Inliners International Forums General Discussion Engines 250 chevy timing

The 250 in my truck has recently begun running sluggishly as though a couple of cylinders aren't firing. I get occasional intake backfiring. I have pulled the plugs and cleaned them. I put the timing at around 8 BTDC. At that timing I get around 12 inches of vacuum, and the engine is on the verge of stalling. I tried setting the timing with a vacuum gauge to the point where I get maximum vac, around 18 inches, but at that point the timing is off the scale and the backfiring is more frequent. The timing is set with the vac advance plugged.
The engine will accelerate smoothly while sitting with the trans mission (T-5) in neutral. When I accelerate in gear the engine bogs. It will move down the road but it does have any spunk. Runs "flat".

Any suggestions?
Thanks.
Ken

What kind of carb are you using? You may have something wrong with the carb. Try turning the idle speed adjustment screw in if runing something like a Holley ar Rochested 2GC. If the motor doesn't start to die then your problem (at least one of them) is something like a blown power valve or clogged passage.

beltfed,
Thanks for getting back to me.
I am running Rochester 2SE, with vacuum secondary. I just put a rebuild kit in the carb. The power valve was not included in the kit so I did not replace it. The sluggish problem was happening before the rebuild. And rebuilding did not improve the problem. I know that does not eliminate the power valve as a problem, I just thought I would mention that.
Ken

Sorry, I have no personel experience with the 2SE so I can't help there. You have a vacuum gauge which is an invaluable troubleshooting tool. Have you used it to check for problems such a burnt valves, vacuum leaks, etc? There are a number of web sites like 'Holley' which will assist in learning how to use the vacuum gauge to troubleshoot the engine. I also recommend you visit the Stovebolt.com site. Look at the engine section under forums. Do a search for engine problems using words that discribe your problems. You will find plenty of reading. If that doesn't put you on the path to happy motoring post a question. Stovebolters will jump on your problem "like a duck on a junebug". I guess technically our 250 engines are not stovebolts but they will still consider you part of their family. Good luck.

At 8 BTDC, and 12 inches of vacuum, is the gauge needle steady or is it fluctuating. Seems to me that it could be a problem with valves, either adjustment or a burnt valve.

Larry

Have you done a compression test?

I don't want to be a pessimist but I had/have a similar problem. I can accelerate nicely from a low RPM to cruising speed if I do it very slowly. If I put my foot into it it sputters and coughs and backfire through he carb. When it gets to speed it runs smoothly.
Problem? Failed cam. I have removed the cam and sure enough all but two lobes are rounded and scuffed. It seems to be common these days with the "new API oil standards". They removed all the zinc and phosphate (ZDDP), which is hell on catalytic converters. Problem is the flat tappets, need the extra lubrication.
If this sounds right then stop driving and check for medal in the oil filter. Cut it open or just pour out some oil though the filter intakes. If it looks like veryfine medalflake paint, then thats probably your problem.
Hope for your sake I'm wrong.
Bill
Inliners # 1783

When you folks set the distributor timing with a timing light, do you leave the vacuum advance in place and functional or do you disconnect and plug the vacuum line?
Thanks.
Ken

Disconnect & plug the line.

Gentlemen,
Thank you for your input. To date I have found that two cylinders #2 and #4 are not firing. # 2 plug was "sooty" and # 4 was clean and wet (Fuel). I have confirmed that the plugs and wiring are good. I opened the oil filter and found no concentration of metallic flakes, although there are few present.
I believe the next step would be to check the rocker arms to esdure that they are being lifted.
Any comments would be appreciated.
Thanks.
Ken

I adjusted the valve lash on each cylinder and there is marginal improvement. I now have 16 inches of vacuum. I still have lousy acceleration. By the way, when I adjusted the valve lash I put cam in the proper position. I tightened the rocker arm nuts to the point where each push rod to remove any "spin". I then tightened the nut an additonal full turn. After doing this my engine would not crank. I went back and loosened and readjusted each push rod to just past the no spin point and it turned over and ran with marginal improvement.

Back to the carb, I ran my idle mixture screw all the way in and the engine would not die.

Any comments?

Next step is compression check. How do I know for certain if the cam is truly flat?

Ken

i have a question, when you say you put the cam in the proper position, does that mean you did that for each individual cylinder? tom

153
624
The way to adjust valves in my book,unless you have a huge cam.

