every 25 years I like to rebuild that 292, whether it needs it or not. |
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#18 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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Re: HEI Compression Test Question
Swapped the old points distributor for an integrated coil HEI.
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Tony J ![]() |
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#19 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: HEI Compression Test Question
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#20 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#21 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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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. |
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#22 |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,872
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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 ?
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#23 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#24 |
Registered User
Join Date: Oct 2015
Location: Upland, CA
Posts: 39
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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.
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Tony J ![]() |
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#25 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: HEI Compression Test Question
He forgot to block the throttle open, or forgot to mention that he did, one or the other.
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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compression test, hei disabling, straight 6 |
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