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

[nicolas72gmc]

Originally Posted by 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?

I did change the module just to eliminate it, I greased the bottom of it and bolted it in. I checked the pick-up coil resistance and it was reading 875. My carb is a 1405 performer. When I did change my plugs before the trip the old ones looked a little black but not bad, when I changed them last week they looked real good. The ones in the 7 and 8 cylinders looked a little dark. I just don't know what it could be. At night I can see the all the plugs firing at the plug and I did check from the distributor to the wire one at a time and all are firing. Thanks.

[70duramax]

you found the problem already (hotter temp in one cyclinder)just dig deeper forget all the timing and fuel b.s. if it was running fine and didnt change anything. burnt valve my guess

[longhorn396]

Did u ever get this fixed

[harshard]

You dont have to tear into it to check the cam, just pull the valve covers and check to see if the valves are opening and closing. That will eaither pin point or x-out the cam.

[nicolas72gmc]

Originally Posted by longhorn396

Did u ever get this fixed

Longhorn, I have not been able to tear into it yet. I am south of Tucson right now and the truck is up near Phx. I hope to get to look at it this weekend.

Originally Posted by harshard

You dont have to tear into it to check the cam, just pull the valve covers and check to see if the valves are opening and closing. That will eaither pin point or x-out the cam.

Thanks harshard, that is what I was planning on doing. I have never really done anything to a motor except normal maint. and some add ons like carbs, exhaust, etc..... I am just worried it is gonna be a lot worse than expected. Maybe I'm just a worry wort!!!

[lovnthe70]

i dont have any answer for you but just had to say that i am in tempe and just had this happen to me today on a recently rebuilt engine! was driving great and then bam! just started chugging like i wasnt getting enough gas whenever i go over 40 or more than 1/2 to WOT! ill let you know if i figure anything out, good luck hope we dont have trashed cams! i have been using straight 30 weight rotella for break in and havent even hit 500 miles. anyways let me know if there is anything i can do to help you, i am here in tempe and got tools and what not if you dont have access to anything up here.

[nicolas72gmc]

Originally Posted by lovnthe70

i dont have any answer for you but just had to say that i am in tempe and just had this happen to me today on a recently rebuilt engine! was driving great and then bam! just started chugging like i wasnt getting enough gas whenever i go over 40 or more than 1/2 to WOT! ill let you know if i figure anything out, good luck hope we dont have trashed cams! i have been using straight 30 weight rotella for break in and havent even hit 500 miles. anyways let me know if there is anything i can do to help you, i am here in tempe and got tools and what not if you dont have access to anything up here.

Thanks Lovnthe70, I may need some help, I just took a job up here and my family and tools are all in my garage in Texas. I feel kinda helpless out here and not knowing anyone. I appreciate the help offer. This is what makes this board so fricken awesome.

[70 Suburban]

your always welcome to come to bryan.. your only abut 1.5 2 hrs from here

[nicolas72gmc]

Originally Posted by 70 Suburban

your always welcome to come to bryan.. your only abut 1.5 2 hrs from here

Thanks 70 Suburban, but I'm in Tucson right now and wont be back to Bremond until x-mas. My truck is out here for good and not doing so good.

[70 Suburban]

lol i know the feeling...

[highperf4x4]

It sounds to me like one of 2 things. If an exhaust valve is not opening far enough due to worn cam lobe or pushrod through a rocker then it will cause it to backfire through the intake eventually becoming a steady "popping" sound each time the intake valve opens. However, you will usually here a strong tapping sound like a stuck lifter to go along with it. My other suggestion would be as stated previously also, a burnt valve. If it's an intake valve you will hear popping from the carb, if it's an exhaust vavle you will have a funky sound from the pipe on that side. Lean conditions will definitely cause excessive heat from a cylinder but the loss of compression will cause a cylinder to be "lean" even when it pulls enough fuel.

The best thing to do is pull the valve cover and check the pushrods/rockers for that cylinder. If either of them have much slack then you are looking at a rebuild. If all pushrods and rockers feel snug and there is no visible wear then start the motor with the valve cover off and listen to them. If they sound ok then it might be a burnt valve which would result in a single head job. Keep your fingers crossed for the burnt valve.

