250ci Exhaust Question
 

So it's been a while, but today we finally got most of the carb issues and such straightened out on the '70 C10. Ended up putting a Holley 350 CFM Street Avenger on it, with a Transdapt 2bbl to 1bbl adapter, and a custom CNCed plate that holds the throttle return springs and throttle cable.

Fought a REALLY stupid battle with what sounded like a timing problem, and turned out to be that I had plugs 2 and 4 reversed. It actually idled really well, and at high RPM with no load, but as soon as it was under load, starting backfiring through the exhaust. Once I figured out that it was 153624 and not 153642, things got a lot better.

I've got a set of Hedman headers on it, and cylinders 1-2-3 go into one pipe, and 4-5-6 into the other. I'm getting what almost feels like 1/2 the flow out of the 4-5-6 pipe compared to the 1-2-3 pipe. We've reset the valves, and they seem to be correct. I have not yet re-run a compression test, which is my next step. We did one a while back and they were about a 5% spread from high to low.

I don't know what the effects of a slightly bent valve are, but I'm not smelling any excessive gas through the exhaust, and it doesn't seem to be blowing back through the carb.

Anyone have any advice and/or thoughts on this? I'd expect the exhaust flow to be fairly balanced between the two pipes, but I've sure been wrong before...

Re: 250ci Exhaust Question
 

Pull a compression check on your cylinders.
Yes, 1-2-3 and 4-5-6 should have equal output.

Re: 250ci Exhaust Question
 

Still not exactly sure what's going on. Checked compression again and got 115, 125, 140, 130, 145, 125. Vacuum is steady at idle, so I don't think it's valve guides or stuck valves. No gassy smell in the exhaust.

Guessing it's a wiped cam lobe on either 4, 5, or 6, probably exhaust?. Next step would be to measure the lift at each rocker, or can I pull the lifter cover plates and see the tops of the lifters there?

Re: 250ci Exhaust Question
 

Are you sure you have and engine problem? Are the exhaust pipes the same length? Same mufflers on each side? I'm not sure you have a cam problem if the engine sounds happy and the compression is that close.

Re: 250ci Exhaust Question
 

Hedman headers, so they're pretty much the same length. Just a LOT less exhaust flow out the 4-5-6 pipe than the 1-2-3 pipe. 1-2-3 is a sharp pulse, 4-5-6 is smooth, but low flow. I mean, I guess the problem could be the 1-2-3 side because of the pulsing, but the 4-5-6 feels more like a small Honda.

250C.I. Exhaust Pulse Question
 

A 20 pound compression difference is nothing to sneeze at and if it's bone stock there may well be additional unequal cylinder power issues apart from the low compression .

For fun get a low co$t dial indicator and check the valve lift as well as cam timing, factory tolerance were _very_ loose on these, why all i6's ran well but some got 20 MPG and others only 12 when new in otherwise identical trucks.....

Unless it runs poorly don't sweat it too much .

We got a new truck in with the main bearing saddles bored .010' off center in the 350 V8's block ~ it ran sick from new , we just scrapped the entire engine around 10,000 miles, plopped in a Target 350 (new, Mexican) 350 4 bolt main and it ran perfect after that .

Plenty of little things like connecting rods having off set saddles cause one or more piston to be .010" lower deck height, little things like this are addressed when "Blueprinting" an engine during overhaul / rebuild .

Few understand what "Blueprinting" means : to follow the engineers design to a "T", no variables .

Re: 250C.I. Exhaust Pulse Question
 

[QUOTE=VWNate1;8616479]For fun get a low co$t dial indicator and check the valve lift as well as cam timing, factory tolerance were _very_ loose on these, why all i6's ran well but some got 20 MPG and others only 12 when new in otherwise identical trucks.....

Good info. I can definitely check the valve lift, but how do I check the cam timing?

Checking Cam Timing
 

If I try to explain it I'll prolly do it wrong .

It's in the FSM and every 1/2 way decent shop manual for the engine you're working on .

This means no using Haynes / Chiltons books .

I'm sure the how to info is in this forum somewhere but I have no idea how to find it .

Re: 250ci Exhaust Question
 

jcwren, I fully agree with what VWNate1 has to say about variations in cylinder power balance.. Several years ago (more than I care to count), As part of my business, I built race engines in the 400 -- 550 HP range.. You would not believe the amount of new OEM engine parts I rejected simply because they were so far out of spec, no amount of machining would bring them back to "blue print" specs..

A "slightly bent valve" will exhibit the same symptoms as a burned valve.. At best that cylinder will have extremely low to no compression.. Your best bet to determine cylinder power balance is to follow VWNate1's advice and check cam timing and cam/valve lift.. It's a technical and complex procedure requiring special tools and some engine building skills. The directions for conducting it are not something that can be accurately done here.. Refer to a Factory Service Manual for specifications and procedures..

Personally, I believe the difference in your exhaust flow and "tone" is attributed to the 20% difference in compression between the high and low readings.. I also believe that until the lowest reading gets below 80 - 95 lbs. there is no reason to be concerned.. 20% difference might produce a slight "flutter" at idle speeds (almost imperceptible) , but nothing that will make you pull your hair out..

You corrected the back firing when you corrected the plug wire firing order.. Any roughness or poor performance you now experience probably is not due to the single low compression.. You're overthinking a lot of this.. My suggestion is --- put it back together, close the hood, and drive the damn thing...

The Few, The FOOLISH
 

Thanx R.P. ;

I expect that like me, you were working on these when new .

