Inliners International Forums General Discussion Hi Performance Slight miss on my Chev 250 L6

Hi all

I'm new here, signed up recently.

I am struggling with a slight miss on my motor. I am using the original Monojet Rochester carb. TI have also removed the points and condenser and installed the ignitor kit in the distributor.

I have also had the tappets adjusted per the book with the motor off (less messy that way).

People tell me that it seems as though the motor is hunting.

I have had the carb overhauled which fixed a slight hestitation on take off.
At higher speeds it is all good, just when i'm standing in traffic I feel as though the motor will stall.

I really do not know the history of this motor. I bought it with the pickup about 3 years ago and she never ran well. I have not overhauled the motor as she has lots of power on the open road.

I also had the compression tested and it was consistent across all 6 cylinders, around 9.

Please help, no one locally can give me any clues as to what the problem could be.

Check timing. Check carburetor float level. Correct as necessary, then adjust idle speed and mixture screw alternatively until you get decent idle speed, smooth and steady. Basically, starting with an almost closed mixture screw, start turning it OUT 1/3 or 1/2 turn at a time and wait to see how the idle changes. If the idle speed kicks up higher, keep turning. Once you find a spot where the idle didn't go up as you turned the last 1/2 turn, go BACK and leave it there. It'll be close enough.

Assuming hydraulic lifters: Adjust valves with the engine running at idle. It's not THAT messy, but it's more accurate. You do NOT want to end up with valves too tight, where the lifters are out of range and holding the valves cracked open when they should be closed. While idling, loosen up the adjuster until you start hearing valve clearance noise. Then turn back in until it gets quiet, and then another 1/2 to full turn in. Idle will change every time you make a change, so wait for it to settle.

That said, I had to hunt for a "miss" for a long time, never got it to run perfect... turned out all the valves were leaking a little and one cam lobe was more than half way worn down. Measure how far the valves actually open, to find any that may differ greatly from others. Hydraulic lifters would mask the wear.

Hopefully others here will add to and correct what I wrote, I'm going by shoddy memory and being too lazy to hit the books. Sorry Got two little kids running around who need attention

When I got a 292 out of a '71 truck the previous owner said the engine sometimes ran great and sometimes not. The first thing I checked was the dist. shaft. The side to side play was equal to or greater than the points gap. Whether this would affect an electronic module I could not say but worth checking.

The electronic module (I use Pertronix) almost eliminates all timing flutter and inconsistencies from distributor wear, love it.

I had a mechanic look at the distributer and he says that the shaft is ok.

The module I use is called the ignitor as that is what we have in South Africa.

I also put the points back in to see if that made a difference and it ran exactly the same.

Another mechanic also told me that I need to set the tappets with the motor running, will give that a try.

I have messed with the carb and timing with no success. Even made sure that I have no vacuum leaks.

Kavesh, spray some carb cleaner around the throttle shaft while the engine is running; if the idle speed increases then you have a vacuum leak around the shaft. As 70 Nova stated; the cam may be worn rendering unequal valve action cylinder to cylinder, when you couple this with the poor distribution in the stock manifold you can get a lean miss. You might also check the timing gears; Pull the distributor cap so you can view the rotor.....turn the engine counterclockwise about 1/4 turn.....mark the dampner at t.d.c.....turn the engine clockwise(normal rotation) untill the rotor just starts to move, mark the dampner at t.d.c. again.... the distance between the two points is the gear wear.Calculate the circumference of the dampner and divide that into 360....that will give you the degrees per inch....everybody has their own cutoff point but 6 degrees is what I consider absolute max on stock engines;even though the ignition timing is adjustable in relation to the crank, the valve action is not in sync. with t.d.c. and performance suffers. fats

Thanks for all the replies guys.

Fats, I have sprayed carb cleaner down the carb shaft and every possible area around the carb and manifold to hopefully find a vacuum leak. Even blocked off all the vacuum pipes and connected only one at a time to see if there may have been an issues with the booster or gearbox modulator, no luck there either.

Fats you did not mention that I must bring the motor up to TDC first before I do what you asked to calculate the gear wear.

Will also have a go trying to set the tappets with the motor running.

I really appreciate the advice, thanks again.

BTW, I am closing in on a deal for a 38 weber carb. I have heard that this carb with an adapter is much better for my motor, will give me better performance and mileage.

Have you checked the distributor vacuum Can to see if it is functioning?

You might try putting a vacuum gauge on your motor. It's sort of a lost art these days but some of the others are right, you may be having a little valve issue. It sounds to me like you have a little intermittent miss. If the vacuum gauge is bouncing around a little, you might be having a valve hanging up, or bad cam lobe

My distributor vac was initially not working. Once I connected the vac hose for the distributor to the lower vacuum port on the carb that sorted out the distributor vacuum. When the motor is gunned one can see the vacuum advance move. (I was surprised to note at the time that the original port midway up the carb was meant to be for the vacuum advance but it did not work even after a good clean).

