Rebuilt 350, hard on fuel

Welcome to the real world. The 350 has never been real good on fuel. Sounds like you may want to take it to a dyno shop and get it set up as close as you can. Then live with the mileage.

What kind of mileage did you get before the rebuild?

~jp

didn't you read?

Its only getting 8 or 9 mpg. Its horrible on fuel. What gives. Is it because of the cam? Carb? Please help. I love my truck(85 GMC 2500

"J>What kind of mileage did you get before the rebuild?

Yes...I did read... I must've missed the "twice as hard on fuel" part. I was asking specifics, but I suppose that'll have to do.

I may have missed it because I read it when I first woke up and was waiting on the coffee to brew.

Regardless...are you contributing anything useful to the thread or were you just lurking waiting on someone to post something you could attack?

~jp

I missed your "twice" part... I assume by that you mean that it got between 16-18mpg before the rebuild?

I put in a new 350 myself a while back. I also used the Edelbrock

If so it's jetted too big right out of the box. I've been battling rich-running conditions and bad mileage since installing it. I have replaced the jets, but in the process did something stupid, and as a result I haven't had the opportunity to see how it runs with the new jets.

Regardless, many folks that I consulted agreed that the jets were too big in that carb for my application (street driven K5 Blazer).

Worth looking into...

~jp

What series is the carb? 1405 or 1406? The 1405 is jetted richer for performance...the 1406 is jetted for economy. Well, according to Edelbrock. Every engine is different. A freshly rebuilt engine will get terrible MPG due to everything being such tight tolerances and needs about 500miles of mild driving to break in. Plan on a few thousand miles to loosen up enough to start seeing a MPG increase. I would leave the carb alone till then. I have seen far to many 'fresh' engines blow up in a few thousand miles or only get 20-30,000miles on em before they are in need of rebuild because of improper break in and/or too lean at the carb.

I usually do a 200-300 mile trip one way down the Freeway keeping RPM's between 2000 and 2500. I change the oil after a cool down after arriving home. Never run synthetic oil for the first 5000miles or so. Don't run Lucas or any other friction reducer either. An engine NEEDS friction to break in and for the bearings, rings, and lifters to take a 'seat'.

One quick note, don't push a .060 overbore engine too hard even after break in. They tend to run hot and are prone to crack cylinder walls. Neither is definite, but I have seen more of them that do one or both than I have that don't. Just my opinion and experiance...

"snay" wrote in message news: snipped-for-privacy@localhost.talkaboutautos.com...

Could be a few things, first thing would be carb jetting. Second, is the crossover exhaust passage in intake open or blocked. Next are you sure cam is timed correctly? Also are you running big tires because if you are thery can murder MPG especailly if engine is lugging a bit. Finallym do you have dual exhausts? Soetimes if these are done wrong you can get worse MPG especailly around town. One more thing, what do you consider a mild cam (duration/lift) and 60 over is asking for trouble with a 350 on block thinness. About 40 over is max and 30 over considered safer.

One more thing, what do

Just curious, small block 400 has a .125 larger bore than a 350. Is it siamesed to allow the larger bore without trouble?

I've been wondering about that term--siamesed...what's it mean?

~jp

It means that there is no water passage between each cylinder. That is the reason for the steam holes in 400's.

Sounds familiar...lol

Hmm...I can't quite visualize the steam holes you're talking about. I've always wanted to see a 400 up close. I've heard good things about them when built correctly.

~jp

The steam holes are nothing more than 1/4"(I think) holes above and below the siamesed area of the cylinders to allow hot pockets(not the microwave kind) to be released. The heads need the same holes. Any Mouse heads can be run on a 400 if the head gasket is used as a template for drilling the holes in the head.

If the stock rods are replaced with at least 5.7 rods, they last allot longer and have a more usable powerband. I Have seen many disappointed with the power of a 400 when they install a hot 350 cam...a 400 tones down cams, but are also easy to overcam. One little known fact about 400's, the 2bolt block is stronger than the 400 block...reason...the 2bolt main webs are thicker than the main webs on the 400. A set of splayed main caps on a 2bolt block make them EXTREMELY strong. 4bolt 400's also have 3 frost plugs per side where the 2bolts have the typical 2. Extra attention needs to be paid with piston selection too. For most street use, on typical pump gas(90octane and under), slightly dished pistons will need to be used in conjunction with larger chamber heads(76cc) to keep the compression down in the 9's.

So what you've heard is correct, their great...IF built right!

Hey boys,

Ok, we found the problem with the mpg issue. I havent tested it yet, but im positive I had a major problem. When I reasembled the carb, the two fuel rods didnt slide into the jets. They glanced off to the side and bent. I knew that engine wasnt working right! I didint have a timing gun, so I timed it by ear. It was at about 12 BTDC. Coulnt tune my carb at all, no wonder.

So once I get that sorted out, the truck should wotk fine. As long as you watch your guages and dont pound on her to much, bored 60 over is a good torker. Keep up on your oil changes and keep the redlining for special occasions and that motor will last me 10 yrs.

Around 300 ponies, 350 tork

later

Yes the block is cast differently and the the cylinders are joinned together in places to add more meat to them in those areas hence the term "siamesed".