The Stovebolt Site Forums The Shop Area Engine Shop 270 bogging me down

Here's my question, it's about what I need to do with my ol' beastie as I seem to be at a point where I need some advice.
So! I have a 270, it's in really good shape so far as the engine goes, recent refurbishing and a fresh set of seals. Stock exhaust manifold has a slight leak on the 4/5 flange but nothing bad, everything else is tight. New pipes, good breathing air filter, timing is dead on the ball at idle and winds up as it should, everything looks perfect.
At idle,...
Purrs like a kitten (a 8,000lb kitten, but yanno!) at idle and accelerates pretty well. Puts out a bit of smoke on startup, but I believe that may be oil gettin into the valves.
Where it's killin me is when I drive her. It's a little dodgy @ about 1500 RPM, seems to miss a little bit, but can get up to steam OK on level ground and runs pretty well. Then she starts to bog down a bit, MUCH worse if climbing even a slight grade, and I get some black smoke out the pipes. At it's worst it felt as if I was hitting on 2-3 pistons until I backed out of it and coaxed her along at lower RPMs.
Thinking I was flooding out so I rebuilt the carb (it's a Rochester C) and checked all the seals, float level etc. Put it back on, seems to be working great, until I get her on the road and expereince the same issue.

SO!
Here's where I am, I've verified everything is working and firing, just doesn't wanna run up all the way.
Thinkin I have too much jetting on the carb, that or the power valve is coming open and staying that way.
Or, I have a stock vaccum timing advance setup I can toss under my dizzy and see if that helps with my burn issue.

Any insights oh gurus of old iron?

OK, update.
I took Tater for a spin this evening to get her warmed up (& show it off to an old freind), and it ran pretty well while it was cold. It was after a couple miles and everything was nice and hot that it really loses its oomph! When it's hot I can barely get out of 3rd on level ground. Still runs smooth at idle and slight throttle, but missing horribly with more pedal, the more pedal the worse the misfire and bogging.

Just for info, here's a couple shots of my engine:
Engine right side
Engine Left side

Another thought I had since heat seems to be an issue, would the fact that I need to either fix or wire open the manifold heat valve be affecting me this way?

Freq2002,

Beautiful engine compartment. Have you thought about the coil. Is your rig positive ground? Is the coil wired for your polarity.

While black smoke and chugging point toward the carburetor, if the ignition system is failing, or the points aren't allowing the coil to fully saturate, your going to get incomplete combustion.

A failing coil, or valves that are too tight would be on the list of things to check. The power valve must have the ability to sense engine vacuum....is the base gasket installed properly.

I don't think the heat riser would have much bearing on your problem. It looks like you have a Hydrovac. Is the vacuum line from the manifold snug and leak free?

Stuart

Thanks for the reply Stuart.

The Hydro-Vac line is tight, that's one of the fixes I made already, and the carb gaskets seem to be OK as well. Dont believe I have issues with vac leaks, but I'll check again.

I checked the valves after I'd gotten it running the 1st couple times, so they are all to spec.

The coil is a newer Bosch Blue coil and it is wired for a negative ground system. Higher output than the old stock GM coil that was on it.
The only issue I can think of with the ignition system is that there's no resistor between the coil and the ignition as the original wiring had. I wouldn't think that an updated coil would need one, might be whats gettin me since I didn't update the condenser on the dizzy.
I should probably do away with the points alltogether and get a optical or magnetic trigger, even thoug I so adore fiddling with my points. :p

Unless the coil is marked and there is some indication that it has an internal resistor, it needs an external resistor. It will not perform correctly with the full 12 volts to the coil.
Fred

Hy Freq2002, New coils can be purchased in either resistor or non resistor type, if you bought it new it should have installation instructions with it, if not sometimes the coil itself has wheather or not a resistor is required stencilled on the outside of of the coil. If the coil is designed for a resistor, and you don't have one the coil will overheat and break down, giving you little or no spark. The coil sould be mounted snuggly in its bracket and on a surface that allows it to disapate heat, hope that helps.

Bosch Blues dont come with a resistor and take 12VDC right from the ignition, taking that into account I'd have to say they have an internal resistor. This is actualy my standby coil I keep in my VW parts box and I have run it before on other engines.
Even though they are notoriously relaible I may pick up a new one just in case this one has seen it's share of sparks and retired on me.

Im still thinking carb issues, just cause I triple checked everything several times on mine and had my carb apart a dozen times before the gunk worked out of one of the passages. When you had it apart did you blow all the passages out with compressed air?