I have tried to look for a vacuum gauge here in South Africa and it seems like hens teeth to find one. None of the local parts shops stock them.

I will definitely redo the tappets with the motor running to see it I can get it to run any better, wish me luck, maybe on Wednesday I will get time to do that.

Kavesh, T.D. C. is the most common reference but if you mark the dampner after you stop @counterclock wise and again at movement point @ clockwise relative to zero on the pointer the distance will be the same. I have a 292 in my shop that is a full 4 degrees off on the scale attached to the front cover; T.D.C. is actually @ the 4degree retarded point, you may have to fabricate a positive stop from an old plug to varify your dampner&pointer. While you have the valve cover off to re-adjust the valves take a hand pump oil can and flood the valve guide on each of the intake valves; since you are without a vacuum gauge you'll have to listen for the idle to improve or the miss to smooth out. If the exhaust begins to smoke on a particular guide then you have a clearance problem there; this is effective if the o.e.m o-ring seals have not been replaced with p/c type seals. The ported vacuum signal can be blocked if an incorrect gasket is installed between the base and mainbody; I don't know about your area but most of the kits I get are universal and come with multiple gaskets so they can be used on several carbs of the same basic design...good luck....fats

Ok so this is what I have done thus far.

Yesterday I reset the tappets with the motor idling with it at operating temperature.

Did as you guys suggested, loosened each in turn, waited for the tappet noise and then tightened so the noise went away and then one full turn from there.

Its made a very marginal difference as the slight miss is still there.

For now I'll just settle for it to be like that.

I am going to swap out the carb for a 38 weber which I just paid for. I should get it in about a week or so. Will need to probably overhaul.

Have any of you done this carb conversion before? I'm sure I will have to play around with different jet sizes. Any advise on this.

I would find the root cause of your miss problem first, get everything running right with the original carb first. Otherwise the problem will follow with the new non-original carb and it will be a nightmare to tune. I think you would just be making it harder for yourself to fix the problem. If it was not the carburetor, it won't go away. And you don't know what it is at this point.
Like I said, I had a slight miss too that never went away no matter what I did. Valve guides were very worn and valves leaking, not enough spring tension, and two cam lobes were worn out. Basically the head needed to be rebuilt and cam replaced. Measure how far the valves open or how much each push rod moves up and down to see if your cam is still ok. A dial indicator would be the best tool for this.

I really worry about the 9 lbs. of compression you got on all six cylinders! Maybe its 90 lbs? Or is it some metric measurement like hectobars or som'thin'?

The vacuum opening partway up the carb was for a timed vacuum advance system. This opening is ABOVE the throttle butterfly and has no vacuum at idle. As the throttle is opened vacuum will appear at this port and thus vacuum for the distributor. I personally like my distributor to be supplied with vacuum from the lower (manifold vacuum) port. This way the motor will have a higher ignition advance at idle and in turn a stronger, more stable idle. Get a Doug Roe Rochester carb book from amazon he has a good section on explaining the Monojet's workings an' you will be able then to "cast out the smog control devils" from it and turn the Mono into a VERY nice carb--try to order a Vacuum gage as you will need it to check and alter the power piston cut-in point which can make it a smooth operating carb. (you dont need no progressive dual named carb when you got a good Rochester around-just ask Jack Benny!) Surely you remember Jack Benny?

Who's Jack Benny?

Thanks for the advice guys. The compression is not in LBS. Not sure what unit of measure, probably metric. I spoke to a few guys who are mechanics and they say that it is perfect. They also mentioned that if a valve was not sealing or seating correctly I would have a much lower reading at that piston. So their conclusion was that the valves may be too tight.

Ok I don't know Jack Benny either. Will try and get the book you mention. Thanks for explaining the higher vac port. Makes more sense now.

I'm sure my issue is with the carb. Carb specialist where I live want almost the price of what one would pay for a new one to overhaul.

Are you guys dead against a weber. I have already committed to it and should be with me soon. At a recent car show in my town I met a guy who had the same motor as me but did the carb conversion I want to do and this guy could not stop talking about how good the conversion setup was. He mentioned that it gives better gas mileage and has a bit more legs.

Standard atmospheric pressure if I remember right is about 1012 millibars or 1.012 bars which is 14.xx psi(don't remember the decimal points). If his compression measurement was 9 it would likely be 9 bars or about 130 psi. Does that sound about right?

edit: After posting this I just had to go look it up. Standard atmosphere is 1013.25 mb or 14.696 psi giving just over 130 psi for 9 bars.