[nicolas72gmc]

I haven't figured this out yet. I did do a compression test on all 8 cylinders and they are all right around 125 psi. Does that sound right for a gm crate? I swapped all of the plugs, after re-checking the gap. I pulled the valve cover on the pass. side and everything is moving up and down like it should be. I didn't measure how much everything is moving but they appear to be moving the same distance. Everything appears to be getting oil and looks good. I took it for another drive to listen to it, misses a little at cruise, @55 loses all balls and starts chugging real bad under throttle, got back to the RV with it and checked header pipe temps and #6 was at 710 degrees. What the hell??? I am thinking swap the whole damn motor with another one and tear into this one. ????? Any AZ board brother engine gurus? Help!!!!

[highperf4x4]

I don't know bro, I think from where I'm sitting you've pretty much narrowed it down to the carb but I don't know why your #6 would be running so hot. It sounds like maybe an intake valve problem. That 125lbs sounds a little low to me for a new 290hp crate engine. But your'e saying that all cylinders are 125 right? Even the number 6. Did you let it spin over several times on each cylinder when you were doing the compression test?

Originally Posted by 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...

Did you check this yet? Just fire it up with the breather off and then watch the carb while you bump it to WOT several times? I'm thinking if this was a mechanical issue it would evidence itself all the time and not just at certain speeds or under load. Got another carb anywhere you can drop on it for a little bit?

[echo72]

Sounds like you timing is off a hair?

[greasemonkey]

Check that the fuel cap isn't pulling a vacuum. Maybe take a short drive with it loose to rule it out.

[CCCC]

Too bad that you are far from home/shop with this mystery issue. The exhaust hot spot at #6 is funky - may or may not be a clue.
After what you have checked and done, and given that it runs OK although a bit rough at lower RPM and then just craps out at 55 mph (which would be more broad than a single cylinder/valve issue) would have to guess that it is a very sudden "lean" situation (bad vac leak or serious drop in fuel delivery) or some odd development in the timing system (retardation) ocurring at that RPM.

Have you checked the pressure from fuel pump? Could be low but still giving enough to run at low speed but insufficient at 55. Low pressure could be faulty pump, line blockage before pump, dirty filter before pump, etc. You can check for general vac leaks with a small butane lighter stick with flame held near possible sucking spots. If the problem is a vac leak or fuel shortage happening from within the carb at a certain rate of flow, will be hard to determine under driving conditions. Someone else suggested trying another carb - very good to do if the other searches do not yield a fix. Some forum guy down there in the valley should be able to lend you a carb.

After the basics, I am not strong on timing issues - especially the vac advance details - but someone else should be able to tell you how a sudden retard or loss of spark could happen. Best wishes with this. Keep us involved.

[Fraser Beatty]

I had a 350 with a worn cam and it did EXACTLY as you are describing.

[crushermechanic]

If you have a lean condition on #6 check your intake runners. i bet your gasket is leaking. this is a bad problem on vw motors. they do the same thing you are describing. take starting fluid and spray the gasket. if it is leaking the engine will rev slightly.

[nicolas72gmc]

Originally Posted by highperf4x4

I don't know bro, I think from where I'm sitting you've pretty much narrowed it down to the carb but I don't know why your #6 would be running so hot. It sounds like maybe an intake valve problem. That 125lbs sounds a little low to me for a new 290hp crate engine. But your'e saying that all cylinders are 125 right? Even the number 6. Did you let it spin over several times on each cylinder when you were doing the compression test?

Travis, I did let it turn over for probably 10 seconds and then I would check it. Somebody I work with said it sounds low also. Yes, they were all around 125 +- 5. Even #6. Am I right when I say the compression test would not tell me if I had a bad intake valve? or would it confirm both exh. and intake are good?



Did you check this yet? Just fire it up with the breather off and then watch the carb while you bump it to WOT several times? I'm thinking if this was a mechanical issue it would evidence itself all the time and not just at certain speeds or under load. Got another carb anywhere you can drop on it for a little bit?

