Hey fellas! I finally finished getting my motor put together last winter and got it broke in. Now come the fun part, tuning, which appears to be a completely lost art with numerous opinions. I'm super new when it comes to all things motor-related, so I'm reaching out the brain trust here to either help solve my issue or offer consolation that everything is fine. I'm running a .060 over 283 with a Rochester 2 jet carb (3x2 intake with dummy secondaries) for now. When running at startup/idle (1,200 rpm) vacuum gauge reads 17 inHg, but the needle fluctuates rapidly about 1inHg. Once the motor is warmed up, the needle fluctuates rapidly about 2~3 inHg. I've gone through the rockers twice now assuming my preload was wrong. Is this normal or is there a larger issue here (blowby at the lifters, valve guides, something else)? I've been told both. I'm thinking my next diagnostic will be a bleed down compression test. Thanks in advance!
A rapid vacuum gauge fluctuation points to an engine miss. I'd definitely do a compression test or a cylinder leakage test first. I don't know what you have for exhaust, but an infrared thermometer might help you find the weak cylinder.
The needle should be steady, 17 in. is fine. Have you fine tuned the idle air mixture screws? Leave the vacuum gauge hooked up, adjust the screws one at a time. With the idle speed set where you want it adjust the first air mixture screw to achieve the highest vacuum reading, This may raise the idle speed, so reset that before moving to the other screw. Now adjust the other screw to achieve maximum vacuum reading. Again, reset the idle speed if needed. Now repeat the whole process at least one more time, wouldn't hurt to do it a few more times. As a kid I used to watch my father do this with just his ears, listening to the engine run; and placing his hand on the fender of the car with an index finger pointing out, watching the finger shake. As he tuned it his finger would shake less and less. I'm not that good, I use vacuum gauge and/or a tachometer. But spending some time getting this right pays dividends in a smooth running engine that transitions well from idle to acceleration.
I did run a total compression test with some variability and one slightly weaker cylinder. That said, I'm torn here because the motor probably has less than an hour of total run time. From what I understand the rings/valves may not be completely set yet.
Short answer... not yet. I could see if the motor had low/high vacuum or maybe not idling correctly maybe that would create an issue. Right now, it seems to idle fine. Would having the idle adjustment screws not dialed in yet create the rapid fluctuations?
You may want to pull the valve covers and make sure all the rockers are moving the same amount. I don't want to be a doomsayer, but yours wouldn't be the first to flatten a cam lobe during intial break-in/tuning. It'll be good to rule that potential issue out.
Yes, that's why I mentioned it. Other things that could cause that, like sticking valves or loose valve guides, are not probable in a fresh engine. Poor idle mixture adjustment is. Do that first.
Worth checking you’ve got the HT leads on in the right order and you’ve not got a dodgy plug or HT lead.
That's not a real answer. In order to help you we need to see numbers. What you consider slightly weak may be your problem. You should get consistant numbers even on an engine that is not fully broken in.
Like @Blues4U says anything related to valves shouldn’t be an issue on a fresh engine but a leaky head gasket could be. As stated get the carb tuned, check the ignition also once all that is set if you still have an issue then you’ll have to dig deeper. Hopefully just mis adjusted carb. Dan
Here are the compression numbers right after the 30 minute break-in. To me it seems pretty variable, but I should probably try again now that its has a little more run time. I probably wont get a chance to play with the carb until later this week. My only time to work on the car is at night, and that doesn't work well with lake pipes, neighbors and sleeping kids. 8 - 145 6 - 160 4 - 165 2 - 135 7 - 155 5 - 148 3 - 160 1 - 145
Where are you connecting your vacuum gauge? A full manifold vacuum source on the carb or in a plenum area of the manifold will usually provide the steadiest, most accurate reading. If connected in a more remote area of the manifold, or near or in an individual runner, you may see a slight pulse or fluctuation in the vacuum reading.