Jack Benny was a preannually 39 year old, frugal, comic (from the Bob Hope,George Burns,Danny Thomas era. Who, as part of his personna, had a black, gravely-voiced valet/cook/housekeeper/mechanic/chauffuer named Rochester who drove/maintained Benny's car, a Maxwell.

The cranking compression in bar, psi, Hg" etc. is not, ever, the compression ratio times atmospheric pressure - it just looks like it, but it's a coincidence.

Compression ratio is a volume thing as I recall, not taking into account density. The ratio is a constant whereas atmospheric pressure is a variable.

There are two different "compression ratios" when it comes to engines. I'll try to clarify the differences between the two most common ones.

The most common one everyone talks about is STATIC compression ratio, or "CR" as it is often abbreviated. It is a calculated number, measuring the volume swept by the piston from bottom to top dead center, plus the unswept area between the top of the piston at TDC and the cylinder head. The number means how much the original combined volume is reduced when the piston moves from bottom dead center to top dead center. It is NOT a measure of pressure. It is a RATIO, like 8.5:1 This value does not take into account ANY environment variables, air density, not even valve overlap or cam profile (compression bleeds off when a valve is open when the piston is traveling UP).

More accurate and useful, yet much harder to calculate, is DYNAMIC Compression ratio. It is basically STATIC compression ratio corrected with pressure bleed-off caused by valve timing. The end result is always lower than static CR. It is still not a unit of pressure, but a ratio just like static CR.


EXAMPLE: An engine is calculated to have 8:1 static compression ratio, and 7:1 dynamic compression ratio. Someone decides that 7:1 DYNAMIC CR is optimal for a good, efficient combustion, and should not be deviated from in either direction. Then they want more power, so they install a race cam that holds the valves open longer than the stock cam, in hopes of making tons and tons of power.. But the valves now bleed off some of the compression, way more than before. Static CR is still 8:1, but dynamic CR has dropped to 5:1 and the engine is a pig to drive, does not respond well to throttle, and has weak low end power.
To correct this, they need to compress the fuel more, and that is achieved by reducing the compressed volume. So they install domed pistons, machine the head so the combustion chambers get smaller, and mill the block so there is less "deck", unswept area above the piston at TDC. Now their STATIC CR is up to scary 10:1, but the dynamic CR is back at a safe 7:1 level and everything is hunky-dory and they have a happy engine.

ACTUAL compression is a pressure reading taken from the spark plug hole with a pressure gauge. It shows true pressure at VERY low rpms, only the starter turning the engine over. Every little leak will reduce this number, and the density of the air does have an effect, however minor in real life.


Clear as mud, huh?

Thanks for that detailed explanation 70Nova.

Just to confirm my actual compression ratio was about 8.7:1

Hi all

I got myself a vacuum gauge recently, got it from an old retired mechanic. I connected it to a spare port on my carb at the base. (Is this ok or must it be connected to the manifold?).

The reading I am getting at idle is about 28 cmHg. Its the blue zone on my gauge which says "Drive" According to the gauge I should be between 45-50 cmHg at idle.

When I gun the motor the needle drops and then it comes back to around the 28 mark.

I fiddled with the air/fuel mixture and timing but the needle does not change much. The needle is also not perfectly steady.


Reading on the web, seem to suggest that my motor is worn out if its giving these readings :oops:

Any advice here appreciated.

Not enough idle spark will look like that.

Hi Panic

You need to explain that to me, although I am handy in the garage I do not have the technical knowledge. Once I fully understand I would be able to investigate further.

Thanks

If the initial spark setting is too far retarded, the throttle disc must be held open farther to get a good idle speed, which reduces idle vacuum.

Kavesh,
Lots of people here run the weber (not me except on the Pinto) it is a great carb. I think the message was just, dont be surprised if the new carb still has the same problem. That would just mean its PROBABLY not carb related.

If it DOES fix things, then the culprit was probably your other carb. So I'd try it, for sure, just be prepared to hear the same miss.

One more thing I'd try.
I've had engines miss when the cam/lifters were worn and tired, lifters acting almost like solids. In that case, they WILL miss if you turn em 1 full turn down.

So you might try, set the lifters running AGAIN (more mess).
But, just set them til they barely stop clattering and still just barely "tick". You dont want to run it that way but see if it makes the miss go away and gives a smooth idle. It did for me. For sure it was a tired engine but you might have the same problem.

Hi all

DeuceCoupe, thanks for your comments. I will look at the lifters. I actually was going to look at it today, but as I was opening the side cover oil began to leak so I tightened the bolts again, I need to drain some oil first I think.

