Left motor has weak idle, turns less rpm's than rt side
07-03-2018 | 06:43 PM
#1
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Left motor has weak idle, turns less rpm's than rt side
Motors were mercruiser 330 hp. 454's, now are 489 strokers, 9.2:1 compression, 049 heads with 2.19 intake, 1.88 exhaust, howards roller cam, part# 120405-10. I was running weiand 256 blowers with 2 holley 600's on them, but took them off at the beginning of this season. I really had too much compression for the blowers and ended up damaging some valves on the left motor. Had all that reworked at the beginning of the season. Bottom end looked fine, went back together with air gap intakes and holley 750's.
The left side has always been weaker on power than the right side, and since I removed the blowers, the left side has developed a weird idle, I have tried adjusting the mixture screws with a vacum gauge to no avail, it is an erratic idle, like it has a vacum leak. You get it idling about 1000 rpm, drop the motors in gear, rt side is strong about 550 in gear, left side wants to die. The left side only wants to do 4500 rpm, rt side has seen upwards of 5300 and wants more. I noticed the idle problem after I took the blowers off, although with the blowers the left side had somewhat of a blower surge at idle, and the right side just idled normally.
Here's more info..
Went from the blower setup to air gap intakes, different carbs on both setups, now 750 holleys, swapped the carbs no difference
Heads on left motor reworked this spring, no difference
Ran two different ignitions, GM HEI, now running DUI 7000 that advance 24* at 3k, they are set to 11* at idle, hasn't affected anything
I've checked the compression, all cylinders are within 2 psi of 150 on the left motor.
I pulled the timing cover last weekend and checked the cam timing, thought maybe the guy that put it together missed it, but it was set at zero like the right side.
The cam is a howards cam roller part#120405-10
Considering swapping the cams lol.
Am I wrong that it just about HAS to be valves/cam??
The left side has always been weaker on power than the right side, and since I removed the blowers, the left side has developed a weird idle, I have tried adjusting the mixture screws with a vacum gauge to no avail, it is an erratic idle, like it has a vacum leak. You get it idling about 1000 rpm, drop the motors in gear, rt side is strong about 550 in gear, left side wants to die. The left side only wants to do 4500 rpm, rt side has seen upwards of 5300 and wants more. I noticed the idle problem after I took the blowers off, although with the blowers the left side had somewhat of a blower surge at idle, and the right side just idled normally.
Here's more info..
Went from the blower setup to air gap intakes, different carbs on both setups, now 750 holleys, swapped the carbs no difference
Heads on left motor reworked this spring, no difference
Ran two different ignitions, GM HEI, now running DUI 7000 that advance 24* at 3k, they are set to 11* at idle, hasn't affected anything
I've checked the compression, all cylinders are within 2 psi of 150 on the left motor.
I pulled the timing cover last weekend and checked the cam timing, thought maybe the guy that put it together missed it, but it was set at zero like the right side.
The cam is a howards cam roller part#120405-10
Considering swapping the cams lol.
Am I wrong that it just about HAS to be valves/cam??
Last edited by hblair; 07-03-2018 at 06:46 PM. Reason: added engine specs to beginning
07-03-2018 | 08:06 PM
#4
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From: Cape coral, FL
I'd look for vacuum leaks. Have you tried simple swaps like cap and rotors? One of mine was acting up on the dyno and we went through everything. Swapped fuel pumps all kinds of stuff. Come to find out it was a weak pin on the inside of the brand new cap
07-03-2018 | 09:03 PM
#5
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Yeah I’ve been looking for vacuum leaks, none to be found. I have inline fuel pressure gauges right outside the carbs, they’re right at 7psi idling. I’ve changed entire distributors. I don’t think it’s electrical. I think it’s it’s valve train, just want to get you guys input. Weak lifter?
07-04-2018 | 12:12 AM
#6
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From: Omaha/LOTO
IMO, 450 rpm drop when put in gear is a lot. You'd be much better off running more base timing with less advance.
I think your carbs are on the small side as well, but neither explain and 800rpm WOT difference.
Do both engines have the exact same valve springs?? Possibly too heavy of valve springs not allowing valve to fully open??
Or valves adjusted too tight and fully compressing the lifter and making it act like a solid lifter??
Another idea is maybe the fuel tank pick up sucking in some air?? I would check WOT fuel pressure.
I think your carbs are on the small side as well, but neither explain and 800rpm WOT difference.
Do both engines have the exact same valve springs?? Possibly too heavy of valve springs not allowing valve to fully open??
Or valves adjusted too tight and fully compressing the lifter and making it act like a solid lifter??
Another idea is maybe the fuel tank pick up sucking in some air?? I would check WOT fuel pressure.
07-05-2018 | 11:29 AM
#8
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One thing I forgot to mention above was the high oil pressure this motor has, like 95 psi at 3500 rpm. I have not verified this with a mechanical gauge yet, but will, my friend was telling me if this is accurate, could very well be the problem, making the lifters act as solid lifters with no lash, causing the valves to stay partially open? Looks like I'll prolly be pulling the motor...