You are right, they are all over the board. That's a lot more variation than I would expect from a newly assembled engine. Number 2 is definitely low. A cylinder leakage test would point you as to why. So the engine is 0.060 over, did you have it bored or was it that way when you got it? Was a machine shop valve job done?
During breaking in how long did you run and at what RPM was it running? During the break in period the rings should be pretty well seated. Most cam manufactures recommend a break in of 2000 to 2500 RPM for 20 to 30 minutes. The valves don’t need a break in. They should seat after valve job. I’m assuming you have hydraulic lifters. https://www.jalopyjournal.com/forum/threads/tech-a-simple-way-to-adjust-valves.168345/ I’ve done it this way but can be kinda messy. With engine running: 1. back off the rocker nut until the rocker clatters. 2. tighten slowly until the rocker stops clattering. 3. turn 1/4 turn tighter. Dan
- I didn't think about that at all. It is a port that was drilled and tapped behind the center carb, which I believe is in line with a runner. I'll need to double check. - Break in consisted of 3 cycles of 10 minutes running at 3,000 rpm, then a cool down. - This is the cam... https://www.compcams.com/xtreme-energy-218-224-hydraulic-flat-cam-for-chevrolet-small-block.html - I had the machine work done by a shop in Indy. The machinist said the heads looked good when he inspected them. I believe he replaced the guides.
20 lbs difference is quite a bit but if it were me I wouldn't sit there thinking of reasons for getting my undies in a knot over it and would wait until I had about 500 road miles on the engine before I did any serious checking.
Nice cam, shouldn't cause rapid needle fluctuation of the vacuum gauge. Should make some good streetable power.
I agree, but it's more like 30 lbs difference between lowest and highest, and at just under 20%, that's a lot. Still, I wouldn't be tearing it down at this point either.
The fluctuation and lower pressures on some cylinders could be mis adjusted valves - too much pre load keeping a few valves from closing completely. Dan
I'd wait until it had some real miles on it, with an actual load against the motor pulling the weight of the car for at least 500 miles before I'd get too excited. Also never seen breaking in a motor that way. Always set them at 2500 rpm for 30 minutes on every motor I've built ( probably 15). Also I built mine tight, then once cam broke in, then I'd drive it in close stop and go driving not holding back on throttle either. I'd let the motor get good and warm and pull into a car wash, run water thru radiator, engine running, until it hit 150 and do it all again, hit 210, hit carwash, cool to 150 repeat. After about 3 or 4 times and it'd never get over 190. Over 100,000 on several motors and never had valve covers off of them.... .
That camshaft in a 283 could definitely cause vac gauge fluctuation at idle. More gauge flutter the slower you idle it. It'll be a fairly quick flutter since all cyls will be doing it. In a 350 it'd be a lot less. I'd drive it a while before worrying about the compression variation. Just be observant of any symptoms and put some miles on.
I put a small clamp on the vacuum line and pinched the tube down till the vacuum became steady; doesn't seem to affect the gauge reading as you manipulate the throttle. As previously said put some miles on it and I would put the vacuum gauge in the vehicle where you can see what it does while engine is under varying loads.
Try setting lash running back off until clicking tighten until quiet + 1/4 turn and see what happens.
A couple of guys have recommended this, and I would do it as well. Get some oil deflectors like these to keep the mess down as you adjust the rocker arms. https://www.summitracing.com/parts/...MIovqv7NqhjQMVFVR_AB1UYCvjEAQYASABEgJ9TPD_BwE
Here's my plan of attack for this weekend, in order. I feel that if I memorialize it in a post it will hold me accountable to following up with ya'll. 1-Relocate the vacuum gauge to the carbs throttle plate port. 2-Recheck timing. 3-Pinch line slightly until gauge has a more steady reading, then fine tune idle adjustment screws. 4-Pull valve covers. With the engine running back off rocker nuts 1/4 turn sequentially and see if I've got too much preload on one or more valves. When I first built then motor I did 3/4 turn. The most recent valve adjustment I used 1/2 turn. 5-Unbunch my panties and just finish building the damn car.
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