Has anyone had a vacuum leak that DID NOT respond to external testing ( wd-40 ...stating fluid etc.) I've been fooling around with this 351w motor off and on for some time . I have the performer rpm cam and the specs call for 11 or 12 lbs. of vacuum but with the timing set right and a very rough idle of roughly 1000 rpm the best vacuum I can get is 8-10. It does not respond to spraying w-d or starting fluid. I rebuilt the motor ( I've done a few without problem). The Performer intake was used. Is it possible the intake is leaking on the underside of the intake . I've never seen this . Has anyone else?
Vaccuum Leak DURATION AS ADVERTISEDINTAKE: 270°EXHAUST: 280°DURATION @ .050INTAKE: 204°EXHAUST: 214°LIFT @ CAM (Lobe)INTAKE: 0.28"EXHAUST: 0.295"LIFT @ VALVEINTAKE: 0.448"EXHAUST: 0.472"LOBE SEPERATION: 112°INTAKE CENTERLINE: 107°IDLE VACUUM @ 1000 RPM: 15 I hope the specs show up, I just copy and pasted the cam card. As far as the vacuum modulator this car has an AOD (they use a TV cable no vacuum connection). And as for elevation, I'm in Michigan we're not far above sea level. I'll be pulling the intake this week . I just wondered if anyone had ever found a case where the intake gasket only leaked on the under of the of the gasket? It seems as though I have a vacuum leak but I can't find it externally.
I just noticed this spec calls for 15 lbs. Another Edelbrock source called for 11. That makes my problem even worse
It may be the valves are too tight. If it is a hydraulic cam then you should have no more than 1/2 to 1 turn on the rocker arm for pre load. (I use the 1/2 option on all mine)
Do you have power brakes? I would also disconnect and block any vacuum ports for testing to rule out any modulator or anything under the dash.
Good thought on the brakes , but no.....I've got manual brakes. The only vacuum port being used is the distibutor vacuum advance. Last night I pulled the intake manifold off to inspect. I saw nothing obviously leaking. I think next I'll pull the timing cover and verify that I got the cam lined up right. This timing set has 3 positions. ( straight up , advanced and retard ) I've always used the straight up. Has anyone used the advanced position. Other forums have sugested that valve clearance may be a problem
you'll usually have oil contamination issues(inside the ports and also the plugs) show up with internal manifold leaks like that. have you verified that timing is steady and running at least 20 degrees at idle speed? you can also do a quick and dirty test on the disty's vac diaphram to make sure it's not leaking internally as well. You can even disconnect it and jack the initial advance up to around 20+ degrees at idle to mimic its additional spark lead while it's disconnected. You'll also want to be sure that the carb's throttle plates are cracked slightly to partially uncover the transition slots and readjust the air bleeds to match up with the adjustament. Hook up the vac guage. Then set the disty's lockdown bolt to the point that you can barely move it and restart the engine. Then add more initial to see if the speed goes up. If it does?.. readjust air bleeds.. rinse and repeat until you get highest numbers. Most are surprised at how much lead a motor will tolerate at idle. I run up to and over 50 degrees on many that I tune. Then tune from there to what the motor will allow under load and at various rpm ranges.
I just installed a similar Crane Cam last week. Advertised Deg Duration 286/296, Deg Duration @.050 226/236, Lobe Separation 110 Deg, Lift .502/.520. At 1000 rpm i have 10 in vacuum In gear it,s 8-9 in vacuum. The Comp Cam i took out was almost exact spec to yours had the same vacuum. Maybe it,s a Michigan thing
I've had 14-15 inches of vacuum on cams larger than that and it's all in the ignition tuning. CR plays a part.. but the general rule is that bigger cams like more initial lead. Very high initial lead.. + lighter spring combined with heavy spring(this allows for an initially quicker mechanical curve that settles down once it hits the heavy spring to limit total advance from going sky high) .. + tighter set vac advance pot running off manifold vac usually always does the trick. Finely tuned air bleeds and screw settings never hurts matter either.
I do have a higher compression ratio 10-7 to 1, I don't know how this will effect the vacuum. I do believe a lot of the tuning issues i'm having are due to the inexpensive distributor i have on it, and my lack of knowledge with tuning a distributor not ashamed to say,,,I'm stumped.
All good advise guys THANKS. I was kinda running out of ideas for troubleshooting.I did play with the timing and the vacuum and could get all the way up 19 lbs vacuum.(over 30 degrees advanced) The idle continued to rise and I adjusted the RPMs down(but at that point I didn't mess with the idle screws).The car was driveable at that point but quite doggy at all RPMs. When I put everything back together this weekend I will use that procedure and readjust the carb as I change the idle speed. I was using the ported vacuum off the carb for my distributor. I'll give the manifold vacuum a chance. I'm frustrated, this motor should run better that it does, compression is very good ,all parts have been purchased new except the intake. But Hey! I'm no quitter I'll get it