I have a manual choke so I don't see how the choke could be it. I will double check it though. I am gonna find a spare carb and try that. Probably a Q-jet. Hell I never had this prob when I had my old Qjet.

Originally Posted by echo72

Sounds like you timing is off a hair?

It is set around 18 degrees advanced.

Originally Posted by greasemonkey

Check that the fuel cap isn't pulling a vacuum. Maybe take a short drive with it loose to rule it out.

I'll check it!!

Thanks for all of the other ideas fellas!!!

[nicolas72gmc]

Originally Posted by CCCC

Too bad that you are far from home/shop with this mystery issue. The exhaust hot spot at #6 is funky - may or may not be a clue.

After what you have checked and done, and given that it runs OK although a bit rough at lower RPM and then just craps out at 55 mph (which would be more broad than a single cylinder/valve issue) would have to guess that it is a very sudden "lean" situation (bad vac leak or serious drop in fuel delivery) or some odd development in the timing system (retardation) ocurring at that RPM.
Have you checked the pressure from fuel pump? Could be low but still giving enough to run at low speed but insufficient at 55. Low pressure could be faulty pump, line blockage before pump, dirty filter before pump, etc. You can check for general vac leaks with a small butane lighter stick with flame held near possible sucking spots. If the problem is a vac leak or fuel shortage happening from within the carb at a certain rate of flow, will be hard to determine under driving conditions. Someone else suggested trying another carb - very good to do if the other searches do not yield a fix. Some forum guy down there in the valley should be able to lend you a carb.




After the basics, I am not strong on timing issues - especially the vac advance details - but someone else should be able to tell you how a sudden retard or loss of spark could happen. Best wishes with this. Keep us involved.

I have not checked the output of the pump. What blows about the whole thing is I am working at one mine in Tucson for and outage and driving to the RV on the weekends In Superior (which is where the other copper mine I am based out of is). I have to sit and think about it all week and then try to work on it for two days, tired and all.

Originally Posted by Fraser Beatty

I had a 350 with a worn cam and it did EXACTLY as you are describing.

That is what I'm afraid of!!!!

Originally Posted by crushermechanic

If you have a lean condition on #6 check your intake runners. i bet your gasket is leaking. this is a bad problem on vw motors. they do the same thing you are describing. take starting fluid and spray the gasket. if it is leaking the engine will rev slightly.

I'll try that too. Thanks bro.

[highperf4x4]

If all cylinders are showing the same compression then your valves are ok. 125 sounds low but it may not be for that engine. Mine is closer to 160lbs but mine is bored .030 over with flat top pistons.

One thing that does surprise me is that you have it timed so fast. If 18 degrees is your "initial" or "base" timing. Did you set the timing with the vacum advance disconnected and the engine idling around 800rpm? My base timing is about 10 degrees. Your vacum advance could be a 10 degree or a 20. For your engine you want no more than 27 degrees of "total" advance including vacum and mechanical. You will be all in at 3000rpm. Do you have access to an advance timing light with a dial so you could check it at 3000? You have checked to make sure the dizzy is not loose right?

I am still thinking this is a carb/fuel issue though. I'd do a carb swap and see what happens.

[70 Suburban]

i say carb/fuel.

[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

[pitbull784]

I think i would check my fuel pressure it sounds like a lean condition along with to much timing your base timing should be around 8- 10 degrees with vacuum advance disconnected and vacuum port plugged

[68C15]

Originally Posted by Fraser Beatty

I had a 350 with a worn cam and it did EXACTLY as you are describing.

this is what i am thinking as well. granted you have 125 PSI cranking, that is only telling you that the valves and pistons are moving. a flat cam will read this way. cranking compression is "static". when running you will actually have MUCH higher pressure which is impossible to measure.
you need to measure, not just eye ball the pushrod movement.depending on your cam the lifter will move about 3/8" total. hard to see while running a 50% difference in this.

I base this all on the modern oils not having sufficient zinc compounded with the fact you stated the engine sat before installation. factor in the #6 being hotter (bigger problem cylinder) exhaust and how you were on a road trip. it all leads to a flat cam and wiped lifters.