Got a good one for you??!!!!

[nicolas72gmc]

Ok fellas, sorry for the book, here is the deal, I drove the GMC all the way to Superior AZ friday. The truck did awesome until about the AZ state line. It started chugging at low cruise (about 35 mph) and at 3/4 all the way to WOT. At 45 to 55 mph it does ok. At 60 to WOT it was cutting out almost to the point of dying, if I let off the pedal a little it would sound like it went back to normal. While it is cutting out if I let off the pedal completely it will back-fire and sounds like it is burning unburned gas in the headers. Sometimes it pops so loud it sounds like the headers are blowing off. I did check the header pipes seperately and #6 is running really hot (600 degrees) compared to 450 or so on the rest. Fuel delivery seems to be fine. I still have the clear fuel filter and can see fuel while running, I even pulled the top of the carb to check for blockage and bowl/float levels. Engine temp stays in the normal range as does oil pressure. I switched over to the HEI this summer and it has ran fine until now. Here is what I have done so far in the last two days:

Changed to MSD cap
Changed to MSD rotor
replaced stock coil
replaced module in Dist.
replaced all spark plugs
added a can of Seafoam to the gas.
Replaced air cleaner
Replaced MSD wires
Purchased the pick-up coil, but don't know how to replace it.

What is weird is that if I put it in park it revs just fine, but put it in drive and take off it does the above again. I am losing my mind here and would like to hear some other suggestions. Thanks for any help Ya'll can provide.

[vectorit]

Vacuum leak?

[oldspowered]

check your pressure on that fuel pump! just because you look like you get fuel don't mean that you get enough fuel. psi will tell you that! i had one do that same thing and after spending alot time and parts that is what mine turned out to be...
just my 2 cents!
David

[nicolas72gmc]

Vectorit, I'll write that one down to check.

Thanks David, I'll have to check on that too!

[basemodel67]

I'd check to see if the distributor is off a tooth or if timing is off. Most fuel problems turn out to be ignition problems.

[dadsworktruck]

If the engine has high mileage i would also check for worn camshaft lobes.

[67ChevyRedneck]

Do you have 12V going to the HEI? The old points wire only has 8.

Does your coil have the sperate metal ground strap that goes inside the distributor cap? That would allow the truck to run fine until it finally started frying the coil/control module.

[nicolas72gmc]

Originally Posted by 67ChevyRedneck

Do you have 12V going to the HEI? The old points wire only has 8.

Does your coil have the sperate metal ground strap that goes inside the distributor cap? That would allow the truck to run fine until it finally started frying the coil/control module.

Yeah I did run a completely new 12v wire to the dist. and I also added the extra ground wire like it described in the "destructions" I did the extra ground wire on the first coil I replaced this summer also. I just changed it again to eliminate that as a problem. Thanks.

[Paul M]

try swapping the bad cylinders spark plug with a known good one.i had a bad plug and it would break down on compression and not fire under load. was fine just idling without a load.

[nicolas72gmc]

Originally Posted by Puropedo

Nico, check for vacuum leaks as suggested earlier, also test the pressure on your fuel pump it should be somewhere in between 5-10 psi. Good luck buddy!

Thanks Jhonny, will do.

Originally Posted by Paul M

try swapping the bad cylinders spark plug with a known good one.i had a bad plug and it would break down on compression and not fire under load. was fine just idling without a load.

Hey Paul, I will try that. What would be causing the one exhaust header pipe temp to be so high? Thanks.

[pat 70/71]

Originally Posted by nicolas72gmc

Thanks Jhonny, will do.



Hey Paul, I will try that. What would be causing the one exhaust header pipe temp to be so high? Thanks.

Nick, I have had this happen to me in the past sounds like a lean condition(low fuel in the carb?) internal fuel leak? my 2 cents, change out the carb to a known good one good luck! ..Pat

[Puropedo]

Nico, check for vacuum leaks as suggested earlier, also test the pressure on your fuel pump it should be somewhere in between 5-10 psi. Good luck buddy!

[67ChevyRedneck]

Well, that eliminates my stuff then

You wouldn't believe how many people post a similar problem and it's one of the things I mentioned, or both! I myself goofed on the ground strap. Learned that lesson the expensive way. Killed a practically new coil and control module

[68 C-10 Panel]

Well these guys already covered most of the possible solutions so I'll toss in my .02 on the header pipe situation. I am a professional motorcycle mechanic so more that 2 cylinders can throw me for a loop but when one pipe is hotter than the rest, it is often an indication of a lean condition. I would get a can of starting fluid or carb cleaner and check the intake and exhaust gasket mating areas. can use the same stuff to help when checking the vacuum hoses to. Also late timing on that cyl or a exhaust valve staying open can cause fuel to burn in the exhaust raising the temp. Well thought it would worth a try so you guys let me know if that stuff works the same in the sbc world as well. Good luck.

[72gmcworker]

water in gas accumulating in the filter and or carb displacing the fuel from getting through or at least the right amount.
or clogged fuel filter ...if one in carb

to much alcohol/ethanol blend in the fuel

????

[D.PASSMORE]

I had a divorced choke on a old holly that I was running on a 71, 350 ci and it would kinda do the same thing. Turned out the choke butterfly would shut nearly close at odd intrevals and make it run crappy. I wired it open until I fixed it...

[70 Suburban]

Sounds like its sucin air.. Check your base plate on your carb

[Tynee]

Originally Posted by 70 Suburban

Sounds like its sucin air.. Check your base plate on your carb

Seems more like it would have to be at the intake/head seal, doesn't it? to only get one hot cylinder?

[70 Suburban]

Originally Posted by tynee

seems more like it would have to be at the intake/head seal, doesn't it? To only get one hot cylinder?

i missed that... Yep i would think so. Also could be a plug with the wrong gap... It its only a slight change....

[hayhauler71]

My 2 cents is to replace the spark plug in #6 hole and check the old plug for cracks.

[nicolas72gmc]

Thanks fellas, I will get to work on the suggestions. I just don't get it. It is very frustrating and I appreciate ya'lls help with this.

[Puropedo]

Nico, let us know the outcome

[knsmodels]

I would not discount the cam even though 7500 miles.....what kind of cam and what kind of oil are you running.....thats about how long it took my new motor to eat a cam from running oil without zinc in it..sympoms very similar......jfyi

[nicolas72gmc]

Originally Posted by knsmodels

I would not discount the cam even though 7500 miles.....what kind of cam and what kind of oil are you running.....thats about how long it took my new motor to eat a cam from running oil without zinc in it..sympoms very similar......jfyi

I don't know what cam is in the motor. It is a 290 hp GM crate with the edelbrock power package which puts it at around 330. I am going off of what my uncle said was done to the motor. I got it for a real good deal and it was used for about 1500 miles. It wasn't enough motor so he took it out and stored it. The Two times I've changed the oil, I put Castrol 10-30 in it. I was kinda hoping it wasn't the cam. I know it could make it do what it is doing if the cam was junk. I guess the only way to find out is to tear into it? Thanks.

[jim zag]

Make sure you have a nice strong spark that is BLUE in color. Buy a spark tester or make one out of an old spark plug. You could have a module problem, sometimes they can act funny when they get HOT. What kind of carb?