Even back then only the gray beards understood, respected and cared about the InLiners .

So many have brought me old dead 235 powered Chevies and asked "how much to rebuild this thing I just bought it not running for twenty years" .

They're always amazed when I do a full tune up and service and they run almost as good as new even when having 95# or so compression (under 90# I tell them it's time to dip in) after proper service .

Re: 250ci Exhaust Question
 

I developed an interest in automobiles when I was just a little kid.. I Rebuilt my first engine in 1957 when I was 14.. With help from a service manual, my Mom and a friend who was a couple years older than me, I rebuilt the engine on my Cushman motor scooter.. I was working on cars at a professional level when I was still in high school.. I worked part time for a friend of my Dad who owned a little "one man operation" automobile repair shop.. Built my first race engine when I was 19.. I served my military time in the mid 60's... When I came home I started working in new car dealerships in order to earn those "specialty" certificates. For the most part my certifications have expired, but I've lost count of just how many fields I earned my certifications. I even worked at a VW dealership and hold certifications on those cars. I was around when Volkswagen rolled out their 12 volt electrical systems and that goofy fuel injection system..

I got to see those first Cs and Ks roll off the transporters.. I've worked in the industry almost all my adult life.. My wife and I owned and operated a shop for almost 20 years.. I retired in 1993, but I have been around cars continuously since then..

Re: The Few, The FOOLISH
 

When I first started racing stock cars (dirt tracks), the class I ran in was populated by small displacement engines limited to (if I remember correctly) 338 cubic inches.. I ran a sbc 283 in the beginning.. Later when the rules gave a weight break to the inliners, I had to really hustle my 283 to just keep up with the Ford 300s and the GM 292s.. A well prepped 292 could really get it on.. After a couple years, it took a 327 to keep up with them.. After about four years, when the larger displacement engines became legal, the inliners were given their own "six cylinder" class..

Old Timers
 

Thanx ~

I'm not many years behind you ans also spent my entire adult life being a Mechanic .

I owned an indie VW Shop during the glory days, buy howdy that was fun .

Re: 250ci Exhaust Question
 

We may get accused of hyjacking jcwren's thread, but I think we've solved his problem..

Oh yeah, they are a blast!!! Those air-cooled Vdubs can be made to run also.. Back in the early 2000's, my son and I built a wicked VW sedan.. If I remember correctly, the engine was an 1835 cc dual port.. Twin IDA Webbers, and a bunch of EMPI stuff.. A hard launch would lift the front wheels and carry them a short distance.. Had to have a special transaxle case built.. Lots of bracing just so the ring gear would stay put..

Do you know or have you heard of Bear Barrillo (pronounced Barry-O).. I worked with him a few years during my stint at the VW dealership.. He has lived in Hawaii for the past several years.. He was our "consultant" on the VW project..

Re: 250ci Exhaust Question
 

I misspelled Bear's last name.. Barrilleaux is the proper spelling..

Re: 250ci Exhaust Question
 

Thanks, guys, I do appreciate it. I have the FSM, and I'll take a look at the cam timing process. Checking the valve lift is easy enough, and unless I've got a wiped cam lobe, I'll leave it as is.

I do have a "spare" 250 that I bought to scavenge a couple parts from (namely the distributor coil bracket... The correct one was impossible to find, and I paid $100 for the 250 + 3-speed), and it had been running not too long before I bought it (guess why he pulled it? LS swap! *bleh*). If I've got a wiped cam, I'll may freshen that one up and drop it in. Mostly because it'd probably take me a least another month to fix the one that's in there with my limited time, and I can still drive the truck in the mean time.

250 C.I. Engine
 

It sounds like you have the right attitude .

Too many build screaming machines that are not fun to drive anywhere and $uck down ma$$ive amounts of fuel then get cranky and try to sell it off only to discover that no one else like that particular look . build / whatever and soon one more nice old truck gets junked or broken for parts .

The thing that brought me to this particular series of light duty trucks was their inherent driveability .

You can make it a soft boulevard cruiser that also hauls the freight or a fun long distance touring rig or a basic work truck that doesn't beat you to death when you're all tuckered out from loading it up....

:chevy: .

Powerful & Smooth Engines
 

How you build any particular engine will have a major effect on how it runs, smoothness as well as power, fuel economy and life span .

When my son was 18 or so he built and "Civic Killer" engine for his 1963 VW # 117 Beetle, he listened to all my old man comments and spent a lot of time and no small amount of money making dead certain everything was perfect ~ every part was balanced to zero grams and close tolerances throughout .

When finished he took it to the local race track and zoomed right past the Hot Rodded Honda Civics and began whipping butt on the American V8's ~ he was very well pleased and surprised .

As he built it he told me he'd be happy if it lasted 10,000 miles (about one year of use) ~ as it turned out it had several years and well over 40,000 very hard miles of racing and daily driving on it when it was stolen never to be seen again . (TIP : don't be a poor winner, be gracious)

My point is : if you're careful and diligent even a 265 or 283 SBC engine will be a stormer, get decent fuel economy and last a long time even if you romp on it a fair bit .

My little old 250 i6 engine is tired yet runs like stink, those behind me always ask what's under the hood and are flabber gasted to see a vintage CHEVROLET ORANGE i6 under the hood .

No, I don't ever spin the tires, that's foolish and wa$te$ money, I prefer to be far out ahead on the open road or in the twisty mountains & canyons where quick driving is a art .