Adjust #1Cyl when #6Cyl is in the rock position Intake & exhaust rockers arms are both moving.

Adjust #5Cyl when #2Cyl is in rock position.
Adjust #3Cyl when #4Cyl is in rock position.
Adjust #6Cyl when #1Cyl is in rock position.
Adjust #2Cyl when #% Cyl is in rock position & so-om.
Hope that makes any sense.
Do it this way takes less turns of the crankshaft to adjust all you rocker arms.
Hope that might help someone.

MBHD

El Viejo, I don't think you are adjusting the valve train correctly. If I remember correctly, after establishing you're at T.D.C. on the cylinder that your working on (see other posts above) and both rockers will move around some. Back the adjustment screw off some. The push rod should spin between the thumb and forefinger. Start to tighten the adjustment screw UNTIL you fill resistance to rod turning. This action has taken up all the slack in the valve train. Grab the rocker arm and give it a good wiggle to see that it is indeed centered. You can tell as the resistance to turning the push rod will change. Tighten the adjustment screw 1/2 to 3/4 more (personal choice) and lock it down without letting it turn further. As far as the carb is concerned...as I stated before, I have no working knowledge of that carb. On other carbs like the Holleys, if you turn the adjustment screw in the engine will die. If it doesn't, then something is amiss. Good luck.

Beltfed,
Thanks for your interest. The method I used to adjust the valves is from the Haynes manual. With the distributor cap off and the # 1 and # 6 firing positions marked I cranked the engine until the rotor was pointing to # 1 and adjusted Intakes on 1-2-4 and exhaust on 1-3-5. Then cranked until rotor pointed at # 6 firing position and adjusted Intakes on 3-5-6 and exhaust on 2-4-6. As I adjusted each valve I tightened the nut until I was unable to spin the rod between my fingers. On the first go 'round after all the spin was gone, I then tightened each nut one full turn. However after I was finished the engine would not crank so I went back and repeated the process but only tightened the nut maybe 1/8 of a turn after all the lash was gone. Fired right up. That "one full turn" has me puzzled. I can't figure why the manual would recommend one full turn if the engine won't crank.

I have just rebuilt another Varajet 2SE carb that I am going to install to see if that makes a difference.

Sorry to drag this out so long. I really appreciate all your help.
Ken

Since there are questions about the accuracy of how the rockers are being adjusted, and since the engine runs, you might consider ditching that method and using the running method. I highly suggest getting an old valve cover and cutting the top out of it to keep oil spillage to a minimum.



While the engine is running back off the rocker until it taps. Tighten until the tapping stops, that is zero lash, then continue to tighten to preload the lifters:

(this is the debated part below that I leave to the experts here...) I have heard to continue 1/4, 1/2, 3/4 and 1 full turn depending on the source, I would trust the replies here most as this is inline specific.

I would also say that depending on the state of your valves if they are possibly burned you may need to try preloading the lifters at different levels to compensate until you find the sweet spot. I have a burned valve on mine and crank mine down 1/4 turn past zero lash, and my engine runs good, any more than that, I lose power. I just think the live action way might work good for you because you can hear what is going on.

hope this helps

In reading the posts,unless I missed it, Ken, you are talking about solid lifters and not hydraulics,right?

Thomas,
Thanks for the suggestion. Will do!

Drew,
It's a 1979, 250 with a stock (probably the original) cam. I assume that the lifters are hydraulics.

Any suggestions on how to tell if my cam is flat?

Thanks gentlemen.

Ken

The method suggested by Thomas is a good one. Problem is a lot of guys don't have that extra valve cover to toast by cutting. Nor do they have a way to cut it. Anyway, if you try it don't rev the motor with the cut cover on, I did and had to clean oil off the inner and outer fenders plus my glasses for my mistake. Also Thomas make another good point, You might think about ditching the lash adjustment method by Haynes. I lost a new cam (CompCam) with less than 3,000 miles to the ZDDP problem and flat tappets. Other than somehow measuring the movement of the rocker/valve or pulling the cam out, I don't know how to check for a flat cam. Very interested to finding out if there is a way for a shade tree mechanic like myself.

About the only is to use a dail indicator Magnetic base holder
and mount it on the valve cover gasket rail. Find the base of what you think is a Good Lobe rotate the motor over and Use that as a base line for the others. You should even be able to do that with out taking anything apart. hope this helps.