Right now my big concern is the low vacuum reading. Will look at the weber once I can figure out the vacuum issue.

I did further testing today and disconnected and blocked off all ports and still the reading was the same. Now I know that its not my vacuum lines or whatever it is connected to.

What Vacuum readings do you get on the stock 250 L6? I was thinking perhaps this motor has a lower reading....maybe wishful thinking.
My vacuum gauge is steady so I cannot determine what could be the problem, like if the needle was bouncing about.

Thanks again for the advice.

Kavesh,
Heres a test drive on youtube

http://www.youtube.com/watch?v=cxQZUYcqm4A

I made a movie to show the carb runs good.
But, at about 1:56 you can see the gages with it in Drive.
The vacuum gage is 2nd from the right and marked 0-10-20-30 inches tho you cant read the numbers. But, you can see it runs 18-19 inches of vacuum in drive. This is a stock cam 292 but the stock cam 250 is similar. The cams in these are very mild so they have a lot of idle vacuum.

If your idle vacuum is low but steady and its not a vacuum leak, I'd suspect either
* VERY wrong timing, way retarded, damper messed up maybe?
* Carb idle circuits badly clogged. Try closing the choke plate almost all the way while its idling warm. If it speeds way up as its almost closed, then you either have a vac leak or dirty idle circuits.
* Cam gear is a tooth off, tho thats unlikely.

Man your motor sounds really good. Surely that is not stock. My stock motor is so much more silent.

I will do as you mentioned with the choke on the carb and see what effect that has.

My timing is not wrong a I did have it checked with a timing light and we had it set originally at 4*, but then we played around a bit over a time going in increments all the way to about 12*. The miss never went away.

You know I was on another forum and a retired mechanic that worked at the GM factory in South Africa, said that the L6 250 had much lower vacuum than other engines. But this contradict what you said above. I don't know if he has tested it on his motor as he also has the same motor as mine.

But you have the gauge connected to your motor and you know its reading much higher than what I am getting. The only question I would ask is did you ever test the vacuum when you had the original Monojet carb, which is what I still have on mine?

Thanks again for the response much appreciated.

Thanks yup its stock 292 (except for the 4bbl), but has a big 3" muffler thats about as big as a bathtub on it, maybe thats the "rumble" you hear.

What I meant by the timing maybe wrong is, the DAMPER may have slipped, sometimes they do that. So the MARKS read 12btdc but it could really be 20 AFTER TDC or 30 atdc or just about anything, who knows.

At idle, just try advancing the timing using the marks, go 12btc, 20btc, 30btc, even 40btc. (Make sure to set it back where you had it before you shut it off). If it runs better and the vacuum goes up, thats a hint that the damper slipped or the pointer is wrong or something.

If the marks are "right", the idle speed and vacuum will go up just a little bit until youre at say 15-20btc, then about the same, then at about 30btc or 40btc at idle it will start to skip and run rough. Thats how a normal engine behaves if the damper/marks are right.

Yup Ive had many of these, 194cid, 250cid, 292cid (never had a 230cid yet), all show about 18-19 inches vacuum at idle no matter what carb is on top.

Im kinda wary of the Monojet carb, never had much luck with em. Some folks here like em, kinda like the Quadrajet you just have to be an expert with em I guess. Maybe its just the carb, have you tried a different one?

All my cars are early Novas and came with the old Rochester "B/BC/BV" carb. Not the greatest on gas but very simple and reliable. Find one of those, or a Carter YF, or a Holley 1940 carb, after all you need a spare carb anyway!

Good morning.

I tried playing with the timing yesterday as you suggested to see what happens to the vacuum reading. I advanced the timing gradually until it wanted to just about cut off the engine an there was no noticeable change in the vacuum reading.
So I guess that my damper has not slipped and is in the right area.

MY monojet is the MV series. I do have a spare but if I have to install that I will need to go buy a new kit as there are absolutely no gaskets or seals in it, its bare.

I would rather wait for installing my 38 weber. Busy hunting for a spare manifold so that I can machine it to increase the aperture for the weber. I have read that this is the preferred method rather than using an adaptor plate. Would you agree with this theory?
I don't want to botch up my only good manifold if the machine shop stuffs up my manifold, just being cautious I think...LOL

You know except for idle, on the open road you cannot feel the miss at all. I can cruise at 120km/h and she rides so smoothly.

My big gripe is that my friend who has a chevy turbo 400 V8 with TH 400 box, highly modded motor gets the same mileage as I do and he drives like a maniac, while I drive like Miss Daisy.

I am sure that my carb has a lot to do with my vacuum and mileage, if only I knew why and how to fix it, short of a complete replacement.