I melted a coil by not having the resistor on it once, Oops...

Now a question for the carb gurus, I thought a rochester would be to small to run a 270??? :confused:

I'm actualy wondering the same thing as RustedNuts myself. I looked at everything I could find and the Rochester B's were only noted on smaller engine, so I'm wondering if this could be a source of my issues as well.

It does run quite well when at lower throttle settings, just cant hack it with more opening, so I'm wondering if my jetting is all messed up in relation to my fuel needs. Still, the black smoke seems to tell me I'm running too rich, not too lean.

I wouldn't mind finding a Holley 1904 to pop on there, any of the three carbs listed in the manual would be good really. At least then I'd have some more info in my factory manual to work with, and better yet, I have the numbers for the jetting and such right there as well. I learned from working with dual Weber setups that there's not much room for guessing games in carburation.
Hey,... There's an idea, maybe I should fab up a manifold and slap on a Weber! :p

Research says that in '56, 270's were usualy topped with a Zenith 228BV11/2 series carb or the Holley 1904HS. No luck finding any of these online or anything more revealing about the Roch I have on it now.

I did get bid in on a NOS Rochester Monojet that's set up for a '69 Chevy 296. Basic carb, manual choke, jetting and airflow should be good for my 270 so I'm putting a bid on it.
Any of the fuel/air guys see any reason why this would be a bad choice?

Thanks!

If you are getting black smoke you are right the carb is rich. What Stuart was refering to on the gasket is the carb has a vacuum hole in the bottom that must have a passage to the intake manifold. If there is not a cuttaway in the gasket or it is not lined up it will cause this problem. Make shure the power valve piston goes up & down & the ball with a spring behind it seals the hole. Take a look at the spark plugs. If they are carboned up it is a good indication it is too rich & compounds the problem. You may have to clean them. The monojet is a good carb.

You might also check to make sure your mechanical distributor advance is working- your problem sounds suspiciously like a timing issue.

I have to check the alignment of the base gasket cutouts, but yes they are there. All of the mechanical parts of the carb work perfectly on the bench, the power valve moves freely & unseats the enrichment ball. I'm wondering if something is keeping the power valve from closing when it should (manual says enrichment opens at > 5"Hg). If it's just staying open it would act like the choke when cold & give extra fuel, but bogging it down when hot. Possible as it seems to not like being choked down for more than a couple seconds even when stone cold.

Timing had occured to me as well, but it I have to wonder as the engine runs well at speed when it's cold, timing issues tend to be the same regardless of fuel/air delivery.
The mech advance on the dizzy was working fine yet I may drop in my other unit to see if there's a signifigant difference. I'm also leaning towards instaling the vac advance mechanism if for no other reason than to improve my beasties fuel economy.

Down to the wire on the Monojet, people have started bidding again. If I dont get it at least I know what to start looking for. The internet can be a great thing.

Lots of black smoke indicate that not all of the fuel provided is being burned. This would probably be either (A) defective ignition, or (B) defective (or wrong) carburetor.

Looking first at ignition:

Since the coil is non-original, it would be nice to know both the inductive reactance of the coil, and the capacitive reactance of the condenser. These should be opposite and equal, thus leaving only a resistive component to the impedance of the ignition system. IT IS POSSIBLE TO INSTALL A HIGH VOLTAGE COIL OF THE WRONG INDUCTANCE, AND HAVE LESS VOLTAGE AVAILABLE THAN WITH THE ORIGINAL COIL!

Looking at the carburetor:

Since the carburetor is non-original, it would be nice to know exactly which Rochester is being used. Quite possible that some previous owner mis-matched a 216 bowl and a 235 throttle body, causing a passage mis-match and a power valve always open. Also, the 216 internal venturi would be too small for the 270 (as would one from a 235).

The fact that the issue seems to be heat related, would cause me to look at the ignition; but too small a carburetor could also be the culprit.

Jon.

Thanks for the input Jon. The condenser has been making me wonder, I can always take the one off my spare dizzy that I used with that particular coil and see if that increases my spark energy.

The carb is an enigma really. The casting numbers all match up, and I verified the passages all seem to line up as well. The onlt thing that boggles me is that it 's very similar to the Roch B series, uses the same rebuild components and all, yet the casting says Roch C on the horn casting.
The info we did find on it in the manual at Napa seemed to point to a smaller passenger car carb. I am hopeful, should I win the bidding, that the Roch Monojet for a 292 will give a more correct mixture.