Engine miss

[mason1968]

Hi

I have a 350 SBC which is experiencing a miss.

I have one cylinder (number 6) which is showing higher exaust temperatures compraed to the other.

I have run a compression test on all cylinder an all 8 cylinders a within 10 lbs (avg 190 psi).

Now what I am guessing is that I have a bad hydraulic lifter, or I might have a bad valve.

Anyone have aany ideas?

Thanks

 

[Sensei]

You did not give a lot of detail, but a hot exhaust can be caused by a lack of spark to the cylinder. Unburnt fuel goes into the exhaust and burns. God bless, Sensei

 

[mason1968]

Hi,

Here is better explanation of my problem.

The engine is late model GM 350 create motor. So the issue of vale seats and leaded fuel should not be a concern. The engine has a B&M 144 supercharger and is carbureted. I recently installed a set of headers with ceramic coating. The coating on the primary pipe for cylinder #6 has burned away.

I just installed the headers about 2 weeks ago and noticed it. I talked with the manufacture of the headers and they are going to honor the warranty and recoat the headers for free. By the way the headers are Doug’s I like the quality and the fact they will re-coat them. The manufacture mentioned that the cylinder might be running lean.

What can cause one cylinder only to be lean?

I do not know if this is a new problem or not. I originally had the stock cast manifolds and never noticed anything.

Last night after driving the car a little I used my multi-meter which had a thermocouple (measure temp) attachment. At idle I checked the temp on the outside of the header near the heads. This is the results: Cyl #1 ~ 350-375 F, cyl #2 & 4 ~ 350-375, Cyl #6 +415 and climbing.

I tend to believe that the flame front is passing by the exhaust valve.

Would a burnt valve do this how about a miss-adjusted rocker arm that is not allowing the valve to fully close????

I have performed a compression test on the passenger side bank
CYL PRESSURE
#2 160 PSI
#4 160 PSI
#6 150 PSI
#8 155 PSI

The compression testing looks good.

I also performed a leak down test, but I am not sure if I am doing it correctly. For cylinder #6 and #8 I apparently have a leak down of 15% (had set gage to 100 psi, connect and read 85 psi) I also hear a leaking sound but not sure from where. I think the sound is coming from the lift valley. I have the valve cover off and I hear the leak but not from the valves them selves. I think I can feel air leaking past the valve stems but again not sure.

I have also removed and examined all of the spark plugs. My first impression is that the engine itself is lean, but I only have about 50 miles on the plugs. The porcelain looks very white; I do not see the combustion ring.

Any ideas?

Thanks
Matt

 

[AKRAY4PLAY]

your compression test is good, if you had a burnt valve the pressure would drop down to around 80 psi or less depending on how bad it was. also lower compression will burn more rich than high compression. there are so many variables to look at, cam timing per cylinder, carb settings, ingition timing curve, vacuum leaks, etc. etc. i would look closely at the intake runner of the intake port on the head. if it is oily, you have a vacuum leak into the port from the lifter valley (bad gasket seal). to test your cylinder with a leak down test, put the given cylinder at TDC and apply constant air. then see where the air is going. coming out the intake port, intake valve and/or seat problem. exhaust port = exhaust valve and crankcase is the rings. since your motor is new, i am pretty certain that the leak is going past the rings from lack of seating and the gaps from the moly rings. as for #6 being lean, what carb are you running?? most blower applications should run a mechanical secondary carb with fairly balanced jetting.

 

[mason1968]

The carb is a endelbrock performer carb (carter). I do not know about the jet sizes. I would like to know why only one cylinder would be running lean???

One thing to mention is that the distributer is new. I replaced the old points with a MSD unit. Initial timing is 10 with total of 36 by 3000 rpm.

One of my friends mentioned that the #6 cylinders exhaust valve hydraulic lifter may not be bleeding done correctly, thereby leaving the exhaust valve open.

 

[AKRAY4PLAY]

you don't want the lifter to bleed down. did you double check the lifter settings after installing the engine?? if not, it's easy to do. just get an old valve cover and cut the top out of it. then set it on the head, you won't be able to tighten it if it's a top bolt retaining head instead of the old school flange tightening head. it's just a splash shield anyway. with the cover on, start the engine and let idle. then back off one adjuster at a time until it starts ticking, then tighten 1/2 turn after the ticking goes away.

now back to #6 running lean, did you check all the other cylinders?? did you check them after a long drive at speed?? are the headers equal length?? header length will effect temps at idle. with a roots blower, fuel can seperate from the air and cause lean streams feeding the intake ports especially if you are not getting good atomization. the #5 & #6 are notorious for running lean compared to the rest of the cylinders. 3, 4, 5 & 6 have straighter intake tracks and better port velocity than 1, 2, 7 & 8. couple that with little to no flow from the secondaries (at idle) and 5 & 6 will run leaner. the best way to run a carbed blower is with two carbs. the double carb balances the front a rear four cylinders a little better. i am also assuming you have a low compression stocker short block?? these are pretty popular with blower guys since they are cheap, tough and excepting to blowers. the only problem is cam quality. these engines were designed for low hp applications and parts are not the best available. low quality control cams are usually installed to keep costs down and performance is not an issue. if your cam was ground with slightly offset lobe timing on #6, it will most certainly effect idle flow characteristics and temps. run your engine at speed and double check the temps, your problem may just be bad coating on the headers.

 

[Boggie]

Misfiring

I have 81' stock (Bowlin' Green), 56,000. Had carb rebuilt because revs wee 2K idle. Within a few days, idle went from 800 to 400, shake, rattle, roll. I'm told cly#8 valve is toast. As a owner with lo technical savvy, I'm trying to build up confidence with local expert. Since I'm looking at head retooled and valve replaced, what else should I throw $$ at while engine is apart? Last year it was 8grand.

 

[Antz81]

I have 81' stock (Bowlin' Green), 56,000. Had carb rebuilt because revs wee 2K idle. Within a few days, idle went from 800 to 400, shake, rattle, roll. I'm told cly#8 valve is toast. As a owner with lo technical savvy, I'm trying to build up confidence with local expert. Since I'm looking at head retooled and valve replaced, what else should I throw $$ at while engine is apart? Last year it was 8grand.

May be best to start a new thread rather than using a 10 year old thread

 

[Hib Halverson]

For now, forget bad lifters. Rule out a burnt #6 exhaust because your compression test indicates that problem does not exist.

If the engine is lean on #6, let's assume there may be other of the cylinders you did not measure which are lean. That indicates a fuel problem not an engine mechanical issue.

Was the Edelbrock carb you installed designed for a supercharged application?

Do you have exh. temp data for 3,5,7 and 8?